Mutants of the Agrobacterium tumefaciens virA Gene Exhibiting Acetosyringone-Independent Expression of the vir Regulon. Robert G. Ankenbauer. Department of Microbiology, SC-42, University of Washington, Seattle, Washington 98195 U.S.A. Elaine A. Best, Corinna A. Palanca, and Eugene W. Nester. Department of Microbiology, SC-42, University of Washington, Seattle, Washington 98195 U.S.A. MPMI 4:400-406. Accepted 29 April 1991. Copyright 1991 The American Phytopathological Society.

Hydroxylamine-induced mutations in the virA gene of Agro-bacterium tumefaciens that do not require the plant phenolic-inducing compound acetosyringone for vir regulon induction were isolated. The isolation was based on the activation of both virB::lacz and virE::cat fusions by mutant virA loci in the absence of acetosyringone. Three of these virA(Ais) (alphacetosyringone-independent signaling) mutants were characterized. All three mutants expressed a virB::lacz fusion at high levels in the absence of acetosyringone. One virA(Ais) mutant, virA112, exhibited vir gene expression in the absence of inducing monosaccharides and acidic growth conditions, both of which are normally required for vir gene induction. The phenotype of the virA112 mutant resulted from a glycine to glutamic acid change near His-474, the site of VirA autophosphorylation.