The diameters of frozen-hydrated chromatin fibers increase with DNA linker length: evidence in support of variable diameter models for chromatin.
The diameters of chromatin fibers from Thyone briareus (sea cucumber) sperm (DNA linker length, n = 87 bp) and Necturus maculosus (mudpuppy) erythrocytes (n = 48 bp) were investigated. Soluble fibers were frozen into vitrified aqueous solutions of physiological ionic strength (124 mM), imaged by cryo-EM, and measured interactively using quantitative computer image-processing techniques. Frozen-hydrated Thyone and Necturus fibers had significantly different mean diameters of 43.5 nm (SD = 4.2 nm; SEM = 0.61 nm) and 32.0 nm (SD = 3.0 nm; SEM = 0.36 nm), respectively. Evaluation of previously published EM data shows that the diameters of chromatin from a large number of sources are proportional to linker length. In addition, the inherent variability in fiber diameter suggests a relationship between fiber structure and the heterogeneity of linker length. The cryo-EM data were in quantitative agreement with space-filling double-helical crossed-linker models of Thyone and Necturus chromatin. The data, however, do not support solenoid or twisted-ribbon models for chromatin that specify a constant 30 nm diameter. To reconcile the concept of solenoidal packing with the data, we propose a variable-diameter solid-solenoid model with a fiber diameter that increases with linker length. In principle, each of the variable diameter models for chromatin can be reconciled with local variations in linker length.
Athey
B D
BD
Biophysics Research Division, University of Michigan, Ann Arbor 48109-2099.
Smith
M F
MF
Rankert
D A
DA
Williams
S P
SP
Langmore
J P
JP
D000818:Animals; D002843:Chromatin / Q000648:ultrastructure*; D003198:Computer Simulation; D004247:DNA / Q000648:ultrastructure; D004447:Echinodermata / Q000235:genetics*; D004912:Erythrocytes / Q000648:ultrastructure; D005615:Freezing; D007091:Image Processing, Computer-Assisted; D008297:Male; D008854:Microscopy, Electron; D008958:Models, Molecular; D009337:Necturus / Q000235:genetics*; D009338:Necturus maculosus / Q000235:genetics*; D009707:Nucleosomes / Q000648:ultrastructure; D012616:Sea Cucumbers / Q000235:genetics*; D013094:Spermatozoa / Q000648:ultrastructure
D002843:Chromatin; D009707:Nucleosomes; D004247:DNA