Chromosome Segregation Laboratory, The Francis Crick Institute, London, UK.
Université Paris Cité and Université Paris-Saclay, Inserm, CEA, Stabilité Génétique Cellules Souches et Radiations, Fontenay-aux-Roses, France.
EMBO J. 2024 Oct;43(19):4173-4196. doi: 10.1038/s44318-024-00202-5. Epub 2024 Aug 19.
The ring-shaped cohesin complex topologically entraps two DNA molecules to establish sister chromatid cohesion. Cohesin also shapes the interphase chromatin landscape with wide-ranging implications for gene regulation, and cohesin is thought to achieve this by actively extruding DNA loops without topologically entrapping DNA. The 'loop extrusion' hypothesis finds motivation from in vitro observations-whether this process underlies in vivo chromatin loop formation remains untested. Here, using the budding yeast S. cerevisiae, we generate cohesin variants that have lost their ability to extrude DNA loops but retain their ability to topologically entrap DNA. Analysis of these variants suggests that in vivo chromatin loops form independently of loop extrusion. Instead, we find that transcription promotes loop formation, and acts as an extrinsic motor that expands these loops and defines their ultimate positions. Our results necessitate a re-evaluation of the loop extrusion hypothesis. We propose that cohesin, akin to sister chromatid cohesion establishment at replication forks, forms chromatin loops by DNA-DNA capture at places of transcription, thus unifying cohesin's two roles in chromosome segregation and interphase genome organisation.
环状黏合蛋白复合物在拓扑学上束缚两个 DNA 分子以建立姐妹染色单体的黏合。黏合蛋白还通过广泛影响基因调控来塑造有丝分裂期染色质景观，并且黏合蛋白被认为通过主动推出 DNA 环而不拓扑学束缚 DNA 来实现这一点。“环推出”假说从体外观察中找到了动力——这一过程是否是体内染色质环形成的基础仍未得到检验。在这里，我们使用酿酒酵母 S. cerevisiae 产生了失去推出 DNA 环能力但保留拓扑学束缚 DNA 能力的黏合蛋白变体。对这些变体的分析表明，体内染色质环的形成独立于环推出。相反，我们发现转录促进了环的形成，并作为一种外在的动力，扩展了这些环并确定了它们的最终位置。我们的结果需要重新评估环推出假说。我们提出，黏合蛋白类似于复制叉处姐妹染色单体黏合的建立，通过在转录部位的 DNA-DNA 捕获形成染色质环，从而将黏合蛋白在染色体分离和有丝分裂期基因组组织中的两个作用统一起来。
