Combinatorial biosynthesis for the engineering of novel fungal natural products.

Natural products are small molecules synthesized by fungi, bacteria and plants, which historically have had a profound effect on human health and quality of life. These natural products have evolved over millions of years resulting in specific biological functions that may be of interest for pharmaceutical, agricultural, or nutraceutical use. Often natural products need to be structurally modified to make them suitable for specific applications. Combinatorial biosynthesis is a method to alter the composition of enzymes needed to synthesize a specific natural product resulting in structurally diversified molecules. In this review we discuss different approaches for combinatorial biosynthesis of natural products via engineering fungal enzymes and biosynthetic pathways. We highlight the biosynthetic knowledge gained from these studies and provide examples of new-to-nature bioactive molecules, including molecules synthesized using combinations of fungal and non-fungal enzymes.

Discovery of unguisin J, a new cyclic peptide from Aspergillus heteromorphus CBS 117.55, and phylogeny-based bioinformatic analysis of UngA NRPS domains.

Several under-explored Aspergillus sp. produce intriguing heptapeptides containing a γ-aminobutyric acid (GABA) residue with as yet unknown biological functions. In this study, a new GABA-containing heptapeptide - unguisin J (1) - along with known unguisin B (2) were isolated from a solid culture of Aspergillus heteromorphus CBS 117.55. The structure of compound 1 was elucidated by extensive 1D and 2D NMR spectroscopic analysis including HSQC, HMBC, COSY, and 2D NOESY as well as HRESIMS. The stereochemistry of 1 and 2 was determined by Marfey's method. A biosynthetic gene cluster (BGC) encoding unguisins B and J was compared to characterized BGCs in other Aspergillus sp. Since the unguisin family of heptapetides incorporate different amino acid residues at different positions of the peptide, the A and C domains of the UngA NRPS were analyzed in an attempt to understand the lack of substrate specificity observed.

Subcellular localization of fungal specialized metabolites.

Fungal specialized metabolites play an important role in the environment and have impacted human health and survival significantly. These specialized metabolites are often the end product of a series of sequential and collaborating biosynthetic enzymes that reside within different subcellular compartments. A wide variety of methods have been developed to understand fungal specialized metabolite biosynthesis in terms of the chemical conversions and the biosynthetic enzymes required, however there are far fewer studies elucidating the compartmentalization of the same enzymes. This review illustrates the biosynthesis of specialized metabolites where the localization of all, or some, of the biosynthetic enzymes have been determined and describes the methods used to identify the sub-cellular localization.

Heterologous Expression of Secondary Metabolite Genes in Trichoderma reesei for Waste Valorization.

Trichoderma reesei (Hypocrea jecorina) was developed as a microbial cell factory for the heterologous expression of fungal secondary metabolites. This was achieved by inactivation of sorbicillinoid biosynthesis and construction of vectors for the rapid cloning and expression of heterologous fungal biosynthetic genes. Two types of megasynth(et)ases were used to test the strain and vectors, namely a non-reducing polyketide synthase (nr-PKS, aspks1) from Acremonium strictum and a hybrid highly-reducing PKS non-ribosomal peptide synthetase (hr-PKS-NRPS, tenS + tenC) from Beauveria bassiana. The resulting engineered T. reesei strains were able to produce the expected natural products 3-methylorcinaldehyde and pretenellin A on waste materials including potato, orange, banana and kiwi peels and barley straw. Developing T. reesei as a heterologous host for secondary metabolite production represents a new method for waste valorization by the direct conversion of waste biomass into secondary metabolites.

Biosynthesis of fungal polyketides by collaborating and trans-acting enzymes.

