Relative distribution of rDNA and proteins of the RNA polymerase I transcription machinery at chromosomal NORs.

Using confocal and immunofluorescence microscopy the relative distribution of the ribosomal chromatin and some proteins of the RNA polymerase I transcription machinery such as upstream binding factor (UBF), RNA polymerase I and DNA topoisomerase I was analyzed on chromosomal nucleolus organizer regions (NORs) of PtK1 cells. Staining with various DNA fluorochromes revealed that the ribosomal chromatin may be found at the axial region of the NOR and also at lateral expansions around the axis that can also be detected by in situ hybridization. It was observed that the transcription machinery shows a crescent-shaped distribution around the axial ribosomal chromatin at the NOR of metaphase and anaphase chromatids. An ultrastructural analysis of serially sectioned NORs supports this crescent-shape organization. Taking into account previous and present results and the loop/scaffold model of chromosome structure, we propose a model of NOR organization. The model proposes that ribosomal genes that were inactive in the preceding interphase would be present as condensed short Q-loops occupying the axial region of the NOR. Ribosomal genes previously active during interphase would be undercondensed as large R-loops associated with the transcription machinery, which is distributed in a crescent-shaped fashion around the previously active ribosomal DNA.