Quorum sensing circuits of <i>Chromobacterium violaceum</i> (A) and <i>Vibrio harveyi</i> (B).

<p>In <i>C</i>. <i>violaceum</i> ATCC 31532 (A), the synthase CviI produces the AI molecule HHL that is recognized by the cytoplasmatic receptor CviR. When bound to HHL, CviR dimerizes and binds DNA, leading to the expression of QS-regulated genes, including those involved in violacein production. In <i>V</i>. <i>harveyi</i> (B), three different AIs, synthesized by LuxM, LuxS and CqsA are recognized by the transmembrane two-component receptors LuxN, LuxPQ and CqsS, respectively. At low AI concentrations, these receptors act as kinases, phosphorylating LuxU and subsequently the σ⁵⁴-dependent response regulator LuxO. The phosphorylated LuxO activate the transcription of Qrr sRNAs that together with the chaperone Hfq, destabilize the mluxR RNA. At high AI concentrations, the receptors switch to phosphatases and the expression of the master regulator LuxR is allowed.