摘要 : 修复双链DNA断裂的第一步分析表明，Mre11酶使得DNA切口从断裂端口中分离，形成一个入口点以供外切核酸酶进一步处理。相关文章发表于2014年9月17日的《Nature》杂志上。
一个DNA双链断裂点上的同源重组利用一个3´-尾分子(3´-tailed molecule)，后者需要将 5´链切除。以前的遗传分析表明，MRX复合物(由修复蛋白Mre11、Rad50 和Xrs2组成)是这种切除所需的。然而其体外活性却令人迷惑，因为只检测到从3´ 到 5´的切除。
现在，Elda Cannavo 和 Petr Cejka 揭开了这个谜团。他们发现，在酵母中，Sae2 核酸酶激活MRX，在5´ 链上产生一个最初的内切核苷酸切口，以便MRX 能以“3´ 到 5´”的方式将5´ 链消化回去。
Elda Cannavo & Petr Cejka
To repair double-strand DNA breaks by homologous recombination, the 5′-terminated DNA strand must first be resected, which generates 3′ single-stranded DNA overhangs. Genetic evidence suggests that this process is initiated by the Mre11–Rad50–Xrs2 (MRX) complex. However, its involvement was puzzling, as the complex possesses exonuclease activity with the opposite (3′ to 5′) polarity from that required for homologous recombination. Consequently, a bidirectional model has been proposed wherby dsDNA is first incised endonucleolytically and MRX then proceeds back to the dsDNA end using its 3′ to 5′ exonuclease. The endonuclease creates entry sites for Sgs1–Dna2 and/or Exo1, which then carry out long-range resection in the 5′ to 3′ direction. However, the identity of the endonuclease remained unclear. Using purified Saccharomyces cerevisiae proteins, we show that Sae2 promotes dsDNA-specific endonuclease activity by the Mre11 subunit within the MRX complex. The endonuclease preferentially cleaves the 5′-terminated dsDNA strand, which explains the polarity paradox. The dsDNA end clipping is strongly stimulated by protein blocks at the DNA end, and requires the ATPase activity of Rad50 and physical interactions between MRX and Sae2. Our results suggest that MRX initiates dsDNA break processing by dsDNA endonuclease rather than exonuclease activity, and that Sae2 is the key regulator of this process. These findings demonstrate a probable mechanism for the initiation of dsDNA break processing in both vegetative and meiotic cells.
来源： Nature中文 浏览次数：86