Hishida Takashi, Kubota Yoshino, Carr Antony M, Iwasaki Hiroshi
Research Institute for Microbial Diseases, Osaka University, 3-1 Yamadaoka, Suita, Osaka 565-0871, Japan.
Nature. 2009 Jan 29;457(7229):612-5. doi: 10.1038/nature07580. Epub 2008 Dec 14.
In nature, organisms are exposed to chronic low-dose ultraviolet light (CLUV) as opposed to the acute high doses common to laboratory experiments. Analysis of the cellular response to acute high-dose exposure has delineated the importance of direct DNA repair by the nucleotide excision repair pathway and for checkpoint-induced cell cycle arrest in promoting cell survival. Here we examine the response of yeast cells to CLUV and identify a key role for the RAD6-RAD18-RAD5 error-free postreplication repair (RAD6 error-free PRR) pathway in promoting cell growth and survival. We show that loss of the RAD6 error-free PRR pathway results in DNA-damage-checkpoint-induced G2 arrest in CLUV-exposed cells, whereas wild-type and nucleotide-excision-repair-deficient cells are largely unaffected. Cell cycle arrest in the absence of the RAD6 error-free PRR pathway was not caused by a repair defect or by the accumulation of ultraviolet-induced photoproducts. Notably, we observed increased replication protein A (RPA)- and Rad52-yellow fluorescent protein foci in the CLUV-exposed rad18Delta cells and demonstrated that Rad52-mediated homologous recombination is required for the viability of the rad18Delta cells after release from CLUV-induced G2 arrest. These and other data presented suggest that, in response to environmental levels of ultraviolet exposure, the RAD6 error-free PRR pathway promotes replication of damaged templates without the generation of extensive single-stranded DNA regions. Thus, the error-free PRR pathway is specifically important during chronic low-dose ultraviolet exposure to prevent counter-productive DNA checkpoint activation and allow cells to proliferate normally.
在自然界中，生物体暴露于慢性低剂量紫外线（CLUV）下，这与实验室实验中常见的急性高剂量紫外线不同。对细胞对急性高剂量暴露的反应分析已经阐明了核苷酸切除修复途径直接进行DNA修复以及检查点诱导的细胞周期停滞在促进细胞存活中的重要性。在这里，我们研究了酵母细胞对CLUV的反应，并确定了RAD6-RAD18-RAD5无差错复制后修复（RAD6无差错PRR）途径在促进细胞生长和存活中的关键作用。我们发现，RAD6无差错PRR途径的缺失会导致CLUV暴露细胞中DNA损伤检查点诱导的G2期停滞，而野生型和核苷酸切除修复缺陷型细胞则基本不受影响。在没有RAD6无差错PRR途径的情况下，细胞周期停滞不是由修复缺陷或紫外线诱导的光产物积累引起的。值得注意的是，我们在CLUV暴露的rad18Delta细胞中观察到复制蛋白A（RPA）和Rad52-黄色荧光蛋白聚集增加，并证明Rad52介导的同源重组是rad18Delta细胞从CLUV诱导的G2期停滞释放后存活所必需的。这些以及其他所呈现的数据表明，响应环境水平的紫外线暴露，RAD6无差错PRR途径促进受损模板的复制，而不会产生广泛的单链DNA区域。因此，在慢性低剂量紫外线暴露期间，无差错PRR途径对于防止适得其反的DNA检查点激活并允许细胞正常增殖特别重要。
