Purification and properties of a new restriction endonuclease from Haemophilus influenzae Rf

Haemophilus influenzae Rf 232, showing the phenomena of restriction and modification, contains an endonuclease that inactivates in vitro the biological activity of DNAs lacking the strain‐specific modification. This specific restriction endonuclease has been purified to near homogeneity by a procedure that includes DNA‐agarose chromatography. This highly purified enzyme requires ATP and Mg2+ for activity and is stimulated by S‐adenosylmethionine. The enzyme seems to cleave DNA at well‐defined sites, since it produces a specific pattern of bands upon agarose gel electrophoresis. The enzyme has no ATPase activity. A methylase activity is observed in the course of the endonucleolytic reaction, which probably protects some of the DNA sites from cleavage.