Virulence Genes Promote Conjugative Transfer of the Ti Plasmid between Agrobacterium Strains

Certain virulence region operons of the Agrobacterium tumefaciens Ti plasmid promoted conjugative Ti plasmid transfer. Mutations in the vir region of pTiC58 inhibited conjugative plasmid transfer between A. tumefaciens strains. Mutations in virA, virG, 5 ' virB, and virE had the greatest effect on plasmid transfer, and mutations in virC had no effect. Transfer inhibition in vir mutants occurred in the presence or absence of acetosyringone. Agrobacterium tumefaciens causes the disease crown gall on a variety of plants (recently reviewed in references 25 and 37). Essential for disease formation are the T-DNA (trans-ferred DNA) and virulence (vir) regions located on the Ti plasmid harbored in this plant pathogen. Genes of the vir region encode proteins that recognize inducing compounds from wounded plant cells, stimulate expression of vir genes, affect host range, and control the processing and transfer of the T-DNA from the bacterial cell to the plant cell. Once transferred, the T-DNA is integrated into the plant genome