Characterization of nucleosome unwrapping within chromatin fibers using magnetic tweezers

Nucleosomal arrays fold into chromatin fibers and the higher-order folding of chromatin plays a strong regulatory role in all processes involving DNA access, such as transcription and replication. A fundamental understanding of such regulation requires insight into the folding properties of the chromatin fiber in molecular detail. Despite this, the structure and the mechanics of chromatin fibers remain highly disputed. Single-molecule force spectroscopy experiments have the potential to provide such insight, but interpretation of the data has been hampered by the large variations in experimental force-extension traces. Here we explore the possibility that chromatin fibers are composed of both single-turn and fully wrapped histone octamers. By characterizing the force-dependent behavior of in vitro reconstituted chromatin fibers and reanalyzing existing data, we show the unwrapping of the outer turn of nucleosomal DNA at 3 pN. We present a model composed of two freely-jointed chains, which reveals that nucleosomes within the chromatin fiber show identical force-extension behavior to mononucleosomes, indicating that nucleosome-nucleosome interactions are orders-of-magnitude smaller than previously reported and therefore can be overcome by thermal fluctuations. We demonstrate that lowering the salt concentration externally increases the wrapping energy significantly, indicative of the electrostatic interaction between the wrapped DNA and the histone octamer surface. We propose that the weak interaction between nucleosomes could allow easy access to nucleosomal DNA, while DNA unwrapping from the histone core could provide a stable yet dynamic structure during DNA maintenance.