A conserved pathway consisting of at least MSS1 and MTO1 modifies the wobble nucleotide uridine-34 in mitochondrial tRNA (Umeda et al. 2005). A methyl group and a glycine group are conjugated to the 5 position of the uracil ring, yielding 5-carboxymethylaminomethyluridine. The details of the reaction mechanism are unknown. Modification of the wobble nucleotide is required for efficient and accurate translation. Mutations in constituents of the pathway cause mitochondrial dysfunction (Umeda et al. 2005, Wang et al. 2010).