Transfer RNA Genes Affect Chromosome Structure and Function via Local Effects
Omar HamdaniNamrita DhillonTsung-Han S. HsiehTakahiro FujitaJosefina OcampoJacob G. KirklandJosh LawrimoreTetsuya KobayashiBarbara FriedmanDerek FultonKenneth WuRăzvan V. CherejiMasaya OkiKerry BloomDavid J. ClarkOliver J. RandoRohinton T. Kamakaka
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Abstract The genome is packaged and organized in an ordered, non-random manner and specific chromatin segments contact nuclear substructures to mediate this organization. While transfer RNA genes (tDNAs) are essential for the generation of tRNAs, these loci are also binding sites for transcription factors and architectural proteins and are thought to play an important role in the organization of the genome. In this study, we investigate the role of tDNAs in genomic organization and chromosome function by editing a chromosome so that it lacks any tDNAs. Surprisingly our analyses of this tDNA-less chromosome show that loss of tDNAs does not grossly affect chromosome folding or chromosome tethering. However, loss of tDNAs affects local nucleosome positioning and the binding of SMC proteins at these loci. The absence of tDNAs also leads to changes in centromere clustering and a reduction in the frequency of long range HML-HMR heterochromatin clustering. We propose that the tDNAs primarily affect local chromatin structure that result in effects on long-range chromosome architecture.Keywords:
Chromosome conformation capture
Chromosome conformation capture
ChIA-PET
Scaffold/matrix attachment region
Bivalent chromatin
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The organization of chromatin structure plays a crucial role in gene expression, DNA replication, and repair. Chromatin alterations influence gene expression, and modifications could be associated with genomic instability in the cells during aging or diseases. Here, we provide a modified protocol to isolate fixed neuronal nuclei from a single mouse cortex to investigate the spatial organization of chromatin structure on a genome-wide scale by ATAC-seq (the assay for transposase-accessible chromatin with high-throughput sequencing) and chromatin conformation by Hi-C (high-throughput chromosome conformation capture).
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ABSTRACT We introduce a computational model to simulate chromatin structure and dynamics. Starting from one-dimensional genomics and epigenomics data that are available for hundreds of cell types, this model enables de novo prediction of chromatin structures at five-kilo-base resolution. Simulated chromatin structures recapitulate known features of genome organization, including the formation of chromatin loops, topologically associating domains (TADs) and compartments, and are in quantitative agreement with chromosome conformation capture experiments and super-resolution microscopy measurements. Detailed characterization of the predicted structural ensemble reveals the dynamical flexibility of chromatin loops and the presence of cross-talk among neighboring TADs. Analysis of the model’s energy function uncovers distinct mechanisms for chromatin folding at various length scales.
Chromosome conformation capture
Epigenomics
Folding (DSP implementation)
Genomic Organization
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Chromosome conformation capture
ChIA-PET
Hypersensitive site
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Centromere is a specialized chromatin domain that plays a vital role in chromosome segregation. In most eukaryotes, centromere is surrounded by the epigenetically distinct heterochromatin domain. Heterochromatin has been shown to contribute to centromere function, but the precise role of heterochromatin in centromere specification remains elusive. Centromeres in most eukaryotes, including fission yeast (Schizosaccharomyces pombe), are defined epigenetically by the histone H3 (H3) variant CENP-A. In contrast, the budding yeast Saccharomyces cerevisiae has genetically-defined point centromeres. The transition between regional centromeres and point centromeres is considered as one of the most dramatic evolutionary events in centromere evolution. Here we demonstrated that Cse4, the budding yeast CENP-A homolog, can localize to centromeres in fission yeast and partially substitute fission yeast CENP-ACnp1. But overexpression of Cse4 results in its localization to heterochromatic regions. Cse4 is subject to efficient ubiquitin-dependent degradation in S. pombe, and its N-terminal domain dictates its centromere distribution via ubiquitination. Notably, without heterochromatin and RNA interference (RNAi), Cse4 fails to associate with centromeres. We showed that RNAi-dependent heterochromatin mediates centromeric localization of Cse4 by protecting Cse4 from ubiquitin-dependent degradation. Heterochromatin also contributes to the association of native CENP-ACnp1 with centromeres via the same mechanism. These findings suggest that protection of CENP-A from degradation by heterochromatin is a general mechanism used for centromere assembly, and also provide novel insights into centromere evolution.
