The RNA polymerase I transcription factor UBF and rDNA are located at the same major sites in both interphase and mitotic pig embryonic kidney (PK) cells

Abstract

Indirect immunolabeling with anti-UBF antibodies, in situ hybridization with an rDNA probe, and confocal scanning laser microscopy were used to study nucleolar organizer regions (NORs) during the cell cycle in pig embryonic kidney (PK) cells. The chromosomal distribution of the polymerase I transcription factor UBF and rDNA was compared with the number of silver-stained NORs (Ag-NORs) present and nucleolar size. It was shown, both at interphase and mitosis, that the majority of UBF and rDNA signals were located at the same foci and that the amounts of UBF and rDNA at any given site were in a striking positive correlation. At mitosis, only the NORs were labeled; at interphase, the signals for both UBF and rDNA were arranged in necklace-like structures around the nucleoli. No chromosomal NORs without Ag-proteins or UBF were present, indicating that all NORs in PK cells are active at interphase. It was concluded that (1) UBF and rDNA co-localize throughout the cell cycle in PK cells; (2) their association with mitotic NORs is determined by the number of rDNA repeats, rather than by any differential ability of NORs to recruit the transcription factor; and (3) the amount of UBF can be correlated with the size and activity of the nucleoli at interphase.

Population differences in the expression of nucleolus organizer regions in the grasshopperEyprepocnemis plorans

“…Although secondary NORs in Moroccan specimens were comparatively as active as primary ones, the size of the nucleoli yielded was rather different, the primary NORs yielding large nucleoli and the secondary NORs very small ones. This is consistent with the existence of a relationship between the number of rRNA genes in a NOR and the amount of rRNA synthesized and, consequently, the nucleolus size (De Capoa et al 1988, Shubert and Ktinzel 1990, Zatsepina et al 1996.…”

Nonrandom Distribution of Metaphase AgNOR Staining Patterns on Human Acrocentric Chromosomes

“…Each transcription factor (UBF and others) may bind randomly to ribosomal cistrons in all clusters, or once the first factors are bound the others may bind in a cooperative manner (Moss 1983;Zurita et al 1998;for review see Jacob 1995). A correlation between the size of AgNORs and the amount of the transcription factor UBF has been shown in several studies (Roussel and Hernandez-Verdun 1994;Zatsepina et al 1996), and stick-like and pair NOR structures are also observed by immunostaining for UBF (Héliot et al 1997). RPI is also associated with rDNA in mitotic NORs (Scheer and Rose 1984;Zatsepina et al 1996) and is stalled after the transcription of a few hundred bases (Weisenberger and Scheer 1995).…”

“…A correlation between the size of AgNORs and the amount of the transcription factor UBF has been shown in several studies (Roussel and Hernandez-Verdun 1994;Zatsepina et al 1996), and stick-like and pair NOR structures are also observed by immunostaining for UBF (Héliot et al 1997). RPI is also associated with rDNA in mitotic NORs (Scheer and Rose 1984;Zatsepina et al 1996) and is stalled after the transcription of a few hundred bases (Weisenberger and Scheer 1995). These data suggest that assembled transcription complexes inherited from the previous interphase may determine the pattern of an AgNOR at the beginning of the next cell cycle (Ferraro et al 1981).…”

The role of UBF in regulating the structure and dynamics of transcriptionally active rDNA chromatin

“…44,51 Importantly, the observation that UBF remains associated with the rRNA genes throughout mitosis even when rDNA transcription is shut down provides a mechanistic explanation for why active NORs remain undercondensed during mitosis and have a distinct chromatin structure that is evident as secondary constriction on metaphase chromosomes. [51][52][53]…”