Environmentally regulated algD promoter is responsive to the cAMP receptor protein in Escherichia coli

ChIP-seq reveals the global regulator AlgR mediating cyclic di-GMP synthesis in Pseudomonas aeruginosa

AlgR is a key transcriptional regulator required for the expression of multiple virulence factors, including type IV pili and alginate in Pseudomonas aeruginosa. However, the regulon and molecular regulatory mechanism of AlgR have yet to be fully elucidated. Here, among 157 loci that were identified by a ChIP-seq assay, we characterized a gene, mucR, which encodes an enzyme that synthesizes the intracellular second messenger cyclic diguanylate (c-di-GMP). A ΔalgR strain produced lesser biofilm than did the wild-type strain, which is consistent with a phenotype controlled by c-di-GMP. AlgR positively regulates mucR via direct binding to its promoter. A ΔalgRΔmucR double mutant produced lesser biofilm than did the single ΔalgR mutant, demonstrating that c-di-GMP is a positive regulator of biofilm formation. AlgR controls the levels of c-di-GMP synthesis via direct regulation of mucR. In addition, the cognate sensor of AlgR, FimS/AlgZ, also plays an important role in P. aeruginosa virulence. Taken together, this study provides new insights into the AlgR regulon and reveals the involvement of c-di-GMP in the mechanism underlying AlgR regulation.

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.

Transient alginate gene expression by Pseudomonas putida biofilm residents under water-limiting conditions reflects adaptation to the local environment

Under water-limiting conditions Pseudomonas putida produces the exopolysaccharide alginate, which influences biofilm development and facilitates maintaining a hydrated microenvironment. Since alginate is a minor biofilm matrix component it is important to determine whether alginate production occurs by all or a subset of residents, and when and to what extent cells contribute to alginate production. To address these questions we employed stable and unstable fluorescent reporters to measure alginate biosynthesis (algD) operon expression and metabolic activity in vivo quantitatively by flow cytometry and visually by microscopy. Here we report that during growth under water-limiting conditions and when biofilms become dehydrated most residents transiently express the alginate biosynthesis genes leading to distinct spatial patterns as the biofilm ages. Transient alginate gene expression was not a consequence of decreased metabolic activity, since metabolic reporters were still expressed, nor was it likely due to transient cytosolic availability of the alternative sigma factor AlgT, based on qRT-PCR. Our findings also indicate that one or more biofilm attribute, other than alginate, provides protection from desiccation stress. Collectively, our findings suggest that differentiated cells dedicated to alginate production are not part of the P. putida biofilm lifestyle under water-limiting conditions. Alternatively, P. putida biofilm cells may be responding to their own local environment, producing alginate because of the fitness advantage it confers under those particular conditions.

Regulation of Pseudomonas aeruginosa ptxR by Vfr

Pseudomonas aeruginosa PtxR enhances the expression of the exotoxin A gene toxA. The expression of ptxR itself, which occurs from two promoters (P1 and P2), is not completely understood. We have recently demonstrated that the ptxR upstream region contains potential binding sites for multiple regulators, including the virulence factor regulator Vfr. In this study, we identified within the ptxR upstream region, a 25 bp sequence to which Vfr specifically binds. The sequence is located 20-44 (32.5) bp 5' of the ptxR P2 promoter, and overlaps a potential binding site for the iron-starvation sigma factor PvdS. We also show that, throughout the growth cycle, deletion of vfr reduces ptxR expression from the P2 promoter in the P. aeruginosa strain PAO1 by four- to eightfold, but does not affect ptxR expression from P1. Further, loss of Vfr eliminates the PtxR-induced enhancement in the synthesis of exotoxin A and the metalloproteinase LasB. Our results suggest that Vfr modulates toxA and lasB expression in PAO1 through PtxR. A model defining the relationships between these different genes is presented.

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.

Characterization of DNA-binding specificity and analysis of binding sites of the Pseudomonas aeruginosa global regulator, Vfr, a homologue of the Escherichia coli cAMP receptor protein

