Histone H3 and H4 N-terminal tails in nucleosome arrays at cellular concentrations probed by magic angle spinning NMR spectroscopy.

Chromatin is a supramolecular assembly of DNA and histone proteins, organized into nucleosome repeat units. The dynamics of chromatin organization regulates DNA accessibility to eukaryotic transcription and DNA repair complexes. Yet, the structural and dynamic properties of chromatin at high concentrations characteristic of the cellular environment (>∼200 mg/mL) are largely unexplored at the molecular level. Here, we apply MAS NMR to directly probe the dynamic histone protein regions in (13)C,(15)N-enriched recombinant nucleosome arrays at cellular chromatin concentrations and conditions designed to emulate distinct states of DNA condensation, with focus on the flexible H3 and H4 N-terminal tails which mediate chromatin compaction. 2D (1)H-(13)C and (1)H-(15)N spectra reveal numerous correlations for H3 and H4 backbone and side-chain atoms, enabling identification of specific residues making up the dynamically disordered N-terminal tail domains. Remarkably, we find that both the H3 and H4 N-terminal tails are overall dynamic even in a highly condensed state. This significant conformational flexibility of the histone tails suggests that they remain available for protein binding in compact chromatin states to enable regulation of heterochromatin. Furthermore, our study provides a foundation for quantitative structural and dynamic investigations of chromatin at physiological concentrations.