| Accurate aminoacylation of tRNA by aminoacyl-tRNA synthetases controls translational fidelity. Although tRNA synthetases are generally highly accurate in tRNA charging, recent results show that the methionyl-tRNA synthetase (MetRS) is an exception. Misacylation of nonmethionyl-tRNAs with methionine occurs at frequencies of up to 10% in mammalian cells; such tRNA mismethionylation has been proposed to serve a beneficial role for cells to protect their own proteins against oxidative damage. The Escherichia coli MetRS mismethionylates specifically two tRNA species in vitro, and these two tRNAs contain identity elements for mismethionylation. We have investigated tRNA mismethionylation in other evolutionary branches distinct from mammals and E. coli, including the yeast Saccharomyces cerevisiae, the hyperthermophile archaea Aeropyrum pernix, the facultatively aerobic archaea Thermoplasma acidophilum, and the Gram-positive bacterium Staphylococcus aureus. tRNA mismethionylation occurs in all of these evolutionary distinct species and occurs at a similar extent in yeast in vivo as in mammalian cells. Based on our findings in yeast, both cognate and mismethionylated tRNAs have similar utilization kinetics upon cycloheximide treatment, and mismethionylated peptides have been detected in proteomic mass spectrometry, suggesting that mismethionylated tRNAs are used in translation. Our results further our understanding of the evolutionary conservation, mechanism, and potential benefits of mismethionylation. |
