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22 RNA Editing—An Evolutionary Perspective

Larry Simpson


RNA editing is a term used to describe a variety of phenomena that involve the modification of nucleotide sequences of RNA transcripts in different organisms. In general, there are two basic types of editing, involving either insertions and deletions or substitutions of nucleotides. The genetic effect of the former, especially within coding regions of mRNAs, is of course more dramatic in that translation frameshifts can be created or corrected, but the effect of nucleotide substitutions can cause changes in the encoded amino acids and subsequent phenotypic consequences. The definition of RNA editing is actually somewhat historical in that nucleotide modifications of tRNAs and rRNAs described prior to the 1986 (Benne et al. 1986) discovery of the uridine (U) insertion/deletion modifications in trypanosome mitochondria are not usually included in this rubric. Editing is used to describe the diverse phenomena represented by trypanosome U-insertions and deletions; cytidine(C)-insertions in Physarum mitochondria; C-to-U substitutions in plant mitochondria and chloroplasts and mammalian apoB mRNA; nucleotide substitutions in Acanthamoeba, marsupial, and rat tRNAs; adenosine(A)-to-inosine(I) substitutions in mammalian glutamate receptor mRNA; and guanosine insertions in negative-strand RNA viruses. In this chapter, I review the trypanosome mitochondrial U-insertion/deletion editing in an evolutionary perspective.

The transcripts of 13 of the 18 structural genes of the maxicircle mitochondrial DNA in trypanosomatid protozoa are modified after transcription by the insertion and occasional deletion of uridine residues, mainly within coding regions (Fig. 1) (Benne et al. 1986; Simpson and Shaw 1989; Simpson and Emeson 1996). These sequence modifications overcome frameshifts (internal-editing), create...

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