Involvement of cpxA, a sensor of a two-component regulatory system, in the pH-dependent regulation of expression of Shigella sonnei virF gene

Shigella: a model of virulence regulation in vivo

Much is known about the molecular effectors of pathogenicity of gram-negative enteric pathogens, among which Shigella can be considered a model. This is due to its capacity to recapitulate the multiple steps required for a pathogenic microbe to survive close to its mucosal target, colonize and then invade its epithelial surface, cause its inflammatory destruction and simultaneously regulate the extent of the elicited innate response to likely survive the encounter and achieve successful subsequent transmission. These various steps of the infectious process represent an array of successive environmental conditions to which the bacteria need to successfully adapt. These conditions represent the selective pressure that triggered the "arms race" in which Shigella acquired the genetic and molecular effectors of its pathogenic armory, including the regulatory hierarchies that regulate the expression and function of these effectors. They also represent cues through which Shigella achieves the temporo-spatial expression and regulation of its virulence effectors. The role of such environmental cues has recently become obvious in the case of the major virulence effector of Shigella, the type three secretion system (T3SS) and its dedicated secreted virulence effectors. It needs to be better defined for other major virulence components such as the LPS and peptidoglycan which are used as examples here, in addition to the T3SS as models of regulation as it relates to the assembly and functional regulation of complex macromolecular systems of the bacterial surface. This review also stresses the need to better define what the true and relevant environmental conditions can be at the various steps of the progression of infection. The "identity" of the pathogen differs depending whether it is cultivated under in vitro or in vivo conditions. Moreover, this "identity" may quickly change during its progression into the infected tissue. Novel concepts and relevant tools are needed to address this challenge in microbial pathogenesis.

A Haemophilus ducreyi CpxR deletion mutant is virulent in human volunteers

Haemophilus ducreyi 35000HP contains a homolog of the CpxRA 2-component signal transduction system, which controls the cell envelope stress response system in other gram-negative bacteria and regulates some important H. ducreyi virulence factors. A H. ducreyi cpxR mutant was compared with its parent for virulence in the human challenge model of experimental chancroid. The pustule formation rate in 5 volunteers was 33% (95% confidence interval [CI], 1.3%-65.3%) at 15 parent sites and 40% (95% CI, 18.1%-61.9%) at 15 mutant sites (P = .35). Thus, the cpxR mutant was not attenuated for virulence. Inactivation of the H. ducreyi cpxR gene did not reduce the ability of this mutant to express certain proven virulence factors, including the DsrA serum resistance protein and the LspA2 protein, which inhibits phagocytosis. These results expand our understanding of the involvement of the CpxRA system in regulating virulence expression in H. ducreyi.

Characterization of combinatorial patterns generated by multiple two-component sensors in E. coli that respond to many stimuli

Two-component systems enable bacteria to sense changes in their environment and adjust gene expression in response. Multiple two-component systems could function as a combinatorial sensor to discriminate environmental conditions. A combinatorial sensor is composed of a set of sensors that are non-specifically activated to different magnitudes by many stimuli, such that their collective activity pattern defines the signal. Using promoter reporters and flow cytometry, we measured the response of three two-component systems in Escherichia coli that have been previously reported to respond to many environmental stimuli (EnvZ/OmpR, CpxA/CpxR, and RcsC/RcsD/RcsB). A chemical library was screened for the ability to activate the sensors and 13 inducers were identified that produce different patterns of sensor activity. The activities of the three systems are uncorrelated with each other and the osmolarity of the inducing media. Five of the seven possible non-trivial patterns generated by three sensors are observed. This data demonstrate one mechanism by which bacteria are able to use a limited set of sensors to identify a diverse set of compounds and environmental conditions.

Managing membrane stress: the phage shock protein (Psp) response, from molecular mechanisms to physiology

The bacterial phage shock protein (Psp) response functions to help cells manage the impacts of agents impairing cell membrane function. The system has relevance to biotechnology and to medicine. Originally discovered in Escherichia coli, Psp proteins and homologues are found in Gram-positive and Gram-negative bacteria, in archaea and in plants. Study of the E. coli and Yersinia enterocolitica Psp systems provides insights into how membrane-associated sensory Psp proteins might perceive membrane stress, signal to the transcription apparatus and use an ATP-hydrolysing transcription activator to produce effector proteins to overcome the stress. Progress in understanding the mechanism of signal transduction by the membrane-bound Psp proteins, regulation of the psp gene-specific transcription activator and the cell biology of the system is presented and discussed. Many features of the action of the Psp system appear to be dominated by states of self-association of the master effector, PspA, and the transcription activator, PspF, alongside a signalling pathway that displays strong conditionality in its requirement.

A member of the peptidase M48 superfamily of Porphyromonas gingivalis is associated with virulence in vitro and in vivo

Background

In vivo-induced antigen technology was previously used to identify 115 genes induced in Porphyromonas gingivalis W83 during human infection. One of these, PG2197, a conserved hypothetical protein which has homology to a Zn-dependent protease, was examined with respect to a role in disease.

Design

The expression of PG2197 in human periodontitis patients was investigated, but as there is increasing evidence of a direct relationship between P. gingivalis and cardiovascular disease, a mutation was constructed in this gene to also determine its role in adherence, invasion, and persistence within human coronary artery endothelial cells (HCAEC) and neutrophil killing susceptibility.

Results

Plaque samples from 20 periodontitis patients were analyzed by real-time PCR, revealing that PG2197 was expressed in 60.0% of diseased sites compared to 15.8% of healthy sites, even though P. gingivalis was detected in equal numbers from both sites. The expression of this gene was also found to be up-regulated in microarrays at 5 and 30 min of invasion of HCAEC. Interestingly, a PG2197 mutant displayed increased adherence, invasion, and persistence within HCAEC when compared to the wild-type strain.

Conclusion

This gene appears to be important for the virulence of P. gingivalis, both in vivo and in vitro.

Three (and more) component regulatory systems - auxiliary regulators of bacterial histidine kinases

Two-component signal transduction (TCST) is the most prevalent mechanism employed by microbes to sense and respond to environmental changes. It is characterized by the signal-induced transfer of phosphate from a sensor histidine kinase (HK) to a response regulator (RR), resulting in a cellular response. An emerging theme in the field of TCST signalling is the discovery of auxiliary factors, distinct from the HK and RR, which are capable of influencing phosphotransfer. One group of TCST auxiliary proteins accomplishes this task by acting on HKs. Auxiliary regulators of HKs are widespread and have been identified in all cellular compartments, where they can influence HK activity through interactions with the sensing, transmembrane or enzymatic domains of the HK. The effects of an auxiliary regulator are controlled by its regulated expression, modification and/or through ligand binding. Ultimately, auxiliary regulators can connect a given TCST system to other regulatory networks in the cell or result in regulation of the TCST system in response to an expanded range of stimuli. The studies highlighted in this review draw attention to an emerging view of bacterial TCST systems as core signalling units upon which auxiliary factors act.

