CO(2)-sensitive tRNA modification associated with human mitochondrial disease

It has been generally thought that tRNA modifications are stable and static, and their frequencies are rarely regulated. N(6)-threonylcarbamoyladenosine (t(6)A) occurs at position 37 of five mitochondrial (mt-)tRNA species. We show that YRDC and OSGEPL1 are responsible for t(6)A37 formation, utilizing L-threonine, ATP, and CO(2)/bicarbonate as substrates. OSGEPL1-knockout cells exhibit respiratory defects and reduced mitochondrial translation. We find low level of t(6)A37 in mutant mt-tRNA isolated from the MERRF-like patient’s cells, indicating that lack of t(6)A37 results in pathological consequences. Kinetic measurements of t(6)A37 formation reveal that the Km value of CO(2)/bicarbonate is extremely high (31 mM), suggesting that CO(2)/bicarbonate is a rate-limiting factor for t(6)A37 formation. Consistent with this, we observe a low frequency of t(6)A37 in mt-tRNAs isolated from human cells cultured without bicarbonate. These findings indicate that t(6)A37 is regulated by sensing intracellular CO(2)/bicarbonate concentration, implying that mitochondrial translation is modulated in a codon-specific manner under physiological conditions.