RNA editing restores critical domains of a group I intron in fern mitochondria.

In the leptosporangiate fern Osmunda regalis, cox1 gene is disrupted by a 1071-nucleotide-long group I intron that is homologous to the Marchantia polymorpha cox1 intron 4 (cox1i395g1). This intron, which shares 89% sequence identity with its bryophyte counterpart, lost the capacity to encode for a maturase due to insertion/deletion mutations. The cox1 coding region is interrupted by a stop codon in both exons. The cox1 transcript undergoes 58 C-to-U and 13 U-to-C conversions, including the suppression of two stop codons that result in the recovery of a functional cox1 ORF. Interestingly, 4 C-to-U conversions found in mRNA precursors showed that the O. regalis cox1i395g1 intron is efficiently edited. These modifications improved the sequence identity with the Marchantia cox1i395 intron. In particular, the RNA editing events affect regions involved in secondary and tertiary structures of the intron, restoring three base pairing in the structural P5a and P9 helices, and correcting a highly conserved U in the P7 helix that contributes to the catalytic core. Moreover, cox1 intron orthologous from three different fern species were found to be edited by both C-to-U and U-to-C conversions in P7 and P9. Thus, RNA editing helps to correct the conserved domains of group I introns in "true ferns", suggesting a possible link between editing and splicing. We present here the first experimental evidence of RNA editing concerning a group I intron in plant organelles.