A Single Module Type I Polyketide Synthase Directs de Novo Macrolactone Biogenesis during Galbonolide Biosynthesis in Streptomyces galbus |
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Authors: | Hyun-Ju Kim Suman Karki So-Yeon Kwon Si-Hyung Park Baek-Hie Nahm Yeon-Ki Kim Hyung-Jin Kwon |
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Affiliation: | From the ‡Department of Biological Science, Myongji University, Yongin 449-728.;the §Department of Oriental Medicine Resources, Mokpo National University, Muan 534-729, and ;¶GreenGene BioTech Inc., Yongin 449-728, Republic of Korea |
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Abstract: | Galbonolide (GAL) A and B are antifungal macrolactone polyketides produced by Streptomyces galbus. During their polyketide chain assembly, GAL-A and -B incorporate methoxymalonate and methylmalonate, respectively, in the fourth chain extension step. The methoxymalonyl-acyl carrier protein biosynthesis locus (galG to K) is specifically involved in GAL-A biosynthesis, and this locus is neighbored by a gene cluster composed of galA-E. GalA-C constitute a single module, highly reducing type I polyketide synthase (PKS). GalD and GalE are cytochrome P450 and Rieske domain protein, respectively. Gene knock-out experiments verified that galB, -C, and -D are essential for GAL biosynthesis. A galD mutant accumulated a GAL-C that lacked two hydroxyl groups and a double bond when compared with GAL-B. A [U-13C]propionate feeding experiment indicated that no rare precursor other than methoxymalonate was incorporated during GAL biogenesis. A search of the S. galbus genome for a modular type I PKS system, the type that was expected to direct GAL biosynthesis, resulted in the identification of only one modular type I PKS gene cluster. Homology analysis indicated that this PKS gene cluster is the locus for vicenistatin biosynthesis. This cluster was previously reported in Streptomyces halstedii. A gene deletion of the vinP2 ortholog clearly demonstrated that this modular type I PKS system is not involved in GAL biosynthesis. Therefore, we propose that GalA-C direct macrolactone polyketide formation for GAL. Our studies provide a glimpse into a novel biochemical strategy used for polyketide synthesis; that is, the iterative assembly of propionates with highly programmed β-keto group modifications. |
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Keywords: | Actinobacteria Bacterial Genetics Gene Knockout Natural Product Biosynthesis Polyketide Streptomyces galbus Galbonolide Highly Reducing Type I Polyketide Synthase |
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