A Restriction enzyme from Hemophilus influenzae

Extracts of Hemophilus influenzae strain Rd contain an endonuclease activity which produces a rapid decrease in the specific viscosity of a variety of foreign native DNA's; the specific viscosity of H. influenzae DNA is not altered under the same conditions. This “restriction” endonuclease activity has been purified approximately 200-fold. The purified enzyme contains no detectable exo- or endonucleolytic activity against H. influenzae DNA. However, with native phage T7 DNA as substrate, it produces about 40 double-strand 5′-phosphoryl, 3′-hydroxyl cleavages. The limit product has an average length of about 1000 nucleotide pairs and contains no single-strand breaks. The enzyme is inactive on denatured DNA and it requires no special co-factors other than magnesium ions.