The Deoxyribonucleic Acid Modification and Restriction Enzymes of Escherichia coli B

The modification methylase of Escherichia coli B has been purified to apparent homogeneity. The enzyme can exist in several forms, each possessing two nonidentical polypeptides, β and γ, of molecular weights of 60,000 and 55,000, respectively. Freshly isolated enzyme has the structure β1γ1, but upon storage at neutral pH and low salt, it disproportionates, producing another form, β3γ1. Treatment at pH 5 converts the mixture of β3γ1 and β1γ1 to a mixture of β1γ1 and another form β2γ1, whereas exposure to high salt breaks down the β3γ1 structure. The enzyme is inhibited by S-adenosylethionine and 5′-methylthioadenosine, but not by S-adenosylhomocysteine. None of these compounds can replace the required cofactor, S-adenosylmethionine. The enzyme can methylate a wide variety of DNAs, but not DNA produced by E. coli B.