ACTIVITY OF THE QUORUM-SENSING REGULATOR TRAR OF AGROBACTERIUM-TUMEFACIENS IS INHIBITED BY A TRUNCATED, DOMINANT DEFECTIVE TRAR-LIKE PROTEIN

Horizontal transfer of Agrobacterium tumefaciens tumour-inducing plasm ids requires opines, which are released from plant tumours as nutrient s for the bacteria. The opine octopine causes synthesis of the quorum- sensing TraR protein, which activates several tra promoters in the pre sence of a pheromone called Agrobacterium autoinducer (AAI). A gene, t raS, was previously found on the same Ti plasmid in an operon that dir ects the uptake of mannopine, another opine. TraS strongly resembles T raR but lacks a DNA-binding module. TraS did not activate a TraR-depen dent promoter and blocked TraR function, probably by forming inactive heteromultimers. Expression of traS was induced by mannopine, although this induction was strongly inhibited by the favoured catabolites suc cinate, glutamine and tryptone. Mannopine inhibited conjugation in a T raS-dependent fashion, and artificial overexpression of TraS also inhi bited conjugation. Favoured catabolites restored tra gene expression i n wild-type strains but not in strains that overexpress TraS. Downstre am of traS is a gene encoding a truncated, defective chemoreceptor who se expression abolished chemotaxis.