Hybrid PKS-NRPS synthetase; part of the gene cluster that mediates the biosynthesis of pyranterreones, a family of antioxidative compounds. The first step of pyranonigrins biosynthesis is performed by the hybrid PKS-NRPS synthetase pytA that condenses 4 malonyl-CoA units ato the acetyl starter unit by the modular PKS of pytA. The acyl chain is then connected to an L-serine through the amide bond by the modular NRPS of pytA. A tetramic acid is formed and released from the PKS-NRPS pytA to give pyranterreone 5 with the help of the thioesterase pytI. Pyranterreone 5 could be methylated by pytC to afford pyranterreone 6 (Probable). Both pyranterreones 5 and 6 are subsequently oxidized by the FAD-linked oxidoreductase pytB and the cytochrome P450 monooxygenase pytD to form the fused gamma-pyrone core, resulting in pyranterreones 7 and 11, respectively. The hydroxy group at C-8 of pyranterreones 7 and 11 are dehydrated by the aspartyl protease pytH to form a delta-7 double bond to give pyranterreones 3 and 1, 2 accordingly. The exo-methylene of pyranterreone 3 could be reduced into a pendant methyl by reductase pytE to provide pyranterreone 4, also known as cordylactam (Probable). Pyranterreone 4 can be reconverted to pyranterreone 3 through pytB-catalyzed dehydrogenation or further oxidized to pyranterreones 9 and 10 (Probable)