On the origins and biosynthesis of tetrodotoxin

The potent neurotoxin tetrodotoxin (TTX) has been identified from taxonomically diverse marine organisms. TTX possesses a unique cage-like structure, however, its biosynthesis has yet to be elucidated. Biosynthetic studies in the TTX-producing newt Taricha torosa, and in bacterial genera, including Vibrio, have proven inconclusive. Indeed, very few studies have been performed that address the cellular production of TTX. Here we review the sources of TTX described to date and provide evidence for the biosynthesis of TTX by symbiotic microorganisms in higher taxa. Chemical and genetic based biosynthesis studies of TTX undertaken thus far are discussed and we outline approaches which may be useful for expanding upon the current body of knowledge. The complex biosynthesis of structurally similar toxins, that reveal clues into the biosynthetic pathway of TTX, is also presented.

Binding of two flaviolin substrate molecules, oxidative coupling, and crystal structure of Streptomyces coelicolor A3(2) cytochrome P450 158A2

Cytochrome P450 158A2 (CYP158A2) is encoded within a three-gene operon (sco1206-sco1208) in the prototypic soil bacterium Streptomyces coelicolor A3(2). This operon is widely conserved among streptomycetes. CYP158A2 has been suggested to produce polymers of flaviolin, a pigment that may protect microbes from UV radiation, in combination with the adjacent rppA gene, which encodes the type III polyketide synthase, 1,3,6,8-tetrahydroxynaphthalene synthase. Following cloning, expression, and purification of this cytochrome P450, we have shown that it can produce dimer and trimer products from the substrate flaviolin and that the structures of two of the dimeric products were established using mass spectrometry and multiple NMR methods. A comparison of the x-ray structures of ligand-free (1.75 angstroms) and flaviolin-bound (1.62 angstroms) forms of CYP158A2 demonstrates a major conformational change upon ligand binding that closes the entry into the active site, partly due to repositioning of the F and G helices. Particularly interesting is the presence of two molecules of flaviolin in the closed active site. The flaviolin molecules form a quasi-planar three-molecule stack including the heme of CYP158A2, suggesting that oxidative C-C coupling of these phenolic molecules leads to the production of flaviolin dimers.

EncM, a versatile enterocin biosynthetic enzyme involved in Favorskii oxidative rearrangement, aldol condensation, and heterocycle-forming reactions

The bacteriostatic natural product enterocin from the marine microbe "Streptomyces maritimus" has an unprecedented carbon skeleton that is derived from an aromatic polyketide biosynthetic pathway. Its caged tricyclic, nonaromatic core is derived from a linear poly-beta-ketide precursor that formally undergoes a Favorskii-like oxidative rearrangement. In vivo characterization of the gene encM through mutagenesis and heterologous biosynthesis demonstrated that its protein product not only is solely responsible for the oxidative C-C rearrangement, but also facilitates two aldol condensations plus two heterocycle forming reactions. In total, at least five chiral centers and four rings are generated by this multifaceted flavoprotein. Heterologous expression of the enterocin biosynthesis genes encABCDLMN in Streptomyces lividans resulted in the formation of the rearranged metabolite desmethyl-5-deoxyenterocin and the shunt products wailupemycins D-G. Addition of the methyltransferase gene encK, which was previously proposed through mutagenesis to additionally assist EncM in the Favorskii rearrangement, shifted the production to the O-methyl derivative 5-deoxyenterocin. The O-methyltransferase EncK seems to be specific for the pyrone ring of enterocin, because bicyclic polyketides bearing pyrone rings are not methylated in vivo. Expression of encM with different combinations of homologous actinorhodin biosynthesis genes did not result in the production of oxidatively rearranged enterocin-actinorhodin hybrid compounds as anticipated, suggesting that wild-type EncM may be specific for its endogenous type II polyketide synthase or for benzoyl-primed polyketide precursors.

