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RNA polymerase II is necessary for spatial chromatin reorganization following exit from mitosis [ChIP-Seq]

ABSTRACT: Mammalian chromosomes are three-dimensional entities shaped by converging and opposing forces acting on chromatin. Mitotic cell division induces drastic chromatin condensation, but following reentry into the G1 cell cycle phase, condensed chromosomes unwind to re-establish interphase organization. Here, we use a cell line allowing auxin-mediated degradation of RNA polymerase II to test its role in this transition. In situ Hi-C and super-resolution imaging showed that RNAPII is required for compartment and loop formation following mitosis. RNAPs also often counteract loop extrusion and, in their absence, longer and more prominent loops arise. Evidence from chromatin fractionation and in silico modeling attribute these effects to RNAPII-mediated cohesin loading at active promoters upon G1 reentry. Our findings reconcile the role of RNAPII in gene expression with that in chromatin architecture.

ORGANISM(S): Homo sapiens

PROVIDER: GSE160317 | GEO | 2021/06/18

REPOSITORIES: GEO

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Project description:Cohesin organizes mammalian interphase chromosomes by reeling chromatin fibers into dynamic loops. "Loop extrusion" is obstructed when cohesin encounters a properly oriented CTCF protein. It has been proposed that transcription relocalizes or interferes with cohesin, and that active transcription start sites function as cohesin loading sites, but how these effects, and transcription in general, shape chromatin is unknown. To determine whether transcription can modulate loop extrusion, we studied cells in which the primary extrusion barriers could be removed by CTCF depletion and cohesin’s residence time and abundance on chromatin could be increased by Wapl knockout. We found evidence that transcription directly interacts with loop extrusion through a novel "moving barrier" mechanism, but not by loading cohesin at active promoters. Hi-C experiments showed intricate, cohesin-dependent genomic contact patterns near actively transcribed genes, and in CTCF-Wapl double knockout (DKO) cells, genomic contacts were enriched between sites of transcription-driven cohesin localization ("cohesin islands"). Similar patterns also emerged in polymer simulations in which transcribing RNA polymerases (RNAPs) acted as "moving barriers" by impeding, slowing, or pushing loop-extruding cohesins. The model predicts that cohesin does not load preferentially at promoters and instead accumulates at TSSs due to the barrier function of RNAPs. We tested this prediction by new ChIP-seq experiments, which revealed that the "cohesin loader" Nipbl co-localizes with cohesin, but, unlike in previous reports, Nipbl did not accumulate at active promoters. We propose that RNAP acts as a new type of barrier to loop extrusion that, unlike CTCF, is not stationary in its precise genomic position, but is itself dynamically translocating and relocalizes cohesin along DNA. In this way, loop extrusion could enable translocating RNAPs to maintain contacts with distal regulatory elements, allowing transcriptional activity to shape genomic functional organization.

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RNA polymerase II is necessary for spatial chromatin reorganization following exit from mitosis [ChIP-Seq]

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