The protein synthetic activity in vitro of ribosomes differing in the extent of phosphorylation of their ribosomal proteins

We describe a re-examination of the cell-free protein synthetic activity of eukaryotic ribosomes having proteins phosphorylated to different extents. Ribosomal 40 S subunits were isolated both from a variety of cells in which there is relatively little phosphorylation of ribosomal protein S6, and from cells subjected in vivo to different stimuli that promote the extensive phosphorylation of protein S6. The ability of these subunits to bind Met-tRNA as well as the second amino acyl-tRNA (Val-tRNA) was compared in the presence of highly purified initiation factors, elongation factor EF-1 at various concentrations of 60 S subunits, 9 S globin mRNA and potassium ions. The ability of the subunits to synthesize polyphenylalanine was also studied using highly purified elongation factors. In no case was any significant difference in activity observed between ribosomes with protein S6 phosphory-lated to different extents. Similar, though less extensive, studies were performed comparing 60 S ribosomal subunits differing in the extent of phosphorylation of the acidic phosphoprotein, Lψ, and of L14. No difference in activity was observed between these ribosomes.