O-methyltransferase, part of the gene cluster that mediates the biosynthesis of melleolides, a range of antifungal and phytotoxic polyketide derivatives composed of an orsellinic acid (OA) moiety esterified to various sesquiterpene alcohols (Probable). The first step in melleolides biosynthesis is performed by the delta(6)-protoilludene synthase PRO1 which catalyzes the cyclization of farnesyl diphosphate to protoilludene (PubMed:21148562). The orsellinic acid synthase armB produces OA by condensing acetyl-CoA with 3 malonyl-CoA units in a three-round chain elongation reaction folowed by a C2-C7 ring closure (By similarity). ArmB further catalyzes the trans-esterification of OA to the various sesquiterpene alcohols resulting from the hydroxylation of protoilludene (By similarity). The melleolides cluster also includes 5 cytochrome P450 monooxygenases, 4 NAD(+)-dependent oxidoreductases, one flavin-dependent oxidoreductase, and one O-methyltransferase (By similarity). The cytochrome P450 monooxygenases may be involved in protoilludene hydroxylation to elaborate melleolides with multiple alcohol groups, such as melleolide D, which carries alcohol functionalities at C-4, C-5, C-10, and C-13 (By similarity). The role of the NAD(+)-dependent enzymes remains unknown (By similarity). Numerous melleolides, including arnamial, show 5'-O-methylation of the aromatic moiety which may be catalyzed by the methyltransferase encoded in the cluster (By similarity). The flavin-dependent oxidoreductase might represent the dehydrogenase yielding the aldehyde in position 1 of arnamial and other melleolides (By similarity). Finally, several halogenase localized outside of the cluster, are able to catalyze the transfer of a single chlorine atom to the melleolide backbone, resulting in a 6'-chloromelleolide product (By similarity). Secondary metabolite biosynthesis. Melleolide sesquiterpene aryl esters are cytotoxic secondary products with anti-cancer potential (PubMed:21376582, PubMed:26952552). Armillaridin shows therapeutic and radiosensitizing effects on human esophageal cancer cells (PubMed:23864890). Armillaridin induces autophagy-associated cell death in human chronic myelogenous leukemia as well as of hepatocellular carcinoma cells (PubMed:27592257, PubMed:31488037). Armillaridin can also inhibit the differentiation and activation of human macrophages and thus might have potential to be developed as a biological response modifier for inflammatory diseases (PubMed:25746621). Armillaria species are both devastating forest pathogens and some of the largest and oldest terrestrial organisms on Earth (PubMed:31746694) (Probable). They forage for hosts and achieve immense colony sizes via rhizomorphs, root-like multicellular structures of clonal dispersal (Probable). One genetic Armillaria gallica individual localized in Michigan's Upper Peninsula stands out as exceptionally large, covering hundreds of tree root systems over approximately 75 hectares of the forest floor (PubMed:30963893). Based on observed growth rates of the fungus, the minimum age of this large individual can be estimated as 2500 years (PubMed:30963893). Belongs to the class I-like SAM-binding methyltransferase superfamily. Cation-independent O-methyltransferase family.