SOME PROPERTIES OF RIBONUCLEOPROTEIN PARTICLES IN ISOLATED NUCLEOLI OF MOUSE ASCITES TUMOR CELLS

1. 1. A polyanion plus anionic detergent lysate of isolated nucleoli was fractionated into a 25 000 × g sediment, a 105 000 × g sediment, and supernatant by means of differential centrifugation. The 105 000 × g sediment, containing RNA and protein, corresponded most closely to the rapidly sedimenting fraction on sucrose zone sedimentation. 2. 2. For the 105 000 × g sediment, both the pattern of basic protein, studied by polyacrylamide gel electrophoresis, and the charge properties, analyzed by Geon column electrophoresis, were similar to those of ribosomes. 3. 3. The nucleolar ribonucleoprotein particles were sensitive to the action of a small amount of ribonuclease (EC 2.7.7.16), whereas ribosomes from the same cell were unaffected except for dissociation of their polysome region. 4. 4. Using CsCl buoyant density analysis, it was possible to separate the rapidly sedimenting fraction into three components having different buoyant densities, all of which were significantly lower than the density of ribosomes. 5. 5. The evidence shows that the ribonucleoprotein particles in the nucleoli, containing both ribosomal RNA and protein, are not necessarily identical to the ribosomes in bio- and physicochemical properties. It appears likely that the nucleolar particles are an incomplete intermediate in the process of ribosome formation in the tumor cell. © 1969.