MAMMALIAN TOPOISOMERASE-I HAS BASE MISMATCH NICKING ACTIVITY

The all-type nicking enzyme (ATE) from human HeLa cells or calf thymus can nick DNA at the first phosphodiester bond 5' to all 8 possible mi smatched bases. The strand disparity of this nicking is influenced by the neighboring nucleotide sequences. After nicking, the ATE covalentl y binds to the 3' end of the DNA product to form a cleavable complex, whose formation is insensitive to camptothecin, a specific inhibitor o f eukaryotic topoisomerase I (Topo-I). During the purification of ATE from calf thymus, a Mg2+-independent relaxation activity, characterist ic of eukaryotic Topo-I, copurifies with the mismatch-nicking activity . The ATE from calf thymus may be a breakdown product of Topo-I. N-ter minal amino acid analysis indicates that one of the polypeptides with ATE activity contains the C-terminal portion of Topo-I. Moreover, acti ve human Topo-I, expressed as a fusion protein in Escherichia coli, is also capable of nicking all 8 base mispairs in the absence of Mg2+. T his mismatch-specific nicking activity may be a novel property of the mammalian Topo-I.