RELATIVE DISTRIBUTION OF RDNA AND PROTEINS OF THE RNA-POLYMERASE-I TRANSCRIPTION MACHINERY AT CHROMOSOMAL NORS

Using confocal and immunofluorescence microscopy the relative distribu tion of the ribosomal chromatin and some proteins of the RNA polymeras e I transcription machinery such as upstream binding factor (UBF), RNA polymerase I and DNA topoisomerase I was analyzed on chromosomal nucl eolus organizer regions (NORs) of PtK1 cells. Staining with various DN A 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 observ ed that the transcription machinery shows a crescent-shaped distributi on around the axial ribosomal chromatin at the NOR of metaphase and an aphase chromatids. An ultrastructural analysis of serially sectioned N ORs supports this crescent-shape organization. Taking into account pre vious and present results and the loop/scaffold model of chromosome st ructure, we propose a model of NOR organization. The model proposes th at ribosomal genes that were inactive in the preceding interphase woul d 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 mach inery, which is distributed in a crescent-shaped fashion around the pr eviously active ribosomal DNA.