Immunocytochemical localization of RNA polymerase I in the fibrillar centers of nucleoli

Abstract

For an understanding of the functional organization of the nucleolus identification of the site of the expression of the rRNA genes is of considerable importance. Although various stages of preribosome formation have been mapped to morphologically distinct components of the nucleolus (Hadjiolov, 1985), the intranucleolar location of transcriptionally active rRNA genes has remained uncertain. From high resolution autoradiographic studies, following pulse labelling of cells with radioactive RNA precursors, it was suggested that transcription of rRNA genes takes place in the “dense fibrillar component”, and possibly also at the periphery of the “fibrillar centers” (Fakan, 1978; Thiry et al., 1985). However, using other techniques such as autoradiography with tritiated thymidine or actinomycin D, in situ-hybridization with radioactive rRNA probes and selective DNA-staining with the osmium-amine Feulgen-like reaction, DNA (including rDNA) could be detected only in the fibrillar centers (for reviews see Hernandez-Verdun, 1983; Goessens, 1984). To reconcile these seemingly contrasting results it was proposed that the fibrillar centers contain inactive rDNA in a relatively high packing density, thereby facilitating its detection, whereas the transcribed rRNA genes loop out into the surrounding dense fibrillar component (Stahl, 1982; Goessens, 1984). According to this concept the dense fibrillar component is formed by the superposition of rRNA-transcription units and newly formed rRNP material.