Zentgraf H, Müller U, Franke W W
Eur J Cell Biol. 1980 Dec;23(1):171-88.
Structural transitions between extended and condensed forms of chromatin have been studied by electron microscopy of whole chicken erythrocyte nuclei incubated in low buffer concentrations (0.5 mM sodium borate; 1 to 5 mM Tris-HCl) with and without additions of monovalent or divalent cations. Both spread preparations and ultrathin sections have been examined. It is shown that the condensed chromatin present in chicken erythrocytes is organized, at the supranucleosomal level, in granular particles (mean diameter 29 +/- 3 nM0 tightly apposed against each other in beaded chains (higher order fibrils). These chains of higher order granules are gradually unravelled in low salt buffers lacking divalent cations into extended nucleosomal filaments. When nuclear chromatin swollen in low salt buffer and extended uniformly into nucleosomal filaments is exposed to buffers containing higher concentrations of monovalent ions (e.g., 100 mM NaCl) or low concentrations of divalent cations (e.g., 0.05-0.5 mM MgCl2) rapid and almost complete recondensation of the entire chromatin of a nucleus into one compact spheroidal aggregate of chains of higher order granules (diameters 25-30 nm) is observed. This transition is reversible and whole nuclear chromatin can be induced, in vitro, to undergo several cycles of such conformational changes. The results show that reconstitution of globular forms of higher order packing of nucleosomes is not restricted to oligonucleosomal units but also takes place, in a rather uniform and regular mode, in long nucleosomal filaments as they are present in whole nuclei. The data are discussed in relation to other studies of chromatin ultrastructure and are interpreted to indicate that the predominant, if not exclusive form of supranucleosomal packing existing in condensed inactive chromatin of living cells is a chromatin fibril made up of serially arranged, close-packed granular particles ("suprabeads" sensu in [64]). In addition, the results suggest that higher order packing of chromatin into such supranucleosomal granules is not simply a function of ionic strength but that the type of ions present strongly influences the state of chromatin packing.
通过对在低缓冲液浓度（0.5 mM硼酸钠；1至5 mM Tris-HCl）中孵育的完整鸡红细胞核进行电子显微镜观察，研究了染色质伸展形式和凝聚形式之间的结构转变，实验中添加或不添加单价或二价阳离子。同时检查了铺展标本和超薄切片。结果表明，鸡红细胞中存在的凝聚染色质在超核小体水平上由颗粒状颗粒（平均直径29 +/- 3 nM）组成，这些颗粒在串珠链（高阶纤维）中彼此紧密相邻。在缺乏二价阳离子的低盐缓冲液中，这些高阶颗粒链会逐渐解开，形成伸展的核小体细丝。当在低盐缓冲液中膨胀并均匀伸展成核小体细丝的核染色质暴露于含有较高浓度单价离子（例如100 mM NaCl）或较低浓度二价阳离子（例如0.05 - 0.5 mM MgCl2）的缓冲液中时，会观察到细胞核的整个染色质迅速且几乎完全重新凝聚成一个由高阶颗粒链组成的紧密球形聚集体（直径25 - 30 nm）。这种转变是可逆的，并且在体外可以诱导整个核染色质经历几个这样的构象变化循环。结果表明，核小体高阶包装的球状形式的重构不仅限于寡核小体单元，而且在整个细胞核中存在的长核小体细丝中也以相当均匀和规则的方式发生。本文结合其他染色质超微结构研究对这些数据进行了讨论，并解释为表明在活细胞的凝聚非活性染色质中存在的超核小体包装的主要（如果不是唯一）形式是由连续排列、紧密堆积的颗粒状颗粒（[64]中所谓的“超珠”）组成的染色质纤维。此外，结果表明染色质包装成这种超核小体颗粒不仅仅是离子强度的函数，而且存在的离子类型强烈影响染色质包装状态。
