pepA, a gene mediating pH regulation of virulence genes in Vibrio cholerae

ToxT, a member of the AraC family of transcriptional regulators, controls t he expression of several virulence factors in Vibrio cholerae. In the class ical biotype of V. cholerae, expression of toxT is regulated by the same en vironmental conditions that control expression of the virulence determinant s cholera toxin and the toxin coregulated pilus. Several genes that activat e toxT expression have been identified. To identify genes that repress toxT expression in nonpermissive environmental conditions, a genetic screen was used to isolate mutations which alter the expression of a toxT-gusA transc riptional fusion. Several mutants were isolated, and the mutants could be d ivided into two classes. One class of mutants exhibited higher expression l evels of toxT-gusA at both the nonpermissive pH and temperature, while the second class showed elevated toxT-gusA expression only at the nonpermissive pH. One mutant from the second class was chosen for further characterizati on. This mutant was found to carry a TnphoA insertion in a homolog of the E scherichia coli pepA gene. Disruption of pepA in V. cholerae resulted in el evated levels of expression of cholera toxin, tcpA, toxT, and tcpP at the n oninducing pH but not at the noninducing temperature. Elevated levels of ex pression of toxT and tcpP at the nonpermissive pH in the pepA mutant were a bolished in tcpP toxR mutant and aphB mutant backgrounds, respectively. A p utative binding site for PepA was identified in the tcpPH-tcpI intergenic r egion, suggesting that PepA may act at the level of tcpPH transcription. Di sruption of pepA caused only partial deregulation at the noninducing pH, su ggesting the involvement of additional factors in the pH regulation of viru lence genes in V. cholerae.