Characterization of a virG mutation that confers constitutive virulence gene expression in Agrobacterium

Transformation of plants by Agrobacterium tumefaciens is mediated by a set of virulence (vir) genes that are specifically induced by plant signal molecules through the VirA/VirG two-component regulatory system. The plant signal is transmitted from VirA to VirG by a cascade of phosphorylation reactions followed by the sequence-specific DNA binding of the VirG protein to the vir gene promoters which then activates their transcription. In this report, we describe a VirG mutant which is able to activate vir gene expression independently of the VirA molecule and the two plant signal molecules, acetosyringone and monosaccharides. A strain of Agrobacterium containing this virG gene but lacking a functional virA gene was able to induce tumours on all three plants that were tested. A single amino acid change of asparagine (N) to aspartate (D) at position 54, adjacent to the site of VirG phosphorylation, aspartate 52, resulted in this constitutive phenotype. In vitro phosphorylation experiments showed that the mutant protein cannot be phosphorylated by VirA, suggesting that the negative charge resulting from the N to D switch mimics the phosphorylated conformation of the VirG molecule. The same amino acid change in the virG gene of the supervirulent strain A281 also resulted in a constitutive phenotype. However, the vir genes were not induced to high levels when compared with the levels of the constitutive virG of strain A348.