Gene algD coding for GDPmannose dehydrogenase is transcriptionally activated in mucoid Pseudomonas aeruginosa

Pseudomonas aeruginosa AlgZ, a ribbon-helix-helix DNA-binding protein, is essential for alginate synthesis and algD transcriptional activation

The Pseudomonas aeruginosa algD gene is the first gene of an operon encoding most of the enzymes necessary for biosynthesis of the exopolysaccharide alginate. Transcriptional activation of algD results in the high-level synthesis of alginate, an important P. aeruginosa virulence factor with antiphagocytic and adherence properties. Previously, we have identified a protein(s), AlgZ, expressed in mucoid P. aeruginosa CF isolates that specifically bound to sequences located 280 bp upstream of the algD promoter. Mutagenesis of the AlgZ DNA binding site and transcription assays were used to show that AlgZ was an activator of algD transcription. In the current study, the monomeric size of AlgZ was estimated to be between 6 kDa and 15 kDa by electroelution of a protein preparation from an SDS-PAGE gel and analysis of the fractions via protein staining and electrophoretic mobility shift assays. A biochemical enrichment procedure, resulting in a 130-fold enrichment for AlgZ, was devised, the protein identified and a partial amino-terminal sequence obtained. Using the P. aeruginosa Genome Project database, a complete sequence was obtained, and algZ was cloned and expressed in Escherichia coli. Expression of algZ was sufficient for the observed AlgZ DNA binding previously observed from extracts of P. aeruginosa. A protein database search revealed that AlgZ is homologous to the Mnt and Arc repressors of the ribbon-helix-helix family of DNA-binding proteins. An algZ deletion mutant was constructed in the mucoid CF isolate FRD1. The resulting strain was non-mucoid and exhibited no detectable algD transcription. As an indirect role in transcription would probably result in some residual algD transcription, these data suggest that AlgZ is an integral activator of algD and support the hypothesis that both AlgZ and the response regulator AlgR are involved in direct contact with RNA polymerase containing the alternative sigma factor, AlgT. The cloning of algZ is a crucial step in determining the mechanism of algD activation.

Genetics of O-antigen biosynthesis in Pseudomonas aeruginosa

Pathogenic bacteria produce an elaborate assortment of extracellular and cell-associated bacterial products that enable colonization and establishment of infection within a host. Lipopolysaccharide (LPS) molecules are cell surface factors that are typically known for their protective role against serum-mediated lysis and their endotoxic properties. The most heterogeneous portion of LPS is the O antigen or O polysaccharide, and it is this region which confers serum resistance to the organism. Pseudomonas aeruginosa is capable of concomitantly synthesizing two types of LPS referred to as A band and B band. The A-band LPS contains a conserved O polysaccharide region composed of D-rhamnose (homopolymer), while the B-band O-antigen (heteropolymer) structure varies among the 20 O serotypes of P. aeruginosa. The genes coding for the enzymes that direct the synthesis of these two O antigens are organized into two separate clusters situated at different chromosomal locations. In this review, we summarize the organization of these two gene clusters to discuss how A-band and B-band O antigens are synthesized and assembled by dedicated enzymes. Examples of unique proteins required for both A-band and B-band O-antigen synthesis and for the synthesis of both LPS and alginate are discussed. The recent identification of additional genes within the P. aeruginosa genome that are homologous to those in the A-band and B-band gene clusters are intriguing since some are able to influence O-antigen synthesis. These studies demonstrate that P. aeruginosa represents a unique model system, allowing studies of heteropolymeric and homopolymeric O-antigen synthesis, as well as permitting an examination of the interrelationship of the synthesis of LPS molecules and other virulence determinants.

Pattern of changes in the activity of enzymes of GDP-D-mannuronic acid synthesis and in the level of transcription of algA, algC and algD genes accompanying the loss and emergence of mucoidy in Pseudomonas aeruginosa

The low activity levels of the four GDP-D-mannuronic acid-forming enzymes, even in highly alginate-producing strains of Pseudomonas aeruginosa, have made it difficult to compare enzyme activities accompanying the loss/acquisition of mucoidy. Using optimized conditions, we compared the specific activity of these enzymes in three different mucoid P. aeruginosa cystic fibrosis isolates, in their nonmucoid spontaneous variants, and in mucoid variants that emerged during extended incubation of these nonmucoid forms in acetamide broth. A correlation was established between the promptness of emergence of the mucoid forms and the differing sensitivity to nutrient-limitation-induced death of the nonmucoid compared with the isogenic mucoid population. Consistent with the undetectable levels of algD mRNA in nonmucoid forms and with the concept that the step catalyzed by the algD-encoded GDP-mannose dehydrogenase (GMD) is a key step in control of the alginate pathway, GMD activity was undetectable or showed negligible values in nonmucoid variants and correlated with alginate production. However, phosphomannose isomerase (PMI), phosphomannomutase (PMM), and GDP-mannose pyrophosphorylase (GMP) activities in the nonmucoid forms were only slightly (40-70%) below the values in the mucoid forms. Nevertheless, no transcripts homologous to algA (encoding a bifunctional enzyme that possesses both PMI and GMP activities) were detected in the nonmucoid form, and the levels of algC (encoding PMM) transcripts, although detectable in the nonmucoid variants, were, in general, much higher in the mucoid forms. These apparently intriguing observations were cleared up by the identification of two algA functional homologues in P. aeruginosa, recently reported by others, and by the identification of one algC homologue, in contig225 of the PAO1 genome sequence, defining a polypeptide with a deduced amino acid sequence that showed significant homology with that of enzymes of the phosphohexomutase family found in databases. Results are also consistent with the requirement of PMI, GMP and PMM activities for the supply of GDP-D-mannose to (at least) A-band lipopolysaccharide synthesis, while GMD channels this precursor into the alginate pathway.

Synthesis of the A-band polysaccharide sugar D-rhamnose requires Rmd and WbpW: identification of multiple AlgA homologues, WbpW and ORF488, in Pseudomonas aeruginosa

Pseudomonas aeruginosa is capable of producing various cell-surface polysaccharides including alginate, A-band and B-band lipopolysaccharides (LPS). The D-mannuronic acid residues of alginate and the D-rhamnose (D-Rha) residues of A-band polysaccharide are both derived from the common sugar nucleotide precursor GDP-D-mannose (D-Man). Three genes, rmd, gmd and wbpW, which encode proteins involved in the synthesis of GDP-D-Rha, have been localized to the 5' end of the A-band gene cluster. In this study, WbpW was found to be homologous to phosphomannose isomerases (PMIs) and GDP-mannose pyrophosphorylases (GMPs) involved in GDP-D-Man biosynthesis. To confirm the enzymatic activity of WbpW, Escherichia coli PMI and GMP mutants deficient in the K30 capsule were complemented with wbpW, and restoration of K30 capsule production was observed. This indicates that WbpW, like AlgA, is a bifunctional enzyme that possesses both PMI and GMP activities for the synthesis of GDP-D-Man. No gene encoding a phosphomannose mutase (PMM) enzyme could be identified within the A-band gene cluster. This suggests that the PMM activity of AlgC may be essential for synthesis of the precursor pool of GDP-D-Man, which is converted to GDP-D-Rha for A-band synthesis. Gmd, a previously reported A-band enzyme, and Rmd are predicted to perform the two-step conversion of GDP-D-Man to GDP-D-Rha. Chromosomal mutants were generated in both rmd and wbpW. The Rmd mutants do not produce A-band LPS, while the WbpW mutants synthesize very low amounts of A band after 18 h of growth. The latter observation was thought to result from the presence of the functional homologue AlgA, which may compensate for the WbpW deficiency in these mutants. Thus, WbpW AlgA double mutants were constructed. These mutants also produced low levels of A-band LPS. A search of the PAO1 genome sequence identified a second AlgA homologue, designated ORF488, which may be responsible for the synthesis of GDP-D-Man in the absence of WbpW and AlgA. Polymerase chain reaction (PCR) amplification and sequence analysis of this region reveals three open reading frames (ORFs), orf477, orf488 and orf303, arranged as an operon. ORF477 is homologous to initiating enzymes that transfer glucose 1-phosphate onto undecaprenol phosphate (Und-P), while ORF303 is homologous to L-rhamnosyltransferases involved in polysaccharide assembly. Chromosomal mapping using pulsed field gel electrophoresis (PFGE) and Southern hybridization places orf477, orf488 and orf303 between 0.3 and 0.9 min on the 75 min map of PAO1, giving it a map location distinct from that of previously described polysaccharide genes. This region may represent a unique locus within P. aeruginosa responsible for the synthesis of another polysaccharide molecule.

Regulation of the spvR gene of the Salmonella typhimurium virulence plasmid during exponential-phase growth in intracellular salts medium and at stationary phase in L broth

The authors previously showed that the SpvR-regulated spvABCD operon of the Salmonella typhimurium virulence plasmid is highly induced during exponential-phase growth by salmonellae intracellularly in mammalian cells and in a medium designed to mimic the intracellular environment of mammalian cells, intracellular salts medium (ISM), as well as at stationary phase in L broth (LB). The most relevant signal(s) for spv gene expression in vivo is not known. To elucidate the means by which salmonellae regulate the spv genes in response to the environment during the disease process, expression of the spvR gene, encoding the positive regulatory protein SpvR, was examined under these same growth conditions by using RNAse-protection analysis. spvR was expressed at a low, basal level during exponential growth in LB but was induced during exponential growth in ISM and during stationary phase in LB, the same conditions that increased expression of the spvABCD operon. Basal expression of spvR during exponential growth in LB was independent of both SpvR and the alternative sigma factor RpoS, whereas maximal induction of spvR was dependent on both SpvR and RpoS. In an RpoS- background, spvR message was decreased in stationary phase, whereas spvR exhibited residual RpoS-independent induction during exponential growth in ISM. Deletion of spvA from the virulence plasmid of S. typhimurium increased expression of spvR during stationary phase in LB, but not during exponential growth in ISM. These results suggest that expression of spvR is controlled by different regulatory factors, depending on the growth conditions encountered by the salmonellae.

