Gene-scrambling mutagenesis: generation and analysis of insertional mutations in the alginate regulatory region of Pseudomonas aeruginosa

Expression analysis of the Pseudomonas aeruginosa AlgZR two-component regulatory system

Pseudomonas aeruginosa virulence components are subject to complex regulatory control primarily through two-component regulatory systems that allow for sensing and responding to environmental stimuli. In this study, the expression and regulation of the P. aeruginosa AlgZR two-component regulatory system were examined. Primer extension and S1 nuclease protection assays were used to identify two transcriptional initiation sites for algR within the algZ coding region, and two additional start sites were identified upstream of the algZ coding region. The two algR transcriptional start sites, RT1 and RT2, are directly regulated by AlgU, consistent with previous reports of increased algR expression in mucoid backgrounds, and RpoS additionally plays a role in algR transcription. The expression of the first algZ promoter, ZT1, is entirely dependent upon Vfr for expression, whereas Vfr, RpoS, or AlgU does not regulate the second algZ promoter, ZT2. Western blot, real-time quantitative PCR (RT-qPCR), and transcriptional fusion analyses show that algZR expression is Vfr dependent. The algZ and algR genes also are cotranscribed in both nonmucoid and mucoid backgrounds. Furthermore, algZR was found to be cotranscribed with hemCD by RT-PCR. RT-qPCR confirmed that hemC transcription in the PAO1 ΔalgZ mutant was 40% of the level of the wild-type strain. Taken together, these results indicate that algZR transcription involves multiple factors at multiple start sites that control individual gene expression as well as coexpression of this two-component system with heme biosynthetic genes.

The Pseudomonas aeruginosa AlgZR two-component system coordinates multiple phenotypes

Pseudomonas aeruginosa is an opportunistic pathogen that causes a multitude of infections. These infections can occur at almost any site in the body and are usually associated with a breach of the innate immune system. One of the prominent sites where P. aeruginosa causes chronic infections is within the lungs of cystic fibrosis patients. P. aeruginosa uses two-component systems that sense environmental changes to differentially express virulence factors that cause both acute and chronic infections. The P. aeruginosa AlgZR two component system is one of its global regulatory systems that affects the organism's fitness in a broad manner. This two-component system is absolutely required for two P. aeruginosa phenotypes: twitching motility and alginate production, indicating its importance in both chronic and acute infections. Additionally, global transcriptome analyses indicate that it regulates the expression of many different genes, including those associated with quorum sensing, type IV pili, type III secretion system, anaerobic metabolism, cyanide and rhamnolipid production. This review examines the complex AlgZR regulatory network, what is known about the structure and function of each protein, and how it relates to the organism's ability to cause infections.

The Pseudomonas aeruginosa sensor kinase KinB negatively controls alginate production through AlgW-dependent MucA proteolysis

Mucoidy, or overproduction of the exopolysaccharide known as alginate, in Pseudomonas aeruginosa is a poor prognosticator for lung infections in cystic fibrosis. Mutation of the anti-sigma factor MucA is a well-accepted mechanism for mucoid conversion. However, certain clinical mucoid strains of P. aeruginosa have a wild-type (wt) mucA. Here, we describe a loss-of-function mutation in kinB that causes overproduction of alginate in the wt mucA strain PAO1. KinB is the cognate histidine kinase for the transcriptional activator AlgB. Increased alginate production due to inactivation of kinB was correlated with high expression at the alginate-related promoters P(algU) and P(algD). Deletion of alternative sigma factor RpoN (sigma(54)) or the response regulator AlgB in kinB mutants decreased alginate production to wt nonmucoid levels. Mucoidy was restored in the kinB algB double mutant by expression of wt AlgB or phosphorylation-defective AlgB.D59N, indicating that phosphorylation of AlgB was not required for alginate overproduction when kinB was inactivated. The inactivation of the DegS-like protease AlgW in the kinB mutant caused loss of alginate production and an accumulation of the hemagglutinin (HA)-tagged MucA. Furthermore, we observed that the kinB mutation increased the rate of HA-MucA degradation. Our results also indicate that AlgW-mediated MucA degradation required algB and rpoN in the kinB mutant. Collectively, these studies indicate that KinB is a negative regulator of alginate production in wt mucA strain PAO1.

