FIG. 5.

Model for the regulation of quorum sensing in V. harveyi. (A) Genetic analysis (18) suggests that under conditions of low cell density, in the absence of autoinducers, the two hybrid sensor kinases LuxN and LuxQ autophosphorylate at conserved histidine residues (H1). Intramolecular phosphotransfer from the sensor kinase domains to conserved aspartate residues (D1) in the response regulator domains of the hybrid proteins occurs next. Intermolecular transfer from D1 of both LuxN and LuxQ to His 58 (H2) of the phosphorelay protein LuxU occurs, with subsequent phosphotransfer to the conserved Asp 47 residue (D2) of the response regulator protein LuxO. Our evidence suggests that phosphorylation of LuxO activates its repressor function, luxCDABEGH transcription is repressed, and no light is produced. (B) Under conditions of high cell density, the presence of autoinducers stimulates the LuxN and LuxQ sensors to switch from kinases to phosphatases. This switch ultimately results in dephosphorylation of LuxO and inactivation of its repressor function. Dephosphorylation could occur by several different mechanisms, and these terminal reactions have not been characterized. Inactivation of LuxO allows transcription of the luxCDABEGH operon and light production. Outer membrane and inner membrane are abbreviated OM and IM, respectively. HTH, helix-turn-helix.