Transcription-mediated supercoiling regulates genome folding and loop formation [ChIP-Seq]

The chromatin fiber folds into loops, however the mechanisms controlling loop extrusion are still poorly understood. Using super-resolution microscopy, we visualized that loops in intact nuclei are formed by a scaffold of cohesin complexes from which the DNA protrudes. RNA polymerase II decorates the top of the loops and is physically segregated from cohesin. Augmented looping upon increased loading of cohesin on chromosomes causes disruption of Lamin at the nuclear rim and chromatin blending, a homogenous distribution of chromatin within the nucleus. Altering supercoiling via either transcription or topoisomerase inhibition counteracts chromatin blending, increases chromatin condensation, disrupts loop formation and leads to altered cohesin distribution and mobility on chromatin. Overall, negative supercoiling generated by transcription is an important regulator of loop formation in vivo. Overall design: ChIP profiling of H3K4me2 histone mark in HeLa cells (control DMSO treated)