Thlo modification of yeast cytosolic tRNA is an iron-sulfur protein-dependent pathway

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Defects in yeast cysteine desulfurase, Nfs1p cause a severe impairment in 2-thio modification of uridine of the mitochondrial (mt-) tRNAs and the cytosolic (cy-) tRNAs. Nfs1 p can also provide the sulfur atoms of the iron-sulfur (Fe/S) clusters generated by the mitochondrial and cytosolic Fe/S cluster assembly machineries termed ISC and CIA, respectively. Therefore, a key question remains as to whether biosynthesis of Fe/S clusters is a prerequisite for the 2-thio modification of tRNAs in both subcellular compartments of yeast cells. To elucidate this question, we asked if mitochondrial ISC and/or cytosolic CIA components except Nfs1p would be involved in 2-thio modification of these tRNAs. We demonstrate here that the three CIA components, Cfd1p, Nbp35p, and Cia1p, are required for the 2-thio modification of cy-tRNAs but not of mt-tRNAs. Interestingly, the mitochondrial scaffold proteins lsu1p and Isu2p are required to the 2-thio modification of the cytosolic tRNAs but not of the mitochondrial tRNAs while mitochondrial Nfs1p is required for both 2-thio modifications. These results clearly indicate that the 2-thio modification of cytosolic tRNAs is Fe/S protein-dependent and thus requires both CIA and ISC machineries but that of mitochondrial tRNAs is Fe/S cluster-independent and does not require key mitochondrial ISC components except Nfs1p.