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Gene expression in the mitochondria of kinetoplastid protists like the human pathogen Trypanosoma brucei occurs via a remarkable pathway involving both the deletion of encoded uridines (Us) and the insertion of extra Us at defined positions within mRNAs synthesized from “cryptogenes” in the maxicircle DNA (see refs. 1 and 2 for recent reviews). In some cases, >50% of the nucleotides present in the mature mRNA are generated by RNA editing events. U addition and removal create sense from nonsense, producing ORFs that encode the proteins involved in oxidative phosphorylation and other mitochondrial functions. Many of these changes are developmentally regulated, occurring only in the procyclic (insect) stage or the bloodstream (mammalian) form. Although the overall mechanism by which this extraordinary phenomenon occurs has been sketched out (3–6), filling in the details has been difficult because of the low abundance and complexity of the editing apparatus, the low efficiency of in vitro editing assays, the lack of assays for processive editing, and difficulties in assigning functions to specific proteins. The article by Aphasizhev et al. (7) in this issue of PNAS provides important information regarding the function of two of the key enzymes involved in this fascinating process. The insertion and deletion of Us into kinetoplast mRNAs occur through the concerted action of a series of enzymes (Fig. 1) (see references in ref. 2). Specificity is provided by small RNA molecules termed guide RNAs (gRNAs), which base-pair to preedited mRNAs just downstream of editing sites (8). gRNAs have three functional domains: an anchor region, which anneals to the substrate; a guiding region, which directs the insertion or deletion of U residues; and an oligo(U) tail, which is added posttranscriptionally and is thought to help tether the purine-rich …

Two distinct roles for terminal uridylyl transferases in RNA editing