The structural consequences of one-sided loop extrusion

SMC complexes, such as condensin or cohesin, organize chromatin in various scenarios across many cell types by a process known as loop extrusion. These complexes reel in DNA and extrude it as a loop in a directed and ATP-dependent manner. Recent in vitro experiments show that, in contrast to existing models, loop extrusion is “one-sided” so that SMC complexes reel in and extrude DNA from only one side. We develop theoretical and simulation models to determine whether one-sided extrusion can explain phenomena such as mitotic chromosome compaction, interphase TAD formation, and juxtaposition of bacterial chromosome arms. We find that purely one-sided extrusion cannot explain these phenomena, but simple variants in which extrusion becomes effectively two-sided can rescue chromosome morphology in these scenarios. Our results suggest that there must be factors in vivo that promote two-sided extrusion or other SMC complex cooperativity.