Negatively supercoiled DNA from plants infected with a single-stranded DNA virus

A method for isolating covalently closed circular double-stranded DNA from plants infected with the geminivirus, tomato golden mosaic virus, is described. Ethidium bromide titration showed this DNA to be negatively supercoiled with a superhelical density of - 0.062. The presence of S1 nuclease-sensitive secondary structure in the supercoiled DNA was demonstrated by its conversion to the open circular and linear DNA forms on treatment with this enzyme