Agrobacterium tumefaciens strain C58 can transform plant cells to produce and secrete the sugar-phosphate conjugate opines agrocinopines A and B. The bacterium then moves in response to the opines and utilizes them as exclusive sources of carbon, energy, and phosphate via the functions encoded by the acc operon. These privileged opine-involved activities contribute to the formation of agrobacterial niches in the environment. We found that the expression of the acc operon is induced by agrocinopines and also by limitation of phosphate. The main promoter is present in front of the first gene, accR, which codes for a repressor. This operon structure enables efficient repression when opine levels are low. The promoter contains two putative operators, one overlapping the ؊10 sequence and the other in the further upstream from it; two partly overlapped putative pho boxes between the two operators; and two consecutive transcription start sites. DNA fragments containing either of the operators bound purified repressor AccR in the absence of agrocinopines but not in the presence of the opines, demonstrating the on-off switch of the promoter. Induction of the acc operon can occur under low-phosphate conditions in the absence of agrocinopines and further increases when the opines also are present. Such opine-phosphate dual regulatory system of the operon may ensure maximum utilization of agrocinopines when available and thereby increase the chances of agrobacterial survival in the highly competitive environment with limited general food sources.During agrobacterial infection of susceptible plants, a copy of the T region in a tumor-inducing plasmid (Ti plasmid), called the T-DNA, is transferred from the bacterium to the plant where it becomes integrated into the chromosome (10)(11)(12)25). Expression of the genes on the integrated T-DNA results in the tumorous phenotype and also the production of unusual low-molecular-weight carbon compounds that belong to a large group of plant tumor-specific metabolites collectively called opines (15,20,58,66).Crown gall tumors induced by the classic nopaline-type Agrobacterium tumefaciens strain C58 synthesize and secrete two families of tumor metabolites: agrocinopines A and B and nopaline (20,43). The genes responsible for the biosynthesis of agrocinopines and of nopaline are carried on the T region of pTiC58, harbored by the strain C58 (7,14,29). Strain C58 can catabolize nopaline and agrocinopines by using functions encoded by two different sets of genes located on the nontransferred region of the Ti plasmid (6, 27, 34, 76). These loci, called acc (agrocinopine catabolism) and noc (nopaline catabolism), code for the transport and catabolism of, as well as chemotaxis to, their cognate opines (34,35,51,76).The process by which A. tumefaciens transforms plant cells to produce opines has been termed genetic colonization of the plant (60). The genetic colonization theory leads to the opine concept, proposed by Tempé et al. (68), which states that the opines are the chemical mediators of t...