Hemoglobins radioactive amino acid incorporation

Some phases of the synthesis of hemoglobin have been studied in vitro and in vivo in both animal and human subjects by various investigators [(G9, Hll, M4, N13) and others]. Labeled amino acids, i.e., leucine- C, phenylalanine- C, and leucine- H, are used for the study of amino acid incorporation. The procedure consists of obtaining either human or animal mRNA, reticulocyte ribosomes from the study case, and radioactive amino acids. The three are combined and incubated for... 

Does this mischarged tRNA recognize the codon for cysteine or for alanine The answer came when the tRNA was added to a cell-free protein-synthesizing system. The template was a random copolymer of U and G in the ratio of 5 1, which normally incorporates cysteine (encoded by UGU) but not alanine (encoded by GCN). However, alanine was incorporated into a polypeptide when Ala-tHNA ys was added to the incubation mixture. The same result was obtained when mRNA for hemoglobin served as the template and [i CJalanyl-tRNACys was used as the mischarged aminoacyl-tRNA. The only radioactive tryptic peptide produced was one that normally contained cysteine but not alanine. Thus, the amino acid in aminoacyl-tRNA does not play a role in selecting a codon. 

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