RAD51 is the central eukaryotic recombinase required for meiotic recombination and mitotic repair of double-strand DNA breaks (DSBs)^1,2. However, the mechanism by which RAD51 functions at DSB sites in chromatin has remained elusive. Here we report the cryo-electron microscopy structures of human RAD51–nucleosome complexes, in which RAD51 forms ring and filament conformations. In the ring forms, the N-terminal lobe domains (NLDs) of RAD51 protomers are aligned on the outside of the RAD51 ring, and directly bind to the nucleosomal DNA. The nucleosomal linker DNA that contains the DSB site is recognized by the L1 and L2 loops—active centres that face the central hole of the RAD51 ring. In the filament form, the nucleosomal DNA is peeled by the RAD51 filament extension, and the NLDs of RAD51 protomers proximal to the nucleosome bind to the remaining nucleosomal DNA and histones. Mutations that affect nucleosome-binding residues of the RAD51 NLD decrease nucleosome binding, but barely affect DNA binding in vitro. Consistently, yeast Rad51 mutants with the corresponding mutations are substantially defective in DNA repair in vivo. These results reveal an unexpected function of the RAD51 NLD, and explain the mechanism by which RAD51 associates with nucleosomes, recognizes DSBs and forms the active filament in chromatin.

RAD51 是减数分裂重组和双链 DNA 断裂 (DSB) 有丝分裂修复所需的核心真核重组酶^1,2。然而，RAD51 在染色质 DSB 位点发挥作用的机制仍不清楚。在这里，我们报告了人类 RAD51-核小体复合物的冷冻电子显微镜结构，其中 RAD51 形成环状和丝状构象。在环形式中，RAD51原聚体的N端叶结构域(NLD)排列在RAD51环的外侧，并直接与核小体DNA结合。包含 DSB 位点的核小体接头 DNA 被 L1 和 L2 环（面向 RAD51 环中心孔的活性中心）识别。在细丝形式中，核小体 DNA 被 RAD51 细丝延伸剥离，并且靠近核小体的 RAD51 原聚体的 NLD 与剩余的核小体 DNA 和组蛋白结合。影响 RAD51 NLD 核小体结合残基的突变会降低核小体结合，但在体外几乎不影响 DNA 结合。一致地，具有相应突变的酵母Rad51突变体在体内DNA修复方面存在显着缺陷。这些结果揭示了 RAD51 NLD 的意想不到的功能，并解释了 RAD51 与核小体结合、识别 DSB 并形成染色质中的活性丝的机制。