MUTANTS OF AGROBACTERIUM VIRA THAT ACTIVATE VIR GENE-EXPRESSION IN THE ABSENCE OF THE INDUCER ACETOSYRINGONE

In the presence of inducer molecules produced by wounded plants, the V irA/VirG two-component positive regulatory system of Agrobacterium tum efaciens initiates transcription of virulence genes required for crown gall tumor formation. Exactly how this system enables the bacterium t o respond to an environmental signal is not known, but phosphorylation of VirA and VirG plays a role. To analyze further the function of Vir A, we chemically mutagenized the virA gene. Two mutants that activate vir transcription without the plant inducer acetosyringone were found; these mutants alter VirA function by distinct mechanisms. One mutant functions entirely independently of acetosyringone, whereas the activi ty of the second mutant is enhanced by acetosyringone. Both mutants fu nction best at acid pH, but respond differently to specific monosaccha rides that stimulate induction by wild-type VirA. Both mutant phenotyp es are dominant over wild-type VirA, and both need the conserved histi dine at the autophosphorylation site for strong inducer-independent vi r transcription.