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Direct Measurement of Local Chromatin Fluidity Using Optical Trap Modulation Force Spectroscopy

T. Roopa ^* and G. V. Shivashankar ^* 

^* Raman Research Institute, Bangalore, India; and National Centre for Biological Sciences, TIFR, Bangalore, India

Correspondence: Address reprint requests to G. V. Shivashankar, E-mail: shiva{at}ncbs.res.in.

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 coverslip 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 also 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.

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