Incorporation of noncoded amino acids by in vitro protein biosynthesis

The method of site-specific mutagenesis with noncoded amino acids using suppression of a nonsense codon by a semi-synthetic tRNA was first introduced in 1989. Initially used to probe the tolerance of the protein biosynthetic machinery for compounds other than the 20 primary amino acids, the method has since been applied to study a widely diverse range of biological problems. The ability to introduce side chains bearing subtle structural and electronic differences, fluorescent probes, isotope labels, photolabile protecting groups, chemical handles and photoactivated cross-linkers at unique sites has facilitated studies not currently accessible by other means. Improvements and alternatives to the early methodology are considered as well as some interesting recent applications.