Project description:The access of Transcription Factors (TFs) to their cognate DNA binding motifs requires a precise control over nucleosome positioning. This is especially important following DNA replication and during mitosis, both resulting in profound changes in nucleosome organization over TF binding regions. Using mouse Embryonic Stem (ES) cells, we show that the TF CTCF displaces nucleosomes from its binding site and locally organizes large and phased nucleosomal arrays, not only in interphase steady-state but also immediately after replication and during mitosis. While regions bound by other TFs, such as Oct4 and Sox2, display major rearrangement, the post-replication and mitotic nucleosome organization activity of CTCF is not likely to be unique: Esrrb binding regions are also characterized by persistent nucleosome positioning. Therefore, selected TFs such as CTCF and Esrrb act as resilient TFs governing the inheritance of nucleosome positioning at gene regulatory regions throughout the ES cell-cycle.
Project description:During mitosis, transcription is globally attenuated and chromatin architecture is dramatically reconfigured. Here we exploited the M-phase to G1-phase progression to interrogate the contributions of the architectural factor CTCF and the process of transcription to re-sculpting the genome in newborn nuclei. While CTCF appears to be dispensable for large scale post-mitotic compartmentalization, depletion of CTCF specifically during the M-phase to G1-phase transition alters the re-establishment of local short-range compartmentalization after mitosis. Without CTCF, structural loops fail to reform, leading to illegitimate contacts between cis-regulatory elements (CREs) and altered gene expression in G1-phase. Transient CRE contacts that are normally resolved after telophase persist deeply into G1-phase in CTCF depleted cells. Boundary reformation is largely disrupted upon CTCF loss. Yet, a subset (~27%) of boundaries emerges normally in the absence of CTCF and is characterized by transitions in chromatin states. Reformation of gene domains can occur prior to the full onset of transcription and can be linked to tri-methylation at lysine 36 of histone 3 (H3K36me3), a mark stable throughout mitosis. The focus on the de novo formation of nuclear architecture during G1 entry yielded novel insights into how CTCF and the process of transcription contribute to the dynamic re-configuration of chromatin architecture during the mitosis to G1 phase progression.
Project description:During mitosis, transcription is globally attenuated and chromatin architecture is dramatically reconfigured. Here we exploited the M-phase to G1-phase progression to interrogate the contributions of the architectural factor CTCF and the process of transcription to re-sculpting the genome in newborn nuclei. While CTCF appears to be dispensable for large scale post-mitotic compartmentalization, depletion of CTCF specifically during the M-phase to G1-phase transition alters the re-establishment of local short-range compartmentalization after mitosis. Without CTCF, structural loops fail to reform, leading to illegitimate contacts between cis-regulatory elements (CREs) and altered gene expression in G1-phase. Transient CRE contacts that are normally resolved after telophase persist deeply into G1-phase in CTCF depleted cells. Boundary reformation is largely disrupted upon CTCF loss. Yet, a subset (~27%) of boundaries emerges normally in the absence of CTCF and is characterized by transitions in chromatin states. Reformation of gene domains can occur prior to the full onset of transcription and can be linked to tri-methylation at lysine 36 of histone 3 (H3K36me3), a mark stable throughout mitosis. The focus on the de novo formation of nuclear architecture during G1 entry yielded novel insights into how CTCF and the process of transcription contribute to the dynamic re-configuration of chromatin architecture during the mitosis to G1 phase progression.
