The role of S-adenosylmethionine in the cleavage of deoxyribonucleic acid by the restriction endonuclease from Escherichia coli K.

The restriction endonuclease from Escherichia coli K specifically cleaves foreign DNA in the presence of S-adenosylmethionine, ATP, and Mg2+. The role of S-adenosylmethionine in this reaction has been studied by following the specific binding of the enzyme to unmodified DNA. The results indicate that S-adenosylmethionine acts as an allosteric effector. However, the rate-limiting step in the activation of the enzyme is not the binding of the effector itself, but an event subsequent to it. The interaction of the S-adenosylmethionine with two mutant K restriction endonucleases isolated previously has also been investigated. One of them, which is defective in restriction, can be activated in a manner similar to the wild type enzyme, while the other one, which lacks both restriction and modification activities (due to a mutation in the subunit responsible for DNA recognition), shows no such effect.