Bg. Mclean et al., MUTANTS OF AGROBACTERIUM VIRA THAT ACTIVATE VIR GENE-EXPRESSION IN THE ABSENCE OF THE INDUCER ACETOSYRINGONE, The Journal of biological chemistry, 269(4), 1994, pp. 2645-2651
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.