Evidence that the granular and fibrillar components of nucleoli contain 28 and 6S RNA, respectively

Abstract

Electron microscopic studies of nucleoli were correlated with sucrose density gradient sedimentation analysis of nucleolar RNA in samples obtained from livers of control rats, rats treated with thioacetamide, rats treated with actinomycin D and rats treated with both compounds. By planimetric analysis the fibrillar elements comprised approximately 20 per cent of the nucleolar area in the controls; the percentages of the total nucleolar area containing fibrillar elements were markedly increased after administration of actinomycin D and markedly decreased after administration of thioacetamide. Corresponding changes were found in 6S RNA following administration of actinomycin D and thioacetamide. When both compounds were administered simultaneously, coalescence of the fibrillar elements was observed and the resulting spherules were apparently extruded from the nucleolus, leaving nucleoli largely composed of granular elements. In these nucleoli, the 6S RNA was almost absent. These data provide evidence that 6S RNA is present in the fibrillar element of the nucleolus and that the 28S RNA is present in granular elements.