Cohesin Can Remain Associated with Chromosomes during DNA Replication

To ensure disjunction to opposite poles during anaphase, sister chromatids must be held together following DNA replication. This is mediated by cohesin, which is thought to entrap sister DNAs inside a tripartite ring composed of its Smc and kleisin (Scc1) subunits. How such structures are created during S phase is poorly understood, in particular whether they are derived from complexes that had entrapped DNAs prior to replication. To address this, we used selective photobleaching to determine whether cohesin associated with chromatin in G1 persists in situ after replication. We developed a non-fluorescent HaloTag ligand to discriminate the fluorescence recovery signal from labeling of newly synthesized Halo-tagged Scc1 protein (pulse-chase or pcFRAP). In cells where cohesin turnover is inactivated by deletion of WAPL, Scc1 can remain associated with chromatin throughout S phase. These findings suggest that cohesion might be generated by cohesin that is already bound to un-replicated DNA. [Display omitted] •Non-fluorescent ligand blocks labeling of newly synthesized HaloTag fusion proteins•pcFRAP makes it possible to observe defined pools of protein over long time periods•Cohesin that is loaded in G1 can remain associated with DNA during DNA replication How sister chromatids become entrapped within cohesin rings is not known. Rhodes et al. show that cohesin that is loaded onto DNA before S phase can remain associated with chromatin throughout DNA replication. This is consistent with the conversion of G1-loaded cohesin into its cohesive form.