ELECTRON TOMOGRAPHY OF METAPHASE NUCLEOLAR ORGANIZER REGIONS - EVIDENCE FOR A TWISTED-LOOP ORGANIZATION

Metaphase nucleolar organizer regions (NORs), one of four types of chr omosome bands, are located on human acrocentric chromosomes. They cont ain r-chromatin, i.e., ribosomal genes complexed with proteins such as upstream binding factor and RNA polymerase I, which are argyrophilic NOR proteins. Immunocytochemical and cytochemical labelings of these p roteins were used to reveal r-chromatin in situ and to investigate its spatial organization within NORs by confocal microscopy and by electr on tomography. For each labeling, confocal microscopy revealed small a nd large double-spotted NORs and crescent-shaped NORs. Their internal three-dimensional (3D) organization was studied by using electron tomo graphy on specifically silver-stained NORs. The 3D reconstructions all ow us to conclude that the argyrophilic NOR proteins are grouped as a fiber of 60-80 nm in diameter that constitutes either one part of a tu rn or two or three turns of a helix within small and large double-spot ted NORs, respectively. Within crescent-shaped NORs, virtual slices re veal that the fiber constitutes several longitudinally twisted loops, grouped as two helical 250- to 300-nm coils, each centered on a nonarg yrophilic axis of condensed chromatin. We propose a model of the 3D or ganization of r-chromatin within elongated NORs, in which loops are tw isted and bent to constitute one basic chromatid coil.