Polyketide synthase; part of the gene cluster that mediates the biosynthesis of cercosporin, a light-activated, non-host-selective toxin (By similarity). The perylenequinone chromophore of cercosporin absorbs light energy to attain an electronically-activated triplet state and produces active oxygen species such as the hydroxyl radical, superoxide, hydrogen peroxide or singlet oxygen upon reaction with oxygen molecules (PubMed:11701851). These reactive oxygen species cause damage to various cellular components including lipids, proteins and nucleic acids (PubMed:11701851). The first step of cercosporin biosynthesis is performed by the polyketide synthase CTB1 which catalyzes the formation of nor-toralactone (By similarity). The starter unit acyltransferase (SAT) domain of CTB1 initiates polyketide extension by the selective utilization of acetyl-CoA, which is elongated to the heptaketide in the beta-ketoacyl synthase (KS) domain by successive condensations with six malonyl units introduced by the malonyl acyltransferase (MAT) domain. The product template (PT) domain catalyzes C4-C9 and C2-C11 aldol cyclizations and dehydrations to a trihydroxynaphthalene, which is thought to be delivered to the thioesterase (TE) domain for product release (By similarity). The bifunctional enzyme CTB3 then methylates nor-toralactone to toralactone before conducting an unusual oxidative aromatic ring opening (By similarity). The O-methyltransferase CTB2 further methylates the nascent OH-6 of the CBT3 product, blocking further oxidation at this site before the reductase CTB6 reduces the 2-oxopropyl ketone at position C7, giving naphthalene (By similarity). The FAD-dependent monooxygenase CTB5 in concert with the multicopper oxidase CTB12 are responsible for homodimerization of naphthalene with CTB7 installing the dioxepine moiety, finally producing cercosporin (By similarity). The fasciclin domain-containing protein CTB11 might act with CTB5 and CTB12 whereas the roles of CTB9 and CTB10 have still to be elucidated (By similarity). acetyl-CoA + 6 H(+) + 6 malonyl-CoA = 6 CO2 + 7 CoA + 2 H2O + nor-toralactone Binds 1 phosphopantetheine covalently. Mycotoxin 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; 2 acyl-carrier protein (ACP) domains that serve as the tether of the growing and completed polyketide via its phosphopantetheinyl arm; and a C-terminal thioesterase (TE) domain that facilitates the release of the final product from the enzyme.