Eukaryotic DNAs in solution contain characteristic components of tertiary structure

For natural eukaryotic DNA in solution, we suggest the existence of secondary-helix components superponed to parts of the DNA double helices. In a previous report we found, for calf thymus DNA in solution and of different mean molar mass Mr, an electrostatically driven rise of the hydrodynamically operative contour length of the double helix. This result was derived from Mr-dependent systematic deviations from the almost but not exactly linear plots of intrinsic viscosity [eta] as a function of 1/cs1/2 (cs = Na+ concentration) accurately determined by a titration technique [K.G. and K.E.R., Nucl. Acids Res. 8, 2807 (1980)]. In order to discriminate between DNA elongation contributions caused by secondary or by tertiary structure effects, respective measurements have now been extended to different temperatures for two eukaryotic and two prokaryotic DNA species. The slope of the curves obtained for the (apparent) gradual elongation effect as a function of temperature is negative for the eukaryotic DNAs investigated and is smaller and positive for the prokaryotic species, thus revealing different underlying main elongation mechanisms. We propose that, for the eukaryotic DNA samples, an electrostatically driven partial abolition of tertiary structure components is responsible for the prevailing part of the DNA elongation effect measured. (A helix elongation of this type may be the result of an abolition of an apparent helix shortening as realized in a very high degree on formation of nucleosome chains or in a less degree by DNA molecules with a respective evolutionarily fitted tertiary structure). For the smaller effects of prokaryotic DNA species something like a base breathing seems to dominate. Recent literature results support such an interpretation.