Metabolic control through ornithine and uracil of epithelial cell invasion by Shigella flexneri

This paper shows that compounds in defined growth media strongly influence the expression of the effectors of virulence in the human invasive pathogen Shigella flexneri. Ornithine in conjunction with uracil reduces the haemolytic ability of wild-type cultures more than 20-fold and the expression of the type III secretion system more than 8-fold, as monitored by an mxiC : : lacZ transcriptional reporter. mxiC gene expression is further decreased by the presence of methionine or branched-chain amino acids (15-fold or 25-fold at least, respectively). Lysine and a few other aminated metabolites (cadaverine, homoserine and diaminopimelate) counteract the ornithine-mediated inhibition of haemolytic activity and of the expression of a transcriptional activator virF reporter. The complete abolition of invasion of HeLa cells by wild-type bacteria by ornithine, uracil, methionine or branched-chain amino acids establishes that these metabolites are powerful effectors of virulence. These findings provide a direct connection between metabolism and virulence in S. flexneri. The inhibitory potential exhibited by the nutritional environment is stronger than temperature, the classical environmental effector of virulence. The implications and practical application of this finding in prophylaxis and treatment of shigellosis are discussed.

Involvement of RNA-binding protein Hfq in the osmotic-response regulation of invE gene expression in Shigella sonnei

Background

The expression of Type III secretion system (TTSS) in Shigella is regulated in response to changes in environmental osmolarity and temperature. Temperature-dependent regulation of virF, the master regulator of TTSS synthesis, is believed to occur at the transcriptional level. We recently demonstrated, however, that TTSS synthesis also involves post-transcriptional regulation of the synthesis of InvE, a target of virF and key regulator of TTSS synthesis. The mRNA levels of invE (virB) are stable at 37°C, but mRNA stability markedly decreases at low temperatures where the TTSS synthesis is tightly repressed. Deletion of hfq, which encodes an RNA chaperone in Gram-negative bacteria, results in the restoration of expression of invE and other TTSS genes at low temperature due to an increase in the stability of invE mRNA. To date, the molecular details of the regulation of TTSS expression in response to osmotic pressure are not known. In the current study, we investigated the mechanism of regulation of TTSS by osmotic pressure.

Results

Transcription of virF, which encodes the master regulator of TTSS expression, was partially repressed under low osmotic conditions. Several lines of evidence indicated that osmolarity-dependent changes in TTSS synthesis are controlled at the post-transcriptional level, through the regulation of InvE synthesis. First, the expression InvE protein was tightly repressed under low osmotic growth conditions, even though invE mRNA transcripts were readily detectable. Second, under low osmotic conditions, invE mRNA was rapidly degraded, whereas deletion of hfq, which encodes an RNA chaperone, resulted in increased invE mRNA stability and the production of InvE protein. Third, the binding of purified Hfq in vitro to invE RNA was stronger in low-salt buffer, as assessed by gel-shift analysis and surface plasmon resonance (Biacore analysis).

Conclusion

Osmolarity-dependent changes in TTSS synthesis in Shigella involve the post-transcriptional regulation of InvE expression, in addition to partial transcriptional activation by virF. The stability of invE mRNA is reduced under low osmotic conditions, similar to the effect of temperature. Deletion of an RNA chaperone gene (hfq) abolished the repression of TTSS synthesis at low osmolarity through a mechanism that involved increased stability of invE mRNA. We propose that the expression of Shigella virulence genes in response to both osmolarity and temperature involves the post-transcriptional regulation of expression of InvE, a critical regulator of TTSS synthesis.

CpxRA influences Xenorhabdus nematophila colonization initiation and outgrowth in Steinernema carpocapsae nematodes through regulation of the nil locus

The gammaproteobacterium Xenorhabdus nematophila mutualistically colonizes an intestinal region of a soil-dwelling nematode and is a blood pathogen of insects. The X. nematophila CpxRA two-component regulatory system is necessary for both of these host interactions (E. Herbert et al., Appl. Environ. Microbiol. 73:7826-7836, 2007). Mutualistic association of X. nematophila with its nematode host consists of two stages: initiation, where a small number of bacterial cells establish themselves in the colonization site, and outgrowth, where these cells grow to fill the space. In this study, we show that the Cpx system is necessary for both of these stages. X. nematophila DeltacpxR1 colonized fewer nematodes than its wild-type parent and did not achieve as high a density as did the wild type within a portion of the colonized nematodes. To test whether the DeltacpxR1 host interaction phenotypes are due to its overexpression of mrxA, encoding the type I pilin subunit protein, we assessed the colonization phenotype of a DeltacpxR1 DeltamrxA1 double mutant. This mutant displayed the same colonization defect as DeltacpxR1, indicating that CpxR negative regulation of mrxA does not play a detectable role in X. nematophila-host interactions. CpxR positively regulates expression of nilA, nilB, and nilC genes necessary for nematode colonization. Here we show that the nematode colonization defect of the DeltacpxR1 mutant is rescued by elevating nil gene expression through mutation of nilR, a negative regulator of nilA, nilB, and nilC. These data suggest that the nematode colonization defect previously observed in DeltacpxR1 is caused, at least in part, by altered regulation of nilA, nilB, and nilC.

The roles of two-component systems in virulence of pathogenic Escherichia coli and Shigella spp

Two-component systems (TCSs) are well conserved among E. coli strains, including pathogenic E. coli and also closely related Shigella spp. Although 25% of the genome of pathogenic E. coli is strain-specific, only small number of strain-specific TCSs is found. Regulation of virulence genes in response to environmental stimuli is partly dependent on TCSs commonly present in nonpathogenic E. coli strains. Some virulence genes are directly regulated by response regulator ofTCS but some are affected at posttranscriptional steps of production or assembly ofmacromolecule by TCS-induced products. In the process ofacquiringvirulence traits, regulatory systems for virulence genes expression seem to be built by integrating E. coli backbone TCSs with the virulence regulatory network via transcription regulatory gene.

Differential expression of the Smb bacteriocin in Streptococcus mutans isolates

The two-component lantibiotic Smb is produced by Streptococcus mutans GS5. In the present study, we identified seven strains of S. mutans containing the smb gene cluster. These strains could be classified into high- and low-level Smb producers relative to the levels of Smb production by indicator strains in vitro. This classification was dependent upon the transcription levels of the structural smbA and smbB genes. Sequence analysis upstream of smbA in the high- and low-level Smb-producing strains revealed differences at nucleotide position -46 relative to the smbA start codon. Interestingly, the transcription start site was present upstream of the point mutation, indicating that both groups of strains have the same promoter constructs and that the differential expression of smbA and smbB mRNA occurred subsequent to transcription initiation. In addition, smbA::lacZ fusion expression was higher when it was regulated by the sequences of strains with high-level Smb activity than when it was regulated by the comparable region from strains with low-level Smb activity. Taken together, we conclude that high- or low-level Smb expression is dependent on the presence of a G or a T nucleotide at position -46 relative to the smbA translational start site in S. mutans Smb producers.

