期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：1972
卷号：69
期号：11
页码：3263-3267
DOI：10.1073/pnas.69.11.3263
语种：English
出版社：The National Academy of Sciences of the United States of America
摘要：We have established empirical limits for the circular dichroism spectra appropriate for aqueous solutions of the B, C, and A forms of calf-thymus DNA and have analyzed DNA conformations in biological structures. The circular dichroism spectrum above 250 nm of purified calf-thymus chromatin can be satisfactorily accounted for as a linear combination of contributions of the B and C reference spectra without invoking higher-order structures such as supercoils. The amount of A contribution, if any, is below the limit of detection ([≤]4%). The fraction of bases in the B conformation depends on the method of isolation of nucleohistone, and ranges from 30-50%. The B content of a given preparation is increased by addition of a chelating agent and decreased by addition of divalent ions. More radical increases ensue upon protein removal. Nuclease treatment results in a dramatic decrease in B content. The fraction of bases melting out in the lower transitions of the complex melting profile of a given chromatin preparation corresponds to its B content. We propose a model for chromatin structure in which part of the DNA duplex is exposed or accessible to the solvent and is in the B conformation. The remainder of the base pairs and ribophosphate backbone are protected from interaction with the solvent by efficient histone coverage and are in the C conformation. Divalent ions modulate the distribution of bases between these two conformations.
