Genes encoding modular polyketide synthase

Fig. 2a-d. The major architectural paridigms of fatty acid and polyketide synthases. Relationships between genes encoding a bacterial aromatic polyketide synthases, b eukaryotic fatty acid synthases and fungal polyketide synthases, c modular polyketide synthases, and d plant polyketide synthases... 

Since the PKS (polyketide synthase) gene cluster for actinorhodin (act), an antibiotic produced by Streptomyces coelicolor[ 109], was cloned, more than 20 different gene clusters encoding polyketide biosynthetic enzymes have been isolated from various organisms, mostly actinomycetes, and characterized [98, 100]. Bacterial PKSs are classified into two broad types based on gene organization and biosynthetic mechanisms [98, 100, 102]. In modular PKSs (or type I), discrete multifunctional enzymes control the sequential addition of thioester units and their subsequent modification to produce macrocyclic compounds (or complex polyketides). Type I PKSs are exemplified by 6-deoxyerythronolide B synthase (DEBS), which catalyzes the formation of the macrolactone portion of erythromycin A, an antibiotic produced by Saccharopolyspora erythraea. There are 7 different active-site domains in DEBS, but a given module contains only 3 to 6 active sites. Three domains, acyl carrier protein (ACP), acyltransferase (AT), and P-ketoacyl-ACP synthase (KS), constitute a minimum module. Some modules contain additional domains for reduction of p-carbons, e.g., P-ketoacyl-ACP reductase (KR), dehydratase (DH), and enoyl reductase (ER). The thioesterase-cyclase (TE) protein is present only at the end of module 6. 

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