Bacterial alginate biosynthesis--recent progress and future prospects

The extracellular polysaccharide alginate has been widely associated with chronic Pseudomonas aeruginosa infections in the cystic fibrosis lung. However, it is clear that alginate biosynthesis is a more widespread phenomenon. Alginate plays a key role as a virulence factor of plant-pathogenic pseudomonads, in the formation of biofilms and with the encystment process of Azotobacter spp.

Phosphorylation-independent activity of the response regulators AlgB and AlgR in promoting alginate biosynthesis in mucoid Pseudomonas aeruginosa

Overproduction of the capsular polysaccharide alginate appears to confer a selective advantage for Pseudomonas aeruginosa in the lungs of cystic fibrosis patients. The regulators AlgB and AlgR, which are both required as positive activators in alginate overproduction, have homology with the regulator class of two-component environmental responsive proteins which coordinate gene expression through signal transduction mechanisms. Signal transduction in this class of proteins generally occurs via autophosphorylation of the sensor kinase protein and phosphotransfer from the sensor to a conserved aspartate residue, which is present in the amino terminus of the response regulator. Recently, kinB was identified downstream of algB and was shown to encode the cognate histidine protein kinase that efficiently phosphorylates AlgB. However, we show here that a null mutation in kinB in a mucoid cystic fibrosis isolate, P. aeruginosa FRD1, did not block alginate production. The role of the conserved aspartate residue in the phosphorylation of AlgB was examined. The predicted phosphorylation site of AlgB (D59) was mutated to asparagine (N), and a derivative of an AlgB lacking the entire amino-terminal phosphorylation domain (AlgB delta1-145) was constructed. A hexahistidine tag was included at the amino terminus of the wild-type (H-AlgB), H-AlgB delta1-145, and mutant (H-AlgB.59N) AlgB proteins. These derivatives were purified by Ni2+ affinity chromatography and examined for in vitro phosphorylation by the purified sensor kinase protein, KinB. The results indicated that while KinB efficiently phosphorylated H-AlgB, no phosphorylation of H-AlgB delta1-145 or H-AlgB.D59N was apparent. An allelic exchange system was developed to transfer mutant algB alleles onto the chromosome of a P. aeruginosa algB mutant to examine the effect on alginate production. Despite the defect in AlgB phosphorylation, P. aeruginosa strains expressing AlgB.D59N or H-AlgB delta1-145 remained mucoid. The roles of the conserved aspartate residues in the phosphorylation of AlgR were also examined. As seen with AlgB, mutations in the predicted phosphorylation site of AlgR (AlgR.D54N and AlgR.D85N) did not affect alginate production. These results indicate that in vivo phosphorylation of AlgB and AlgR are not required for their roles in alginate production. Thus, the mechanism by which these response regulators activate alginate genes in mucoid P. aeruginosa appears not to be mediated by conventional phosphorylation-dependent signal transduction.

Exponential-phase expression of spvA of the Salmonella typhimurium virulence plasmid: induction in intracellular salts medium and intracellularly in mice and cultured mammalian cells

The spv genes of Salmonella typhimurium and other non-typhoidal Salmonella serovars are essential for efficient systemic infection beyond the intestines in orally inoculated mice as a model for enteric fever. These virulence genes are not significantly expressed by salmonellae during exponential growth in L broth but are induced when the bacteria enter the stationary phase of growth. Using RNase protection analysis to directly measure spvA mRNA from the virulence plasmid of S. typhimurium, we found that spvA was maximally induced in an SpvR- and RpoS-dependent manner during exponential growth in intracellular Salts Medium, which mimics the intracellular environment of mammalian cells. A cloned spvA-lacZ operon fusion in S. typhimurium was induced intracellularly in periotoneal cells of mice, correlating in vivo intracellular gene expression with intracellular function of the spv genes in infected mice. spvA was also induced intracellularly in vitro within both Henle-407 intestinal epithelial cells and J774.A1 macrophage-like cells when the bacteria were replicating with exponential kinetics. Prevention of invasion of salmonellae with cytochalasin D inhibited spvA induction within tissue culture cells, indicating that salmonellae must be internalized for spvA to be induced. The spvA-lacZ fusion was not induced by salmonellae in extracellular fluid of the peritoneal cavity or in serum. Since induction of the spv genes occurs intracellularly during exponential growth of salmonellae, cessation of growth may not be the most relevant inducing signal for spv gene expression.

A transcriptional activator, FleQ, regulates mucin adhesion and flagellar gene expression in Pseudomonas aeruginosa in a cascade manner

Previous work has demonstrated that fleR, the gene for a transcriptional activator belonging to the NtrC subfamily of response regulators, is involved in the regulation of mucin adhesion and flagellar expression by Pseudomonas aeruginosa. This report describes the identification and characterization of fleQ, the gene for another transcriptional regulator which also regulates mucin adhesion and motility in this organism. The complete nucleotide sequence of the fleQ gene was determined on both DNA strands, and an open reading frame (ORF) consisting of 1,493 nucleotides was identified. This ORF coded for a gene product of predicted molecular weight, as confirmed by the overexpression of the fleQ gene as a fusion protein under an inducible promoter. The fleQ gene is flanked by a flagellar operon, fliDSorf126, at the 5' end and the fleSR operon on the 3' end. FleQ also had striking homology to a number of proteins belonging to the NtrC subfamily of response regulators, which work in concert with the alternate sigma factor RpoN (sigma54) to activate transcription. However, FleQ lacks the residues corresponding to Asp-54 and Lys-104 of the NtrC protein which are conserved in most of the members belonging to this subfamily of regulators. In addition, unlike some of the other transcriptional activators of this group, FleQ does not appear to have a cognate sensor kinase. A chromosomal insertional mutation in the fleQ gene abolished mucin adhesion and motility of P. aeruginosa PAK and PAK-NP. Both of these functions were regained by providing the complete fleQ gene on a multicopy plasmid. The location of fleQ immediately upstream of the fleSR operon, which is also necessary for the same process, suggested that these regulators may interact in some way. We therefore examined the regulation of the fleSR operon by fleQ and vice versa. Promoter fusion experiments showed that the fleSR operon was regulated by RpoN and FleQ. On the other hand, the fleQ promoter was independent of RpoN and FleR. FleQ, thus, adds another level of regulation to motility and adhesion in P. aeruginosa, above that of fleSR. We therefore propose the existence of a regulatory cascade which consists of at least two transcriptional regulators, FleQ and FleR, in the control of motility and adhesion in P. aeruginosa.

Roles of Pseudomonas aeruginosa las and rhl quorum-sensing systems in control of elastase and rhamnolipid biosynthesis genes

Two quorum-sensing systems (las and rhl) regulate virulence gene expression in Pseudomonas aeruginosa. The las system consists of a transcriptional activator, LasR, and LasI, which directs the synthesis of the autoinducer N-(3-oxododecanoyl) homoserine lactone (PAI-1). Induction of lasB (encoding elastase) and other virulence genes requires LasR and PAI-1. The rhl system consists of a putative transcriptional activator, RhlR, and RhlI, which directs the synthesis of N-butyryl homoserine lactone (PAI-2). Rhamnolipid production in P. aeruginosa has been reported to require both the rhl system and rhlAB (encoding a rhamnosyltransferase). Here we report the generation of a delta lasI mutant and both delta lasI delta rhlI and delta lasR rhlR::Tn501 double mutants of strain PAO1. Rhamnolipid production and elastolysis were reduced in the delta lasI single mutant and abolished in the double-mutant strains. rhlAB mRNA was not detected in these strains at mid-logarithmic phase but was abundant in the parental strain. Further RNA analysis of the wild-type strain revealed that rhlAB is organized as an operon. The rhlAB transcriptional start was mapped, and putative sigma 54 and sigma 70 promoters were identified upstream. To define components required for rhlAB expression, we developed a bioassay in Escherichia coli and demonstrated that PAI-2 and RhlR are required and sufficient for expression of rhlA. To characterize the putative interaction between PAI-2 and RhlR, we demonstrated that [3H]PAI-2 binds to E. coli cells expressing RhlR and not to those expressing LasR. Finally, the specificity of the las and rhl systems was examined in E. coli bioassays. The las system was capable of mildly activating rhlA, and similarly, the rhl system partly activated lasB. However; these effects were much less than the activation of rhlA by the rhl system and lasB by the las system. The results presented here further characterize the roles of the rhl and las quorum-sensing systems in virulence gene expression.

Mucoid Pseudomonas aeruginosa in cystic fibrosis: characterization of muc mutations in clinical isolates and analysis of clearance in a mouse model of respiratory infection

A distinguishing feature of Pseudomonas aeruginosa isolates from cystic fibrosis (CF) patients is their mucoid, exopolysaccharide alginate-overproducing phenotype. One mechanism of conversion to mucoidy is based on mutations in the algU mucABCD cluster, encoding the stress sigma factor AlgU and its regulators. However, conversion to mucoidy in laboratory strains can be achieved via mutations in other chromosomal sites. Here, we investigated mechanisms of the emergence of mucoid P. aeruginosa in CF by analyzing the status of mucA in a collection of mucoid P. aeruginosa isolates from 53 CF patients. This negative regulator of algU, when inactivated under laboratory conditions, causes conversion to mucoidy. The overall frequency of mucA alterations in mucoid CF isolates was 84%. Nucleotide sequence analyses revealed that the majority of the alterations caused premature termination of the mucA coding sequence. Comparison of paired nonmucoid and mucoid P. aeruginosa isolates from three CF patients indicated the presence of mucA mutations only in the mucoid strains. Interestingly, mucoid P. aeruginosa isolates from urinary tract infections also had mutations in the mucA gene. Clearance of CF isolates from the murine lung was investigated in an aerosol infection model with C57BL/6J, BALB/c, and DBA/2NHsd mice. Two CF strains, selected for further study based on the dependence of their alginate production on the concentration of salt in the medium, were used to examine the effects of mucoidy on pulmonary clearance. Statistically significant improvement in recovery from the murine lung of viable mucoid P. aeruginosa cells relative to the nonmucoid bacteria was observed in the majority of mouse strains tested. Collectively, the results reported here suggest that mucA is most likely the preferential site for conversion to mucoidy in CF and that alginate overproduction in mucA-mutant P. aeruginosa improves its resistance to the innate clearance mechanisms in the lung.

