Methyltransferase; part of the gene cluster that mediates the biosynthesis of equisetin, a trans-fused decalin-containing tetramic acid with antimicrobial activity (PubMed:23614392). The PKS module of eqxS together with the enoylreductase eqxC catalyze the formation of the polyketide unit which is then conjugated to L-serine by the condensation domain of the eqxS NRPS module (PubMed:23614392). Activity of the Dieckmann cyclase domain (RED) results in release of the Dieckmann product intermediate (PubMed:23614392, PubMed:18652469). Diels-Alderase eqx3 is involved in endo-selective Diels-Alder cycloaddition to form the decalin ring, leading to the production of N-desmethylequisetin also called trichosetin (By similarity). Subsequent N-methylation is carried out by eqxD to give equisetin (PubMed:23614392). S-adenosyl-L-methionine + trichosetin = equisetin + H(+) + S-adenosyl-L-homocysteine Mycotoxin biosynthesis. Abolishes the production of equisetin and accumulates the intermediate trichosetin (PubMed:23614392). Belongs to the class I-like SAM-binding methyltransferase superfamily. Cation-independent O-methyltransferase family.