Chromosomes are organized as chromatin loops that promote segregation, enhancer-promoter interactions, and other genomic functions. Loops were hypothesized to form by 'loop extrusion,' by which structural maintenance of chromosomes (SMC) complexes, such as condensin and cohesin, bind to chromatin, reel it in, and extrude it as a loop. However, such exotic motor activity had never been observed. Following an explosion of indirect evidence, recent single-molecule experiments directly imaged DNA loop extrusion by condensin and cohesin in vitro. These experiments observe rapid (kb/s) extrusion that requires ATP hydrolysis and stalls under pN forces. Surprisingly, condensin extrudes loops asymmetrically, challenging previous models. Extrusion by cohesin is symmetric but requires the protein Nipbl. We discuss how SMC complexes may perform their functions on chromatin in vivo.

中文翻译：

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环挤出：理论符合单分子实验。

染色体被组织成促进分离、增强子-启动子相互作用和其他基因组功能的染色质环。假设环是通过“环挤压”形成的，通过这种方式，染色体 (SMC) 复合物的结构维持 (SMC) 复合物，例如凝聚素和粘着蛋白，与染色质结合，将其卷入并将其作为环挤压。然而，从未观察到这种奇异的运动活动。随着间接证据的激增，最近的单分子实验直接成像了体外凝聚素和凝聚素对 DNA 环的挤压。这些实验观察到需要 ATP 水解和在 pN 力下停滞的快速 (kb/s) 挤出。令人惊讶的是，condensin 不对称地挤出循环，挑战了以前的模型。cohesin 的挤出是对称的，但需要蛋白质 Nipbl。