title: Mixed conformations of deoxyribonucleic acid in chromatin: a preliminary report.
abstract: 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.
mesh_terms: D000818:Animals; D002417:Cattle; D002712:Chlorides; D002843:Chromatin / Q000032:analysis*; D002942:Circular Dichroism; D004247:DNA / Q000032:analysis; D003851:Deoxyribonucleases / Q000378:metabolism; D006657:Histones / Q000032:analysis* / Q000378:metabolism; D008094:Lithium; D008961:Models, Structural; D009698:Nucleoproteins / Q000032:analysis; D010727:Phosphoric Diester Hydrolases / Q000378:metabolism; D011402:Pronase / Q000378:metabolism; D011487:Protein Conformation; D013552:Swine; D013950:Thymus Gland / Q000032:analysis
chemical_list: D002712:Chlorides; D002843:Chromatin; D006657:Histones; D009698:Nucleoproteins; D004247:DNA; D008094:Lithium; D003851:Deoxyribonucleases; D010727:Phosphoric Diester Hydrolases; D011402:Pronase