Methyltransferase that methylates adenine residues in the ssDNA and dsDNA sequence 5'-GATC-3' (PubMed:10581668). May prevent degradation of viral DNA by the host restriction-modification antiviral defense system (Probable). a 2'-deoxyadenosine in DNA + S-adenosyl-L-methionine = an N(6)-methyl-2'-deoxyadenosine in DNA + H(+) + S-adenosyl-L-homocysteine Belongs to the N(4)/N(6)-methyltransferase family.