S-adenosyl-L-methionine-dependent methyltransferase involved in the biosynthesis of desosamine, found in certain macrolide antibiotics such as erthyromycin, azithromycin, clarithromycin, and methymycin. Catalyzes the last step in the biosynthesis of dTDP-desosamine, i.e. the N,N-dimethylation of the 3-amino group of dTDP-3-amino-3,4,6-trideoxy-alpha-D-glucose. dTDP-3-amino-3,4,6-trideoxy-alpha-D-glucose + 2 S-adenosyl-L-methionine = dTDP-alpha-D-desosamine + 2 H(+) + 2 S-adenosyl-L-homocysteine kcat is 92 min(-1) with dTDP-3-amino-3,4,6-trideoxy-alpha-D-glucopyranose as substrate. kcat is 4.2 min(-1) with dTDP-3-amino-3,6-dideoxy-alpha-D-glucopyranose as substrate. Antibiotic biosynthesis. Homodimer. Belongs to the methyltransferase TylM1/DesVI family.