Purification and properties of the P15 specific restriction endonuclease from Escherichia coli

The specific restriction endonuclease of the Escherichia coli plasmid, P15, has been purified to apparent homogeneity by a procedure that includes DNA-cellulose chromatography as well as a new endonuclease assay. Sedimentation on glycerol gradients showed two peaks of activity with values of 11.3 S and 15.7 S. The highly purified enzyme requires ATP and Mg2+ for activity and is stimulated by S-adenosylmethionine. A methylase activity is observed in the course of the endonucleolytic reaction which protects some of the DNA sites from cleavage.