Molecular arrangement of DNA in nucleohistone.

Several optical and hydrodynamic properties of purified nucleohistone have indicated that the secondary structure of DNA in nucleohistone is slightly different both from the structures of native and denatured DNA in solution. It has been suggested that while histones stabilize the overall DNA-structure they incorporate into the latter some denaturation defects. Rotational diffusion studies of acridine-orange complexes of whole and dehistonized nucleohistones have indicated that nucleohistone should contain a single continuous DNA molecule. The contour length of the DNA molecule would be about 2 to 3 times the end-to-end distance of nucleohistone. Polarized fluorescence microscopy and linear dichroism studies of whole nucleohistone have given an average estimate (about 30% of the total CNA) of oriented DNA in nucleohistone. The degree and the direction of maximum polarization of fluorescence from the dye complexes of whole and partial (i.e. partially dissociated) nucleohistones have been studied. From these studies, it is proposed that no single supercoiled arrangement of the DNA in nucleohistone is tenable. The combined results of all these studies suggest that the basic nucleohistone molecule contains coiled and extended DNA regions. The proportion of the lengths of the DNA constrained in the coiled and the extended regions is about 7:3. Only two probable models or classes have this quantitative feature of the DNA-arrangement and have been discussed.