tRNA^Tyr has an unusually short half-life in Trypanosoma brucei

Following transcription, tRNAs undergo a series of processing and modification events to become functional adaptors in protein synthesis. Eukaryotes have also evolved intracellular transport systems whereby nucleus-encoded tRNAs may travel out and into the nucleus. In trypanosomes, nearly all tRNAs are also imported from the cytoplasm into the mitochondrion, which lacks tRNA genes. Differential subcellular localization of the cytoplasmic splicing machinery and a nuclear enzyme responsible for queuosine modification at the anticodon "wobble" position appear to be important quality control mechanisms for tRNA(Tyr), the only intron-containing tRNA in T. brucei. Since tRNA-guanine transglycosylase (TGT), the enzyme responsible for Q formation, cannot act on an intron-containing tRNA, retrograde nuclear transport is an essential step in maturation. Unlike maturation/processing pathways, the general mechanisms of tRNA stabilization and degradation in T. brucei are poorly understood. Using a combination of cellular and molecular approaches, we show that tRNA(Tyr) has an unusually short half-life. tRNA(Tyr), and in addition tRNA(Asp), also show the presence of slow-migrating bands during electrophoresis; we term these conformers: alt-tRNA(Tyr) and alt-tRNA(Asp), respectively. Although we do not know the chemical or structural nature of these conformers, alt-tRNA(Tyr) has a short half-life resembling that of tRNA(Tyr); the same is not true for alt-tRNA(Asp). We also show that RRP44, which is usually an exosome subunit in other organisms, is involved in tRNA degradation of the only intron-containing tRNA in T. brucei and is partly responsible for its unusually short half-life.