Polyketide methyltransferase; part of the gene cluster that mediates the biosynthesis of sordarin and hypoxysordarin, glycoside antibiotics with a unique tetracyclic diterpene aglycone structure (PubMed:27072286). First, the geranylgeranyl diphosphate synthase sdnC constructs GGDP from farnesyl diphosphate and isopentenyl diphosphate (PubMed:27072286). The diterpene cyclase sdnA then catalyzes the cyclization of GGDP to afford cycloaraneosene (PubMed:27072286). Cycloaraneosene is then hydroxylated four times by the putative cytochrome P450 monooxygenases sdnB, sdnE, sdnF and sdnH to give a hydroxylated cycloaraneosene derivative such as cycloaraneosene-8,9,13,19-tetraol (PubMed:27072286). Although the order of the hydroxylations is unclear, at least C8, C9 and C13 of the cycloaraneosene skeleton are hydroxylated before the sordaricin formation (PubMed:27072286). Dehydration of the 13-hydroxy group of the hydroxylated cycloaraneosene derivative might be catalyzed by an unassigned hypothetical protein such as sdnG and sdnP to construct the cyclopentadiene moiety (PubMed:27072286). The FAD-dependent oxidoreductase sdnN is proposed to catalyze the oxidation at C9 of the hydroxylated cycloaraneosene derivative and also catalyze the Baeyer-Villiger oxidation to give the lactone intermediate (PubMed:27072286). The presumed lactone intermediate would be hydrolyzed to give an acrolein moiety and a carboxylate moiety (PubMed:27072286). Then, [4+2]cycloaddition would occur between the acrolein moiety and the cyclopentadiene moiety to give sordaricin (PubMed:27072286). SdnN might also be involved in the [4+2]cycloaddition after the hypothesized oxidation to accommodate the oxidized product and prompt the [4+2]cycloaddition (PubMed:27072286). GDP-6-deoxy-D-altrose may be biosynthesized from GDP-D-mannose by the putative GDP-mannose-4,6-dehydratase sdnI and the short-chain dehydrogenase sdnK (PubMed:27072286). The glycosyltransferase sdnJ catalyzes the attachment of 6-deoxy-D-altrose onto the 19-hydroxy group of sordaricin to give 4'-O-demethylsordarin (PubMed:27072286). The methyltransferase sdnD would complete the biosynthesis of sordarin (PubMed:27072286). Sordarin can be further modified into hypoxysordarin (PubMed:27072286). The unique acyl chain at the 3'-hydroxy group of hypoxysordarin would be constructed by an iterative type I PKS sdnO and the trans-acting polyketide methyltransferase sdnL. SdnL would be responsible for the introduction of an alpha-methyl group of the polyketide chain (PubMed:27072286). Alternatively, the beta-lactamase-like protein sdnR might be responsible for the cleavage and transfer of the polyketide chain from the PKS sdnO to sordarin (PubMed:27072286). Two putative cytochrome P450 monooxygenases, sdnQ and sdnT, might catalyze the epoxidations of the polyketide chain to complete the biosynthesis of hypoxysordarin (PubMed:27072286). Transcriptional regulators sdnM and sdnS are presumably encoded for the transcriptional regulation of the expression of the sdn gene cluster (PubMed:27072286).