Spontaneous access to DNA target sites in folded chromatin fibers

DNA wrapped in nucleosomes is sterically occluded from many protein complexes that must act on it; how such complexes gain access to nucleosomal DNA is not known. In vitro studies on isolated nucleosomes show that they undergo spontaneous partial unwrapping conformational transitions, which make the wrapped nucleosomal DNA transiently accessible. Thus, site exposure might provide a general mechanism allowing access of protein complexes to nucleosomal DNA. However, existing quantitative analyses of site exposure focused on single nucleosomes, while the presence of neighbor nucleosomes and concomitant chromatin folding might significantly influence site exposure. In this work, we carried out quantitative studies on the accessibility of nucleosomal DNA in homogeneous nucleosome arrays. Two striking findings emerged. Organization into chromatin fibers changes the accessibility of nucleosomal DNA only modestly, from approximately 3-fold decreases to approximately 8-fold increases in accessibility. This means that nucleosome arrays are intrinsically dynamic and accessible even when they are visibly condensed. In contrast, chromatin folding decreases the accessibility of linker DNA by as much as approximately 50-fold. Thus, nucleosome positioning dramatically influences the accessibility of target sites located inside nucleosomes, while chromatin folding dramatically regulates access to target sites in linker DNA.