Amino acid substitutions in protein biosynthesis. Poly(A)-directed polyphenylalanine synthesis.

The fidelity of protein biosynthesis in vitro was studied quantitatively in a well defined system that employed poly(A) as a message for the elaboration of phenylalanine-containing polypeptides from Escherichia coli phenylalanyl-tRNALys. Admixture of phenylalanyl-tRNALys and lysyl-tRNALys in three different ratios resulted in the efficient formation of peptides that contained the amino acids in the same ratios in which they had been utilized in the individual incubation mixtures. The incorporation was also shown to be codon-specific in a quantitative sense; the poly(A)-directed incorporation of [14C]phenylalanine from phenylalanyl-tRNALys was unaffected by [3H]phenylalanyl tRNAPhe, by arginyl-tRNA (one species of which responds to the codon triplet AGA) or by unfractionated E. coli tRNA. These findings suggest that the transfer of amino acids from (misacylated) tRNAs into polypeptides in vitro is as predicted by the adapter hypothesis and that such systems can operate with sufficient fidelity to permit the preparation of proteins having defined amino acid substitutions.