Structural maintenance of chromosomes (SMC) protein complexes are key determinants of chromosome conformation. Using Hi-C and polymer modelling, we study how cohesin and condensin, two deeply conserved SMC complexes, organize chromosomes in the budding yeast Saccharomyces cerevisiae. The canonical role of cohesin is to co-align sister chromatids, while condensin generally compacts mitotic chromosomes. We find strikingly different roles for the two complexes in budding yeast mitosis. First, cohesin is responsible for compacting mitotic chromosome arms, independently of sister chromatid cohesion. Polymer simulations demonstrate that this role can be fully accounted for through cis-looping of chromatin. Second, condensin is generally dispensable for compaction along chromosome arms. Instead, it plays a targeted role compacting the rDNA proximal regions and promoting resolution of peri-centromeric regions. Our results argue that the conserved mechanism of SMC complexes is to form chromatin loops and that distinct SMC-dependent looping activities are selectively deployed to appropriately compact chromosomes.

中文翻译：

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SMC 根据基因组背景复合不同压缩的有丝分裂染色体。


染色体结构维持 (SMC) 蛋白复合物是染色体构象的关键决定因素。使用 Hi-C 和聚合物模型，我们研究了粘连蛋白和凝缩蛋白（两种高度保守的 SMC 复合物）如何组织芽殖酵母酿酒酵母中的染色体。粘连蛋白的典型作用是共同排列姐妹染色单体，而凝缩蛋白通常压缩有丝分裂染色体。我们发现这两种复合物在芽殖酵母有丝分裂中的作用截然不同。首先，粘连蛋白负责压缩有丝分裂染色体臂，独立于姐妹染色单体的粘连。聚合物模拟表明，这种作用可以通过染色质的顺式环化来充分解释。其次，对于沿染色体臂的压缩，凝聚素通常是不必要的。相反，它起到压缩 rDNA 近端区域并促进着丝粒周围区域解析的目标作用。我们的结果表明，SMC 复合物的保守机制是形成染色质环，并且不同的 SMC 依赖性环活性被选择性地部署到适当压缩的染色体上。