A LuxR-LuxI type regulatory system activates Agrobacterium Ti plasmid conjugal transfer in the presence of a plant tumor metabolite.

Conjugal transfer of Agrobacterium octopine-type Ti plasmids is activated by octopine, a metabolite released from plant tumors. Octopine causes conjugal donors to secrete a pheromone, Agrobacterium autoinducer (AAI), and exogenous AAI further stimulates conjugation. The putative AAI synthase and an AAI-responsive transcriptional regulator were found to be encoded by the Ti plasmid traI and traR genes, respectively, and the expression of traR was induced by octopine. The octopine-type traR gene product is highly homologous to the TraR protein recently characterized from a nopaline-type Ti plasmid. TraR and TraI are homologous to the LuxR and LuxI regulatory proteins of Vibrio fischeri, and AAI is similar in structure to the diffusable V. fischeri autoinducer, the inducing ligand of LuxR. TraR activated target genes in the presence of AAI and also activated traR and traI themselves, creating two positive-feedback loops. TraR-AAI-mediated activation in wild-type Agrobacterium strains was dramatically enhanced by culturing on solid media, suggesting a possible role in cell density sensing.