J. Am. Chem. Soc., published online 8 August 2011, doi:10.1021/ja2054034

Credit: JASON CHIN

Reprogramming of translational stop codons to direct the incorporation of non-natural amino acids into proteins—or 'genetic code expansion', as it is sometimes called—has provided a useful method for the site-specific engineering of proteins in bacteria, yeast and mammalian cells. These cellular approaches require an orthogonal coding system that makes use of an engineered mRNA, a suppressor tRNA and a matched aminoacyl-tRNA synthetase (AARS) that uniquely charges the suppressor tRNA with the non-natural amino acid of interest. Greiss and Chin now tackle the challenge of extending this technology to multicellular organisms by expanding the genetic code of the nematode Caenorhabditis elegans. Using a C. elegans strain in which the nonsense-mediated decay (NMD) pathway had been disabled, the authors adapted an orthogonal tRNA-AARS system from the archaeobacter Methanosarcina mazei that had been previously optimized to encode non-natural lysine analogs. An mRNA construct that produced green fluorescent protein (GFP) in the absence and mCherry in the presence of amber codon suppression by the orthogonal tRNA demonstrated that codon reprogramming takes place in the worm cells and requires the presence of the non-natural amino acid. The incorporation of the modified lysine analogs was also verified directly by protein immunoblotting. The study provides the first example of genetic code expansion in an animal, representing a technology that enables more direct analysis of molecular events in living organisms.