BPS2026 – Histone-histone vs. histone-DNA interactions within the nucleosome probed at the single-molecule level

Typical single-molecule experiments probe the stability of nucleosomes by disrupting DNA-histone contacts, unwinding DNA from the octamer. Yet, the fate of the disrupted histones is often unclear in these experiments. Here, we utilize a combination of optical tweezers, atomic force microscopy, and confocal imaging to probe both nucleosome stability and histone dynamics under varying solution conditions. We find that nucleosome stability decreases with increasing monovalent salt, as expected. Specifically, we see increased spontaneous unwinding of DNA and a decrease in the free-energy change associated with the release of the central core of DNA. Increasing divalent salt concentrations cause weaker effects on stability than expected, though the energy landscape is affected. Multicolor confocal imaging reveals that once octamers are unwound, they remain attached at the central dyad and are then released from DNA. Increasing salt accelerates this rate. Furthermore, if tension is released, DNA may rewind around the octamer, though high salt inhibits this process. However, low salt appears to reduce the stability of the octamer. Thus, solution conditions alter the relative strengths of histone-histone and histone-DNA interactions, influencing histone mobility and affinity.