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. N6 -threonylcarbamoyladenosine (t6 A) occurs at position 37 of five mitochondrial (mt-)tRNA species. We show that YRDC and OSGEPL1 are responsible for t6 A37 formation, utilizing L-threonine, ATP, and CO2 /bicarbonate as substrates. OSGEPL1-knockout cells exhibit respiratory defects and reduced mitochondrial translation. We find low level of t6 A37 in mutant mt-tRNA isolated from the MERRF-like patient's cells, indicating that lack of t6 A37 results in pathological consequences. Kinetic measurements of t6 A37 formation reveal that the Km value of CO2 /bicarbonate is extremely high (31 mM), suggesting that CO2 /bicarbonate is a rate-limiting factor for t6 A37 formation. Consistent with this, we observe a low frequency of t6 A37 in mt-tRNAs isolated from human cells cultured without bicarbonate. These findings indicate that t6 A37 is regulated by sensing intracellular CO2 /bicarbonate concentration, implying that mitochondrial translation is modulated in a codon-specific manner under physiological conditions.