Sister chromatid cohesion ensures proper chromosome segregation and is mediated by the cohesin complex forming a ring around sister chromatids. How DNA replication forks go through cohesin-associated obstructions to achieve genome replication remains unclear. Prasad Jallepalli and colleagues now report that fork advancement depends on cohesin acetylation, which might affect cohesin conformation.

Credit: COMSTOCK IMAGES/ImageSource

Cohesion requires the acetylation of cohesin's structural maintenance of chromosomes 3 (SMC3) subunit, entrapment of nascent DNA strands in the cohesin ring and the chromosome transmission fidelity 18-containing replication factor C complex (RFCCTF18). To assess the contribution of RFCCTF18 to replication fork dynamics, the authors created RFCCTF18-deficient human cells, pulse-labelled them with halogenated nucleosides (to label DNA) and used them for single-molecule analysis of DNA fibres. They found that lack of RFCCTF18 decreases fork velocity and increases fork density (owing to reactivation of dormant origins) and the frequency at which forks arrest or collapse. Importantly, loss of RFCCTF18 activity also leads to cohesion loss and premature chromatid separation.

As cells expressing non-acetylatable SMC3 also show loss of cohesion, the authors investigated whether cohesin acetylation requires RFCCTF18. Indeed, RFCCTF18-deficient cells showed a 70% reduction of SMC3 acetylation. RFCCTF18 thus affects both replication fork processivity and cohesin acetylation, but are these processes linked?

“cohesin acetylation regulates replication fork progression.”

Fork velocity is reduced in cells derived from patients with Roberts syndrome, who lack the SMC3-specific acetyltransferase ESCO2. Furthermore, fork speed is reduced in cells expressing non-acetylatable SMC3, suggesting that cohesin acetylation regulates replication fork progression.

But what is the mechanism by which cohesin acetylation promotes fork advancement? Acetylation promotes the dissociation of two cohesin cofactors — PDS5A and Wings apart-like (WAPL) — which regulate cohesin association with chromosomes. So, PDS5A and WAPL might keep cohesin in an occlusive conformation that is released by acetylation.

Together, these results show that RFCCTF18-dependent cohesin acetylation is required to switch cohesin from a configuration that obstructs the fork to one that allows its advancement. Further studies are needed to clarify how this is regulated.