More than a million Okazaki fragments are synthesized, processed and joined during replication of the human genome. After synthesis of an RNA-DNA oligonucleotide by DNA polymerase α holoenzyme, proliferating cell nuclear antigen (PCNA), a homotrimeric DNA sliding clamp and polymerase processivity factor, is loaded onto the primer-template junction by replication factor C (RFC). Although PCNA interacts with the enzymes DNA polymerase δ (Pol δ), flap endonuclease 1 (FEN1) and DNA ligase I (LigI) that complete Okazaki fragment processing and joining, it is not known how the activities of these enzymes are coordinated. Here we describe a novel interaction between Pol δ and LigI that is critical for Okazaki fragment joining in vitro. Both LigI and FEN1 associate with PCNA-Pol δ during gap-filling synthesis, suggesting that gap-filling synthesis is carried out by a complex of PCNA, Pol δ, FEN1 and LigI. Following ligation, PCNA and LigI remain on the DNA, indicating that Pol δ and FEN1 dissociate during 5′ end processing and that LigI engages PCNA at the DNA nick generated by FEN1 and Pol δ. Thus, dynamic PCNA complexes coordinate Okazaki fragment synthesis and processing with PCNA and LigI forming a terminal structure of two linked protein rings encircling the ligated DNA.

在人类基因组的复制过程中，超过一百万个冈崎片段被合成、加工和连接。在通过 DNA 聚合酶 α 全酶合成 RNA-DNA 寡核苷酸后，增殖细胞核抗原 (PCNA)、同源三聚体 DNA 滑动钳和聚合酶持续合成因子通过复制因子 C (RFC) 加载到引物-模板连接处。尽管 PCNA 与完成 Okazaki 片段加工和连接的酶 DNA 聚合酶 δ (Pol δ)、瓣内切酶 1 (FEN1) 和 DNA 连接酶 I (LigI) 相互作用，但尚不清楚这些酶的活性是如何协调的。在这里，我们描述了 Pol δ 和 LigI 之间的一种新的相互作用，这对于冈崎片段的体外连接至关重要。LigI 和 FEN1 在间隙填充合成过程中与 PCNA-Pol δ 结合，表明间隙填充合成是由 PCNA、Pol δ、FEN1 和 LigI 的复合物进行的。连接后，PCNA 和 LigI 保留在 DNA 上，表明 Pol δ 和 FEN1 在 5' 末端加工过程中解离，并且 LigI 在 FEN1 和 Pol δ 产生的 DNA 缺口处与 PCNA 接合。因此，动态 PCNA 复合物与 PCNA 和 LigI 协调 Okazaki 片段的合成和加工，形成环绕连接 DNA 的两个连接蛋白环的末端结构。