Fungal polyketide, known with structural deversity and various biological properties, are synthesized by iteractive type I polyketide synthase (PKS). We focused on the PKS which synthesizes emotion, ATEG_08451 in a filamentous fungi, Aspergillus terreus, here named "atrochrysone carboxylic acid synthase (ACAS)", is a non-reducing iterative type I PKS (NR-PKS) that does not contain Claisen cyclase/thioesterase domain (CLC/TE) domain. Because a CLC/TE domain is responsible for product release from NR-PKS, ACAS was predicted to have novel mechanism for product release. In vitro, reactions of ACAS wigh malonyl-CoA yielded a polyketide intermediate, probably attached to its acyl carrier protein (ACP). The addition of ATEG_08450, here named "Atrochrysone carboxyl ACP thioesterase" (ACTE), to the reaction resulted in the release of products derived from atrochrysone carboxylic acid, such as atrochrysone, endocrocin anthrone, endocrocin, emodin anthrone and emodin. Atrochrysone carboxyliic acid decarboxylates to yield atrochrysone, and dehydrates to yield indocrocin anthrone. Atrochrysone dehydrates to yield emodin anthrone. Endocrocin anthrone and emodin anthrone auto-oxidize to form endocrocin and emodin. ACTE, belonging to the β-Lactamase superfamily, thus appears to be a novel type of thioesterase responsible for product release in polyketide biosynthesis. These findings show that ACAS synthesizes the scaffold of atrochrysone carboxylic acid from malonyl-CoA and that ACTE hydrolyzes the thioester bond between the ACP of ACAS and the intermediate to release atrochrysone carboxylic acid as the reaction product.
