SummaryThe phage-shock-protein (Psp) system responds to extracytoplasmic stress that may reduce the energy status of the cell. It is conserved in many different bacteria and has been linked to several important phenotypes. Escherichia coli psp mutants have defects in maintenance of the proton-motive force, protein export by the sec and tat pathways, survival in stationary phase at alkaline pH, and biofilm formation. Yersinia enterocolitica psp mutants cannot grow when the secretin component of a type III secretion system is mislocalized, and have a severe virulence defect in animals. A Salmonella enterica psp mutation exacerbates some phenotypes of an rpoE null mutant and the psp genes of S. enterica and Shigella flexneri are highly induced during macrophage infection. PspA, the most abundant of the Psp proteins, is required for most of the phenotypes associated with the Psp system. Therefore, PspA is probably an effector that may play a role in maintaining cytoplasmic membrane integrity and/or the proton-motive force. However, PspA is not required for the ability to tolerate secretin mislocalization, which suggests an important physiological role for other Psp proteins. This article summarizes our current understanding of the Psp system: inducing signals, the underlying signal transduction mechanisms, the physiological roles it may play, and a genomic analysis of its conservation.

Pathogenic Yersinia species are associated with both localized and systemic infections in mammalian hosts. In this study, signature‐tagged transposon mutagenesis was used to identify Yersinia enterocolitica genes required for survival in a mouse model of infection. Approximately 2000 transposon insertion mutants were screened for attenuation. This led to the identification of 55 mutants defective for survival in the animal host, as judged by their ability to compete with the wild‐type strain in mixed infections. A total of 28 mutants had transposon insertions in the virulence plasmid, validating the screen. Two of the plasmid mutants with severe virulence defects had insertions in an uncharacterized region. Several of the chromosomal insertions were in a gene cluster involved in O‐antigen biosynthesis. Other chromosomal insertions identified genes not previously demonstrated as being required for in vivo survival of Y. enterocolitica. These include genes involved in the synthesis of outer membrane components, stress response and nutrient acquisition. One severely attenuated mutant had an insertion in a homologue of the pspC gene (phage shock protein C) of Escherichia coli. The phage shock protein operon has no known biochemical or physiological function in E. coli, but is apparently essential for the survival of Y. enterocolitica during infection.

The phage shock protein locus (pspFpspABCDE) of Escherichia coli has proved to be something of an enigma since its discovery. The physiological functions of the psp locus, including those of the predicted effector protein PspA, are unknown. In a previous genetic screen, we determined that a Yersinia enterocolitica pspC mutant was severely attenuated for virulence. In this study, the psp locus of Y. enterocolitica was characterized further. The pspC gene of Y. enterocolitica was found to be important for normal growth when the Ysc type III secretion system was expressed in the laboratory. This growth defect was specifically caused by production of the secretin protein, YscC. Expression of the psp genes was induced when the type III secretion system was functional or when only the yscC gene was expressed. This induction of psp gene expression required a functional pspC gene. Most significantly, evidence suggests that the expression of at least one gene that is not part of the psp locus is regulated by Psp proteins. This unidentified gene (or genes) may also be important for growth when the type III secretion system is expressed. These conclusions are supported by the effects of various psp mutations on virulence. This is the first indication that Psp proteins might be involved in the regulation of genes besides the psp locus itself.

SummaryThe phage-shock-protein (Psp) stress-response system is conserved in many bacteria and has been linked to important phenotypes in Escherichia coli , Salmonella enterica and also Yersinia enterocolitica , where it is essential for virulence. It is activated by specific extracytoplasmic stress events such as the mislocalization of secretin proteins. From studies of the Psp system in E. coli , the cytoplasmic membrane proteins PspB and PspC have only been proposed to act as positive regulators of psp gene expression. However, in this study we show that PspB and PspC of Y. enterocolitica are dual function proteins, acting both as regulators and effectors of the Psp system. Consistent with the current model, they positively control psp gene expression in response to diverse inducing cues. PspB and PspC must work together to achieve this regulatory function, and bacterial twohybrid (BACTH) analysis demonstrated a specific interaction between them, which was confirmed by in vivo cross-linking. We also show that PspB and PspC play a second role in supporting growth when a secretin protein is overexpressed. This function is independent from their role as regulators of psp gene expression. Furthermore, whereas PspB and PspC must work together for their regulatory function, they can apparently act independently to support growth during secretin production. This study expands the current understanding of the roles played by PspB and PspC, and demonstrates that they cannot be considered only as positive regulators of psp gene expression in Y. enterocolitica .

