Dissimilarity in protein chain elongation factor requirements between yeast and rat liver ribosomes

Factor requirements for yeast and rat liver ribosomes were determined in several different reactions using either yeast or liver factors. In polymerization assays yeast ribosomes required a factor in addition to elongation factor 1 (EF-1) and elongation factor 2 (EP-2). The third factor (EF-3) requirement was observed with EFs from either yeast or liver for both poly(U)-directed polyphenylalanine synthesis and elongation of endogenous peptidyl-tRNA. No significant effect of EF-3 was observed with liver risomes in either assay. In contrast to results with polypeptide synthesis EF-3 was not required for EF-1 dependent binding of [3H]Phe-tRNA or the translocation-dependent formation of N-acetylphenylalanylpuromycin. Up to 2-fold stimulation of the binding reaction was observed with saturating levels of either yeast or liver EF-1. No effect of EF-3 was observed on ribosome-EF-2-GDP-fusidic acid complex formation. The data suggest that the yeast EF-3 may be a loosely bound ribosomal protein which is not required for a specific step in the elongation cycle but is involved in the coordination of the partial reactions required for polymerization.