RNA Editing

RNA editing is a post-transcriptional chemical modification of an mRNA molecule -- through base modification, nucleotide insertion, nucleotide deletion, or nucleotide replacement -- that alters the amino acid sequence the transcript ultimately specifies, without changing the underlying DNA sequence itself. In plants, this altered coding capacity is confined to two organelles that carry their own separate genomes, mitochondria and chloroplasts (plastids), and it coordinates the expression of those organelle genes with signals arriving from the nucleus, from the environment, and between the organelles themselves. Two main types of RNA editing occur in plants: substitution editing, which alters individual nucleotide bases (the dominant form documented in plants is a site-specific cytosine-to-uracil, C to U, pyrimidine transition occurring in both mitochondrial and chloroplast RNA), and insertion/deletion editing, in which nucleotides are added to or removed from the total base count of the transcript. RNA editing carries real functional significance: in the chloroplasts of higher plants, it helps restore the correct codons for amino acids that are conserved across species, including, in some cases, restoring proper initiation and termination codons that would otherwise be disrupted; it plays a regulatory role in coordinating organellar gene expression generally; and, because it restores codons for amino acid residues that are conserved across a wide range of related species, it carries clear phylogenetic significance, helping researchers reconstruct evolutionary relationships. The earliest documented cases established the pattern: a 1989 report found that C-to-U editing in plant mitochondria …