The Staphylococcus aureus allelic genetic loci for serotype 5 and 8 capsule expression contain the type-specific genes flanked by common genes

The nucleotide sequences of two gene clusters, cap5 and cap8, involved in the synthesis of Staphylococcus aureus type 5 and type 8 capsular polysaccharides (CPs), respectively were determined. Each gene cluster contained 16 ORFs, which were named cap5A through cap5P for type 5 CP and cap8A through cap8P for type 8 CP. The cap5 and cap8 loci were allelic and were mapped to the SmaI-G fragment in the standard SmaI map of Staph. aureus strain NCTC 8325. The predicted gene products of cap5A through cap5G and cap5L through cap5P are essentially identical to those of cap8A through cap8G and cap8L through cap8P, respectively, with very few amino acid substitutions. Four ORFs located in the central region of each locus are type-specific. A comparison of the predicted amino acid sequences of cap5 and cap8 with sequences found in the databases allowed tentative assignment of functions to 15 of the 16 ORFs. The majority of the capsule genes are likely to be involved in amino sugar synthesis; the remainder are likely to be involved in sugar transfer, capsule chain-length regulation, polymerization and transport.

Posttranslational control of the algT (algU)-encoded sigma22 for expression of the alginate regulon in Pseudomonas aeruginosa and localization of its antagonist proteins MucA and MucB (AlgN)

Pseudomonas aeruginosa strains associated with cystic fibrosis are often mucoid due to the copious production of alginate, an exopolysaccharide and virulence factor. Alginate gene expression is transcriptionally controlled by a gene cluster at 68 min on the chromosome: algT (algU)-mucA-mucB (algN)-mucC (algM)-mucD (algY). The algT gene encodes a 22-kDa alternative sigma factor (sigma22) that autoregulates its own promoter (PalgT) as well as the promoters of algR, algB, and algD. The other genes in the algT cluster appear to regulate the expression or activity of sigma22. The goal of this study was to better understand the functional interactions between sigma22 and its antagonist regulators during alginate production. Nonmucoid strain PAO1 was made to overproduce alginate (indicating high algD promoter activity) through increasing sigma22 in the cell by introducing a plasmid clone containing algT from mucA22(Def) strain FRD1. However, the bacterial cells remained nonmucoid if the transcriptionally coupled mucB on the clone remained intact. This suggested that a stoichiometric relationship between sigma22 and MucB may be required to control sigma factor activity. When the transcription and translational initiation of algT were measured with lacZ fusions, alginate production correlated with only about a 1.2- to 1.7-fold increase in algT-lacZ activity, respectively. An algR-lacZ transcriptional fusion showed a 2.8-fold increase in transcription with alginate production under the same conditions. A Western blot analysis of total cell extracts showed that sigma22 was approximately 10-fold higher in strains that overproduced alginate, even though algT expression increased less than 2-fold. This suggested that a post-transcriptional mechanism may exist to destabilize sigma22 in order to control certain sigma22-dependent promoters like algD. By Western blotting and phoA fusion analyses, the MucB antagonist of sigma22 was found to localize to the periplasm of the cell. Similar experiments suggest that MucA localizes to the inner membrane via one transmembrane domain with amino- and carboxy-terminal domains in the cytoplasm and periplasm, respectively. These data were used to propose a model in which MucB-MucA-sigma22 interact via an inner membrane complex that controls the stability of sigma22 protein in order to control alginate biosynthesis.

Vfr controls quorum sensing in Pseudomonas aeruginosa

Pseudomonas aeruginosa controls several genes in a cell density-dependent manner through a phenomenon termed quorum sensing. The transcriptional activator protein of the las quorum-sensing system is encoded for by the lasR gene, which is at the top of a quorum-sensing hierarchy. The activation of LasR as a transcriptional activator induces the expression of multiple genes that code for factors important for virulence, and rhlR, which encodes the transcriptional activator protein of the P. aeruginosa rhl quorum-sensing system. Elucidating the method of lasR regulation is crucial to understanding P. aeruginosa quorum sensing. In this report, we present studies on the transcriptional control of lasR. We identified two distinct transcriptional start sites for lasR that were located 201 bp (transcript T1) and 231 bp (transcript T2) upstream from the lasR start of translation. With the use of transcriptional lasRp-lacZ fusions, we showed that in P. aeruginosa, lasR expression is cell density dependent. This gene was expressed at a basal level until it was induced during the second half of log-phase growth, with expression becoming maximal during stationary-phase growth. We also showed that lasR expression was regulated through the cyclic AMP receptor protein (CRP)-binding consensus sequence in its promoter region. Our results from P. aeruginosa mutant studies and gel retardation assays indicated that this regulation was mediated by Vfr, a homolog of the Escherichia coli CRP.

Glucose stimulates alginate production and algD transcription in Pseudomonas aeruginosa

A previous study [DeVault et al. (1991) Mol. Microbiol. 5, 2503-2509] suggested that growth of Pseudomonas aeruginosa in glucose-containing medium represses algD gene transcription. In this study, growth of P. aeruginosa in rich medium containing glucose or gluconate increased alginate production and algD transcription at concentrations ranging from 1 to 5%.

An operon containing fumC and sodA encoding fumarase C and manganese superoxide dismutase is controlled by the ferric uptake regulator in Pseudomonas aeruginosa: fur mutants produce elevated alginate levels

The activities of fumarase- and manganese-cofactored superoxide dismutase (SOD), encoded by the fumC and sodA genes in Pseudomonas aeruginosa, are elevated in mucoid, alginate-producing bacteria and in response to iron deprivation (D. J. Hassett, M. L. Howell, P. A. Sokol, M. L. Vasil, and G. E. Dean, J. Bacteriol. 179:1442-1451, 1997). In this study, a 393-bp open reading frame, fagA (Fur-associated gene), was identified immediately upstream of fumC, in an operon with orfX and sodA. Two iron boxes or Fur (ferric uptake regulatory protein) binding sites were discovered just upstream of fagA. Purified P. aeruginosa Fur caused a gel mobility shift of a PCR product containing these iron box regions. DNA footprinting analysis revealed a 37-bp region that included the Fur binding sites and was protected by Fur. Primer extension analysis and RNase protection assays revealed that the operon is composed of at least three major iron-regulated transcripts. Four mucoid fur mutants produced 1.7- to 2.6-fold-greater fumarase activity and 1.7- to 2.3-greater amounts of alginate than wild-type organisms. A strain devoid of the alternative sigma factor AlgT(U) produced elevated levels of one major transcript and fumarase C and manganase-cofactored SOD activity, suggesting that AlgT(U) may either play a role in regulating this transcript or function in some facet of iron metabolism. These data suggest that the P. aeruginosa fagA, fumC, orfX, and sodA genes reside together on a small operon that is regulated by Fur and is transcribed in response to iron limitation in mucoid, alginate-producing bacteria.

Effects of growth-inhibitory concentrations of copper on alginate biosynthesis in highly mucoid Pseudomonas aeruginosa

Alginate production and degree of polymerization were affected when the highly mucoid Pseudomonas aeruginosa 8821M was grown with growth-inhibitory concentrations of Cu2+ (supplied as CuCl2; 1-5 mM). The inhibition of alginate biosynthesis was consistent with the decreased activity in Cu2+-stressed cells of phosphomannose isomerase/GDP-mannose pyrophosphorylase (encoded by algA), phosphomannomutase (encoded by algC) and GDP-mannose dehydrogenase (encoded by algD). However, in cells grown with concentrations of CuCl2 below 2 mM, the steady-state mRNA levels from algA, algC, algD and from the regulatory gene algR1 increased moderately. This observation is consistent with the suggested linkage between the control of alginate gene expression and the global regulation involved in the oxidative stress response. At highly inhibitory concentrations the levels of the four alginate gene transcripts decreased from maximal values. The bell-shaped curves, representing the effect of Cu2+ concentration on mRNA levels from the four alginate genes, exhibited similar patterns but did not concur. The decrease of the specific activity of enzymes necessary for GDP-mannuronic acid synthesis in Cu2+-grown cells was correlated with changes in gene expression, with the inhibitory effect of Cu2+ on enzyme activities and with Cu2+-induced oxidative inactivation of enzymes, especially the particularly sensitive phosphomannose isomerase activity.

Oxygen-dependent upregulation of transcription of alginate genes algA, algC and algD in Pseudomonas aeruginosa

The mRNA levels of algA, algC and algD genes increased, coordinately, in cells of the highly mucoid Pseudomonas aeruginosa 8821M grown under increasing dissolved oxygen tensions (DOT) of up to 70% of air saturation. These genes encode the bifunctional protein with phosphomannose isomerase (PMI) and GDP-mannose pyrophosphorylase (GMP) activities (algA), the phosphomannomutase (PMM) (algC) and the GDP-mannose dehydrogenase (GMD) (algD). These four enzyme activities are necessary for the synthesis of GDP-mannuronic acid, which is the activated sugar precursor for alginate polymerization. For growth-limiting DOT--lower than 10% of air saturation--the increase in mRNA levels of algA, algC and algD with oxygen concentration was accompanied by a strong increase in the activity of the encoded enzymes and the consequent increase in alginate synthesis. However, and despite the upregulation of alginate gene transcription by DOT above 10% of air saturation, the activities of the encoded enzymes either maintained (GMP and GMD) or decreased (PMI and PMM) their levels at high oxygen tensions, leading to a slight decrease in alginate synthesis. This has previously been attributed to the oxidative inactivation of alginate enzymes, particularly of PMM and PMI activities.

