The histone variant CENP-A is the epigenetic determinant for the centromere, where it is interspersed with canonical H3 to form a specialized chromatin structure that nucleates the kinetochore. How nucleosomes at the centromere arrange into higher order structures is unknown. Here we demonstrate that the human CENP-A-interacting protein CENP-N promotes the stacking of CENP-A-containing mononucleosomes and nucleosomal arrays through a previously undefined interaction between the α6 helix of CENP-N with the DNA of a neighboring nucleosome. We describe the cryo-EM structures and biophysical characterization of such CENP-N-mediated nucleosome stacks and nucleosomal arrays and demonstrate that this interaction is responsible for the formation of densely packed chromatin at the centromere in the cell. Our results provide first evidence that CENP-A, together with CENP-N, promotes specific chromatin higher order structure at the centromere.

Linker histones are epigenetic regulators that bind to nucleosomes and alter chromatin structures and dynamics. Biophysical studies have revealed two binding modes in the linker histone/nucleosome complex, the chromatosome, where the linker histone is either centered on or askew from the dyad axis. Each has been posited to have distinct effects on chromatin, however the molecular and thermodynamic mechanisms that drive them and their dependence on linker histone compositions remain poorly understood. We present molecular dynamics simulations of chromatosomes with the globular domain of two linker histone variants, generic H1 (genGH1) and H1.0 (GH1.0), to determine how their differences influence chromatosome structures, energetics and dynamics. Results show that both unbound linker histones adopt a single compact conformation. Upon binding, DNA flexibility is reduced, resulting in increased chromatosome compaction. While both variants enthalpically favor on-dyad binding, energetic benefits are significantly higher for GH1.0, suggesting that GH1.0 is more capable than genGH1 of overcoming the large entropic reduction required for on-dyad binding which helps rationalize experiments that have consistently demonstrated GH1.0 in on-dyad states but that show genGH1 in both locations. These simulations highlight the thermodynamic basis for different linker histone binding motifs, and details their physical and chemical effects on chromatosomes.

Linker histones are epigentic regulators that bind to nucleosomes and alter chromatin structures and dynamics. Biophysical studies have revealed two binding modes in the linker histone/nucleosome complex, the chromatosome, where the linker histone is either centered on or askew from the dyad axis. Each has been posited to have distinct effects on chromatin, however the molecular and thermodynamic mechanisms that drive them and their dependence on linker histone compositions remain poorly understood. We present molecular dynamics simulations of chromatosomes with two linker histone isoforms, globular H1 (GH1) and H5 (GH5), to determine how their differences influence chromatosome structures, energetics, and dynamics.Results show that both linker histones adopt a single compact conformation in solution. Upon binding, DNA flexibility is reduced and there is increased chromatosome compaction. While both isoforms favor on-dyad binding, the enthalpic benefit is significantly higher for GH5. This suggests that GH5 is more capable of overcoming the large entropic reduction required for on-dyad binding than GH1, which helps rationalize experiments that have consistently demonstrated GH5 in on-dyad states but that show GH1 in both locations.These simulations highlights the thermodynamic basis for different linker histone binding motifs, and details their physical and chemical effects on chromatosomes.

Purpose To evaluate the efficacy of vitrectomy with vancomycin for the treatment of experimental Bacillus cereus endophthalmitis. Methods Endophthalmitis was initiated in rabbits via intravitreal injection of 100 CFU B. cereus. Treatment groups included included 25-gauge transconjunctival sutureless vitrectomy with intravitreal vancomycin (1 mg) or vancomycin alone. Groups were treated at 4 h, 5 h, or 6 h postinfection. At 48 h (for 4 h and 5 h groups) or 36 h (for the 6 h group) postinfection, eyes were analyzed by electroretinography, histology, and inflammatory cell counts. Results Treatment with vitrectomy/vancomycin at 4 h resulted in significantly greater retinal function compared to that of vancomycin alone. Intraocular inflammation following treatment at 4 h was minimal for both treatment groups. Treatment with vitrectomy/vancomycin or vancomycin alone at 5 h or 6 h postinfection resulted in similar levels of retinal function loss (i.e. >90%) and significant intraocular inflammation. Conclusions These results demonstrate that vitrectomy may be of therapeutic benefit in the treatment of B. cereus endophthalmitis, but only during the early stages of infection.

Linker histones bind to nucleosomes and modify chromatin structure and dynamics as a means of epigenetic regulation. Biophysical studies have shown that chromatin fibers can adopt a plethora of conformations with varying levels of compaction. Linker histone condensation, and its specific binding disposition, has been associated with directly tuning this ensemble of states. However, the atomistic dynamics and quantification of this mechanism remains poorly understood. Here, we present molecular dynamics simulations of octa-nucleosome arrays, based on a cryo-EM structure of the 30-nm chromatin fiber, with and without the globular domains of the H1 linker histone to determine how they influence fiber structures and dynamics. Results show that when bound, linker histones inhibit DNA flexibility and stabilize repeating tetra-nucleosomal units, giving rise to increased chromatin compaction. Furthermore, upon the removal of H1, there is a significant destabilization of this compact structure as the fiber adopts less strained and untwisted states. Interestingly, linker DNA sampling in the octa-nucleosome is exaggerated compared to its mono-nucleosome counterparts, suggesting that chromatin architecture plays a significant role in DNA strain even in the absence of linker histones. Moreover, H1-bound states are shown to have increased stiffness within tetra-nucleosomes, but not between them. This increased stiffness leads to stronger long-range correlations within the fiber, which may result in the propagation of epigenetic signals over longer spatial ranges. These simulations highlight the effects of linker histone binding on the internal dynamics and global structure of poly-nucleosome arrays, while providing physical insight into a mechanism of chromatin compaction. SignificanceLinker histones dynamically bind to DNA in chromatin fibers and serve as epigentic regulators. However, the extent to which they influence the gamut of chromatin architecture is still not well understood. Using molecular dynamics simulations, we studied compact octa-nucleosome arrays with and without the H1 linker histone to better understand the mechanisms dictating the structure of the chromatin fiber. Inclusion of H1 results in stabilization of the compact chromatin structure, while its removal results in a major conformational change towards an untwisted ladder-like state. The increased rigidity and correlations within the H1-bound array suggests that H1-saturated chromatin fibers are better suited to transferring long-range epigentic information. 2.