THE TI PLASMID INCREASES THE EFFICIENCY OF AGROBACTERIUM-TUMEFACIENS AS A RECIPIENT IN VIRB-MEDIATED CONJUGAL TRANSFER OF AN INCQ PLASMID

The T-DNA transfer apparatus of Agrobacterium tumefaciens mediates the delivery of the T-DNA into plant cells, the transfer of the IncQ plas mid RSF1010 into plant cells, and the conjugal transfer of RSF1010 bet ween Agrobacteria, We show in this report that the Agrobacterium-to-Ag robacterium conjugal transfer efficiencies of RSF1010 increase dramati cally if the recipient strain, as well as the donor strain, carries a wild-type Ti plasmid and is capable of vir gene expression. Investigat ion of possible mechanisms that could account for this increased effic iency revealed that the VirB proteins encoded by the Ti plasmid were r equired. Although, with the exception of VirB1, all of the proteins th at form the putative T-DNA transfer apparatus (VirB1-11, VirD4) are re quired for an Agrobacterium strain to serve as an RSF1010 donor, expre ssion of only a subset of these proteins is required for the increase in conjugal transfer mediated by the recipient. Specifically, VirB5, 6 , 11, and VirD4 are essential donor components but are dispensable for the increased recipient capacity. Defined point mutations in virB9 af fected donor and recipient capacities to the same relative extent, sug gesting that similar functions of VirB9 are important in both of these contexts.