Highly reducing polyketide synthase; part of the pkh gene cluster that mediates the biosynthesis of 2,4-dihydroxy-6-[(3E,5E,7E)-2-oxonona-3,5,7-trienyl]benzaldehyde (PubMed:22510154). The highly reducing polyketide synthase pkhB first produces the (2E,4E,6E)-octa-2,4,6-trienyl strater unit for the non-reducing polyketide synthase pkhA (PubMed:22510154). This octatrienoyl starter is then loaded onto the SAT domain of the NR-PKS pkhA to be condensed with 4 malonyl-CoA units to yield 2,4-dihydroxy-6-[(3E,5E,7E)-2-oxonona-3,5,7-trienyl]benzaldehyde (PubMed:22510154). Binds 1 phosphopantetheine covalently. Secondary metabolite biosynthesis. 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; a methyltransferase (CMeT) domain responsible for the incorporation of methyl groups; an enoylreductase (ER) domain that reduces enoyl groups to alkyl group; 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.