Purification and properties of a topoisomerase from Ustilago maydis

The lower eukaryote Ustilago maydis contains a topoisomerase that removes supercoils from negative and positive superhelical DNA. The enzyme may be a multimeric protein or an aggregate of polypeptides with a native molecular weight of 270,000 as estimated by gel filtration. No cofactors are required by the enzyme, but activity is enhanced by Mg2+. Dependence of activity upon enzyme concentration is not linear. Below a threshold level where topoisomerase cannot ordinarily be detected, addition of H1 histone sharply stimulates activity. ATP and a number of structural analogues inhibit the enzyme.