The Deoxyribonucleic Acid Modification and Restriction Enzymes of Escherichia coli B

The restriction endonuclease of Escherichia coli B catalyzes a massive hydrolysis of ATP to ADP and Pi. The ATPase requires S-adenosylmethionine and DNA containing unmodified restriction sites. The apparent Km for fd replicative form DNA is 20 µm DNA nucleotide, and ATP is halfsaturating at 100 µm. Like the nuclease, the ATPase is inhibited strongly by S-adenosylethionine and 5'-methylthioadenosine, but only weakly by S-adenosylhomocysteine. The hydrolysis of ATP continues long after DNA degradation has ceased. Whereas no ATPase is observed when restricted DNA but no unmodified DNA is present in a reaction mixture, restricted DNA is required for the maintenance of ATPase once initiated. A hypothetical scheme is presented which involves the conversion of the enzyme during DNA hydrolysis from a form capable of nuclease activity to one catalyzing the breakdown of ATP.