Virulence genes A, G , and D mediate the double-stranded border cleavage of T-DNA from the Agrobacterium Ti plasmid

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Abstract
Agrobacterium tumefaciens transfers the T-DNA portion of its Ti plasmid to the nuclear genome of plant cells. Upon cocultivation of A. tumefaciens strain A348 with regenerating tobacco leaf protoplasts, restriction endonuclease fragments of the T-DNA were generated that are consistent with double-stranded cleavage of the T-DNA at the border sequences. The T-DNA border cleavage was also induced by acetosyringone, a compound that induces many of the virulence genes. T-DNA cleavage did not occur in Agrobacterium strains harboring Tn 3 -HoHo1 insertions in the virA, -D , or - G genes. Insertion mutations in virB, -C , or - E did not have any effect on the T-DNA cleavage. Complementation of the mutations in virA, -D , or - G with cosmids containing the respective wild-type genes restored the T-DNA cleavage. Since virA and - G are essential in regulating the expression of other vir genes in response to plant signal molecules, the virD gene product(s) appear to mediate double-stranded T-DNA border cleavage.