SPECIFIC STRUCTURAL FEATURES OF THE RAT-L IVER POLYRIBOSOMES AS STUDIED BY THE METHOD OF TRITIUM LABELING

Polyribosomes isolated from the rat liver in a medium with low ionic s trength were irradiated by ''hot'' tritium atoms under conditions prov iding for the replacement of the hydrogen atoms located at the surface of polyribosomes by tritium. After fractionation of such polyribosome s, the radioactivity of the obtained fractions was measured and their proportions were calculated for the total surface accessible for the t ritium atoms (in %), as well as their specific radioactivity. The mate rial loosely associated with the polyribosomes and containing amino ac yl-tRNA-synthetases is more radioactive than rRNA and r-proteins, espe cially concerning their specific radioactivity. This suggests that the material is organized as individual molecules located on the surface of ribosomes. The specific radioactivity of the RNA-component of this material (tRNA) is twice that of proteins, thus suggesting its surface localization in the composition of loosely associated material. Based on the pattern of labeling of rRNA and r-proteins of the native and p reliminarily dissociated polyribosomes, we propose that the material, loosely associated with the polyribosomes, has affinity to both rRNA a nd r-proteins.