Fibrillar nucleolar remnants do not contain macromolecular precursors of ribosomal RNA : Demonstration by the effects of -galactosamine

Administration of -galactosamine to rats produces inhibition of liver nuclear RNA synthesis and associated alterations in the structure of the nucleolus. Polyacrylamide gel electrophoretic analysis of liver nuclear RNA from galactosamine-treated rats has shown the virtual complete absence of ribosomal RNA (rRNA) precursor molecules at a time when the nucleolus consists solely of a dense fibrillar core devoid of granules. No evidence for an artefactual, preferential breakdown of nuclear RNA during extraction could be obtained from either 2.7 or 8% acrylamide gels. Furthermore, the almost complete cessation of nuclear RNA synthesis makes the possibility of there being rapid synthesis and degradation of ribosomal precursor molecules in vivo unlikely. With toluidine blue stains for RNA with nuclei isolated from galactosamine-treated animals, the large, brightly staining area associated with the normal nucleolus was not seen. On the basis of these observations, it is concluded that an RNA-depleted nucleolus appears fibrillar. It is suggested that the fibrillar material of a normal nucleolus may not itself be RNA even though this region does contain RNA precursor molecules.