Initiation of Hemoglobin Synthesis with Rabbit and E. coli tRNA in a Reticulocyte Cell‐Free System

The mechanism of chain initiation has been investigated in experiments with intact reticulocytes and with a cell‐free system prepared from these cells. Polyribosomes were isolated from reticulocytes incubated with labeled amino acids. The NH2‐terminal amino acid of polyribosome‐bound peptide chains has been analyzed by different methods. No evidence has been obtained for the presence of a blocked NH2‐terminal in polyribosome‐bound peptide chains. Moreover, only valine could be detected at the NH2‐terminal. Globin chains were labeled in cell‐free synthesis with [14C]valyl‐tRNA prepared from rabbit liver or E. coli. Valine was incorporated into the NH2‐terminal as well as into internal positions with both tRNAs. This indicated that a valine codon corresponds to the NH2‐terminal amino acids of rabbit globin since this codon can be recognized by E. coli tRNA. Acetyl‐valyl‐tRNA was not utilized by the cell‐free system nor had it any stimulatory activity on the incorporation of valine. These results are discussed in view of the observation that acetyl‐valyl‐tRNA seems to be required for the translation of hemoglobin messenger RNA in a cell‐free system prepared from E. coli.