The three-dimensional architecture of chromatin in situ: electron tomography reveals fibers composed of a continuously variable zig-zag nucleosomal ribbon.
The three dimensional (3D) structure of chromatin fibers in sections of nuclei has been determined using electron tomography. Low temperature embedding and nucleic acid-specific staining allowed individual nucleosomes to be clearly seen, and the tomographic data collection parameters provided a reconstruction resolution of 2.5 nm. Chromatin fibers have complex 3D trajectories, with smoothly bending regions interspersed with abrupt changes in direction, and U turns. Nucleosomes are located predominantly at the fiber periphery, and linker DNA tends to project toward the fiber interior. Within the fibers, a unifying structural motif is a two nucleosome-wide ribbon that is variably bent and twisted, and in which there is little face-to-face contact between nucleosomes. It is suggested that this asymmetric 3D zig-zag of nucleosomes and linker DNA represents a basic principle of chromatin folding that is determined by the properties of the nucleosome-linker unit. This concept of chromatin fiber architecture is contrasted with helical models in which specific nucleosome-nucleosome contacts play a major role in generating a symmetrical higher order structure. The transcriptional control implications of a more open and irregular chromatin structure are discussed.
Horowitz
R A
RA
Department of Biology, University of Massachusetts, Amherst 01003.
Agard
D A
DA
Sedat
J W
JW
Woodcock
C L
CL
D000818:Animals; D002467:Cell Nucleus / Q000648:ultrastructure*; D002645:Chickens; D002843:Chromatin / Q000648:ultrastructure*; D004912:Erythrocytes / Q000648:ultrastructure; D007091:Image Processing, Computer-Assisted; D008297:Male; D008854:Microscopy, Electron; D009707:Nucleosomes / Q000648:ultrastructure*; D013077:Sperm Head / Q000648:ultrastructure; D013215:Starfish
D002843:Chromatin; D009707:Nucleosomes