The R1 conjugative plasmid increases Escherichia coli biofilm formation through an envelope stress response

Differential gene expression in biofilm cells suggests that adding the derepressed conjugative plasmid R1drd19 increases biofilm formation by affecting genes related to envelope stress (rseA and cpxAR), biofilm formation (bssR and cstA), energy production (glpDFK), acid resistance (gadABCEX and hdeABD), and cell motility (csgBEFG, yehCD, yadC, and yfcV); genes encoding outer membrane proteins (ompACF), phage shock proteins (pspABCDE), and cold shock proteins (cspACDEG); and phage-related genes. To investigate the link between the identified genes and biofilm formation upon the addition of R1drd19, 40 isogenic mutants were classified according to their different biofilm formation phenotypes. Cells with class I mutations (those in rseA, bssR, cpxA, and ompA) exhibited no difference from the wild-type strain in biofilm formation and no increase in biofilm formation upon the addition of R1drd19. Cells with class II mutations (those in gatC, yagI, ompC, cspA, pspD, pspB, ymgB, gadC, pspC, ymgA, slp, cpxP, cpxR, cstA, rseC, ompF, and yqjD) displayed increased biofilm formation compared to the wild-type strain but decreased biofilm formation upon the addition of R1drd19. Class III mutants showed increased biofilm formation compared to the wild-type strain and increased biofilm formation upon the addition of R1drd19. Cells with class IV mutations displayed increased biofilm formation compared to the wild-type strain but little difference upon the addition of R1drd19, and class V mutants exhibited no difference from the wild-type strain but increased biofilm formation upon the addition of R1drd19. Therefore, proteins encoded by the genes corresponding to the class I mutant phenotype are involved in R1drd19-promoted biofilm formation, primarily through their impact on cell motility. We hypothesize that the pili formed upon the addition of the conjugative plasmid disrupt the membrane (induce ompA) and activate the two-component system CpxAR as well as the other envelope stress response system, RseA-sigma(E), both of which, along with BssR, play a key role in bacterial biofilm formation.

Molecular pathogenesis of Shigella spp.: controlling host cell signaling, invasion, and death by type III secretion

Shigella spp. are gram-negative pathogenic bacteria that evolved from harmless enterobacterial relatives and may cause devastating diarrhea upon ingestion. Research performed over the last 25 years revealed that a type III secretion system (T3SS) encoded on a large plasmid is a key virulence factor of Shigella flexneri. The T3SS determines the interactions of S. flexneri with intestinal cells by consecutively translocating two sets of effector proteins into the target cells. Thus, S. flexneri controls invasion into EC, intra- and intercellular spread, macrophage cell death, as well as host inflammatory responses. Some of the translocated effector proteins show novel biochemical activities by which they intercept host cell signal transduction pathways. An understanding of the molecular mechanisms underlying Shigella pathogenesis will foster the development of a safe and efficient vaccine, which, in parallel with improved hygiene, should curb infections by this widespread pathogen.

Activation of the Cpx envelope stress response down-regulates expression of several locus of enterocyte effacement-encoded genes in enteropathogenic Escherichia coli

The Cpx two-component system regulates an extracytoplasmic stress response that functions to rid the envelope of misfolded and mislocalized proteins that may interfere with normal cellular processes. The Cpx pathway is also involved in pathogenesis. This study investigated the role of the Cpx response in enteropathogenic Escherichia coli (EPEC) type III secretion (T3S). It was determined that a functional Cpx pathway is not required for T3S but that pathway activation inhibits secretion by reducing the cellular pools of T3S substrates. The EPEC T3S system structural components, as well as a number of its substrates, are encoded on the locus of enterocyte effacement (LEE) pathogenicity island. Transcriptional fusions to the five major operons of the LEE were constructed and examined under Cpx pathway-activating conditions. Induction of the Cpx response caused a decrease in the transcription of several LEE operons, with the most pronounced effect on LEE4 and LEE5. Collectively, these two operons encode components of the T3S translocation apparatus, the bacterial adhesin intimin, and the translocated bacterial receptor Tir. These data show for the first time that activation of the Cpx envelope stress response in EPEC inhibits T3S of both translocators and effectors, likely through down regulation of LEE transcription. Coupled with recent findings, our results suggest that Cpx-mediated down regulation of virulence is a conserved theme in a number of bacterial pathogens.

Signal integration by the two-component signal transduction response regulator CpxR

The CpxAR two-component signal transduction system in Escherichia coli and other pathogens senses diverse envelope stresses and promotes the transcription of a variety of genes that remedy these stresses. An important member of the CpxAR regulon is cpxP. The CpxA-dependent transcription of cpxP has been linked to stresses such as misfolded proteins and alkaline pH. It also has been proposed that acetyl phosphate, the intermediate of the phosphotransacetylase (Pta)-acetate kinase (AckA) pathway, can activate the transcription of cpxP in a CpxA-independent manner by donating its phosphoryl group to CpxR. We tested this hypothesis by measuring the transcription of cpxP using mutants with mutations in the CpxAR pathway, mutants with mutations in the Pta-AckA pathway, and mutants with a combination of both types of mutations. From this epistasis analysis, we learned that CpxR integrates diverse stimuli. The stimuli that originate in the envelope depend on CpxA, while those associated with growth and central metabolism depend on the Pta-AckA pathway. While CpxR could receive a phosphoryl group from acetyl phosphate, this global signal was not the primary trigger for CpxR activation associated with the Pta-AckA pathway. On the strength of these results, we contend that the interactions between central metabolism and signal transduction can be quite complex and that successful investigations of such interactions must include a complete epistatic analysis.

CpxRA regulates mutualism and pathogenesis in Xenorhabdus nematophila

The CpxRA signal transduction system, which in Escherichia coli regulates surface structure assembly and envelope maintenance, is involved in the pathogenic and mutualistic interactions of the entomopathogenic bacterium Xenorhabdus nematophila. When DeltacpxR1 cells were injected into Manduca sexta insects, the time required to kill 50% of the insects was twofold longer than the time observed for wild-type cells and the DeltacpxR1 cells ultimately killed 16% fewer insects than wild-type cells killed. During mutualistic colonization of Steinernema carpocapsae nematodes, the DeltacpxR1 mutant achieved colonization levels that were only 38% of the wild-type levels. DeltacpxR1 cells exhibited an extended lag phase when they were grown in liquid LB or hemolymph, formed irregular colonies on solid medium, and had a filamentous cell morphology. A mutant with a cpxRp-lacZ fusion had peaks of expression in the log and stationary phases that were conversely influenced by CpxR; the DeltacpxR1 mutant produced 130 and 17% of the wild-type beta-galactosidase activity in the log and stationary phases, respectively. CpxR positively influences motility and secreted lipase activity, as well as transcription of genes necessary for mutualistic colonization of nematodes. CpxR negatively influences the production of secreted hemolysin, protease, and antibiotic activities, as well as the expression of mrxA, encoding the pilin subunit. Thus, X. nematophila CpxRA controls expression of envelope-localized and secreted products, and its activity is necessary for both mutualistic and pathogenic functions.

