Evidence for short-range helical order in the 30-nm chromatin fibers of erythrocyte nuclei

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 108; no. 41; pp. 16992 - 16997
• Main Authors: Scheffer, Margot P, Eltsov, Mikhail, Frangakis, Achilleas S
• Format: Journal Article
• Language: English
• Published: United States National Academy of Sciences 11.10.2011; National Acad Sciences
• Subjects: Animals; Biological Sciences; Biophysical Phenomena; Cell Nucleus - chemistry; Cell Nucleus - ultrastructure; Cells; Chickens; Chromatin; Chromatin - chemistry; Chromatin - ultrastructure; Cryoelectron Microscopy; Deoxyribonucleic acid; DNA; DNA biosynthesis; DNA repair; DNA replication; Electron Microscope Tomography; Erythrocytes; Erythrocytes - chemistry; Erythrocytes - ultrastructure; Eukaryotes; Fibers; genome; Genomes; Gyres; Handedness; Heterochromatin; Histones; Imaging, Three-Dimensional; Models, Molecular; Molecular Conformation; Nuclei; Nucleosomes; Nucleosomes - chemistry; Nucleosomes - ultrastructure; Packing; Replication; Rotation; Superhelical DNA; Symmetry; Tomography; Transcription; transcription (genetics); Translation
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.1108268108

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.