The effect of superhelicity on the interaction of histone f1 with closed circular duplex DNA.

A set of covalently closed circular duplex simian virus 40 DNA preparations of varying superhelical densities was prepared by closure of nicked duplex DNA with polynucleotide ligase in the presence of varying amounts of ethidium. The resulting molecules were tested for complex formation with the lysine-rich histone f1. The results confirmed earlier experiments in demonstrating that f1 histone reacts preferentially with superhelical DNA compared to relaxed circular DNA. Furthermore, the extent of the reaction is demonstrated to depend on the superhelical density. At the relatively low ratios of histone to DNA used in these experiments, the product of the interaction of f1 histone with superhelical DNA does not precipitate. At higher ratios of histone to DNA, an insoluble aggregate is formed.