RELAXING THE SUBSTRATE-SPECIFICITY OF AN AMINOACYL-TRANSFER-RNA SYNTHETASE ALLOWS IN-VITRO AND IN-VIVO SYNTHESIS OF PROTEINS CONTAINING UNNATURAL AMINO-ACIDS

It has previously been demonstrated that the unnatural amino acid p-Q-phenylalanine can be attached to tRNA(Phe) by a modified phenlalanyl-tRNA synthetase with relaxed amino acid substrate specificity, We show that this modification to the translational machinery of Escherichia coli is the only requirement for the incorporation of either p-Cl- or p-Br-phenylalanine into full-length luciferase in vitro, The incorporation of p-Cl-phenylalanine was also demonstrated in vivo using a suitably modified host strain. These results represent the first description of the incorporation into a protein in vivo of an unnatural amino acid which is normally rejected by the cellular translational machinery.