Abstract:

Sister chromatid cohesion essential for mitotic chromosome segregation is thought to involve the co-entrapment of sister DNAs within cohesin rings. Though cohesin can load onto chromosomes throughout the cell cycle, it normally only builds cohesion during S phase. A key question is whether cohesion is generated by conversion of cohesin complexes associate with un-replicated DNAs ahead of replication forks into cohesive structures behind them, or from nucleoplasmic cohesin that is loaded de novo onto nascent DNAs associated with forks, a process that would be dependent on cohesin’s Scc2 subunit. We show here that in S. cerevisiae, both mechanisms exist and that each requires a different set of non-essential replisome-associated proteins. Cohesion produced by cohesin conversion requires Tof1/Csm3, Ctf4 and Chl1 (TCCC) but not Scc2 while that created by Scc2-dependent de novo loading at replication forks requires the Ctf18-RFC complex. Though inactivation of either pathway individually merely reduces the efficiency of cohesion establishment, simultaneous inactivation resembles the effect of cohesin ablation and is lethal. The association of specific replisome proteins with different types of cohesion establishment opens the way to a mechanistic understanding of an aspect of DNA replication unique to eukaryotic cells.