Dual function of PilS during transcriptional activation of the Pseudomonas aeruginosa pilin subunit gene

Hyperpiliation, not loss of pilus retraction, reduces Pseudomonas aeruginosa pathogenicity

Type IVa pili (T4aP) are important virulence factors for many bacterial pathogens. Previous studies suggested that the retraction ATPase, PilT, modulates pathogenicity due to its critical role in pilus dynamics and twitching motility. Here we use a Caenorhabditis elegans slow-killing model to show that hyperpiliation, not loss of pilus retraction, reduces virulence of Pseudomonas aeruginosa strains PAK and PA14. Hyperactivating point mutations in the P. aeruginosa PilSR two-component system that controls transcription of the major pilin gene, pilA, increased levels of surface pili to the same extent as deleting pilT, without impairing twitching motility. These functionally hyperpiliated PilSR mutants had significant defects in pathogenicity that were rescued by deleting pilA or through disruption of hyperpiliation via deletion of the type III secretion system needle-length regulator, PscP. Hyperpiliated pilT deletion or pilO point mutants showed similar PilA-dependent impairments in virulence, validating the phenotype. Together, our data support a model where a surfeit of pili reduces virulence, potentially through the prevention of effective engagement of contact-dependent virulence factors. These findings suggest that the role of T4aP retraction in virulence should be revised.IMPORTANCEPseudomonas aeruginosa is a major contributor to hospital-acquired infections and particularly problematic due to its intrinsic resistance to many front-line antibiotics. Strategies to combat this and other important pathogens include the development of anti-virulence therapeutics. We show that the pathogenicity of P. aeruginosa is impaired when the amount of T4aP expressed on the cell surface increases, independent of the bacteria's ability to twitch. We propose that having excess T4aP on the cell surface may physically interfere with productive engagement of the contact-dependent type III secretion toxin delivery system. A better understanding of how T4aP modulate interaction of bacteria with target cells will improve the design of therapeutics targeting components involved in the regulation of T4aP expression and function to reduce the clinical burden of P. aeruginosa and other T4aP-expressing bacteria.

A molecular mechanism for how sigma factor AlgT and transcriptional regulator AmrZ inhibit twitching motility in Pseudomonas aeruginosa

Pseudomonas aeruginosa isolates from cystic fibrosis patients are often mucoid (due to the overexpression of exopolysaccharide alginate) yet lost motility. It remains unclear about how P. aeruginosa coordinately regulates alginate production and the type IV pili-driven twitching motility. Here we showed that sigma 22 factor (AlgT/U), an activator of alginate biosynthesis, repressed twitching motility by inhibiting the expression of pilin (PilA) through the intermediate transcriptional regulator AmrZ, which directly bound to the promoter region of pilA in both mucoid strain FRD1 and non-mucoid strain PAO1. Four conserved AmrZ-binding sites were found in pilA promoters among 10 P. aeruginosa strains although their entire pilA promoters had low identity. AmrZ has been reported to be essential for twitching in PAO1. We found that AmrZ was also required for twitching in mucoid FRD1, yet a high level of AmrZ inhibited twitching motility. This result was consistent with the phenomenon that twitching is frequently repressed in mucoid strains, in which the expression of AmrZ was highly activated by AlgT. Additionally, AlgT also inhibited the transcription of pilMNOP operon, which is involved in efficient pilus assembly. Our data elucidated a mechanism for how AlgT and AmrZ coordinately controlled twitching motility in P. aeruginosa.

The Pseudomonas aeruginosa PilSR Two-Component System Regulates Both Twitching and Swimming Motilities

Motility is an important virulence trait for many bacterial pathogens, allowing them to position themselves in appropriate locations at appropriate times. The motility structures type IV pili and flagella are also involved in sensing surface contact, which modulates pathogenicity. In Pseudomonas aeruginosa, the PilS-PilR two-component system (TCS) regulates expression of the type IV pilus (T4P) major subunit PilA, while biosynthesis of the single polar flagellum is regulated by a hierarchical system that includes the FleSR TCS. Previous studies of Geobacter sulfurreducens and Dichelobacter nodosus implicated PilR in regulation of non-T4P-related genes, including some involved in flagellar biosynthesis. Here we used transcriptome sequencing (RNA-seq) analysis to identify genes in addition to pilA with changes in expression in the absence of pilR. Among the genes identified were 10 genes whose transcription increased in the pilA mutant but decreased in the pilR mutant, despite both mutants lacking T4P and pilus-related phenotypes. The products of these inversely dysregulated genes, many of which were hypothetical, may be important for virulence and surface-associated behaviors, as mutants had altered swarming motility, biofilm formation, type VI secretion system expression, and pathogenicity in a nematode model. Further, the PilSR TCS positively regulated transcription of fleSR, and thus many genes in the FleSR regulon. As a result, pilSR deletion mutants had defects in swimming motility that were independent of the loss of PilA. Together, these data suggest that in addition to controlling T4P expression, PilSR could have a broader role in the regulation of P. aeruginosa motility and surface sensing behaviors.

