Action of bleomycin on structural mimics of intermediates in DNA double-strand cleavage

Bleomycin-induced cleavage was examined in several nicked, gapped, or intact duplex DNA substrates, including a structure designed to mimic a proposed singly nicked intermediate in double-strand cleavage. This nicked structure appeared to correctly target the second cleavage event in the complementary strand, resulting in a blunt-ended double-strand break, similar to that induced directly by bleomycin alone in an intact duplex of the same sequence. A one-base-gapped structure was markedly less efficient in correctly targeting bleomycin attack in the complementary strand. The results are consistent with a model of bleomycin-induced double-strand cleavage in which the nick formed by the initial bleomycin attack serves to target secondary attack to a specific position in the complementary strand, resulting in a double-strand break with a defined geometry.