Four mononucleosome preparations were isolated from micrococcal nuclease digests of chicken erythrocyte nuclei which differed in average deoxyribonucleic acid (DNA) length and in HI and H5 content. The circular dichroism properties of the unperturbed mononucleosome preparations and the corresponding HI - and H5-depleted species demonstrate that the nucleoprotein spectra above 250 nm are all altered relative to protein-free DNA by the addition of a single negative band at 275 nm, similar to the band observed for Ψ-DNA. The quantitative analysis of the Ψ-type band intensity for any of the higher molecular weight unperturbed samples relative to core particle mononucleosomes yielded a constant number of DNA base pairs (~140) contributing to this new band. Upon removal of H1 and H5 from the mononucleosome preparations which have sufficiently long linker DNA, the SP-type band intensity indicates an ~30 base pair reduction in the number of core DNA base pairs contributing to the altered circular dichroism properties. The Ψ-type band is proposed to be due to the compact DNA tertiary structure, i.e., the manner in which the DNA is wound around the histone core allowing interactions between adjacent turns of the superhelix. This interpretation implies that ~30 base pairs of core DNA are removed from the unique core tertiary structure when the linker DNA is not bound by H1 or H5. The circular dichroism analysis correlates well with the thermal denaturation properties of mononucleosomes. Removal of H1 and H5 causes an overall reduction in the thermal stability of both core and linker DNA. The degree of destabilization is greatest when the average DNA length is maximum. Some core DNA is lost from the highest temperature melting bands when histone-free DNA is present. These results indicate two regions of different conformational and thermodynamic stability in core DNA. The length of attached linker DNA and its histone content influence the two regions of the core to differing extents.
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