Electron tomography of metaphase nucleolar organizer regions: evidence for a twisted-loop organization.
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.
Heliot
L
L
Unité 314 Institut National de la Santé et de la Recherche Médicale, Laboratoire Pol Bouin, Reims, France.
Kaplan
H
H
Lucas
L
L
Klein
C
C
Beorchia
A
A
Doco-Fenzy
M
M
Menager
M
M
Thiry
M
M
O'Donohue
M F
MF
Ploton
D
D
D000818:Animals; D002286:Carcinoma, Ehrlich Tumor; D002843:Chromatin / Q000737:chemistry / Q000648:ultrastructure; D002868:Chromosomal Proteins, Non-Histone / Q000032:analysis; D004275:DNA, Ribosomal / Q000032:analysis; D006801:Humans; D007091:Image Processing, Computer-Assisted; D007624:KB Cells; D004915:Leukemia, Erythroblastic, Acute; D008677:Metaphase; D051379:Mice; D018613:Microscopy, Confocal; D017348:Microscopy, Electron, Scanning Transmission; D008958:Models, Molecular; D009690:Nucleic Acid Conformation; D009697:Nucleolus Organizer Region / Q000737:chemistry* / Q000648:ultrastructure*; D011487:Protein Conformation; D014407:Tumor Cells, Cultured
D002843:Chromatin; D002868:Chromosomal Proteins, Non-Histone; D004275:DNA, Ribosomal