C²-methyladenosine in tRNA promotes protein translation by facilitating the decoding of tandem m²A-tRNA-dependent codons.

Abstract

RNA modification C²-methyladenosine (m²A) exists in both rRNA and tRNA of Escherichia coli (E. coli), installed by the methyltransferase RlmN using a radical-S-adenosylmethionine (SAM) mechanism. However, the precise function of m²A in tRNA and its ubiquity in plants have remained unclear. Here we discover the presence of m²A in chloroplast rRNA and tRNA, as well as cytosolic tRNA, in multiple plant species. We identify six m²A-modified chloroplast tRNAs and two m²A-modified cytosolic tRNAs across different plants. Furthermore, we characterize three Arabidopsis m²A methyltransferases-RLMNL1, RLMNL2, and RLMNL3-which methylate chloroplast rRNA, chloroplast tRNA, and cytosolic tRNA, respectively. Our findings demonstrate that m²A37 promotes a relaxed conformation of tRNA, enhancing translation efficiency in chloroplast and cytosol by facilitating decoding of tandem m²A-tRNA-dependent codons. This study provides insights into the molecular function and biological significance of m²A, uncovering a layer of translation regulation in plants.