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Nucleosome dynamics render heterochromatin generally accessible in living human cells (MNase-seq in MCF7 nuclei)

ABSTRACT: The eukaryotic genome is packaged into chromatin, which resembles beads on a string. Each bead consists of a nucleosome containing ~147 bp of DNA wrapped around a histone octamer. The nucleosomal filament undergoes additional folding to create a complex genomic organisation which is thought to restrict the access of gene-regulatory transcription factors to the genome. Consequently, it is widely accepted that DNA accessibility, modulated by chromatin architecture, is pivotal in gene regulation. Here, we measure genome accessibility at all GATC sites in living human MCF7 and MCF10A cells, using an adenovirus vector to express the sequence-specific dam DNA adenine methyltransferase. We find that the human genome is globally accessible in living cells, unlike in isolated nuclei. Active promoters are methylated somewhat faster than gene bodies and inactive promoters. Remarkably, both constitutive and facultative heterochromatic sites are methylated only marginally more slowly than euchromatic sites. In contrast, sites in centromeric chromatin are methylated slowly and are partly inaccessible. We conclude that nucleosomes in euchromatin and heterochromatin are highly dynamic in living cells, whereas nucleosomes in centromeric asatellite chromatin are static. A dynamic chromatin architecture implies that simple occlusion of transcription factor binding sites is unlikely to be critical in gene regulation.

ORGANISM(S): Homo sapiens

PROVIDER: GSE282874 | GEO | 2025/05/01

REPOSITORIES: GEO

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Project description:Specialized chromatin containing CENP-A nucleosomes instead of H3 nucleosomes is found at all centromeres. However, the mechanisms that specify the locations at which CENP-A chromatin is assembled remain elusive in organisms with regional, epigenetically regulated centromeres. It is known that normal centromeric DNA is transcribed in several systems including the fission yeast, Schizosaccharomyces pombe. Here, we show that factors which preserve stable histone H3 chromatin during transcription also play a role in preventing promiscuous CENP-A(Cnp1) deposition in fission yeast. Mutations in the histone chaperone FACT impair the maintenance of H3 chromatin on transcribed regions and promote widespread CENP-A(Cnp1) incorporation at non-centromeric sites. FACT has little or no effect on CENP-A(Cnp1) assembly at endogenous centromeres where CENP-A(Cnp1) is normally assembled. In contrast, Clr6 complex II (Clr6-CII; equivalent to Rpd3S) histone deacetylase function has a more subtle impact on the stability of transcribed H3 chromatin and acts to prevent the ectopic accumulation of CENP-A(Cnp1) at specific loci, including subtelomeric regions, where CENP-A(Cnp1) is preferentially assembled. Moreover, defective Clr6-CII function allows the de novo assembly of CENP-A(Cnp1) chromatin on centromeric DNA, bypassing the normal requirement for heterochromatin. Thus, our analyses show that alterations in the process of chromatin assembly during transcription can destabilize H3 nucleosomes and thereby allow CENP-A(Cnp1) to assemble in its place. We propose that normal centromeres provide a specific chromatin context that limits reassembly of H3 chromatin during transcription and thereby promotes the establishment of CENP-A(Cnp1) chromatin and associated kinetochores. These findings have important implications for genetic and epigenetic processes involved in centromere specification. In total, 24 samples: 22 ChIP DNA files (10 different conditions), 2 Input files.

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Nucleosome dynamics render heterochromatin generally accessible in living human cells (MNase-seq in MCF7 nuclei)

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