DNA replication perturbs chromatin by triggering the eviction, replacement, and incorporation of nucleosomes. How this dynamic is orchestrated in time and space is poorly understood. Here, we apply a genetically encoded sensor for histone exchange to follow the time-resolved histone H3 exchange profile in budding yeast cells undergoing slow synchronous replication in nucleotide-limiting conditions. We find that new histones are incorporated not only behind, but also ahead of the replication fork. We provide evidence that Rtt109, the S-phase-induced acetyltransferase, stabilizes nucleosomes behind the fork but promotes H3 replacement ahead of the fork. Increased replacement ahead of the fork is independent of the primary Rtt109 acetylation target H3K56 and rather results from Vps75-dependent Rtt109 activity toward the H3 N terminus. Our results suggest that, at least under nucleotide-limiting conditions, selective incorporation of differentially modified H3s behind and ahead of the replication fork results in opposing effects on histone exchange, likely reflecting the distinct challenges for genome stability at these different regions.

中文翻译：

---

Rtt109 促进复制叉之前的核小体替换


DNA 复制通过触发核小体的驱逐、替换和掺入来扰乱染色质。人们对这种动态在时间和空间上是如何协调的知之甚少。在这里，我们应用基因编码的组蛋白交换传感器来跟踪在核苷酸限制条件下进行缓慢同步复制的芽殖酵母细胞中的时间分辨组蛋白 H3 交换谱。我们发现新的组蛋白不仅在复制叉后面而且也在复制叉之前掺入。我们提供的证据表明，S 期诱导的乙酰转移酶 Rtt109 可以稳定叉后的核小体，但促进叉前的 H3 替换。分叉前替换的增加与主要 Rtt109 乙酰化靶点 H3K56 无关，而是由 Vps75 依赖性 Rtt109 对 H3 N 末端的活性所致。我们的结果表明，至少在核苷酸限制条件下，复制叉前后差异修饰的 H3 的选择性掺入会对组蛋白交换产生相反的影响，这可能反映了这些不同区域的基因组稳定性面临的独特挑战。