Quantitative analysis of single-molecule force spectroscopy on folded chromatin fibers.
Bottom Line: Comparison of force-extension curves between single nucleosomes and chromatin fibers shows that embedding nucleosomes in a fiber stabilizes the nucleosome by 10 kBT.Chromatin fibers with 20- and 50-bp linker DNA follow a different unfolding pathway.These results have implications for accessibility of DNA in fully folded and partially unwrapped chromatin fibers and are vital for understanding force unfolding experiments on nucleosome arrays.
Affiliation: Biological and Soft Matter Physics, Huygens-Kamerlingh Onnes Laboratory, Leiden University, Leiden, The Netherlands.Show MeSH
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Mentions: Whereas all chromatin fibers feature similar unfolding characteristics, we observed a rather large variation in the force-extension behavior between fibers. Figure 4 shows the force-extension curves of 10 different fibers. Because the last unfolding transition is not in thermodynamic equilibrium, the rupture forces for this transition are distributed stochastically. Nevertheless, all curves align well with the set of unfolding states that contain extended and fully unwrapped nucleosomes, indicated with the gray dashed lines. The first two transitions at forces below 7 pN on the other hand are fully reversible, resulting in overlapping pull and release curves (Figure 1A, inset). Despite the highly reproducible curves that we obtain from individual fibers, we observe large variations in extension between fibers in this low-force range. We attribute these differences to the variations in the composition of the fiber.
Affiliation: Biological and Soft Matter Physics, Huygens-Kamerlingh Onnes Laboratory, Leiden University, Leiden, The Netherlands.