GENETIC-EVIDENCE FOR DIRECT SENSING OF PHENOLIC-COMPOUNDS BY THE VIRAPROTEIN OF AGROBACTERIUM-TUMEFACIENS

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, However, it is not clear how the phenolic compounds are sensed by the VirA/VirG system, We tested the vir-inducing abilities of 15 diffe rent phenolic compounds using four wildtype strains of A. tumefaciens- KU12, C58, A6, and Bo542, We analyzed the relationship between structu res of the phenolic compounds and levels of vir gene expression in the se strains. In strain KU12, vir genes were not induced by phenolic com pounds containing 4'-hydroxy, 3'-methoxy, and 5'-methoxy groups, such as acetosyringone, which strongly induced vir genes of the other three strains, On the other hand, vir genes of strain KU12 were induced by phenolic compounds containing only a 4'-hydroxy group, such as 4-hydro xyacetophenone, which did not induce vir genes of the other three stra ins. The vir genes of strains KU12, A6, and Bo542 were all induced by phenolic compounds containing 4'-hydroxy and 3'-methoxy groups, such a s acetovanillone. By transferring different Ti plasmids into isogenic chromosomal backgrounds, we showed that the phenolic-sensing determina nt is associated with Ti plasmid, Subcloning of Ti plasmid indicates t hat the virA locus determines which phenolic compounds can function as vir gene inducers, These results suggest that the VirA protein direct ly senses the phenolic compounds for vir gene activation.