Identification and characterization of AlgZ, an AlgT-dependent DNA-binding protein required for Pseudomonas aeruginosa algD transcription

Transcriptional activation of the Pseudomonas aeruginosa algD gene results in high-level synthesis of the capsular polysaccharide alginate, an important P. aeruginosa virulence factor expressed in cystic fibrosis (CF) patients with chronic pulmonary disease. In this study, electrophoretic mobility-shift assays were used to identify a novel protein (AlgZ), which binds specifically to a sequence located 280 bp upstream of the algD promoter. While AlgZ-binding activity did not require the response regulators AlgB or AlgR, expression of AlgZ was found to be absolutely dependent on the alternative sigma factor AlgT. Electrophoretic mobility-shift assays and copper-phenanthroline footprinting localized AlgZ binding to a 36 bp algD region, which includes several helical repeats. A collection of alginate-producing (mucoid) and non-mucoid P. aeruginosa strains, derived from CF patients, was characterized for AlgZ-binding activity. In all cases, AlgZ binding to algD sequences was observed when extracts derived from mucoid P. aeruginosa CF isolates were examined. However, this binding activity was not present when extracts from non-mucoid P. aeruginosa CF isolates were tested. Oligonucleotide mutagenesis was employed to create an algD allele with a 4 bp mutation in the predicted AlgZ-binding site (algD38) and a heterologous substitution allele (algD40), in which the entire AlgZ-binding site was replaced with a non-specific DNA sequence of identical size. When the algD38 mutation was cloned into an algD-cat transcriptional fusion, this resulted in a 28-fold reduction in algD expression, whereas the algD40 mutation abolished algD transcription, indicating that AlgZ acts as an activator of algD transcription. These results support the hypothesis that activation of algD involves the formation of a high-order looped structure allowing for multivalent contacts between AlgZ, AlgR and RNA polymerase containing the alternative sigma factor AlgT. Characterization of the molecular details of algD activation will provide insights into the control of other prokaryotic and eukaryotic promoters that utilize multiple activators.

Microbial pathogenesis in cystic fibrosis: mucoid Pseudomonas aeruginosa and Burkholderia cepacia

Respiratory infections with Pseudomonas aeruginosa and Burkholderia cepacia play a major role in the pathogenesis of cystic fibrosis (CF). This review summarizes the latest advances in understanding host-pathogen interactions in CF with an emphasis on the role and control of conversion to mucoidy in P. aeruginosa, a phenomenon epitomizing the adaptation of this opportunistic pathogen to the chronic chourse of infection in CF, and on the innate resistance to antibiotics of B. cepacia, person-to-person spread, and sometimes rapidly fatal disease caused by this organism. While understanding the mechanism of conversion to mucoidy in P. aeruginosa has progressed to the point where this phenomenon has evolved into a model system for studying bacterial stress response in microbial pathogenesis, the more recent challenge with B. cepacia, which has emerged as a potent bona fide CF pathogen, is discussed in the context of clinical issues, taxonomy, transmission, and potential modes of pathogenicity.

Analysis of the Pseudomonas aeruginosa elastase (lasB) regulatory region

The enzyme elastase is an important virulence factor of the opportunistic human pathogen Pseudomonas aeruginosa. Previous studies have shown that expression of the P. aeruginosa elastase gene (lasB) requires both an activator protein, LasR, and an N-acylhomoserine lactone compound termed Pseudomonas autoinducer (PAI). In this study, we analyzed the lasB promoter region to learn more about lasB activation by LasR and PAI. We report that the lasB transcriptional start is located 141 nucleotides upstream from the lasB translational start. It was also discovered that the lasB promoter region contains two putative operator sequences (OP1 and OP2) that are similar to each other and the Vibrio fischeri lux operator. OP1 is located directly upstream from, and may overlap with, the lasB promoter region, and OP2 is centered 102 nucleotides upstream from the lasB transcriptional start site. To study the effects of these putative operators and other sequences upstream from the lasB transcriptional start site on lasB activation, a series of transcriptional lasBp-lacZ gene fusions was constructed. Data from these fusions indicate that both putative operators are involved in LasR- and PAI-mediated lasB activation, with OP1 being more important than OP2.

Alginate synthesis in Pseudomonas aeruginosa: the role of AlgL (alginate lyase) and AlgX

Previous studies localized an alginate lyase gene (algL) within the alginate biosynthetic gene cluster at 34 min on the Pseudomonas aeruginosa chromosome. Insertion of a Tn501 polar transposon in a gene (algX) directly upstream of algL in mucoid P. aeruginosa FRD1 inactivated expression of algX, algL, and other downstream genes, including algA. This strain is phenotypically nonmucoid; however, alginate production could be restored by complementation in trans with a plasmid carrying all of the genes inactivated by the insertion, including algL and algX. Alginate production was also recovered when a merodiploid that generated a complete alginate gene cluster on the chromosome was constructed. However, alginate production by merodiploids formed in the algX::Tn501 mutant using an alginate cluster with an algL deletion was not restored to wild-type levels unless algL was provided on a plasmid in trans. In addition, complementation studies of Tn501 mutants using plasmids containing specific deletions in either algL or algX revealed that both genes were required to restore the mucoid phenotype. Escherichia coli strains which expressed algX produced a unique protein of approximately 53 kDa, consistent with the gene product predicted from the DNA sequencing data. These studies demonstrate that AlgX, whose biochemical function remains to be defined, and AlgL, which has alginate lyase activity, are both involved in alginate production by P. aeruginosa.

Multiple promoters and induction by heat shock of the gene encoding the alternative sigma factor AlgU (sigma E) which controls mucoidy in cystic fibrosis isolates of Pseudomonas aeruginosa

Overproduction of the exopolysaccharide alginate causes mucoid colony morphology in Pseudomonas aeruginosa and is considered a major virulence determinant expressed by this organism during chronic respiratory infections in cystic fibrosis. One of the principal regulatory elements governing conversion to mucoidy in P. aeruginosa is AlgU, an alternative sigma factor which is 66% identical to and functionally interchangeable with sigma E from Escherichia coli and Salmonella typhimurium. sigma E has been implicated in the expression of systems enhancing bacterial resistance to environmental stress. In this study, we report that the gene encoding AlgU is transcribed in wild-type nonmucoid P. aeruginosa from multiple promoters (P1 through P5) that fall into three categories: (i) the P1 and P3 promoters, which display strong similarity to the -35 and -10 canonical sequences of sigma E promoters and were found to be absolutely dependent on AlgU; (ii) the P2 promoter, which was less active in algU mutants, but transcription of which was not completely abrogated in algU::Tcr cells; and (iii) the transcripts corresponding to P4 and P5, which were not affected by inactivation of algU. Introduction of E. coli rpoE (encoding sigma E) or algU into P. aeruginosa algU::Tcr strains restored P1 and P3 transcription and brought the P2 signal back to the wild-type level. The AlgU-dependent promoters P1 and P3 were inducible by heat shock in wild-type nonmucoid P. aeruginosa PAO1. At the protein level, induction of AlgU synthesis under conditions of extreme heat shock was detected by metabolic labeling of newly synthesized proteins, two-dimensional gel analysis, and reaction with polyclonal antibodies raised against an AlgU peptide. Another AlgU-dependent promoter, the proximal promoter of algR, was also found to be induced by heat shock. Under conditions of high osmolarity, growth at elevated temperature induced alginate synthesis in the wild-type nonmucoid P. aeruginosa PAO1. Cumulatively, these results suggest that algU itself is subject to complex regulation and is inducible by extreme heat shock, that the alginate system is a subset of the stress-responsive elements controlled by AlgU, and that AlgU and, by extension, its homologs in other organisms (e.g., sigma E in S. typhimurium) may play a role in bacterial virulence and adjustments to adverse growth conditions.

Functional equivalence of Escherichia coli sigma E and Pseudomonas aeruginosa AlgU: E. coli rpoE restores mucoidy and reduces sensitivity to reactive oxygen intermediates in algU mutants of P. aeruginosa

Mucoid colony morphology is the result of the overproduction of the exopolysaccharide alginate and is considered to be a major pathogenic determinant expressed by Pseudomonas aeruginosa during chronic respiratory infections in cystic fibrosis. Conversion to mucoidy can be caused by mutations in the second or third gene of the stress-responsive system algU mucA mucB. AlgU is 66% identical to the alternative sigma factor RpoE (sigma E) from Escherichia coli and Salmonella typhimurium and directs transcription of several critical alginate biosynthetic and regulatory genes. AlgU is also required for the full resistance of P. aeruginosa to reactive oxygen intermediates and heat killing. In this work, we report that E. coli sigma E can complement phenotypic defects of algU inactivation in P. aeruginosa: (i) the rpoE gene from E. coli complemented an algU null mutant of P. aeruginosa to mucoidy; (ii) the presence of the E. coli rpoE gene in P. aeruginosa induced alginate production in the standard genetic nonmucoid strain PAO1; (iii) the plasmid-borne E. coli rpoE gene induced transcription of algD, a critical algU-dependent alginate biosynthetic gene; and (iv) when present in algU::Tcr mutants, E. coli rpoE partially restored resistance to paraquat, a redox cycling compound that increases intracellular levels of superoxide radicals. A new gene, mclA, encoding a polypeptide with an apparent molecular mass of 27.7 kDa was identified immediately downstream of rpoE in E. coli. The predicted product of this gene is 28% identical (72% similar) to MucA, a negative regulator of AlgU activity in P. aeruginosa. The results reported in this study demonstrate that RpoE and AlgU are functionally interchangeable in P. aeruginosa and suggest that elements showing sequence similarity to those known to regulate AlgU activity in P. aeruginosa are also present in other bacteria.

