11月16日，DNA Repair在线发表了北京生命科学研究所研究员杜立林实验室的最新研究成果，首次报道了一个在裂殖酵母中检测染色体上进行的末端连接修复的方法，并利用此方法分析了机理不同的三类末端连接修复方式。
非同源末端连接（NHEJ）是哺乳动物中修复DNA双链断裂的主要方式。参与NHEJ的修复蛋白如Ku，ligase IV，XLF，和X家族DNA聚合酶在裂殖酵母中都是保守的。通过表达一个高特异性的核酸内切酶来产生基因组上一个特定位置的双链断裂，作者建立了结合传统测序和高通量测序全面分析不精确修复产物序列的方法，对突变体中修复产物序列的分析验证了Ku，ligase IV和XLF在经典NHEJ中的核心作用，以及X家族DNA聚合酶Pol4在间隙填补(Gap Filling)中的重要作用。研究发现在Ku或ligase IV缺失的突变体中，两种非NHEJ的另类末端连接仍能发生。其中一种方式利用了DNA断裂位置两侧的微同源（microhomology）序列并造成几百至几千碱基的缺失，另外一种方式则只在酶切位点处引入碱基置换。
本文的实验结果主要由北京大学、清华大学和北京生命科学研究所联合培养博士研究生项目（PTN-BBS）2011级研究生李鹏在轮转期间完成，其他贡献者有研究生李俊，李明，窦坤，和生物信息分析员张美俊和索芳。此项研究由科技部和北京市资助，在北京生命科学研究所完成。（生物谷Bioon.com）
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doi:10.1016/j.ndarep.2011.10.011
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Multiple end joining mechanisms repair a chromosomal DNA break in fission yeast
Li P, Li J, Li M, Dou K, Zhang MJ, Suo F, Du LL.
Non-homologous end joining (NHEJ) is an important mechanism for repairing DNA double-strand breaks (DSBs). The fission yeast Schizosaccharomyces pombe has a conserved set of NHEJ factors including Ku, DNA ligase IV, Xlf1, and Pol4. Their roles in chromosomal DSB repair have not been directly characterized before. Here we used HO endonuclease to create a specific chromosomal DSB in fission yeast and examined the imprecise end joining events allowing cells to survive the continuous expression of HO. Our analysis showed that cell survival was significantly reduced in mutants defective for Ku, ligase IV, or Xlf1. Using Sanger sequencing and Illumina sequencing, we have characterized in depth the repair junction sequences in HO survivors. In wild type cells the majority of repair events were one-nucleotide insertions dependent on Ku, ligase IV, and Pol4. Our data suggest that fission yeast Pol4 is important for gap filling during NHEJ repair and can extend primers in the absence of terminal base pairing with the templates. In Ku and ligase IV mutants, the survivors mainly resulted from two types of alternative end joining events: one used microhomology flanking the HO site to delete sequences of hundreds to thousands of base pairs, the other rejoined the break using the HO-generated overhangs but also introduced one- or two-nucleotide base substitutions. The chromosomal repair assay we describe here should provide a useful tool for further exploration of the end joining repair mechanisms in fission yeast.
