Mitochondrial translation requires folate-dependent tRNA methylation

Folates enable the activation and transfer of one-carbon units for biosynthesis of purines, thymidine and methionine(1–3). Antifolates are important immunosuppressive(4) and anticancer agents(5). In proliferating lymphocytes(6) and human cancers(7,8), folate enzymes localizing to the mitochondria are particularly strongly upregulated. This in part reflects the need for mitochondria to generate one-carbon units and export them to the cytosol for anabolic metabolism(2,9). The full range of uses of folate-bound one-carbon units in the mitochondrial compartment itself, however, has not been thoroughly explored. Here we show that loss of catalytic activity of the mitochondrial folate enzyme serine hydroxymethyltransferase 2 (SHMT2), but not other folate enzymes, leads to defective oxidative phosphorylation due to impaired mitochondrial translation. We find that SHMT2, presumably by generating mitochondrial 5,10-methylenetetrahydrofolate, provides methyl donors for producing the taurinomethyluridine base at the wobble position of select mitochondrial tRNAs. Mitochondrial ribosome profiling reveals that SHMT2 knockout cells, due to lack of this modified base, suffer from defective translation with preferential mitochondrial ribosome stalling at certain lysine (AAG) and leucine (UUG) codons. This results in impaired respiratory chain enzyme expression. Stalling at these specific codons also occurs in certain mitochondrial inborn errors of metabolism. Disrupting whole-cell folate metabolism, by folate deficiency or antifolate therapy, also impairs the respiratory chain. In summary, mammalian mitochondria use folate-bound one-carbon units to methylate tRNA, and this modification is required for respiratory chain translation and thus oxidative phosphorylation.