Thiolation controls cytoplasmic tRNA stability and acts as a negative determinant for tRNA editing in mitochondria

Kinetoplastids encode a single nuclear tryptophanyl tRNA that contains a CCA anticodon able to decode the UGG codons used in cytoplasmic protein synthesis but cannot decode the mitochondrial UGA codons. Following mitochondrial import, this problem is circumvented in Trypanosoma brucei by specifically editing the tRNA(Trp) anticodon to UCA, which can now decode the predominant mitochondrial UGA tryptophan codons. This tRNA also undergoes an unusual thiolation at position 33 of the anticodon loop, the only known modification at U33 in any tRNA. In other organisms, tRNA thiolation is mediated by the cysteine desulfurase, Nfs1 (IscS). However, T. brucei encodes two Nfs homologues, one cytoplasmic and the other mitochondrial. We show by a combination of RNA interference and Northern and Western analyses that the mitochondria-targeted TbNfs, and not TbNfs-like protein, is essential for thiolation of both cytosolic and mitochondrial tRNAs. Given the exclusive mitochondrial localization of TbNfs, how it mediates thiolation in the cytoplasm remains unclear. Furthermore, thiolation specifically affects thiolated tRNA stability in the cytoplasm but more surprisingly acts as a negative determinant for the essential C to U editing in T. brucei. This provides a first line of evidence for mitochondrial C to U editing regulation in this system.