The action of γ -rays on deoxyribonucleohistone in solution

Abstract

Measurements have been made by light scattering of the decrease, resulting from γ -irradiation, in the molecular weight and radius of gyration of deoxyribonucleohistone ( DNH ) in 0·7 mM phosphate buffer (pH 6·8). The number of fractures produced depended on a power of the dosage intermediate between one and two. The relative constancy of the radius of gyration at doses up to about 1 eV/P atom, while the mean molecular weight decreased, suggested that dissociation of the histone from the DNA core might be occurring. Similar measure­ments on DNH dissolved in 2M potassium chloride solutions showed that the DNA core was degraded during irradiation and this was confirmed by the increase observed in the width of the band which was obtained when the DNH was centrifuged to equilibrium in a density-gradient of caesium chloride. Irradiation also caused denaturation, since the buoyant density of the DNA component of the DNH increased and there was a decrease in ultra-violet hypochromicity, as determined after heating the irradiated solutions. From integral sedimentation-distribution curves at zero concentration it was inferred that all the DNH molecules, apart from some aggregated material, had undergone the same fractional decrease in sedimentation coefficient, and hence in molecular weight, irrespective of their original size. The irradiated DNH also showed an increased anionic electrophoretic mobility, which is consistent with the loss of a cationic fragment from the molecule. Calculations have shown that the observed molecular-weight changes could be explained by a mechanism which combines dissociation of histone by a ‘one-hit’ process and degradation of the DNA by a ‘two-hit’ process. It is suggested that an attack on the DNA , resulting in the breakage of one of the two chains, and the consequent disorganization of the helix in the vicinity of the break, would be the most likely cause of dissociation of the histone.