Functional replacement of the endogenous tyrosyl-tRNA synthetase-tRNATyr pair by the archaeal tyrosine pair in Escherichia coli for genetic code expansion.

Non-natural amino acids have been genetically encoded in living cells, using aminoacyl-tRNA synthetase-tRNA pairs orthogonal to the host translation system. In the present study, we engineered Escherichia coli cells with a translation system orthogonal to the E. coli tyrosyl-tRNA synthetase (TyrRS)-tRNA(Tyr) pair, to use E. coli TyrRS variants for non-natural amino acids in the cells without interfering with tyrosine incorporation. We showed that the E. coli TyrRS-tRNA(Tyr) pair can be functionally replaced by the Methanocaldococcus jannaschii and Saccharomyces cerevisiae tyrosine pairs, which do not cross-react with E. coli TyrRS or tRNA(Tyr). The endogenous TyrRS and tRNA(Tyr) genes were then removed from the chromosome of the E. coli cells expressing the archaeal TyrRS-tRNA(Tyr) pair. In this engineered strain, 3-iodo-L-tyrosine and 3-azido-L-tyrosine were each successfully encoded with the amber codon, using the E. coli amber suppressor tRNATyr and a TyrRS variant, which was previously developed for 3-iodo-L-tyrosine and was also found to recognize 3-azido-L-tyrosine. The structural basis for the 3-azido-L-tyrosine recognition was revealed by X-ray crystallography. The present engineering allows E. coli TyrRS variants for non-natural amino acids to be developed in E. coli, for use in both eukaryotic and bacterial cells for genetic code expansion.