Highly reducing polyketide synthase; part of the gene cluster that mediates the biosynthesis of asperlin, a polyketide showing anti-inflammatory, antitumor and antibiotic activities (PubMed:30339758). The first step of the asperlin biosynthesis is the production of the intermediate 2,4,6-octatrienoic acid by the highly redusing polyketide synthase alnA with cleavage of the PKS product by the esterase alnB (PubMed:30339758). 2,4,6-octatrienoic acid is further converted to asperlin via several steps involving the remaining enzymes from the cluster (PubMed:30339758). Polyketide biosynthesis. Expression is controlled by the asperlin biosynthesis cluster-specific transcription factor alnR. 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 dehydratase (DH) domain that reduces hydroxyl groups to enoyl groups; an enoylreductase (ER) domain that reduces enoyl groups to alkyl groups; a ketoreductase (KR) domain that catalyzes beta-ketoreduction steps; and an acyl-carrier protein (ACP) that serves as the tether of the growing and completed polyketide via its phosphopantetheinyl arm. Fully eliminates the production of asperlin. 2,4,6-octatrienoic acid is a particularly interesting compound because of its potential as a natural tanning agent for human skin. It is a promoter of melanogenesis and antioxidant defense in melanocytes in vitro and in vivo, topically and systemically. Pigmentation is a powerful protectant from UV damage and skin cancer, and octatrienoic acid is an attractive natural photoprotectant.