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 i nduction of vir is markedly enhanced by specific monosaccharides. Sign als generated by both types of compound are transduced into Agrobacter ium 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 invest igate the signal pathway of the enhancement by the sugars, we first is olated a virA mutant of Agrobacterium with a base substitution mutatio n that caused a single amino acid change in the periplasmic domain. Th e mutant exhibited no enhancement of vir induction by sugar and had se verely 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 o f sugar enhancement, restored the tumorigenicity of the virA mutant. W ild-type and suppressor ChvE proteins were localized in the periplasmi c space. These results provide genetic evidence for the physical inter action between VirA and ChvE proteins in the periplasmic space of Agro bacterium, which enhances the cytoplasmic signal generated by phenolic s. We also discuss the molecular architecture of the operon to which t he chvE gene belongs.