Johnson, D. B. F. et al. Nat. Chem. Biol. 7, 779–786 (2011).

Probes can be site-specifically introduced into proteins by reassigning the stop codon UAG to encode an unnatural amino acid. But incorporation efficiency is low because the stop codon continues to serve as a stop signal. Johnson et al. fully reassign the UAG codon in Escherichia coli by knocking out the essential release factor 1 and fixing the expression of release factor 2. This greatly improves incorporation of unnatural amino acids, allowing incorporation at multiple sites.