Pruitt S C, Grainger R M
Cell. 1981 Mar;23(3):711-20. doi: 10.1016/0092-8674(81)90434-7.
We report an analysis by electron microscopy of the differences in the folding of ribosomal gene and adjacent nontranscribed spacer DNA of Xenopus laevis oocytes into supranucleosomal chromatin structures. The chromatin structures identified in gene and spacer regions of transcriptionally active nucleoli (from stage 5 oocytes) were compared with those found in nucleoli prior to transcription (from stage 2 oocytes) to determine whether changes in the chromatin structure occur when transcription is initiated. Chromatin structures were characterized by their morphology and by the extent of folding of DNA in chromatin. Nontranscribed spacer regions from both transcriptionally active and inactive nucleoli appear to be packaged into supranucleosomal structures and are contracted by a factor of at least 20 from the length of B form DNA. The structure of the adjacent gene region, both before and during transcription, is much more extended; the only structures observed are the size of nucleosomes, and the DNA is contracted by a factor of 1.4 from its B form length. Thus a mosaicism in the higher order structure of gene and spacer rDNA is established days or weeks prior to the initiation of transcription of these genes and maintained during transcriptionally active stages.
我们报告了一项通过电子显微镜分析非洲爪蟾卵母细胞核糖体基因及相邻非转录间隔区DNA折叠成超核小体染色质结构的差异。将转录活跃的核仁（来自5期卵母细胞）的基因和间隔区中鉴定出的染色质结构与转录前的核仁（来自2期卵母细胞）中的染色质结构进行比较，以确定转录开始时染色质结构是否发生变化。染色质结构通过其形态以及染色质中DNA的折叠程度来表征。来自转录活跃和不活跃核仁的非转录间隔区似乎都被包装成超核小体结构，并且从B型DNA的长度收缩了至少20倍。相邻基因区域在转录前和转录过程中的结构更为伸展；观察到的唯一结构是核小体大小，并且DNA从其B型长度收缩了1.4倍。因此，在这些基因转录开始前数天或数周就建立了基因和间隔区核糖体DNA高阶结构的镶嵌性，并在转录活跃阶段得以维持。