Plant-like biosynthetic pathways in bacteria: from benzoic acid to chalcone

Although phenylpropanoids and flavonoids are common plant natural products, these major classes of biologically active secondary metabolites are largely absent from bacteria. The ubiquitous plant enzymes phenylalanine ammonia-lyase (PAL) and chalcone synthase (CHS) are key biosynthetic catalysts in phenylpropanoid and flavonoid assembly, respectively. Until recently, few bacterial counterparts were known, thus reflecting the dearth of these plant natural products in bacteria. This review highlights our progress on the biochemical and genetic characterization of recently identified streptomycete biosynthetic pathways to benzoic acid and type III polyketide synthase (PKS)-derived products. The sediment-derived bacterium "Streptomyces maritimus" produces benzoyl-CoA in a plant-like manner from phenylalanine involving a PAL-mediated reaction through cinnamic acid during the biosynthesis of the polyketide antibiotic enterocin. All but one of the genes encoding benzoyl-CoA biosynthesis in "S. maritimus" have been cloned, sequenced, and inactivated, providing a model for benzoate biosynthesis not only in this bacterium, but in plants where benzoic acid is an important constituent of many products. The recent discovery that bacteria harbor homodimeric PKSs belonging to the plant CHS superfamily of condensing enzymes has further linked the biosynthetic capabilities of plants and bacteria. A bioinformatics approach led to the prediction that the model actinomycete Streptomyces coelicolor A3(2) contains up to three type III PKSs. Biochemical analysis of one of the recombinant type III PKSs from S. coelicolor demonstrated activity as a 1,3,6,8-tetrahydroxynaphthalene synthase (THNS). A homology model of THNS based upon the known three-dimensional structure of CHS was constructed to explore the structural and mechanistic details of this new subclass of bacterial PKSs.

Context-Dependent Behavior of the Enterocin Iterative Polyketide Synthase

Heterologous expression and mutagenesis of the enterocin type II polyketide synthase (PKS) system suggest for the first time that the association of an extended set of proteins and substrates is needed for the effective production of the enterocin-wailupemycin polyketides. In the absence of its endogenous ketoreductase (KR) EncD in either the enterocin producer "Streptomyces maritimus" or the engineered host S. lividans K4-114, the enterocin minimal PKS is unable to produce benzoate-primed polyketides, even when complemented with the homologous actinorhodin KR ActIII or with EncD active site mutants. These data suggest that the enterocin PKS requires EncD to serve a catalytic and not just a structural role in the functional PKS enzyme complex. This strongly implies that EncD reduces the polyketide chain during elongation rather than after its complete assembly, as suggested for most type II PKSs.

Mutational analysis of the enterocin favorskii biosynthetic rearrangement

[reaction: see text] A mutational analysis of the enterocin biosynthesis genes revealed that the putative oxygenase and the methyltransferase gene products EncM and EncK, respectively, jointly catalyze a biosynthetic Favorskii-like rearrangement. Inactivation of either gene terminated enterocin production and caused the accumulation of four nonrearranged, nonmethylated polyketides. The structure elucidation of the new wailupemycins E-G is reported.

Biosynthesis and Structural Revision of Neomarinone

[reaction: see text] The biosynthesis of the meroterpenoid neomarinone from a marine actinomycete was probed through feeding experiments with (13)C-labeled precursors. NMR characterization of [U-(13)C(6)]glucose-enriched neomarinone led to the structural revision of structure 4a to 4b, which was confirmed by extensive 2D NMR spectrometry with unlabeled compound.

Mutasynthesis of enterocin and wailupemycin analogues

Inactivation of the novel phenylalanine ammonia lyase gene encP, whose product is a key component in the biosynthetic pathway to benzoyl-coenzyme A (CoA) in the bacterium Streptomyces maritimus, resulted in the loss of production of the benzoate-primed polyketides enterocin and wailupemycin G. A series of cinnamate and benzoate derivatives were administered to the DeltaencP mutant, resulting in the formation of novel analogues bearing p-fluorobenzoate, 2- and 3-thiophenecarboxylate, and cyclohex-1-enecarboxylate residues. Given that the benzoate:CoA ligase EncN was evaluated to have broad in vitro substrate specificity towards aryl acids, the strict starter unit specificity observed in vivo indicates that the enterocin type II polyketide synthase (PKS) exerts selective control over the choice of starter units. This study represents the first mutasynthesis experiments with iterative type II PKSs.

