Labhart P, Koller T
Cell. 1982 Feb;28(2):279-92. doi: 10.1016/0092-8674(82)90346-4.
Active nucleolar chromatin of Xenopus laevis oocytes was prepared for electron microscopy by a step gradient method, which separates the chromatin from proteins and other constituents that might nonspecifically bind at low ionic strength. Between putative RNA polymerases and within the nontranscribed spacer region, the chromatin appears as smooth, thin filaments. For the first time, it is shown here that these filaments are indistinguishable from pure DNA absorbed to the same specimen, even when the ionic strength is raised up to 100 mM NaCl. Bulk rat liver chromatin, however, which was coprepared as a biochemically well-characterized standard with the active nucleolar chromatin, shows nucleosomes containing fibers, which condense into supranucleosomal structures with increasing ionic strength. Since the appearance and the behavior of active nucleolar chromatin at different ionic strengths and pHs resembles tht of pure DNA, but not of any known type of chromatin, it is suggested that, except for the transcription apparatus, very few macromolecular constituents are associated with ribosomal DNA during transcription. The observations described in this paper explain most of the published and partly conflicting results obtained by electron microscopy of nucleolar chromatin.
通过梯度离心法制备非洲爪蟾卵母细胞的活性核仁染色质用于电子显微镜观察，该方法可在低离子强度下将染色质与可能非特异性结合的蛋白质及其他成分分离。在假定的RNA聚合酶之间以及非转录间隔区内，染色质呈现为光滑的细丝状。本文首次表明，即使将离子强度提高到100 mM NaCl，这些细丝与吸附在同一标本上的纯DNA并无区别。然而，与活性核仁染色质一同作为生化特性良好的标准品共同制备的大鼠肝脏整体染色质，显示出含有纤维的核小体，随着离子强度的增加，这些核小体会凝聚成超核小体结构。由于活性核仁染色质在不同离子强度和pH值下的外观及行为类似于纯DNA，而非任何已知类型的染色质，因此表明，除转录装置外，转录过程中与核糖体DNA相关的大分子成分极少。本文所述的观察结果解释了通过核仁染色质电子显微镜观察所获得的大部分已发表且部分相互矛盾的结果。
