Spadafora C, Crippa M
Nucleic Acids Res. 1984 Mar 26;12(6):2691-704. doi: 10.1093/nar/12.6.2691.
Micrococcal nuclease digestion was used as a tool to study the organization of the ribosomal chromatin in liver, blood and embryo cells of X. laevis. It was found that in liver and blood cells, ribosomal DNA is efficiently protected from nuclease attack in comparison to bulk chromatin. Although ribosomal chromatin is fragmented in a typical nucleosomal pattern, a considerable portion of ribosomal DNA retains a high molecular weight even after extensive digestion. A greater accessibility of the coding region in comparison to the non-coding spacer was found. In embryos, when ribosomal DNA is fully transcribed, these genes are even more highly protected than in adult tissues: in fact, the nucleosomal ladder can hardly be detected and rDNA is preserved in high molecular weight. Treatment of chromatin with 0.8 M NaCl abolishes the specific resistance of the ribosomal chromatin to digestion. The ribosomal chromatin, particularly in its active state, seems to be therefore tightly complexed with chromosomal proteins which protect its DNA from nuclease degradation.
微球菌核酸酶消化被用作一种工具来研究非洲爪蟾肝脏、血液和胚胎细胞中核糖体染色质的组织情况。研究发现，在肝脏和血细胞中，与整体染色质相比，核糖体DNA能有效抵御核酸酶的攻击。尽管核糖体染色质以典型的核小体模式断裂，但即使经过长时间消化，相当一部分核糖体DNA仍保留高分子量。与非编码间隔区相比，编码区的可及性更高。在胚胎中，当核糖体DNA完全转录时，这些基因比成年组织中受到的保护更高：事实上，几乎检测不到核小体梯带，核糖体DNA以高分子量形式保存。用0.8M NaCl处理染色质会消除核糖体染色质对消化的特异性抗性。因此，核糖体染色质，特别是处于活跃状态时，似乎与染色体蛋白紧密结合，从而保护其DNA免受核酸酶降解。
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