Photoinduced Protein‐RNA Cross‐Linking in Mammalian 80‐S Ribosomes

RNA‐protein interactions in the 80‐S rat liver ribosomes were studied by measuring cross‐linking of proteins to rRNAs induced by ultraviolet radiation, as already reported for free 40‐S and 60‐S subunits. Our results are compatible with the model in which most of the ribosomal proteins are accessible to rRNAs in the native conformational state of the ribosomes. Subunit association in 80‐S ribosomes does not seem to induce modifications in protein‐RNA interactions as measured by this irradiation technique. However, two proteins, S9 and S13, appeared to be significantly less cross‐linked with RNA in ribosomes than in free subunits.Ribosomes which had been frozen and thawed several times were highly sensitive to ultraviolet radiation. Such treatment in the cold chiefly modified their 60‐S subunit moiety.