Cech T R, Brehm S L
Nucleic Acids Res. 1981 Jul 24;9(14):3531-43. doi: 10.1093/nar/9.14.3531.
Cultures of Tetrahymena thermophila were deprived of nutrients and later refed with enriched medium to obtain partial synchrony of DNA replication. Preferential replication of the extrachromosomal, macronuclear ribosomal RNA genes (rDNA) was found to occur at 40-80 min after refeeding. The rDNA accounted for one half of the label incorporated into cellular DNA during this period. Electron microscopy of the purified rDNA showed 1% replicative intermediates. Their structure was that expected for bidirectional replication of the linear rDNA from an origin or origins located in the central nontranscribed region of the palindromic molecule. Similar forms had previously been observed for the rDNA of a related species, Tetrahymena pyriformis. The electron microscopic data was consistent with an origin of replication located approximatley 600 base pairs from the center of the rDNA of T. thermophila, in contrast to a more central location in the rDNA of T. pyriformis. One implication of an off-center origin of replication is that there are two such sequences per palindromic molecule.
嗜热四膜虫的培养物被剥夺营养，随后用富集培养基重新喂养以获得DNA复制的部分同步性。发现染色体外的大核核糖体RNA基因（rDNA）在重新喂养后40 - 80分钟优先复制。在此期间，rDNA占掺入细胞DNA的标记物的一半。纯化的rDNA的电子显微镜检查显示有1%的复制中间体。它们的结构是来自位于回文分子中央非转录区域的一个或多个起始点的线性rDNA双向复制所预期的结构。以前在相关物种梨形四膜虫的rDNA中也观察到过类似形式。电子显微镜数据与复制起始点位于嗜热四膜虫rDNA中心约600个碱基对处一致，这与梨形四膜虫rDNA中更居中的位置形成对比。复制起始点偏离中心的一个含义是每个回文分子有两个这样的序列。
