Ultrastructural and biochemical studies of the isolated fibrillar component of nucleoli from Novikoff hepatoma ascites cells

Abstract

Nucleoli of Novikoff hepatoma ascites cells were fractionated into granular and fibrillar components with solutions containing polyvinsulphate and Mg 2+ (PVS/Mg 2+) or solutions containing EDTA. After treatment with the PVS/Mg 2+ solutions, the morphology of the fibrillar component was similar to that in situ inasmuch as they contain fibrillar clusters as well as other fibrillar elements ranging in diameter from 150–700 Å. The nucleotide composition of the RNA of the fibrillar and granular fractions was similar to that of ribosomal RNA. In agreement with earlier autoradiographic studies, the kinetics of incorporation of 32P-orthophosphate into RNA in both fractions showed that the specific activity of the RNA of the fibrillar fraction was fourfold greater than that of RNA in the granular region at 15 min. After 2 h the specific activity of the RNA in the granular region was 1.5 times greater than that in the fibrillar region. Two-dimensional polyacrylamide gel electrophoresis showed that the fibrillar fraction contained fewer proteins than the granular fraction. Some of the proteins were common to both the nucleolar granular particles and cytoplasmic ribosomes while others appeared to be unique to this fraction.