Conformational changes of histones and DNA during the thermal denaturation of nucleoprotein.

Abstract

The conformational changes of histones and DNA during the thermal denaturation of native and partially histone-depleted nucleoprotein (DNA · protein) was studied by following the variations of absorbance and circular dichroism with temperature. All the measurements were made in 1 mM NaCl, 0.2 mM EDTA pH 5.6. Absorbance was measured at 260 nm and circular dichroism (C.D.) at 280 nm and 227 nm, in order to follow the conformational changes of DNA and histones separately. Besides the premelting effect (which is not observed with native DNA · protein), the second maximum which takes place, in the 280-nm C.D. melting profile, at a temperature close to 65 °C, can be interpreted as a change from the C form to the B form of DNA. If non-histone proteins are assumed to have no effect upon the structure of DNA, the height of the second maximum varies almost linearly with the histone content except, may be, in the case of F2a1. Any histone would thus be equivalent to induce conformational changes of DNA. This change is always preceded and probably triggered by the cooperative disruption of a large amount of α-helical regions of the histones which in their new tertiary structure are no more able to maintain the DNA in a C conformation. In the model of the structural changes of DNA · protein with temperature, which is tentatively proposed, histone-histone interactions would play a role as important as histone-DNA interactions.