Chromatin dynamics: Nucleosome occupancy and sensitivity as determinants of gene expression and cell fate.

The nucleosome, consisting of ~150bp of DNA wrapped around a core histone octamer, is a regulator of nuclear events that contributes to gene expression and cell fate. Nucleosome organization at promoters and their associated remodeling events are important regulators of access to the genome. Occupancy alone, however, is not the only nucleosomal characteristic that plays a role in genome regulation. Nucleosomes at the transcription start sites (TSSs) of genes show differential sensitivity to micrococcal nuclease (MNase) and this differential sensitivity is linked to transcription and regulatory factor binding events. Recently, lymphoblastoid cells treated with heat-killed Salmonella typhimurium were shown to exhibit increased MNase sensitivity specifically at genes implicated in immune responses. Increased sensitivity at the -1-nucleosome permitted transcription factor and RNA Pol II binding events. This system illustrates how cytoplasmic signals induce altered chromatin states to produce a specific cellular response to a stimulus. Innate immune activation is a longstanding model for inducible promoters, transcriptional activation, and differential nucleosomal sensitivity in response to immune activation and offers a model that may be largely applicable to other specific cellular responses including viral infection and cancer. Previous work has shown that early transformation events are associated with prolonged nucleosome occupancy changes that are not observed later in cancer progression. Herein, we propose a model in which we suggest that detailed studies of nucleosomal occupancy and sensitivity in response to specific stimuli will provide insight into the regulation of nuclear events in cancer and other biological processes.