Smc2 and Smc4 form a V-shaped heterodimer in which two ATPase heads face each other at the apices of two 50-nm-long coiled-coil arms. Eukaryotic condensins are composed of the Smc2 and Smc4 ATPase subunits, called Cut14 and Cut3 in fission yeast, and three non-SMC subunits. In contrast, how condensins reconfigure chromosome structure in a cell cycle-regulated manner remains poorly understood.Ĭondensins are ring-shaped ATPases that belong to the family of SMC (Structural Maintenance of Chromosomes) protein complexes, which also include cohesin, responsible for sister chromatid cohesion, and the Smc5/Smc6 complex, which is implicated in DNA damage repair (Thadani et al, 2012 Aragon et al, 2013 Hirano, 2016). It is widely accepted that Topo II ensures decatenation of sister chromatids and chromosomes (Baxter et al, 2011 Charbin et al, 2014). From yeasts to human, chromosome condensation relies upon condensin complexes and topoisomerase II-α (Topo II). This profound reorganization, called chromosome condensation, is a strict prerequisite for the accurate segregation of chromosomes. In most eukaryotes, chromatin fibres metamorphose into compact and individualized rod-shaped chromosomes during mitosis and meiosis.
#CHROMOSOMES CONDENSE FREE#
We propose that nucleosomes act as a barrier for the initial binding of condensin and that nucleosome-depleted regions formed at highly expressed genes by transcriptional coactivators constitute access points into chromosomes where condensin binds free genomic DNA. The function of Gcn5 is closely linked to condensin positioning, since neither the localization of topoisomerase II nor that of the cohesin loader Mis4 is altered in gcn5 mutant cells. Two transcriptional coactivators, the Gcn5 histone acetyltransferase and the RSC chromatin-remodelling complex, bind to promoters adjoining condensin-binding sites and locally evict nucleosomes to facilitate condensin binding and allow efficient mitotic chromosome condensation.
![chromosomes condense chromosomes condense](http://biology4ibdp.weebly.com/uploads/9/0/8/0/9080078/617361490.jpg)
Here, we report that condensin accumulates at or in the immediate vicinity of nucleosome-depleted regions during fission yeast mitosis. Condensins are enriched nearby highly expressed genes during mitosis, but how this binding is achieved and what features associated with transcription attract condensins remain unclear.
![chromosomes condense chromosomes condense](https://c2.staticflickr.com/4/3085/3111568678_5705ba59d5_z.jpg)
Condensins associate with DNA and shape mitotic chromosomes.