Structural Analysis of the Hexasome, Lacking One Histone H2A/H2B Dimer from the Conventional Nucleosome

Genomic DNA is packaged into chromatin in eukaryotes, and the nucleosome is the fundamental unit of chromatin. The canonical nucleosome is the octasome, which is composed of two H2A/H2B dimers and two H3/H4 dimers. During transcription elongation, one of the H2A/H2B dimers is removed from the octasome. The depletion of the H2A/H2B dimer is also suggested to occur during DNA replication and repair. The remaining histone components are believed to maintain a nucleosomal structure called a hexasome, which is probably important for the regulation of gene expression, DNA replication, and repair in chromatin. However, hexasomes are currently poorly understood, due to the lack of in vivo and in vitro studies. Biochemical and structural studies of hexasomes have been hampered by the difficulty of preparing purified hexasomes. In the present study, we successfully reconstituted hexasomes, using recombinant human histones. A micrococcal nuclease treatment and in vitro reconstitution assays revealed that the hexasome tightly wraps approximately 110 base-pairs of DNA, about 40 base-pairs shorter than the length of the DNA wrapped within the canonical nucleosome. A small-angle X-ray scattering analysis revealed that the global structure of the hexasome is similar to that of the canonical nucleosome. Our studies suggest that octasomes can be converted into hexasomes by the eviction of one of the H2A/H2B dimers, and the release of about 40 base-pairs of DNA, without involving large structural changes in the nucleosome core particle.