Heterochromatin protein 1
Schizosaccharomyces
Chromatin immunoprecipitation
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The centromeric nucleosome of budding yeast is perfectly positioned and covers the entire centromere
The centromeres of budding yeast are ∼120 bp in size and contain three functional elements: an AT-rich region flanked by binding sites for Cbf1 and CBF3. A specialized nucleosome containing the H3 variant Cse4 (CenH3) is formed at the centromere. Our genome-wide paired-end sequencing of nucleosomal DNA reveals that the centromeric nucleosome contains a micrococcal nuclease-resistant kernel of 123–135 bp, depending on the centromere, and is therefore significantly shorter than the canonical nucleosome. Unlike canonical nucleosomes, the centromeric nucleosome is essentially perfectly positioned. The entire centromere is included, together with at least 1 bp of DNA upstream of the Cbf1 site and at least 4 bp downstream of the CBF3 site. The fact that the binding sites for Cbf1 and CBF3 are included within the centromeric nucleosome has important implications for models of the centromeric nucleosome and for kinetochore function.
Micrococcal nuclease
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Chromosome conformation capture
ChIA-PET
Scaffold/matrix attachment region
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The establishment of centromere-specific CENP-A chromatin is influenced by epigenetic and genetic processes. Central domain sequences from fission yeast centromeres are preferred substrates for CENP-ACnp1 incorporation, but their use is context dependent, requiring adjacent heterochromatin. CENP-ACnp1 overexpression bypasses heterochromatin dependency, suggesting that heterochromatin ensures exposure to conditions or locations permissive for CENP-ACnp1 assembly. Centromeres cluster around spindle-pole bodies (SPBs). We show that heterochromatin-bearing minichromosomes localize close to SPBs, consistent with this location promoting CENP-ACnp1 incorporation. We demonstrate that heterochromatin-independent de novo CENP-ACnp1 chromatin assembly occurs when central domain DNA is placed near, but not far from, endogenous centromeres or neocentromeres. Moreover, direct tethering of central domain DNA at SPBs permits CENP-ACnp1 assembly, suggesting that the nuclear compartment surrounding SPBs is permissive for CENP-ACnp1 incorporation because target sequences are exposed to high levels of CENP-ACnp1 and associated assembly factors. Thus, nuclear spatial organization is a key epigenetic factor that influences centromere identity.
Heterochromatin protein 1
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The present study is to determine the effects of centromeric heterochromatin on centromere separation. Amniotic cell cultures in which the centromeric heterochromatin of one chromosome was at least twice as large (qh+) as the heterochromatin (qh) in the homologous chromosome were selected. Fifteen amniotic cell samples with 1qh+, 9qh+ or 16qh+ were studied. The size of the centromeric heterochromatin was directly correlated with the delay in centromere separation. The chromosome with the smaller centromeric heterochromatin tended to show earlier centromere separation than the homologue with the larger heterochromatin. Our results suggest that the quantity of centromeric heterochromatin may influence the genetic control of centromere separation.
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The long-range interactions of cis-regulatory elements (cREs) play a central role in regulating the spatial-temporal gene expression program of multi-cellular organism. cREs are characterized by the presence of accessible (or open) chromatin, which can be identified at genome-wide scale with assays such as ATAC-seq, DHS-seq, and FAIRE-seq. However, it remains technically challenging to comprehensively identify the long-range physical interactions that occur between cREs, especially in a cost effective manner using low-input samples. Here, we report HiCAR (High-throughput Chromosome conformation capture on Accessible DNA with mRNA-seq co-assay), a method that enables simultaneous assessment of cis-regulatory chromatin interactions and chromatin accessibility, as well as evaluation of the transcriptome, which represents the functional output of chromatin structure and accessibility. Unlike immunoprecipitation-based methods such as HiChIP, PLAC-seq, and ChIA-PET, HiCAR does not require target-specific antibodies and thus can comprehensively capture the cis-regulatory chromatin contacts anchored at accessible regulatory DNA regions and associated with diverse epigenetic modifications and transcription factor binding. Compared to Trac-looping, another method designed to capture interactions between accessible chromatin regions, HiCAR produced a 17-fold greater yield of informative long-range cis- reads at a similar sequencing depth and required 1,000-fold fewer cells as input. Applying HiCAR to H1 human embryonic stem cells (hESCs) revealed 46,792 cis-regulatory chromatin interactions at 5kb resolution. Interestingly, we found that epigenetically poised, bivalent, and repressed cREs exhibit comparable spatial interaction activity to those transcriptionally activated cREs. Using machine learning approaches, we predicated 22 epigenome features that are potentially important for the spatial interaction activity of cREs in H1 hESC. Lastly, we also identified long-range cis-regulatory chromatin interactions in GM12878 and mouse embryonic stem cells with HiCAR. Our results demonstrate that HiCAR is a robust and cost-effective multi-omics assay, which is broadly applicable for simultaneous analysis of genome architecture, chromatin accessibility, and the transcriptome using low-input samples.
Chromosome conformation capture
ChIA-PET
Chromatin immunoprecipitation
ChIP-sequencing
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