Magnesium binding and conformational change of DNA in chromatin.
The structure of chromatin in the presence of Mg2+ ions was examined by circular dichroism and equilibrium dialysis. Circular dichroism (CD) shows that above 260 nm the intensity of the spectrum of DNA in nucleoproteins decreases as the Mg2+ concentration increases. This change is an intrinsic characteristic of DNA since it is also observed in protein-free DNA and has been attributed to a change in the winding angle of base pairs around the DNA axis. Some structural elements of the DNA in the nucleosome core, therefore, are as movable as those of protein-free DNA. The basic organization of H1-depleted chromatin, 146 base pairs (bp) of DNA wound around core histones and a residual 49 bp in the linker region in the repeating unit, is maintained both in the presence and in the absence of Mg2+ ions, as shown by the fact that the CD spectrum of H1-depleted chromatin has the same type of linear combination between the spectrum of protein-free DNA and that of the nucleosome core in 0.2 mM MgCl2-10 mM triethanolamine (pH 7.8) as it has in 1 mM ethylenediaminetetraacetic acid-10 mM tris(hydroxymethyl) aminomethane (pH 7.8). The ellipticity of chromatin shows a smaller decrease relative to the other nucleoproteins and protein-free DNA upon the addition of Mg2+ ions. Therefore, some structural elements of chromatin are apparently somewhat protected against the conformational change induced by these ions. The spectrum of chromatin becomes almost indistinguishable from that of H1-depleted chromatin in 0.2 mM MgCl2.(ABSTRACT TRUNCATED AT 250 WORDS)
D000818:Animals; D002417:Cattle; D002843:Chromatin / Q000648:ultrastructure*; D002942:Circular Dichroism; D004247:DNA / Q000378:metabolism; D006657:Histones / Q000502:physiology*; D008274:Magnesium / Q000502:physiology*; D009690:Nucleic Acid Conformation; D009707:Nucleosomes / Q000648:ultrastructure*; D010710:Phosphates; D011487:Protein Conformation
D002843:Chromatin; D006657:Histones; D009707:Nucleosomes; D010710:Phosphates; D004247:DNA; D008274:Magnesium