Analysis of the Yersinia enterocolitica PspBC proteins defines functional domains, essential amino acids and new roles within the phage-shock-protein response.

The Yersinia enterocolitica phage-shock-protein (Psp) stress response system is activated by mislocalized outer-membrane secretin components of protein export systems and is essential for virulence. The cytoplasmic membrane proteins PspB and PspC were proposed to be dual function components of the system, acting both as positive regulators of psp gene expression and to support survival during secretin-induced stress. In this study we have uncoupled the regulatory and physiological functions of PspBC and discovered unexpected new roles, functional domains and essential amino acids. First, we showed that PspB controls PspC concentration by both pre- and post-transcriptional mechanisms. We then screened for PspBC mutants with altered transcriptional regulatory function. Unexpectedly, we identified PspB and PspC mutants that activated psp gene expression in the absence of secretin-induced stress. Together with a subsequent truncation analysis, this revealed that the PspC cytoplasmic domain plays an unforeseen role in negatively regulating psp gene expression. Conversely, mutations within the PspC periplasmic domain abolished its ability to activate psp gene expression. Significantly, PspC mutants unable to activate psp gene expression retained their ability to support survival during secretin-induced stress. These data provide compelling support for the proposal that these two functions are independent.