Quorum Sensing Regulators AphA and OpaR Control Expression of the Operon Responsible for Biosynthesis of the Compatible Solute Ectoine.

To maintain the turgor pressure of the cell under high osmolarity, bacteria accumulate small organic compounds called compatible solutes, either through uptake or biosynthesis. , a marine halophile and an important human and shellfish pathogen, has to adapt to abiotic stresses such as changing salinity. contains multiple compatible solute biosynthesis and transporter systems, including the operon required for ectoine biosynthesis. Ectoine biosynthesis genes are present in many halotolerant bacteria; however, little is known about the mechanism of regulation. We investigated the role of the quorum sensing master regulators OpaR and AphA in gene regulation. In an deletion mutant, transcriptional reporter assays demonstrated that expression was induced. In an electrophoretic mobility shift assay, we showed that purified OpaR bound to the regulatory region indicating direct regulation by OpaR. In an deletion mutant, expression of the genes was repressed, and purified AphA bound upstream of the genes. These data indicate that AphA is a direct positive regulator. CosR, a Mar-type regulator known to repress expression in , was found to repress expression in In addition, we identified a feed-forward loop in which OpaR is a direct activator of , while AphA is an indirect activator of Regulation of the ectoine biosynthesis pathway via this feed-forward loop allows for precise control of ectoine biosynthesis genes throughout the growth cycle to maximize fitness. Accumulation of compatible solutes within the cell allows bacteria to maintain intracellular turgor pressure and prevent water efflux. ectoine production is widespread among bacteria, and the operon encoding the biosynthetic enzymes is induced by increased salinity. Here, we demonstrate that the quorum sensing regulators AphA and OpaR integrate with the osmotic stress response pathway to control transcription of ectoine biosynthesis genes in We uncovered a feed-forward loop wherein quorum sensing regulators also control transcription of , which encodes a negative regulator of the operon. Moreover, our data suggest that this mechanism may be widespread in species.