Interaction of topoisomerase 1 with the transcribed region of the Drosophila HSP 70 heat shock gene.

Topoisomerase I cleavage sites have been mapped in vivo on the Hsp70 heat shock gene of Drosophila melanogaster cells using the drug camptothecin. Topoisomerase I cleavage was only observed when the Hsp70 gene was transcriptionally active. Site-specific single-strand DNA cleavage by topoisomerase I was confined to the transcribed region of the Hsp70 gene and occurred on both the transcribed and nontranscribed DNA strands. A number of the single-strand breaks on the complementary DNA strands occurred in close proximity giving rise to double-stranded DNA breaks. Inhibition of heat-induced Hsp70 transcription by either Actinomycin D (Act D) or 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole (DRB) inhibited topoisomerase I cleavage except at the 5' and to a lesser extent the 3' end of the gene. Camptothecin (100 microM) inhibited transcription of the Hsp70 gene greater than 95%. These results suggest that topoisomerase I is intimately associated with and has an integral part in Hsp70 gene transcription.