Functional Analysis of the VirG-Like Domain Contained in the Agrobacterium VirA Protein That Senses Plant Factors

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The virulence regulon of the Agrobacterium hairy-root-inducing plasmid A4 is controlled by a two-component regulatory system, the VirA-VirG system. The VirA protein senses plant factors and transfers its signal to the transcriptional regulator VirG. Unlike sensor members of other two-component regulatory system, the VirA protein contains at its C-terminal portion and extra domain similar to the N-terminal half of the VirG protein (VirG-like, VGL, domain). Toward understanding of its function, effects by mutations within the VGL domain mutations showed poorly inducible expression of the virulence genes by plant factors, and generated no or reduced tumorigenic symptoms on Kalanchoe leaves and stems. Autophosphorylation of VirA and phosphortransfer reaction, however, was made significantly inefficient by the VGL domain mutations. Therefore, the VGL domain appears to encourage interaction of VirA with VirG for the phosphortransfer process. Molecular mechanisms of signal transduction from VirA to VirG were discussed.