Transcriptional analysis of the amidase operon from Pseudomonas aeruginosa

The transcriptional start point for the amidase structural gene (amiE) of Pseudomonas aeruginosa has been identified, and the promoter (pE) has been shown to function constitutively, as predicted for a system regulated by transcription antitermination. Northern (RNA) analysis results show that in cells grown under inducing conditions, a major 1.3-kb amiE transcript arises from pE, and in addition, a larger transcript of approximately 5.0 kb in length has been shown to derive from the same promoter, encoding all of the genes of the operon. DNA sequencing and S1 nuclease mapping have located a transcription terminator downstream of amiE, which terminates approximately half of the pE transcripts. Previously, two RpoN-dependent promoter-like sequences (pN1 and pN2) were identified upstream of the negative regulator gene, amiC, and we have now constructed a promoter probe vector which shows weak constitutive promoter activity within this region. This promoter would be expected to provide basal levels of expression of the amiC and amiR regulatory genes to allow induction of the system.

The Escherichia coli genes sspA and rnk can functionally replace the Pseudomonas aeruginosa alginate regulatory gene algR2

The algR2 (also known as algQ) gene of Pseudomonas aeruginosa has previously been identified as being necessary for alginate production at 37 degrees C. We have cloned two genes, from a cosmid library of Escherichia coli, which can restore mucoidy to an algR2 mutant of P. aeruginosa. The complementing regions of both cosmids were localized by subcloning restriction fragments. One of the E. coli genes identified here has not previously been described; we have named this gene rnk (regulator of nucleoside diphosphate kinase). It encodes a 14.9 kDa protein with no homology to any other protein. The other gene, sspA, is a regulator involved in stationary-phase regulation in E. coli. Either gene will restore mucoidy to an algR2-deficient strain of P. aeruginosa. AlgR2 has been shown to regulate at least two enzymes, succinyl-CoA synthetase (Scs) and nucleoside diphosphate kinase (Ndk), which form a complex in P. aeruginosa. When we examined the ability of the E. coli analogues to regulate Ndk, we found that rnk but not sspA was able to restore Ndk activity to the P. aeruginosa algR2 mutant. Furthermore, rnk was able to restore growth of the algR2 mutant in the presence of Tween 20, which inhibits other Ndk-like activities.

Alginate regulatory and biosynthetic gene homologs in Pseudomonas putida WCS358: correlation with the siderophore regulatory gene pfrA

A previous study [Venturi et al., Mol. Microbiol. 10 (1993) 63-73] demonstrated that the siderophore regulatory gene pfrA of Pseudomonas putida (Pp) WCS358 is highly similar and interchangeable with the alginate regulatory gene algQ (algR2) of P. aeruginosa (Pa). The algQ gene is physically linked to two other alginate regulators in the Pa chromosome, namely algR (algR1), a response regulator, and algP (algR3), a histone-like gene. In this study, we have identified the same genes and a similar genetic organization in the Pp chromosome. The two genes linked to pfrA, designated pprA and pprB, are similar to algR and algP, respectively. Chromosomal mutants of pprA and pprB were constructed showing that unlike pfrA, the two newly identified regulators are not involved in siderophore regulation. The pprA gene complemented a Pa algR mutant phenotype, suggesting that it could be involved in alginate gene regulation. The WCS358 strain is not producing alginate, but we demonstrated by Southern analysis that it also possesses, in addition to pprA and pprB, algD and algU (algT) gene homologs, two genes essential for alginate biosynthesis. Using an algD-xylE transcriptional fusion, we observed that the algD promoter is active in strain WCS358 and absolutely requires pfrA. The possibility that all five genes of Pp WCS358 are involved in alginate biosynthesis is discussed.

Growth-phase-dependent alginate synthesis, activity of biosynthetic enzymes and transcription of alginate genes in Pseudomonas aeruginosa

Alginate synthesis by the highly mucoid Pseudomonas aeruginosa 8821 M is growth-phase-dependent, and the alginate produced per unit of biomass reaches maximum values in the deceleration phase of growth. However, the degree of polymerization increases as batch growth proceeds, reaching maximum values at the stationary phase of growth. The activity of the four enzymes leading to GDP-mannuronic acid formation, phosphomannose isomerase, phosphomannomutase, GDP-mannose pyrophosphorylase and GDP-mannose dehydrogenase peaked earlier at the late exponential phase. Growth-phase-dependent activity of alginate biosynthetic enzymes correlates with the level of transcription of the encoding alginate genes algA, algC and algD during growth, as indicated by Northern blot hybridization experiments. The pattern of coordinate transcriptional growth-phase regulation of these alginate structural genes concurs with the growth-dependent transcription of the regulatory gene algR1.

Activation of the Pseudomonas aeruginosa lasI gene by LasR and the Pseudomonas autoinducer PAI: an autoinduction regulatory hierarchy

In Pseudomonas aeruginosa, the transcriptional activator LasR and the Pseudomonas autoinducer PAI, are necessary for efficient transcriptional activation of the lasB gene, encoding elastase (L. Passador, J. M. Cook, M.J. Gambello, L. Rust, and B. H. Iglewski, Science 260:1127-1130, 1993). The transcriptional start points of lasI in Escherichia coli and P. aeruginosa were determined by S1 nuclease mapping. In the presence of both LasR and PAI, the start site, T1, is located at position -25 relative to the ATG translational start codon. A minor transcriptional start, T2, is found at position -13 when lasI is transcribed in the absence of either LasR or PAI in P. aeruginosa and E. coli, respectively. To begin to closely examine the regulation of lasI, whose product is involved in the synthesis of PAI, a lasI-lacZ fusion on a lambda phage was constructed to form monolysogens of E. coli MG4. Lysogens supplied only with either lasI or lasR via multicopy plasmids demonstrated no significant increase in beta-galactosidase expression compared with control levels. Lysogens in which both lasR and lasI were supplied in multicopy exhibited a 62-fold increase in expression, and a lysogen in which lasR was supplied in trans and which was grown in the presence of exogenous PAI exhibited a 60-fold increase. Thus, LasR and PAI are necessary for the full expression of lasI in E. coli. The interchangeability of the P. aeruginosa and Vibrio fischeri homologs LasR and LuxR and their respective autoinducers, PAI and VAI, as activators of lasI-lacZ was examined. Only the combination of LasR and PAI significantly increased the expression of lasI. The comparison of lasI-lacZ and lasB-lacZ expression lysogens grown in the presence of lasR and PAI revealed that half-maximal expression of lasI required 0.1 nM PAI, in contrast to the 1.0 nM PAI necessary for lasB half-maximal expression. These results suggest an autoinduction regulatory hierarchy in which LasR and low PAI concentrations primarily activate lasI expression in a regulatory loop. With the accumulation of PAI, secondary activation of virulence product genes such as lasB occurs.

Analysis of promoters controlled by the putative sigma factor AlgU regulating conversion to mucoidy in Pseudomonas aeruginosa: relationship to sigma E and stress response

Alginate overproducition by mucoid Pseudomonas aeruginosa is a critical pathogenic determinant expressed by this organism during chronic infections in cystic fibrosis. Conversion to mucoidy and a subsequent loss of mucoid character can occur via different mutations in the algU mucA mucB gene cluster. The algU gene encodes a 22.2-kDa putative alternative sigma factor required for expression of the critical alginate biosynthetic gene algD. In this work, algU transcription was studied by S1 nuclease protection analysis. Transcription from the promoter proximal to the algU coding region was found to be dependent on AlgU. The -35 and -10 sequences of this newly mapped promoter showed strong similarity ot the promoters of two other critical alg genes: algD and algR. The proximal promoter of algR was also shown to depend on algU. Interestingly, the putative -35 and -10 regions of all three promoters displayed striking similarity to the consensus sequence of the sigma E-dependent promoters in Escherichia coli and Salmonella typhimurium. This 24-kDa sigma factor, controlling genes participating in resistance to high temperatures and oxidative stress, has been previously biochemically characterized, but the gene for sigma E remained unidentified. To examine whether AlgU is related to sigma E, the effect of algU inactivation on the sensitivity of P. aeruginosa to killing by heat and reactive oxygen intermediates was tested. Two isogenic pairs of algU+ and algU mutant strains were compared. The algU mutants, irrespective of the mucoid status of the parental strains, displayed increased sensitivity to killing by paraquat, known to generate intracellular superoxide radicals, and heat. Further lgobal homology searches revealed the presence of a previously unrecognized E. coli gene with the predicted gene product showing a striking 66% identity to AlgU. The corresponding gene from S. typhimurium was cloned and sequenced, and it is displayed one amino acid substitution relative to its E. coli equivalent. AlgU and its close homologs in E. coli and S. typhimurium may be functionally related.

Transcriptional analysis of the Pseudomonas aeruginosa genes algR, algB, and algD reveals a hierarchy of alginate gene expression which is modulated by algT

Strains of Pseudomonas aeruginosa which colonize and infect the lungs of cystic fibrosis patients have a mucoid colony morphology due to the overproduction of the exopolysaccharide alginate. The response regulators AlgB and AlgR are required for the transcription of algD, a tightly regulated gene encoding GDP-mannose dehydrogenase, which is critical for P. aeruginosa alginate biosynthesis. Previous studies indicated that mutations in the algT gene of mucoid FRD1 P. aeruginosa result in nonmucoid derivatives. However, the specific role for algT in alginate gene regulation has not been elucidated. In this study, transcription of algB, algD, and algR was characterized by gene fusion and primer extension analysis. Expression of algR and algD was abolished in P. aeruginosa strains containing algT::Tn501 insertions because of lack of transcription initiation at the algR and algD promoters. An algR mutation was constructed in FRD1, and this resulted in the loss of alginate production and a dramatic decrease in algD transcription. RNA and gene fusion analysis revealed that algB is not required for algR expression, nor is algR necessary for transcription of algB. Thus, with the exception of a requirement for AlgT, the AlgB and AlgR pathways appear to be independent of each other. In gel band mobility shift assays, a protein(s) present in extracts from mucoid and algB and algR mutant P. aeruginosa strains formed a specific complex with algD sequences located immediately upstream of the start of transcription. No binding to these sequences was observed when extracts from algT mutant strains were examined. A model proposed suggests that a hierarchy of alginate gene expression exists in which AlgT is required for transcription of the response regulators algB and algR, which in turn are necessary for algD expression. AlgT or a protein under algT control also binds to sequences located within the algD promoter.

