Mechanism and Function of Chromatin Positional Dynamics

Abstract

Recent studies of the spatial organization of chromatin in interphase showed with 1Mb resolution that DNA is not as intertwined as expected but consists of knot-free globules [1]. Chromatin is territorialized with respect to the chromosomes, but also with respect to the expressed and silenced genomic regions. The globular organization facilitates the rapid folding and unfolding of local territories allowing a gene to be accessible for expression at any time [2].In the present work we studied the dynamics of chromatin territories in interphase by measuring positional fluctuations of GFP-labeled histones H2B in mammalian cells (HeLa) using fluorescence intensity fluctuations spectroscopy. Spatio-temporal fluctuation maps of chromatin dynamics in single nuclei were determined by cross-correlation spectroscopy (STICS, [3]). We show that small territories move independently from each other confirming the blob-like structure observed in the static measurement [1].In a second set of experiments, we studied the dynamics of chromatin on a single filament level by single particle tracking of GFP-tagged specific positions on chromosomal DNA (such as telomeres, centromeres). We hypothesize that chromatin is a kind of “active material” that combines passive (thermally driven) and active (motor-driven, ATP-dissipating) dynamics. We also hypothesize that chromatin is composed of blobs which move independently. The mean square displacement of locally labeled chromosomal DNA loci obeys a power law ∼ t3/4 as expected for semiflexible macromolecules. A.Z. is Damon Runyon Fellow supported by Damon Runyon Cancer Research Foundation (DRG 2040-10).[1] E. Liebermann-Aiden et al., 2009, Science, 326: 289.[2] L. Mirny, 2011, Chromosome Res. 19:37.[3] B. Hebert et al., 2005, Biophys. J., 88: 3601.