Pestalactams A-C: novel caprolactams from the endophytic fungus Pestalotiopsis sp

Chemical investigations of a fermentation culture from the endophytic fungus Pestalotiopsis sp. yielded three novel caprolactams, pestalactams A-C (). The structures of were determined by analysis of 1D and 2D-NMR, UV, IR, and MS data. The structure of pestalactam A was confirmed following single crystal X-ray diffraction analysis. Pestalactams A-C are the first C-7 alkylated caprolactam natural products to be reported. Pestalactams A () and B () were tested against two different strains of the malaria parasite Plasmodium falciparum (3D7 and Dd2), and the mammalian cell lines, MCF-7 and NFF, and showed modest in vitro activity in all assays.

Exploiting marine actinomycete biosynthetic pathways for drug discovery

Drug discovery relies on the generation of large numbers of structurally diverse compounds from which a potential candidate can be identified. To this end, actinomycetes have often been exploited because of their ability to biosynthesize an impressive array of novel metabolites particularly polyketides. The genetic organization of polyketide synthases (PKSs) makes them readily amenable to manipulation, and thus re-engineering artificial or hybrid PKSs to produce unnatural natural products is a reality. This review highlights two approaches we have used to generate novel polyketides by manipulating genes responsible for starter unit biosynthesis in the 'Streptomyces maritimus' enterocin type II PKS. Our preliminary investigation into the biosynthesis of neomarinone, a rare marine actinomycete-derived meroterpenoid, is also presented.

Heterologous biosynthesis of truncated hexaketides derived from the actinorhodin polyketide synthase

Heterologous expression of the actinorhodin polyketide synthase in the recombinant host Streptomyces lividans K4-114 led to the characterization of three new minor polyketides, the novel hexaketides BSM1 and BSM3 and 9'-hydroxyaloesaponarin II, in addition to known anthraquinone and aromatic octaketides. The structures of BSM1 and BSM3 imply that these compounds are derived from a C-5-reduced hexaketide intermediate, suggesting that the timing of the ketoreduction reaction in the actinorhodin biosynthetic pathway may take place during the polyketide elongation process rather than after the completion of the octaketide chain as previously suggested.

In vitro biosynthesis of unnatural enterocin and wailupemycin polyketides

Nature has evolved finely tuned strategies to synthesize rare and complex natural products such as the enterocin family of polyketides from the marine bacterium Streptomyces maritimus. Herein we report the directed ex vivo multienzyme syntheses of 24 unnatural 5-deoxyenterocin and wailupemycin F and G analogues, 18 of which are new. We have generated molecular diversity by priming the enterocin biosynthesis enzymes with unnatural substrates and have illustrated further the uniqueness of this type II polyketide synthase by way of exploiting its unusual starter unit biosynthesis pathways.

Adociasulfates 1, 7, and 8: New Bioactive Hexaprenoid Hydroquinones from the Marine Sponge Adocia sp

Adociasulfate 1 (1), adociasulfate 7 (2), and adociasulfate 8 (3), which are inhibitors of proton pump activity in hen bone-derived membrane vesicles, were isolated from an extract of the sponge Adocia sp. (Chalinidae). Structure elucidation by 2D-NMR spectroscopy revealed that they are novel hexaprenoid hydroquinone sulfates.

Diversity and biosynthetic potential of culturable microbes associated with toxic marine animals

Tetrodotoxin (TTX) is a neurotoxin that has been reported from taxonomically diverse organisms across 14 different phyla. The biogenic origin of tetrodotoxin is still disputed, however, TTX biosynthesis by host-associated bacteria has been reported. An investigation into the culturable microbial populations from the TTX-associated blue-ringed octopus Hapalochlaena sp. and sea slug Pleurobranchaea maculata revealed a surprisingly high microbial diversity. Although TTX was not detected among the cultured isolates, PCR screening identifiedsome natural product biosynthesis genes putatively involved in its assembly. This study is the first to report on the microbial diversity of culturable communities from H. maculosa and P. maculata and common natural product biosynthesis genes from their microbiota. We also reassess the production of TTX reported from three bacterial strains isolated from the TTX-containing gastropod Nassarius semiplicatus.

