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Originally published as Biophys J. BioFAST on February 4, 2005.
doi:10.1529/biophysj.104.053074
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Biophysical Journal 88:3572-3583 (2005)
© 2005 The Biophysical Society

Single Chromatin Fiber Stretching Reveals Physically Distinct Populations of Disassembly Events

L. H. Pope *, M. L. Bennink *, K. A. van Leijenhorst-Groener *, D. Nikova *, J. Greve * and J. F. Marko {dagger}

* Biophysical Techniques, Department of Science and Technology and MESA+ Institute for Nanotechnology, University of Twente, 7500 AE Enschede, The Netherlands; and {dagger} University of Illinois at Chicago, Department of Physics, Chicago, Illinois 60607-7059 USA

Correspondence: Address reprint requests to J. F. Marko, University of Illinois at Chicago, Dept. of Physics, 845 W. Taylor St., Chicago, IL 60607-7059. Tel.: 312-996-3416; Fax: 312-996-9016; E-mail jmarko{at}uic.edu.

Eukaryotic DNA is packaged into the cell nucleus as a nucleoprotein complex, chromatin. Despite this condensed state, access to the DNA sequence must occur during gene expression and other essential genetic events. Here we employ optical tweezers stretching of reconstituted chromatin fibers to investigate the release of DNA from its protein-bound structure. Analysis of fiber length increase per unbinding event revealed discrete values of ~30 and ~60 nm. Furthermore, a loading rate analysis of the disruption forces revealed three individual energy barriers. The heights of these barriers were found to be ~20 kBT, ~25 kBT, and ~28 kBT. For subsequent stretches of the fiber it was found that events corresponding to the ~28 kBT energy barrier were significantly reduced. No correlation between energy barrier crossed and DNA length release was found. These studies clearly demonstrate that optical tweezers stretching of chromatin provides insight into the energetic penalties imposed by chromatin structure. Furthermore these studies reveal possible pathways via which chromatin may be disrupted during genetic code access.




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