Abstract
The transfer of 14C-amino acids from 14C-aminoacyl-RNA into globin polypeptides on reticulocyte ribosomes required guanosine 5′-triphosphate (GTP) and two soluble enzyme fractions. The detailed purification of these transfer enzymes is described. The transfer reaction occurred at concentrations of MgCl 2 and KCl which were optimal for cell-free synthesis of hemoglobin. Other nucleoside triphosphates could not substitute for GTP. About 70% of the radioactivity of the ribosome-bound product was found in acid-precipitable polypeptides. The remaining 30% was found in an acid-soluble fraction containing mostly small peptides. A comparison suggests that amino acids are added on to existing polypeptide chains on the ribosomes in this hemoglobin transfer system, whereas chain initiation is the predominant reaction in a transfer system stimulated with polyuridylic acid.
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