Laird P W, Zomerdijk J C, de Korte D, Borst P
EMBO J. 1987 Apr;6(4):1055-62. doi: 10.1002/j.1460-2075.1987.tb04858.x.
Discontinuous mRNA synthesis in trypanosomes is thought to involve a 140-nucleotide precursor, called the mini-exon-derived RNA or medRNA, which contributes its 5' 35 nucleotides to the 5' end of nascent mRNAs. We used in vivo labelling of RNA to show that medRNA has a half-life of less than 6 min, whereas putative high mol. wt intermediates containing the 3' part of the medRNA have an average half-life of less than 1 min. This eliminates priming of pre-mRNA synthesis by intact medRNA as the main mode of discontinuous mRNA synthesis. Potential intermediates of 35 and 105 nucleotides were labelled in parallel with medRNA, but their significance could not be assessed in RNA preparations containing medRNA, as they are also produced by artefactual cleavage of medRNA. We show, however, that high mol. wt RNA, free of medRNA, can release medRNA segments upon a debranching treatment. These results are consistent with a trans splicing mechanism involving short-lived forked intermediates, analogous to lariats in cis splicing systems.
锥虫中的不连续mRNA合成被认为涉及一种140个核苷酸的前体，称为微型外显子衍生RNA或medRNA，它将其5'端的35个核苷酸贡献给新生mRNA的5'端。我们利用RNA的体内标记显示，medRNA的半衰期小于6分钟，而含有medRNA 3'部分的假定高分子量中间体的平均半衰期小于1分钟。这排除了完整的medRNA作为不连续mRNA合成的主要模式对前体mRNA合成的引发作用。35和105个核苷酸的潜在中间体与medRNA同时被标记，但在含有medRNA的RNA制剂中无法评估它们的意义，因为它们也是由medRNA的人为切割产生的。然而，我们表明，不含medRNA的高分子量RNA在去分支处理后可以释放medRNA片段。这些结果与一种涉及短寿命叉状中间体的反式剪接机制一致，类似于顺式剪接系统中的套索结构。