Post-transcriptional regulation of the Pseudomonas aeruginosa algC gene

The algC gene (encoding phosphomannomutase) of Pseudomonas aeruginosa, similarly to the algD gene, is environmentally regulated through transcriptional activation of its promoter. This gene, like algD, has a long (244 bp) 5' untranslated leader region (5' UTR). Using transcriptional and translational algC::lacZ fusions, we show that even though the transcript levels are similar, the beta-galactosidase-specific activities of the translational fusions are much higher than those of the transcriptional fusions during the entire growth phase. Both the 5' UTR and the ribosomal-binding site are shown to be important for efficient translation of the algC mRNA.

Integration host factor and sequences downstream of the Pseudomonas aeruginosa algD transcription start site are required for expression

Pseudomonas aeruginosa is an extremely important opportunistic pathogen in immunocompromised individuals. Strains of P. aeruginosa isolated from chronic lung infections in patients with the genetic disease cystic fibrosis have a mucoid colony morphology. This phenotype is due to overproduction of the exopolysaccharide alginate, which is believed to confer a selective advantage on P. aeruginosa in cystic fibrosis lungs. Alginate biosynthesis is controlled by a complex regulatory mechanism. Genes located in the 34-min region of the P. aeruginosa chromosome form an operon which encodes most of the biosynthetic enzymes necessary for alginate production. algD, the first gene in the operon and a critical point for the transcriptional regulation of alginate biosynthesis, is controlled by several trans, cis, and environmental factors. In this study, the involvement of the histone-like protein integration host factor (IHF) in algD expression was examined. Sequences with similarity to consensus IHF-binding sites of Escherichia coli were identified 75 bp upstream (site 1) and 90 bp downstream (site 2) of the start of algD transcription. In gel band mobility shift assays, DNA fragments containing either site bind IHF but site 2 has an approximately 90-fold higher affinity for IHF. Mutations in each of the elements were generated, and they resulted in the reduction or loss of in vitro IHF binding and a three- to fourfold decrease in algD-cat expression. This indicates that IHF binding is necessary for high-level algD transcription. The presence of a high-affinity IHF-binding site located 3' of the algD transcription start site suggested that sequences further downstream of this element are involved in algD expression. When a fragment located downstream of site 2 and upstream of the promoterless cat gene (+110 to +835) was deleted, algD-cat expression was reduced 10-fold supporting the notion that 3' enhancer elements are required for algD transcription. This is the first direct evidence of a 3' element involved in the control of a P. aeruginosa gene. It is postulated that IHF mediates the formation of a higher-order looped structure which is necessary for efficient algD transcription.

Cloning and characterization of a gene affecting the methicillin resistance level and the autolysis rate in Staphylococcus aureus

Tn918 mutagenesis of a high-level methicillin-resistant Staphylococcus aureus (methicillin MIC, 800 micrograms/ml) led to the isolation of a low-resistance mutant. The Tn918 insert was transferred back to the parent to produce strain SRM563 (methicillin MIC, 12.5 micrograms/ml), which showed heterogeneous resistance. Twenty-two clinical isolates of methicillin-resistant S. aureus were transformed with DNA of SRM563. In the transformants of most strains, instances of reduced resistance were observed with concomitant increases of autolysis rate induced by Triton X-100 and were generally more profound in high-resistance strains. Two transformants exhibited a decrease of the autolysis rate and little reduction of resistance. In the transformant of methicillin-susceptible strain RN2677, an increase of the autolysis rate and little reduction of resistance were observed. The production of low-affinity penicillin-binding protein (PBP2') did not significantly decrease in the mutants. Insertion of Tn918 occurred within the 3'-terminal region of a novel gene designated llm, which was cloned and sequenced. RNA blot analysis demonstrated that the gene was transcribed. The encoded protein was composed of 351 amino acid residues with a molecular weight of 38,512 and was hydrophobic, suggesting its location on the membrane. The gene was detected by PCR in all S. aureus strains tested but not in the other 26 staphylococcal species. Comparison of the 3'-terminal sequences of the gene among several S. aureus strains showed that, whereas nucleotide substitutions occurred at the third position in seven of eight 3'-terminal codons, only C-terminal amino acid variation of glutamate or aspartate was observed.

Gene cluster controlling conversion to alginate-overproducing phenotype in Pseudomonas aeruginosa: functional analysis in a heterologous host and role in the instability of mucoidy

Conversion to mucoidy, caused by the overproduction of the exopolysaccharide alginate in laboratory and cystic fibrosis strains of Pseudomonas aeruginosa, can occur via frameshift or nonsense mutations in the second gene of the algU mucA mucB cluster. The first gene of the cluster, algU, encodes a putative alternative sigma factor required for algD transcription. The algD gene encodes a critical alginate biosynthetic enzyme and is invariably activated in mucoid P. aeruginosa cells. To investigate the function of the genes controlling conversion to mucoidy, the wild-type algU mucA mucB cluster from the standard genetic strain PAO1 was used to reconstitute algD transcription in Escherichia coli. Transcription of an algD-lacZ chromosomal fusion in E. coli was detected upon introduction of plasmid-borne algU mucA mucB. Moreover, insertional inactivation of either mucA or mucB resulted in further stimulation of transcriptional activity from the algD promoter. This activation was dependent on algU, since a double algU mucA mutation abrogated transcription of algD. These experiments suggest that the phenotypic manifestations of muc mutations, i.e., increased algD expression and mucoid phenotype, depend on the presence of an active algU gene and that this regulator and the factors encoded by the downstream genes interact. Further support for these conclusions came from the investigations of the mechanism of reversion to nonmucoidy in P. aeruginosa, a phenomenon frequently referred to as the instability of mucoid phenotype. Spontaneous nonmucoid derivatives of the mucoid strain PAO578 carrying the mucA22 mutation were examined for the presence of alterations within the algU mucA mucB locus. Point mutations which inactivated algU were detected in some, but not all, nonmucoid revertants. No reversion of the original mucA22 mutation (a deletion of one C) was observed in any of the investigated strains. This observation suggests that the process of conversion to nonmucoidy ban be explained, at least partially, by second-site suppressor mutations and that a fraction of such mutations occurs in algU.

Distribution of alginate genes in bacterial isolates from corroded metal surfaces

The distribution of alginate genes encoding biosynthesis of alginate was examined for bacterial isolates associated with corrosive biofilms recovered from source water, cooling lines, and reactor surfaces of a nuclear power plant. A total of 120 diverse Gram-positive and -negative isolates were obtained. Using DNA:DNA hybridization, 11 isolates were shown to contain sequences homologous to structural (algD, algG, alg-76) and/or regulatory (albB) alginate biosynthetic genes derived from an alginate-producing cystic fibrosis isolate of Pseudomonas aeruginosa (FRD1). Identification of isolates was accomplished by fatty acids methyl esters (FAME) analysis and the Biolog identification system. Nine of the twelve isolates were identified as various Pseudomonas spp., and two additional Gram-negative isolates were tentatively identified as Aeromonas veronii and Stenotrophomonas maltophilia. The remaining isolate was identified as a Gram-positive Bacillus pumilus. The results of the investigation extend current knowledge on the distribution of alginate biosynthetic genes in environmental isolates and permits the development of a more environmentally realistic model system to investigate the role of exopolymer production in biofilm formation and biocorrosion processes.

An algD-bioluminescent reporter plasmid to monitor alginate production in biofilms

A broad-host range algD-lux bioluminescent reporter plasmid was developed to examine the role of exopolymer production in biofilm function. The algD-lux reporter plasmid will allow rapid on-line in situ detection of environmental factors that induce alginate biosynthesis. The algD promoter was stimulated by factors previously reported to induce alginate production, including ethanol and NaCl, and differences were observed with different nitrogen sources. With growth on minimal media with either glucose or succinate as a carbon source, succinate had a greater inductive effect on the algD promoter. An increase in light output of 1.3-fold and 1.7-fold was seen with cultures amended with 50 and 150 mM NaCI, respectively, compared to cultures with succinate alone. NaCl induction of the algD promoter was confirmed by algD RNA slot blots. Light output increased 2.0-fold and 1.7-fold with 0.25% and 0.5% ethanol, respectively, compared with controls grown with succinate only. While the rate of algD promoter response was initially similar when either NH4 or NO3 was used as a nitrogen source, NH4-grown cultures maintained a higher light output during late log phase compared to NO3-grown cultures.

lasA and lasB genes of Pseudomonas aeruginosa: analysis of transcription and gene product activity

The lasA gene was the first of the Pseudomonas aeruginosa genes involved in proteolysis and elastolysis to be cloned and sequenced. Its function and significance have been studied by genetic approaches (D. S. Toder, M. J. Gambello, and B. H. Iglewski, Mol. Microbiol. 5:2003-2010, 1991) and by attempts to purify an active fragment of the protein (J. E. Peters and D. R. Galloway, J. Bacteriol. 172:2236-2240, 1990). To further study LasA in vivo, we have constructed and characterized an insertional mutant in the lasA gene in strain PAO1 (PAO-A1) and in the lasB insertional mutant, PAO-B1. Analysis of these isogenic strains demonstrates that the lasA lesion diminished elastolysis more than proteolysis and that LasA is required for staphylolytic activity. Despite previous suggestions that lasB elastase cleaves the LasA protein, the size of the LasA protein was the same whether or not lasB elastase was present. Expression of lasA in a lasR-negative mutant, PAO-R1, demonstrated that the LasA protein is produced in an active form in the absence of (lasB) elastase or alkaline protease and is itself a protease with elastolytic activity. We also observed that PAO-A1 was closer to the parental phenotype, with respect to elastolytic and proteolytic activities, than the previously characterized, chemically induced lasA mutant PAO-E64. Quantification of promoter activity with lasA::lacZ and lasB::lacZ fusions suggests that PAO-E64 harbors a mutation in a gene which regulates expression of both lasA and lasB.

