THE ROLE OF VIRA AND VIRG PHOSPHORYLATION IN CHEMOTAXIS TOWARDS ACETOSYRINGONE BY AGROBACTERIUM-TUMEFACIENS

The Ti-plasmid-encoded two-component sensor-regulator system comprising VirA and VirG confers upon Agrobacterium tumefaciens the ability to respond chemotactically to nanomolar concentrations of vir-inducing phenolics such as acetosyringone. Non-phosphorylatable, mutant VirA and VirG proteins are incapable of replacing their wild-type counterparts in conferring this phenotype. This indicates that, like vir-gene induction in response to acetosyringone, chemotaxis to the same ligand involves phosphorylation of VirA and VirG. However, unlike vir-induction, deletion of the periplasmic domain of VirA severely curtails acetosyringone chemotaxis, suggesting that acetosyringone may mediate effects through more than one region of VirA. When introduced into strains expressing wild-type VirA and VirG, the non-phosphorylatable versions suppress chemotaxis towards acetosyringone, implying that mutant copies of VirA and VirG compete with their wild-type counterparts in interactions between VirA and VirG.