Malek O, Knoop V
Allgemeine Botanik, Universität Ulm, Germany.
RNA. 1998 Dec;4(12):1599-609. doi: 10.1017/s1355838298981262.
The fragmentation of group II introns without concomitant loss of splicing competence is illustrated by extraordinary gene arrangements in plant mitochondrial genomes. The mitochondrial genes nad1, nad2, and nad5, all encoding subunits of the NADH dehydrogenase, require trans-splicing for functional assembly of their mRNAs in flowering plants. Tracing the origins of trans-splicing group II introns shows that they have evolved from formerly cis-arranged homologs whose descendants can still be identified in lineages of early branching land plants. In this contribution we present the full set of ancestor introns for all five conserved mitochondrial trans-splicing positions. These introns are strikingly small in the quillwort Isoetes lacustris, the continuous nad2 gene intron in this species representing the smallest (389 nt) land plant group II intron yet identified. cDNA analysis shows correct splicing of the introns in vivo and also identifies frequent RNA editing events in the flanking nad gene exons. Other representatives of the ancestral cis-arranged introns are identified in the fern Osmunda regalis, the horsetail Equisetum telmateia, and the hornwort Anthoceros crispulus. Only the now identified intron in Osmunda carries significant traces of a former maturase reading frame. The identification of a continuous homolog in Anthoceros demonstrates that intron invasion into the affected genes in some cases predated the split of vascular and nonvascular plants more than 400 million years ago. As an alternative to disruption after size increase, the respective introns can get secondarily lost in certain lineages.
植物线粒体基因组中非同寻常的基因排列说明了II类内含子的片段化，而拼接能力并未随之丧失。线粒体基因nad1、nad2和nad5均编码NADH脱氢酶的亚基，在开花植物中，它们的mRNA功能组装需要反式剪接。追溯反式剪接II类内含子的起源表明，它们是从以前顺式排列的同源物进化而来的，在早期分支陆生植物的谱系中仍可识别出其后代。在本论文中，我们展示了所有五个保守线粒体反式剪接位置的完整祖先内含子集合。这些内含子在水韭中非常小，该物种中连续的nad2基因内含子是迄今发现的最小的（389个核苷酸）陆生植物II类内含子。cDNA分析表明内含子在体内能够正确剪接，并且还识别出侧翼nad基因外显子中频繁的RNA编辑事件。在蕨类植物王紫萁、木贼和角苔中鉴定出了祖先顺式排列内含子的其他代表。只有现在在王紫萁中鉴定出的内含子带有以前成熟酶阅读框的显著痕迹。在角苔中鉴定出连续的同源物表明，在某些情况下，内含子侵入受影响的基因早于4亿多年前维管植物和非维管植物的分化。作为大小增加后破坏的替代方式，相应的内含子在某些谱系中可能会二次丢失。
