Active Remodeling of Chromatin and Implications for In Vivo Folding

Building on the observation that chromatin compaction can be locally modulated by activity, we propose a model of in vivo chromatin as an active polymer and study its large scale conformations. In particular, we study an active mechanochemical model of chromosomal folding based on the interplay among polymer elasticity, confinement, topological constraints, and fluctuating active stresses arising from the ATP-dependent action of a variety of chromatin-associated protein machines and chromatin-remodeling proteins and their stochastic turnover. We find that activity drives the chromatin to a nonequilibrium steady state; the statistics of conformations in this nonequilibrium steady state are consistent with recent measurements on intrachromosomal contact probabilities and chromosomal compaction. The contact exponents at steady state show a systematic variation with changes in the nature of activity and the rates of turnover. The steady state configuration of the active chromatin in two dimensions resembles a space-filling Peano curve, which might have implications for the optimization of genome information storage.