Real-time imaging of DNA loop extrusion by condensin

Live imaging of DNA loop extrusion To spatially organize chromosomes, ring-shaped protein complexes including condensin and cohesin have been hypothesized to extrude DNA loops. Condensin has been shown to exhibit a DNA-translocating motor function, but extrusion has not been observed directly. Using single-molecule imaging, Ganji et al. visualized in real time a condensin-mediated, adenosine triphosphate-dependent, fast DNA loop extrusion process. Loop extrusion occurred asymmetrically, with condensin reeling in only one end of the DNA. These data provide unambiguous evidence of a loop extrusion mechanism for chromosome organization. Science, this issue p. 102 Single-molecule imaging supports a loop extrusion mechanism for the spatial organization of chromosomes. It has been hypothesized that SMC protein complexes such as condensin and cohesin spatially organize chromosomes by extruding DNA into large loops. We directly visualized the formation and processive extension of DNA loops by yeast condensin in real time. Our findings constitute unambiguous evidence for loop extrusion. We observed that a single condensin complex is able to extrude tens of kilobase pairs of DNA at a force-dependent speed of up to 1500 base pairs per second, using the energy of adenosine triphosphate hydrolysis. Condensin-induced loop extrusion was strictly asymmetric, which demonstrates that condensin anchors onto DNA and reels it in from only one side. Active DNA loop extrusion by SMC complexes may provide the universal unifying principle for genome organization.