Electron Tomography of Metaphase Nucleolar Organizer Regions: Evidence for a Twisted-Loop Organization

Author:

Heliot Laurent1,Kaplan Hervé1,Lucas Laurent1,Klein Christophe1,Beorchia Adrien1,Doco-Fenzy Martine1,Menager Monique1,Thiry Marc2,O’Donohue Marie-Françoise1,Ploton Dominique1

Affiliation:

1. Unité 314 Institut National de la Santé et de la Recherche Médicale, Laboratoire Pol Bouin, and Institut Féderatif de Recherche 53, Centre Hospitalier Regional Maison Blanche, 51092 Reims Cedex, France; and

2. Laboratoire de Biologie Cellulaire, Institut Pitteurs, Liege, Belgique

Abstract

Metaphase nucleolar organizer regions (NORs), one of four types of chromosome bands, are located on human acrocentric chromosomes. They contain 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 proteins were used to reveal r-chromatin in situ and to investigate its spatial organization within NORs by confocal microscopy and by electron tomography. For each labeling, confocal microscopy revealed small and large double-spotted NORs and crescent-shaped NORs. Their internal three-dimensional (3D) organization was studied by using electron tomography on specifically silver-stained NORs. The 3D reconstructions allow 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 turn or two or three turns of a helix within small and large double-spotted NORs, respectively. Within crescent-shaped NORs, virtual slices reveal that the fiber constitutes several longitudinally twisted loops, grouped as two helical 250- to 300-nm coils, each centered on a nonargyrophilic axis of condensed chromatin. We propose a model of the 3D organization of r-chromatin within elongated NORs, in which loops are twisted and bent to constitute one basic chromatid coil.

Publisher

American Society for Cell Biology (ASCB)

Subject

Cell Biology,Molecular Biology

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