Genetic evidence for an interaction between the VirA sensor protein and the ChvE sugar-binding protein of Agrobacterium.

Most vir genes of Agrobacterium, which are required for tumorigenicity of the bacterium, are expressed in response to plant phenolics. The induction of vir is markedly enhanced by specific monosaccharides. Signals generated by both types of compound are transduced into Agrobacterium cells via the functions of the VirA membrane-bound sensor protein. A putative sugar-binding protein, known as ChvE, also functions at a step of the enhancement of vir induction by monosaccharides. To investigate the signal pathway of the enhancement by the sugars, we first isolated a virA mutant of Agrobacterium with a base substitution mutation that caused a single amino acid change in the periplasmic domain. The mutant exhibited no enhancement of vir induction by sugar and had severely attenuated tumorigenicity on Kalanchoe leaves. We then isolated two chvE mutants that restored sugar enhancement on the background of this virA mutation. One chvE mutant, which exhibited a higher level of sugar enhancement, restored the tumorigenicity of the virA mutant. Wild-type and suppressor ChvE proteins were localized in the periplasmic space. These results provide genetic evidence for the physical interaction between VirA and ChvE proteins in the periplasmic space of Agrobacterium, which enhances the cytoplasmic signal generated by phenolics. We also discuss the molecular architecture of the operon to which the chvE gene belongs.