The relative affinity of histones for DNA was studied by the analysis of competitive histone binding to DNA in whole histone/DNA mixtures at physiological and low ionic strengths as well as in water. Use of polyphosphate in similar experiments, as a model of DNA deprived of hydrophobic functional groups allowed us to reject the hypothesis that hydrophobic DNA-histone interaction plays a decisive role in the determination of the relative affinity of histones for DNA, because the orders of histone preference for DNA and for polyphosphate were the same. The relative histone affinity for DNA does not depend on the secondary structure of DNA or on the ionic strength of salt solutions, though the differences in the histone affinities for DNA decrease on lowering the salt concentration. The binding orders of the first and the last molecules of histone type to DNA, studied at various DNA/histone ratios in the medium of physiological ionic strength, are the following: H3+H4, H2A+H2B, H1 and H3+H4, H2A, H2B, H1. In water the binding orders of the first and the last histone molecules to DNA are identical : H 3 + H 4, H 2A, H 2 B + H 1. It is concluded that the relative histone affinity for DNA in water/salt solutions is determined by non-ionic interactions between histones bound to DNA. The folding of DNA induced by histone-histone interaction seems to lead to the increase in the correlation between amino acid residues in the histone regions bound to DNA and the ionic DNA-histone interaction becoming stronger.Since the appearance of the hypothesis about the repressive role of histones in chromosomes [l], for over a quarter of a century DNA-histone interactions have been a matter of steady interest to researchers studying the mechanisms of gene regulation in eucaryotic cells. The discovery of chemical heterogeneity of the histone component of chromatin has made it necessary to explore separate histone fraction affinities for DNA, because the principle of the biological molecule affinity lies at the foundation of all living processes [2] and determines biological specificity [3]. The histone affinity for DNA has been studied in two principal ways : (a) by investigation of DNA-histone complex decomposition, i. e. of histone dissociation from DNA, by modification of the medium into which the extracted chromatin is inserted [4-171; and (b) investigation of DNA-histone complex formation in the mixtures of DNA and whole histone [9,18,19] molecules without change in their charges, alterations (near neutral pH) of ionic strength and ionic composition of solvent [4-6,8,9,13 -15,18,20-221, addition of urea [7 -9,191, of alcohols [8,23,24], and of neutral detergents [13,14] to water/salt solutions; (b) the modifications leading to variation in the electrostatic charges of interacting components, addition of ionic detergents to water/salt solutions [lo-12,15,25], acid titration [16], alkaline titration [17].By now essential differences have been found both in the strength of histone fraction binding to DNA, and in the influence o...
