Vibrio cholerae VttRA and VttRB Regulatory Influences Extend beyond the Type 3 Secretion System Genomic Island

A subset of non-O1/non-O139 serogroup strains of Vibrio cholerae cause disease using type 3 secretion system (T3SS)-mediated mechanisms. An similar to 50-kb genomic island carries genes encoding the T3SS structural apparatus, effector proteins, and two transmembrane transcriptional regulators, VttR(A) and VttR(B), which are ToxR homologues. Previous experiments demonstrated that VttR(A) and VttR(B) are necessary for colonization in vivo and promote bile-dependent T3SS gene expression in vitro. To better understand the scope of genes that are potential targets of VttR(A) and VttR(B) regulation, we performed deep RNA sequencing using O39 serogroup strain AM-19226 and derivatives carrying deletions in vttR(A) and vttR(B) grown in bile. Comparison of the transcript profiles from Delta vttR(A) and Delta vttR(B) mutant strains to the isogenic parent strain confirmed that VttR(A) and VttR(B) regulate expression of some T3SS island genes and provided additional information about relative expression levels and operon organization. Interestingly, the data also suggested that additional genes, located outside the T3SS island and encoding functions involved in motility, chemotaxis, type 6 secretion, transcriptional regulation, and stress responses, may also by regulated by VttR(A) and VttR(B). We verified transcript levels for selected genes by quantitative reverse transcription (RT)-PCR and then focused additional studies on motility and biofilm formation. The results suggest that VttR(A) and VttR(B) act as part of a complex transcriptional network that coordinates virulence gene expression with multiple cellular phenotypes. VttR(A) and VttR(B) therefore represent horizontally acquired transcriptional regulators with the ability to influence global gene expression in addition to modulating gene expression within the T3SS genomic island.