Optimization of the inefficient translation initiation region of the cpxP gene from Escherichia coli

The Escherichia coli Cpx envelope stress system is comprised of three proteins; the periplasmic regulatory CpxP, the inner membrane sensor kinase CpxA, and the cytoplasmic transcriptional activator CpxR. Although misfolded envelope proteins activate the Cpx system, the molecular mechanism by which this signal is sensed remains largely unknown. In an attempt to reconstitute the Cpx system from purified proteins, we failed to produce the small CpxP protein in its natural periplasmic compartment, but a high protein level was achieved when it was produced in the cytoplasm. Silent base mutations in the first codons of the cpxP gene encoding the signal sequence or substitution by two well-characterized signal sequences, those of MalE and DsbA, resulted in a large increase of the CpxP level in the periplasm. Our results support the hypothesis that periplasmic expression could be inhibited by sequence elements in the early coding signal sequence region of cpxP.

EfeUOB (YcdNOB) is a tripartite, acid-induced and CpxAR-regulated, low-pH Fe2+ transporter that is cryptic in Escherichia coli K-12 but functional in E. coli O157:H7

Escherichia coli possesses iron transporters specific for either Fe2+ or Fe3+. Although Fe2+ is far more soluble than Fe3+, it rapidly oxidizes aerobically at pH > or = 7. Thus, FeoAB, the major Fe2+ transporter of E. coli, operates anaerobically. However, Fe2+ remains stable aerobically under acidic conditions, although a low-pH Fe2+ importer has not been previously identified. Here we show that ycdNOB (efeUOB) specifies the first such transporter. efeUOB is repressed at high pH by CpxAR, and is Fe2+-Fur repressed. EfeU is homologous to the high-affinity iron permease, Ftr1p, of Saccharomyces cerevisiae and other fungi. EfeO is periplasmic with a cupredoxin N-terminal domain; EfeB is also periplasmic and is haem peroxidase-like. All three Efe proteins are required for Efe function. The efeU gene of E. coli K-12 is cryptic due to a frameshift mutation - repair of the single-base-pair deletion generates a functional EfeUOB system. In contrast, the efeUOB operon of the enterohaemorrhagic strain, O157:H7, lacks any frameshift and is functional. A 'wild-type' K-12 strain bearing a functional EfeUOB displays a major growth advantage under aerobic, low-pH, low-iron conditions when a competing metal is provided. 55Fe transport assays confirm the ferrous iron specificity of EfeUOB.

Influence of the Cpx extracytoplasmic-stress-responsive pathway on Yersinia sp.-eukaryotic cell contact

The extracytoplasmic-stress-responsive CpxRA two-component signal transduction pathway allows bacteria to adapt to growth in extreme environments. It controls the production of periplasmic protein folding and degradation factors, which aids in the biogenesis of multicomponent virulence determinants that span the bacterial envelope. This is true of the Yersinia pseudotuberculosis Ysc-Yop type III secretion system. However, despite using a second-site suppressor mutation to restore Yop effector secretion by yersiniae defective in the CpxA sensor kinase, these bacteria poorly translocated Yops into target eukaryotic cells. Investigation of this phenotype herein revealed that the expression of genes which encode several surface-located adhesins is also influenced by the Cpx pathway. In particular, the expression and surface localization of invasin, an adhesin that engages beta1-integrins on the eukaryotic cell surface, are severely restricted by the removal of CpxA. This reduces bacterial association with eukaryotic cells, which could be suppressed by the ectopic production of CpxA, invasin, or RovA, a positive activator of inv expression. In turn, these infected eukaryotic cells then became susceptible to intoxication by translocated Yop effectors. In contrast, bacteria harboring an in-frame deletion of cpxR, which encodes the cognate response regulator, displayed an enhanced ability to interact with cell monolayers, as well as elevated inv and rovA transcription. This phenotype could be drastically suppressed by providing a wild-type copy of cpxR in trans. We propose a mechanism of inv regulation influenced by the direct negative effects of phosphorylated CpxR on inv and rovA transcription. In this fashion, sensing of extracytoplasmic stress by CpxAR contributes to productive Yersinia sp.-eukaryotic cell interactions.

Extracytoplasmic-stress-responsive pathways modulate type III secretion in Yersinia pseudotuberculosis

Three signal transduction pathways, the two-component systems CpxRA and BaeSR and the alternative sigma factor sigma(E), respond to extracytoplasmic stress that facilitates bacterial adaptation to changing environments. At least the CpxRA and sigma(E) pathways control the production of protein-folding and degradation factors that counter the effects of protein misfolding in the periplasm. This function also influences the biogenesis of multicomponent extracellular appendages that span the bacterial envelope, such as various forms of pili. Herein, we investigated whether any of these regulatory pathways in the enteropathogen Yersinia pseudotuberculosis affect the functionality of the Ysc-Yop type III secretion system. This is a multicomponent molecular syringe spanning the bacterial envelope used to inject effector proteins directly into eukaryotic cells. Disruption of individual components revealed that the Cpx and sigma(E) pathways are important for Y. pseudotuberculosis type III secretion of Yops (Yersinia outer proteins). In particular, a loss of CpxA, a sensor kinase, reduced levels of structural Ysc (Yersinia secretion) components in bacterial membranes, suggesting that these mutant bacteria are less able to assemble a functional secretion apparatus. Moreover, these bacteria were no longer capable of localizing Yops into the eukaryotic cell interior. In addition, a cpxA lcrQ double mutant engineered to overproduce and secrete Yops was still impaired in intoxicating cells. Thus, the Cpx pathway might mediate multiple influences on bacterium-target cell contact that modulate Yersinia type III secretion-dependent host cell cytotoxicity.

Pushing the envelope: extracytoplasmic stress responses in bacterial pathogens

Despite being nutrient rich, the tissues and fluids of vertebrates are hostile to microorganisms, and most bacteria that attempt to take advantage of this environment are rapidly eliminated by host defences. Pathogens have evolved various means to promote their survival in host tissues, including stress responses that enable bacteria to sense and adapt to adverse conditions. Many different stress responses have been described, some of which are responsive to one or a small number of cues, whereas others are activated by a broad range of insults. The surface layers of pathogenic bacteria directly interface with the host and can bear the brunt of the attack by the host armoury. Several stress systems that respond to perturbations in the microbial cell outside of the cytoplasm have been described and are known collectively as extracytoplasmic or envelope stress responses (ESRs). Here, we review the role of the ESRs in the pathogenesis of Gram-negative bacterial pathogens.

Expression of and secretion through the Aeromonas salmonicida type III secretion system

Aeromonas salmonicida subsp. salmonicida is the aetiological agent of furunculosis, a disease of farmed and wild salmonids. The type III secretion system (TTSS) is one of the primary virulence factors in A. salmonicida. Using a combination of differential proteomic analysis and reverse transcriptase (RT)-PCR, it is shown that A. salmonicida A449 induces the expression of TTSS proteins at 28 °C, but not at its more natural growth temperature of 17 °C. More modest increases in expression occur at 24 °C. This temperature-induced up-regulation of the TTSS in A. salmonicida A449 occurs within 30 min of a growth temperature increase from 16 to 28 °C. Growth conditions such as low-iron, low pH, low calcium, growth within the peritoneal cavity of salmon and growth to high cell densities do not induce the expression of the TTSS in A. salmonicida A449. The only other known growth condition that induces expression of the TTSS is growth of the bacterium at 16 °C in salt concentrations ranging from 0·19 to 0·38 M NaCl. It is also shown that growth at 28 °C followed by exposure to low calcium results in the secretion of one of the TTSS effector proteins. This study presents a simple in vitro model for the expression of TTSS proteins in A. salmonicida.