Project description:The experiment was performed to assess the importance of nucleosome eviction for the binding of condensin to chromatin during mitosis. The condensin complex associates with chromatin during mitosis to ensure chromosome condensation and accurate segregation, but how this binding is achieved remains poorly understood. Our study indicates that transcription co-activators Gcn5 and Mst2 assist condensin binding during mitosis by evicting nucleosomes. To reach this conclusion, we mapped nucleosomes, during mitosis, in wild-type and gcn5mst2 mutant strains. This experiment allowed us (1) to identify nucleosome-depleted regions (ndrs) during mitosis and to show that condensin tends to colocalize with ndr at the 3ends of genes, and (2) to confirm the importance of nucleosome eviction from ndrs by gcn5 and mst2, during mitosis, for the binding of condensin to chromatin.this experiment determines the patterns of nucleosomes in wild type and mutant fission yeast cells arrested in early mitosis. We analysed wild-type cells, cells lacking Gcn5 histone acetylatransferase (gcn5D) and cells lacking both Gcn5 and Mst2 histone acetyltransferases (Gcn5Dmst2D). All cells used in this study expressed a GFP-tagged version of condensin (Cnd2-GFP) and were blocked in early mitosis at 19C by the nda3-KM311 mutation. Mitotic indexes were determined by scoring the accumulation of Cnd2-GFP in the nucleus. Mitotic cells were collected and chromatin was digested by increasing amount of MNase to produce mononucleosomes. Mononucleosomal DNA was purified on an agarose gel and sequence on an Illumina Nextseq 500 apparatus. Three replicates of wild type, gcn5D and gcn5Dmst2D were analysed. Nucleosome patterns were determined in wild-type cells, cells lacking Gcn5 and cells lacking both Gcn5 and Mst2.
Project description:Catalytic activity of the ISWI family of remodelers is critical for nucleosomal organization and transcription factor binding, including the insulator protein CTCF. To define which subcomplex mediates these diverse functions we phenotyped a panel of isogenic mouse stem cell lines each lacking one of six ISWI accessory subunits. Individual deletions of either CERF, RSF1, ACF, WICH or NoRC subcomplexes only moderately affect the chromatin landscape, while removal of the NURF-specific subunit BPTF leads to drastic reduction in chromatin accessibility and Snf2h ATPase localization around CTCF sites. While this reduces distances to the adjacent nucleosomes it only modestly impacts CTCF binding itself. In absence of accessibility, the insulator function of CTCF is nevertheless impaired resulting in lower occupancy of cohesin and cohesin-loading factors, and reduced insulation at these sites, highlighting the need of NURF-mediated remodeling for open chromatin and proper CTCF function. Our comprehensive analysis reveals a specific role for NURF in mediating Snf2h localization and chromatin opening at bound CTCF sites showing that local accessibility is critical for cohesin binding and insulator function.
Project description:Mitotic bookmarking transcription factors (BFs) maintain the capacity to bind to their targets during mitosis, despite major rearrangements of the chromatin. While they were thought to propagate gene regulatory information through mitosis by statically occupying their DNA targets, it has recently become clear that BFs are highly dynamic in mitotic cells. This represents both a technical and a conceptual challenge to study and understand the function of BFs: first, formaldehyde has been suggested to be unable to efficiently capture these transient interactions, leading to profound contradictions in the literature; second, if BFs are not permanently bound to their targets during mitosis, it becomes unclear how they convey regulatory information to daughter cells. Here, comparing formaldehyde to alternative fixatives we clarify the nature of the chromosomal association of previously proposed BFs in embryonic stem cells: while ESRRB can be considered as a canonical BF that binds at selected regulatory regions in mitosis, SOX2 and OCT4 establish DNA sequence independent interactions with the mitotic chromosomes, either throughout the chromosomal arms (SOX2) or at pericentromeric regions (OCT4). Moreover, we show that ordered nucleosomal arrays are retained during mitosis at ESRRB bookmarked sites, whereas regions losing transcription factor binding display a profound loss of order. By maintaining nucleosome positioning during mitosis, ESRRB might ensure the rapid post-mitotic re-establishment of functional regulatory complexes at selected enhancers and promoters. Our results provide a mechanistic framework that reconciles dynamic mitotic binding with the transmission of gene regulatory information across cell division.
Project description:We report genome wide mapping of the histone variant H2A.Z during G0/G1 and mitosis in T24 bladder cancer cells. The results show that the broad enrichment pattern of H2A.Z near transcription start sites of active genes is maintained during mitosis. Furthermore, using H2A.Z localization to visualize nucleosome positioning near the start site, we see that the +1 nucleosome of active genes shifts upstream to occupy the transcription start sites during mitosis and the nucleosome depleted region is shortened. H2A.Z is also maintained on the -2 nucleosome which also shifts towrds the transcription start site during mitosis, further contributing to the shorteneing of the nucleosome depleted region. Examination of H2A.Z duing G0/G1 and mitosis in bladder cancer cells