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Polycomb repressive complex 1 regulates the nucleosome landscape but not accessibility at target genes [RNA-seq]

ABSTRACT: Polycomb repressive complexes 1 and 2 (PRC1/2) act to limit transcriptional activity at gene promoters. One of the mechanisms by which they are proposed to achieve this is through the compaction of nucleosomal chromatin. However, this model is based largely upon in vitro studies and has yet to be explored sufficiently in living cells. We therefore characterised the chromatin accessibility of Polycomb-occupied gene promoters using assay for transposase accessible chromatin (ATAC-seq). Although Polycomb-occupied promoters exist in a less accessible state compared to Polycomb-free promoters, we were surprised to observe that deletion of either PRC1, PRC2 or both PRC1/2 did not result in any increases in chromatin accessibility. This was in contrast to widespread increases in the transcriptional activity of Polycomb target genes. Together with experiments examining chromatin accessibility in the context of RNA polymerase II inhibition, we therefore demonstrate an apparent uncoupling of the chromatin accessibility and transcriptional activity of gene promoters.

ORGANISM(S): Mus musculus

PROVIDER: GSE98400 | GEO | 2018/03/19

REPOSITORIES: GEO

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Polycomb repressive complex 1 regulates the nucleosome landscape but not accessibility at target genes [ATAC-seq]

Project description:Polycomb repressive complexes 1 and 2 (PRC1/2) act to limit transcriptional activity at gene promoters. One of the mechanisms by which they are proposed to achieve this is through the compaction of nucleosomal chromatin. However, this model is based largely upon in vitro studies and has yet to be explored sufficiently in living cells. We therefore characterised the chromatin accessibility of Polycomb-occupied gene promoters using assay for transposase accessible chromatin (ATAC-seq). Although Polycomb-occupied promoters exist in a less accessible state compared to Polycomb-free promoters, we were surprised to observe that deletion of either PRC1, PRC2 or both PRC1/2 did not result in any increases in chromatin accessibility. This was in contrast to widespread increases in the transcriptional activity of Polycomb target genes. Together with experiments examining chromatin accessibility in the context of RNA polymerase II inhibition, we therefore demonstrate an apparent uncoupling of the chromatin accessibility and transcriptional activity of gene promoters.

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Mapping polycomb complexes in human and mouse embryonic stem cells

Project description:In embryonic stem (ES) cells, bivalent chromatin domains with overlapping repressive (H3 lysine 27 tri-methylation) and activating (H3 lysine 4 tri-methylation) histone modifications mark the promoters of more than 2000 genes. To gain insight into the structure and function of bivalent domains, we mapped key histone modifications and subunits of Polycomb repressive complexes 1 and 2 (PRC1 and PRC2) genomewide in human and mouse ES cells by chromatin immunoprecipitation followed by ultra high-throughput sequencing. We find that bivalent domains can be segregated into two classes: the first occupied by both PRC2 and PRC1 (PRC1-positive) and the second specifically bound by PRC2 (PRC2-only). PRC1-positive bivalent domains appear functionally distinct as they more efficiently retain lysine 27 tri-methylation upon differentiation, show stringent conservation of chromatin state, and associate with an overwhelming number of developmental regulator gene promoters. We also used computational genomics to search for sequence determinants of Polycomb binding. This analysis revealed that the genomewide locations of PRC2 and PRC1 can be largely predicted from the locations, sizes and underlying motif contents of CpG islands. We propose that large CpG islands depleted of activating motifs confer epigenetic memory by recruiting the full repertoire of Polycomb complexes. Keywords: cell type comparison Suz12, Ezh2, Ring1b ChIP-Seq in singlicate from mouse embryonic stem (mES) cells. H3K4me3, H3K27me3, H3K36me3, Ezh2 and Ring1b ChIP-Seq in singlicate from human embyonic stem cells (hES; H9).

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Project description:R-loops are three-stranded nucleic acid structures that form naturally during transcription, especially over unmethylated CpG-rich promoters. In mESC, such promoters of developmental regulator genes are occupied by the Polycomb-repressor complexes PRC1 and PRC2. Here we have explored the possibility that R-loops form over Polycomb-repressed genes and play a role in their transcriptional silencing. Using single gene and genome-wide analyses, we show that R-loops form at a specific subset of PRC-target genes and contribute to Polycomb occupancy on chromatin. Removal of R-loops leads to an up-regulation of nascent and processed transcripts and the appearance of the elongating form of RNA polymerase II. In contrast, removal of PRC2 does not influence R-loop formation, transcriptional repression and PRC1 recruitment. We finally show that R-loops and PRC1 can repress Polycomb-target genes in the absence of PRC2. Our results uncover an unanticipated synergy between R-loops and PRC1 in Polycomb repression mechanisms.

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