Description:
<jats:p>The nucleoprotein of chicken erythrocytes was dissociated by NaCl. The free histone and the residual complex were separated by centrifugation or gel filtration in order to obtain a series of complexes with variable protein content. For each complex the viscosity, circular dichroïsm spectrum and thermal profile were determined and discussed in terms of the configuration and conformation of DNA inside the nucleoprotein.</jats:p><jats:p>A very small amount of protein is able to provoke a folding of the DNA which then behaves as a rigid rod. Subsequent binding of histones (up to 20—30%) is accompanied by an increased flexibility and a coiled configuration.</jats:p><jats:p>In the case of a protein/DNA ratio of 0.9, the intrinsic viscosity of the nucleoprotein is four times smaller than the viscosity of DNA in the same condition. Such a decrease could indicate an organization of the DNA in a supercoiled structure.</jats:p><jats:p>The circular dichroïsm spectrum of the DNA inside the nucleoprotein is very peculiar and bears no resemblance to any known state of DNA (A form or denatured). It is only the positive band between 250 nm and 300 nm which is strongly reduced, the negative one being practically unaffected. Similar results were obtained with solutions of DNA of high salt content.</jats:p><jats:p>During the progressive stripping of histones the intensity of the positive band remains unchanged with the release of F1 and begins to increase when F2 and F3 are simultaneously liberated.</jats:p><jats:p>In contrast to this absence of any structural change of DNA with the binding of F1, it is remarkable that the same binding (or release) affects the stability of roughly 50% of the DNA molecule. Although F1 represents about 20% of the total histone, and owing to the cooperative nature of the melting, it seems clear that histones are unequally distributed along the DNA and that F1‐type histones predominantly cover large sequences of nucleotides.</jats:p>