An Agrobacterium virulence factor encoded by a Ti plasmid gene or a chromosomal gene is required for T-DNA transfer into plants.

Mutagenesis of the vir region on the Ti plasmid of Agrobacterium tumefaciens revealed a new locus, virJ, that is induced by the plant-wound signal molecule, acetosyringone (AS). virJ lies between virA and virB, and is transcribed in the same direction. The amino acid sequence of virJ is similar to a region of a previously characterized chromosomal gene, acvB, required for virulence. virJ can complement the avirulent phenotype of an acvB mutant, indicating that virJ and acvB encode the same factor required for tumorigenesis. Southern analysis revealed that virJ is present on the Ti plasmid of an octopine but not a nopaline strain whereas acvB is present on the chromosomes of both octopine and nopaline strains. While virJ is regulated by AS under the control of the virA/virG two-component regulatory system, acvB is not induced by AS. VirJ possesses a putative signal peptide and was found predominantly in the periplasmic fraction. The strain lacking both acvB and virJ had an impaired ability to transfer T-DNA into plant cells, suggesting that the factor encoded by virJ or acvB is required for T-DNA transfer from A. tumefaciens to plant cells. acvB is the first chromosomal gene implicated in T-DNA transfer, but whether it functions specifically for this process is not clear. We hypothesize that virJ evolved from acvB, presumably for a more specialized role in tumorigenesis.