Mechanical Stability of Mononucleosome Revealed by Optical Torque Wrench

The fundamental chromatin packing unit in eukaryotes is the nucleosome, where ∼147 base pairs of DNA are wrapped in ∼1.7 turns around a core histone octamer. A crucial question in biology is to explain how proteins are able to access DNA which is tightly bound in chromatin. For example, RNA polymerase must navigate through the nucleosome while transcribing DNA. Hence, the DNA-histone interactions play a key role in gene regulation. Single-molecule force spectroscopy is a powerful tool to probe this system. Prior studies have exerted linear tension to stretch both chromatin fibers and mononucleosome molecules, which have given information on the nature of the free-energy barrier for a particular disruption pathway. We develop a theoretical model including torsional constraints, which suggests that the disruption pathway may be strongly sensitive to the torsional loading of the nucleosome. This is of interest because helicases, polymerases, or other motor proteins may use a combination of force and torque to disrupt chromatin. Experimentally we apply force and torque simultaneously to disrupt a mononucleosome structure using an optical torque wrench. Positive supercoiling is found to destabilize the nucleosome while negative supercoiling has little effect, which is consistent with our model. By determining the influence of supercoiling density on the disruption barrier we obtain more detailed information about DNA-protein interaction strength in nucleosomes.