Incorporation of Cy4-amino Acids into Ribonuct,eo- Protein Particles from the Ehrlich Mouse Ascites Tumor*

There have been many indications from earlier studies that ribonucleic acid’ is involved in protein synthesis, perhaps as the template for the formation of specific proteins (see Brachet (2) for a recent review). In studies on the mechanism of protein synthesis we have found that, when a C14amino acid is injected intravenously into a rat, the initial incorporation into the cytoplasmic ribonucleoprotein particles of the liver is several times greater than into other cell fractions (3). The rapid initial incorporation into these particles could represent the formation of new polypeptide chains on the surface of the RNA. In these experiments in tivo, the Cl4 in the ribonucleoprotein declined after the hrst few minutes if the specific activity of the free intracellular amino acid decreased. If, however, the latter was kept constant, the labeling of the nucleoprotein remained constant, suggesting a steady state in which the rate of formation and release of polypeptide chains were equal. Only 1 per cent of the proteinin the ribonucleoprotein particles became labeled in this process. Earlier, Petermann et al. had shown by ultracentrifugal and electrophoretie analyses that there is in the cytoplasm of liver and other cells a variety of ribonucleoprotein particles containing equal amounts of RNA and protein (4). The major type of particle inliver (Component B) hasa sedimentation rate of 50 S. In tumors and other rapidly growing tissues another type of particle (Component C, 40 S) is prominent, as if it were somehow connected with cell division (5). Perhaps Component C contains the templates for the structural and enzymatic proteins of the cell formed during growth. These several ribonucleoprotein particles were presumably identi-