Surface appendages such as type IV pili and flagella are important for establishing surface attachment and infection in a host in response to appropriate cues. The PilSR regulatory system that controls type IV pilus expression in Pseudomonas aeruginosa has an established role in expression of the major pilin PilA. Here we provide evidence supporting a new role for PilSR in regulating flagellum-dependent swimming motility in addition to pilus-dependent twitching motility. Further, even though both pilA and pilR mutants lack PilA and pili, we identified sets of genes downregulated in the pilR mutant and upregulated in a pilA mutant as well as genes downregulated only in a pilR mutant, independent of pilus expression. This finding suggests that change in the inner membrane levels of PilA is only one of the cues to which PilR responds to modulate gene expression. Identification of PilR as a regulator of multiple motility pathways may make it an interesting therapeutic target for antivirulence compounds.

Mycobacterium tuberculosis DevR/DosR Dormancy Regulator Activation Mechanism: Dispensability of Phosphorylation, Cooperativity and Essentiality of α10 Helix

DevR/DosR is a well-characterized regulator in Mycobacterium tuberculosis which is implicated in various processes ranging from dormancy/persistence to drug tolerance. DevR induces the expression of an ~48-gene dormancy regulon in response to gaseous stresses, including hypoxia. Strains of the Beijing lineage constitutively express this regulon, which may confer upon them a significant advantage, since they would be ‘pre-adapted’ to the environmental stresses that predominate during infection. Aerobic DevR regulon expression in laboratory-manipulated overexpression strains is also reported. In both instances, the need for an inducing signal is bypassed. While a phosphorylation-mediated conformational change in DevR was proposed as the activation mechanism under hypoxia, the mechanism underlying constitutive expression is not understood. Because DevR is implicated in bacterial dormancy/persistence and is a promising drug target, it is relevant to resolve the mechanistic puzzle of hypoxic activation on one hand and constitutive expression under ‘non-inducing’ conditions on the other. Here, an overexpression strategy was employed to elucidate the DevR activation mechanism. Using a panel of kinase and transcription factor mutants, we establish that DevR, upon overexpression, circumvents DevS/DosT sensor kinase-mediated or small molecule phosphodonor-dependent activation, and also cooperativity-mediated effects, which are key aspects of hypoxic activation mechanism. However, overexpression failed to rescue the defect of C-terminal-truncated DevR lacking the α10 helix, establishing the α10 helix as an indispensable component of DevR activation mechanism. We propose that aerobic overexpression of DevR likely increases the concentration of α10 helix-mediated active dimer species to above the threshold level, as during hypoxia, and enables regulon expression. This advance in the understanding of DevR activation mechanism clarifies a long standing question as to the mechanism of DevR overexpression-mediated induction of the regulon in the absence of the normal environmental cue and establishes the α10 helix as an universal and pivotal targeting interface for DevR inhibitor development.

Type IV pilins regulate their own expression via direct intramembrane interactions with the sensor kinase PilS

Type IV pili are important virulence factors for many pathogens, including Pseudomonas aeruginosa Transcription of the major pilin gene-pilA-is controlled by the PilS-PilR two-component system in response to unknown signals. The absence of a periplasmic sensing domain suggested that PilS may sense an intramembrane signal, possibly PilA. We suggest that direct interactions between PilA and PilS in the inner membrane reduce pilA transcription when PilA levels are high. Overexpression in trans of PilA proteins with diverse and/or truncated C termini decreased native pilA transcription, suggesting that the highly conserved N terminus of PilA was the regulatory signal. Point mutations in PilA or PilS that disrupted their interaction prevented autoregulation of pilA transcription. A subset of PilA point mutants retained the ability to interact with PilS but could no longer decrease pilA transcription, suggesting that interaction between the pilin and sensor kinase is necessary but not sufficient for pilA autoregulation. Furthermore, PilS's phosphatase motif was required for the autoregulation of pilA transcription, suggesting that under conditions where PilA is abundant, the PilA-PilS interaction promotes PilR dephosphorylation and thus down-regulation of further pilA transcription. These data reveal a clever bacterial inventory control strategy in which the major subunit of an important P. aeruginosa virulence factor controls its own expression.

senX3-independent contribution of regX3 to Mycobacterium tuberculosis virulence

Background

Mycobacterium tuberculosis (Mtb) must adapt to various stress conditions during host infection. The two-component regulatory system (2CRS) SenX3-RegX3 is required for Mtb virulence. We showed recently that the senX3-regX3 intergenic region contains promoter activity, driving senX3-independent regX3 expression. In the current study, we tested the hypothesis that RegX3 has a SenX3-independent role in Mtb virulence. The gene expression patterns, growth, and survival of mutants containing transposon insertions in senX3 (senX3::Tn) and regX3 (regX3::Tn) were compared to those of their respective complemented strains and the isogenic wild-type parent strain during axenic growth in nutrient-rich broth, phosphate depletion, nutrient starvation, and in the lungs of BALB/c mice.

