Histone Depletion Facilitates Chromatin Loops on the Kilobasepair Scale

The packing of eukaryotic DNA in the nucleus is decisive for its function; for instance, contact between remote genome sites constitutes a basic feature of gene regulation. Interactions among regulatory proteins, DNA binding, and transcription activation are facilitated by looping of the intervening chromatin. Such long-range interactions depend on the bending flexibility of chromatin, i.e., the ring-closure probability is a directly measurable indicator of polymer flexibility. The applicability of a wormlike chain model to naked DNA has been widely accepted. However, whether this model also suffices to describe the flexibility of eukaryotic interphase chromatin is still a matter of discussion. Here we compare both 5C data from a gene desert and data from fluorescence in situ hybridization with the results of a Monte Carlo simulation of chromatin fibers with and without histone depletion. We then estimate the ring-closure probabilities of simulated fibers with estimates from analytical calculations and show that the wormlike chain model grossly underestimates chromatin flexibility for sharp bends. Most importantly, we find that only fibers with random depletion of linker histones or nucleosomes can explain the probability of random chromatin contacts on small length scales that play an important role in gene regulation. It is possible that missing linker histones and nucleosomes are not just simple, unavoidable, randomly occurring defects, but instead play a regulatory role in gene expression.