Figure 2. Quorum sensing circuits in V. cholerae. At low cell density, the membrane proteins CqsS and LuxQ act as kinases to initiate transfer of phosphate through LuxU to LuxO. LuxO-P, with σ⁵⁴ of RNA polymerase, serve as a transcription factor to produce qrr1–4. Qrr1–4 and Hfq inhibit HapR. At high cell density,^21-26 CqsS and LuxO, through the binding of AI molecules, act as phosphatases to reverse the flow of phosphate through the regulatory circuit. LuxO becomes deactivated, qrr1–4 expression becomes repressed, and HapR is expressed.^22,27-29