Compensatory role of PspA, a member of the phage shock protein operon, in rpoE mutant Salmonella enterica serovar Typhimurium

Abstract

σE is an alternative sigma factor that responds to and ameliorates extracytoplasmic stress. In Salmonella enterica serovar Typhimurium (S. Typhimurium), σE is required for oxidative stress resistance, stationary-phase survival and virulence in mice. Microarray analysis of stationary-phase gene expression in rpoE mutant bacteria revealed a dramatic increase in expression of pspA, a member of the phage shock protein (psp) operon. The psp operon can be induced by filamentous bacteriophages or by perturbations of protein secretion, and is believed to facilitate the maintenance of proton motive force (PMF). We hypothesized that increased pspA expression may represent a compensatory response to the loss of σE function. Increased pspA expression was confirmed in rpoE mutant Salmonella and also observed in a mutant lacking the F1F 0 ATPase. Alternatively, expression of pspA could be induced by exposure to CCCP, a protonophore that disrupts PMF. An rpoE pspA double mutant strain was found to have a stationary-phase survival defect more pronounced than that of isogenic strains harbouring single mutations. The double mutant strains were also more susceptible to killing by CCCP or by a bactericidal/ permeability-increasing protein (BPI)-derived anti-microbial peptide. Using fluorescence ratio imaging, differences were observed in the Δψ of wild-type and rpoE or pspA mutant bacteria. These findings suggest that pspA expression in S. Typhimurium is induced by alterations in PMF and a functional σE regulon is essential for the maintenance of PMF. © 2005 Blackwell Publishing Ltd.