| Abstract: | 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. |
