Figure 3: Quorum sensing circuit in V. cholera. Two kinds of AIs, CAI-1 (synthesized by CqsA) and AI-2 (produced by LuxS), which are detected by their respective receptors, CqsS and LuxPQ, are found in V. cholerae. In the absence of AIs at a low cell density, the receptors function as kinases leading to phosphorylation of LuxO via LuxU and activation of transcription of qrr1-4 genes. The sRNAs, Qrr1-4 together with Hfq, influence the stability of mRNA available for LuxR translation. At a high cell population, AIs bind the receptors inhibiting their kinase activity and converting them to phosphatases. The phosphate group is removed from LuxO, thereby the qrr gene expression is stopped and hapR expression is increased to initiate or repress the target gene expression.