Abstract
In order to elucidate the site of the synthesis of ribosomal proteins and of their assembly with ribosomal RNA in liver cells, labeling experiments were performed in vivo, employing regenerating rat liver and the following results were obtained. 1 The relative synthetic rates of 28-S ribosomal RNA and ribosomal proteins at various time after partial hepatectomy, which were expressed as the ratios to those of the normal rat liver, ran parallel with each other and had maximal values, more than 10-times higher than those of normal rats, at 12 h after partial hepatectomy. 2 Disc electrophoresis of the acid-soluble proteins extracted from nucleolar detergent extract or 60-S particles by the procedures of Narayan and Birnstiel [1] revealed that the protein fraction contained most species of structural proteins of cytoplasmic ribosomes. Furthermore, the profiles of radioactivity on disc electrophoresis of the mixture of acid-soluble proteins of nucleolar 60-S particles and those of the large subunit of cytoplasmic ribosomes which were labeled with 3H and 14C-labeled amino acids, respectively, showed that most of the peaks of 3H and 14C radioactivities coincided with each other. The results may indicate the similarities between the protein moieties of these two particles. 3 The time courses of the incorporation of [3H]leucine into acid-soluble proteins of the nucleolar detergent extract and into the basic structural proteins of the cytoplasmic ribosomes suggested the precursor nature of proteins of the nucleolar extracts to structural proteins of the cytoplasmic ribosomes. Similarly the time-course study on the incorporation of [14C]orotic acid into RNA moieties and that of [3H]leucine into protein moieties of both nucleolar 60-S particles and the large subunit of the cytoplasmic ribosomes, indicated that 60-S nucleolar particles may be the precursor to the large subunit of cytoplasmic ribosomes. 4 The nascent ribosomal structural protein fraction, which was purified from the nascent protein fraction of regenerating-liver ribosomes labeled with [3H]leucine was subjected to disc electrophoresis. The radioactive proteins were distributed in the region of ribosomal structural proteins in addition to some amounts of radioactivity in more fast-moving regions. The time course of the incorporation of [3H]leucine into this nascent ribosomal proteins showed a sharp increase after the injection, reached a maximal value after 5 min and thereafter decreased rapidly, while that into proteins of nucleolar 60-S particles took the maximal value after 20 min. 5 These results may indicate that most of structural proteins of liver ribosomes are synthesized on cytoplasmic ribosomes, then transferred to the nucleolus where their assembly with ribosomal precursor RNA occurs to form 60-S nucleolar particles. The nucleolar 60-S particles may be the precursor to the large subunit of rat liver ribosomes.
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