title: Evidence for short-range helical order in the 30-nm chromatin fibers of erythrocyte nuclei.
abstract: Chromatin folding in eukaryotes fits the genome into the limited volume of the cell nucleus. Formation of higher-order chromatin structures attenuates DNA accessibility, thus contributing to the control of essential genome functions such as transcription, DNA replication, and repair. The 30-nm fiber is thought to be the first hierarchical level of chromatin folding, but the nucleosome arrangement in the compact 30-nm fiber was previously unknown. We used cryoelectron tomography of vitreous sections to determine the structure of the compact, native 30-nm fiber of avian erythrocyte nuclei. The predominant geometry of the 30-nm fiber revealed by subtomogram averaging is a left-handed two-start helix with approximately 6.5 nucleosomes per 11 nm, in which the nucleosomes are juxtaposed face-to-face but are shifted off their superhelical axes with an axial translation of approximately 3.4 nm and an azimuthal rotation of approximately 54°. The nucleosomes produce a checkerboard pattern when observed in the direction perpendicular to the fiber axis but are not interdigitated. The nucleosome packing within the fibers shows larger center-to-center internucleosomal distances than previously anticipated, thus excluding the possibility of core-to-core interactions, explaining how transcription and regulation factors can access nucleosomes.
authors.affiliation: European Molecular Biology Laboratory, Meyerhofstr 1, 69117 Heidelberg, Germany.
authors.forename: Margot P
authors.initials: MP
authors.lastname: Scheffer
authors.forename: Mikhail
authors.initials: M
authors.lastname: Eltsov
authors.forename: Achilleas S
authors.initials: AS
authors.lastname: Frangakis
mesh_terms: D000818:Animals; D055592:Biophysical Phenomena; D002467:Cell Nucleus / Q000737:chemistry / Q000648:ultrastructure; D002645:Chickens; D002843:Chromatin / Q000737:chemistry* / Q000648:ultrastructure; D020285:Cryoelectron Microscopy; D055032:Electron Microscope Tomography; D004912:Erythrocytes / Q000737:chemistry* / Q000648:ultrastructure; D021621:Imaging, Three-Dimensional; D008958:Models, Molecular; D008968:Molecular Conformation; D009707:Nucleosomes / Q000737:chemistry / Q000648:ultrastructure
chemical_list: D002843:Chromatin; D009707:Nucleosomes