Molecular mechanism of parental H3/H4 recycling at a replication fork

In eukaryotic chromatin replication, faithful recycling of histones from parental DNA to replicated leading and lagging strands is essential for maintaining epigenetic information across generations. A recent experimental study has revealed that disrupting interactions between the N-terminal disordered tail of Mcm2, a subunit in DNA replication machinery, and a histone H3/H4 tetramer, a carrier of epigenetic modifications, perturbs their faithful recycling. However, the molecular pathways via which the H3/H4 tetramer attached to Mcm2 is recycled to the replicated strands have yet to be deciphered. Furthermore, the factors that regulate the ratio recycled to each strand and the destination location still need to be discovered. The elucidation requires visualization of a structural trajectory from H3/H4 bound to Mcm2 until recycled to the replicated strands. In this study, we performed molecular dynamics simulations of yeast DNA replication machinery (Mcm2-7, Cdc45, GINS, Pol ε, and RPA), an H3/H4 tetramer, and replicated DNA strands. The simulations demonstrated that histones are recycled via Cdc45-mediated and unmediated pathways even without histone chaperones, as our in vitro biochemical assays supported. Also, RPA binding to the ssDNA portion of the lagging strand regulated the ratio recycled to each strand, whereas DNA bending by Pol ε modulated the destination location. Together, the simulations in this study provided testable hypotheses, which are vital for clarifying intracellular histone recycling controlled by the cooperation of many histone chaperones.