Genetic Code: What Nature Missed Dispatch

The genetic code encodes 20 amino acids in almost allliving cells; the only known exceptions are a few organ-isms that use selenocysteine [1] or pirrolysine [2]. Inorder to acquire chemical properties not representedin this set, proteins must be modified post-translation-ally, be made non-ribosomally, or use co-factors. In aseries of papers, Peter Schultz’s group has describedhow they carried out a directed expansion of thegenetic code to include unnatural amino acids in livingprokaryotic [3,4] and eukaryotic [5,6] cells. The mostrecent of these papers [6] reports the incorporation offive artificial amino acids into the yeast genetic code(Figure 1). In this study, Chin