Quantifying the effect of magnesium ions on nucleosome stability in chromatin fibers

Chromatin structure governs DNA accessibility and thereby a wide range of DNA-mediated processes. Magnesium ions are known to influence chromatin structure across organizational scales, but a thorough quantification of the effect on nucleosomes in the context of individual chromatin fibers is currently lacking. Using single-molecule force spectroscopy, we determined how Mg²⁺ affects nucleosome stability in reconstituted chromatin fibers by assessing two force regimes that represent distinct phases of force-induced fiber unfolding. We found that Mg²⁺ destabilizes the nucleosome inner turn in the extended conformation of the chromatin fiber. In contrast, Mg²⁺ stabilizes compacted chromatin fibers in the low-force regime, indicating stabilization of the nucleosome outer turns. Taken together, these results reveal that Mg²⁺ has differential effects on the inner and outer nucleosomal turn.