Single-molecule force spectroscopy reveals a highly compliant helical folding for the 30-nm chromatin fiber.
The compaction of eukaryotic DNA into chromatin has been implicated in the regulation of all DNA processes. To unravel the higher-order folding of chromatin, we used magnetic tweezers and probed the mechanical properties of single 197-bp repeat length arrays of 25 nucleosomes. At forces up to 4 pN, the 30-nm fiber stretches like a Hookian spring, resulting in a three-fold extension. Together with a high nucleosome-nucleosome stacking energy, this points to a solenoid as the underlying topology of the 30-nm fiber. Unexpectedly, linker histones do not affect the length or stiffness of the fiber but stabilize its folding. Fibers with a nucleosome repeat length of 167 bp are stiffer, consistent with a two-start helical arrangement. The observed high compliance causes extensive thermal breathing, which forms a physical basis for the balance between DNA condensation and accessibility.
Leiden University, The Netherlands.
Maarten
M
Kruithof
Fan-Tso
FT
Chien
Andrew
A
Routh
Colin
C
Logie
Daniela
D
Rhodes
John
J
van Noort
D000818:Animals; D002645:Chickens; D002843:Chromatin / Q000737:chemistry*; D006657:Histones / Q000378:metabolism; D008274:Magnesium; D008958:Models, Molecular; D009690:Nucleic Acid Conformation; D009707:Nucleosomes / Q000737:chemistry; D013057:Spectrum Analysis / Q000379:methods*; D013816:Thermodynamics
D002843:Chromatin; D006657:Histones; D009707:Nucleosomes; D008274:Magnesium