Nanangenines: drimane sesquiterpenoids as the dominant metabolite cohort of a novel Australian fungus, Aspergillus nanangensis

Chemical investigation of an undescribed Australian fungus, Aspergillus nanangensis, led to the identification of the nanangenines - a family of seven new and three previously reported drimane sesquiterpenoids. The structures of the nanangenines were elucidated by detailed spectroscopic analysis supported by single crystal X-ray diffraction studies. The compounds were assayed for in vitro activity against bacteria, fungi, mammalian cells and plants. Bioinformatics analysis, including comparative analysis with other acyl drimenol-producing Aspergilli, led to the identification of a putative nanangenine biosynthetic gene cluster that corresponds to the proposed biosynthetic pathway for nanangenines.

Harnessing long-read amplicon sequencing to uncover NRPS and Type I PKS gene sequence diversity in polar desert soils

The severity of environmental conditions at Earth's frigid zones present attractive opportunities for microbial biomining due to their heightened potential as reservoirs for novel secondary metabolites. Arid soil microbiomes within the Antarctic and Arctic circles are remarkably rich in Actinobacteria and Proteobacteria, bacterial phyla known to be prolific producers of natural products. Yet the diversity of secondary metabolite genes within these cold, extreme environments remain largely unknown. Here, we employed amplicon sequencing using PacBio RS II, a third generation long-read platform, to survey over 200 soils spanning twelve east Antarctic and high Arctic sites for natural product-encoding genes, specifically targeting non-ribosomal peptides (NRPS) and Type I polyketides (PKS). NRPS-encoding genes were more widespread across the Antarctic, whereas PKS genes were only recoverable from a handful of sites. Many recovered sequences were deemed novel due to their low amino acid sequence similarity to known protein sequences, particularly throughout the east Antarctic sites. Phylogenetic analysis revealed that a high proportion were most similar to antifungal and biosurfactant-type clusters. Multivariate analysis showed that soil fertility factors of carbon, nitrogen and moisture displayed significant negative relationships with natural product gene richness. Our combined results suggest that secondary metabolite production is likely to play an important physiological component of survival for microorganisms inhabiting arid, nutrient-starved soils.

Policing starter unit selection of the enterocin type II polyketide synthase by the type II thioesterase EncL

Enterocin is an atypical type II polyketide synthase (PKS) product from the marine actinomycete 'Streptomyces maritimus'. The enterocin biosynthesis gene cluster (enc) codes for proteins involved in the assembly and attachment of the rare benzoate primer that initiates polyketide assembly with the addition of seven malonate molecules and culminates in a Favorskii-like rearrangement of the linear poly-β-ketone to give its distinctive non-aromatic, caged core structure. Fundamental to enterocin biosynthesis, which utilizes a single acyl carrier protein (ACP), EncC, for both priming with benzoate and elongating with malonate, involves maintaining the correct balance of acyl-EncC substrates for efficient polyketide assembly. Here, we report the characterization of EncL as a type II thioesterase that functions to edit starter unit (mis)priming of EncC. We performed a series of in vivo mutational studies, heterologous expression experiments, in vitro reconstitution studies, and Fourier-transform mass spectrometry-monitored competitive enzyme assays that together support the proposed selective hydrolase activity of EncL toward misprimed acetyl-ACP over benzoyl-ACP to facilitate benzoyl priming of the enterocin PKS complex. While this system resembles the R1128 PKS that also utilizes an editing thioesterase (ZhuC) to purge acetate molecules from its initiation module ACP in favor of alkylacyl groups, the enterocin system is distinct in its usage of a single ACP for both priming and elongating reactions with different substrates.

Hancockiamides: phenylpropanoid piperazines from Aspergillus hancockii are biosynthesised by a versatile dual single-module NRPS pathway

The hancockiamides are an unusual new family of N-cinnamoylated piperazines from the Australian soil fungus Aspergillus hancockii, originating from mixed nonribosomal peptide and phenylpropanoid pathways.

