Accurate transfer RNA (tRNA) aminoacylation by aminoacyl-tRNA synthetases controls translational fidelity. Although tRNA synthetases are generally highly accurate, recent results show that the methionyl-tRNA synthetase (MetRS) is an exception. MetRS readily misacylates non-methionyl tRNAs at frequencies of up to 10% in mammalian cells; such mismethionylation may serve a beneficial role for cells to protect their own proteins against oxidative damage. The Escherichia coli MetRS mismethionylates two E. coli tRNA species in vitro, and these two tRNAs contain identity elements for mismethionylation. Here we investigate tRNA mismethionylation in Saccharomyces cerevisiae. tRNA mismethionylation occurs at a similar extent in vivo as in mammalian cells. Both cognate and mismethionylated tRNAs have similar turnover kinetics upon cycloheximide treatment. We identify specific arginine/lysine to methionine-substituted peptides in proteomic mass spectrometry, indicating that mismethionylated tRNAs are used in translation. The yeast MetRS is part of a complex containing the anchoring protein Arc1p and the glutamyl-tRNA synthetase (GluRS). The recombinant Arc1p-MetRS-GluRS complex binds and mismethionylates many tRNA species in vitro. Our results indicate that the yeast MetRS is responsible for extensive misacylation of non-methionyl tRNAs, and mismethionylation also occurs in this evolutionary branch.
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