Most laboratory strains of E. coli contain three site-specific DNA
methylases.
• The methylase encoded by the dam gene (Dam methylase) transfers a
methyl group from S-adenosylmethionine (SAM) to the N0 position of
the adenine residues in the sequence GATC.
• The Dcm methylase (encoded by the dcm gene) methylates the
internal (second) cytosine residues in the sequences CCAGG and
CCTGG at the C0 position.
• The EcoKI methylase modifies adenine residues in the sequences
AAC(N0)GTGC and GCAC(N0)GTT.
• EcoKI sites (~0 site per 0 kb) are much less common than Dam sites (~0
site per 000 bp) or Dcm sites (~0 site per 000 bp) in DNA of random
sequence (GC=AT).