Chi et al. BMC Molecular Biol (2019) 20:16 https://doi.org/10.1186/s12867-019-0134-y BMC Molecular Biology RESEARCH ARTICLE Open Access Giant group I intron in a mitochondrial genome is removed by RNA back-splicing Sylvia Ighem Chi1, Mikael Dahl1, Åse Emblem1 and Steinar D. Johansen1,2* Abstract Background: 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. Results: We obtained the complete mitochondrial genome sequences and corresponding mitochondrial transcrip- tomes of the two distantly related corallimorpharian species Ricordea yuma and Amplexidiscus fenestrafer. All mito- chondrial genes were found to be expressed at the RNA-level. Both introns were perfectly removed by autocatalytic splicing, but COI-884 excision appeared more efcient 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. Conclusion: 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 frst reported natural group I intron that becomes excised by back-splicing from a permuted precursor RNA. Back-splicing may explain why Corallimorpharia mitochon- drial genomes tolerate giant group I introns. Keywords: Amplexidiscus, Back-splicing, Catalytic RNA, Group I intron, Intron retention, Mitochondrial RNA, Ribozyme, Ricordea Background Te mitochondrial genomes (mtDNAs) of species rep- Hexacorallia represents an important subclass of marine resenting most hexacorallian orders have been sequence cnidarians that hosts several well-known orders like characterized, and subsequently applied in phylogenetic Scleractinia (stony corals), Actiniaria (sea anemones), analyses and molecular studies [4–7]. Te mtDNAs are Zoantharia (colonial anemones), and Corallimorpharia vertebrate-like, which include a small-sized (17–22 kb) (mushroom corals). Corallimorpharia appears closely circular organization with a high gene density [8]. Tey related to Scleractinia, but if they represent true naked harbor the same set of ribosomal RNA (rRNA) genes and corals is currently under debate [1, 2]. Te order Coral- the 13 protein-coding genes of the oxidative phosphoryl- limorpharia consists of about 50 valid species organ- ation (OxPhos) system as most metazoan mitochondrial ized into seven families [3]. Tey are solitary polyps that genomes, but optional genes may occur in some species usually occur in larger groups in tropical habitats, and [7–11]. Hexacoral mtDNA contains a highly reduced morphologically they have short tentacles and no stony tRNA gene repertoire, and all the conventional mito- skeletons [3]. chondrial genes are encoded by the same DNA strand [8, 12]. Te most remarkable feature, however, is the pres- *Correspondence: [email protected] ence of autocatalytic group I introns [4, 13]. 2 Genomics Group, Faculty of Biosciences and Aquaculture, Nord Group I introns are intervening sequences that encode University, Bodø, Norway self-splicing ribozymes responsible for the RNA process- Full list of author information is available at the end of the article ing reactions [14]. Intron sequences are removed from © The Author(s) 2019. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/ publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated. Chi et al. BMC Molecular Biol (2019) 20:16 Page 2 of 12 the precursor RNA in a two-step transesterifcation Here, we have characterized the mito-transcriptomes pathway that involves RNA cleavage and ligation activi- of the two distantly related corallimorpharian species ties [15]. Splicing is initiated by a nucleophilic attach at Ricordea yuma and Amplexidiscus fenestrafer. All mito- the 5′ splice site (SS) by a noncoded guanosine cofac- chondrial genes were found expressed, and their introns tor (exo-G). Tis reaction leads to covalent attachment (ND5-717 and COI-884) were perfectly removed by RNA of exo-G to the released intron’s 5′ end. In a subsequent splicing. We further investigated the ND5 mRNA precur- reaction, the free 3′ end of the upstream exon attacks the sor and the ND5-717 intron RNA splicing intermediate, 3′ SS, resulting in exon ligation and release of the linear and concluded that the intron was excised by back-splic- intron RNA. Te catalytic property of group I intron ing. Back-splicing is relatively common in vertebrate RNA is funded