Results

regX3 expression was reduced in senX3::Tn during phosphate depletion and nutrient starvation, and expression of the phosphate-specific transport gene pstC2 was reduced similarly in senX3::Tn and regX3::Tn during phosphate depletion. Although senX3 and regX3 were each dispensable for Mtb growth in nutrient-rich broth, disruption of senX3 or regX3 caused a similar growth defect during phosphate depletion. Interestingly, senX3::Tn, in which monocistronic regX3 expression is preserved, showed significantly higher survival relative to regX3::Tn after 7 days of nutrient starvation (p <0.01), and in mouse lungs at Day 31 (p < 0.01), Day 62 (p < 0.01), and Day 124 (p = 0.05) after aerosol infection.

Conclusion

Our data demonstrate the specificity of the senX3-regX3 2CRS for sensing and responding to low ambient phosphate, but also raise the possibility that RegX3 may function independently of its cognate sensor histidine kinase.

Type IV pilin proteins: versatile molecular modules

Type IV pili (T4P) are multifunctional protein fibers produced on the surfaces of a wide variety of bacteria and archaea. The major subunit of T4P is the type IV pilin, and structurally related proteins are found as components of the type II secretion (T2S) system, where they are called pseudopilins; of DNA uptake/competence systems in both Gram-negative and Gram-positive species; and of flagella, pili, and sugar-binding systems in the archaea. This broad distribution of a single protein family implies both a common evolutionary origin and a highly adaptable functional plan. The type IV pilin is a remarkably versatile architectural module that has been adopted widely for a variety of functions, including motility, attachment to chemically diverse surfaces, electrical conductance, acquisition of DNA, and secretion of a broad range of structurally distinct protein substrates. In this review, we consider recent advances in this research area, from structural revelations to insights into diversity, posttranslational modifications, regulation, and function.

Two isoforms of Geobacter sulfurreducens PilA have distinct roles in pilus biogenesis, cytochrome localization, extracellular electron transfer, and biofilm formation

Type IV pili of Geobacter sulfurreducens are composed of PilA monomers and are essential for long-range extracellular electron transfer to insoluble Fe(III) oxides and graphite anodes. A previous analysis of pilA expression indicated that transcription was initiated at two positions, with two predicted ribosome-binding sites and translation start codons, potentially producing two PilA preprotein isoforms. The present study supports the existence of two functional translation start codons for pilA and identifies two isoforms (short and long) of the PilA preprotein. The short PilA isoform is found predominantly in an intracellular fraction. It seems to stabilize the long isoform and to influence the secretion of several outer-surface c-type cytochromes. The long PilA isoform is required for secretion of PilA to the outer cell surface, a process that requires coexpression of pilA with nine downstream genes. The long isoform was determined to be essential for biofilm formation on certain surfaces, for optimum current production in microbial fuel cells, and for growth on insoluble Fe(III) oxides.

Crystal structure and regulation mechanisms of the CyaB adenylyl cyclase from the human pathogen Pseudomonas aeruginosa

Pseudomonas aeruginosa is an opportunistic bacterial pathogen and a major cause of healthcare-associated infections. While the organism's intrinsic and acquired resistance to most antibiotics hinders treatment of P. aeruginosa infections, the regulatory networks controlling its virulence provide novel targets for drug development. CyaB, a key regulator of P. aeruginosa virulence, belongs to the Class III adenylyl cyclase (AC) family of enzymes that synthesize the second messenger cyclic adenosine 3',5'-monophosphate. These enzymes consist of a conserved catalytic domain fused to one or more regulatory domains. We describe here the biochemical and structural characterization of CyaB and its inhibition by small molecules. We show that CyaB belongs to the Class IIIb subfamily, and like other subfamily members, its activity is stimulated by inorganic carbon. CyaB is also regulated by its N-terminal MASE2 (membrane-associated sensor 2) domain, which acts as a membrane anchor. Using a genetic screen, we identified activating mutations in CyaB. By solving the crystal structure of the CyaB catalytic domain, we rationalized the effects of these mutations and propose that CyaB employs regulatory mechanisms similar to other Class III ACs. The CyaB structure further indicates subtle differences compared to other Class III ACs in both the active site and the inhibitor binding pocket. Consistent with these differences, we observed a unique inhibition profile, including identification of a CyaB selective compound. Overall, our results reveal mechanistic details of the physiological and pharmacological regulation of CyaB and provide the basis for its exploitation as a therapeutic drug target.

Proteomic characterization of a pilR regulatory mutant of Azoarcus sp. strain BH72 with the aid of gel-based and gel-free approaches

The proteome of the grass endophyte Azoarcus sp. strain BH72 was analyzed by a combination of gel-based methods by means of 2-DE and MS and a gel-free approach via LC-MS/MS. Among the identified 785 proteins, synthesis of around 100 conserved hypothetical proteins could be confirmed. Membrane proteins were detected at a higher rate in the gel-free than in the gel-based approach. The abundance of proteins in the constructed proteome reference map was analyzed and the 30 most abundant proteins were determined. The reference map was then used as a starting point to characterize the regulon under control of the response regulator PilR. PilR is part of the two-component regulatory system PilSR controlling type IV pilin gene (pilAB) expression in strain BH72, which was strongly decreased (19.7-fold) in the pilR-mutant. Changes of protein composition in the wild type and the regulatory mutant were compared by the gel-based and gel-free analyses. Proteins responsible for amino acid and energy metabolism, chaperones as well as proteins that are involved in iron metabolism and iron storage were present in a pilR-mutant at different levels than in the wild-type strain. Levels of the transcriptional regulator Flp were also dependent on PilR, indicating that PilR might be part of a hierarchical regulatory cascade.

