CONDENSATION OF DNA BY BASIC-PROTEINS DOES NOT DEPEND ON PROTEIN-COMPOSITION

We have studied by electron microscopy the size and morphology of the complexes obtained with different DNAs (between 500 and 5243 base pairs long) and four different proteins : sea urchin histone H1, sea cucumber histone phi(0), chicken erythrocyte histone H5, and clupeine. Surprisingly, the type of protein used has only a marginal influence on the complexes formed. The molecular weight and topology of DNA do not show any influence. The size of the complexes depends strongly on the ratio of positive to negative charges and also on the ionic conditions. Our studies have been mainly carried out at a ratio of 0.4. Under these conditions the average thickness of rods and toroids observed varies between 165 Angstrom at 1.5 mM salt to 290 Angstrom at 100 mM salt, with minor variations around these values depending on the type of DNA and protein used. We conclude that the formation of DNA condensates is mainly determined by a balance of electrostatic and intermolecular forces, the influence of specific interactions is only marginal. This conclusion seems to apply not only to the complexes described here, but also to chromatin fibers and to DNA condensed by low molecular weight counterions and other compounds (polyamines, inorganic ions, ethanol, etc.). (C) 1994 John Wiley & Sons, Inc.