Vfr, a global regulator of Pseudomonas aeruginosa virulence factors, is a homologue of the Escherichia coli cAMP receptor protein, CRP. Vfr is 91% similar to CRP and maintains many residues important for CRP to bind cAMP, bind DNA, and interact with RNA polymerase at target promoters. While vfr can complement an E. coli crp mutant in beta-galactosidase production, tryptophanase production and catabolite repression, crp can only complement a subset of Vfr-dependent phenotypes in P. aeruginosa. Using specific CRP binding site mutations, it is shown that Vfr requires the same nucleotides as CRP for optimal transcriptional activity from the E. coli lac promoter. In contrast, CRP did not bind Vfr target sequences in the promoters of the toxA and regA genes. Footprinting analysis revealed Vfr protected sequences upstream of toxA, regA, and the quorum sensing regulator lasR, that are similar to but significantly divergent from the CRP consensus binding sequence, and Vfr causes similar DNA bending to CRP in bound target sequences. Using a preliminary Vfr consensus binding sequence deduced from the Vfr-protected sites, Vfr target sequences were identified upstream of the virulence-associated genes plcN, plcHR, pbpG, prpL and algD, and in the vfr/orfX, argH/fimS, pilM/ponA intergenic regions. From these sequences the Vfr consensus binding sequence, 5'-ANWWTGNGAWNY : AGWTCACAT-3', was formulated. This study suggests that Vfr shares many of the same functions as CRP, but has specialized functions, at least in terms of DNA target sequence binding, required for regulation of a subset of genes in its regulon.

PsrA is a positive transcriptional regulator of the type III secretion system in Pseudomonas aeruginosa

The type III secretion system (TTSS) of Pseudomonas aeruginosa is induced in vivo upon contact with eukaryotic cells and in vitro by calcium depletion in culture medium. We have observed a previously identified protein, PsrA, necessary for full activation of TTSS gene expression in P. aeruginosa. Electrophoretic mobility shift assays showed that recombinant PsrA could bind to the exsCEBA promoter region. A mutant with a deletion in the psrA gene was constructed. Using transcriptional fusions, we demonstrated that PsrA is required for the full activation of transcription of the TTSS regulatory operon exsCEBA and effector exoS, although the deletion mutant still responded to calcium depletion, to serum, and to host cell contact. The psrA mutant showed a marked decrease in the secretion of the type III effectors and weak resistance to phagocyte-like PLB-985 cells. The defect in TTSS transcription and secretion in the psrA mutant could be complemented by expression in trans of psrA. PsrA was previously identified as a transcriptional activator of RpoS, a central regulator during stationary phase. We confirmed with our strain that RpoS has a negative effect on TTSS gene expression. Taken altogether, these results suggest that PsrA is a newly identified activator that is involved in the expression of the TTSS by enhancing the exsCEBA transcriptional level.

Understanding the control of Pseudomonas aeruginosa alginate synthesis and the prospects for management of chronic infections in cystic fibrosis

Decades of research have been dedicated to the study of the opportunistic pathogen Pseudomonas aeruginosa, a Gram-negative, environmental bacterium that secretes the exopolysaccharide alginate during chronic lung infection of cystic fibrosis (CF) patients. Although P. aeruginosa utilizes a variety of factors to establish a successful infection in the lungs of CF patients, alginate has stood out as one of the best-studied prognostic indicators of chronic lung infection. While the genetics, biosynthesis and regulation of alginate are well understood, questions still remain concerning its role in biofilm development and its potential as a therapeutic target. The purpose of this review is to provide a brief summary of alginate biosynthesis and regulation, and to highlight recent discoveries in the areas of alginate production, biofilm formation and vaccine design. This information is placed in context with a proposed P. aeruginosa infectious pathway, highlighting avenues for the use of existing therapies as well as the potential for novel agents to reduce or eliminate chronic infections in CF patients.

fleQ, the gene encoding the major flagellar regulator of Pseudomonas aeruginosa, is sigma70 dependent and is downregulated by Vfr, a homolog of Escherichia coli cyclic AMP receptor protein

The flagellar transcriptional regulator FleQ appears to be the highest-level regulator in the hierarchical regulatory cascade of flagellar biogenesis in Pseudomonas aeruginosa. Except for the posttranslational downregulation of FleQ activity by FleN, an antiactivator, not much is known about the regulation of the fleQ gene or its gene product. Some FleQ homologs in other bacterial species either are positively regulated by another regulator (e.g., CtrA, the master regulator regulating FlbD in Caulobacter crescentus) or are expressed from a sigma70-dependent promoter (e.g., FlgR of Helicobacter pylori). In this study we demonstrated that Vfr, an Escherichia coli CRP homolog known to function as an activator for various genes, including lasR, regA, and toxA, in P. aeruginosa, is capable of repressing fleQ transcription by binding to its consensus sequence in the fleQ promoter. In a DNase I footprint assay, purified Vfr protected the sequence 5'-AATTGACTAATCGTTCACATTTG-3'. When this putative Vfr binding site in the fleQ promoter was mutated, Vfr was unable to bind the fleQ promoter fragment and did not repress fleQ transcription effectively. Primer extension analysis of the fleQ transcript revealed two transcriptional start sites, t1 and t2, that map within the Vfr binding site. A putative -10 region (TAAAAT) for the t2 transcript, with a five-of-six match with the E. coli sigma70 binding consensus, overlaps with one end of the Vfr binding site. A 4-bp mutation and an 8-bp mutation in this -10 region markedly reduced the activity of the fleQ promoter. The same mutations led to the disappearance of the 203-nucleotide fleQ transcript in an in vitro transcription assay. Vfr probably represses fleQ transcription by binding to the Vfr binding site in the fleQ promoter and preventing the sigma factor from binding to the -10 region to initiate transcription.

