Structural Characterization of Histone H2A Variants

Eukaryotic DNA associates with an equal amount ofprotein to form chromatin, the fundamental unit of whichis the nucleosome core particle (NCP). An NCP consistsof two copies each of the four core histones H2A, H2B,H3, and H4. This histone octamer binds 147 base pairs ofDNA around its outer surface in 1.65 tight superhelicalturns (Fig. 1A) (Luger et al. 1997; Richmond and Davey2003). Linker histones and other nonhistone proteins promote or stabilize the folding of nucleosomal arrays intosuperstructures of increasing complexity and largely unknown architecture (Hansen 2002). Covalent modification of the core histones and variations in the fundamental biochemical composition of nucleosomes distinguishtranscriptionally active from inactive chromatin regions,by either changing the structure of the nucleosomes, altering their ability to interact with other protein factors, ormodifying their propensity to fold into varying degrees ofhigher-order structures (or by any combination of theabove). Studying the mechanism for establishing distinctchromatin domains is essential to understanding differential regulation of gene expression and all other DNA-d...