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In eukaryotes, the nucleosome serves as the primary unit of DNA packaging, with the negatively charged
DNA wrapped around a positively charged histone core. Nucleosomes undergo site exposure to recruit
DNA-binding factors to naked DNA, enabling biological processes. Previous studies had reported the
exchange of a less stable histone dimer variant inside an NCP with a freely diffusing stable dimer variant
in vitro. In this presentation, I will discuss the fate of site exposure resulting in spontaneous histone dimer
exchange between nucleosomes. We investigated spontaneous histone dimer exchange within nucleosomes
using three-color single-molecule FRET (smFRET). Our findings reveal that histone exchange occurs on a
timescale of tens of seconds at physiological nucleosome concentrations, with faster exchange rates
observed at higher monovalent salt concentrations, in the presence of acetylated histone H3K56ac, or when
assisted by histone chaperone Nap1. Exchange rates were unaffected by higher temperatures but decreased
with DNA CpG methylation, suggesting that exchange kinetics are influenced by alterations in the site
exposure dynamics. Histone diffusion in a compact chromatin context can modulate local concentrations
of post-translational histone modifications and histone variants. We also explored the conformational
dynamics of the disordered H1 C-terminal domain (CTD), which stabilizes higher-order chromatin
structures. smFRET analysis showed dynamic structural fluctuations in about half of the H1-CTDs, while
crosslinking identified two CTD regions involved in organizing linker DNA. Histone H3 acetylation
induced H1-CTD decondensation, highlighting the role of epigenetic modifications in chromatin
condensation and accessibility. |