The conformation of the DNA and histones from calf thymus has been studied both in deoxyribonucleoprotein (DNA -protein) and in the free state using circular dichroism over a wide range of ionic strength, pH and temperature conditions. The relationships between protein content and conformational state of DNA and histones in DNAeprotein has been also investigated.The dependence of the circular dichroism on pH and temperature in solutions of DNA, histones and DNA -protein, indicates that DNA and histones in DNA -protein mutually stabilize the secondary structure of each other against denaturation. Changes in the DNA structure within DNA * protein are accompanied by structural transformations of the histones. The circular dichroism study of free DNA and histones in solutions of various ionic strengths shows that the effects produced by DNA on the histone conformation and by histones on the structure of DNA in DNA -protein are similar to that of high ionic strength.The circular dichroism measurements of partial DNA -protein of various protein content show that the structural changes of DNA in DNA * protein are caused by histone fractions dissociated in the range of 0.7-1.0 M NaC1.It is well known that DNA in the cell nuclei of eukaryotes exists in a form of a deoxyribonucleoprotein complex (DNA * protein) which serves as the basis for the structure of chromatin. The bulk of the protein in DNA -protein (80-86°/0) is histones.The structure of DNA and histones in DNA * protein is not fully understood yet. The results of hydrodynamic, X-ray and electron microscopic investigations indicate that DNA within DNA -protein is more compact than free DNA [1-41. Nevertheless, such data yield no information concerning the secondary structure of the DNA -protein components. Optical rotatory dispersion in the visible region of the spectrum [l] and in the region of intrinsic absorption bands [5-71 has been used to determine the conformation of DNA and histones in DNA * protein. However, interpretation of the information obtained is controversial since the optical rotatory dispersion curves of DNA and histones overlap. When the optical rotatory dispersion curves for DNA -protein undergo changes it is difficult to decide whether these are due to alterations in the structure of DNA or histones in DNA protein or of both components together.Less ambiguous information about the structure of DNA and histones in the DNA protein may be obtained by the method of circular dichroism. While studying the circular dichroism of the DNA protein in the region of intrinsic absorption bands of DNA (the optically active band near 260nm due to a TC -+ x* transition in the nitrogeneous bases) and histone (the band near 220nm due to a n + n * transition in the peptide groups of proteins) one may distinguish effects produced by DNA and histones.It is well known that the circular dichroism bands a t about 260 nm for DNA and 220 nm for protein are characteristic and directly related to the structure of DNA and proteins [8-111. Therefore the study of circular...

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