Drug-induced Stabilization of Covalent DNA Topoisomerase I-DNA Intermediates: DNA Cleavage Assays

Eukaryotic DNA topoisomerase I is a highly conserved enzyme that catalyzes the relaxation of positively and negatively supercoiled DNA (1 –4 ). The enzyme binds duplex DNA and transiently cleaves a single DNA strand. This is accompanied by the formation of a phospho-tyrosyl linkage between the active site tyrosine and the 3′-phosphate of the cleaved strand. The presence of this protein-linked nick in the DNA presumably allows the rotation of the nicked DNA end around its complementary strand to effect changes in DNA linking number. A second transesterification reaction religates the nicked DNA and restores the active site tyrosine.