Chromatin Fiber Folding Represses Transcription and Loop Extrusion in Quiescent Cells

Determining the conformation of chromatin in cells at the nucleosome level and its relationship to cellular processes has been a major challenge in biology. We have found that in quiescent yeast, widespread transcriptional repression coincides with the compaction of chromatin fibers into structures that are less condensed and more heteromorphic than canonical 30-nanometer fibers, but are similarly dependent on the histone H4 tail. Acetylation or substitution of H4 residues decompacts fibers and results in global transcriptional de-repression. Unexpectedly, fiber decompaction also increases the rate of loop extrusion by condensin. These findings establish a role for chromatin fiber folding in regulating transcription and loop extrusion in cells. They also demonstrate the relevance of biochemically-determined mechanisms of chromatin folding even in the absence of regular 30-nanometer structures.