PilM/N/O/P proteins form an inner membrane complex that affects the stability of the Pseudomonas aeruginosa type IV pilus secretin

The highly conserved pilM/N/O/P/Q gene cluster is among the core set of genes required for cell surface expression of type IV pili and associated twitching motility. With the exception of the outer membrane secretin, a multimer of PilQ subunits, the specific functions of the products encoded by this gene cluster are poorly characterized. Orthologous proteins in the related bacterial type II secretion system have been shown to interact to form an inner membrane complex required for protein secretion. In this study, we provide evidence that the PilM/N/O/P proteins form a functionally equivalent type IVa pilus complex. Using Pseudomonas aeruginosa as model organism, we found that all four proteins, including the nominally cytoplasmic PilM, colocalized to the inner membrane. Stability studies via Western blot analyses revealed that loss of one component has a negative impact on the levels of other members of the putative complex. Furthermore, complementation studies revealed that the stoichiometry of the components is important for the correct formation of a stable complex in vivo. We provide evidence that an intact inner membrane complex is required for optimal formation of the outer membrane complex of the type IVa pilus system in P. aeruginosa, as PilQ stability is negatively affected in its absence. Finally, we show that, in the absence of the pilin subunit, the levels of membrane-bound components of the inner membrane complex are negatively regulated by the PilR/S two-component system, suggesting a role for PilR/S in sensing the piliation status of the cell.

Sigma factors in Pseudomonas aeruginosa

In Pseudomonas aeruginosa, as in most bacterial species, the expression of genes is tightly controlled by a repertoire of transcriptional regulators, particularly the so-called sigma (sigma) factors. The basic understanding of these proteins in bacteria has initially been described in Escherichia coli where seven sigma factors are involved in core RNA polymerase interactions and promoter recognition. Now, 7 years have passed since the completion of the first genome sequence of the opportunistic pathogen P. aeruginosa. Information from the genome of P. aeruginosa PAO1 identified 550 transcriptional regulators and 24 putative sigma factors. Of the 24 sigma, 19 were of extracytoplasmic function (ECF). Here, basic knowledge of sigma and ECF proteins was reviewed with particular emphasis on their role in P. aeruginosa global gene regulation. Summarized data are obtained from in silico analysis of P. aeruginosasigma and ECF including rpoD (sigma(70)), RpoH (sigma(32)), RpoF (FliA or sigma(28)), RpoS (sigma(S) or sigma(38)), RpoN (NtrA, sigma(54) or sigma(N)), ECF including AlgU (RpoE or sigma(22)), PvdS, SigX and a collection of uncharacterized sigma ECF, some of which are implicated in iron transport. Coupled to systems biology, identification and functional genomics analysis of P. aeruginosasigma and ECF are expected to provide new means to prevent infection, new targets for antimicrobial therapy, as well as new insights into the infection process.

Regulation of type IV fimbrial biogenesis in Dichelobacter nodosus

Type IV fimbriae are expressed by several bacterial pathogens and are essential for virulence in Dichelobacter nodosus, which causes ovine footrot. We have identified a two-component signal transduction system (PilR/S) and an alternative sigma factor (sigma 54) that were shown by insertional inactivation to be required for the regulation of fimbrial biogenesis in D. nodosus. Western blots showed that in both pilR and rpoN mutants, fimbrial subunit production was significantly reduced by a process that was shown to occur at a PilR- and sigma 54-dependent promoter. The mutants lacked surface fimbriae, which were shown to be required for the adherence of D. nodosus cells to tissue culture monolayers. The reduction in fimbrial subunit production in these mutants also resulted in a concomitant loss of the ability to secrete extracellular proteases. A maltose binding protein-PilR fusion protein was purified and was shown to bind specifically to a region located 234 to 594 bp upstream of the fimA transcriptional start point. To determine additional targets of PilR and sigma 54, genome-wide transcriptional profiling was performed using a whole-genome oligonucleotide microarray. The results indicated that PilR and sigma 54 regulated genes other than fimA; these genes appear to encode surface-exposed proteins whose role in virulence is unknown. In conclusion, this study represents a significant advancement in our understanding of how the ability of D. nodosus to cause ovine footrot is regulated, as we have shown that the biogenesis of type IV fimbriae in D. nodosus is regulated by a sigma 54-dependent PilR/S system that also indirectly controls protease secretion.

