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Type-II Restriction and Modification Enzymes

Paul Modrich, Richard J. Roberts


DNA restriction endonucleases and their companion modification methylases have been identified in a wide variety of prokaryotes (Roberts 1982). Three distinct classes of restriction-modification systems have been found that differ in their cofactor requirements. The type-I and type-III enzymes have rather specific cofactor requirements and have been studied in detail, both biochemically and genetically (see Chapter 5). Although they recognize specific sequences on DNA, they cleave randomly at sites remote from the recognition sequence and so do not produce specific fragments. In contrast, the type-II restriction enzymes require only Mg2+ as a cofactor and they both recognize and cleave a specific sequence within the DNA molecule. It is this property that has rendered these enzymes so useful to the molecular biologists. Recombinant DNA technology and the present rapid methods for DNA sequence analysis are two important areas that have become possible because of the existence of these enzymes.

As with their type-I and type-III counterparts, the biological phenomenon of restriction and modification can be catalyzed by the type-II enzymes. However, it is important to note that for the vast majority of these enzymes there is no genetic evidence showing that they are involved in host-controlled restriction-modification, and it is quite possible that some of them have other, as yet undiscovered, roles in the cell. Whereas interest in the type-I and type-III enzymes centers around their biological properties, this is not true of the type-II enzymes. In this case, interest has focused on their use as tools with which to...

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