Spectrofluorometric Measurement of the Binding of Ethidium to Superhelical DNA from Cell Nuclei

Structures retaining many of the morphological features of nuclei may be released by lysing HeLa cells in solutions containing non‐ionic detergents and high concentrations of salt. These nucleoids contain few chromatin proteins. We have shown that the DNA of nucleoids is quasi‐circular and supercoiled by measuring spectrofluorometrically the amount of the intercalating dye, ethidium, bound to unirradiated and γ‐irradiated nucleoids. Ethidium binds to nucleoids in the manner characteristic of the binding to superhelical DNA: at low concentrations more ethidium binds to unirradiated nucleoids than to their γ‐irradiated counterparts with broken DNA, and at higher concentrations less ethidium binds to the unirradiated nucleoids. The quasi‐circles in nucleoids are 22 times less sensitive to γ‐irradiation than are circles of pure PM2 DNA: they must contain about 2.2 × 10 5 base pairs. The constraints that maintain the quasi‐circularity of nucleoid DNA are very resistant to extremes of temperature and alkali; some remain under conditions in which the duplex is denatured. The constraints are destabilised by ethidium suggesting that they are stabilised by free energy of supercoiling. Proteolytic enzymes, but not ribonucleases, remove the constraints. Possible structures for the constraining mechanism are discussed.