A novel sensor kinase-response regulator hybrid regulates type III secretion and is required for virulence in Pseudomonas aeruginosa

The type III secretion system (TTSS) of Pseudomonas aeruginosa is induced by contact with eukaryotic cells and by growth in low-calcium media. We have identified a protein, RtsM, that is necessary for expression of the TTSS genes in P. aeruginosa. RtsM possesses both histidine kinase and response regulator domains common to two-component signalling proteins, as well as a large predicted periplasmic domain and seven transmembrane domains. Deletion of rtsM resulted in a defect in production and secretion of the type III effectors. Northern blot analysis revealed that mRNAs encoding the effectors ExoT and ExoU are absent in the DeltartsM strain under TTSS-inducing conditions. Using transcriptional fusions, we demonstrated that RtsM is required for transcription of the operons encoding the TTSS effectors and apparatus in response to calcium limitation or to host cell contact. The operon encoding the TTSS regulator ExsA does not respond to calcium limitation, but the basal transcription rate of this operon was lower in deltartsM than in the wild-type parent, PA103. The defect in TTSS effector production and secretion of deltartsM could be complemented by overexpressing ExsA or Vfr, two transcriptional activators involved in TTSS regulation. DeltartsM was markedly less virulent than PA103 in a murine model of acute pneumonia, demonstrating that RtsM is required in vivo. We propose that RtsM is a sensor protein at the start of a signalling cascade that induces expression of the TTSS in response to environmental signals.

Disparate subcellular localization patterns of Pseudomonas aeruginosa Type IV pilus ATPases involved in twitching motility

The opportunistic pathogen Pseudomonas aeruginosa expresses polar type IV pili (TFP), which are responsible for adhesion to various materials and twitching motility on surfaces. Twitching occurs by alternate extension and retraction of TFP, which arise from assembly and disassembly of pilin subunits at the base of the pilus. The ATPase PilB promotes pilin assembly, while the ATPase PilT or PilU or both promote pilin dissociation. Fluorescent fusions to two of the three ATPases (PilT and PilU) were functional, as shown by complementation of the corresponding mutants. PilB and PilT fusions localized to both poles, while PilU fusions localized only to the piliated pole. To identify the portion of the ATPases required for localization, sequential C-terminal deletions of PilT and PilU were generated. The conserved His and Walker B boxes were dispensable for polar localization but were required for twitching motility, showing that localization and function could be uncoupled. Truncated fusions that retained polar localization maintained their distinctive distribution patterns. To dissect the cellular factors involved in establishing polarity, fusion protein localization was monitored with a panel of TFP mutants. The localization of yellow fluorescent protein (YFP)-PilT and YFP-PilU was independent of the subunit PilA, other TFP ATPases, and TFP-associated proteins previously shown to be associated with the membrane or exhibiting polar localization. In contrast, YFP-PilB exhibited diffuse cytoplasmic localization in a pilC mutant, suggesting that PilC is required for polar localization of PilB. Finally, localization studies performed with fluorescent ATPase chimeras of PilT and PilU demonstrated that information responsible for the characteristic localization patterns of the ATPases likely resides in their N termini.

The Pseudomonas aeruginosa PA01 gene collection

Pseudomonas aeruginosa, a common inhabitant of soil and water, is an opportunistic pathogen of growing clinical relevance. Its genome, one of the largest among bacteria [5570 open reading frames (ORFs)] approaches that of simple eukaryotes. We have constructed a comprehensive gene collection for this organism utilizing the annotated genome of P. aeruginosa PA01 and a highly automated and laboratory information management system (LIMS)-supported production line. All the individual ORFs have been successfully PCR-amplified and cloned into a recombination-based cloning system. We have isolated and archived four independent isolates of each individual ORF. Full sequence analysis of the first isolate for one-third of the ORFs in the collection has been completed. We used two sets of genes from this repository for high-throughput expression and purification of recombinant proteins in different systems. The purified proteins have been used to set up biochemical and immunological assays directed towards characterization of histidine kinases and identification of bacterial proteins involved in the immune response of cystic fibrosis patients. This gene repository provides a powerful tool for proteome- and genome-scale research of this organism, and the strategies adopted to generate this repository serve as a model for building clone sets for other bacteria.

Phosphorylation of the Pseudomonas aeruginosa response regulator AlgR is essential for type IV fimbria-mediated twitching motility

The response regulator AlgR is required for both alginate biosynthesis and type IV fimbria-mediated twitching motility in Pseudomonas aeruginosa. In this study, the roles of AlgR signal transduction and phosphorylation in twitching motility and biofilm formation were examined. The predicted phosphorylation site of AlgR (aspartate 54) and a second aspartate (aspartate 85) in the receiver domain of AlgR were mutated to asparagine, and mutant algR alleles were introduced into the chromosome of P. aeruginosa strains PAK and PAO1. Assays of these mutants demonstrated that aspartate 54 but not aspartate 85 of AlgR is required for twitching motility and biofilm initiation. However, strains expressing AlgR D85N were found to be hyperfimbriate, indicating that both aspartate 54 and aspartate 85 are involved in fimbrial biogenesis and function. algD mutants were observed to have wild-type twitching motility, indicating that AlgR control of twitching motility is not mediated via its role in the control of alginate biosynthesis. In vitro phosphorylation assays showed that AlgR D54N is not phosphorylated by the enteric histidine kinase CheA. These findings indicate that phosphorylation of AlgR most likely occurs at aspartate 54 and that aspartate 54 and aspartate 85 of AlgR are required for the control of the molecular events governing fimbrial biogenesis, twitching motility, and biofilm formation in P. aeruginosa.

