Cohesin chromatin loop formation by an extrinsic motor (ChIP-Seq)
The ring-shaped cohesin complex topologically entraps two DNAs to establish sister chromatid cohesion. Cohesin also shapes the interphase chromatin landscape, with wide-ranging implications for gene regulation, which cohesin is thought to achieve by actively extruding DNA loops without topologically entrapping DNA. The 'loop extrusion' model find motivation from in vitro observations - whether this process indeed underlies chromatin loop formation in vivo 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, likely by generating DNA substrates for topological loop capture. Transcription furthermore acts as an extrinsic motor that, by pushing cohesin along transcription units, extends chromatin loops and defines their ultimate positions. Our results necessitate a re-evaluation of the loop extrusion hypothesis and point to an alternative mechanism for cohesin-dependent chromatin organisation. Loop formation by DNA-DNA capture, akin to sister chromatid cohesion establishment at replication forks, unifies cohesin's two roles in chromosome segregation and interphase genome organisation. Overall design: Cohesin calibrated ChIP sequencing and Micro-C. Two independent replicates are provided