STUDIES ON BINDING OF PHENYLALANYL TRANSFER RNA TO RAT-LIVER RIBOSOMES

When rat-liver ribosomes, free of peptidyl-tRNA and mRNA, are incubated with [14C]phenylalanyl-tRNA in relatively low MgCl2-containing solutions, polyphenylalanine is synthesized in the presence of purified transferase I (aminoacyl-tRNA-binding factor), transferase II (translocation factor), GTP and poly U. N-Acetylphenylalanyl-tRNA has no effect, and high MgCl2 concentrations markedly inhibit polyphenylalanine synthesis. At low Mg2+ concentrations, binding of aminoacyl-tRNA to ribosomes stringently requires transferase I and GTP. At high Mg2+ concentrations, binding occurs in the absence of the factor or the nucleotide. Both the enzymatic and nonenzymatic binding reactions are codon-specific interactions. Most of the ribosome-bound product is amino acid; negligible amounts of dipeptide are detected. This finding indicates that only the aminoacyl site on rat-liver ribosomes is receptive to aminoacyl-tRNA, and differs somewhat from those obtained with Escherichia coli ribosomes in certain respects: polyphenylalanine synthesis from aminoacyl-tRNA occurs at low MgCl2 concentrations in the absence of additional initiators; aminoacyl-tRNA binding is specific for the aminoacyl site; the peptidyl site can be filled only as the results of translocation. © 1969.