THE RNA-POLYMERASE-I TRANSCRIPTION FACTOR UBF AND RDNA ARE LOCATED ATTHE SAME MAJOR SITES IN BOTH INTERPHASE AND MITOTIC PIG EMBRYONIC KIDNEY (PK) CELLS

Indirect immunolabeling with anti-UBF antibodies, in situ hybridizatio n 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 p olymerase I transcription factor UBF and rDNA was compared with the nu mber of silver-stained NORs (Ag-NORs) present and nucleolar size. It w as 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 UB F 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 th e nucleoli. No chromosomal NORs without Ag-proteins or UBF were presen t, indicating that all NORs in PK cells are active at interphase. It w as concluded that (1) UBF and rDNA co-localize throughout the cell cyc le in PIC cells; (2) their association with mitotic NORs is determined by the number of rDNA repeats, rather than by any differential abilit y of NORs to recruit the transcription factor; and (3) the amount of U BF can be correlated with the size and activity of the nucleoli at int erphase.