Thoma F, Koller T, Klug A
J Cell Biol. 1979 Nov;83(2 Pt 1):403-27. doi: 10.1083/jcb.83.2.403.
We describe the results of a systematic study, using electron microscopy, of the effects of ionic strength on the morphology of chromatin and of H1-depleted chromatin. With increasing ionic strength, chromatin folds up progressively from a filament of nucleosomes at approximately 1 mM monovalent salt through some intermediate higher-order helical structures (Thoma, F., and T. Koller, 1977, Cell 12:101-107) with a fairly constant pitch but increasing numbers of nucleosomes per turn, until finally at 60 mM (or else in approximately 0.3 mM Mg++) a thick fiber of 250 A diameter is formed, corresponding to a structurally well-organized but not perfectly regular superhelix or solenoid of pitch approximately 110 A as described by Finch and Klug (1976, Proc. Natl. Acad. Sci. U.S.A. 73:1897-1901). The numbers of nucleosomes per turn of the helical structures agree well with those which can be calculated from the light-scattering data of Campbell et al. (1978, Nucleic Acids Res. 5:1571-1580). H1-depleted chromatin also condenses with increasing ionic strength but not so densely as chromatin and not into a definite structure with a well-defined fiber direction. At very low ionic strengths, nucleosomes are present in chromatin but not in H1-depleted chromatin which has the form of an unravelled filament. At somewhat higher ionic strengths (greater than 5 mM triethanolamine chloride), nucleosomes are visible in both types of specimen but the fine details are different. In chromatin containing H1, the DNA enters and leaves the nucleosome on the same side but in chromatin depleted of H1 the entrance and exit points are much more random and more or less on opposite sides of the nucleosome. We conclude that H1 stabilizes the nucleosome and is located in the region of the exit and entry points of the DNA. This result is correlated with biochemical and x-ray crystallographic results on the internal structure of the nucleosome core to give a picture of a nucleosome in which H1 is bound to the unique region on a complete two-turn, 166 base pair particle (Fig. 15). In the formation of higher-order structures, these regions on neighboring nucleosomes come closer together so that an H1 polymer may be formed in the center of the superhelical structures.
我们描述了一项系统研究的结果，该研究使用电子显微镜来观察离子强度对染色质以及H1缺失染色质形态的影响。随着离子强度的增加，染色质从大约1 mM单价盐时的核小体细丝开始逐渐折叠，经过一些中间的高阶螺旋结构（托马，F.，和T. 科勒，1977年，《细胞》12:101 - 107），这些螺旋结构具有相当恒定的螺距，但每圈的核小体数量增加，直到最终在60 mM（或者大约在0.3 mM Mg++中）形成直径为250 Å的粗纤维，这对应于芬奇和克鲁格（1976年，《美国国家科学院院刊》73:1897 - 1901）所描述的结构良好但并非完全规则的超螺旋或螺线管，其螺距约为110 Å。螺旋结构每圈的核小体数量与根据坎贝尔等人（1978年，《核酸研究》5:1571 - 1580）的光散射数据计算得出的数量非常吻合。H1缺失的染色质也会随着离子强度的增加而凝聚，但不像染色质那样紧密，也不会形成具有明确纤维方向的确定结构。在非常低的离子强度下，核小体存在于染色质中，但不存在于呈解开细丝形式的H1缺失染色质中。在稍高的离子强度（大于5 mM氯化三乙醇胺）下，在两种类型的样本中都能看到核小体，但细节有所不同。在含有H1的染色质中，DNA从核小体的同一侧进入和离开，但在H1缺失的染色质中，进出点更加随机，或多或少位于核小体的相对两侧。我们得出结论，H1稳定核小体，并位于DNA的进出点区域。这一结果与关于核小体核心内部结构的生化和X射线晶体学结果相关联，从而描绘出一幅H1与完整的两圈、166个碱基对颗粒上的独特区域结合的核小体图景（图15）。在高阶结构的形成过程中，相邻核小体上的这些区域靠得更近，从而可能在超螺旋结构的中心形成H1聚合物。
