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 structur e appeared to correctly target the second cleavage event in the complementa ry strand, resulting in a blunt-ended double-strand break, similar to that Induced directly Gy bleomycin alone in an intact duplex of the same sequenc e. A one-base-gapped structure was markedly Less efficient in correctly tar geting bleomycin attack in the complementary strand. The results are consis tent with a model of bleomycin-induced double-strand cleavage in which the nick formed by the initial bleomycin attack serves to target secondary atta ck to a specific position in the complementary strand, resulting in a doubl e-strand break with a defined geometry.