Ecological physiology of a coral pathogen and the coral reef environment

Laboratory studies on the ecological physiology of a coral pathogen were carried out to investigate growth potential in terms of environmental factors that may control coral diseases on reefs. The disease chosen for this study, white plague type II, is considered to be one of the major diseases of Caribbean scleractinian corals, affecting a wide range of coral hosts and causing rapid and widespread tissue loss. It is caused by a single pathogen, the bacterium Aurantimonas coralicida. A series of laboratory experiments using a pure culture of the pathogen was carried out to examine the roles of temperature, pH, and O(2) concentration on growth rate. Results revealed optimal growth between 30 and 35 degrees Celsius, and between pH values of 6 and 8. There was a distinctive synergistic relationship between pH and temperature. Increasing temperature from 25 to 35 degrees Celsius expanded the range of pH tolerance from a minimum of 6.0 down to 5.0. O(2) concentration directly affected growth rate, which increased with increasing O(2). The combined effects of increasing O(2) and increasing temperature resulted in a synergistic effect of more rapid growth. These laboratory results are discussed in terms of the coral host and the range of the environmental factors that occur on coral reefs. We conclude that changing environmental conditions in the reef environment, in particular observed increases in water temperature, may be promoting coral diseases by allowing coral pathogens to expand their ecological niches. In the case of the white plague type II pathogen, elevated temperature would allow A. coralicida to colonize the low pH environment of the coral surface mucopolysaccharide layer as an initial stage of infection. The synergistic effect between temperature and oxygen concentration appeared to be less environmentally relevant for this coral pathogen.

Two-component signal transduction systems, environmental signals, and virulence

The relevance toward virulence of a variety of two-component signal transduction systems is reviewed for 16 pathogenic bacteria, together with the wide array of environmental signals or conditions that have been implicated in their regulation. A series of issues is raised, concerning the need to understand the environmental cues that determine their regulation in the infected host and in the environment outside the laboratory, which shall contribute toward the bridging of bacterial pathogenesis and microbial ecology.

Adenylate cyclase mutations rescue the degP temperature-sensitive phenotype and induce the sigma E and Cpx extracytoplasmic stress regulons in Escherichia coli

Inactivation of the gene encoding the periplasmic protease DegP confers a high-temperature-sensitive phenotype in Escherichia coli. We have previously demonstrated that a degP mutant of E. coli strain CBM (W3110 pldA1) is not temperature sensitive and showed that this was most likely due to constitutive activation of the sigma E and Cpx extracytoplasmic stress regulons in the parent strain. In this study, further characterization of this strain revealed a previously unknown cryptic mutation that rescued the degP temperature-sensitive phenotype by inducing the extracytoplasmic stress regulons. We identified the cryptic mutation as an 11-bp deletion of nucleotides 1884 to 1894 of the adenylate cyclase-encoding cyaA gene (cyaAdelta11). The mechanism in which cyaAdelta11 induces the sigma E and Cpx regulons involves decreased activity of the mutant adenylate cyclase. Addition of exogenous cyclic AMP (cAMP) to the growth medium of a cyaAdelta11 mutant strain that contains a Cpx- and sigma E-inducible degP-lacZ reporter fusion decreased beta-galactosidase expression to levels observed in a cyaA+ strain. We also found that a cyaA null mutant displayed even higher levels of extracytoplasmic stress regulon activation compared to a cyaAdelta11 mutant. Thus, we conclude that the lowered concentration of cAMP in cyaA mutants induces both sigma E and Cpx extracytoplasmic stress regulons and thereby rescues the degP temperature-sensitive phenotype.

Cpx signal transduction is influenced by a conserved N-terminal domain in the novel inhibitor CpxP and the periplasmic protease DegP

In Escherichia coli, envelope stress can be overcome by three different envelope stress responses: the sigma(E) stress response and the Bae and Cpx two-component systems. The Cpx envelope stress response is controlled by the sensor kinase CpxA, the response regulator CpxR, and the novel periplasmic protein CpxP. CpxP mediates feedback inhibition of the Cpx pathway through a hypothetical interaction with the sensing domain of CpxA. No informative homologues of CpxP are known, and thus it is unclear how CpxP exerts this inhibition. Here, we identified six cpxP loss-of-function mutations using a CpxP-beta-lactamase (CpxP'-'Bla) translational fusion construct. These loss-of-function mutations identified a highly conserved, predicted alpha-helix in the N-terminal domain of CpxP that affects both the function and the stability of the protein. In the course of this study, we also found that CpxP'-'Bla stability is differentially controlled by the periplasmic protease DegP in response to inducing cues and that mutation of degP diminishes Cpx pathway activity. We propose that the N-terminal alpha-helix is an important functional domain for inhibition of the Cpx pathway and that CpxP is subject to DegP-dependent proteolysis.

Envelope stress responses and Gram-negative bacterial pathogenesis

The sigma(E), Cpx and Bae envelope stress responses of Escherichia coli are involved in the maintenance, adaptation and protection of the bacterial envelope in response to a variety of stressors. Recent studies indicate that the Cpx and sigma(E) stress responses exist in many Gram-negative bacterial pathogens. The envelope is of particular importance to these organisms because most virulence determinants reside in, or must transit through, this cellular compartment. The Cpx system has been implicated in expression of pili, type IV secretion systems and key virulence regulators, while the sigma(E) pathway has been shown to be critical for protection from oxidative stress and intracellular survival. Homologues of the sigma(E)- and Cpx-regulated protease DegP are essential for full virulence in numerous pathogens, and, like sigma(E), DegP appears to confer resistance to oxidative stress and intracellular survival capacity. Some pathogens contain multiple homologues of the Cpx-regulated, disulphide bond catalyst DsbA protein, which has been demonstrated to play roles in the expression of secreted virulence determinants, type III secretion systems and pili. This review highlights recent studies that indicate roles for the sigma(E), Cpx and Bae envelope stress responses in Gram-negative bacterial pathogenesis.

Sensing external stress: watchdogs of the Escherichia coli cell envelope

The Cpx and sigmaE signaling systems monitor the cell envelope in Escherichia coli. When induced, each system triggers a signaling cascade that leads to the upregulation of factors needed to combat envelope damage. Although each system is distinct and can be uniquely induced by certain cues, they also share striking similarities. In this review, we discuss the recent progress in our understanding of the Cpx and sigmaE systems and compare how both function to maintain the integrity of the cell envelope.