Covering: 1999 up to 2021Fungal polyketides encompass a range of structurally diverse molecules with a wide variety of biological activities. The giant multifunctional enzymes that synthesize polyketide backbones remain enigmatic, as do many of the tailoring enzymes involved in functional modifications. Recent advances in elucidating biosynthetic gene clusters (BGCs) have revealed numerous examples of fungal polyketide synthases that require the action of collaborating enzymes to synthesize the carbon backbone. This review will discuss collaborating and trans-acting enzymes involved in loading, extending, and releasing polyketide intermediates from fungal polyketide synthases, and additional modifications introduced by trans-acting enzymes demonstrating the complexity encountered when investigating natural product biosynthesis in fungi.

Engineering Aspergillus oryzae for the Heterologous Expression of a Bacterial Modular Polyketide Synthase.

Microbial natural products have had phenomenal success in drug discovery and development yet form distinct classes based on the origin of their native producer. Methods that enable metabolic engineers to combine the most useful features of the different classes of natural products may lead to molecules with enhanced biological activities. In this study, we modified the metabolism of the fungus Aspergillus oryzae to enable the synthesis of triketide lactone (TKL), the product of the modular polyketide synthase DEBS1-TE engineered from bacteria. We established (2S)-methylmalonyl-CoA biosynthesis via introducing a propionyl-CoA carboxylase complex (PCC); reassembled the 11.2 kb DEBS1-TE coding region from synthetic codon-optimized gene fragments using yeast recombination; introduced bacterial phosphopantetheinyltransferase SePptII; investigated propionyl-CoA synthesis and degradation pathways; and developed improved delivery of exogenous propionate. Depending on the conditions used titers of TKL ranged from <0.01-7.4 mg/L. In conclusion, we have demonstrated that A. oryzae can be used as an alternative host for the synthesis of polyketides from bacteria, even those that require toxic or non-native substrates. Our metabolically engineered A. oryzae may offer advantages over current heterologous platforms for producing valuable and complex natural products.

Analysis of the Secondary Metabolism in Magnaporthe oryzae.

Magnaporthe oryzae produces a number of secondary metabolites, some of which are thought to be responsible for the virulence of this fungus toward rice. Due to the importance of understanding plant-pathogen interactions, several of these metabolites have been investigated chemically and biosynthetically. This chapter provides an overview of the secondary metabolites isolated from M. oryzae and describes a general method for metabolite extraction, followed by an analysis using high-performance liquid chromatography (HPLC) combined with mass spectrometry (LCMS).

Early Oxidative Transformations During the Biosynthesis of Terrein and Related Natural Products.

The mycotoxin terrein is derived from the C10 -precursor 6-hydroxymellein (6-HM) via an oxidative ring contraction. Although the corresponding biosynthetic gene cluster (BGC) has been identified, details of the enzymatic oxidative transformations are lacking. Combining heterologous expression and in vitro studies we show that the flavin-dependent monooxygenase (FMO) TerC catalyzes the initial oxidative decarboxylation of 6-HM. The reactive intermediate is further hydroxylated by the second FMO TerD to yield a highly oxygenated aromatic species, but further reconstitution of the pathway was hampered. A related BGC was identified in the marine-derived Roussoella sp. DLM33 and confirmed by heterologous expression. These studies demonstrate that the biosynthetic pathways of terrein and related (polychlorinated) congeners diverge after oxidative decarboxylation of the lactone precursor that is catalyzed by a conserved FMO and further indicate that early dehydration of the side chain is an essential step.

Biosynthesis of 6-Hydroxymellein Requires a Collaborating Polyketide Synthase-like Enzyme.

The polyketide synthase (PKS)-like protein TerB, consisting of inactive dehydratase, inactive C-methyltransferase, and functional ketoreductase domains collaborates with the iterative non reducing PKS TerA to produce 6-hydroxymellein, a key pathway intermediate during the biosynthesis of various fungal natural products. The catalytically inactive dehydratase domain of TerB appears to mediate productive interactions with TerA, demonstrating a new mode of trans-interaction between iterative PKS components.

Chemical and Genetic Studies on the Formation of Pyrrolones During the Biosynthesis of Cytochalasans.

