Purification and characterization of wheat germ DNA topoisomerase I (nicking-closing enzyme)

Wheat germ contains an enzyme capable of removing supercoils from circular DNA. We have purified this enzyme using Polymin P fractionation, ammonium sulfate precipitation, and chromatography on Bio-Rex 70 and phenyl-Sepharose. Renaturation after electrophoresis on sodium dodecyl sulfate-polyacrylamide gels shows that topoisomerase activity is associated with a polypeptide with a Mr = about 111,000. The enzyme is similar to other eukaryotic type I DNA topoisomerases (nicking-closing enzymes) by the following criteria: it is capable of increasing or decreasing the topological linking number of covalently closed DNA substrate; it is capable of restoring an equilibrium distribution of linking numbers to DNA substrate with a single unique linking number; and it does not require magnesium ion or ATP for activity.