MTU1 (TRMU) is a mitochondrial
enzyme responsible for the 2-thiolation of the wobble U in
tRNA(Lys),
tRNA(Glu) and
tRNA(Gln), a post-transcriptional modification believed to be important for accurate and efficient synthesis of the 13 respiratory chain subunits encoded by
mtDNA. Mutations in MTU1 are associated with acute infantile
liver failure, and this has been ascribed to a transient lack of
cysteine, the
sulfur donor for the thiouridylation reaction, resulting in a mitochondrial translation defect during early development. A mutation in
tRNA(Lys) that causes
myoclonic epilepsy with ragged-red fibers (
MERRF) is also reported to prevent modification of the wobble U. Here we show that mitochondrial translation is unaffected in fibroblasts from an MTU1 patient, in which MTU1 is undetectable by immunoblotting, despite the severe reduction in the 2-thiolation of mitochondrial
tRNA(Lys),
tRNA(Glu) and
tRNA(Gln). The only respiratory chain abnormality that we could observe in these cells was an accumulation of a Complex II assembly intermediate, which, however, did not affect the level of the fully assembled
enzyme. The identical phenotype was observed by
siRNA-mediated knockdown of MTU1 in HEK 293 cells. Further, the mitochondrial translation deficiencies present in myoblasts from
mitochondrial encephalomyopathy,
lactic acidosis and
stroke-like episode and
MERRF patients, which are associated with defects in post-transcriptional modification of mitochondrial tRNAs, did not worsen following knockdown of MTU1 in these cells. This study demonstrates that MTU1 is not required for mitochondrial translation at normal steady-state levels of tRNAs, and that it may possess an as yet uncharacterized function in another
sulfur-trafficking pathway.