Genetics of bacterial alginate: alginate genes distribution, organization and biosynthesis in bacteria

Bacterial alginate genes are chromosomal and fairly widespread among rRNA homology group I Pseudomonads and Azotobacter. In both genera, the genetic pathway of alginate biosynthesis is mostly similar and the identified genes are identically organized into biosynthetic, regulatory and genetic switching clusters. In spite of these similarities,still there are transcriptional and functional variations between P. aeruginosa and A. vinelandii. In P. aeruginosa all biosynthetic genes except algC transcribe in polycistronic manner under the control of algD promoter while in A. vinelandii, these are organized into many transcriptional units. Of these, algA and algC are transcribed each from two different and algD from three different promoters. Unlike P. aeruginosa, the promoters of these transcriptional units except one of algC and algD are algT-independent. Both bacterial species carry homologous algG gene for Ca(2+)-independent epimerization. But besides algG, A. vinelandii also has algE1-7 genes which encode C-5-epimerases involved in the complex steps of Ca(2+)-dependent epimerization. A hierarchy of alginate genes expression under sigma(22)(algT) control exists in P. aeruginosa where algT is required for transcription of the response regulators algB and algR, which in turn are necessary for expression of algD and its downstream biosynthetic genes. Although algTmucABCD genes cluster play similar regulatory roles in both P. aeruginosa and A. vinelandii but unlike, transcription of A. vinelandii, algR is independent of sigma(22). These differences could be due to the fact that in A. vinelandii alginate plays a role as an integrated part in desiccation-resistant cyst which is not found in P. aeruginosa.

The Pseudomonas aeruginosa algC gene product participates in rhamnolipid biosynthesis

Pseudomonas aeruginosa produces exoproducts correlated with its pathogenicity. One of these virulence-associated traits is the surfactant rhamnolipid. The production of alginate and lipopolysaccharide (LPS) are also of importance for P. aeruginosa virulence. The product of the algC gene (which is involved in alginate production through its phosphomannomutase activity and in LPS synthesis through its phosphoglucomutase activity) participates in rhamnolipid production, presumably catalyzing the first step in the deoxy-thymidine-diphospho-L-rhamnose (dTDP-L-rhamnose) pathway, the conversion of glucose-6-phosphate to glucose-1-phosphate. Other structural alg genes, encoded in the alg operon, are not involved in rhamnolipid nor LPS production. These results show that the AlgC protein plays a central role in the production of the three P. aeruginosa virulence-associated saccharides: alginate, LPS and rhamnolipid.

Regulation of alginate biosynthesis in Pseudomonas syringae pv. syringae

Both Pseudomonas aeruginosa and the phytopathogen P. syringae produce the exopolysaccharide alginate. However, the environmental signals that trigger alginate gene expression in P. syringae are different from those in P. aeruginosa with copper being a major signal in P. syringae. In P. aeruginosa, the alternate sigma factor encoded by algT (sigma22) and the response regulator AlgR1 are required for transcription of algD, a gene which encodes a key enzyme in the alginate biosynthetic pathway. In the present study, we cloned and characterized the gene encoding AlgR1 from P. syringae. The deduced amino acid sequence of AlgR1 from P. syringae showed 86% identity to its P. aeruginosa counterpart. Sequence analysis of the region flanking algR1 in P. syringae revealed the presence of argH, algZ, and hemC in an arrangement virtually identical to that reported in P. aeruginosa. An algR1 mutant, P. syringae FF5.32, was defective in alginate production but could be complemented when algR1 was expressed in trans. The algD promoter region in P. syringae (PsalgD) was also characterized and shown to diverge significantly from the algD promoter in P. aeruginosa. Unlike P. aeruginosa, algR1 was not required for the transcription of algD in P. syringae, and PsalgD lacked the consensus sequence recognized by AlgR1. However, both the algD and algR1 upstream regions in P. syringae contained the consensus sequence recognized by sigma22, suggesting that algT is required for transcription of both genes.

Pseudomonas aeruginosa in cystic fibrosis: role of mucC in the regulation of alginate production and stress sensitivity

Alginate production in Pseudomonas aeruginosa and the associated mucoid phenotype of isolates from cystic fibrosis patients are under the control of the algU mucABCD cluster. This group of genes encodes AlgU, the P. aeruginosa equivalent of the extreme heat shock sigma factor sigma E in Gram-negative bacteria, the AlgU-cognate anti-sigma factor MucA, the periplasmic protein MucB and a serine protease homologue, MucD. While mucA, mucB or mucD act as negative regulators of AlgU, the function of mucC is not known. In this study the role of mucC in P. aeruginosa physiology and alginate production has been addressed. Insertional inactivation of mucC in the wild-type P. aeruginosa strain PAO1 did not cause any overt effects on alginate synthesis. However, it affected growth of P. aeruginosa under conditions of combined elevated temperature and increased ionic strength or osmolarity. Inactivation of mucC in mucA or mucB mutant backgrounds resulted in a mucoid phenotype when the cells were grown under combined stress conditions of elevated temperature and osmolarity. Each of the stress factors tested separately did not cause comparable effects. The combined stress factors were not sufficient to cause phenotypically appreciable enhancement of alginate production in mucA or mucB mutants unless mucC was also inactivated. These findings support a negative regulatory role of mucC in alginate production by P. aeruginosa, indicate additive effects of muc genes in the regulation of mucoidy in this organism and suggest that multiple stress signals and recognition systems participate in the regulation of algU-dependent functions.

