THE SENSING OF PLANT SIGNAL MOLECULES BY AGROBACTERIUM - GENETIC-EVIDENCE FOR DIRECT RECOGNITION OF PHENOLIC INDUCERS BY THE VIRA PROTEIN

The virulence (vir) genes of Agrobacterium tumefaciens are induced by low-molecular-weight phenolic compounds and monosaccharides through a two-component regulatory system consisting of the VirA and VirG protei ns. Although it is clear that the monosaccharides require binding to a periplasmic binding protein before they can interact with the sensor VirA protein, it is not certain whether the phenolic compounds also in teract with a binding protein or directly interact with the sensor pro tein. To shed light on this question, we tested the vir-inducing abili ties of several different phenolic compounds using two wild-type strai ns of A. tumefaciens, KU12 and A6. We found that several compounds suc h as 4-hydroxyacetophone and p-coumaric acid induced the vir of KU12, but not A6. On the other hand, acetosyringone and several other phenol ic compounds induced the vir of A6, but not KU12. By transferring diff erent Ti plasmids into isogenic chromosomal backgrounds, we showed tha t the phenolic sensing determinant is associated with the Ti plasmid. Subcloning of the Ti plasmid indicated that the virA locus determines which phenolic compounds can function as vir inducers. These results s uggest that VirA directly senses the phenolic compounds for vir activa tion.