Tauchi Hiroshi, Kobayashi Junya, Morishima Ken-ichi, van Gent Dik C, Shiraishi Takahiro, Verkaik Nicole S, vanHeems Diana, Ito Emi, Nakamura Asako, Sonoda Eiichiro, Takata Minoru, Takeda Shunichi, Matsuura Shinya, Komatsu Kenshi
Department of Environmental Sciences, Faculty of Science, Ibaraki University, Bunkyo 2-1-1, Mito, Ibaraki 310-8512 Japan.
Nature. 2002 Nov 7;420(6911):93-8. doi: 10.1038/nature01125.
Double-strand breaks occur during DNA replication and are also induced by ionizing radiation. There are at least two pathways which can repair such breaks: non-homologous end joining and homologous recombination (HR). Although these pathways are essentially independent of one another, it is possible that the proteins Mre11, Rad50 and Xrs2 are involved in both pathways in Saccharomyces cerevisiae. In vertebrate cells, little is known about the exact function of the Mre11-Rad50-Nbs1 complex in the repair of double-strand breaks because Mre11- and Rad50-null mutations are lethal. Here we show that Nbs1 is essential for HR-mediated repair in higher vertebrate cells. The disruption of Nbs1 reduces gene conversion and sister chromatid exchanges, similar to other HR-deficient mutants. In fact, a site-specific double-strand break repair assay showed a notable reduction of HR events following generation of such breaks in Nbs1-disrupted cells. The rare recombinants observed in the Nbs1-disrupted cells were frequently found to have aberrant structures, which possibly arise from unusual crossover events, suggesting that the Nbs1 complex might be required to process recombination intermediates.
双链断裂发生在DNA复制过程中，也可由电离辐射诱导产生。至少有两条途径可以修复此类断裂：非同源末端连接和同源重组（HR）。尽管这些途径本质上相互独立，但在酿酒酵母中，蛋白质Mre11、Rad50和Xrs2可能参与了这两条途径。在脊椎动物细胞中，关于Mre11-Rad50-Nbs1复合物在双链断裂修复中的确切功能知之甚少，因为Mre11和Rad50基因敲除突变是致死性的。在此我们表明，Nbs1对于高等脊椎动物细胞中HR介导的修复至关重要。Nbs1的破坏会减少基因转换和姐妹染色单体交换，这与其他HR缺陷型突变体类似。事实上，一项位点特异性双链断裂修复试验表明，在Nbs1破坏的细胞中产生此类断裂后，HR事件显著减少。在Nbs1破坏的细胞中观察到的罕见重组体经常被发现具有异常结构，这可能源于异常的交叉事件，表明Nbs1复合物可能需要处理重组中间体。
