INTEGRATION OF COMPLETE TRANSFERRED DNA UNITS IS DEPENDENT ON THE ACTIVITY OF VIRULENCE E2 PROTEIN OF AGROBACTERIUM-TUMEFACIENS

Agrobacterium tumefaciens transfers transferred DNA (T-DNA), a single- stranded segment of its tumor-inducing (Ti) plasmid, to the plant cell nucleus. The Ti-plasmid-encoded virulence E2 (VirE2) protein expresse d in the bacterium has single-stranded DNA (ssDNA)-binding properties and has been reported to act in the plant cell. This protein is though t to exert its influence on transfer efficiency by coating and accompa nying the single-stranded T-DNA (ss-T-DNA) to the plant cell genome. H ere, we analyze different putative roles of the VirE2 protein in the p lant fell. In the absence of VirE2 protein, mainly truncated versions of the T-DNA are integrated. We infer that VirE2 protects the ss-T-DNA against nucleolytic attack during the transfer process and that it is interacting with the ss-T-DNA on its way to the plant cell nucleus, F urthermore, the VirE2 protein was found not to be involved in directin g the ss-T-DNA to the plant cell nucleus in a manner dependent on a nu clear localization signal, a function which is carried by the NLS of V irD2. In addition, the efficiency of T-DNA integration into the plant genome was found to be VirE2 independent, We conclude that the VirE2 p rotein of A. tumefaciens is required to preserve the integrity of the T-DNA but does not contribute to the efficiency of the integration ste p per se.