Atrochrysone carboxylic acid synthase; part of the gene cluster that mediates the biosynthesis of monodictyphenone, a prenyl xanthone derivative (PubMed:20139316, PubMed:21351751). The pathway begins with the synthesis of atrochrysone thioester by the polyketide synthase (PKS) mdpG (PubMed:20139316). The atrochrysone carboxyl ACP thioesterase mdpF then breaks the thioester bond and releases the atrochrysone carboxylic acid from mdpG (PubMed:20139316). The atrochrysone carboxylic acid is then converted to atrochrysone which is further transformed into emodin anthrone (PubMed:20139316). The next step is performed by the anthrone oxygenase mdpH that catalyzes the oxidation of emodinanthrone to emodin (By similarity). Emodin is further modified to yield monodictyphenone via several steps involving mdpB, mdpC mdpJ, mdpK and mdpL (PubMed:20139316, PubMed:21351751, PubMed:22909031). The short chain dehydrogenase mdpC converts the tautomers of emodin hydroquinone into the 3-hydroxy-3,4-dihydroan-thracen-1(2H)-one derivative (PubMed:22909031, PubMed:26266881). These enzymes with xptA, xptB and xptC are also proposed to be involved in the synthesis of shamixanthone from emodin (PubMed:22730213). Especially, direct reduction of emodin by the short chain dehydrogenase mdpC followed by dehydration catalyzed by the scytalone dehydratase-like protein mdpB gives loss of oxygen and formation of chrysophanol intermediate in two simple steps (PubMed:22730213). 8 H(+) + holo-[ACP] + 8 malonyl-CoA = atrochrysone carboxyl-[ACP] + 8 CO2 + 8 CoA + 2 H2O 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). Impairs the production of monodictyphenone and any of the intermediates of the pathway (PubMed:20139316, PubMed:21351751).