3-Substituted 2-oxindoles are important structural motifs found in many biologically active natural products and pharmaceutical lead compounds. Here, we report an enzymatic formation of the 3-substituted 2-oxindoles catalyzed by MarE in the maremycin biosynthetic pathway in Streptomyces sp. B9173. MarE is a homologue of FeII/heme-dependent tryptophan 2,3-dioxygenases (TDOs). Typical TDOs usually catalyze the insertion of two oxygen atoms from O2 into an indole ring to generate N-formylkynurenine (NFK)-like products. In contrast, MarE catalyzes the insertion of a single oxygen atom from O2 into an indole ring, to probably generate an epoxyindole intermediate that undergoes an unprecedented 2,3-hydride migration to form 2-oxindole structure. MarE shows substrate robustness to catalyze the conversion of a series of 3-substituted indoles into their corresponding 3-substituted 2-oxindoles. Although containing most key amino acid residues conserved in well-known TDO homologues, MarE falls into a separate new subgroup in the phylogenetic tree. The characterization of MarE and its homologue enriches the functional diversities of TDO superfamily and provides a new strategy for discovering novel natural products containing 3-substituted 2-oxindole pharmacophores by genome mining.

•Identification of the entire murayaquinone biosynthetic gene cluster•Anthrene-type model compounds were identified from biosynthetic mutants•A linear 1(4H)-anthracenone epoxide is key intermediate for murayaquinone formation•Three oxidoreductases catalyze skeleton rearrangement of the tricyclic intermediateBacterial aromatic polyketides are a group of natural products synthesized by polyketide synthases (PKSs) that show diverse structures and biological activities. They are structurally subclassified into linear, angular, and discoid aromatic polyketides, the formation of which is commonly determined by the shaping and folding of the poly-β-keto intermediates under the concerted actions of the minimal PKSs, cyclases and ketoreductases. Murayaquinone, found in several streptomycetes, possesses an unusual tricyclic angular aromatic polyketide core containing a 9,10-phenanthraquinone. In this study, genes essential for murayaquinone biosynthesis were identified, and a linear anthraoxirene intermediate was discovered. A unique biosynthetic model for the angular aromatic polyketide formation was discovered and confirmed through in vivo and in vitro studies. Three oxidoreductases, MrqO3, MrqO6, and MrqO7, were identified to catalyze the conversion of the linear aromatic polyketide intermediate into the final angularly arranged framework, which exemplifies a novel strategy for the biosynthesis of angular aromatic polyketides.Download high-res image (121KB)Download full-size image

In the flavin-reductase-catalyzed reducing condition, a mild and efficient α-hydroxylation and decarboxylation procedure using natural flavins as a catalyst and atmospheric oxygen as an external oxidizing agent has been successfully developed and applied to the synthesis of streptonigrone from streptonigrin. This reaction was achieved not only with streptonigrin analogues but also with structurally diverse electron-rich picolinic acid derivatives. The hydroxylation and decarboxylation may take place in a concerted manner.Keywords: flavin reductase; oxidative decarboxylation; picolinic acid derivatives; streptonigrin; streptonigrone

MarH, a small protein (129 amino acids) belonging to the cupin superfamily, was previously characterized as an epimerase involved in the (2S,3S)-β-methyltryptophan formation in the maremycin biosynthesis. Here, MarH was discovered to act as an acyltransferase that can catalyze the 3-O-acylation of chloramphenicol. Furthermore, MarH can catalyze N-acylation of deacylated chloramphenicol analogue thereby activating them for 3-O-acylation. By systematic site-directed mutagenesis, H64 was revealed as a potential catalytic base that deprotonates the acyl acceptor substrate. Nucleophilic attack at the carbonyl carbon of the acyl donor then gives the acylation product.Keywords: acyltransferase; chloramphenicol acetyltransferase; cupin protein; MarH; multifunctional enzyme

The maremycin biosynthetic gene cluster has been identified in Streptomyces sp. B9173. Comparative metabolic profiling with knockout mutant strains led to the identification of new products correlated to the maremycin biosynthesis, in particular the “demethyl”-maremycins with an unexpected D-tryptophan unit. A biosynthetic pathway for the maremycins is proposed and plausible reasoning for tryptophan epimerization in the demethylmaremycin biosynthesis is also provided.

Sparsomycin is a model protein synthesis inhibitor that blocks peptide bond formation by binding to the large ribosome subunit. It is a unique dipeptidyl alcohol, consisting of a uracil acrylic acid moiety and a monooxo-dithioacetal group. To elucidate the biosynthetic logic of sparsomycin, a biosynthetic gene cluster for sparsomycin was identified from the producer Streptomyces sparsogenes by genome mining, targeted gene mutations, and heterologous expression. Both the genetic and enzymatic studies revealed a minimum set of non-ribosomal peptide synthetases needed for generating the dipeptidyl alcohol scaffold of sparsomycin, featuring unusual mechanisms in dipeptidyl assembly and off-loading.

Streptonigrin (STN, 1) is a highly functionalized aminoquinone alkaloid with broad and potent antitumor activity. Here, we reported the biosynthetic gene cluster of STN identified by genome scanning of a STN producer Streptomyces flocculus CGMCC4.1223. This cluster consists of 48 genes determined by a series of gene inactivations. On the basis of the structures of intermediates and shunt products accumulated from five specific gene inactivation mutants and feeding experiments, the biosynthetic pathway was proposed, and the sequence of tailoring steps was preliminarily determined. In this pathway, a cryptic methylation of lavendamycin was genetically and biochemically characterized to be catalyzed by a leucine carboxyl methyltransferase StnF2. A [2Fe–2S]2+ cluster-containing aromatic ring dioxygenase StnB1/B2 system was biochemically characterized to catalyze a regiospecific cleavage of the N–C8′ bond of the indole ring of the methyl ester of lavendamycin. This work provides opportunities to illuminate the enzymology of novel reactions involved in this pathway and to create, using genetic and chemo-enzymatic methods, new streptonigrinoid analogues as potential therapeutic agents.

Polyene macrolides are a large family of natural products typically produced by soil actinomycetes. Polyene macrolides are usually biosynthesized by modular and large type I polyketide synthases (PKSs), followed by several steps of sequential post-PKS modifications such as region-specific oxidations and glycosylations. Although known as powerful antibiotics containing potent antifungal activities (along with additional activities against parasites, enveloped viruses and prion diseases), their high toxicity toward mammalian cells and poor distribution in tissues have led to the continuous identification and structural modification of polyene macrolides to expand their general uses. Advances in in-depth investigations of the biosynthetic mechanism of polyene macrolides and the genetic manipulations of the polyene biosynthetic pathways provide great opportunities to generate new analogues. Recently, a novel class of polyene antibiotics was discovered (a disaccharide-containing NPP) that displays better pharmacological properties such as improved water-solubility and reduced hemolysis. In this review, we summarize the recent advances in the biosynthesis, pathway engineering, and regulation of polyene antibiotics in actinomycetes.