Single-Stranded DNA Binding Protein Encoded by the virE Locus of Agrobacterium tumefaciens

The transfer process of T (transfer)-DNA of Agrobacterium tumefaciens is activated after the induction of the expression of the Ti plasmid virulence (vir) loci by plant signal molecules such as acetosyringone. The vir gene products then act to generate a free transferable single-stranded copy of the T-DNA, designated the T-strand. Although some vir proteins are responsible for the synthesis of the T-strand, others may mediate T-strand transfer to plant cells as part of a DNA-protein complex. Here, a novel 69-kilodalton vir-specific single-stranded DNA binding protein is identified in Agrobacterium harboring a nopaline-type Ti plasmid. This protein binds single-stranded but not double-stranded DNA regardless of nucleotide sequence composition. The molecular size of the vir-specific single-stranded DNA binding protein and its relative abundance in acetosyringone-induced Agrobacterium suggested that it might be the product of the virE locus; molecular cloning and expression of the virE region in Escherichia coli confirmed this prediction.