on a highly organized and complex three- spliceosomal introns, resulting in exon permutation and dimensional ribozyme structure that consists of a cata- covalently closed circular RNAs [22–24]. However, the lytic domain, a folding domain, and a substrate domain ND5-717 introns in R. yuma and A. fenestrafer are the [16, 17]. Each domain contains a conserved subset of frst reported examples of group I intron back-splicing. secondary structure paired elements (P1 to P9), where P7 harbors the catalytic core structure designed to bind Results diferent guanosine factors in Step 1 (exo-G) and Step 2 Mitochondrial genome sequences of R. yuma and A. (ɷG) of the splicing reaction [14]. fenestrafer All hexacoral species and isolates investigated to date Te circular mitochondrial genomes of R. yuma harbor a group I intron in the mitochondrial NADH (21,430 bp) and A. fenestrafer (20,054) were determined dehydrogenase 5 (ND5) gene at position 717 [4]. Tis on both strands using a combined strategy of Ion Torrent obligatory intron (ND5-717) is strictly vertically inher- PGM (average coverage of 126× and 70×, respectively) ited, and appears to have a fungal origin [4]. One unu- and Sanger sequencing. All canonical mitochondrial sual feature of hexacoral ND5-717 is the large insertion genes were coded on the same strand and in a simi- in one of its ribozyme paired segments (P8) that includes lar order compared to those of most corallimorphar- several cognate mitochondrial gene sequences, from two ian mtDNAs investigated (Fig. 1a) [1, 5]. In addition to genes in Actiniaria and Zoantharia [7, 10] to 16 genes in the conserved set of 13 OxPhos protein coding genes, R. Corallimorpharia [1, 5]. Te mitochondrial cytochrome yuma and A. fenestrafer encoded a homing endonuclease oxidase I (COI) gene contains diferent, but optional, and an antisense open reading frame (aORF) transcript. group I introns at positions 720, 867, and 884 [10, 18, 19]. Several notable sequence features related to the protein- Whereas COI-720 and COI-867 appeared confned to coded genes were observed. Firstly, a signifcant frac- the Indo-Pacifc scleractinians [20] and to zoantharians tion (approximately 40%) of the reading frames contain [10], respectively, COI-884 is more widely distributed a GTG initiation codon (Additional fle 1: Table S1), sug- in species belonging to several hexacorallian orders. All gesting that tRNA-fMet recognizes both ATG and GTG COI introns contain homing endonuclease gene (HEG) at this position. Tis feature was conserved among all sequences, encoding a LAG-type (LAGLIDADG) hom- corallimorpharian mtDNAs [5]. Secondly, group I introns ing endonuclease [18, 21]. were present at conserved positions (884 and 717) within Corallimorpharian mitochondrial genomes are slightly the COI and ND5 genes, respectively. Tirdly, the ND5- larger in size (20 to 22 kb) compared to other hexacor- 717 was noted as a giant intron in R. yuma (19.3 kb) allian orders and contain two group I introns, ND5-717 and A. fenestrafer (18.9 kb), harboring almost the com- and COI-884 [1, 5]. We have previously noted a highly plete mtDNA sequence between its 5′ and 3′ splice sites complex organization of ND5-717 in Corallimorpha- (Fig. 1a). Finally, COI-884 was identifed as a 1.2 kb group ria that involves 16 genes embedded in P8, including I intron containing a LAG-type HEG. the COI-884 group I intron containing COI gene [17]. We recognized 18 intergenic regions (IGRs) in the Recently, we reported RNAseq-based mitochondrial R. yuma and A. fenestrafer mtDNAs (Additional fle 1: transcriptome (mito-transcriptome) profling in several Table S1). Most IGRs were found to be identical, or actiniarian and zoantharian species [7, 10, 11]. Difer- almost identical, in size among the corallimorphar- ent from the Corallimorpharia, these anemones harbor ian mtDNAs available in the NCBI database. We found only two mitochondrial genes (ND1 and ND3) embed- the R. yuma mtDNA to be 1376 bp larger than that of ded within P8 of their respective ND5-717 introns. We A. fenestrafer, and this size diference was mainly due found all mitochondrial protein genes to be expressed to IGR-5, IGR-7, IGR-11 and IGR-13. No recognizable at