Palen T E, Cech T R
Cell. 1984 Apr;36(4):933-42. doi: 10.1016/0092-8674(84)90043-6.
The chromatin structure of regulatory regions of the extrachromosomal rRNA genes of Tetrahymena thermophila was probed by nuclease treatment of isolated nuclei. The chromatin near the origins of replication contains hypersensitive sites for micrococcal nuclease, DNAase I, and DNAase II. These sites persist in starved cells, consistent with the origins' being maintained in an altered chromatin structure independent of DNA replication. The region between the two origins of replication is organized into a phased array of seven nucleosomes, the fourth of which is centered at the axis of symmetry of the palindromic rDNA. The entire transcribed region and 150 bp upstream from the initiation site are generally accessible to nucleases; any histone proteins associated with these regions are clearly not in a highly organized nucleosomal array as seen in the central region. Comparison of the chromatin structures of the central spacer of T. thermophila and T. pyriformis rDNA reveals that deletion or insertion of DNA has occurred in increments of 200 bp. This is taken to imply that there are constraints on the evolution of spacer DNA sequences at the level of the nucleosome.
通过对分离出的细胞核进行核酸酶处理，探究了嗜热四膜虫染色体外rRNA基因调控区的染色质结构。复制起点附近的染色质含有对微球菌核酸酶、DNA酶I和DNA酶II敏感的位点。这些位点在饥饿细胞中持续存在，这与复制起点保持在与DNA复制无关的改变的染色质结构中一致。两个复制起点之间的区域被组织成由七个核小体组成的相位阵列，其中第四个核小体以回文rDNA的对称轴为中心。整个转录区域和起始位点上游150 bp通常可被核酸酶接近；与这些区域相关的任何组蛋白显然不像在中心区域那样处于高度有组织的核小体阵列中。嗜热四膜虫和梨形四膜虫rDNA中央间隔区染色质结构的比较表明，DNA的缺失或插入以200 bp的增量发生。这意味着在核小体水平上，间隔DNA序列的进化存在限制。
