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This polyketide synthase, characterized from the bacteria Streptomyces orinoci and Streptomyces spectabilis, generates the backbone of the antibiotic spectinabilin. It is composed of 6 modules that total 28 domains and is encoded by four genes. The enzyme accepts the unusual starter unit 4-nitrobenzoyl-CoA and extends it by 6 molecules of (S)-methylmalonyl-CoA and a single molecule of malonyl-CoA. The first module (encoded by norA) is used twice in an iterative fashion, so that the seven Claisen condensation reactions are catalysed by only six modules. The iteration becomes possible by the transfer of the [acp]-bound polyketide intermediate back to the ketosynthase (KS) domain on the opposite polyketide synthase strand (polyketides are homodimeric). |
