Competition between the Elongation Factors 1 and 2, and Phenylalanyl Transfer Ribonucleic Acid for the Ribosomal Binding Sites in a Polypeptide-synthesizing System from Brain*

80 S ribosomes and elongation factors 1 and 2 (EF-1 and EF-2), isolated from freeze-dried porcine brain of the hypothalamic region, were used to study the interrelation between the ribosomal binding of the two factors and of phenylalanyltRNA and N-acetyl-phenylalanyl-tRNA. Complexing of ribosomes with EF-2 + /3, y-methylene-guanosine S’-triphosphate or EF-2 + GTP + fusidic acid prevented or inhibited EF-l-dependent binding of phenylalanyl-tRNA as well as EF-l-independent binding at 20 mu Mg*+. On the other hand, when ribosomes were preincubated with EF-1, GTP, and phenylalanyl-tRNA, EFS-dependent GDP binding became inhibited. Furthermore, ribosomes nonenzymatically charged with phenylalanyl-tRNA or N-acetylphenylalanyl-tRNA, presumably in the acceptor position, likewise did not accept EF-2 and GDP. These results indicate that EF-1 and EF-2 have an overlapping region on 80 S ribosomes that includes the acceptor site for nonenzymatic binding of phenylalanyl-tRNA or N-acetyl-phenylalanyl-tRNA.