Involved in the biosynthesis of the tetracycline antibiotic, oxytetracycline. Catalyzes the dimethylation of 4-amino-4-de(dimethylamino)anhydrotetracycline (4-amino-ATC) to yield anhydrotetracycline (ATC) (PubMed:1134373, PubMed:18422316). Also able to catalyze the dimethylation of 7-chloro-, 6-demethyl-, 2-decarboxamido-2-nitrile-, and 4-methylamino-derivatives of 4-amino-4-de(dimethylamino)anhydrotetracycline (PubMed:1134373). 4-amino-4-dedimethylamino-anhydrotetracycline + S-adenosyl-L-methionine = 4-methylamino-4-dedimethylamino-anhydrotetracycline + H(+) + S-adenosyl-L-homocysteine 4-methylamino-4-dedimethylamino-anhydrotetracycline + S-adenosyl-L-methionine = anhydrotetracycline + H(+) + S-adenosyl-L-homocysteine Antibiotic biosynthesis; oxytetracycline biosynthesis. Cells lacking this gene are unable to produce the tetracycline intermediate anhydrotetracycline (ATC). Belongs to the class I-like SAM-binding methyltransferase superfamily. Cation-independent O-methyltransferase family.