Studies on the “independence” of nucleolar ribonucleic acid synthesis

Using [¹⁴C]orotic acid as a precursor, the time course of labeling of nuclear and nucleolar RNA of rat liver has been studied in vivo. RNA from both nuclear and nucleolar preparations was fractionated into two classes with the aid of phenol. Each class of RNA was found to have a relatively specific kinetic profile after administration of the tracer. Labeling of nucleolar RNA was as rapid as nuclear RNA indicating that the extranucleolar RNA could not be the precursor of the nucleolar RNA. Actinomycin D inhibited nucleolar-RNA synthesis to a much greater degree than the synthesis of extranucleolar RNA. These data, together with previous observations on the base compositions of nucleolar-RNA fractions and other types of experiments provide evidence that nucleolar-RNA synthesis largely proceeds “independently” of extranucleolar-RNA synthesis.