DOES THE SYNTHESIS OF RIBOSOMAL-RNA TAKE PLACE WITHIN NUCLEOLAR CENTERS OR DENSE FIBRILLAR COMPONENTS - A CRITICAL-APPRAISAL

The localization of transcribing rRNA genes within nucleoli of mammali an cells, although intensively studied, has not been established. Most published papers on this topic situate transcribing ribosomal genes e ither to nucleolar fibrillar centers or to nucleolar dense fibrillar c omponents. To clarify this point, we have generated the electron micro scopic affinity cytochemistry picture of the nucleolus of cultured mam malian cells. Three kinds of affinity probes have been used: (1) probe s to nucleolar chromatin, including rDNA sequences; (2) probes to a nu mber of macromolecules (such as RNA polymerase I) which are directly, or indirectly, involved in the synthesis and processing of rRNA and fo rmation of preribosomes; (3) antibodies to bromouridine for a recently standardized nonisotopical method depicting incorporated bromouridine within RNA. The results suggest the localization of transcription sit es not only to dense fibrillar components but also to the border regio n between these components and fibrillar centers. Our data support a h ypothesis that in metabolically active mammalian nucleoli, fibrillar c enters and dense fibrillar components form a single functional domain for the transcription of rRNA genes, with nascent transcripts generati ng ''automatically'' dense fibrillar components. Through the active pr ocess of transcription, individual rRNA genes thus become engulfed wit hin dense fibrillar components. (C) 1995 Academic Press, Inc.