Lettier Gaëlle, Feng Qi, de Mayolo Adriana Antúnez, Erdeniz Naz, Reid Robert J D, Lisby Michael, Mortensen Uffe H, Rothstein Rodney
Center for Microbial Biotechnology, BioCentrum-DTU, Technical University of Denmark, Lyngby, Denmark.
PLoS Genet. 2006 Nov 10;2(11):e194. doi: 10.1371/journal.pgen.0020194. Epub 2006 Oct 5.
Homologous recombination (HR) is a source of genomic instability and the loss of heterozygosity in mitotic cells. Since these events pose a severe health risk, it is important to understand the molecular events that cause spontaneous HR. In eukaryotes, high levels of HR are a normal feature of meiosis and result from the induction of a large number of DNA double-strand breaks (DSBs). By analogy, it is generally believed that the rare spontaneous mitotic HR events are due to repair of DNA DSBs that accidentally occur during mitotic growth. Here we provide the first direct evidence that most spontaneous mitotic HR in Saccharomyces cerevisiae is initiated by DNA lesions other than DSBs. Specifically, we describe a class of rad52 mutants that are fully proficient in inter- and intra-chromosomal mitotic HR, yet at the same time fail to repair DNA DSBs. The conclusions are drawn from genetic analyses, evaluation of the consequences of DSB repair failure at the DNA level, and examination of the cellular re-localization of Rad51 and mutant Rad52 proteins after introduction of specific DSBs. In further support of our conclusions, we show that, as in wild-type strains, UV-irradiation induces HR in these rad52 mutants, supporting the view that DNA nicks and single-stranded gaps, rather than DSBs, are major sources of spontaneous HR in mitotic yeast cells.
同源重组（HR）是基因组不稳定的一个来源，也是有丝分裂细胞中杂合性丧失的原因。由于这些事件会带来严重的健康风险，因此了解导致自发HR的分子事件非常重要。在真核生物中，高水平的HR是减数分裂的正常特征，是由大量DNA双链断裂（DSB）的诱导所致。以此类推，人们普遍认为，罕见的自发有丝分裂HR事件是由于在有丝分裂生长过程中偶然发生的DNA DSB的修复。在这里，我们提供了首个直接证据，即酿酒酵母中大多数自发有丝分裂HR是由DSB以外的DNA损伤引发的。具体而言，我们描述了一类rad52突变体，它们在染色体间和染色体内有丝分裂HR方面完全正常，但同时无法修复DNA DSB。这些结论来自遗传分析、在DNA水平上对DSB修复失败后果的评估，以及在引入特定DSB后对Rad51和突变型Rad52蛋白细胞重新定位的检查。为进一步支持我们的结论，我们表明，与野生型菌株一样，紫外线照射会在这些rad52突变体中诱导HR，这支持了DNA切口和单链缺口而非DSB是有丝分裂酵母细胞中自发HR主要来源的观点。
