SUBSTITUTION OF 5-METHYLCYTOSINES FOR CYTOSINES ENHANCES THE STABILITY OF TOPOISOMERASE I-DNA COMPLEXES AND MODULATES THE SEQUENCE SELECTIVITY OF CAMPTOTHECIN-INDUCED DNA CLEAVAGE

We have investigated the binding and cleavage of DNA by human topoisom erase I using a 160 bp restriction fragment containing either natural bases or 5-methylcytosine residues in place of cytosines. Experiments were performed in the presence and absence of the antitumour drug camp tothecin which specifically inhibits topoisomerase I. Replacement of a ll cytosines with 5-methylcytosine residues (i) reinforces the enzyme- DNA interaction, (ii) enhances the stability of topoisomerase I-DNA co mplexes and (iii) modulates the sequence selectivity of camptothecin-i nduced DNA cleavage. The methyl group exposed in the major groove of t he double helix is identified as a critical element for the interactio n between topoisomerase I and DNA. (C) 1998 Federation of European Bio chemical Societies.