Quality control in the bacterial periplasm

Studies of the mechanisms that Gram-negative bacteria use to sense and respond to stress have led to a greater understanding of protein folding in both cytoplasmic and extracytoplasmic locations. In response to stressful conditions, bacteria induce a variety of stress response systems, examples of which are the sigma(E) and Cpx systems in Escherichia coli. Induction of these stress response systems results in upregulation of several gene targets that have been shown to be important for protein folding under normal conditions. Here we review the identification of stress response systems and their corresponding gene targets in E. coli. In addition, we discuss the apparent redundancy of the folding factors in the periplasm, and we consider the potential importance of the functional overlap that exists.

The Cpx envelope stress response affects expression of the type IV bundle-forming pili of enteropathogenic Escherichia coli

The Cpx envelope stress response mediates adaptation to potentially lethal envelope stresses in Escherichia coli. The two-component regulatory system consisting of the sensor kinase CpxA and the response regulator CpxR senses and mediates adaptation to envelope insults believed to result in protein misfolding in this compartment. Recently, a role was demonstrated for the Cpx response in the biogenesis of P pili, attachment organelles expressed by uropathogenic E. coli. CpxA senses misfolded P pilus assembly intermediates and initiates increased expression of both assembly and regulatory factors required for P pilus elaboration. In this report, we demonstrate that the Cpx response is also involved in the expression of the type IV bundle-forming pili of enteropathogenic E. coli (EPEC). Bundle-forming pili were not elaborated from an exogenous promoter in E. coli laboratory strain MC4100 unless the Cpx pathway was constitutively activated. Further, an EPEC cpxR mutant synthesized diminished levels of bundle-forming pili and was significantly affected in adherence to epithelial cells. Since type IV bundle-forming pili are very different from chaperone-usher-type P pili in both form and biogenesis, our results suggest that the Cpx envelope stress response plays a general role in the expression of envelope-localized organelles with diverse structures and assembly pathways.

A sensor of the two-component system CpxA affects expression of the type III secretion system through posttranscriptional processing of InvE

The chief function of the Cpx two-component system is perceiving various cell envelope stresses, but CpxR is also known to regulate the expression of the type III secretion system (TTSS) of Shigella sonnei through transcription of the primary regulator virF. Here, we have isolated novel cpxA mutants that exhibited decreased TTSS expression from Escherichia coli HW1273, which carries the virulence plasmid of S. sonnei. The cpxA deletion strain of HW1273 expressed beta-galactosidase activity levels from the virF-lacZ fusion similar to those of HW1273. However, the second regulator InvE (VirB) and the TTSS component IpaB proteins were apparently expressed at a low level. In the cpxA strain, beta-galactosidase activity levels from the invE-lacZ transcriptional fusion remained similar to those of HW1273, whereas the beta-galactosidase activity level from the translational fusion of invE-lacZ was reduced to 21% of that of HW1273. Therefore, the deletion of the cpxA gene influenced TTSS expression chiefly at the posttranscriptional processing of InvE. In addition, the cpxA deletion strain of S. sonnei showed the same phenotype. These results indicate that the Cpx two-component system is involved in virulence expression through posttranscriptional processing of the regulatory protein InvE, a novel feature of the Cpx two-component system in posttranscriptional processing and virulence expression of Shigella.

Transcriptional adaptation of Shigella flexneri during infection of macrophages and epithelial cells: insights into the strategies of a cytosolic bacterial pathogen

Shigella flexneri, the etiologic agent of bacillary dysentery, invades epithelial cells as well as macrophages and dendritic cells and escapes into the cytosol soon after invasion. Dissection of the global gene expression profile of the bacterium in its intracellular niche is essential to fully understand the biology of Shigella infection. We have determined the complete gene expression profiles for S. flexneri infecting human epithelial HeLa cells and human macrophage-like U937 cells. Approximately one quarter of the S. flexneri genes showed significant transcriptional adaptation during infection; 929 and 1,060 genes were up- or down-regulated within HeLa cells and U937 cells, respectively. The key S. flexneri virulence genes, ipa-mxi-spa and icsA, were drastically down-regulated during intracellular growth. This theme seems to be common in bacterial infection, because the Ipa-Mxi-Spa-like type III secretion systems were also down-regulated during mammalian cell infection by Salmonella enterica serovar Typhimurium and Escherichia coli O157. The bacteria experienced restricted levels of iron, magnesium, and phosphate in both host cell types, as shown by up-regulation of the sitABCD system, the mgtA gene, and genes of the phoBR regulon. Interestingly, ydeO and other acid-induced genes were up-regulated only in U937 cells and not in HeLa cells, suggesting that the cytosol of U937 cells is acidic. Comparison with the gene expression of intracellular Salmonella serovar Typhimurium, which resides within the Salmonella-containing vacuole, indicated that S. flexneri is exposed to oxidative stress in U937 cells. This work will facilitate functional studies of hundreds of novel intracellularly regulated genes that may be important for the survival and growth strategies of Shigella in the human host.

Role of the two-component regulator CpxAR in the virulence of Salmonella enterica serotype Typhimurium

The CpxAR (Cpx) two-component regulator controls the expression of genes in response to a variety of environmental cues. The Cpx regulator has been implicated in the virulence of several gram-negative pathogens, although a role for Cpx in vivo has not been demonstrated directly. Here we investigate whether positive or negative control of gene expression by Cpx is important for the pathogenesis of Salmonella enterica serotype Typhimurium. The Cpx signal pathway in serotype Typhimurium was disrupted by insertional inactivation of the cpxA and cpxR genes. We also constitutively activated the Cpx pathway by making an internal in-frame deletion in cpxA (a cpxA* mutation). Activation of the Cpx pathway inhibited induction of the envelope stress response pathway controlled by the alternative sigma factor sigma(E) (encoded by rpoE). Conversely, the Cpx pathway was highly up-regulated (>40-fold) in a serotype Typhimurium rpoE mutant. The cpxA* mutation, but not the cpxA or the cpxR mutation, significantly reduced the capacity of serotype Typhimurium to adhere to and invade eucaryotic cells, although intracellular replication was not affected. The cpxA and cpxA* mutations significantly impaired the ability of serotype Typhimurium to grow in vivo in mice. To our knowledge, this is the first demonstration that the Cpx system is important for a bacterial pathogen in vivo.

P pilus assembly motif necessary for activation of the CpxRA pathway by PapE in Escherichia coli

P pilus biogenesis occurs via the highly conserved chaperone-usher pathway, and assembly is monitored by the CpxRA two-component signal transduction pathway. Structural pilus subunits consist of an N-terminal extension followed by an incomplete immunoglobulin-like fold that is missing a C-terminal seventh beta strand. In the pilus fiber, the immunoglobulin-like fold of each pilin is completed by the N-terminal extension of its neighbor. Subunits that do not get incorporated into the pilus fiber are driven "OFF-pathway." In this study, we found that PapE was the only OFF-pathway nonadhesin P pilus subunit capable of activating Cpx. Manipulation of the PapE structure by removing, relocating within the protein, or swapping its N-terminal extension with that of other subunits altered the protein's self-associative and Cpx-activating properties. The self-association properties of the new subunits were dictated by the specific N-terminal extension provided and were consistent with the order of the subunits in the pilus fiber. However, these aggregation properties did not directly correlate with Cpx induction. Cpx activation instead correlated with the presence or absence of an N-terminal extension in the PapE pilin structure. Removal of the N-terminal extension of PapE was sufficient to abolish Cpx activation. Replacement of an N-terminal extension at either the amino or carboxyl terminus restored Cpx induction. Thus, the data presented in this study argue that PapE has features inherent in its structure or during its folding that act as specific inducers of Cpx signal transduction.

