Structure of ribosomal genes of mammalian cells in situ.

The structural and functional organization of ribosomal genes was investigated in situ in human circulating lymphocytes and a human tumour cell line, TG cells, Stereo-pair electron micrographs revealed that this ribosomal chromatin is not structured into nucleosomes, but composed of completely extended filaments, 2-3 nm thick. Despite its homogeneous morphological structure only a small portion of ribosomal chromatin present in the dense fibrillar component is transcriptionally active. This was demonstrated in TG cells by exclusive autoradiographic labelling on serial sections of the dense fibrillar component with 3H-uridine and by the distribution of RNase-gold particles in all the ribonucleoprotein (RNP) structures but not in the fibrillar centres. The extended, non-nucleosomal configuration of both transcriptionally inactive and active ribosomal chromatin could be explained by the peculiar protein composition of this chromatin. Staining with the acrolein-silver-methenamine technique for basic proteins indicated that all the completely extended ribosomal chromatin is devoid of histones, even after inactivation of transcription by actinomycin D. Stereo-electron-microscopical visualisation of the Ag-NOR proteins revealed a thread-like structural organization of these proteins with a spatial distribution superimposable on that of the ribosomal chromatin filaments.