Digestion of Methylated DNA

DNA methylation is the process of transferring a methyl group from a donor molecule to either a cytosine or an adenine by DNA methyltransferases. Such methylation is the most common and abundant DNA modification process in living organisms. Three types of methylated bases are predominantly found in DNA:

• 5-methylcytosine (m5C)
• N4-methylcytosine (m4C)
• N6-methyladenine (m6A)

Other modified bases, such as 5-hydroxymethylcytosine (hm5C) and 5-hydroxymethyluracil (hm5U), have also been described. The organism-specific pattern of methylation depends on the methyltransferases' specificity.

In prokaryotes, DNA cleavage by a cognate restriction enzyme is prevented by the methylation of DNA by a sequence-specific methyltransferase which is an integral component of every restriction-modification (R-M) system (1, 2).

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9. Bird, A.P., et al., Methylated and unmethylated DNA compartments in the sea urchin genome, Cell, 17, 889-902, 1979.
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12. Butkus, V. et al., Investigation of restriction-modification enzymes from M.varians RFL19 with a new type of specificity toward modification of substrate, Nucleic Acids Res., 13, 5727-5746, 1985.