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The highly purified restriction endonucleases of E. coli K and coliphage P1 transfer methyl groups from S-adenosylmethionine to adenine residues of unmodified DNA. Incubation of unmodified DNA with endonucleases K or P and S-adenosylmethionine renders the DNA resistant to restriction. The enzymes, therefore, have both restriction endonuclease and modification methylase activities.Restriction endonucleases are strain-specific enzymes that enable bacteria to recognize and rapidly destroy DNA introduced from foreign strains (1). They make double-chain scissions at a limited number of specific sites on the DNA molecule, rendering it susceptible to further degradation by nonspecific nucleases. Cells possessing a restriction enzyme of given specificity are also furnished with a specific methylase that transfers methyl groups from S-adenosylmethionine (SAM) to specific adenine residues located at or near the sites where the restriction enzyme acts. By this means, cells are protected against their own restriction enzyme, for such methylation protects a site against cleavage. DNA introduced from any strain lacking the specific methylase of the host will be degraded.DNA made insensitive to restriction is said to have undergone modification. Methylases that perform this reaction are called modification methylases. Modification and restriction are highly specific, in the sense that the modification imparted to DNA by a given bacterial strain protects it only against the restriction endonuclease of the same strain. Several different restriction and modification specificities are known, and several of the enzymes involved have been isolated. The experiments reported here deal with the specificities designated K and P (2-8). These are determined by genes of Escherichia coli strain K and coliphage P1, respectively.We wish to present evidence that DNA restriction endonucleases K afid P are also modification methylases. This is shown by the ability of the purified enzymes selectively to transfer the methyl group from SAM to unmodified DNA and by the specific resistance the DNA thereby acquires to endonucleolytic attack by the same enzyme.DNA cleavage by restriction endonuclease K requires the presence of Mg++, ATP, and SAM. Cleavage by endonuclease P requires Mg++ and ATP and is stimulated by SAM. However, the methyl transferase reactions require only SAM. This makes it possible to study methylation and modification without the occurrence of the nucleolytic reaction. Ci/mmol, concentration checked by isotope dilution). To each 0.1 ml of reaction mixture were added 1010 phage units of unmodified or modified X DNA, and about 0.3 j.g of restriction endonuclease P from the glycerol gradient fraction (2). The tubes were incubated at 300 for the times indicated. X [32P]DNA (400 cpm) was added to each tube as a recovery marker, and 250 lig of calf-thymus DNA per reaction mixture was added as a carrier. The DNA was then precipitated in 5% trichloroacetic acid-10 mM sodium pyrophosphate-1 M NaCl at 00. After centrifugation at 6000...

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