Zhu J, Schiestl R H
Department of Molecular and Cellular Toxicology, Harvard School of Public Health, Boston, Massachusetts 02115, USA.
Mol Cell Biol. 1996 Apr;16(4):1805-12. doi: 10.1128/MCB.16.4.1805.
Chromosome aberrations may cause cancer and many heritable diseases. Topoisomerase I has been suspected of causing chromosome aberrations by mediating illegitimate recombination. The effects of deletion and of overexpression of the topoisomerase I gene on illegitimate recombination in the yeast Saccharomyces cerevisiae have been studied. Yeast transformations were carried out with DNA fragments that did not have any homology to the genomic DNA. The frequency of illegitimate integration was 6- to 12-fold increased in a strain overexpressing topoisomerase I compared with that in isogenic control strains. Hot spot sequences [(G/C)(A/T)T] for illegitimate integration target sites accounted for the majority of the additional events after overexpression of topoisomerase I. These hot spot sequences correspond to sequences previously identified in vitro as topoisomerase I preferred cleavage sequences in other organisms. Furthermore, such hot spot sequences were found in 44% of the integration events present in the TOP1 wild-type strain and at a significantly lower frequency in the top1delta strain. Our results provide in vivo evidence that a general eukaryotic topoisomerase I enzyme nicks DNA and ligates nonhomologous ends, leading to illegitimate recombination.
染色体畸变可能导致癌症和许多遗传性疾病。拓扑异构酶I一直被怀疑通过介导非法重组而导致染色体畸变。人们已经研究了拓扑异构酶I基因的缺失和过表达对酿酒酵母中非法重组的影响。用与基因组DNA没有任何同源性的DNA片段进行酵母转化。与同基因对照菌株相比，过表达拓扑异构酶I的菌株中非法整合的频率增加了6至12倍。非法整合靶位点的热点序列[(G/C)(A/T)T]占拓扑异构酶I过表达后额外事件的大部分。这些热点序列对应于先前在体外鉴定为其他生物体中拓扑异构酶I优先切割序列的序列。此外，在TOP1野生型菌株中存在的44%的整合事件中发现了此类热点序列，而在top1delta菌株中的频率明显较低。我们的结果提供了体内证据，表明一般的真核拓扑异构酶I酶会切割DNA并连接非同源末端，从而导致非法重组。