Synthesis, characterization, and anti-melanoma activity of tetra-O-substituted analogs of nordihydroguaiaretic acid

Synthesis of seven semi-synthetic analogs of NDGA is described. An approach to NDGA derivatization is described in which the ortho-phenolic groups are tethered together by one atom, forming a 5-membered heterocyclic ring. The analogs were evaluated for cytotoxicity in four cancer cell lines and compared to NDGA and tetra-O-methyl-NDGA (M4N) (1a). NDGA bis-cyclic sulfate (2a), NDGA bis-cyclic carbonate (2b), and methylenedioxyphenyl-NDGA (2d) and NDGA tetra acetate (1b) showed anti-cancer activity in vitro. Two compounds, (1b) and (2b), were evaluated for anticancer activity in a mouse xenograft model of human melanoma and showed dose-dependent activity.

Bioactive natural products from Papua New Guinea marine sponges

The discovery of novel natural products for drug development relies heavily upon a rich biodiversity, of which the marine environment is an obvious example. Marine natural product research has spawned several drugs and many other candidates, some of which are the focus of current clinical trials. The sponge megadiversity of Papua New Guinea is a rich but underexplored source of bioactive natural products. Here, we review some of the many natural products derived from PNG sponges with an emphasis on those with interesting biological activity and, therefore, drug potential. Many bioactive natural products discussed here appear to be derived from non-ribosomal peptide and polyketide biosynthesis pathways, strongly suggesting a microbial origin of these compounds. With this in mind, we also explore the notion of sponge-symbiont biosynthesis of these bioactive compounds and present examples to support the working hypothesis.

Genome mining of a fungal endophyte of Taxus yunnanensis (Chinese yew) leads to the discovery of a novel azaphilone polyketide, lijiquinone

Genome mining of Ascomycete sp. F53 (F53), a fungal endophyte of the traditional Chinese medicinal plant Taxus yunnanensis (Chinese yew), revealed 35 putative specialized metabolite biosynthesis gene clusters, one of which encodes a rarely seen tandem polyketide synthase pathway with close homology to azaphilone biosynthesis pathways. A novel compound, lijiquinone 1, was subsequently isolated from F53 and structurally and functionally characterized. The m/z 385 [M + H+ ]+ compound, comprised of a cyclohexenone side group attached to a core bicyclic ring, displayed cytotoxicity against human myeloma cells (IC50  = 129 μM), as well as antifungal activity against Candida albicans (IC50  = 79 μM) and Cryptococcus albidus (IC50  = 141 μM). Our results suggest that enzymes encoded on the lij gene cluster are responsible for the synthesis of 1 and that the medicinal properties of T. yunnanensis could be partially mediated by this novel azaphilone. This study highlights the utility of combining traditional knowledge with contemporary genomic approaches for the discovery of new bioactive compounds.

Characterisation and heterologous biosynthesis of burnettiene A, a new polyene-decalin polyketide from Aspergillus burnettii

Chemical exploration of the recently described Australian fungus, Aspergillus burnettii, uncovered a new metabolite, burnettiene A. Here, we characterise the structure of burnettiene A as a polyene-decalin polyketide. Bioinformatic analysis of the genome of A. burnettii identified a putative biosynthetic gene cluster for burnettiene A (bue), consisting of eight genes and sharing similarity to the fusarielin gene cluster. Introduction of the reassembled bue gene cluster into Aspergillus nidulans for heterologous expression resulted in the production of burnettiene A under native promoters. Omission of bueE encoding a cytochrome P450 led to the production of preburnettiene A, confirming that BueE is responsible for catalysing the regiospecific multi-oxidation of terminal methyl groups to carboxylic acids. Similarly, bueF was shown to encode an ester-forming methyltransferase, with its omission resulting in the production of the tricarboxylic acid, preburnettiene B. Introduction of an additional copy of the transcription factor bueR under the regulation of the gpdA promoter significantly improved the heterologous production of the burnettienes. Burnettiene A displayed strong in vitro cytotoxicity against mouse myeloma NS-1 cells (MIC 0.8 μg mL-1).