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Heterocycles play an important role in many biological processes and are widespread among natural products. Oxazole-containing natural products possess a broad range of bioactivities and are of great interest in the pharmaceutical and agrochemical industries. Herein, the biosynthetic routes to the oxazole-containing phthoxazolins and the bis(benzoxaozle) AJI9561, were investigated. Phthoxazolins A-D are a group of oxazole trienes produced by a polyketide synthase-nonribosomal peptide synthetase (PKS-NRPS) pathway in Streptomyces sp. KO-7888 and Streptomyces sp. OM-5714. The phthoxazolin pathway was used as a model to study 5-oxazole and primary amide formation in PKS-NRPS pathways. An unusually large gene cluster for phthoxazolin biosynthesis was identified from the complete genome sequence of the producer strains and various gene deletions were performed to define the minimal gene cluster. PhoxP was proposed to encode an ATP-dependent cyclodehydratase for 5-oxazole formation on an enzyme-bound N-formylglycylacyl-intermediate, and its deletion abolished phthoxazolin production. In vitro reconstitution of the early steps of phthoxazolin biosynthesis was attempted to validate the role of PhoxP, but was unsuccessful. Furthermore, Orf3515, a putative flavin-dependent monooxygenase coded by a remote gene, was proposed to hydroxylate glycine-extended polyketide-peptide chain(s) at the α-position to yield phthoxazolins with the primary amide moiety. On the other hand, an in vitro approach was employed to establish the enzymatic logic of the biosynthesis of AJI9561, a bis(benzoxazole) antibiotic isolated from Streptomyces sp. AJ9561. The AJI9561 pathway was reconstituted using the precursors 3-hydroxyanthranilic acid and 6-methylsalicylic acid and five purified enzymes previously identified from the pathway as key enzymes for benzoxazole formation, including two adenylation enzymes for precursor activation, an acyl carrier protein (ACP), a 3-oxoacyl-ACP synthase and an amidohydrolase-like cyclase. Intermediates and shunt products isolated from enzymatic reactions containing different enzyme and precursor combinations were assessed for their competence for various steps of AJI9561 biosynthesis. Further bioinformatic analysis and in silico modelling of the amidohydrolase-like cyclase shed light on the oxazole cyclisation that represents a novel catalytic function of the amidohydrolase superfamily., Henry Lester Trust, Herchel Smith Foundation