Polyketide synthase that produces (R)-mellein, a secondary metabolite that inhibits the germination of wheat (Triticum aestivum) and barrel medic (Medicago truncatula) seeds (PubMed:25326302). Condensates 1 acetate starter unit and 4 extender malonate units (PubMed:25326302). The nascent pentaketide intermediate then undergoes an aldol cyclization and is aromatized via dehydration (PubMed:25326302). The (R)-O-methylmellein isolated from P.nodorum is most likely to be derived from (R)-mellein via an additional methylation at the hydroxyl group. Interestingly, no O-methyltransferase gene is encoded in the vicinity of MLNS on the chromosome. Thus, the O-methylation is likely to be catalyzed by an endogenous O-methyltransferase encoded elsewhere in the genome of P.nodorum (PubMed:25326302). Secondary metabolite biosynthesis. Expression is highly induced during plant infection. Multidomain protein; including 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 dehydrogenase (DH) domain that reduces hydroxyl groups to enoyl groups, a beta-ketoreductase domain (KR) that reduces beta-ketone groups to hydroxyl groups; and an acyl-carrier protein (ACP) that serves as the tether of the growing and completed polyketide via its phosphopantetheinyl arm. Does not induce any observable growth defect but impairs the production of both mellein and O-methylmellein. Truncated N-terminus.