Practice question

T-DNA excision requires site-specific nicking at highly conserved 25-base border repeats. VirD1 acts as accessory factor recognizing border topology, unwinding DNA and positioning the cleavage site for efficient cutting. VirD2 is the真正的 catalytic endonuclease that nicks the bottom strand precisely between the third and fourth bases of right border, remaining covalently attached via tyrosine linkage to the 5' end. This VirD2-T-strand adduct functions as pilot complex guiding transport through VirB/D4 channel into plant nuclei. Without both proteins, single-stranded T-DNA cannot be generated, abolishing transformation.