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Deacylated Transfer Ribonucleic Acid as a Factor for Peptide Chain Initiation in Rabbit Reticulocyte Systems

From the ¹ From the Clayton Foundation Biochemical Institute and the Department of Chemistry, The University of Texas, Austin, Texas 78712

Evidence is presented that is interpreted as reflecting a codon-directed specific requirement for deacylated transfer RNA in reactions associated in sequence with the initiation of polyphenylalanine synthesis in a fractionated transfer system derived from rabbit reticulocytes. The ability of deacylated transfer RNA to participate in these early reactions, as measured by a shift in the optimum Mg⁺⁺ concentration for the synthesis of polyphenylalanine, is destroyed by mild oxidation with sodium periodate under conditions which destroy the ability of transfer RNA to accept amino acids. After periodate oxidation (to remove activity due to contaminating deacylated transfer RNA), preparations of transfer RNA containing phenylalanyl-tRNA or N-acetylphenylalanyl-tRNA are also inactive in these early reactions of chain initiation.

Evidence is also presented that is interpreted as indicating that N-acylated amino acids may play no role in the initiation of and ß chains of globin. N-Formylmethionyl-tRNA was not detected in preparations containing methionyl-tRNA formed in intact reticulocytes. Similar results were obtained for the N-acetyl and N-formyl derivatives of phenylalanine, valine, alanine, leucine, and serine. Furthermore, very little formate, if any, is present as the NH₂-terminal residues of incomplete globin peptide chains that are initiated in intact cells and that remain attached to polysomes during isolation. Most, if not all, of these nascent globin chains exist with valine with a free amino group as the NH₂-terminal amino acid. It is suggested that the initiation of synthesis of globin peptides may involve a codon-directed requirement for deacylated transfer RNA, by a mechanism similar to that involved in the initiation of polyphenylalanine synthesis.

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