Topoisomerase I interaction with SV40 DNA in the presence and absence of camptothecin.

Camptothecin is an antitumor drug, which is a specific inhibitor of eukaryotic topoisomerase I. Enzyme inhibition is related to the stabilization of cleavable complexes between topoisomerase I and DNA. The genomic and DNA sequence localization of L1210 topoisomerase I-mediated DNA breaks produced by camptothecin were determined in the SV40 genome. DNA cleavage was predominantly single-stranded and localized in selective regions of the DNA. A major cleavage site was found at nucleotide 4995 on the coding strand in the early transcription region. The DNA sequence was determined at prominent cleavage sites (nucleotides 127 and 199 in the two 72 bp repeats and nucleotide 4955). A DNA consensus sequence 5'-GATG-3' was found in SV40 DNA. Cleavage occurred between the T and the G and topoisomerase I was linked to the 3'-DNA terminus at the T position. The sequence GATG is more frequent in the non transcribed strand of the early and late transcription of SV40 than in the transcribed strands. This finding is consistent with the role of topoisomerase I in transcription.