Trendelenburg M F
Chromosoma. 1982;86(5):703-15. doi: 10.1007/BF00285612.
The ultrastructure of Xenopus laevis (X.l.) 40S pre-rRNA transcription termination sites was investigated by electron microscopy. Amplified nucleolar chromatin was rapidly dispersed and processed for chromatin spread preparations. This type of preparation revealed that many rDNA termination sites appeared as 50 nm long segments of chromatin axis covered by a complex of three closely spaced RNA polymerase particles. Particle (Ptl) was characterized by the association with the terminal full-length pre-rRNP fibril; particles (Pt2) and (Pt3) are located downstream from (Ptl) and appear to be devoid of transcript fibrils. This particular structural arrangement as well as sequence homology analyses of 3' adjacent spacer rDNA segments indicate that transcript release and dissociation of polymerase particles are not necessarily coupled and termination of X.l. rDNA transcription may occur as a two step process. The structural data are correlated with homologies of DNA sequences at Xenopus rDNA transcription termination regions and are discussed with respect to sequence data of 3' termination sites of rRNA genes from other species.
通过电子显微镜研究了非洲爪蟾（Xenopus laevis, X.l.）40S前体rRNA转录终止位点的超微结构。扩增的核仁染色质迅速分散并用于染色质铺展制备。这种制备方式显示，许多rDNA终止位点表现为染色质轴上50纳米长的片段，被三个紧密间隔的RNA聚合酶颗粒复合体覆盖。颗粒（Ptl）的特征是与末端全长前体rRNP纤维相关联；颗粒（Pt2）和（Pt3）位于（Ptl）的下游，似乎没有转录本纤维。这种特殊的结构排列以及3'相邻间隔rDNA片段的序列同源性分析表明，转录本释放和聚合酶颗粒解离不一定耦合，非洲爪蟾rDNA转录的终止可能是一个两步过程。结构数据与非洲爪蟾rDNA转录终止区域的DNA序列同源性相关，并结合其他物种rRNA基因3'终止位点的序列数据进行了讨论。