Effect of vfr mutation on global gene expression and catabolite repression control of Pseudomonas aeruginosa

Vfr of Pseudomonas aeruginosa is 91% similar to the cAMP receptor protein (CRP) of Escherichia coli. Based on the high degree of sequence homology between the two proteins, the question arose whether Vfr had a global regulatory effect on gene expression for P. aeruginosa as CRP did for E. coli. This report provides two-dimensional polyacrylamide gel electrophoretic evidence that Vfr is a global regulator of gene expression in P. aeruginosa. In a vfr101::aacC1 null mutant, at least 43 protein spots were absent or decreased when compared to the proteome pattern of the parent strain. In contrast, 17 protein spots were absent or decreased in the parent strain when compared to the vfr101::aacC1 mutant. Thus, a mutation in vfr affected production of at least 60 proteins in P. aeruginosa. In addition, the question whether Vfr and CRP shared similar mechanistic characteristics was addressed. To ascertain whether Vfr, like CRP, can bind cAMP, Vfr and CRP were purified to homogeneity and their apparent dissociation constants (K(d)) for binding to cAMP were determined. The K(d) values were 1.6 microM for Vfr and 0.4 microM for CRP, suggesting that these proteins have a similar affinity for cAMP. Previously the authors had demonstrated that Vfr could complement a crp mutation and modulate catabolite repression in E. coli. This study presents evidence that Vfr binds to the E. coli lac promoter and that this binding requires the presence of cAMP. Finally, the possible involvement of Vfr in catabolite repression control in P. aeruginosa was investigated. It was found that succinate repressed production of mannitol dehydrogenase, glucose-6-phosphate dehydrogenase, amidase and urocanase both in the parent and in two vfr null mutants. This implied that catabolite repression control was not affected by the vfr null mutation. In support of this, the cloned vfr gene failed to complement a mutation in the P. aeruginosa crc gene. Thus, although Vfr is structurally similar to CRP, and is a global regulator of gene expression in P. aeruginosa, Vfr is not required for catabolite repression control in this bacterium.

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.

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.

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%.

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.

The vfr gene product, required for Pseudomonas aeruginosa exotoxin A and protease production, belongs to the cyclic AMP receptor protein family

The synthesis of exotoxin A (ETA) by Pseudomonas aeruginosa is a complex, regulated event. Several ETA putative regulatory mutants of P. aeruginosa PA103 have previously been characterized (S. E. H. West, S. A. Kaye, A. N. Hamood, and B. H. Iglewski, Infect. Immun. 62:897-903, 1994). In addition to ETA production, these mutants, PA103-15, PA103-16, and PA103-19, were also deficient in the production of protease and in regA P1 promoter activity. RegA is a positive regulator of ETA transcription. We cloned a gene, designated vfr for virulence factor regulator, that restored ETA and protease production to parental levels in these mutants. In addition, transcription from the regA P1 promoter was restored. In Escherichia coli, when vfr was overexpressed from a phage T7 promoter, a protein with an apparent molecular mass of 28.5 kDa was produced. Analysis of the deduced amino acid sequence of vfr revealed that the expected protein is 67% identical and 91% similar over a 202-amino-acid overlap to the E. coli cyclic AMP receptor protein (CAP or Crp). The cloned vfr gene complemented the beta-galactosidase- and tryptophanase-deficient phenotypes of E. coli RZ1331, a crp deletion mutant. However, the E. coli crp gene under the control of the tac promoter did not complement the ETA-deficient or protease-deficient phenotype of PA103-15 or PA103-16. The ability of vfr to restore both ETA and protease production to these mutants suggests that vfr is a global regulator of virulence factor expression in P. aeruginosa.

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.

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.

