Size changes in eukaryotic ribosomes.

Evidence is presented that ribosomes active in protein synthesis and attached to messenger RNA on polysomes have a smaller diameter than free cytoplasmic single ribosomes. Measurements have been made on these two types of ribosomes of differences in sedimentation velocity and diffusion constant. Differences in these quantities suggest about a 20-A decrease in the diameter of the ribosomes from chick embryo muscles when they are attached to messenger RNA. Similar differences are also observed in rabbit reticulocytes and mouse ascites tumor cells. These two ribosomal states have different sensitivity to Pronase digestion and dissociate into ribosomal subunits at different KCl concentrations. This size difference is not associated with a significant difference in overall ribosomal mass and appears not to be dependent upon the presence of nascent polypeptide chains.