Department of Biochemistry, Dalhousie University, Halifax, Nova Scotia B3H 4H7, Canada.
EMBO J. 1987 Apr;6(4):1063-71. doi: 10.1002/j.1460-2075.1987.tb04859.x.
In Crithidia fasciculata, a trypanosomatid protozoan, the nuclear-encoded ;28S' rRNA is multiply fragmented, comprising two large (c and d) and four small (e, f, g and j) RNA species. We have determined that the coding sequences for these RNAs (and that of the 5.8S rRNA, species i) are separated from one another by spacer sequences ranging in size from 31 to 416 bp. Coding and spacer sequences are presumably co-transcribed, with excision of the latter during post-transcriptional processing generating a highly fragmented large subunit (LSU) rRNA. Secondary structure modelling indicates that the C. fasciculata LSU rRNA complex (seven segments, including 5.8S rRNA) is held together in part by long-range intermolecular base pairing interactions that are characteristic of intramolecular interactions in the covalently continuous LSU (23S) rRNA of Escherichia coli. At least one functionally critical region (encompassing the alpha-sarcin cleavage site) is contained in a small RNA species (f) rather than in one of the two large RNAs. Within a proposed secondary structure model of C. fasciculata LSU rRNA, discontinuities between the different segments (created by spacer excision) map to regions that are highly variable in structure in covalently continuous LSU rRNAs. We suggest that ;rRNA genes in pieces' and discontinuous rRNAs may represent an evolutionarily ancient pattern.
在纤毛原生动物（一种原生动物）中，核编码的 ;28S' rRNA 被多次碎片化，包括两个大（c 和 d）和四个小（e、f、g 和 j）RNA 物种。我们已经确定，这些 RNA 的编码序列（以及 5.8S rRNA，物种 i）彼此之间被大小在 31 到 416bp 之间的间隔序列隔开。编码序列和间隔序列可能是共转录的，在后转录加工过程中切除后者产生高度碎片化的大亚基（LSU）rRNA。二级结构建模表明，C. fasciculata LSU rRNA 复合物（七个片段，包括 5.8S rRNA）部分通过长程分子间碱基配对相互作用结合在一起，这些相互作用是大肠杆菌共价连续 LSU（23S）rRNA 中分子内相互作用的特征。至少一个功能关键区域（包含 alpha-sarcin 切割位点）包含在一个小 RNA 物种（f）中，而不是两个大 RNA 中的一个。在 C. fasciculata LSU rRNA 的建议二级结构模型中，不同片段之间的不连续性（由间隔序列切除产生）映射到共价连续 LSU rRNA 中结构高度可变的区域。我们认为，“碎片 rRNA 基因”和不连续 rRNA 可能代表一种古老的进化模式。
