Aminoacyl-tRNA synthetases: potential markers of genetic code development.

Aminoacylation of tRNAs, catalyzed by 20 aminoacyl-tRNA synthetases, is responsible for establishing the genetic code. The enzymes are divided into two 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 relates these specific pairings to the organization of the table of codons that defines the universal genetic code.