FIG 1.

Model of the V. harveyi quorum sensing system. The autoinducer synthases LuxM, CqsS, and LuxS produce AI-1, CAI-1, and AI-2, respectively. At LCD, autoinducers are at low concentrations in the external environment. LuxO∼P activates qrr gene expression. The Qrr sRNAs activate the expression of AphA and repress the expression of LuxR, LuxM, and LuxO. High levels of AphA and low levels of LuxR together regulate individual behavior genes. AphA autorepresses its expression and feeds back to repress the expression of the qrr genes and luxR. LuxR autorepresses its expression, represses aphA, and activates the qrr genes. At HCD, autoinducer concentrations are high, and AI-1, CAI-1, and AI-2 bind to the LuxN, CqsS, and LuxPQ receptors, respectively. The receptors dephosphorylate LuxU. Thus, LuxU does not phosphorylate LuxO, and the Qrrs are not expressed. LuxR is expressed at high levels, and AphA is not expressed. LuxR regulates group behavior genes. LuxR autorepresses its own expression and feeds back to repress aphA transcription and to activate qrr transcription.