The Higher Order Structure of Chromatin and Histone H1
The Higher Order Structure of Chromatin and Histone H1
复制标题
染色质和组蛋白 H1 的高阶结构
DOI：
10.1242/jcs.1984.supplement_1.1
复制
发表时间：
1984
影响因子：
4
通讯作者：
Jean O. Thomas
中科院分区：
文献类型：
--
作者：
Jean O. Thomas
基本染色质纤维（直径10 nm的纤维）是核小体或核小体丝的线性重复阵列。每个盘状核小体的核心是一个楔形的蛋白八聚体，包含四种核心组蛋白（H3，H4，H2 A和H2 B）中的每一种的两个分子，两圈DNA以每圈约80个碱基对的左手超螺旋缠绕在其周围。这两个转弯被第五组蛋白H1（或有核红细胞中的H5）分子密封。在不同来源的染色质中，连接一个两圈颗粒与下一个两圈颗粒的接头DNA从基本上零到大约80个碱基对不等。这种变化的确切意义尚不清楚。间期染色质主要以30 nm纤维的形式存在。在一价阳离子或浓度低得多的二价阳离子存在下，核小体细丝折叠成非常相似的30 nm纤维，这是H1依赖性的，发生在体外。这些高级结构可能是由核小体丝螺旋盘绕成螺线管而产生的。染色质在溶液中折叠的系统研究，染色质片段大小和离子强度的范围内，揭示了两个不连续的行为，反映了两个结构转变。一个被解释为在离子强度为25 mm时形成一圈约有6个核小体的螺线管，并且是来自三种不同来源的染色质的共同特征，这三种不同来源的染色质在DNA重复长度以及H1的类型和数量上不同。另一种转变是根据低离子强度（低于45 mM）下长螺线管的流体动力学剪切来解释的。这表明鸡红细胞染色质比大鼠肝染色质更稳定的高阶结构（主要归因于H5的存在），并可能被证明是一个有用的一般测定不同染色质的相对稳定性，这可能与其易于解开转录。对大脑皮层神经元的短（165个碱基对）重复染色质的研究表明，在一般情况下，连接DNA可能位于螺线管的中心孔中，可能与H1一起。伸展和浓缩染色质中的H1分子（尽管当从其解离时不是）足够接近以与跨度为2-12 μ m的试剂化学交联。这表明染色质中的H1聚合物，其可能在染色质凝聚中起作用。使用可裂解的双功能氨基试剂的化学交联研究已经允许确定在延伸和缩合状态中接近的H1分子的结构域，并确定折叠时发生的变化。在另一种理解H1的作用和行为的方法中，我们进一步研究了H1分子在不同染色质片段上的位点之间迁移的能力。H1和H5（一种极端的H1变体）也会交换，但表现出不同的行为，这可能反映了它们稳定高级结构的不同能力，这在上文概述的沉积研究中有所揭示。特别是，H5显示出更大的偏好比H1的网站在更高阶的结构。
SUMMARY The basic chromatin fibre (the 10 nm diameter fibre) is a linear repeating array of nucleosomes, or nucleosome filament. The core of each disk-like nucleosome is a wedge-shaped protein octamer containing two molecules of each of the four core histones (H3, H4, H2A and H2B) around which two turns of DNA are wound in a left-handed superhelix with about 80 base-pairs per turn. The two turns are sealed by a molecule of the fifth histone H1 (or H5 in nucleated erythrocytes). The linker DNA that connects one two-turn particle to the next varies from essentially zero to about 80 base-pairs in chromatins from different sources. The exact significance of this variation is unclear. Interphase chromatin exists largely in the form of 30 nm fibres. Folding of the nucleosome filament into very similar 30 nm fibres, which is H1-dependent, occurs in vitro in the presence of monovalent cations or much lower concentrations of divalent cations. These higher-order structures probably arise by helical coiling of the nucleosome filament into a solenoid. Systematic studies of chromatin folding in solution, for a range of chromatin fragment sizes and ionic strengths, reveal two discontinuities in behaviour that reflect two structural transitions. One is interpreted as the formation of a turn of a solenoid with about six nucleosomes, at ionic strength 25 mm, and is a common feature of chromatin from three different sources, which differ in DNA repeat length, and type and amount of H1. The other transition is interpreted in terms of hydrodynamic shearing of long solenoids at low ionic strengths (below ∼45mM). It suggests a more stable higher-order structure for chicken erythrocyte chromatin than for rat liver chromatin (attributed largely to the presence of H5), and may prove to be a useful general assay for the relative stabilities of different chromatins, which might be relevant to their ease of unravelling for transcription. A study of short (165 base-pair) repeat chromatin from cerebral cortex neurons has led to the suggestion that in the general case the linker DNA might be located, perhaps with H1, in the central hole in the solenoid. H1 molecules in both extended and condensed chromatin (although not when dissociated from it) are close enough to be chemically cross-linked with reagents of span 2–12 Å. This suggests an H1 polymer in chromatin, which might have a role in chromatin condensation. Chemical cross-linking studies using cleavable, bifunctional amino-group reagents have permitted the domains of H1 molecules that are in proximity in the extended and condensed states to be determined, and changes occurring on folding to be identified. In another approach towards understanding the role and behaviour of H1 we have studied further the ability of H1 molecules to migrate between sites on different chromatin fragments. H1 and H5 (an extreme H1 variant) also exchange, but show differences in behaviour that may reflect their different abilities to stabilize higher-order structures, revealed in the sedimentation studies outlined above. In particular, H5 shows a greater preference than H1 for sites in higher-order structures.