Topological analysis and role of the transmembrane domain in polar targeting of PilS, a Pseudomonas aeruginosa sensor kinase

In Pseudomonas aeruginosa, synthesis of pilin, the major protein subunit of the pili, is regulated by a two-component signal transduction system in which PilS is the sensor kinase. PilS is an inner membrane protein found at the poles of the bacterial cell. It is composed of three domains: an N-terminal hydrophobic domain; a central cytoplasmic linker region; and the C-terminal transmitter region conserved among other sensor kinases. The signal that activates PilS and, consequently, pilin transcription remains unknown. The membrane topology of the hydrophobic domain was determined using the lacZ and phoA gene fusion approach. In this report, we describe a topological model for PilS in which the hydrophobic domain forms six transmembrane helices, whereas the N- and C-termini are cytoplasmic. This topology is very stable, and the cytoplasmic C-terminus cannot cross the inner membrane. We also show that two of the six transmembrane segments are sufficient for membrane anchoring and polar localization of PilS.

Swarming of Pseudomonas aeruginosa is dependent on cell-to-cell signaling and requires flagella and pili

We describe swarming in Pseudomonas aeruginosa as a third mode of surface translocation in addition to the previously described swimming and twitching motilities. Swarming in P. aeruginosa is induced on semisolid surfaces (0.5 to 0.7% agar) under conditions of nitrogen limitation and in response to certain amino acids. Glutamate, aspartate, histidine, or proline, when provided as the sole source of nitrogen, induced swarming, while arginine, asparagine, and glutamine, among other amino acids, did not sustain swarming. Cells from the edge of the swarm were about twice as long as cells from the swarm center. In both instances, bacteria possessing two polar flagella were observed by light and electron microscopy. While a fliC mutant of P. aeruginosa displayed slightly diminished swarming, a pilR and a pilA mutant, both deficient in type IV pili, were unable to swarm. Furthermore, cells with mutations in the las cell-to-cell signaling system showed diminished swarming behavior, while rhl mutants were completely unable to swarm. Evidence is presented for rhamnolipids being the actual surfactant involved in swarming motility, which explains the involvement of the cell-to-cell signaling circuitry of P. aeruginosa in this type of surface motility.

Identification of a two-component signal transduction system involved in fimbriation of Porphyromonas gingivalis

Porphyromonas gingivalis, a periodontopathogen, is an oral anaerobic gram-negative bacterium with numerous fimbriae on the cell surface. Fimbriae have been considered to be an important virulence factor in this organism. We analyzed the genomic DNA of transposon-induced, fimbria-deficient mutants derived from ATCC 33277 and found that seven independent mutants had transposon insertions within the same restriction fragment. Cloning and sequencing of the disrupted region from one of the mutants revealed two adjacent open reading frames (ORFs) which seemed to encode a two-component signal transduction system. We also found that six of the mutants had insertions in a gene, fimS, a homologue of the genes encoding sensor kinase, and that the insertion in the remaining one disrupted the gene immediately downstream, fimR, a homologue of the response regulator genes in other bacteria. These findings suggest that this two-component regulatory system is involved in fimbriation of P. gingivalis.

Localization of the histidine kinase PilS to the poles of Pseudomonas aeruginosa and identification of a localization domain

Transcription of the type IV pilus subunit gene of Pseudomonas aeruginosa is controlled by a two-component signal transduction system. PilS, the histidine kinase, is membrane bound and PilR, its cognate response regulator, is cytoplasmic. The signal that activates PilS is unknown. PilS has three domains: (i) The N-terminus, predicted to form six transmembrane (TM) helices; (ii) a central linker domain; and (iii) the C-terminal transmitter domain containing all the conserved residues of sensor kinases. A translational fusion of the gfp gene (green fluorescent protein) to the 3' end of pilS was used to determine the position of PilS in the bacterial cell. Epifluorescence microscopy revealed that PilS is retained to the poles of P. aeruginosa but is distributed evenly about the membrane of Escherichia coli. Deletions of the PilS-GFP fusion revealed that the TM domain was sufficient and necessary to bring GFP to the membrane of P. aeruginosa and E. coli but was not sufficient to confine GFP to the poles. Retention to the poles of P. aeruginosa required both the TM and linker domains. Replacement of the PilS TM domain with an E. coli membrane protein, MalG, still allowed polar localization. Therefore, the PilS TM domain positions the linker domain close to the membrane allowing it to interact with the putative polar anchor which is specific to P. aeruginosa.

