Regulatory Small RNA Qrr2 Is Expressed Independently of Sigma Factor-54 and Can Function as the Sole Qrr Small RNA To Control Quorum Sensing in Vibrio parahaemolyticus.

Bacterial cells alter gene expression in response to changes in population density in a process called quorum sensing (QS). In Vibrio harveyi, LuxO, a low-cell-density activator of sigma factor-54 (RpoN), is required for transcription of five noncoding regulatory small RNAs (sRNAs), Qrr1 to Qrr5, which each repress translation of the master QS regulator, LuxR. Vibrio parahaemolyticus, the leading cause of bacterial seafoodborne gastroenteritis, also contains five Qrr sRNAs that control OpaR (the LuxR homolog), controlling capsule polysaccharide (CPS), motility, and metabolism. We show that in a Δ deletion mutant, was derepressed and CPS and biofilm were produced. However, in a Δ mutant, was repressed, no CPS was produced, and less biofilm production was observed than in the wild type. To determine why was repressed, expression analysis in Δ showed that all five genes were repressed, while in Δ the gene was significantly derepressed. Reporter assays and mutant analysis showed that Qrr2 sRNA can act alone to control OpaR. Bioinformatics analysis identified a sigma-70 (RpoD) -35 -10 promoter overlapping the canonical sigma-54 (RpoN) -24 -12 promoter in the regulatory region. The sigma-70 promoter element was also present in additional species, indicating that it is widespread. Mutagenesis of the sigma-70 -10 promoter site in the Δ mutant background resulted in repression of Analysis of quadruple deletion mutants, in which only a single gene is present, showed that only Qrr2 sRNA can act independently to regulate . Mutant and expression data also demonstrated that RpoN and the global regulator, Fis, act additively to repress . Our data have uncovered a new mechanism of expression and show that Qrr2 sRNA is sufficient for OpaR regulation. The quorum sensing noncoding small RNAs (sRNAs) are present in all species but vary in number and regulatory roles among species. In the Harveyi clade, all species contain five genes, and in Vibrio harveyi these are transcribed by sigma-54 and are additive in function. In the Cholerae clade, four genes are present, and in Vibrio cholerae the genes are redundant in function. In Vibrio parahaemolyticus, is controlled by two overlapping promoters. In an mutant, is transcribed from a sigma-70 promoter that is present in all V. parahaemolyticus strains and in other species of the Harveyi clade, suggesting a conserved mechanism of regulation. Qrr2 sRNA can function as the sole Qrr sRNA to control OpaR.