The Vibrio harveyi quorum-sensing system uses shared regulatory components to discriminate between multiple autoinducers

The quorum-sensing bacterium Vibrio harveyi produces and responds to three autoinducers (AIs), and this sensory information converges to control the expression of bioluminescence, biofilm formation, type III secretion (TTS), and protease production. The AIs are detected by cognate sensor histidine kinases that all relay phosphate to the shared response regulator LuxO. LuxO indirectly represses the master regulator of quorum sensing, LuxR, through the activation of multiple genes encoding small regulatory RNAs (called qrr genes for Quorum Regulatory RNA). Here we use differential fluorescence induction to identify 50 quorum-sensing-controlled promoters. Some promoters only showed significant responses in the simultaneous presence of all three AIs, while others displayed substantial responses to the individual AIs. A differential response to each AI input state was also observed for qrr and luxR expression and LuxR protein production. Individual cell analyses revealed that, in each case, all the bacteria in the population respond in unison to the various AI inputs. We propose that the V. harveyi quorum-sensing transition is not switch-like but rather operates in a graded manner, and that this signaling arrangement, which uses shared regulatory proteins, nonetheless provides V. harveyi a mechanism to respond uniquely to different AI input states.