A New Histone Structure Which Binds DNA at Its Eight Subunit N-Termini

A new model for the nucleosome is presented. The histone octamer core is unchanged, but the location of the DNA is different. Since the highest number, and highest concentration of positively- charged amino acid residues is located not in the “superhelical ramp” of the octamer core, but rather in the domain of the eight histone subunit N-termini collectively, the DNA is therefore placed there. The role models for the protein and DNA structures in the N-terminal domain are taken from the comparable role models for protein and DNA in the protamine-DNA complex in sperm cells. The histone subunit N-termini are each modeled as beta-strands, with psi/phi values of approximately /﹣130.5° respectively, which gives a straight chain. The DNA is modeled according to the “straight ladder” model of Tai Te Wu. Each DNA phosphate group is bound to a lysine or arginine residue of histone by a 3 A salt bridge. The new model lends itself so readily to further models of higher-order chromatin structure that the problem shifts entirely, from one of deducing any higher-order structure at all, to one of distinguishing between several models which compete for our attention.