InterPro: IPR003356 DNA methylase, adenine-specific

This domain is fpound in N-6 adenine-specific DNA methylase (EC:2.1.1.72) from Type I and Type IC restriction systems. These enzymes are responsible for the methylation of specific DNA sequences in order to prevent the host from digesting its own genome via its restriction enzymes. These methylases have the same sequence specificity as their corresponding restriction enzymes. The type I restriction and modification system is composed of three polypeptides R, M and S. The M and S subunits together form a methyltransferase that methylates two adenine residues in complementary strands of a bipartite DNA recognition sequence. In the presence of the R subunit, the complex can also act as an endonuclease, binding to the same target sequence but cutting the DNA some distance from this site. Whether the DNA is cut or modified depends on the methylation state of the target sequence. When the target site is unmodified, the DNA is cut. When the target site is hemimethylated, the complex acts as a maintenance methyltransferase, modifying the DNA so that both strands become methylated.