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 biogenesis. For the mapping of ribosomal (r)DNA transcription loci, we combined non-autoradiographic in situ transcription assays with the immunological analysis of the ultrastructural distribution of transcription-associated proteins, i.e., polymerase I, the human polymerase I-specific upstream binding factor, and topoisomerase I. Furthermore, we visualized the nascent transcripts simultaneously with the rDNA in the nucleoli. All tested transcription proteins were found in both the fibrillar center and the dense fibrillar component (DF) of nucleoli in human cells. In the DF the nascent transcripts, detected by bromouridine incorporation, were found colocalized with the transcription proteins only within circumscribed regions. We did not observe colocalization of rDNA with nascent transcripts within the fibrillar centers, which corroborates the view that transcription proteins in this component are rather inactive. Our results suggests that only a minor portion of the DF is involved in transcriptional activity. Transcription appears to be confined 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 functions which are localized in subcompartments of the DF.