Promoter architecture of the Porphyromonas gingivalis fimbrillin gene

Porphyromonas gingivalis fimbriae can mediate adherence to many of the available substrates in the oral cavity. Expression of P. gingivalis fimbriae is regulated at the transcriptional level by environmental signals, such as temperature and hemin concentration. The arrangement of the upstream promoter and regulatory sequences required for transcription and control of the fimbrial structural gene (fimA) was investigated. Primer extension analysis demonstrated that the transcriptional start site of the fimA gene is located 41 bp upstream from the translational start codon. A region (upf) spanning 648 bp upstream of the start codon to 44 bp downstream of the translational start site was cloned upstream of a promoterless lacZ reporter gene. A series of deletion and base substitution mutations were then generated in the upf region. The constructs were introduced into the chromosome of P. gingivalis, and promoter activity measured by assaying levels of beta-galactosidase. The results showed that fimA contains sequences resembling sigma70 promoter consensus sequences, consisting of a -10 region (TATGAC) located at -18 to -23 and a -35 region (TTGTTG) located at -41 to -46 from the transcriptional start point. The AT-rich upstream sequences spanning bases -48 to -85 and bases -90 to -240 were required for full expression of the fimA gene, indicating the existence of positive regulation regions. Moreover, the -48 to -64 region may constitute an UP element, contributing to promoter activity in P. gingivalis. Thus, our data suggest that the P. gingivalis fimA gene has a transcription complex consisting of -10 and -35 sequences, an UP element, and additional AT-rich upstream regulatory sequences.

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.

The secretion apparatus of Pseudomonas aeruginosa: identification of a fifth pseudopilin, XcpX (GspK family)

The xcp gene products in Pseudomonas aeruginosa are required for the secretion of proteins across the outer membrane. Four of the Xcp proteins, XcpT, U, V and W, present sequence homology to the subunits of type IV pili at their N-termini, and they were therefore designated pseudopilins. In this study, we characterized the xcpX gene product, a bitopic cytoplasmic membrane protein. Remarkably, amino acid sequence comparisons also suggested that the XcpX protein resembles the pilins and pseudopilins at the N-terminus. We show that XcpX could be processed by the prepilin peptidase, PilD/XcpA, and that the highly conserved glycine residue preceding the hydrophobic segment could not be mutated without loss of the XcpX function. We, therefore, classified XcpX (GspK) as the fifth pseudopilin of the system.

Complementation analysis of the Dichelobacter nodosus fimN, fimO, and fimP genes in Pseudomonas aeruginosa and transcriptional analysis of the fimNOP gene region

The causative agent of ovine footrot, the gram-negative anaerobe Dichelobacter nodosus, produces polar type IV fimbriae, which are the major protective antigens. The D. nodosus genes fimN, fimO, and fimP are homologs of the Pseudomonas aeruginosa fimbrial assembly genes, pilB, pilC, and pilD, respectively. Both the pilD and fimP genes encode prepilin peptidases that are responsible for cleavage of the leader sequence from the immature fimbrial subunit. To investigate the functional similarity of the fimbrial biogenesis systems from these organisms, the D. nodosus genes were introduced into P. aeruginosa strains carrying mutations in the homologous genes. Analysis of the resultant derivatives showed that the fimP gene complemented a pilD mutant of P. aeruginosa for both fimbrial assembly and protein secretion. However, the fimN and fimO genes did not complement pilB or pilC mutants, respectively. These results suggest that although the PilD prepilin peptidase can be functionally replaced by the heterologous FimP protein, the function of the PilB and PilC proteins may require binding or catalytic domains specific for the P. aeruginosa fimbrial assembly system. The transcriptional organization and regulation of the fimNOP gene region were also examined. The results of reverse transcriptase PCR and primer extension analysis suggested that these genes form an operon transcribed from two sigma70-type promoters located upstream of ORFM, an open reading frame proximal to fimN. Transcription of the D. nodosus fimbrial subunit was found to increase in cells grown on solid media, and it was postulated that this regulatory effect may be of significance in the infected footrot lesion.

A regulatory locus, pehSR, controls polygalacturonase production and other virulence functions in Ralstonia solanacearum

We previously identified a locus that regulates production of polygalacturonase (PG), an extracellular plant cell wall-degrading enzyme important in bacterial wilt of plants caused by Ralstonia (Pseudomonas) solanacearum. The DNA sequence of this locus, called pehSR, was determined and two consecutive open reading frames (ORFs) of 1,905 and 1,680 bp were identified. The amino acid sequences predicted to be encoded by these ORFs are similar to those of regulators of pilin synthesis in Pseudomonas aeruginosa and Myxococcus xanthus and to a regulator of flagellin synthesis and adhesion in P. aeruginosa, as well as to other two-component regulators of the NtrB/C subfamily. pehSR mutants produced negligible levels of endo-PG activity, while exo-PG activity was reduced by 50%. Northern (RNA) blot analysis showed that PehSR regulates endo-PG expression at the transcriptional level. pehSR mutants grew normally in culture and in planta but were dramatically reduced in virulence; this loss of virulence was substantially greater than that observed for endo-PG structural gene mutants, suggesting that pehSR regulates additional factors important in virulence. Although pehSR mutants were essentially nonmotile, like the wild-type strain, multiple copies of pehSR conferred motility on the bacterium. Reporter gene studies indicated that pehSR expression increased when bacteria grew in plant tissue, and that the pehSR locus was itself negatively regulated by the global virulence gene regulator PhcA.

