Direct measurement of local chromatin fluidity using optical trap modulation force spectroscopy

¹ Raman Research Institute
² National Centre for Biological Sciences

^* To whom correspondence should be addressed. E-mail: shiva{at}ncbs.res.in.

Submitted on April 9, 2006
Revised on June 26, 2006
Accepted on 13 September 2006

	Abstract

Chromatin assembly is condensed by histone tail-tail interactions and other nuclear proteins into a highly compact structure. Using an optical trap modulation force spectroscopy, we probe the effect of tail interactions on local chromatin fluidity. Chromatin fibers, purified from mammalian cells are tethered between a microscope cover-slip and a glass micropipette. Mechanical unzipping of tail interactions, using the micropipette, lead to the enhancement of local fluidity. This is measured using an intensity-modulated optically trapped bead, positioned as a force sensor on the chromatin fiber. Enzymatic digestion of the histone tail interactions of tethered chromatin fiber as well leads to a similar increase in fluidity. Our experiments show that an initial increase in the local fluidity precedes chromatin decompaction suggesting possible mechanisms by which chromatin remodeling machines access regulatory sites.

Key Words: Chromatin, Fluidity, Optical tweezer, histone tail-tail interactions, mechanical unfolding, nucleosomes

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