Role of the Agrobacterium tumefaciens VirD2 protein in T-DNA transfer and integration

VirD2 is one of the key Agrobacterium tumefaciens proteins involved in T-DNA processing and transfer. In addition to its endonuclease domain, VirD2 contains a bipartite C-terminal nuclear localization sequence (NLS) and a conserved region called omega that is important for virulence. Previous results from our laboratory indicated that the C-terminal, bipartite NLS and the omega region are not essential for nuclear uptake of T-DNA, and further suggested that the omega domain may be required for efficient integration of T-DNA into the plant genome. In this study, we took two approaches to investigate the importance of the omega domain in T-DNA integration. Using the first approach, we constructed a T-DNA binary vector containing a promoterless gusA-intron gene just inside the right T-DNA border. The expression of beta-glucuronidase (GUS) activity in plant cells transformed by this T-DNA would indicate that the T-DNA integrated downstream of a plant promoter. Approximately 0.4% of the tobacco cell clusters infected by a wild-type A. tumefaciens strain harboring this vector stained blue with 5-bromo-4-chloro-3-indolyl beta-D-glucuronic acid (X-gluc). However, using an omega-mutant A. tumefaciens strain harboring the same binary vector, we did not detect any blue staining. Using the second approach, we directly demonstrated that more T-DNA is integrated into high-molecular-weight plant DNA after infection of Arabidopsis thaliana cells with a wild-type A. tumefaciens strain than with a strain containing a VirD2 omega deletion/substitution. Taken together, these data indicate that the VirD2 omega domain is important for efficient T-DNA integration. To determine whether the use of the T-DNA right border is altered in those few tumors generated by A. tumefaciens strains harboring the omega mutation, we analyzed DNA extracted from these tumors. Our data indicate that the right border was used to integrate the T-DNA in a similar manner regardless of whether the VirD2 protein encoded by the inciting A. tumefaciens was wild-type or contained an omega mutation. In addition, a mutant VirD2 protein lacking the omega domain was as least as active in cleaving a T-DNA border in vitro as was the wild-type protein. Finally, we investigated the role of various amino acids of the omega and bipartite NLS domains in the targeting of a GUS-VirD2 fusion protein to the nucleus of electroporated tobacco protoplasts. Deletion of the omega domain, or mutation of the 10-amino-acid region between the two components of the bipartite NLS, had little effect upon the nuclear targeting of the GUS-VirD2 fusion protein. Mutation of both components of the NLS reduced, but did not eliminate, targeting of the fusion protein to the nucleus.