DNA double-strand break (DSB) repair is essential for maintaining our genomes. Mre11-Rad50-Nbs1 (MRN) and Ku70-Ku80 (Ku) direct distinct DSB repair pathways, but the interplay between these complexes at a DSB remains unclear. Here, we use high-throughput single-molecule microscopy to show that MRN searches for free DNA ends by one-dimensional facilitated diffusion, even on nucleosome-coated DNA. Rad50 binds homoduplex DNA and promotes facilitated diffusion, whereas Mre11 is required for DNA end recognition and nuclease activities. MRN gains access to occluded DNA ends by removing Ku or other DNA adducts via an Mre11-dependent nucleolytic reaction. Next, MRN loads exonuclease 1 (Exo1) onto the free DNA ends to initiate DNA resection. In the presence of replication protein A (RPA), MRN acts as a processivity factor for Exo1, retaining the exonuclease on DNA for long-range resection. Our results provide a mechanism for how MRN promotes homologous recombination on nucleosome-coated DNA.

中文翻译：

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单分子成像揭示了Mre11-Rad50-Nbs1如何启动DNA断裂修复。

DNA双链断裂（DSB）修复对于维持我们的基因组至关重要。Mre11-Rad50-Nbs1（MRN）和Ku70-Ku80（Ku）指导不同的DSB修复途径，但尚不清楚这些复合物在DSB上的相互作用。在这里，我们使用高通量单分子显微镜显示，即使是在核小体包被的DNA上，MRN也可以通过一维促进扩散来寻找自由的DNA末端。Rad50结合同源双链DNA并促进促进扩散，而Mre11是DNA末端识别和核酸酶活性所必需的。MRN通过依赖于Mre11的核酸分解反应去除Ku或其他DNA加合物，从而获得了封闭的DNA末端。接下来，MRN将核酸外切酶1（Exo1）加载到游离DNA末端以启动DNA切除。在存在复制蛋白A（RPA）的情况下，MRN充当Exo1的持续合成因子，将核酸外切酶保留在DNA上以进行远距离切除。我们的结果为MRN如何促进核小体包被的DNA上的同源重组提供了一种机制。