FIG 1 .

Model of quorum-sensing systems in V. fischeri. When the levels of octanoyl homoserine lactone (C8; octagons) and AI-2 (pentagons) are low, AinR and the LuxP/Q complex phosphorylate LuxU. Phosphorylated LuxU, in turn, phosphorylates LuxO, which activates the expression of the small RNA (sRNA) Qrr1. Qrr1 destabilizes the mRNA encoding LitR. Conversely, when the level of C8 and AI-2 autoinducers is high, the signaling cascade is inactivated, resulting in stable LitR production. LitR transcriptionally activates luxR, which encodes the transcriptional regulator LuxR. In the presence of the accumulating autoinducer 3-oxo-hexanoyl homoserine lactone (3-oxo-C6; hexagons), LuxR activates several genes, including the luxCDABEG locus, which is responsible for luciferase production and activity. The dashed arrows indicate phosphoryl group transfers. The question mark indicates an unknown number of steps.