FIG. 1.

Quorum-sensing model in P. fischeri. At low cell densities, transcription of the luxICDABE operon occurs at basal levels. LuxI encodes the AHL synthase, which synthesizes 3-oxo-C₆-HSL from acyl-ACP and SAM substrates. High cell densities lead to the accumulation of AHLs, which bind and activate the LuxR transcriptional activator. LuxR binds to an inverted repeat referred to as the lux box, which is centered at −42.5 from the transcriptional start site, and makes contact with the RNA polymerase to stimulate the expression of the luxICDABE genes. Expression of the luxR gene is regulated by several factors such as heat shock, catabolite repression, and even LuxR itself (only at very high cell densities).