Histone packing in the nucleosome core particle of chromatin

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Abstract
The chromatin core particle DNA conformation deduced by Finch et al. can be described using other experimental results. Histone binding sites compatible with the pattern of pancreatic DNase I digestion lend to core particle DNA pseudosymmetry characteristic of molecular point group D3. DNA symmetry and pseudosymmetry imply equivalence and quasi-equivalence properties of the histone packing arrangement that support the following deductions. One and only 1 .alpha.2.beta.2 histone tetramer, presumably (H3)2(H4)2, can serve as a stable subassembly within the histone octamer. There is a unique, strand-specific way to assign DNA binding domains to the arginine-rich histones (H3 and H4). Histones H3 and H4 alone should suffice to impose a supercoiled structure on DNA, as is observed experimentally, because only the tetramer can mimic a screw dislocation and thereby complement the screw symmetry of the DNA supercoil. The 2 slightly lysine-rich histones H2A and H2B are probably responsible, each in a different way, for dividing the eukaryotic chromatin fiber into discrete subunits. The proposed arrangement of 4 distinct proteins appears to be a minimum formal requirement for making nucleosomes; that is, for introducing regularly spaced supercoiled DNA folds without also allowing formation of an indefinitely long (and genetically inert) DNA superhelix.