Non-reducing polyketide synthase; part of the gene cluster that mediates the biosynthesis of the bicoumarin desertorin (PubMed:26389790). The non-reducing polyketide synthase desS first catalyzes the formation of the pentaketidic 4,7-dihydroxy-5-methylcoumarin from acetyl coenzyme A and 4 malonyl coenzyme A molecules (PubMed:26389790). Further O-methylation by desB leads to the formation of 7-demethylsiderin (PubMed:26389790). Then, an oxidative phenol coupling catalyzed by the cytochrome P450 monooxygenase desC forms the 6,8'-dimer M-desertorin A via dimerization the monomeric precursor, 7-demethylsiderin (PubMed:26389790). M-desertorin A is further converted to M-desertorin C (PubMed:26389790). Binds 1 phosphopantetheine covalently. Secondary metabolite biosynthesis. Multidomain protein; including a starter unit:ACP transacylase (SAT) that selects the starter unit; a ketosynthase (KS) that catalyzes repeated decarboxylative condensation to elongate the polyketide backbone; a malonyl-CoA:ACP transacylase (MAT) that selects and transfers the extender unit malonyl-CoA; a product template (PT) domain that controls the immediate cyclization regioselectivity of the reactive polyketide backbone; and an acyl-carrier protein (ACP) that serves as the tether of the growing and completed polyketide via its phosphopantetheinyl arm (By similarity).