Gilmour D S, Elgin S C
NCI Monogr. 1987(4):17-21.
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.
果蝇多线染色体的免疫荧光染色显示，拓扑异构酶I在转录活跃区域高度富集。一种光交联技术证明，在活跃的热休克基因转录区域存在拓扑异构酶I。喜树碱可稳定在扭转应变DNA松弛过程中形成的拓扑异构酶I-DNA共价中间体。通过绘制所得DNA切口的位置，发现拓扑异构酶I在热休克后与转录活跃基因hsp23、hsp26和hsp28相互作用，但在热休克前不与非活跃基因相互作用。这种相互作用主要发生在转录区域内，在DNA的转录链和非转录链上均观察到特定位点。与非转录侧翼序列几乎没有相互作用。喜树碱在热休克期间仅部分抑制hsp28基因的转录，导致转录本水平降低，但转录本仍为全长。结果表明，在活跃基因座存在一组动态的相互作用。
