Nucleosomes are arranged on DNA into a ‘beads on a string’ conformation approximately 10 nanometers (nm) in diameter called the 10 nm fiber ( Kornberg, 1974 Davies and Haynes, 1976 Finch and Klug, 1976). The basic unit of chromatin is the nucleosome, 147 base pairs (bp) of DNA wrapped around an octamer of histone proteins containing two copies each of histones H2A, H2B, H3, and H4 ( Kornberg, 1974 Luger et al., 1997 Davey et al., 2002). The genetic material of eukaryotic cells is organized into a nucleoprotein complex called chromatin, which regulates the accessibility of the underlying sequence to all DNA-dependent processes. These results reveal that global changes in chromatin fiber compaction can occur during cell state transitions, and establish physiological roles for local chromatin fiber folding in regulating transcription and chromatin domain formation. This quiescence-specific fiber folding globally represses transcription and inhibits chromatin loop extrusion by condensin. ![]() Unlike actively dividing cells, inter-nucleosomal interactions in quiescent cells require a basic patch in the histone H4 tail. We show that an increase in the range of local inter-nucleosomal contacts in quiescent yeast drives the compaction of chromatin fibers genome-wide. Further, global changes in fiber diameters have not been observed, even between interphase and mitotic chromosomes. ![]() ![]() However, it has been difficult to determine the relationship between local chromatin fiber compaction and transcription in cells. A longstanding hypothesis is that chromatin fiber folding mediated by interactions between nearby nucleosomes represses transcription.
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