Condensin, a structural maintenance of chromosomes (SMC) complex, has been shown to be a molecular motor protein that organizes chromosomes by extruding loops of DNA. In cells, such loop extrusion is challenged by many potential conflicts, for example, the torsional stresses that are generated by other DNA-processing enzymes. It has so far remained unclear how DNA supercoiling affects loop extrusion. Here, we use time-lapse single-molecule imaging to study condensin-driven DNA loop extrusion on supercoiled DNA. We find that condensin binding and DNA looping are stimulated by positively supercoiled DNA, and condensin preferentially binds near the tips of supercoiled plectonemes. Upon loop extrusion, condensin collects nearby plectonemes into a single supercoiled loop that is highly stable. Atomic force microscopy imaging shows that condensin generates supercoils in the presence of ATP. Our findings provide insight into the topology-regulated loading and formation of supercoiled loops by SMC complexes and clarify the interplay of loop extrusion and supercoiling.

凝缩蛋白是一种染色体结构维护 (SMC) 复合物，已被证明是一种分子马达蛋白，可通过挤压 DNA 环来组织染色体。在细胞中，这种环挤压受到许多潜在冲突的挑战，例如，由其他 DNA 加工酶产生的扭转应力。迄今为止仍不清楚 DNA 超螺旋如何影响环挤压。在这里，我们使用延时单分子成像来研究超螺旋 DNA 上凝聚素驱动的 DNA 环挤压。我们发现凝缩蛋白结合和 DNA 环化受到正超螺旋 DNA 的刺激，并且凝缩蛋白优先结合超螺旋 plectonemes 的尖端附近。在环挤出时，凝缩素将附近的多聚体收集到一个高度稳定的超螺旋环中。原子力显微镜成像显示凝缩蛋白在 ATP 存在的情况下产生超螺旋。我们的研究结果提供了对 SMC 复合物的拓扑调节负载和超螺旋环形成的深入了解，并阐明了环挤压和超螺旋的相互作用。