Studies on the processing of 45-S RNA in rat liver nucleolus with specific reference to 29.5-S RNA

Abstract

Nucleolar RNAs prepared from rat liver labelled with [ 14C]- or [ 3H]orotic acid with cold and hot sodium dodecyl sulphate-phenol extraction, were analysed by means of polyacrylamide gel electrophoresis and the following results have been obtained. 1. 1. In nucleolar RNAs which are prepared with cold sodium dodecyl sulphatephenol extraction not only from normal, but also from thioacetamide-treated and regenerating rat livers, 29.5-S RNA is found in addition to 45-, 37-, 36-, 32-, 28-, 21.5- and 18-S RNAs, whereas in those with hot sodium dodecyl sulphate-phenol extracttion, 29.5-S RNA is not observed in all cases. Nucleolar 28-S RNA and cytoplasmic 28-S RNA of rat liver co-migrate in acrylamide gel electrophoresis and do not change their electrophoretic mobilities with hot sodium dodecyl sulphate-phenol extraction or heat treatment, indicating that the presence of 29.5-S RNA is not due to the conformational change of 28-S RNA. The 37-S minor component is very unstable in the case of normal rat liver, but stable in the case of thioacetamide-treated rat liver. 29.5-S and 37-S RNAs are thought to be newly found RNAs using the cold sodium dodecyl sulphate-phenol extraction method. 2. 2. Taking the kinetics of labelling, the results of actinomycin D chase experiment and the molecular weights of the nucleolar RNA components together, may indicate that 29.5-S RNA is the metabolic intermediate between 32-S and 28-S RNAs, and 37-S RNA is suggested to be the direct precursor of 36-S RNA. The following scheme for the processing of 45-S RNA in rat liver nucleoli is proposed: ▪