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 plasmid RSF1010 into plant cells, and the conjugal transfer of RSF1010 between Agrobacteria. We show in this report that the Agrobacterium-to-Agrobacterium conjugal transfer efficiencies of RSF1010 increase dramatically if the recipient strain, as well as the donor strain, carries a wild-type Ti plasmid and is capable of vir gene expression. Investigation of possible mechanisms that could account for this increased efficiency revealed that the VirB proteins encoded by the Ti plasmid were required. Although, with the exception of VirB1, all of the proteins that form the putative T-DNA transfer apparatus (VirB1–11, VirD4) are required for an Agrobacterium strain to serve as an RSF1010 donor, expression 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 affected donor and recipient capacities to the same relative extent, suggesting that similar functions of VirB9 are important in both of these contexts.

Adaptive reversion of an episomal frameshift mutation in Escherichia coli requires conjugal functions but not actual conjugation.

Adaptive reversion of a lac- frameshift mutation in Escherichia coli appears to be due to DNA polymerase errors, implying that DNA is being synthesized although the cells are not dividing. Here we report that the production of adaptive lac+ revertants (i) is much higher when the mutational target is on the F' episome than when it is on the bacterial chromosome; (ii) is enhanced by functions required for conjugation; but (iii) does not require conjugation per se. These results suggest that, in static cells, DNA synthesis is initiated from the conjugal origin of transfer. Mutations may arise as polymerase errors during this synthesis or during synthesis stimulated by recombination among the multiple gene copies.