The mitochondrial genomes of mushroom corals (Corallimorpharia) are remarkable for harboring two complex group I introns; ND5-717 and COI-884. How these autocatalytic RNA elements interfere with mitochondrial RNA processing is currently not known. Here, we report experimental support for unconventional processing events of ND5-717 containing RNA. We obtained the complete mitochondrial genome sequences and corresponding mitochondrial transcriptomes of the two distantly related corallimorpharian species Ricordea yuma and Amplexidiscus fenestrafer. All mitochondrial genes were found to be expressed at the RNA-level. Both introns were perfectly removed by autocatalytic splicing, but COI-884 excision appeared more efficient than ND5-717. ND5-717 was organized into giant group I intron elements of 18.1 kb and 19.3 kb in A. fenestrafer and R. yuma, respectively. The intron harbored almost the entire mitochondrial genome embedded within the P8 peripheral segment. ND5-717 was removed by group I intron splicing from a small primary transcript that contained a permutated intron–exon arrangement. The splicing pathway involved a circular exon-containing RNA intermediate, which is a hallmark of RNA back-splicing. ND5-717 represents the first reported natural group I intron that becomes excised by back-splicing from a permuted precursor RNA. Back-splicing may explain why Corallimorpharia mitochondrial genomes tolerate giant group I introns.

中文翻译：

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线粒体基因组中的巨型I组内含子通过RNA反剪技术去除。

蘑菇珊瑚（Corallimorpharia）的线粒体基因组具有两个复杂的I类内含子。ND5-717和COI-884。目前尚不清楚这些自催化RNA元素如何干扰线粒体RNA加工。在这里，我们报告了对含有RNA的ND5-717非常规加工事件的实验支持。我们获得了两个远缘相关的珊瑚亚种Ricordea yuma和Amplexidiscus fenestrafer的完整线粒体基因组序列和相应的线粒体转录组。发现所有线粒体基因均在RNA水平表达。通过自动催化剪接，两个内含子均被完美去除，但COI-884切除似乎比ND5-717更有效。ND5-717在A. fenestrafer和R. yuma中被分为18.1 kb和19.3 kb的第一类巨型内含子元件，分别。内含子几乎包藏了嵌入P8外围区段内的整个线粒体基因组。Ⅰ型内含子剪接从一个小的初级转录本中除去了ND5-717，该转录本包含一个排列的内含子-外显子排列。剪接途径涉及含环状外显子的RNA中间体，这是RNA反向剪接的标志。ND5-717代表第一个报道的天然I类内含子，该内含子通过从置换的前体RNA中进行反向剪接而被切除。反向拼接可能解释了为什么珊瑚拟南芥线粒体基因组能够耐受第一类内含子。剪接途径涉及含环状外显子的RNA中间体，这是RNA反向剪接的标志。ND5-717代表第一个报道的天然I类内含子，该内含子通过从置换的前体RNA中进行反向剪接而被切除。反向拼接可能解释了为什么珊瑚拟南芥线粒体基因组能够耐受第一类内含子。剪接途径涉及含环状外显子的RNA中间体，这是RNA反向剪接的标志。ND5-717代表了第一个报道的天然I类内含子，该内含子通过从置换的前体RNA反向剪接而被切除。反向拼接可能解释了为什么珊瑚拟南芥线粒体基因组能够耐受第一类内含子。