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.

Identification of the algZ gene upstream of the response regulator algR and its participation in control of alginate production in Pseudomonas aeruginosa

Alginate production in mucoid Pseudomonas aeruginosa isolates from cystic fibrosis patients is under direct control by AlgU, the P. aeruginosa equivalent of the extreme heat shock sigma factor sigma(E) in gram-negative bacteria, and AlgR, a response regulator from the superfamily of two-component signal transduction systems. In this report, we describe the identification of the algZ gene, located immediately upstream of algR, which is involved in the control of alginate production. The predicted product of the algZ gene showed similarity to a subset of sensory components from the superfamily of signal transduction systems but lacked several of the highly conserved motifs typical of histidine protein kinases. Inactivation of algZ in the wild-type standard genetic strain PAO1 did not affect its nonmucoid morphology. However, inactivation of algZ in a mucoid mutant P. aeruginosa strain, which had AlgU freed from control by the anti-sigma factor MucA, resulted in increased alginate production under growth conditions which did not permit expression of mucoidy in the parental algZ+ strain. The observed effects were abrogated when algR was inactivated in the algZ::Tc(r) background. These findings indicate that algZ plays a regulatory role in alginate production, possibly interacting with AlgR, and that it may have negative effects on expression of the mucoid phenotype under the conditions tested. The presented results suggest that elements of negative regulation exist at the levels of both the alternative sigma factor AlgU and the transcriptional activator AlgR which, once relieved from that suppression, cooperate to bring about the expression of the alginate system.

The alginate regulator AlgR and an associated sensor FimS are required for twitching motility in Pseudomonas aeruginosa

Mucoid strains of Pseudomonas aeruginosa isolated from the lungs of cystic fibrosis patients produce large amounts of the exopolysaccharide alginate. AlgR has long been considered a key regulator of alginate production, but its cognate sensor has not been identified. Here we show that AlgR is required for twitching motility, which is a form of bacterial surface translocation mediated by type 4 fimbriae. Adjacent to algR we have identified a sensor gene (fimS), which is also required for twitching motility. However, FimS does not appear to be required for alginate production in mucoid strains. FimS and AlgR are representative of a new subclass of two-component transmitter-receiver regulatory systems. The alternative sigma factor AlgU also affects both alginate production and twitching motility. Therefore, these two virulence determinants appear to be closely associated and coordinately regulated.

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.

Two distinct loci affecting conversion to mucoidy in Pseudomonas aeruginosa in cystic fibrosis encode homologs of the serine protease HtrA

Conversion to a mucoid, exopolysaccharide alginate-overproducing phenotype in Pseudomonas aeruginosa is associated with chronic respiratory infections in cystic fibrosis. Mucoidy is caused by muc mutations that derepress the alternative sigma factor AlgU, which in turn activates alginate biosynthetic and ancillary regulatory genes. Here we report the molecular characterization of two newly identified genes, algW and mucD, that affect expression of mucoidy. The algW gene, mapping at 69 min, was isolated on the basis of its ability to suppress mucoidy and reduce transcription of the alginate biosynthetic gene algD. The predicted primary structure of AlgW displayed similarity to HtrA (DegP), a serine protease involved in proteolysis of abnormal proteins and required for resistance to oxidative and heat stress in enteric bacteria. Inactivation of algW on the chromosome of the wild-type nonmucoid strain PAO1 caused increased sensitivity to heat, H2O2, and paraquat, a redox cycling compound inducing intracellular levels of superoxide. This mutation also permitted significant induction of alginate production in the presence of subinhibitory concentrations of paraquat. Two new genes, mucC and mucD, were identified immediately downstream of the previously characterized portion (algU mucA mucB) of the gene cluster at 67.5 min encoding the alternative sigma factor AlgU and its regulators. Interestingly, the predicted gene product of mucD also showed similarities to HtrA. Inactivation of mucD on the PAO1 chromosome resulted in conversion to the mucoid phenotype. The mutation in mucD also caused increased sensitivity to H2O2 and heat killing. However, in contrast to algW mutants, no increase in susceptibility to paraquat was observed in mucD mutants. These findings indicate that algW and mucD play partially overlapping but distinct roles in P. aeruginosa resistance to reactive oxygen intermediates and heat. In addition, since mutations in mucD and algW cause conversion to mucoidy or lower the threshold for its induction by reactive oxygen intermediates, these factors may repress alginate synthesis either directly by acting on AlgU or its regulators or indirectly by removing physiological signals that may activate this stress response system.

