Aminoacyl-tRNA synthetases: potential markers of genetic code development

Aminoacylation of tRNAs, catalyzed by 20 aminoacyl-tRNA synthetases, is res ponsible for establishing the genetic code. The enzymes are divided into tw o classes on the basis of the architectures of their active sites. Members of the two classes also differ in that they bind opposite sides of the tRNA acceptor stem. Importantly, specific pairs of synthetases - one from each class - can be docked simultaneously onto the acceptor stem. This article r elates these specific pairings to the organization of the table of codons t hat defines the universal genetic code.