A key step during the biosynthesis of cytochalasans is a proposed Knoevenagel condensation to form the pyrrolone core, enabling the subsequent 4+2 cycloaddition reaction that results in the characteristic octahydroisoindolone motif of all cytochalasans. In this work, we investigate the role of the highly conserved α,ß-hydrolase enzymes PyiE and ORFZ during the biosynthesis of pyrichalasin H and the ACE1 metabolite, respectively, using gene knockout and complementation techniques. Using synthetic aldehyde models we demonstrate that the Knoevenagel condensation proceeds spontaneously but results in the 1,3-dihydro-2H-pyrrol-2-one tautomer, rather than the required 1,5-dihydro-2H-pyrrol-2-one tautomer. Taken together our results suggest that the α,ß-hydrolase enzymes are essential for first ring cyclisation, but the precise nature of the intermediates remains to be determined.

The same but different: multiple functions of the fungal flavin dependent monooxygenase SorD from Penicillium chrysogenum.

Sorbicillinoids are a large family of fungal secondary metabolites with a diverse range of structures and numerous bioactivites, some of which have pharmaceutical potential. The flavin-dependent monooxygenase SorD from Penicillium chrysogenum (PcSorD) utilizes sorbicillinol to catalyze a broad scope of reactions: formation of oxosorbicillinol and epoxysorbicillinol; intermolecular Diels-Alder and Michael-addition dimerization reactions; and dimerization of a sorbicillinol derivative with oxosorbicillinol. PcSorD shares only 18.3% sequence identity with SorD from Trichoderma reesei (TrSorD) and yet unexpectedly catalyzes many of the same reactions, however, the formation of oxosorbicillinol and bisvertinolone by PcSorD extends the range of reactions catalyzed by a single enzyme. Phylogenetic analysis indicates that PcSorD and TrSorD bind the flavin cofactor covalently but via different residues and point mutations confirm these residues are essential for activity.

Diversely Functionalised Cytochalasins through Mutasynthesis and Semi-Synthesis.

Mutasynthesis of pyrichalasin H from Magnaporthe grisea NI980 yielded a series of unprecedented 4'-substituted cytochalasin analogues in titres as high as the wild-type system (≈60 mg L-1 ). Halogenated, O-alkyl, O-allyl and O-propargyl examples were formed, as well as a 4'-azido analogue. 4'-O-Propargyl and 4'-azido analogues reacted smoothly in Huisgen cycloaddition reactions, whereas p-Br and p-I compounds reacted in Pd-catalysed cross-coupling reactions. A series of examples of biotin-linked, dye-linked and dimeric cytochalasins was rapidly created. In vitro and in vivo bioassays of these compounds showed that the 4'-halogenated and azido derivatives retained their cytotoxicity and antifungal activities; but a unique 4'-amino analogue was inactive. Attachment of larger substituents attenuated the bioactivities. In vivo actin-binding studies with adherent mammalian cells showed that actin remains the likely intracellular target. Dye-linked compounds revealed visualisation of intracellular actin structures even in the absence of phalloidin, thus constituting a potential new class of actin-visualisation tools with filament-barbed end-binding specificity.

Evidence for enzyme catalysed intramolecular [4+2] Diels-Alder cyclization during the biosynthesis of pyrichalasin H.

Cytochalasans are highly complex fungal metabolites which exhibit diverse biological activities. Little is known of the chemical steps involved in the construction of the tricyclic core, which consists of an octahydro-isoindole skeleton fused to a macrocyclic ring. Here, using a directed gene knockout and complementation strategy, we show that PyiF is implicated as the proposed intramolecular [4+2] Diels-Alderase required for construction of the tricyclic core of pyrichalasin H 1.

Diels-Alder Reactions During the Biosynthesis of Sorbicillinoids.

