Examination of circular dichroic and phosphorus nuclear magnetic resonance spectra showed that poly(dA-dT)-poly(dA-dT) exhibited an ethanol-induced transition to the A form in an Na+ containing medium like natural DNAs. A mere replacement of the Na+ by Cs+ counterions meant that the polynucleotide was with a little cooperativity transformed into a novel conformation displaying a deep negative band in the long wavelength part of the CD spectrum. The presence of very low concentration of Cs2+ shifted the midpoint of the transition to a lower content of ethanol.
Circular dichroism was used to study changes in conformation of poly(dA-dC).poly(dG-dT) caused by a high concentration of various monovalent salts. It was found that CsF induced the gradual appearance of a negative band in the long wavelength part of the CD spectrum of poly(dA-dC).poly(dG-dT), which might reflect a transition of this DNA toward a Z-like structure.
Aggregated complexes of histone F1 with DNA of different composition and origin were prepared by gradual dialysis from a high salt concentration to 0.15M NaCl. Analysis of the DNA composition both in the supernatant and in the precipitate showed that in equilibrium complexes in this medium, histone Fl selectively prefers sequences of native DNA rich in adenine and thymine. The mechanism of the selective binding is discussed.Study of complexes reconstituted from isolated DNA and from histone fractions represents one of the possible approaches to the investigation of the structure of chromatin and of chromosomes. Progress in this field appears t o be essential for a better understanding of the mechanism of genetic processes in higher organisms. It was pointed out before [1,2] that the hydrodynamic and the optical properties of native and reconstituted complexes in their soluble form tally. It is an advantage of the reconstitution procedure that one can independently alter the character of the components of the complex, to study their interaction and the effect on the final structure of the complex.Recently the view was advanced [3] that native nucleohistone molecularly dispersed at Iow ionic strength partly loses the assumed superhelix structure. A structural transition on decreasing the ionic strength of the medium was also observed with complexes reconstituted from DNA and from the lysine-rich fraction F1 [4,5]. This histone fraction appears to be very interesting since it differs significantly in its properties from other fractions [6] and some authors assume that it substantially affects the structure of chromosomes [7].In the present paper we studied the interaction of histone F1 with DNA of different composition and origin in a medium of approximately physiological ionic strength (0.15M NaC1). It was intended to contribute to the elucidation of the origin of specific complexes of histone F1 * DNA under conditions resembling those in vivo and hence to contribute to determining the biological function of this histone fraction. A similar problem was examined in a greater number of papers [S] and some of their results indicate that a certain specificity of the binding of Abbreviation. CD, circular dichroism. 7.histones to DNA exists [9,10]. It was of interest in this context to note the interactions of DNA with synthetic polypeptides [S, 111. Still, the binding of histones to DNA is generally considered to be rather nonspecific, perhaps also because most of the studies conducted on reconstituted complexes as well as on native nucleohistones were done with soluble nucleohistones, i.e. under conditions rather different from physiological. The results of analysis of DNA composition both in the supernatant and in the aggregated phase presented here indicate that histone F1 in aggregated complexes prepared by the equilibrium method selectively prefers sequences of native DNA rich in adenine plus thymine.
Circular dichroism spectroscopy, absorption spectroscopy, measurements of Tm values, sedimentation analysis and electron microscopy were used to study properties of calf thymus DNA in methanol-water mixtures as a function of monovalent cation (Na+ or Cs+) concentration and also in the presence of divalent cations Ca2+, Mg2+, and Mn2+. In the absence of divalent cations only slight conformational changes occurred and no condensation and/or aggregation could be detected. The Tm values depend on the amount of methanol and on the nature and concentration of cations. In methanol-water mixtures higher thermal stability was observed in solutions containing Cs+ ions. Up to 40% (v/v) methanol the addition of divalent ions leads to DNA stabilization. At methanol concentration higher than 50% the presence of divalent cations causes DNA condensation and denaturation even at room temperature. The denaturation is reversible with respect to EDTA addition indicating that no separation of complementary strands occurred and the resulting form of DNA is probably similar to the P form. DNA destacking appears to be a direct consequence of stronger cation binding by the condensed DNA in methanol-water mixtures.
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