DNA polymerase δ subunit Pol32 binds histone H3-H4 and couples nucleosome assembly with Okazaki fragment processing

During lagging strand chromatin replication, multiple Okazaki fragments (OFs) require processing and nucleosome assembly, but the mechanisms linking these processes remain unclear. Here, using transmission electron microscopy and rapid degradation of DNA ligase Cdc9, we observed flap structures accumulated on lagging strands, controlled by both Pol δ's strand displacement activity and Fen1's nuclease digestion. The distance between neighboring flap structures exhibits a regular pattern, indicative of matured OF length. While fen1Δ or enhanced strand displacement activities by polymerase δ (Pol δ; pol3^exo-) minimally affect inter-flap distance, mutants affecting replication-coupled nucleosome assembly, such as cac1Δ and mcm2-3A, do significantly alter it. Deletion of Pol32, a subunit of DNA Pol δ, significantly increases this distance. Mechanistically, Pol32 binds to histone H3-H4 and is critical for nucleosome assembly on the lagging strand. Together, we propose that Pol32 establishes a connection between nucleosome assembly and the processing of OFs on lagging strands.