Levin N A, Bjornsti M A, Fink G R
Whitehead Institute for Biomedical Research, Cambridge, Massachusetts 02142.
Genetics. 1993 Apr;133(4):799-814. doi: 10.1093/genetics/133.4.799.
DNA topoisomerases, enzymes that alter the superhelicity of DNA, have been implicated in such critical cellular functions as transcription, DNA replication, and recombination. In the yeast Saccharomyces cerevisiae, a null mutation in the gene encoding topoisomerase I (TOP1) causes elevated levels of mitotic recombination in the ribosomal DNA (rDNA), but has little effect on growth. We have isolated a missense mutation in TOP1 that causes mitotic hyper-recombination not only in the rDNA, but also at other loci, in addition to causing a number of other unexpected phenotypes. This topoisomerase I mutation (top1-103) causes slow growth, constitutive expression of DNA damage-inducible genes, and inviability in the absence of the double-strand break repair system. Overexpression of top1-103 causes RAD9-dependent cell cycle arrest in G2. We show that the Top1-103 enzyme nicks DNA in vitro, suggesting that it damages DNA directly. We propose that Top1-103 mimics the action of wild-type topoisomerase I in the presence of the anti-tumor drug, camptothecin.
DNA拓扑异构酶是一类能够改变DNA超螺旋状态的酶，它们参与了转录、DNA复制和重组等关键的细胞功能。在酿酒酵母中，编码拓扑异构酶I（TOP1）的基因发生无效突变会导致核糖体DNA（rDNA）中减数分裂重组水平升高，但对生长影响较小。我们分离出了TOP1中的一个错义突变，该突变不仅会导致rDNA中的有丝分裂超重组，还会导致其他位点的有丝分裂超重组，此外还会引发许多其他意想不到的表型。这种拓扑异构酶I突变（top1-103）会导致生长缓慢、DNA损伤诱导基因的组成型表达，并且在缺乏双链断裂修复系统的情况下无法存活。top1-103的过表达会导致G2期依赖RAD9的细胞周期停滞。我们发现Top1-103酶在体外会切割DNA，这表明它会直接损伤DNA。我们推测Top1-103在存在抗肿瘤药物喜树碱的情况下模拟了野生型拓扑异构酶I的作用。
