tRNA N6-adenosine threonylcarbamoyltransferase, mitochondrial
tRNA N(6)-L-threonylcarbamoyladenine synthase activity
CO<sub>2</sub>-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<sup>6</sup>-threonylcarbamoyladenosine (t<sup>6</sup>A) occurs at position 37 of five mitochondrial (mt-)tRNA species. We show that YRDC and OSGEPL1 are responsible for t<sup>6</sup>A37 formation, utilizing L-threonine, ATP, and CO<sub>2</sub>/bicarbonate as substrates. OSGEPL1-knockout cells exhibit respiratory defects and reduced mitochondrial translation. We find low level of t<sup>6</sup>A37 in mutant mt-tRNA isolated from the MERRF-like patient's cells, indicating that lack of t<sup>6</sup>A37 results in pathological consequences. Kinetic measurements of t<sup>6</sup>A37 formation reveal that the Km value of CO<sub>2</sub>/bicarbonate is extremely high (31 mM), suggesting that CO<sub>2</sub>/bicarbonate is a rate-limiting factor for t<sup>6</sup>A37 formation. Consistent with this, we observe a low frequency of t<sup>6</sup>A37 in mt-tRNAs isolated from human cells cultured without bicarbonate. These findings indicate that t<sup>6</sup>A37 is regulated by sensing intracellular CO<sub>2</sub>/bicarbonate concentration, implying that mitochondrial translation is modulated in a codon-specific manner under physiological conditions.