RIBOSOMAL GENE-TRANSCRIPTION IS ORGANIZED IN FOCI WITHIN NUCLEOLAR COMPONENTS

We investigated how only three morphologically distinguished nucleolar components can integrate the many necessary tasks in ribosome biogene sis. For the mapping of ribosomal (r)DNA transcription loci, we combin ed non-autoradiographic in situ transcription assays with the immunolo gical analysis of the ultrastructural distribution of transcription-as sociated proteins, i.e., polymerase I, the human polymerase I-specific upstream binding factor, and topoisomerase I. Furthermore, we visuali zed the nascent transcripts simultaneously with the rDNA in the nucleo li. All tested transcription proteins were found in both the fibrillar center and the dense fibrillar component (DF) of nucleoli in human ce lls. In the DF the nascent transcripts, detected by bromouridine incor poration, were found colocalized with the transcription proteins only within circumscribed regions. We did not observe colocalization of rDN A with nascent transcripts within the fibrillar centers, which corrobo rates the view that transcription proteins in this component are rathe r inactive. Our results suggests that only a minor portion of the DF i s involved in transcriptional activity. Transcription appears to be co nfined to small foci, which exist close to or associated with the DF. Our results are in favor of the view that the DF has different functio ns which are localized in subcompartments of the DF.