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Proc Natl Acad Sci U S A. 2009 Jun 16;106(24):9643-8. doi: 10.1073/pnas.0812199106. Epub 2009 May 29.

Agrobacterium tumefaciens VirC2 enhances T-DNA transfer and virulence through its C-terminal ribbon-helix-helix DNA-binding fold.

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Department of Biochemistry, University of Alberta, Edmonton, Alberta, Canada T6G 2H7.


Agrobacterium tumefaciens VirC2 stimulates processing of single-stranded T-DNA that is translocated into plants to induce tumor formation, but how VirC2 functions is unclear. Here, we report the 1.7-A X-ray crystal structure of its trypsin-resistant C-terminal domain, VirC2(82-202), which reveals a form of the ribbon-helix-helix (RHH) DNA-binding fold contained within a single polypeptide chain. DNA-binding assays and mutagenesis indicate that VirC2 uses this RHH fold to bind double-stranded DNA but not single-stranded DNA. Mutations that severely affect VirC2 DNA binding are highly deleterious for both T-DNA transfer into yeast and the virulence of A. tumefaciens in different plants including Nicotiana glauca and Kalanchoe daigremontiana. These data suggest that VirC2 enhances T-DNA transfer and virulence through DNA binding with its RHH fold. The RHH fold of VirC2 is the first crystal structure representing a group of predicted RHH proteins that facilitate endonucleolytic processing of DNA for horizontal gene transfer.

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