RNA editing of mat-r transcripts in maize and soybean increases similarity of the encoded protein to fungal and bryophyte group II intron maturases: evidence that mat-r encodes a functional protein

We present evidence that transcripts of the mat-r (maturase-related) genes of maize and soybean contain 15 and 14 urldlnes (U), respectively, at positions occupied by cytoslnes (C) In the mat-r gene sequences. Eleven and twelve of these C→U edits result In an amlno acid replacement. Ten C→U edits are at corresponding nucleotldes in the maize and soybean transcripts and, except for a single silent edit, the remainder are at positions In one species that are Us in the other species. This results In an Increase In amino acid sequence similarity of the maize and soybean MAT-R proteins. Further, of those amlno acids In maize and soybean MAT-R proteins specified by edited codons, ten are conserved In the reverse transcriptase-assoclated and RNA spllcing-associated sequences of the cox1-/2 and/or the coxT-h maturases of the fungus Saccharomyces cerevlslae and the bryophyte, Marchantla polymorpha, respectively. The Implied strong selection for amlno acid sequence conservation Indicates that the MAT-R protein Is functional. The possibility Is discussed that initiation of translation of the mat-r transcripts Is at a four nucleotlde codon, ATAA or ATGA.