In eukaryotes, DNA compacts into chromatin through nucleosomes^1,2. Replication of the eukaryotic genome must be coupled to the transmission of the epigenome encoded in the chromatin^3,4. Here we report cryo-electron microscopy structures of yeast (Saccharomyces cerevisiae) replisomes associated with the FACT (facilitates chromatin transactions) complex (comprising Spt16 and Pob3) and an evicted histone hexamer. In these structures, FACT is positioned at the front end of the replisome by engaging with the parental DNA duplex to capture the histones through the middle domain and the acidic carboxyl-terminal domain of Spt16. The H2A–H2B dimer chaperoned by the carboxyl-terminal domain of Spt16 is stably tethered to the H3–H4 tetramer, while the vacant H2A–H2B site is occupied by the histone-binding domain of Mcm2. The Mcm2 histone-binding domain wraps around the DNA-binding surface of one H3–H4 dimer and extends across the tetramerization interface of the H3–H4 tetramer to the binding site of Spt16 middle domain before becoming disordered. This arrangement leaves the remaining DNA-binding surface of the other H3–H4 dimer exposed to additional interactions for further processing. The Mcm2 histone-binding domain and its downstream linker region are nested on top of Tof1, relocating the parental histones to the replisome front for transfer to the newly synthesized lagging-strand DNA. Our findings offer crucial structural insights into the mechanism of replication-coupled histone recycling for maintaining epigenetic inheritance.

在真核生物中，DNA 通过核小体压缩成染色质^1,2 。真核基因组的复制必须与染色质编码的表观基因组的传输相结合^3,4 。在这里，我们报告了与 FACT（促进染色质交易）复合物（包括 Spt16 和 Pob3）和驱逐的组蛋白六聚体相关的酵母（酿酒酵母）复制体的冷冻电子显微镜结构。在这些结构中，FACT 位于复制体的前端，通过与亲本 DNA 双链体结合，通过 Spt16 的中间结构域和酸性羧基末端结构域捕获组蛋白。由 Spt16 羧基末端结构域陪伴的 H2A-H2B 二聚体稳定地连接到 H3-H4 四聚体，而空的 H2A-H2B 位点被 Mcm2 的组蛋白结合结构域占据。 Mcm2 组蛋白结合结构域包裹一个 H3-H4 二聚体的 DNA 结合表面，并穿过 H3-H4 四聚体的四聚化界面延伸到 Spt16 中间结构域的结合位点，然后变得无序。这种排列使得另一个 H3-H4 二聚体的剩余 DNA 结合表面暴露于额外的相互作用以进行进一步处理。 Mcm2 组蛋白结合结构域及其下游接头区域嵌套在 Tof1 顶部，将亲代组蛋白重新定位到复制体前端，以便转移到新合成的滞后链 DNA。我们的研究结果为复制偶联组蛋白回收维持表观遗传的机制提供了重要的结构见解。