DNA stainability with base-specific fluorochromes: dependence on the DNA topology in situ.

The influence of DNA topology on stainability with the externally binding fluorochromes Hoechst 33258 (HO) and mithramycin (MI) was investigated in HeLa nuclei in comparison with the intercalating dye propidium iodide (PI). Changes in DNA topology were induced with a mild DNAse I treatment. Stainability properties of untreated and nuclease-treated nuclei were compared with those of the supercoiled-circular and the relaxed-linear forms of the plasmid pBR322. DNAse-treated nuclei stained with HO showed a higher fluorescence intensity than control samples, independently of the dye concentration, in contrast with the findings obtained with PI. Similar behaviour was observed with the relaxed-linear form of pBR322, compared with the supercoiled-circular molecule. With MI, the stainability of HeLa nuclei did not depend on the DNA topology, whereas the stainability of the plasmid was similar to that of HO. In order to assess whether this discrepancy depended on differences in the availability of DNAse-sensitive sites to the fluorochromes, fluorescence resonance energy transfer (FRET) studies were performed in nuclei stained with HO+PI, or with HO+MI dye pairs. After DNAse I digestion, the relative FRET efficiency between donor (HO) and acceptor molecules (PI or MI) was reduced significantly only when MI was the acceptor. This result may be due to greater stainability of DNAse-sensitive sites with HO than with MI. These findings indicate that DNA stainability with base-specific fluorochromes may be affected by the topology of chromatin regions.