THE PHENOLIC RECOGNITION PROFILES OF THE AGROBACTERIUM-TUMEFACIENS VIRA PROTEIN ARE BROADENED BY A HIGH-LEVEL OF THE SUGAR BINDING-PROTEIN CHVE

The formation of crown gall tumors by Agrobacterium tumefaciens requir es that the virulence (vir) genes be induced by chemical signals which consist of specific phenolic compounds and monosaccharides, synthesiz ed at plant wound sites. Signal transduction in the activation of thes e genes is mediated by the VirA-VirG two-component regulatory system, together with ChvE, a glucose-galactose binding protein which interact s with VirA. We have previously presented genetic evidence that virA s enses phenolic compounds directly (Y.-W. Lee, S. Jin, W.-S. Sim, and E . W. Nester, Proc. NatI. Acad. Sci. USA 92:12245-12249, 1995). The vir genes of strain KU12 can be induced by 4-hydroxyacetophenone, p-couma ric acid, and phenol, whereas these same phenolic compounds are weak i nducers of the vir genes of strain A6. In this report, we show that a specific inducing sugar can broaden the specificity of the phenolic co mpound which VirA senses. 4-Hydroxyacetophenone and other related phen olic compounds function as inducing phenolic compounds with the virA g ene of A6 if arabinose replaces glucose as the inducing sugar. We furt her demonstrate that this broadened specificity for phenolic inducers results from the increased level of ChvE through induction by arabinos e via the regulatory protein GbpR. If high levels of ChvE are present, then poorly inducing phenolic compounds can induce the vir genes to h igh levels in combination with glucose. Comparing the induction respon se of the wild type and that of a VirA mutant with a mutation in its r eceiver domain revealed that the activity of the receiver domain is co ntrolled by the periplasmic domain. We discuss these observations in t erms of how VirA senses and transduces signals elicited by the two cla sses of plant signal molecules.