Transcriptional regulation of type 4 pilin genes and the site-specific recombinase gene, piv, in Moraxella lacunata and Moraxella bovis

Moraxella lacunata and Moraxella bovis use type 4 pili to adhere to epithelial tissues of the cornea and conjunctiva. Primer extension analyses were used to map the transcriptional start sites for the genes encoding the major pilin subunits (tfpQ/I) and the DNA invertase (piv), which determines pilin type expression. tfpQ/I transcription starts at a sigma54-dependent promoter (tfpQ/Ip2) and, under certain growth conditions, this transcription is accompanied by weaker upstream transcription that starts at a potential sigma70-dependent promoter (tfpQ/Ip1). piv is expressed in both M. lacunata and M. bovis from a putative sigma70-dependent promoter (pivp) under all conditions assayed. Sigma54-dependent promoters require activators in order to initiate transcription; therefore, it is likely that tfpQ/Ip2 is also regulated by an activator in Moraxella. Primer extension assays with RNA isolated from Escherichia coli containing the subcloned pilin inversion region from M. lacunata showed that pivp is used for the expression of piv; however, tfpQ/Ip2 is not used for the transcription of tfpQ/I. Transcription from tfpQ/Ip2 was activated in E. coli when the sensor (PilS) and response regulator (PilR) proteins of type 4 pilin transcription in Pseudomonas aeruginosa were expressed from a plasmid. These results suggest that the expression of the type 4 pilin in M. lacunata and M. bovis is regulated not only by a site-specific DNA inversion system but also by a regulatory system which is functionally analogous to the PilS-PilR two-component system of P. aeruginosa.

Regulation of expression of the pilA gene in Myxococcus xanthus

Type IV pili are required for social gliding motility in Myxococcus xanthus. In this work, the expression of pilin (the pilA gene product) during vegetative growth and fruiting-body development was examined. A polyclonal antibody against the pilA gene product (prepilin) was prepared, along with a pilA-lacZ fusion, and was used to assay expression of pilA in M. xanthus in different mutant backgrounds. pilA expression required the response regulator pilR but was negatively regulated by the putative sensor kinase pilS. pilA expression did not require pilB, pilC, or pilT. pilA was also autoregulated; a mutation which altered an invariant glutamate five residues from the presumed prepilin processing site eliminated this autoregulation, as did a deletion of the pilA gene. Primer extension and S1 nuclease analysis identified a sigma54 promoter upstream of pilA, consistent with the homology of pilR to the NtrC family of response regulators. Expression of pilA was found to be developmentally regulated; however, the timing of this expression pattern was not entirely dependent on pilS or pilR. Finally, pilA expression was induced by high nutrient concentrations, an effect that was also not dependent on pilS or pilR.

Molecular genetic analysis of type-4 pilus biogenesis and twitching motility using Pseudomonas aeruginosa as a model system--a review

Genetic analysis of Pseudomonas aeruginosa pilus biogenesis and twitching motility has revealed the requirement for several pil loci which have been localized to different regions of the chromosome. One pil locus, designated pilE, resides at approx. 71 min on the PAO genetic map, a region of the chromosome previously shown to harbor a number of genes required for pilus assembly (i.e., pilA, -B, -C, -D, -R and -S). The PilE protein shows significant sequence identity to the N-terminal domain of PilA as well as to the pilin precursors from a variety of type-4 pilus producers. Included within this homologous region is a short, positively charged leader sequence followed by a prepilin peptidase cleavage site and a largely hydrophobic region. Additionally, an unlinked set of pil genes, designated pilG, -H, -I, -J and -K, has been localized to the SpeI fragment H which corresponds to approx. 20 min on the PAO genetic map. This gene cluster encodes proteins that demonstrate remarkable similarity to the chemotaxis proteins of enterics and the gliding bacterium Myxococcus xanthus and are thought to be part of a signal transduction system that controls P. aeruginosa pilus biosynthesis and twitching motility.

New classes of mutants in complementary chromatic adaptation provide evidence for a novel four-step phosphorelay system

Complementary chromatic adaptation appears to be controlled by a complex regulatory system with similarity to four-step phosphorelays. Such pathways utilize two histidine and two aspartate residues for signal transduction. Previous studies of the signaling system controlling complementary chromatic adaptation have uncovered two elements of this pathway, a putative sensor, RcaE, and a response regulator, RcaC. In this work, we describe a second response regulator controlling complementary chromatic adaptation, RcaF, and identify putative DNA binding and histidine phosphoacceptor domains within RcaC. RcaF is a small response regulator with similarity to SpoOF of Bacillus subtilis; the latter functions in the four-step phosphorelay system controlling sporulation. We have also determined that within this phosphorelay pathway, RcaE precedes RcaF, and RcaC is probably downstream of RcaE and RcaF. This signal transduction pathway is novel because it appears to use at least five, instead of four, phosphoacceptor domains in the phosphorelay circuit.

Markerless deletions of pil genes in Myxococcus xanthus generated by counterselection with the Bacillus subtilis sacB gene

In-frame deletions of pilA and pilS were constructed in Myxococcus xanthus with a plasmid integration-excision strategy facilitated by sacB. sacB conferred sucrose sensitivity upon its M. xanthus host only when it lay in the same orientation as adjacent M. xanthus genes. Gene orientation also affected the efficiency of sucrose counterselection in the sucrose-sensitive strains. The deltapilA mutant lacked pili and social motility, while the deltapilS mutant showed no defect in either phenotype.