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 VirA/VirG two-component positive regulatory system of Agrobacterium tumefaciens initiates transcription of virulence genes required for crown gall tumor formation. Exactly how this system enables the bacterium to respond to an environmental signal is not known, but phosphorylation of VirA and VirG plays a role. To analyze further the function of VirA, 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 activity of the second mutant is enhanced by acetosyringone. Both mutants function best at acid pH, but respond differently to specific monosaccharides that stimulate induction by wild-type VirA. Both mutant phenotypes are dominant over wild-type VirA, and both need the conserved histidine at the autophosphorylation site for strong inducer-independent vir transcription.