CsrA and three redundant small RNAs regulate quorum sensing in Vibrio cholerae

Bacteria communicate using a process called quorum sensing which involves production, secretion and detection of signalling molecules called autoinducers. Quorum sensing allows populations of bacteria to simultaneously regulate gene expression in response to changes in cell density. The human pathogen, Vibrio cholerae, uses a quorum-sensing circuit composed of parallel systems that transduce information through four redundant regulatory small RNAs (sRNAs) called quorum regulatory RNAs (Qrr) to control the expression of numerous genes, most notably those required for virulence. We show that the VarS/VarA two-component sensory system comprises an additional regulatory input controlling quorum-sensing-dependent gene expression in V. cholerae. VarS/VarA controls transcription of three previously unidentified small regulatory RNAs (sRNAs) that are similar to the sRNAs CsrB and CsrC of Escherichia coli. The three V. cholerae sRNAs, which we name CsrB, CsrC and CsrD, act redundantly to control the activity of the global regulatory protein, CsrA. The VarS/VarA-CsrA/BCD system converges with the V. cholerae quorum-sensing systems to regulate the expression of the Qrr sRNAs, and thus, the entire quorum-sensing regulon.