During replication, nucleosomes are disrupted ahead of the replication fork, followed by their reassembly on daughter strands from the pool of recycled parental and new histones. However, because no previous studies have managed to capture the moment that replication forks encounter nucleosomes, the mechanism of recycling has remained unclear. Here, through real-time single-molecule visualization of replication fork progression in Xenopus egg extracts, we determine explicitly the outcome of fork collisions with nucleosomes. Most of the parental histones are evicted from the DNA, with histone recycling, nucleosome sliding, and replication fork stalling also occurring but at lower frequencies. Critically, we find that local histone recycling becomes dominant upon depletion of endogenous histones from extracts, revealing that free histone concentration is a key modulator of parental histone dynamics at the replication fork. The mechanistic details revealed by these studies have major implications for our understanding of epigenetic inheritance.

在复制过程中，核小体在复制叉之前被破坏，然后从回收的亲代组蛋白和新组蛋白池中重新组装到子链上。然而，由于之前的研究未能捕捉到复制叉遇到核小体的时刻，因此回收机制仍不清楚。在这里，通过非洲爪蟾卵提取物中复制叉进展的实时单分子可视化，我们明确确定了复制叉与核小体碰撞的结果。大多数亲代组蛋白被从 DNA 中逐出，组蛋白回收、核小体滑动和复制叉停滞也会发生，但频率较低。重要的是，我们发现，当提取物中的内源组蛋白耗尽时，局部组蛋白回收变得占主导地位，这表明游离组蛋白浓度是复制叉处亲代组蛋白动力学的关键调节剂。这些研究揭示的机制细节对于我们对表观遗传的理解具有重大意义。