Pseudomonas aeruginosa AlgG is a polymer level alginate C5-mannuronan epimerase

Alginate is a viscous extracellular polymer produced by mucoid strains of Pseudomonas aeruginosa that cause chronic pulmonary infections in patients with cystic fibrosis. Alginate is polymerized from GDP-mannuronate to a linear polymer of beta-1-4-linked residues of D-mannuronate and its C5-epimer, L-guluronate. We previously identified a gene called algG in the alginate biosynthetic operon that is required for incorporation of L-guluronate residues into alginate. In this study, we tested the hypothesis that the product of algG is a C5-epimerase that directly converts D-mannuronate to L-guluronate. The DNA sequence of algG was determined, and an open reading frame encoding a protein (AlgG) of approximately 60 kDa was identified. The inferred amino terminus of AlgG protein contained a putative signal sequence of 35 amino acids. Expression of algG in Escherichia coli demonstrated both 60-kDa pre-AlgG and 55-kDa mature AlgG proteins, the latter of which was localized to the periplasm. An N-terminal analysis of AlgG showed that the signal sequence was removed in the mature form. Pulse-chase experiments in both E. coli and P. aeruginosa provided evidence for conversion of the 60- to the 55-kDa size in vivo. Expression of algG from a plasmid inan algG (i.e., polymannuronate-producing) mutant of P. aeruginosa restored production of an alginate containing L-guluronate residues. The observation that AlgG is apparently processed and exported from the cytoplasm suggested that it may act as a polymer-level mannuronan C5-epimerase. An in vitro assay for mannuronan C5 epimerization was developed wherein extracts of E. coli expressing high levels of AlgG were incubated with polymannuronate. Epimerization of D-mannuronate to L-guluronate residues in the polymer was detected enzymatically, using a L-guluronate-specific alginate lyase of Klebsiella aerogenes. Epimerization was also detected in the in vitro reaction between recombinant AlgG and poly-D-mannuronate, using high-performance anion-exchange chromatography. The epimerization reaction was detected only when acetyl groups were removed from the poly-D-mannuronate substrate, suggesting that AlgG epimerization activity in vivo may be sensitive to acetylation of the D-mannuronan residues. These results demonstrate that AlgG has polymer-level mannuronan C5-epimerase activity.

Elastase gene expression in non-elastase-producing Pseudomonas aeruginosa strains using novel shuttle vector systems

In order to determine whether non-elastase-producing strains of Pseudomonas aeruginosa such as N-10, PA103 and IFO3080 can express foreign elastase genes, we introduced elastase genes from P. aeruginosa IFO3455 (elastase-producing) as well as from PA103 and N-10 into non-elastase-producing P. aeruginosa strains. Results suggested that gene expression, secretion, and precursor processing systems of elastase were essentially normal in P. aeruginosa N-10 and IFO3080. Our studies using various elastase genes showed that both the elastase structural gene and 5'-upstream regions of P. aeruginosa PA103 were also normal. This was confirmed by the finding that P. aeruginosa N-10 and IFO3080 which carry the PA103 elastase gene produced elastase. Several deleted or chimeric genes were constructed using the 5'-upstream regions of elastase genes from P. aeruginosa N-10 or PA103 and studies of expression revealed that two individual DNA bases seem to be important in suppressing P. aeruginosa N-10 elastase gene expression. Possible reasons for the lack of elastase expression in these non-elastase-producing strains are discussed.

Identification of a putative alternate sigma factor and characterization of a multicomponent regulatory cascade controlling the expression of Pseudomonas syringae pv. syringae Pss61 hrp and hrmA genes

The Pseudomonas syringae hrp and hrmA genes controlling pathogenicity and elicitation of the hypersensitive response and the avr genes controlling host range have been shown previously to be regulated by carbon, nitrogen, pH, osmolarity, and hypothetical plant factors. In P. syringae pv. syringae Pss61, inactivation of hrp complementation groups II and XIII reduced expression of a plasmid-borne hrmA'-lacZ fusion. The hrp regions II and XIII were cloned on separate plasmids and shown to enhance the activity of the hrmA promoter in Escherichia coli MC4100 transformants at least 100-fold. The nucleotide sequence of region XIII revealed two open reading frames (hrpR and hrpS) whose deduced products share homology with P. syringae pv. phaseolicola NPS3121 HrpS and are both related to the NtrC family of two-component signal transduction systems. HrpR and HrpS differ from most members of the protein family by lacking an amino-terminal domain which modulates the regulatory activity. A single open reading frame, hrpL, whose product shares homology with AlgU, a putative alternate sigma factor of P. aeruginosa, as well as with the related alternate sigma factors was identified within region II. Key domains are partially conserved. Inactivation of hrpS in Pss61 repressed expression of a plasmid-borne hrpL'-lacZ fusion carried by pYXPL1R, and transformation of MC4100(pYXPL1R) with a plasmid carrying hrpRS increased hrpL promoter activity at least 200-fold. Neither hrpS nor hrpR, when cloned on separate plasmids, activated the hrpL promoter activity individually. The expression of hrpL when directed by a lac promoter was sufficient to express a set of plasmid-borne hrmA'-, hrpJ'-, and hrpZ'-lacZ fusions independently of other hrp genes. The results indicate that hrpRS and hrpL are part of a regulatory cascade in which HrpR and HrpS activate expression of hrpL and HrpL, a putative sigma factor, induces expression of HrpL-responsive genes.

The Pseudomonas aeruginosa homologs of hemC and hemD are linked to the gene encoding the regulator of mucoidy AlgR

The algD gene encodes NAD-linked GDPmannose dehydrogenase, which is essential for the mucoid phenotype, an important virulence factor expressed by Pseudomonas aeruginosa in cystic fibrosis patients. AlgR, a response regulator controlling mucoidy, is required for high level expression of algD. Inactivation of algR completely abrogates algD expression while mutations immediately downstream of algR affect induction of the algD promoter. In order to examine the nature of genetic elements located downstream of algR, the complete nucleotide sequence of this region was determined. This analysis revealed the presence of two newly identified P. aeruginosa genes with predicted gene products homologous to known porphobilinogen deaminases (HemC) from other organisms, and uroporphyrinogen III cosynthase (HemD) from Escherichia coli. The concerted action of both of these enzymes is essential for the synthesis of heme precursors. Mutations within the region containing the P. aeruginosa homologs of hemC and hemD affect algD promoter activity during growth on nitrate. Furthermore, transcriptional analyses indicated that hemC was cotranscribed with algR at detectable levels in mucoid cells. These results suggest a link between physiological processes dependent on heme and conditions conductive to algD expression and mucoidy.

Cloning and characterization of the Pseudomonas aeruginosa sodA and sodB genes encoding manganese- and iron-cofactored superoxide dismutase: demonstration of increased manganese superoxide dismutase activity in alginate-producing bacteria

Pseudomonas aeruginosa is a strict aerobe which is likely exposed to oxygen reduction products including superoxide and hydrogen peroxide during the metabolism of molecular oxygen. To counterbalance the potentially hazardous effects of elevated endogenous levels of superoxide, most aerobic organisms possess one or more superoxide dismutases or compounds capable of scavenging superoxide. We have previously shown that P. aeruginosa possesses both an iron- and a manganese-cofactored superoxide dismutase (D. J. Hassett, L. Charniga, K. A. Bean, D. E. Ohman, and M. S. Cohen, Infect. Immun. 60:328-336, 1992). In this study, the genes encoding manganese (sodA)- and iron (sodB)- cofactored superoxide dismutase were cloned by using a cosmid library of P. aeruginosa FRD which complemented an Escherichia coli (JI132) strain devoid of superoxide dismutase activity. The sodA and sodB genes of P. aeruginosa, when cloned into a high-copy-number vector (pKS-), partially restored the aerobic growth rate defect, characteristic of the Sod- strain, to that of the wild type (AB1157) when grown in Luria broth. The nucleotide sequences of sodA and sodB have open reading frames of 612 and 579 bp that encode dimeric proteins of 22.9 and 21.2 kDa, respectively. These data were also supported by the results of in vitro expression studies. The deduced amino acid sequence of the P. aeruginosa manganese and iron superoxide dismutase revealed approximately 50 and 67% similarity with manganese and iron superoxide dismutases from E. coli, respectively. There was also remarkable similarity with iron and manganese superoxide dismutases from other phyla. The mRNA start site of sodB was mapped to 174 bp upstream of the ATG codon. A likely promoter with similarity to the -10 and -35 consensus sequence of E. coli was observed upstream of the ATG start codon of sodB. Regions sequenced 519 bp upstream of the sodA electrophoresis, sodA gene revealed no such promoter, suggesting an alternative mode of control for sodA. By transverse field electrophoresis, sodA and sodB were mapped to the 71- to 75-min region on the P. aeruginosa PAO1 chromosome. Strikingly, mucoid alginate-producing bacteria generated greater levels of manganese superoxide dismutase than nonmucoid revertants, suggesting that mucoid P. aeruginosa is responding to oxidative stress and/or changes in the redox status of the cell.

