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Maintaining the Canonical Amino Acid Alphabet: a Story about Aminoacyl-tRNA Synthetases

Natural evolution has selected a set of 20 canonical genetically encoded amino acids to build a plethora of proteins that provide the biochemical foundations for life. To participate in protein synthesis amino acids require attachment to transfer RNA molecules (tRNAs). This reaction is accomplished by aminoacyl-tRNA synthetases (AARSs), enzymes that use ATP to activate the matching amino acid and transfer it to the cognate tRNA, which, in turn, harbors the corresponding anticodon triplet of the amino acid code. Some AARSs are not able to discriminate against highly similar near-cognate amino acids in the synthetic reaction alone, and so have evolved hydrolytic editing to clear mistakenly activated or aminoacylated amino acids. Preventing the incorporation of protein-building amino acids at the wrong positions is not the only task of AARS quality control. Recent findings have indicated that editing may also operate to block the infiltration of natural amino acids that are not coded for protein synthesis. Using genetic, biochemical and mass spectrometry approaches we demonstrated that in the case of leucyl-tRNA synthetase the main physiological role of editing is to preclude toxic misincorporation of the non- canonical amino acid norvaline at protein positions coded for leucine [1, 2]. Norvaline accumulates under oxygen limiting conditions arguing for a critical role for editing in cellular adaptation to the various conditions of growth. Given the lack of evolutionary pressure against unnatural compounds, it had been assumed that editing would not be an obstacle for the use of artificial amino acids as protein building blocks. Yet, we have recently demonstrated that participation of trifluorethylglycine in protein translation takes place only after silencing of weak editing by isoleucyl-tRNA synthetase [3]. Thus, the advanced rational design that relies on misincorporation of artificial amino acids to create proteins with novel features relies on our ability to reengineer the AARS synthetic and editing pathways that act as powerful gatekeepers of the canonical amino acid alphabet. References [1] N. Cvetesic, A. Palencia, I. Halsz, S. Cusack and I Gruic-Sovulj, EMBO J, 33 (2014) 1639-53. [2] N. Cvetesic, M. Semanjski, B. Soufi, K. Krug, I. Gruic-Sovulj and B. Macek, Sci Rep, 6 (2016) 28631. [3] J.S.A Völler, M. Dulic and U.I. Gerling- Driessen, H. Biava, T. Baumann, N. Budisa, I. Gruic-Sovulj, B. Koksch, ACS Central Science, 3 (2017)73-80.

aminoacyl-tRNA synthetases, editing, norvaline, protein quality control

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