Physical properties of inner histone-DNA complexes

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Abstract
Chicken erythrocyte inner histone tetramer has been complexed with several natural and synthetic DNA duplexes by salt-gradient dialysis at various protein/DNA ratios. The resulting complexes, in low-ionic-strength buffer, have been examined by electron microscopy, circular dichroism, and thermal denaturation. Electron microscopy reveals nucleosomes (ɛ bodies) randomly arranged along DNA fibers, including poly(dA-dT).poly(dA-dT), poly(dI-dC.poly(dI-dC), but not poly(dA).poly(dT). Circular dichroism studies showed prominent histone α-helix and “suppression” of nucleic acid ellipticity (λ>240 nm). Thermal denaturation experiments revealed Tm behavior comparable to that of H1- (or H5-) depleted chromatin. Tm III and Tm IV increased linearly with G+C% (natural DNAs), but were virtually independent of the histone/DNA ratio; therefore, the melting of nucleosomes along a DNA chain is insensitive to adjacent “spacer” DNA lengths. This suggests that Tm III and Tm IV arise from the melting of different domains of DNA associated with the core ɛ body.