Codon specificity of UGA suppressor tRNATrp from Escherichia coli.

A synthetic polyribonucleotide, poly(U5,G), was used to study the codon specificity of wild-type and UGA suppressor tRNATrp from Escherichia coli. Phe (UUU) incorporation directed by this synthetic messenger is reduced somewhat by omission from the incubation mixtures of Val (GUU), Leu (UUG), or Cys (UGU). In contrast, omission of Cys stimulates Trp incorporation, and this effect is much more pronounced with the UGA suppressor tRNATrp than with wild-type tRNA. The apparent replacement of Cys by Trp is specific, because the omission of Val or Leu slightly inhibits Trp incorporation. These data suggest that the UGA suppressor tRNATrp can translate codons of the form UGN (N is any ribonucleotide). In other words, the suppressor tRNATrp translates codons that properly match two out of the three anticodon nucleotides.