title: Asymmetric unwrapping of nucleosomal DNA propagates asymmetric opening and dissociation of the histone core.
abstract: The nucleosome core particle (NCP) is the basic structural unit for genome packaging in eukaryotic cells and consists of DNA wound around a core of eight histone proteins. DNA access is modulated through dynamic processes of NCP disassembly. Partly disassembled structures, such as the hexasome (containing six histones) and the tetrasome (four histones), are important for transcription regulation in vivo. However, the pathways for their formation have been difficult to characterize. We combine time-resolved (TR) small-angle X-ray scattering and TR-FRET to correlate changes in the DNA conformations with composition of the histone core during salt-induced disassembly of canonical NCPs. We find that H2A-H2B histone dimers are released sequentially, with the first dimer being released after the DNA has formed an asymmetrically unwrapped, teardrop-shape DNA structure. This finding suggests that the octasome-to-hexasome transition is guided by the asymmetric unwrapping of the DNA. The link between DNA structure and histone composition suggests a potential mechanism for the action of proteins that alter nucleosome configurations such as histone chaperones and chromatin remodeling complexes.
authors.affiliation: School of Applied and Engineering Physics, Cornell University, Ithaca, NY 14853.
authors.forename: Yujie
authors.initials: Y
authors.lastname: Chen
authors.affiliation: School of Applied and Engineering Physics, Cornell University, Ithaca, NY 14853.
authors.forename: Joshua M
authors.initials: JM
authors.lastname: Tokuda
authors.affiliation: School of Molecular Biosciences, Washington State University, Pullman, WA 99164.
authors.forename: Traci
authors.initials: T
authors.lastname: Topping
authors.affiliation: School of Applied and Engineering Physics, Cornell University, Ithaca, NY 14853.
authors.forename: Steve P
authors.initials: SP
authors.lastname: Meisburger
authors.affiliation: School of Applied and Engineering Physics, Cornell University, Ithaca, NY 14853.
authors.forename: Suzette A
authors.initials: SA
authors.lastname: Pabit
authors.affiliation: School of Molecular Biosciences, Washington State University, Pullman, WA 99164 LP26@cornell.edu lmgloss@wsu.edu.
authors.forename: Lisa M
authors.initials: LM
authors.lastname: Gloss
authors.affiliation: School of Applied and Engineering Physics, Cornell University, Ithaca, NY 14853; LP26@cornell.edu lmgloss@wsu.edu.
authors.forename: Lois
authors.initials: L
authors.lastname: Pollack
mesh_terms: D000818:Animals; D002843:Chromatin / Q000378:metabolism; D004247:DNA / Q000378:metabolism*; D006657:Histones / Q000378:metabolism*; D009690:Nucleic Acid Conformation; D009707:Nucleosomes / Q000378:metabolism*; D014982:Xenopus laevis / Q000378:metabolism
chemical_list: D002843:Chromatin; D006657:Histones; D009707:Nucleosomes; D004247:DNA
keywords: FRET; contrast variation SAXS; hexasome; nucleosomes; time resolved