Modification of PM2 DNA with N-acetoxy-2-acetylaminofluorene: changes in buoyant density, electrophoretic mobility and electron microscopic structure.

PM2 DNA, forms I and II, were chemically modified with the ultimate carcinogen N-acetoxy-2-acetylaminofluorene. The products were subjected to 3 analytical procedures: (i) buoyant density centrifugation in the presence of the intercalating dyes ethidium bromide or propidium diiodide; (ii) agarose gel electrophoresis: and (iii) electron microscopy. It was found that carcinogen-modified DNAs banded at higher densities in the presence of either dye. This effect was far more pronounced with form I DNA than with form II. Form I DNA showed a concentration-dependent decrease of migration velocity in agarose gel electrophoresis after modification; form II DNA exhibited a similar but smaller effect. Electron microscopy of form I DNA did not reveal a consistent correlation between the extent of chemical modification and the loss of superhelical turns. Form II DNA exhibited characteristic alterations such as aggregation of several PM2 molecules into network-like structures, kinky configuration, and probably single-stranded bubbles extending over 600-1500 bases.