REGULATION OF QUORUM SENSING IN VIBRIO HARVEYI AND VIBRIO CHOLERAE

Abstract

Quorum sensing is a cell-to-cell communication process that bacteria use to monitor changes in cell population density. By producing, releasing, and detecting extracellular signal molecules called autoinducers, bacteria ensure that collective behaviors such as bioluminescence, biofilm formation, and virulence factor production are only executed at appropriate cell densities. In Vibrio harveyi and Vibrio cholerae, the quorum regulatory RNAs (Qrr sRNAs) function at the center of the quorum-sensing circuit. Qrr sRNA production is activated by the phosphorylated response regulator protein LuxO at low cell density (LCD) and the Qrr sRNAs repress production of the high cell density (HCD) master regulator LuxR/HapR. This thesis focuses on the regulatory functions of and mechanisms used by the Qrr sRNAs, as well as the roles of master regulator proteins. Genetic screens and microarray analyses identified AphA as the LCD master regulator. The Qrr sRNAs activate AphA production through direct base pairing to the aphA mRNA 5'UTR. Positive regulation of AphA and negative regulation of LuxR/HapR ensures that maximum AphA is produced at LCD, while maximum LuxR/HapR exists at HCD. AphA and LuxR/HapR, in turn, direct the proper LCD to HCD quorum-sensing gene expression patterns. Pulse-expression of each individual Qrr sRNA revealed sixteen novel Qrr sRNA target genes. The Qrr sRNAs use unique sets of pairing regions to differentially regulate those targets. Particular sequence differences among the Qrr sRNAs define the target regulation specificity. The contribution to base pairing and to RNA stability conferred by each portion of the Qrr sRNAs was also characterized. Of all of the genes in the quorum-sensing regulon, those that are directly controlled by the Qrr sRNAs are the most rapid to respond during cell density transitions. Lastly, regulation of the type VI secretion system (T6SS) in V. cholerae by the Qrr sRNAs was investigated. We conclude that in V. harveyi and V. cholerae, the Qrr sRNAs together with AphA and LuxR/HapR form the core quorum-sensing regulatory module.