|Abstract: ||The precise division of DNA into two daughter nuclei is a critical event in every cell cycle. The pairing of sister chromatids in order to allow accurate segregation, is achieved by two primary mechanisms – the SMC complex cohesin binds as replication ends and holds sisters together with protein-mediated links; and the process of replication intertwines sister chromatids in a way that results in DNA-DNA mediated links called catenations. At the point of segregation, it is crucial that cohesin is destroyed and all catenations are resolved.
Topoisomerase 2 (Top2) is the primary actor in S. cerevisiae able to resolve DNA inter-
twines. This enzyme appears to maintain an equilibrium level of catenations between S phase and mitosis and it has been shown to be capable of both introducing and removing catenations by its strand passage mechanism. At the metaphase to anaphase transition, a shift to positive supercoiling may help to direct the action of topoisomerase enzymes towards full decatenation at the crucial moment of DNA segregation. Further, it has been shown that condensin, another SMC complex, has a pivotal role in this supercoiling transition.
Here, work is presented to link this change in supercoiling to physical tension across sister chromatids aligned on the metaphase plate. The regulation of this transition requires Cdc5 kinase, Ipl1 kinase and acts independently of the Spindle Assembly Checkpoint. Furthermore, the supercoiling transition is linked to a global enrichment of Smc2 across the entire length of chromosomes as the spindles form and anaphase progresses. This enrichment is matched by Top2 binding and this Top2 enrichment depends directly on the action of Smc2. It is hypothesised that this recruitment of Top2 is dependent on catenations being revealed by the supercoiling action of Smc2. If Top2 is not active, then Smc2 continues to bind globally to DNA and further increase the level of positive supercoiling, suggesting that the complex is able to sense the presence of remaining catenations. Finally, a secondary last minute catenane resolution pathway may have been identified that occurs after the supercoiling transition and is regulated by the Anaphase Promoting Complex.|