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: Comparison between force-extension curves of (A) a chromatin fiber and (B) a mononucleosome. Dark circles represent the pulling trace, light gray circles represent the release trace. All force-extension measurements are reversible, but a significant hysteresis is observed when the the force exceeds 6 pN. The inset in (A) shows a force-extension experiment in which the force was limited to 6 pN; no hysteresis is observed. Light gray dashed lines represent WLC descriptions of the bare DNA and the state in which all nucleosomes are in the extended conformation (see Figure 2). A third dashed line in (B) represents a WLC with a contour length 147 bp shorter than the bare DNA. Black lines are fits to Equation (8) yielding for (A) nfiber = 13, nunfolded = 4, k = 0.28 pN/nm, ext = 4.6 nm, ΔG1 = 20.6 kBT and ΔG2 = 5.5 kBT. For (B): ext = 6.5 nm, ΔG1 = 8.8 kBT and ΔG2 = 3.5 kBT.
Affiliation: Biological and Soft Matter Physics, Huygens-Kamerlingh Onnes Laboratory, Leiden University, Leiden, The Netherlands.