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). (2E,4E,6E)-octa-2,4,6-trienyl-[ACP] + AH2 + 3 H(+) + 4 malonyl-CoA = 2,4-dihydroxy-6-[(3E,5E,7E)-2-oxonona-3,5,7-trienyl]benzaldehyde + A + 4 CO2 + 4 CoA + H2O + holo-[ACP] Binds 1 phosphopantetheine covalently. Secondary metabolite biosynthesis. Multidomain protein; including an N-terminal starter unit:ACP transacylase (SAT) domain, a beta-ketoacyl synthase (KS) domain, a malonyl-CoA:ACP transacylase (MAT) domain, a product template (PT) domain that controls the immediate cyclization regioselectivity of the reactive polyketide backbone, an acyl carrier protein (ACP) domain, and a reductive NADPH-binding domain that is required for NADPH-dependent product release.