Hwang, S.-P.; Liao, H.; Barondeau, K.; Han, X.; Herbert, C.; McConnie, H.; Shekar, A.; Pestov, D.; Limbach, P. A.; Chang, J. T.; Denicourt, C.

Mapping of the epitranscriptome has revealed the chemical diversity of RNA modifications and their functional importance in regulating gene expression. Transfer RNAs (tRNAs) are one of the most modified cellular RNAs, containing on average 10-13 modifications per molecule. These modifications have been shown to be critical for several aspects of tRNA functions, such as decoding, folding, and stability. Here we report that the human RNA methyltransferase TRMT1L associates with components of the Rix1 ribosome biogenesis complex and co-sediments with pre-60S ribosomes. Using eCLIP-Seq, we show that TRMT1L binds to a subset of tRNAs and to the 28S rRNA. Additionally, we demonstrate that TRMT1L is responsible for catalyzing N2, N2-dimethylguanosine (m22G) solely at position 27 of tRNA-Tyr-GUA by Nano-tRNAseq and RNA LC-MS. Surprisingly, TRMT1L depletion also impaired the deposition of acp3U and dihydrouridine on tRNA-Tyr-GUA, Cys-GCA, and Ala-CGC. TRMT1L knockout cells have a marked decrease in tRNA-Tyr-GUA levels, coinciding with a reduction in global translation rates and hypersensitivity of oxidative stress. Our results establish TRMT1L as the elusive methyltransferase catalyzing the m22G27 modification on tRNA Tyr, resolving a long-standing gap of knowledge and highlighting its potential role in a tRNA modification circuit crucial for translation regulation and stress response.