Copyright © 1998, American Society for Microbiology Role of Agrobacterium virB Genes in Transfer of T Complexes

Nonpolar virB mutants of Agrobacterium tumefaciens were tested for RSF1010 mobilization and extracellular complementation. virB2 to virB11 were essential for transfer in both assays. virB1 was essential only for high frequency transfer of RSF1010 and VirE2. Coordinated transfer of a preassembled T complex is supported by these data and competition studies. During infection of dicotyledonous plants, Agrobacterium tumefaciens transfers a segment of its Ti plasmid, the trans-ferred DNA (T-DNA), into the plant cell. The transfer inter-mediate, which is termed the T strand, is composed of the single-stranded T-DNA covalently attached to the protein VirD2 (23). At least 7 of the 11 genes of the virB operon and virD4 are essential for transfer of T strands (5, 15, 16, 27). These genes, along with virE1, are also essential for transfer of the virE2-encoded single-stranded binding protein into plant cells (5, 8, 15, 25). VirE2 is essential for tumorigenesis because it coats the T strand, protecting it from degradation and as