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.

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.

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.

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.

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.

Characterization of a locus determining the mucoid status of Pseudomonas aeruginosa: AlgU shows sequence similarities with a Bacillus sigma factor

Overproduction of the exopolysaccharide alginate by Pseudomonas aeruginosa results in mucoid colony morphology and is an important virulence determinant expressed by this organism in cystic fibrosis. Mucoidy is transcriptionally regulated by signal transduction systems and histone-like elements. One point of convergence of regulatory elements controlling mucoidy is the algD promoter. A newly described genetic locus required for algD transcription was characterized in this study. This DNA region, cloned from a nonmucoid PAO strain, was initially isolated on the basis of its ability to suppress mucoidy when present on a plasmid. The suppressing activity was observed in several mucoid PAO derivatives, including strain PAO568, in which the mapped muc-2 mutation is responsible for its mucoid phenotype, and in close to 40% of cystic fibrosis strains tested. Protein expression studies detected two polypeptides with apparent molecular masses of 27.5 and 20 kDa encoded by the region required for the suppression activity. The gene encoding the polypeptide with an apparent molecular mass of 27.5 kDa, termed algU, was further characterized. A functional chromosomal copy of algU was found to be necessary for the expression of mucoidy. Insertional inactivation of algU on the chromosome of the mucoid strain PAO568 abrogated alginate production and algD transcription. DNA sequence analysis revealed sequence similarity of the predicted algU gene product with sigma H (Spo0H), a sigma factor involved in the control of sporulation and competence in Bacillus spp. Physical mapping revealed that algU resided on the same SpeI fragment (F) as did the pruAB locus, known to be tightly linked with genetic determinants (muc) which can confer mucoidy in genetic crosses. When the chromosomal algU copy was tagged with a Tcr cassette (algU::Tcr), a tight genetic linkage of algU with pruAB was demonstrated by F116L-mediated generalized transduction. Moreover, algU::Tcr derivatives of PAO568 (originally carrying the muc-2 marker) lost the ability to transfer mucoidy in genetic crosses. These results suggest that algU, a regulator of algD transcription showing sequence similarity to an alternative sigma factor, and the genes immediately downstream of algU may be associated with a locus participating in the differentiation into the mucoid phenotype.

Immunocytochemical analysis of AlgP (Hp1), a histonelike element participating in control of mucoidy in Pseudomonas aeruginosa

AlgP, a protein with an unusual carboxy-terminal domain resembling the tails of eukaryotic H1 histones, was detected in whole-cell extracts and within the cells of Pseudomonas aeruginosa by using immunoblotting and immunoelectron microscopy analyses. One known function of AlgP is its participation in the transcriptional activation of the algD gene. This is a pivotal step in the establishment of mucoidy in P. aeruginosa; mucoidy is a critical virulence factor expressed during respiratory infections in patients with cystic fibrosis. Polyclonal and monoclonal antibodies were raised against a synthetic 50-mer peptide containing two sets of six tandem repeats of the motif Lys-Pro-Ala-Ala (and its single-amino-acid substitution variants), based on the sequence of the algP gene from the standard genetic strain PAO. Western immunoblots with these antibodies and total protein extracts from P. aeruginosa revealed two polypeptides that reacted with the antibodies in all of the P. aeruginosa strains tested. The detected polypeptides displayed strain-dependent variability in their electrophoretic mobility, in accordance with the previously noted variability of the algP repeats at the DNA level. In strain PAO, the recognized polypeptides had apparent masses of 46.4 and 41.6 kDa. Immunoelectron microscopy revealed that AlgP is an intracellular protein with a wide distribution suggestive of its more general role. To indicate that fact, AlgP is given here an alternative name, Hp1. Since AlgP (Hp1) is a eubacterial histonelike element displaying sequence and domanial similarity with eukaryotic H1 histones, these findings may have implications on the understanding of the organization of the prokaryotic nucleoid and its role in the control of gene expression and bacterial virulence.

