Coupling of initiation of DNA replication and establishment of sister chromatid cohesion in Xenopus egg extracts

Abstract

Sister chromatid cohesion, which is mediated by a multi‐protein complex cohesin, is established during S‐phase. Recent studies have clarified that a cohesin ring concatenates sister DNA molecules, and that the cohesion is established during S‐phase through acetylation of cohesin subunit. However, little is known about how DNA replication is coupled to the establishment reaction. Since cohesin is loaded onto chromatin before the initiation of replication, the cohesion establishment reaction should involve the passage of replication fork through a cohesin ring. Accumulating evidences suggest that a fairly large complex called replisome progression complex (RPC) is formed at the replication fork. Yeast RPC contains essential components of DNA helicase (CMG complex) and highly conserved but non‐essential components such as Ctf4, Tof1 and Csm3 that are implicated in the establishment process. With Xenopus egg extracts, we found that Xenopus homolog of yeast Ctf4 (AND‐1) and Tof1‐Csm3 (Tim1‐Tipin) are assembled into Xenopus RPC depending on CMG complex formation, and they are required for proper establishment of the cohesion. Double depletion of AND‐1 and Tim1 lead to aberrant sister chromatid cohesion which is similarly induced by depleting cohesin from the extracts. These results demonstrate that AND‐1 and Tim1‐Tipin are key factors for connecting DNA replication and the establishment of cohesion.