The ToxR-Mediated Organic Acid Tolerance Response of Vibrio cholerae Requires OmpU

It was previously demonstrated that the intestinal pathogen Vibrio cholerae could undergo an adaptive stress response known as the acid tolerance response (ATR). The ATR is subdivided into two branches, inorganic ATR and organic ATR. The transcriptional regulator ToxR, while not involved in inorganic ATR, is required for organic ATR in a ToxT-independent manner. Herein, we investigate the effect of organic acid stress on global protein synthesis in V. cholerae and show by two-dimensional gel electrophoresis that the stress response alters the expression of more than 100 polypeptide species. The expression of more than 20 polypeptide species is altered in a toxR strain compared to the wild type. Despite this, ectopic expression of the porin OmpU from an inducible promoter is shown to be sufficient to bypass the toxR organic ATR defect. Characterization of the effect of organic acid stress on ompU and ompT transcription reveals that while ompU transcription remains virtually unaffected, ompT transcription is repressed in a ToxR-independent manner. These transcript levels are similarly reflected in the extent of accumulation of OmpU and OmpT. Possible roles for OmpU in organic acid resistance are discussed.