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普遍存在的人类“主”复制起点通过核小体排除能量屏障的局部富集编码于DNA序列中。

Ubiquitous human 'master' origins of replication are encoded in the DNA sequence via a local enrichment in nucleosome excluding energy barriers.

作者信息

Drillon Guénola, Audit Benjamin, Argoul Françoise, Arneodo Alain

机构信息

Université de Lyon, F-69000 Lyon, France. Laboratoire de Physique, CNRS UMR 5672, École Normale Supérieure de Lyon, F-69007 Lyon, France.

出版信息

J Phys Condens Matter. 2015 Feb 18;27(6):064102. doi: 10.1088/0953-8984/27/6/064102. Epub 2015 Jan 7.

Abstract

As the elementary building block of eukaryotic chromatin, the nucleosome is at the heart of the compromise between the necessity of compacting DNA in the cell nucleus and the required accessibility to regulatory proteins. The recent availability of genome-wide experimental maps of nucleosome positions for many different organisms and cell types has provided an unprecedented opportunity to elucidate to what extent the DNA sequence conditions the primary structure of chromatin and in turn participates in the chromatin-mediated regulation of nuclear functions, such as gene expression and DNA replication. In this study, we use in vivo and in vitro genome-wide nucleosome occupancy data together with the set of nucleosome-free regions (NFRs) predicted by a physical model of nucleosome formation based on sequence-dependent bending properties of the DNA double-helix, to investigate the role of intrinsic nucleosome occupancy in the regulation of the replication spatio-temporal programme in human. We focus our analysis on the so-called replication U/N-domains that were shown to cover about half of the human genome in the germline (skew-N domains) as well as in embryonic stem cells, somatic and HeLa cells (mean replication timing U-domains). The 'master' origins of replication (MaOris) that border these megabase-sized U/N-domains were found to be specified by a few hundred kb wide regions that are hyper-sensitive to DNase I cleavage, hypomethylated, and enriched in epigenetic marks involved in transcription regulation, the hallmarks of localized open chromatin structures. Here we show that replication U/N-domain borders that are conserved in all considered cell lines have an environment highly enriched in nucleosome-excluding-energy barriers, suggesting that these ubiquitous MaOris have been selected during evolution. In contrast, MaOris that are cell-type-specific are mainly regulated epigenetically and are no longer favoured by a local abundance of intrinsic NFRs encoded in the DNA sequence. At the smaller few hundred bp scale of gene promoters, CpG-rich promoters of housekeeping genes found nearby ubiquitous MaOris as well as CpG-poor promoters of tissue-specific genes found nearby cell-type-specific MaOris, both correspond to in vivo NFRs that are not coded as nucleosome-excluding-energy barriers. Whereas the former promoters are likely to correspond to high occupancy transcription factor binding regions, the latter are an illustration that gene regulation in human is typically cell-type-specific.

摘要

作为真核染色质的基本构建单元,核小体处于细胞核中DNA压缩需求与调节蛋白所需可及性之间权衡的核心位置。近期,许多不同生物体和细胞类型的全基因组核小体位置实验图谱的可得性,为阐明DNA序列在多大程度上决定染色质的一级结构,并进而参与染色质介导的核功能调节(如基因表达和DNA复制)提供了前所未有的机会。在本研究中,我们使用体内和体外全基因组核小体占有率数据,以及基于DNA双螺旋序列依赖性弯曲特性的核小体形成物理模型预测的无核小体区域(NFRs)集合,来研究内在核小体占有率在人类复制时空程序调节中的作用。我们将分析重点放在所谓的复制U/N结构域上,这些结构域在种系中(偏斜N结构域)以及胚胎干细胞、体细胞和HeLa细胞中(平均复制时间U结构域)覆盖了约一半的人类基因组。发现与这些兆碱基大小的U/N结构域相邻的“主”复制起点(MaOris)由几百千碱基宽的区域指定,这些区域对DNase I切割高度敏感、低甲基化,并富含参与转录调节的表观遗传标记,即局部开放染色质结构的特征。在这里,我们表明在所有考虑的细胞系中保守的复制U/N结构域边界具有高度富含核小体排除能量屏障的环境,这表明这些普遍存在的MaOris在进化过程中被选择。相比之下,细胞类型特异性的MaOris主要受表观遗传调节,不再受DNA序列中编码的局部大量内在NFRs的青睐。在基因启动子几百碱基的较小尺度上,在普遍存在的MaOris附近发现的管家基因富含CpG的启动子以及在细胞类型特异性MaOris附近发现的组织特异性基因贫CpG的启动子,都对应于体内不是作为核小体排除能量屏障编码的NFRs。前者的启动子可能对应于高占有率转录因子结合区域,而后者说明人类的基因调节通常是细胞类型特异性的。

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