The purification from Escherichia coli of a protein relaxing superhelical DNA

The Escherichia coli omega protein was first described by Wang (Wang J.C.: J. Mol. Biol. 55, 523-533 (1971)) as having the ability to relax supercoiled covalently-closed circular DNA by changing the topological winding number, alpha. We have developed a rapid assay for omega activity which has allowed us to purify the protein to homogeneity. It appears to be an alphabeta-type subunit protein with a molecular weight of the intact protein of about 80,000 (determined by gel filtration) and of the individual subunits of 56000 and 31000 (sodium dodecyl sulfate polyacrylamide gels). We have confirmed Wang's observation that it only partly relaxes negative supercoils, and is not active on a positive supercoils. Its characteristics with respect to pH, salts, temperature and chromatography are described. A method for rapid screening of E. coli for omega mutants is described.