C to U editing stimulates A to I editing in the anticodon loop of a cytoplasmic threonyl tRNA in Trypanosoma brucei

Editing of tRNAs is widespread in nature and either changes the decoding properties or restores the folding of a tRNA. Unlike the phylogenetically disperse adenosine ( A) to inosine ( I) editing, cytosine ( C) to uridine ( U) editing has only been previously described in organellar tRNAs. We have shown that cytoplasmic tRNA(Thr)( AGU) undergoes two distinct editing events in the anticodon loop: C to U and A to I. In vivo, every inosine- containing tRNA(Thr) is also C to U edited at position 32. In vitro, C to U editing stimulates conversion of A to I at the wobble base. Although the in vivo and in vitro requirements differ, in both cases, the C to U change plays a key role in A to I editing. Due to an unusual abundance of A34- containing tRNAs, our results also suggest that the unedited and edited tRNAs are functional, each dedicated to decoding a specific threonine codon. C to U editing of cytoplasmic tRNA expands the editing repertoire in eukaryotic cells, and when coupled to A to I changes, leads to an interrelation between editing sites.