Known inducers of the phage shock protein (Psp) system suggest that it is an extracytoplasmic stress response, as are the well-studied RpoE and Cpx systems. However, a random approach to identify conditions and proteins that induce the Psp system has not been attempted. It is also unknown whether the proteins or mutations that induce Psp are specific or if they also activate the RpoE and Cpx systems. This study addressed these issues for the Yersinia enterocolitica Psp system. Random transposon mutagenesis identified null mutations and overexpression mutations that increase ⌽(pspA-lacZ) operon fusion expression. The results suggest that Psp may respond exclusively to extracytoplasmic stress. Null mutations affected glucosamine-6-phosphate synthetase (glmS), which plays a role in cell envelope biosynthesis, and the F 0 F 1 ATPase (atp operon). The screen also revealed that in addition to several secretins, the overexpression of three novel putative inner membrane proteins ( Misfolding and/or mislocalization of envelope proteins induce extracytoplasmic stress responses in bacteria. The RpoE and Cpx systems of Escherichia coli and its relatives are wellstudied examples (reviewed in references 41 and 42). These systems control many genes, with some overlap between their regulons, which encode proteases, envelope protein folding factors, and several proteins of unknown function (10,41,43). Mounting a response to extracytoplasmic stress is extremely important. rpoE is an essential gene in E. coli (14) and Yersinia enterocolitica (21), although this is apparently not the case in the closely related Salmonella genus (23).The RpoE response is important for virulence in Salmonella enterica serovar Typhimurium (23,45) and Vibrio cholerae (30). In addition, the RpoE and Cpx responses are induced by the overproduction of P pilus subunits from uropathogenic E. coli (25), and the Cpx system affects assembly and expression of the P pilus (24). The Cpx system is also important for the attachment of E. coli to surfaces (37), which is a critical step during biofilm formation.The phage shock protein (Psp) system may be another example of an extracytoplasmic stress response. pspA operon expression, studied most extensively in E. coli K-12, is induced by the mislocalization of secretin proteins and by environmental conditions that induce the RpoE response (reviewed in reference 35).

Periplasmic nitrate reductase (NapABC enzyme) has been characterized from a variety of proteobacteria, especially Paracoccus pantotrophus. Whole-genome sequencing of Escherichia coli revealed the structural genes napFDAGHBC, which encode NapABC enzyme and associated electron transfer components. E. coli also expresses two membrane-bound proton-translocating nitrate reductases, encoded by the narGHJI and narZYWV operons. We measured reduced viologen-dependent nitrate reductase activity in a series of strains with combinations of nar and nap null alleles. The napF operon-encoded nitrate reductase activity was not sensitive to azide, as shown previously for the P. pantotrophus NapA enzyme. A strain carrying null alleles of narG and narZ grew exponentially on glycerol with nitrate as the respiratory oxidant (anaerobic respiration), whereas a strain also carrying a null allele of napA did not. By contrast, the presence of napA ؉ had no influence on the more rapid growth of narG ؉ strains. These results indicate that periplasmic nitrate reductase, like fumarate reductase, can function in anaerobic respiration but does not constitute a site for generating proton motive force. The time course of ⌽(napF-lacZ) expression during growth in batch culture displayed a complex pattern in response to the dynamic nitrate/nitrite ratio. Our results are consistent with the observation that ⌽(napFlacZ) is expressed preferentially at relatively low nitrate concentrations in continuous cultures (H. Wang, C.-P. Tseng, and R. P. Gunsalus, J. Bacteriol. 181:5303-5308, 1999). This finding and other considerations support the hypothesis that NapABC enzyme may function in E. coli when low nitrate concentrations limit the bioenergetic efficiency of nitrate respiration via NarGHI enzyme. Nitrate (NO 3Ϫ ), which is relatively abundant in many environments, has three functions in bacterial physiology. Nitrate assimilation provides a source of ammonium for biosynthesis (reviewed in reference 29), nitrate respiration generates proton motive force for energy (reviewed in references 4, 21, and 65), and nitrate dissimilation oxidizes excess reducing equivalents (reviewed in reference 36).Membrane-bound nitrate reductase (NarGHI enzyme; nitrate reductase A) employs a redox loop to couple quinol oxidation with proton translocation, thereby generating proton motive force for anaerobic respiration. The Escherichia coli and Paracoccus denitrificans enzymes have been the focus of most biochemical studies (reviewed in references 4, 21, and 65). This enzyme contains Mo-molybdopterin guanine dinucleotide, five iron-sulfur clusters, and diheme cytochrome b 556 . NarGHI enzyme activity is inhibited by azide (N 3 Ϫ ). Enzyme synthesis is maximally induced during anaerobic growth in the presence of nitrate.Periplasmic nitrate reductase (NapABC enzyme; nitrate reductase P) also oxidizes quinol, but it is thought that this enzyme is not a coupling site for proton translocation (reviewed in reference 4). Therefore, this enzyme is responsible for nitrate dissimilati...

The phage shock protein (Psp) system was identified as a response to phage infection in Escherichia coli, but rather than being a specific response to a phage, it detects and mitigates various problems that could increase inner-membrane (IM) permeability. Interest in the Psp system has increased significantly in recent years due to appreciation that Psp-like proteins are found in all three domains of life and because the bacterial Psp response has been linked to virulence and other important phenotypes. In this article, we summarize our current understanding of what the Psp system detects and how it detects it, how four core Psp proteins form a signal transduction cascade between the IM and the cytoplasm, and current ideas that explain how the Psp response keeps bacterial cells alive. Although recent studies have significantly improved our understanding of this system, it is an understanding that is still far from complete.

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