VirD proteins of Agrobacterium tumefaciens are required for the formation of a covalent DNA--protein complex at the 5' terminus of T-strand molecules.

The T-DNA transfer process of Agrobacterium tumefaciens is activated by the induction of the Ti plasmid virulence (vir) loci by plant signal molecules such as acetosyringone. Upon initiation of the T-DNA transfer process, site-specific nicks occur at the 25-bp border sequences. This cleavage leads to the generation of a free, linear ssT-DNA molecule which is bound by sequence non-specific VirE proteins. Here we present evidence for the involvement of other acetosyringone-induced proteins in the formation of a covalent complex between the T-strand and protein, designated the T-complex. Alkaline gel-electrophoretic analysis showed that proteins specifically bind to the 5' termini of nicked T-DNA molecules. The T-complex can be formed in Escherichia coli when the VirD1 and VirD2 proteins are expressed.