Characterization of the virA locus of Agrobacterium tumefaciens: a transcriptional regulator and host range determinant.

The virulence (vir) region of Agrobacterium tumefaciens mediates the transfer of a defined segment of plasmid DNA (the T-DNA) into the plant genome. The vir genes are specifically induced by molecules produced by wounded plant cells, and virA is required for this induction. We have determined the nucleotide sequence of virA loci from limited (pTiAg162) and wide (pTiA6) host range tumor-inducing (Ti) plasmids, each of which encodes a single protein of 92,000 daltons. Using antibody directed against the virA gene product, we have localized the VirA protein to the bacterial inner membrane. VirA is homologous to at least four bacterial proteins which play a role in the transcriptional regulation of diverse families of genes. Based on its role in vir gene induction, homology to transcriptional regulators and membrane localization, we propose that VirA acts as an environmental sensor of plant-derived inducer molecules and transmits this information to the level of vir gene expression. The pTiAg162 virA locus was shown to be ineffective at directing vir gene induction, suggesting that this may in part contribute to the narrow host range conferred by this plasmid.