Protein quality control in the bacterial periplasm

The proper functioning of extracytoplasmic proteins requires their export to, and productive folding in, the correct cellular compartment. All proteins in Escherichia coli are initially synthesized in the cytoplasm, then follow a pathway that depends upon their ultimate cellular destination. Many proteins destined for the periplasm are synthesized as precursors carrying an N-terminal signal sequence that directs them to the general secretion machinery at the inner membrane. After translocation and signal sequence cleavage, the newly exported mature proteins are folded and assembled in the periplasm. Maintaining quality control over these processes depends on chaperones, folding catalysts, and proteases. This article summarizes the general principles which control protein folding in the bacterial periplasm by focusing on the periplasmic maltose-binding protein.

Temperature effect on inclusion body formation and stress response in the periplasm of Escherichia coli

We previously characterized a defective-folding mutant of maltose-binding protein of Escherichia coli, MalE31, which formed periplasmic inclusion bodies. Here, we show that MalE31 aggregation does not affect bacterial growth at 30 degrees C but is lethal at 37 degrees C. Surprisingly, under mild heat shock conditions at 42 degrees C, inclusion bodies are degraded and bacterial growth is restored. One physiological consequence for the cells overproducing MalE31 was to induce an extracytoplasmic stress response by increasing the expression of the heat shock protease DegP via the CpxA/CpxR two-component signalling pathway. Furthermore, we show that the Cpx response is required to rescue the cells from the toxicity mediated by MalE31. Finally, expression of highly destabilized MalE variants that do not aggregate in the periplasm also induces the Cpx pathway, indicating that inclusion body formation is not necessary to activate this specific extracytoplasmic stress regulatory system.

Activation of hilA expression at low pH requires the signal sensor CpxA, but not the cognate response regulator CpxR, in Salmonella enterica serovar Typhimurium

A two-component regulatory system, cpxR-cpxA, plays an important role in the pH-dependent regulation of virF, a global activator for virulence determinants including invasion genes, in Shigella sonnei. The authors examined whether the cpxR-cpxA homologues have some function in the expression of Salmonella enterica serovar Typhimurium invasion genes via the regulation of hilA, an activator for these genes. In a Salmonella cpxA mutant, the hilA expression level was reduced to less than 10 % of that in the parent strain at pH 6.0. This mutant strain also showed undetectable synthesis of an invasion gene product, SipC, at pH 6.0 and reduced cell invasion capacity - as low as 20 % of that of the parent. In this mutant, the reduction in hilA expression was much less marked at pH 8.0 than at pH 6.0 - no less than 50 % of that in the parent, and no significant reduction was observed in either SipC synthesis or cell invasion rate, compared to the parent. Unexpectedly, a Salmonella cpxR mutant strain and the parent showed no apparent difference in all three characteristics described above at either pH. These results indicate that in Salmonella, the sensor kinase CpxA activates hilA, and consequently, invasion genes and cell invasion capacity at pH 6.0. At pH 8.0, however, CpxA does not seem to have a large role in activation of these factors. Further, the results show that this CpxA-mediated activation does not require its putative cognate response regulator, CpxR. This suggests that CpxA may interact with regulator(s) other than CpxR to achieve activation at low pH.

Shigella flexneri 2a strain 2457T expresses three members of the H-NS-like protein family: characterization of the Sfh protein

Shigella flexneri 2a is known to express the H-NS nucleoid-structuring protein and the paralogous protein StpA. Using bioinformatic analysis we have now discovered a third member of the H-NS protein family, Sfh (Shigella flexneri H-NS-like protein), in strain 2457T. This protein is encoded by the sfh gene, which is located on a high-molecular-mass plasmid that is closely related to the self-transmissible plasmid R27. When expressed in Escherichia coli, the Sfh protein can complement an hns null mutation, restoring wild-type Bgl, porin protein, and mucoidy phenotypes, and wild-type expression of the fliC and proU genes. While a knockout mutation in the sfh gene alone had no effect on the expression of virulence genes in S. flexneri, an additive effect on virulence gene derepression was seen when the sfh lesion was combined with a mutation in hns. Over-expression of the sfh gene repressed expression of the VirB virulence regulatory protein and transcription of a VirB-dependent structural gene promoter. The purified Sfh protein bound specifically to DNA sequences containing the promoters of the virF and virB virulence regulatory genes. These findings show that Sfh has the ability to influence genetic events beyond the genetic element that encodes it, including the expression of the S. flexneri virulence genes. They raise the possibility of a triangular relationship among three closely related proteins with broad consequences for genetic events in the bacterium that harbours them.

Identification of CpxR as a positive regulator of icm and dot virulence genes of Legionella pneumophila

To date, 24 Legionella pneumophila genes (icm and dot genes) have been shown to be required for intercellular growth and host cell killing. A previous report indicated that the regulation of these genes is complicated and probably involves several regulatory proteins. In this study, a genetic screen performed in Escherichia coli identified the CpxR response regulator as an activator of the L. pneumophila icmR gene. Construction of an L. pneumophila cpxR insertion mutant showed that the expression of icmR is regulated by CpxR. In addition, a conserved CpxR binding site (GTAAA) was identified in the icmR regulatory region and L. pneumophila His-tagged CpxR protein was shown to bind to the icmR regulatory region using a mobility shift assay. Besides its dramatic effect on the icmR level of expression, the CpxR regulator was also found to affect the expression of the icmV-dotA and icmW-icmX operons, but to a lesser extent. The role of CpxA, the cognate sensor kinase of CpxR, was also examined and its effect on the icmR level of expression was found to be less pronounced than the effect of CpxR. The RpoE sigma factor, which was shown to coregulate genes together with CpxR, was examined as well, but it did not influence icm and dot gene expression. In addition, when the cpxR mutant strain, in which the expression of the icmR gene was dramatically reduced, and the cpxA and rpoE mutant strains were examined for their ability to grow inside Acanthamoeba castellanii and HL-60-derived human macrophages, no intracellular growth defect was observed. This study presents the first evidence for a direct regulator (CpxR) of an icm-dot virulence gene (icmR). The CpxR regulator together with other regulatory factors probably concerts with the expression of icm and dot genes to result in successful infection.

