An Orthogonal Tyrosyl-tRNA Synthetase/tRNA Pair from a Thermophilic Bacterium for an Expanded Eukaryotic Genetic Code

The Escherichia coli-derived tyrosyl-tRNA synthetase was the first enzyme engineered for genetic code expansion in a eukaryotic system but can charge only a limited set of structurally simple noncanonical amino acids. In contrast, the thermophilic Methanocaldococcus jannaschii-derived tyrosyl-tRNA synthetase mutants, used in only a prokaryotic system, can charge a surprisingly large set of structurally diverse ncAAs, due to their remarkable structural ability to tolerate mutations. Inspired by this, we characterized a new class of tyrosyl-tRNA synthetase/tRNA^Tyr pairs from thermophilic bacterium Geobacillus stearothermophilus, which is homologous to the E. coli tyrosyl-tRNA synthetase but with better thermostability. This new pair is both orthogonal in mammalian cells and in Saccharomyces cerevisiae for genetic code expansion and can charge a diverse set of ncAAs with a comparable cellular efficiency, better specificity, and lower background, as compared to those of its E. coli homologue. This thermostable enzyme provides an alternative scaffold for synthetase library screening or evolution to genetically encode more structurally complex ncAAs in eukaryotic cells.