Polyketide synthase; part of the gene cluster that mediates the biosynthesis of cercosporin, a light-activated, non-host-selective toxin (PubMed:12937958, PubMed:15915645, PubMed:26938470, PubMed:29610486). 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 (PubMed:23108075, PubMed:26938470, PubMed:29610486). 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 (PubMed:23108075, PubMed:29610486). The bifunctional enzyme CTB3 then methylates nor-toralactone to toralactone before conducting an unusual oxidative aromatic ring opening (PubMed:17074519, PubMed:26938470). 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 (PubMed:17660442, PubMed:26938470). 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 (PubMed:17660442, PubMed:30809363, PubMed:26938470). 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. Expression is positively regulated by the cercosporin cluster-specific transcription factor CTB8 (PubMed:17462021). Expression is also affected by nitrogen and carbon sources and pH, and is also controlled by another transcription activator, CRG1, previously shown to regulate cercosporin production and resistance (PubMed:17462021). 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. Abolishes the production of cercosporin (PubMed:12937958, PubMed:15915645, PubMed:26938470). Does not display any pigmentation (PubMed:26938470). Leads to fewer necrotic lesions on inoculated tobacco leaves compared with the wild type (PubMed:15915645).