Mechanism of conversion to mucoidy in Pseudomonas aeruginosa infecting cystic fibrosis patients

Chronic respiratory infections with mucoid Pseudomonas aeruginosa are the leading cause of high mortality and morbidity in cystic fibrosis (CF). The initially colonizing strains are nonmucoid, but in the CF lung they invariably convert into the mucoid, exopolysaccharide alginate-overproducing form causing further deterioration and poor prognosis. Here we report the molecular basis of conversion to mucoidy. The algU gene is required for expression of the key alginate biosynthetic gene algD and encodes a protein homologous to sigma H, an alternative sigma factor regulating sporulation and other post-exponential-phase processes in Bacillus. The algU gene and the negative regulators mucA and mucB constitute the gene cluster controlling conversion to mucoidy. We demonstrate a critical role of mucA in this process based on (i) the presence of frameshift mutations disrupting the mucA coding region in mucoid cells that were absent in nonmucoid parental strains, (ii) genetic complementation of mucA mutations with the mucA+ gene, (iii) allelic replacements with specific mutant mucA genes causing conversion to mucoidy in previously nonmucoid cells, and (iv) detection of identical and additional mucA mutations in clinical mucoid strains isolated from the lungs of CF patients. These results suggest that the switch from the nonmucoid to mucoid state can be caused by inactivation of mucA, resulting in constitutive expression of alginate biosynthetic genes dependent on algU for transcription and that such mutants may be selected in vivo during chronic infections in CF.

Enhancer-like activity of A1gR1-binding site in alginate gene activation: positional, orientational, and sequence specificity

Significant activation of promoters of alginate genes such as algD or algC occurs in mucoid Pseudomonas aeruginosa during its proliferation in the lungs of cystic fibrosis patients. These promoters have been shown to be responsive to environmental signals such as high osmolarity. The signaling is mediated by a so-called two-component signal transduction system, in which a soluble protein, AlgR2, undergoes autophosphorylation and transfers the phosphate to a DNA-binding response regulator protein, AlgR1. The phosphorylated form of AlgR1 has a high affinity for binding at upstream sequences of both the algC and algD promoters. Two AlgR1-binding sites (ABS) have been reported upstream of the algC gene. One of the two ABSs (algC-ABS1, located at -94 to -81) is critical for the algC activation process, while the second ABS (algC-ABS2, located at +161 to +174) is only weakly active. We now report the presence of a third ABS within the structural gene of algC, and this ABS (algC-ABS3) is also important for algC promoter activation. algC-ABS1 can be replaced functionally by algC-ABS2, algD-ABS1, or algD-ABS2 and somewhat weakly by algD-ABS3. Introduction of a half-integral turn in the DNA helix between the algC site of transcription initiation and algC-ABS1 allowed only slight reduction of promoter activity, suggesting that the binding site could be appreciably functional even when present in the opposite face of the helix. Activation of the algC promoter is independent of the relative location (upstream or downstream of the mRNA start site), the number of copies, or the orientation of algC-ABS1, suggesting that it behaves like a eukaryotic enhancer element in promoting transcription from the algC promoter.

Characterization of the Pseudomonas aeruginosa alginate lyase gene (algL): cloning, sequencing, and expression in Escherichia coli

Mucoid strains of Pseudomonas aeruginosa produce a viscous exopolysaccharide called alginate and also express alginate lyase activity which can degrade this polymer. By transposon mutagenesis and gene replacement techniques, the algL gene encoding a P. aeruginosa alginate lyase enzyme was found to reside between algG and algA within the alginate biosynthetic gene cluster at 35 min on the P. aeruginosa chromosome. DNA sequencing data for algL predicted a protein product of ca. 41 kDa, including a 27-amino-acid signal sequence, which would be consistent with its possible localization in the periplasmic space. Expression of the algL gene in Escherichia coli cells resulted in the expression of alginate lyase activity and the appearance of a new protein of ca. 39 kDa detected on sodium dodecyl sulfate-polyacrylamide gels. In mucoid P. aeruginosa strains, expression of algL was regulated by AlgB, which also controls expression of other genes within the alginate gene cluster. Since alginate lyase activity is associated with the ability to produce and secrete alginate polymers, alginate lyase may play a role in alginate production.

Identification of algF in the alginate biosynthetic gene cluster of Pseudomonas aeruginosa which is required for alginate acetylation

Mucoid strains of Pseudomonas aeruginosa produce a high-molecular-weight exopolysaccharide called alginate that is modified by the addition of O-acetyl groups. To better understand the acetylation process, a gene involved in alginate acetylation called algF was identified in this study. We hypothesized that a gene involved in alginate acetylation would be located within the alginate biosynthetic gene cluster at 34 min on the P. aeruginosa chromosome. To isolate algF mutants, a procedure for localized mutagenesis was developed to introduce random chemical mutations into the P. aeruginosa alginate biosynthetic operon on the chromosome. For this, a DNA fragment containing the alginate biosynthetic operon and adjacent argF gene in a gene replacement cosmid vector was utilized. The plasmid was packaged in vivo into lambda phage particles, mutagenized in vitro with hydroxylamine, transduced into Escherichia coli, and mobilized to an argF auxotroph of P. aeruginosa FRD. Arg+ recombinants coinherited the mutagenized alginate gene cluster and were screened for defects in alginate acetylation by testing for increased sensitivity to an alginate lyase produced by Klebsiella aerogenes. Alginates from recombinants which showed increased sensitivity to alginate lyase were tested for acetylation by a colorimetric assay and infrared spectroscopy. Two algF mutants that produced alginates reduced more than sixfold in acetyl groups were obtained. The acetylation defect was complemented in trans by a 3.8-kb XbaI-BamHI fragment from the alginate gene cluster when placed in the correct orientation under a trc promoter. By a merodiploid analysis, the algF gene was further mapped to a region directly upstream of algA by examining the polar effect of Tn501 insertions. By gene replacement, DNA with a Tn501 insertion directly upstream of algA was recombined with the chromosome of mucoid strain FRD1. The resulting strain, FRD1003, was nonmucoid because of the polar effect of the transposon on the downstream algA gene. By providing algA in trans under the tac promoter, FRD1003 produced nonacetylated alginate, indicating that the transposon was within or just upstream of algF. These results demonstrated that algF, a gene involved in alginate acetylation, is located directly upstream of algA.

Differentiation of Pseudomonas aeruginosa into the alginate-producing form: inactivation of mucB causes conversion to mucoidy

Mucoidy in Pseudomonas aeruginosa is a critical virulence factor associated with chronic respiratory infections in cystic fibrosis. A cluster of three tightly linked genes, algU, mucA and mucB located at 67.5 min, controls development of mucoid phenotype. This locus is allelic with a group of mutations (muc) associated with conversion into constitutively mucoid forms. One of the genes previously characterized in this region, algU, is absolutely required for the transcriptional activation of algD, a critical event in the establishment of mucoidy. AlgU is homologous to the alternative sigma factor sigma H (Spo0H) controlling sporulation and competence in Bacillus. Two genes downstream of algU, mucA and mucB were further characterized in this study. Previous complementation studies have demonstrated that mucA is required for suppression of mucoidy in the muc-2 strain PAO568. In this work, complementation analysis indicated that, in addition, mucB was required for suppression of mucoidy in the muc-25 strain PAO581, and for enhanced complementation of the muc-2 mutation in PAO568. The complete nucleotide sequence of mucA and mucB was determined. Insertional inactivation of mucB on the chromosome of the standard genetic strain PAO resulted in mucoid phenotype, and in a strong transcriptional activation of algD. Thus, a loss of mucB function is sufficient to cause conversion of P. aeruginosa into the mucoid phenotype. Since the algU-mucA-mucB region is a general site where muc mutations have been mapped, it is likely that mucB participates in the emergence of mucoid forms. Both mucA and mucB play a regulatory role in concert with the sigma-like factor AlgU; all three genes, along with signal transduction and histone-like elements, control differentiation of P. aeruginosa into the mucoid phenotype.

Involvement of the alginate algT gene and integration host factor in the regulation of the Pseudomonas aeruginosa algB gene

Strains of Pseudomonas aeruginosa causing pulmonary infection in cystic fibrosis patients are often mucoid because of the synthesis of a capsular polysaccharide called alginate. Regulation of alginate biosynthesis includes the algB gene product (AlgB), which belongs to a class of proteins that control gene transcription in response to environmental stimuli. In this study, a homolog of the DNA-binding-and-bending protein integration host factor (IHF) and the positive regulatory gene algT were shown to be involved in algB expression. An algB-cat gene fusion was constructed on a low-copy-number, broad-host-range plasmid. In alginate-producing (Alg+) P. aeruginosa, levels of chloramphenicol acetyltransferase from algB-cat were twofold higher than in spontaneous Alg- or algT::Tn501 mutant strains, indicating that the mucoid status of the cell influences algB transcription. An algB transcription initiation site was identified 286 nucleotides upstream of translation initiation and revealed an Escherichia coli sigma 70-like promoter. Sequences in the algB promoter region were highly similar to the consensus E. coli IHF binding site. In DNA gel band mobility shift assays, a protein present in extracts from IHF+ E. coli strains and IHF purified from E. coli bound specifically to these algB DNA fragments, while extracts prepared from isogenic IHF- E. coli strains failed to alter the mobility of algB DNA fragments containing the consensus IHF binding site. A protein in cell extracts prepared from P. aeruginosa strains also demonstrated binding to algB fragments containing the IHF binding site, and the position of the complex formed with these extracts was identical to that of the complex formed with purified IHF. Moreover, this binding could be inhibited by anti-IHF antibodies. To test the role of the IHF site in algB regulation, site-specific mutations in the algB IHF site, based on changes which severely affect IHF binding in E. coli, were generated. When either purified E. coli IHF or extracts from P. aeruginosa were used in DNA binding studies, the algB mutant DNAs were severely reduced in IHF binding. Mutations affecting IHF binding at the algB promoter were introduced into the algB-cat plasmid, and all resulted in severely impaired transcriptional activity in Alg- and algT mutant strains of P. aeruginosa. However, these mutations resulted in similar or slightly reduced algB-cat transcription in Alg+ and algB::Tn501 mutant strains. Thus, the algT product plays a positive role in the high-level expression of algB in mucoid cells, whereas as protein present in P.aeruginosa extracts which is likely an IHF homolog plays a positive role in maintaining a basal level of algB expression in nonmucoid strains.