The N-terminal domain enhances the formation of an early, partially cyclized intermediate, while the C-terminal domain catalyzes the 3-O-methylation of one or more later intermediates in the biosynthetic pathway. Catalyzes the methylation of tetracenomycin D3 (Tcm D3) to yield Tcm B3. Catalyzes as well the following side reactions: methylation of 8-O-methyl-Tcm D3 to yield Tcm E; and of 9-carboxymethyl-Tcm B3 to yield Tcm A2. Antibiotic biosynthesis; tetracenomycin C biosynthesis. This multifunction enzyme is comprised of two structurally and functionally independent domains. Belongs to the class I-like SAM-binding methyltransferase superfamily. Cation-independent O-methyltransferase family.