A Chromosomal Virulence Gene (acvB) Product of Agrobacterium tumefaciens That Binds to a T-Strand to Mediate Its Transfer to Host Plant Cells

Abstract

It has been proposed that during tumorigenesis of plants by Agrobacterium tumefaciens, the T-strand is transferred into the host plant cell in the form of a complex consisting of VirD2 and VirE2 proteins. In this paper, we present data supporting the hypothesis that another protein, AcvB (a gene product of a chromosomal virulence gene, acvB^-), also binds to the T-strand in the periplasm to mediate its transfer into the plant cell. An avirulent mutant strain (B119 strain, acvB-) of A. tumefaciens was shown to be impaired in the transfer of T-strands into the cytoplasm of host plant cell by an experiment using a binary Ti plasmid containing aβ-glucuronidase (GUS) reporter gene with an intron. The immunoprecipitation experiment demonstrated the in vitro formation of a complex of single stranded (ss)T-DNA and AcvB. In the periplasm of A. tumefaciens (A208 strain, virulent, acvB⁺) induced with acetosyringone, the T-strand complex containing AcvB and VirE2 proteins was detected immunochemically, while such a complex was not detected with B119 strain. When complexed with AcvB, ssDNA was incorporated into tomato leaf protoplasts with 4.5 times higher efficiency than ssDNA alone. Thus, AcvB protein seems to mediate the transfer of T-complex into the cytoplasm of host plant cell by forming a complex with T-strand DNA.