Association of topoisomerase I with transcriptionally active loci in Drosophila.

Immunofluorescence staining of the polytene chromosomes of Drosophila shows high levels of topoisomerase I associated with transcriptionally active regions. A photocrosslinking technique demonstrates the presence of topoisomerase I in the region of transcription of the active heat-shock genes. Camptothecin stabilizes the topoisomerase I-DNA covalent intermediate that forms during the relaxation of torsionally strained DNA. By mapping the position of the resultant DNA nicks, topoisomerase I is found to interact with the transcriptionally active genes hsp23, hsp26, and hsp28 after heat shock but not with the inactive genes prior to heat shock. The interaction occurs predominantly within the transcribed region, with specific sites observed on both the transcribed and nontranscribed strands of the DNA. Little interaction is seen with nontranscribed flanking sequences. Camptothecin only partially inhibits transcription of the hsp28 gene during heat shock, causing a reduced level of transcripts which are nonetheless full length. The results point to a dynamic set of interactions at the active locus.