Detection of size heterogeneity in the supercoiled fraction of polyoma virus DNA

Abstract

The supercoiled 20 s component of polyoma virus DNA did not behave as a single molecular species when analyzed by agar gel electrophoresis and sedimentation velocity. The observed heterogeneity was not eliminated by temporary heating to 100°C or adjustment to pH 12·5, high ionic strength, dialysis or exposure to chelating agent or incubation with proteolytic enzymes and ribonuclease. The possibility that attached protein is responsible for the heterogeneity seems unlikely, since DNA fractions from the extremes of the 20 s peak in zone sedimentation had indistinguishable densities as determined by equilibrium centrifugation in caesium chloride, and analysis of DNA from virus preparations labelled with radioactive amino acids indicated that contamination with virus protein was at a negligible level. Differences in sedimentation velocity were also observed between double-stranded circular “16 s” forms derived from the extreme fractions of the 20 s peak by the action of deoxyribonuclease and the corresponding circular and linear single-stranded forms produced by alkali denaturation. These results indicate that the supercoiled fraction of polyoma virus DNA can be heterogeneous in molecular weight.