Three-way interactions among the Sfh, StpA and H-NS nucleoid-structuring proteins of Shigella flexneri 2a strain 2457T

Shigella flexneri 2a strain 2457T has been found to express Sfh, a new member of the H-NS-like family of nucleoid-structuring proteins. With H-NS and its paralogue, StpA, this brings to three the number of these proteins expressed in this bacterium. This raises the possibility that three-way interactions may occur in S. flexneri among these proteins and between the proteins and each other's genes. Such three-way interactions among H-NS-like proteins have not been described previously. The expression of the sfh, stpA and hns genes was studied at the transcriptional and post-transcriptional levels. The Sfh protein displays growth phase-dependent regulation that distinguishes it from both H-NS and StpA. Like H-NS and StpA, Sfh can bind to its own promoter region, it negatively autoregulates transcription of its own gene, and when overexpressed all three proteins cross-repress transcription of each other's genes. The presence of highly conserved oligomerization domains within these molecules suggested the possibility of protein-protein interactions. Like H-NS and StpA, the purified Sfh protein forms homodimers in solution. Using the yeast two-hybrid assay we show that each of the three proteins also forms homodimers in vivo and, additionally, each protein can form heterodimers with either of its homologues. This raises the possibility that Sfh may modulate the activities of H-NS and StpA, and vice versa.

Mechanism of pH regulation of connexin 43 expression in MC3T3-E1 cells

Gap junction (GJ) expression and function allowing cell-cell communication among osteoblasts may be important in new bone formation. An alkaline milieu stimulates mineralization, while extracellular acidification leads to demineralization of bone. It was previously demonstrated that alkaline pH increases, while acid pH decreases GJ intercellular communication by an increase in steady-state GJ connexin 43 (Cx43) mRNA and protein expression. At pH 7.6, transcription of new Cx43 mRNA was significantly higher than that at pH 6.9 but not significantly different at pH 7.2, as assessed by nuclear run-on assay. Transcription of new Cx43 mRNA was higher at pH 7.2 compared to that at pH 6.9. Although Cx43 mRNA half-life tended to be longer at pH 7.6, analysis of variance did not yield a significant difference of the Cx43 mRNA half-life at any of the pHs tested. Likewise, the half-life of Cx43 protein at pHs of 6.9, 7.2, and 7.6 was not significantly different. Plasma membrane and cytosolic Cx43 fractions were proportionately similar at pH 7.2 and 6.9. Thus, the decrease in Cx43 mRNA at low pH compared to high pH is due to a decrease in the transcription rate of Cx43 but not due to an alteration of message stability. The early uncoupling of gap junctions by low pH found previously does not appear to be due to changes in the half-life nor distribution of Cx43 protein between cytosolic and plasma membrane compartments.

An extended role for the nucleoid structuring protein H-NS in the virulence gene regulatory cascade of Shigella flexneri

The H-NS nucleoid structuring protein has been shown previously to play a negative role in controlling virulence gene expression in Shigella flexneri by repressing transcription of the virF and virB regulatory genes and the VirF-dependent icsA structural gene under non-permissive growth conditions. Here, we show that H-NS also acts at the promoters of the VirB-dependent structural genes in the regulatory cascade. H-NS protein binds to the promoter regions in vivo and in vitro. The promoters were shown physically and by in silico analysis to contain regions of DNA curvature, a feature of H-NS binding sites. H-NS binding sites were determined by DNase I footprinting at the icsB and the virA promoters. The locations of these sites were consistent with a role for H-NS as a transcription repressor. The VirB-dependent structural gene promoters were found to respond directly to the H-NS repressor, revealing a level of control that is additional to that exerted by the H-NS-dependent virB activator gene. Moreover, the promoters were sensitive to the level of VirB protein in the cell, requiring a threshold level of VirB to be reached before becoming active. A model is discussed in which the levels of expression of the structural genes reflect the outcome of competition between the countervailing regulatory activities of the H-NS and VirB proteins.

Putrescine or a combination of methionine and arginine restores virulence gene expression in a tRNA modification-deficient mutant of Shigella flexneri: a possible role in adaptation of virulence

The wild-type strain YSH6000 of Shigella flexneri growing in minimal medium contains the modified nucleoside epoxy-Q (oQ) in a subset of tRNAs. This nucleoside is lacking in tRNA from a tgt mutant of this bacterium. When these bacteria are growing in minimal medium, the expression of virulence genes is 10-fold lower in the tgt mutant than in the wild type, although only a twofold reduction in the expression of these virulence factors is observed in broth. Such a strong media-dependant expression of virulence genes was not observed in the wild type. Accordingly, the level of the positive regulator of virulence, VirF, is much lower in the mutant than in the wild type. However, the transcription of the virF gene in minimal medium is the same in the wild type and in the tgt mutant. As the undermodification of tRNA is not affected by the quality of the growth medium, we conclude that such an environmental change in growth conditions partly restores virulence gene expression by counteracting poor translation of the virF mRNA mediated by an oQ-deficient tRNA. Virulence gene expression is partly restored in the tgt mutant by the addition of a mixture of arginine and methionine. Addition of the polyamine putrescine, synthesis of which is metabolically related to that of arginine and methionine, has a comparable stimulatory effect on virulence gene expression. These results not only suggest a role for amino acids and polyamines in the environmental regulation of virulence gene expression in S. flexneri, but also demonstrate a strong and specific involvement of tRNA modifications, and especially oQ, in the adaptation of virulence gene expression to the nutritional quality of the growth medium.

Salmonella enterica serovar typhimurium rdoA is growth phase regulated and involved in relaying Cpx-induced signals

The disulfide oxidoreductase, DsbA, mediates disulfide bond formation in proteins as they enter or pass through the periplasm of gram-negative bacteria. Although DsbA function has been well characterized, less is known about the factors that control its expression. Previous studies with Escherichia coli demonstrated that dsbA is part of a two-gene operon that includes an uncharacterized, upstream gene, yihE, that is positively regulated via the Cpx stress response pathway. To clarify the role of the yihE homologue on dsbA expression in Salmonella enterica serovar Typhimurium, the effect of this gene (termed rdoA) on the regulation of dsbA expression was investigated. Transcriptional assays assessing rdoA promoter activity showed growth phase-dependent expression with maximal activity in stationary phase. Significant quantities of rdoA and dsbA transcripts exist in serovar Typhimurium, but only extremely low levels of rdoA-dsbA cotranscript were detected. Activation of the Cpx system in serovar Typhimurium increased synthesis of both rdoA- and dsbA-specific transcripts but did not significantly alter the levels of detectable cotranscript. These results indicate that Cpx-mediated induction of dsbA transcription in serovar Typhimurium does not occur through an rdoA-dsbA cotranscript. A deletion of the rdoA coding region was constructed to definitively test the relevance of the rdoA-dsbA cotranscript to dsbA expression. The absence of RdoA affects DsbA expression levels when the Cpx system is activated, and providing rdoA in trans complements this phenotype, supporting the hypothesis that a bicistronic mechanism is not involved in serovar Typhimurium dsbA regulation. The rdoA null strain was also shown to be altered in flagellar phase variation. First it was found that induction of the Cpx stress response pathway switched flagellar synthesis to primarily phase 2 flagellin, and this effect was then found to be abrogated in the rdoA null strain, suggesting the involvement of RdoA in mediating Cpx-related signaling.