EFFECT OF DIFFERENTIAL METHYLATION BY ESCHERICHIA-COLI OF PLASMID DNA AND PHAGE-T7 AND LAMBDA-DNA ON THE CLEAVAGE BY RESTRICTION ENDONUCLEASE-MBOI FROM MORAXELLA-BOVIS

The nucleotide sequence recognized and cleaved by the restriction endonuclease MboI is 5′↓ GATC and is identical to the central tetranucleotide of the restriction sites of BamHI and BglII. Experiments on the restriction of DNA from Escherichia coli dam and dam+ confirm the notion that GATC sequences are adenosyl-methylated by the dam function of E. coli and thereby are made refractory to cleavage by MboI. On the basis of this observation the degree of dam methylation of various DNAs was examined by cleavage with MboI and other restriction endonucleases. In plasmid DNA essentially all of the GATC sequences are methylated by the dam function. The DNA of phage λ is only partially methylated. extended methylation is observed in the DNA of a substitution mutant of λ, λ gal8bio256, and in the λ derived plasmid, λdv93, which is completely methylated. In contrast, phage T7 DNA is not methylated by dam. A suppression of dam methylation of T7 DNA appears to act only in cis since plasmid DNA replicated in a T7-infected cell is completely methylated. The results are discussed with respect to the participation of the dam methylase in different replication systems. © 1979.