A complex multienzyme system encoded by five polyketide synthase genes is involved in the biosynthesis of the 26-membered polyene macrolide pimaricinin Streptomyces natalensis

Background: Polyene macrolides are a class of large macrocyclic polyketides that interact with membrane sterols, having antibiotic activity against fu ngi but not bacteria. Their rings include a chromophore of 3-7 conjugated d ouble bonds which constitute the distinct polyene structure. Pimaricin is a n archetype polyene, important in the food industry as a preservative to pr event mould contamination of foods, produced by Streptomyces natalensis. We set out to clone, sequence and analyse the gene cluster responsible for th e biosynthesis of this tetraene. Results: A large cluster of 16 open reading frames spanning 84 985 bp of th e S. natalensis genome has been sequenced and found to encode 13 homologous sets of enzyme activities (modules) of a polyketide synthase (PKS) distrib uted within five giant multienzyme proteins (PIMS0-PIMS4). The total of 60 constituent active sites, 25 of them on a single enzyme (PIMS2), make this an exceptional multienzyme system. Eleven additional genes appear to govern modification of the polyketide-derived framework and export. Disruption of the genes encoding the PKS abolished pimaricin production. Conclusions: The overall architecture of the PKS gene cluster responsible f or the biosynthesis of the 26-membered polyene macrolide pimaricin has been determined. Eleven additional tailoring genes have been cloned and analyse d. The availability of the PKS cluster will facilitate the generation of de signer pimaricins by combinatorial biosynthesis approaches. This work repre sents the extensive description of a second polyene macrolide biosynthetic gene cluster after the one for the antifungal nystatin.