Scheer U
Cell. 1978 Mar;13(3):535-49. doi: 10.1016/0092-8674(78)90327-6.
The morphology of nucleolar and non-nucleolar (lampbrush chromosome loops) chromatin was studied in the electron microscope during states of reduced transcriptional activity in amphibian oocytes (Xenopus laevis, Triturus alpestris, T. cristatus). Reduced transcriptional activity was observed in maturing stages of oocyte development and after treatment with an inhibitor, actinomycin D. Strands of nucleolar chromatin appear smooth and thin, and contain only few, if any, nucleosomal particles in the transcribed units. This is true whether they are densely or only sparsely covered with lateral ribonucleoprotein fibrils. This smooth and non-nucleosomal character is also predominant in the interspersed, apparently nontranscribed rDNA spacer regions. During inactivation, however, nucleolar chromatin frequently and progressively assumes a beaded appearance in extended fibril-free--that is, apparently nontranscribed--regions. In either full-grown oocytes or late after drug treatment, most of the nucleolar chromatin is no longer smooth and thin, but rather shows a beaded configuration indistinguishable from inactive non-nucleolar chromatin. In many chromatin strands, transitions of fibril-associated regions of smooth character into beaded regions without lateral fibrils are seen. Similarly, in the non-nucleolar chromatin of the retracting lampbrush chromosome loops, reduced transcriptional activity is correlated with a change from smooth to beaded morphology. Here, however, beaded regions are also commonly found interspersed between the more or less distant bases of the lateral fibrils, the putative transcriptional complexes. In both sorts of chromatin, detergents (in particular Sarkosyl) that remove most of the chromatin proteins including histones from the DNA axis but leave the RNA polymerases of the transcriptional complexes attached were used to discriminate between polymerases and nucleosomal particles. The results suggest that nucleosomes are absent in heavily transcribed chromatin regions but are reformed after inactivation. In contrast to the findings with inactivated nucleolar genes, in lampbrush chromosome loops the beaded nucleosomal configuration appears to be assumed also in regions within transcriptional units that, perhaps temporarily, are not involved in transcription.
在两栖类卵母细胞（非洲爪蟾、高山蝾螈、有冠蝾螈）转录活性降低的状态下，利用电子显微镜研究了核仁染色质和非核仁染色质（灯刷染色体环）的形态。在卵母细胞发育的成熟阶段以及用放线菌素D处理后，观察到转录活性降低。核仁染色质的链显得平滑且细，并且在转录单元中即使有核小体颗粒也很少。无论它们是被侧向核糖核蛋白纤维密集覆盖还是只是稀疏覆盖，情况都是如此。这种平滑且无核小体的特征在散布的、显然不转录的核糖体DNA间隔区也占主导。然而，在失活过程中，核仁染色质在没有纤维的伸展区域（即显然不转录的区域）经常且逐渐呈现出串珠状外观。在完全成熟的卵母细胞或药物处理后期，大多数核仁染色质不再平滑且细，而是呈现出与无活性的非核仁染色质难以区分的串珠状结构。在许多染色质链中，可以看到具有平滑特征的纤维相关区域向没有侧向纤维的串珠状区域的转变。同样，在收缩的灯刷染色体环的非核仁染色质中，转录活性降低与从平滑形态向串珠状形态的变化相关。然而，在这里，串珠状区域也通常散布在侧向纤维（假定的转录复合物）或多或少相隔较远的基部之间。在这两种染色质中，使用去污剂（特别是十二烷基肌氨酸钠）从DNA轴上去除包括组蛋白在内的大多数染色质蛋白，但使转录复合物的RNA聚合酶附着，以区分聚合酶和核小体颗粒。结果表明，在高度转录的染色质区域不存在核小体，但在失活后会重新形成。与失活的核仁基因的研究结果相反，在灯刷染色体环中，串珠状核小体结构似乎也出现在转录单元内可能暂时不参与转录的区域。
