Methyltransferase that methylates adenine residues in the ssDNA and dsDNA sequence 5'-AGACC-3' (PubMed:14715260, PubMed:21048863). Essential for genome packaging because methylation within the pac site makes the latter cleavable (PubMed:2236019). 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 Presence of Mg(2+) enhances the ssDNA binding ability of the enzyme. Optimum pH is 8.0. Homodimer. Belongs to the class I-like SAM-binding methyltransferase superfamily. C5-methyltransferase family.