Amiram, M. et al. Nat. Biotechnol. 33, 1272–1279 (2015).

The ability to incorporate unnatural amino acids with unique chemistries into proteins by reprogramming the cell's translational machinery has had an impact on multiple areas of biological research. However, the technique has suffered from problems of inefficiency, especially when it comes to incorporating unnatural amino acids at multiple sites in a protein. Amiram et al. describe a greatly improved approach that allowed them to incorporate the unnatural amino acids p-acetyl-L-phenylalanine and p-azido-L-phenylalanine (which contain useful chemical handles for labeling) at multiple sites in a protein by evolving new aminoacyl-tRNA synthetases with up to 25-fold–improved activities. They also report aminoacyl-tRNA synthetases with readily tunable specificities for 14 additional unnatural amino acids. The researchers demonstrated the ability to synthesize elastin-like polypeptide fusion proteins containing an unnatural amino acid at up to 30 sites, at high yield and with high fidelity.