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Recruitment of HMGN By H3K27ac Modified Nucleosomes to Regulatory Sites Modulates Transcription Factor Binding to Chromatin

ABSTRACT: H3K27ac modified nucleosomes are a major epigenetic mark of active chromatin that can be used to identify cell type specific chromatin regulatory regions which serve as binding sites for transcription factors. Here we show that H3K27ac modified nucleosomes recruit the ubiquitous nucleosome binding proteins HMGN1 and HMGN2 to cell-type specific chromatin regulatory regions. We find that HMGNs bind directly to the acetylated nucleosome and that the H3K27ac residue and linker DNA are necessary and sufficient for the preferential binding of HMGNs to the modified nucleosomes. Loss of HMGNs increases the levels of H3K27me3 and the histone H1 occupancy at enhancers and promoters and alters the interaction of several transcription factors with chromatin. These experiments identify an additional mechanism whereby the H3K27ac epigenetic mark promotes chromatin decompaction and provide insights into the molecular mechanism whereby HMGN proteins, which bind to chromatin without DNA sequence specificity, modulate cell type specific gene expression.

ORGANISM(S): Mus musculus

PROVIDER: GSE156697 | GEO | 2020/09/20

REPOSITORIES: GEO

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Project description:In mammalian cells transcription factors (TFs) preferentially bind sites contained in regions of high nucleosomal occupancy, as determined by nucleotide-dependent computational analysis. This observation suggests that nucleosomes may act as gatekeepers of TF binding sites. We hypothesized that in mammalian genomes the information controlling nucleosome assembly may partially coincide with the information that enables TFs to recognize cognate sites in cis-regulatory elements while ignoring the myriad of non-functional, randomly occurring consensus binding sites. This way, nucleosome-mediated masking would be coupled to TF binding site functionality. The hematopoietic master regulator Pu.1 maintained nucleosome depletion at macrophage-specific enhancers that were otherwise occupied by nucleosomes in other cell types and in reconstituted chromatin. We identified a minimal set of DNA sequence and shape features that predicted Pu.1 binding with 78% accuracy. The same features predicted nucleosome occupancy in cells where Pu.1 was not expressed with higher accuracy than specifically designed models. Control of nucleosome deposition by DNA sequence and shape features that also specify TF consensus site functionality may allow maintaining the gatekeeper function of nucleosomes during evolution of cis-regulatory elements. Chromatin immuno-precipitations of the transcription factor Pu.1 followed by multiparallel sequencing performed in murine bone marrow-derived macrophages. Experiments carried out in cells infected either with a retroviral vector containing a short hairpin targeting Pu.1 or with the empty vector as control. The shPU.1 hairpin (sequence available upon request) was selected among five designed using a publicly available software (http://katahdin.mssm.edu/siRNA) and was cloned in a modified version of TtRMPVIR inducible retroviral vector (Genbank HQ456318) in which the puromycin resistance gene was inserted. The empty vector, containing an sh-Renilla sequence, was used as control. Chromatin immuno-precipitations of the transcription factor PU.1 binding to in vitro reconstituted chromatin followed by multiparallel sequencing Micrococcal nuclease digestion of chromatin extracted from bone marrow derived macrophages followed by multiparallel sequencing . Experiments were carried out in untreated cells (4 replicates) and cells infected either with a retroviral vector containing a short hairpin targeting Pu.1 or with the empty vector as control (2 replicates). The shPU.1 hairpin (sequence available upon request) was selected among five designed using a publicly available software (http://katahdin.mssm.edu/siRNA) and was cloned in a modified version of TtRMPVIR inducible retroviral vector (Genbank HQ456318) in which the puromycin resistance gene was inserted. The empty vector, containing an sh-Renilla sequence, was used as control. Micrococcal nuclease digestion of in vitro reconstituted chromatin followed by multiparallel sequencing

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Recruitment of HMGN By H3K27ac Modified Nucleosomes to Regulatory Sites Modulates Transcription Factor Binding to Chromatin

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