LasR of Pseudomonas aeruginosa is a transcriptional activator of the alkaline protease gene (apr) and an enhancer of exotoxin A expression

The lasR gene of Pseudomonas aeruginosa is required for transcription of the genes for elastase (lasB) and LasA protease (lasA), two proteases associated with virulence. We report here that the alkaline protease gene (apr) also requires the lasR gene for transcription. Alkaline protease mRNA was absent in the lasR mutant PAO-R1 and present when an intact lasR gene was supplied in trans as determined by Northern (RNA) analysis. The lasR gene also enhances exotoxin A production. Exotoxin A activity in supernatants of PAO-R1 were 30% less than in supernatants of the parental strain, PAO-SR. Multiple copies of lasR in trans in PAO-R1 in increased toxin A activity to twice the parental levels. Analysis of PAO-R1 containing the toxA promoter fused to beta-galactosidase suggests that LasR acts at the toxA promoter or at upstream toxA mRNA sequences. beta-Galactosidase activity was approximately 40% lower in PAO-R1 than in the parental strain, PAO-SR. Furthermore, the effect of LasR on the toxA promoter is not due to the stimulation of transcription of regA, a transcriptional activator of toxA. No difference in chloramphenicol acetyltransferase (CAT) activity was noted between PAO-SR and PAO-R1 containing transcriptional regA promoter-CAT gene fusions. These results broaden the regulatory dominion of lasR and suggest that the lasR gene plays a global role in P. aeruginosa pathogenesis.

Inhibitors of two-component signal transduction systems: inhibition of alginate gene activation in Pseudomonas aeruginosa

Pseudomonas aeruginosa strains infecting cystic fibrosis patients often produce copious amounts of the exopolysaccharide alginate. Expression of alginate genes in P. aeruginosa is regulated by several proteins including members of the two-component bacterial signal transduction systems. Two of these regulatory proteins are AlgR1, the DNA-binding response regulator that transcriptionally activates alginate gene expression, and AlgR2, the kinase that modifies AlgR1 via phosphorylation to enhance its activity. In this paper, we report the identification of compounds that inhibit alginate gene expression by inhibiting (i) the phosphorylation/dephosphorylation of AlgR2 and (ii) the DNA-binding activity of AlgR1. Compounds with these activities may have potential as components of therapy for eliminating P. aeruginosa infection from the cystic fibrosis lung. In addition, we describe the effect of these compounds on the autophosphorylation activity of other known two-component kinases and show the ability of one compound to significantly inhibit the kinase activities of CheA, NRII, and KinA.

In vitro phosphorylation of AlgR, a regulator of mucoidy in Pseudomonas aeruginosa, by a histidine protein kinase and effects of small phospho-donor molecules

AlgR is a transcriptional regulator of mucoidy in Pseudomonas aeruginosa, a critical virulence factor expressed in cystic fibrosis. AlgR belongs to the superfamily of bacterial signal transduction systems, and has been shown to bind to the algD promoter, a critical point in the regulation of mucoidy. This protein, like other typical response regulators, contains highly conserved residues known to be critical for the phosphorylation and signal transduction processes. However, a typical second component interacting with AlgR has not been identified. Here we demonstrate that AlgR undergoes phosphorylation in vitro when interacting with the well-characterized histidine protein kinase CheA. These results indicate that AlgR is capable of undergoing phosphorylation typical of other two-component signal transduction systems. Moreover, the phosphotransfer reaction between CheA and AlgR was found to be affected by the presence of carbamoyl phosphate, acetyl phosphate, and salts of phosphoramidic acid, recently shown to act as small-molecular-weight phospho-donors in the process of phosphorylation of several response regulators. These findings suggest that AlgR may react with intermediary metabolites such as carbamoyl phosphate and acetyl phosphate, and that these processes may play a role in the control of mucoidy in P. aeruginosa.

AlgR2 is an ATP/GTP-dependent protein kinase involved in alginate synthesis by Pseudomonas aeruginosa

The exopolysaccharide alginate is a major virulence factor in the pathogenicity of Pseudomonas aeruginosa infecting the lungs of cystic fibrosis (CF) patients. Alginate synthesis by P. aeruginosa is believed to occur in response to environmental signals present in the CF lung. Transcription of a critical alginate biosynthetic gene, algD, is triggered by environmental signals and is known to be controlled by regulatory proteins AlgR1, AlgR2, and AlgR3. AlgR1 is a member of the family of response regulators of the phosphorylation-dependent two-component bacterial signal transduction systems. In this report, we describe the characterization of AlgR2 as the kinase involved in phosphorylation of AlgR1. AlgR2, an 18-kDa soluble protein undergoes rapid autophosphorylation in the presence of either ATP or GTP and transfers the phosphate to AlgR1. AlgR2 retains high affinity for both ATP and GTP with an apparent Km of 137 and 249 nM, respectively, for phosphorylation by these two substrates. ADP and GDP exhibit competitive inhibition with an apparent Ki of 94 and 314 nM, respectively, during phosphorylation by ATP and 481 and 273 nM during phosphorylation by GTP. AlgR1 and AlgR2 can be isolated in the form of an 80-kDa complex that is capable of undergoing phosphorylation and intracomplex phosphotransfer in vitro. A 16-kDa AlgR2 analog, capable of autophosphorylation in the presence of ATP or GTP and transferring the phosphate moiety to AlgR1, has been characterized in Escherichia coli.