Enzymatic Control of the Related Pathways of Fatty Acid and Undecylprodiginine Biosynthesis in Streptomyces coelicolor

Streptomyces coelicolor produces fatty acids for both primary metabolism and for production of the components of natural products such as undecylprodiginine. Primary metabolism makes the longer and predominantly branched-chain fatty acids, while undecylprodiginine utilizes shorter and almost exclusively straight chain fatty acids. The first step in fatty acid biosynthetic process is catalyzed by FabH (β-ketoacyl synthase III), which catalyzes a decarboxylative condensation of an acyl-CoA primer with malonyl-acyl carrier protein (ACP). The resulting 3-ketoacyl-ACP product is reduced by NADPH-dependent FabG into 3-hydroxyacyl-ACP, which is dehydrated by FabA to form enoyl-ACP. The NADH-dependent FabI (InhA) completes the cycle. Subsequent rounds of elongations in the pathways are catalyzed by the condensing enzyme FabF. For undecylprodiginine biosynthesis in S. coelicolor, homologues of the condensing enzymes (FabH and FabF) and the ACP (FabC) are encoded by redP, redR and redQ respectively in the red gene cluster. The genes encoding 3-ketoacyl-ACP reductase (FabG), 3-hydroxyacyl-ACP dehydratase (FabA), and enoyl-ACP reductase (FabI), are putatively shared between fatty acid and undecylprodigine biosynthesis, since the corresponding genes are not present within the red gene cluster of S. coelicolor. RedP is proposed to initiate biosynthesis of undecylprodiginine alkane chain by condensing an acetyl-CoA with a malonyl-RedQ, in contrast to FabH which process a broad range of acyl-CoA with malonyl-FabC. The 3-keto group of the resulting 3-ketoacyl-RedQ is then reduced to provide butyryl-RedQ, presumably by the type II FAS enzymes FabG, FabA and FabI. These enzymes would not differentiate between straight and branched-chain substrates, and have equal preference for FabC and RedQ ACPs. RedR would then catalyze four subsequent elongation steps with malonyl-RedQ, with appropriate 3-keto group processing after each step. The proposed role and substrate specificities of condensing enzymes RedP and FabH have not been investigated in S. coelicolor. The genes encoding FabG, FabA, and FabI have not been characterized in Streptomyces. Analysis of the S. coelicolor genome sequence has revealed the presence of one fabI gene (SCO1814, encoding an enoyl-ACP reductase), and three likely fabG genes (SCO1815, SCO1345, and SCO1346, encoding β-ketoacyl-ACP reductase). In the current study the substrates specificities of both RedP and FabH were determined from assays using pairings of two acyl-CoA substrates (acetyl-CoA and isobutyryl-CoA) and two malonyl-ACP substrates (malonyl-RedQ and malonyl-FabC) (FabC is a dedicated ACP for fatty acid biosynthesis and RedQ for undecylprodiginine biosynthesis in S. coelicolor). For RedP, activity was only observed with a pairing of acetyl-CoA and malonyl-RedQ. No activity was observed