MTU1 transfers a sulfur atom from L-cysteine to the 2 position of 5-carboxymethylaminomethyluridine-34 in tRNAs (Umeda et al. 2005). In Escherichia coli the sulfur is transferred along a relay system of proteins from L-cysteine to uridine. It is unknown if such a relay system also exists in yeast. Mutations in MTU1 act synergistically with mutations in MSS1 (the homolog of GTPBP3) and MTO1 to impair mitochondrial function (Umeda et al. 2005, Wang et al. 2010). Deletion of MTU1 causes mitochondrial failure (Wang et al. 2007) and mutations in MTU1 modify the phenotype produced by mutations in mitochondrial 12S rRNA (Yan et al. 2005).