Novel features in a combined polyketide synthase/non-ribosomal peptide synthetase: the myxalamid biosynthetic gene cluster of the myxobacterium Stigmatella aurantiaca Sga15

Background: Myxobacteria have been well established as a potent sourer: for natural products with biological activity. They produce a considerable var iety of compounds which represent typical polyketide structures with incorp orated amino acids (e.g. the epothilons. the myxothiazols and the myxalamid s). Several of these secondary metabolites are effective inhibitors of the electron transport via the respiratory chain and have been widely used. Mol ecular cloning and characterization of the genes governing the biosynthesis of these structures is of considerable interest, because such information adds to the limited knowledge as to how polyketide synthases (PKSs) and non -ribosomal peptide synthetases (NRPSs) interact and how they might be manip ulated in order to form novel antibiotics. Results: A DNA region of approximately 50 000 base pairs from Stigmatella a urantiaca Sga15 was sequenced and shown by gene disruption to be involved i n myxalamid biosynthesis. Sequence analysis reveals that the myxalamids are formed by a combined PKS/NRPS system. The terminal NRPS MxaA extends the a ssembled polyketide chain of the myxalamids with alanine. MxaA contains an N-terminal domain with homology to NAD binding proteins, which is responsib le during the biogenesis for a novel type of reductive chain release giving rise to the 2-aminopropanol moiety of the myxalamids. The last module of t he PKS reveals an unprecedented genetic organization: it is encoded on two genes (mxaB1 and mxaB2). subdividing the domains of one module from each ot her. A sequence comparison of myxobacterial acyl-transferase domains with k nown systems from streptomycetes and bacilli reveals that consensus sequenc es proposed to be specific for methylmalonyl-CoA and malonyl-CoA are not al ways reliable. Conclusions: The complete biosynthetic gene cluster of the myxalamid-type e lectron transport inhibitor From S, aurantiaca Sga15 has been cloned and an alyzed. It represents one of the few examples of combined PKS/NRPS systems. the analysis and manipulation of which has the potential to generate novel hybrid structures via combinatorial biosynthesis (e.g. via module-swapping techniques). Additionally, a neu type of reductive release from PKS/NRPS s ystems is described. (C) 2001 Elsevier Science Ltd. All rights reserved.