Unexpected Role of a Short-Chain Dehydrogenase/Reductase Family Protein in Type II Polyketide Biosynthesis

An unusual aromatase/cyclase programs the formation of the phenyldimethylanthrone framework in anthrabenzoxocinones and fasamycin

Aromatic polyketides are renowned for their wide-ranging pharmaceutical activities. Their structural diversity is mainly produced via modification of limited types of basic frameworks. In this study, we characterized the biosynthesis of a unique basic aromatic framework, phenyldimethylanthrone (PDA) found in (+)/(-)-anthrabenzoxocinones (ABXs) and fasamycin (FAS). Its biosynthesis employs a methyltransferase (Abx(+)M/Abx(-)M/FasT) and an unusual TcmI-like aromatase/cyclase (ARO/CYC, Abx(+)D/Abx(-)D/FasL) as well as a nonessential helper ARO/CYC (Abx(+)C/Abx(-)C/FasD) to catalyze the aromatization/cyclization of polyketide chain, leading to the formation of all four aromatic rings of the PDA framework, including the C9 to C14 ring and a rare angular benzene ring. Biochemical and structural analysis of Abx(+)D reveals a unique loop region, giving rise to its distinct acyl carrier protein-dependent specificity compared to other conventional TcmI-type ARO/CYCs, all of which impose on free molecules. Mutagenic analysis discloses critical residues of Abx(+)D for its catalytic activity and indicates that the size and shape of its interior pocket determine the orientation of aromatization/cyclization. This study unveils the tetracyclic and non-TcmN type C9 to C14 ARO/CYC, significantly expanding our cognition of ARO/CYCs and the biosynthesis of aromatic polyketide framework.

Biosynthesis of trialkyl-substituted aromatic polyketide NFAT-133 involves unusual P450 monooxygenase-mediating aromatization and a putative metallo-beta-lactamase fold hydrolase

The bacterial trialkyl-substituted aromatic polyketides are structurally featured with the unusual aromatic core in the middle of polyketide chain such as TM-123 (1), veramycin A (2), NFAT-133 (3) and benwamycin I (4), which were discovered from Streptomyces species and demonstrated with antidiabetic and immunosuppressant activities. Though the biosynthetic pathway of 1-3 was reported as a type I polyketide synthase (PKS), the PKS assembly line was interpreted inconsistently, and it remains a mystery how the compound 3 was generated. Herein, the PKS assembly logic of 1-4 was revised by site-mutagenetic analysis of the PKS dehydratase domains. Based on gene deletion and complementation, the putative P450 monooxygenase nftE₁ and metallo-beta-lactamase (MBL) fold hydrolase nftF₁ were verified as essential genes for the biosynthesis of 1-4. The absence of nftE₁ led to abolishment of 1-4 and accumulation of new products (5-8). Structural elucidation reveals 5-8 as the non-aromatic analogs of 1, suggesting the NftE₁-catalyzed aromatic core formation. Deletion of nftF₁ resulted in disappearance of 3 and 4 with the compounds 1 and 2 unaffected. As a rare MBL-fold hydrolase from type I PKSs, NftF₁ potentially generates the compound 3 through two strategies: catalyze premature chain-offloading as a trans-acting thioesterase or hydrolyze the lactone-bond of compound 1 as an esterase.

Six Sets of Aromatic Polyketides Differing in Size and Shape Derive from a Single Biosynthetic Gene Cluster

One new aromatic polyketide, prealnumycin B (1), and four known aromatic polyketides, K1115A (2), 1,6-dihydroxy-8-propylanthraquinone (DHPA, 3), phaeochromycin B (4), and (R)-7-acetyl-3,6-dihydroxy-8-propyl-3,4dihydronaphthalen-1(2H)-one (5), were isolated from the marine-derived Streptomyces sundarbansensis SCSIO NS01; these compounds represent four sets of aromatic polyketides differing in size and shape. A type II polyketide synthase (PKS) cluster, als, was identified by complete genome sequencing and was shown, by in vivo gene inactivation experiments in the wild-type (WT) NS01 strain and heterologous expression experiments, to encode the biosynthesis of compounds 1-5. Moreover, heterologous expression of the als cluster afforded three additional aromatic polyketides representing two different carbon skeletons, the new phaeochromycin L (6) and two known aromatic polyketides, phaeochromycins D (7) and E (8). These findings expand our knowledge of type II PKS machineries and their versatility in generating structurally diverse aromatic polyketides and highlight the power of type II PKSs in accessing new polyketides via ectopic expression in heterologous hosts.

A Nonfunctional Halogenase Masquerades as an Aromatizing Dehydratase in Biosynthesis of Pyrrolic Polyketides by Type I Polyketide Synthases

The bacterial modular type I polyketide synthases (PKSs) typically furnish nonaromatic lactone and lactam natural products. Here, by the complete in vitro enzymatic production of the polyketide antibiotic pyoluteorin, we describe the biosynthetic mechanism for the construction of an aromatic resorcylic ring by a type I PKS. We find that the pyoluteorin type I PKS does not produce an aromatic product, rather furnishing an alicyclic dihydrophloroglucinol that is later enzymatically dehydrated and aromatized. The aromatizing dehydratase is encoded in the pyoluteorin biosynthetic gene cluster (BGC), and its presence is conserved in other BGCs encoding production of pyrrolic polyketides. Sequence similarity and mutational analysis demonstrates that the overall structure and position of the active site for the aromatizing dehydratase is shared with flavin-dependent halogenases albeit with a loss in ability to perform redox catalysis. We demonstrate that the post-PKS dehydrative aromatization is critical for the antibiotic activity of pyoluteorin.