Divalent metal ions have a profound influence on the structures of many natural nucleoproteins, including ribosomes and chromosomes; however the proteins comprising these substances are heterogeneous. A study was therefore made of the effects of divalent metal ions on mixtures of deoxyribonucleic acid (DNA) with homogeneous globular proteins. The most basic of the chicken hemoglobins complexes with DNA in the presence of Mg2+, Ca2+, or Mn2+ to form insoluble fibers. Polarization and electron microscopic observations establish that the DNA molecules are oriented parallel to the fiber axis. Fibrous horse heart cytochrome c-DNA complexes, structurally similar to the hemoglobin-DNA complexes, also are formed only in the presence of divalent cations. Polarized visible absorption spectra of the hemoglobin-DNA and cytochrome c-DNA fibers were used to determine orientation of the hemoproteins relative to the fiber axes. As controls, polarized spectra of monoclinic crystals of horse methemoglobin, oxyhemoglobin, and methemoglobin azide and also of tetragonal crystals of horse heart ferricytpchrome c were obtained. The chicken hemoglobin molecules in the nucleoprotein fibers do not appear to be strongly oriented with respect to the fiber axis. However in the horse heart cytochrome c-DNA fibers, the heme normals are oriented at 60° to the fiber axis. With both hemoproteins, binding to DNA does not measurably alter the visible spectrum, indicating that gross changes of polypeptide folding do not accompany the interactions. A mechanism is proposed to explain the stabilization of the tightly packed fiber structures by divalent metal ions; it is argued that this mechanism might be operative also in determining ribosome and chromosome structures.
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