A simple method for the preparation of the covalently closed circular form of plasmid DNA.

We describe a simple, rapid, inexpensive method for isolation of covalently closed circular plasmid DNA. The method involves the electrophoresis of crude DNA preparations in an agarose gel, electrotransfer onto a dialysis membrane and elution of the highly purified circular covalently closed plasmid DNA. Native and recombinant plasmid DNA have been purified by this method and shown to be suitable for restriction enzyme digestion and transformation of bacteria. The yield of this rapid purification procedure makes it a good alternative method to standard centrifugation in cesium chloride ethidium bromide gradients.