Methyltransferase; part of the gene cluster that mediates the biosynthesis of HIV-1 integrase inhibitor equisetin and of fusarisetin A, both trans-fused decalin-containing tetramic acids showing also antimicrobial activity (PubMed:25770422). The PKS module of fsa1 together with the enoylreductase fsa3 catalyze the formation of the polyketide unit which is then conjugated to L-serine by the condensation domain of the fsa1 NRPS module (PubMed:25770422). Activity of the Dieckmann cyclase domain (RED) results in release of the Dieckmann product intermediate (PubMed:25770422). Diels-Alderase fsa2 is involved in endo-selective Diels-Alder cycloaddition to form the decalin ring, leading to the production of N-desmethylequisetin also called trichosetin (PubMed:25770422, PubMed:28401214, PubMed:29972614, PubMed:34121297). Subsequent N-methylation is carried out by fsa4 to give equisetin (PubMed:25770422). The enzymatic gene responsible for the conversion of equisetin to fusarisetin A has not been identified yet and is probably located outside of the fsa cluster (PubMed:28401214). Mycotoxin biosynthesis. Results in the loss of production of equisetin and fusarisetin A (PubMed:25770422). Belongs to the class I-like SAM-binding methyltransferase superfamily. Cation-independent O-methyltransferase family.