Abstract: Eukaryotic genomes are organised into nucleosomes via histone proteins, and arranged into a higher-order structure called chromatin. Chromatin harbours the epigenome, which comprises the nucleosome landscape and set of chemical modifications that encode transcriptional programmes while protecting and regulating access to the genome. Through sequential cell cycles, chromatin must be duplicated from a parental template and inherited alongside the underlying DNA sequence to maintain genome stability and cell identity. The main player in this process is the replisome, comprised of the 11-subunit replicative helicase CMG, DNA polymerases and various accessory factors, including the essential histone chaperone FACT (Facilitates Chromatin Transactions). These elements must cooperate to disrupt and traverse nucleosomes, redeposit histones onto newly synthesized DNA, and facilitate the maturation of nascent chromatin into a copy of the parental template.

We have taken an in vitro reconstitution approach to characterise the role of replisome-associated components to chromatin traversal — the first step in this process. By visualising replisome-chromatin encounters by cryo-EM, we show that nucleosome processing can occur at distinct sites on the replisome, with different elements directing parental histones towards their destinations on leading and lagging strands. We have also characterised the role of FACT, and find that this is important to mitigate the rate limiting step during single nucleosome traversal. Our study provides the first glimpse of how the replisome handles the chromatin barrier en route to histone redeposition and chromatin maturation.

By: Paula Amiet-West
Last updated: Thursday, 23 October 2025