The mtDNA rns gene landscape in the Ophiostomatales and other fungal taxa: Twintrons, introns, and intron-encoded proteins

Convergent evolution of twintron-like configurations: One is never enough

“…The possibility of a twintron configuration was first recognized for a complex group II intron in the psbF gene of Euglena gracilis by Michel et al 1 but the term twintron was first applied for group II introns inserted within group II intron or group III introns. [2][3][4][5][6][7] Later other twintron variations were noted such as group I introns within group I introns, 8,9 group II introns within group I introns, 9 lariat capping twin ribozyme introns, 10 tRNA introns within tRNA introns 11 and various combinations of spliceosomal introns embedded within spliceosomal introns. [12][13][14][15][16] In addition there are composite intron arrangements such as spliceosomal type introns inserted within a group I introns.…”

“…In other examples the invading intron may insert into segments not essential for splicing thus the external intron can splice independent of the internal intron. 9,17,18 In addition twintrons have been noted that are nested arrays of repeated insertions of an intron into another intron. 11,19 Finally, some group I or group II introns encode open reading frames (ORFs) and these ORFs can be invaded by introns.…”

“…3B,C&D; 9,22 ). In Cryphonectria parasitica (Chestnut blight) and Ophiocordyceps tricentri within the mtDNA rns gene at position S917 a group ID intron has been invaded by another group ID intron that has an ORF embedded within its P2.1a loop.…”

“…In Chaetomium thermophilum the mS1247 twintron is composed of an external group IC2 intron encoding an ORF that is interrupted by an ORF-less group II A1 intron. 9 It was shown under in vitro conditions that the internal group IIA1 The internal intron has been co-opted into a lariat cap (LC) forming ribozyme.…”

Three new active members of the I-OnuI family of homing endonucleases

“…Hafez et al (2013) reannotated this intron and noted that it is actually inserted at position mS917; this study showed that this intron is a nested (or twintron) group ID intron where the internal group ID intron encodes a double motif LAGLIDADG-type ORF and it is inserted into an ORF-less external group ID intron. Further, this study noted that orthologues of this HEG are present in other flanking the intron insertion sites (~200 nt) were combined, synthesised, and inserted into the pUC57 (2.7 kb) vector (GenScript, New Jersey, USA).…”