AlgR, a response regulator controlling mucoidy in Pseudomonas aeruginosa, binds to the FUS sites of the algD promoter located unusually far upstream from the mRNA start site

Strong transcriptional activation of algD, a key event in the overproduction of alginate and establishment of mucoidy in Pseudomonas aeruginosa, depends on the functional algR gene. The predicted gene product of algR shows homologies to response regulators from bacterial signal transduction systems. The algR gene was overexpressed in Escherichia coli, its product (AlgR) was purified by utilizing its apparent affinity for heparin, and its sequence was verified by partial amino acid sequence analysis. AlgR was found to interact directly with the algD promoter. Deletion mapping analysis, in conjunction with mobility shift DNA-binding assays, indicated the presence of three regions within the algD promoter capable of specifically binding AlgR. A relatively weak interaction was observed with the algD promoter fragment containing the region immediately upstream of the algD mRNA start site (-144 to +11). However, when fragments spanning regions located very far upstream from the algD mRNA initiation site (-533 and -332) were used, strong specific binding was observed. These regions were separated by a DNA segment not binding AlgR and spanning positions -332 to -144. DNase I footprinting analysis further established the presence of discrete AlgR binding sites overlapping with FUS, the far-upstream sites required for full induction of algD transcription and its environmental modulation. There were two distinct binding sites: RB1, spanning nucleotides -479 to -457, and RB2, spanning nucleotides -400 to -380. Both of these sequences shared a highly conserved core region, ACCGTTCGTC. These results established a direct interaction of AlgR with the algD promoter and revealed an arrangement of binding sites highly unusual for response regulators of the AlgR type.

Mucoid Pseudomonas aeruginosa in cystic fibrosis: signal transduction and histone-like elements in the regulation of bacterial virulence

The profuse production of the exopolysaccharide alginate results in mucoidy, a critical virulence factor expressed by Pseudomonas aeruginosa during chronic respiratory tract infections in cystic fibrosis. Studies of the regulation of this pathogenic determinant have unravelled at least two levels of control, including bacterial signal transduction systems and histone-like elements. Although only in its initial phase, an understanding of the dual control of mucoidy may help to illuminate adaptive processes that depend on the combination of these regulatory factors. Integration of specific signals transduced by the two-component systems with inputs generated by the general state of bacterial nucleoids may govern the expression of certain virulence determinants and provide a framework facilitating selection of phenotypes successful under particular environmental conditions and selective pressures.

Expression patterns of genes encoding elastase and controlling mucoidy: co-ordinate regulation of two virulence factors in Pseudomonas aeruginosa isolates from cystic fibrosis

Transcriptional patterns of lasB and algD were compared in isogenic mucoid and non-mucoid Pseudomonas aeruginosa isolates from cystic fibrosis patients. The lasB gene encodes elastase, a major proteolytic enzyme secreted by P. aeruginosa, while algD is required for the synthesis of alginate, an exopolysaccharide frequently overproduced by strains infecting cystic fibrosis patients. A possible coregulation at the transcriptional level of these major virulence determinants was analysed. The lasB and algD genes showed inverse levels of promoter activity. The lasB promoter was active in non-mucoid cells and inactive in mucoid cells (in four out of five tested pairs), while the algD promoter was active in mucoid cells and silent in non-mucoid cells in all cases. When PAO568, a model strain for the analysis of control of the alginate system, was grown under conditions promoting mucoidy, the algD promoter was activated, whereas lasB mRNA could not be detected. This effect was reversed when the cells were grown in a medium suppressing mucoidy. Insertional inactivation of algR, a member of the signal-transduction systems regulating algD transcription, although abolishing algD expression and rendering cells non-mucoid, did not alter the nature of the induction and repression patterns of lasB seen in the parental strain PAO568. These results suggest that the lasB gene and the alginate system are co-ordinately regulated at a level parallel to or above the algR gene.

Role of the far-upstream sites of the algD promoter and the algR and rpoN genes in environmental modulation of mucoidy in Pseudomonas aeruginosa

The role of several regulatory elements in environmental modulation of mucoidy in Pseudomonas aeruginosa was studied. Transcriptional activation of algD, necessary for the mucoid phenotype, was found to depend on FUS, the newly identified far-upstream sites of the algD promoter. The FUS were delimited to a region spanning nucleotides -432 to -332 relative to the algD mRNA start site. Insertional inactivation of algR in PAO568 abolished the algD promoter response to nitrogen availability and greatly diminished but did not completely eliminate reactivity to changes in salt concentration. Insertional inactivation of rpoN (ntrA) in PAO568 did not affect algR and algD transcription.