Expression of the genetic code depends on the recognition of specific tRNAs by the enzymes that aminoacylate them. A computer comparison of tRNA sequences coupled with analysis of mutant nonsense-suppressor tRNAs has revealed the structural features that distinguish the acceptor identity of Escherichia coli tRNA(Gly) from tRNAs that accept phenylalanine, arginine, lysine, and glutamine. On replacement of several nucleotides in the acceptor stem and anticodon of the latter tRNAs with tRNA(Gly)-derived residues, the resulting molecules acquired a tRNA(Gly) identity.