The sorbicillinoids are a class of biologically active and structurally diverse fungal polyketides arising from sorbicillin. Through co-expression of sorA, sorB, sorC, and sorD from Trichoderma reesei QM6a, the biosynthetic pathway to epoxysorbicillinol and dimeric sorbicillinoids, which resemble Diels-Alder-like and Michael-addition-like products, was reconstituted in Aspergillus oryzae NSAR1. Expression and feeding experiments demonstrated the crucial requirement of the flavin-dependent monooxygenase SorD for the formation of dimeric sorbicillinoids, hybrid sorbicillinoids, and epoxysorbicillinol in vivo. In contrast to prior reports, SorD catalyses neither the oxidation of 2',3'-dihydrosorbicillin to sorbicillin nor the oxidation of sorbicillinol to oxosorbicillinol. This is the first report that both the intermolecular Diels-Alder and Michael dimerization reactions, as well as the epoxidation of sorbicillinol are catalysed in vivo by SorD.

Investigating the Function of Cryptic Cytochalasan Cytochrome P450 Monooxygenases Using Combinatorial Biosynthesis.

Tailoring enzymes in cytochalasan biosynthesis are relatively promiscuous. Exploiting this property, we deduced the function of four cryptic cytochrome P450 monooxygenases via heterologous expression of six cytochrome P450-encoding genes, originating from Hypoxylon fragiforme and Pyricularia oryzae, in pyrichalasin H ΔP450 strains. Three cryptic cytochrome P450 enzymes (HffD, HffG, and CYP1) restored pyrichalasin H production in mutant strains, while CYP3 catalyzed a site-selective epoxidation leading to the isolation of three novel cytochalasans.

Targeted Gene Inactivations Expose Silent Cytochalasans in Magnaporthe grisea NI980.

The biosynthetic gene cluster encoding the phytotoxin pyrichalasin H 5 was discovered in Magnaporthe grisea NI980, and the late-stage biosynthetic pathway of 5 was fully elucidated using targeted gene inactivations resulting in the isolation of 13 novel cytochalasans. This study reveals that the nonproteinogenic amino acid O-methyltyrosine is the true precursor of 5, and other cryptic cytochalasans and mutasynthesis experiments produce novel halogenated pyrichalasin H analogues.

Strategies for Engineering Natural Product Biosynthesis in Fungi.

Fungi are a prolific source of bioactive compounds, some of which have been developed as essential medicines and life-enhancing drugs. Genome sequencing has revealed that fungi have the potential to produce considerably more natural products (NPs) than are typically observed in the laboratory. Recently, there have been significant advances in the identification, understanding, and engineering of fungal biosynthetic gene clusters (BGCs). This review briefly describes examples of the engineering of fungal NP biosynthesis at the global, pathway, and enzyme level using in vivo and in vitro approaches and refers to the range and scale of heterologous expression systems available, developments in combinatorial biosynthesis, progress in understanding how fungal BGCs are regulated, and the applications of these novel biosynthetic enzymes as biocatalysts.

Function of pathway specific regulators in the ACE1 and pyrichalasin H biosynthetic gene clusters.

Ectopic expression of BC1 which encodes a putative pathway specific transcription factor from the ACE1 biosynthetic gene cluster of the rice pathogen Pyricularia oryzae Guy11 did not lead to the production of ACE1-related compounds. However the known compound hinnulin A was formed. A putative partial gene cluster potentially involved in the biosynthesis of hinnulin A and DHN melanin was validated by RT-PCR and a possible biosynthetic pathway is proposed. Ectopic expression of pyiR which encodes a pathway specific transcription factor from the pyrichalasin H biosynthetic gene cluster in Magnaporthe grisea NI980 led to the apparent up-regulation of the pyi cluster and a 3-fold increase in pyrichalasin production under standard fermentation conditions, but did not lead to the formation of new compounds.

The biosynthesis of cytochalasans.

Covering: up to 2017Cytochalasans are a class of natural products possessing a wide range of important biological activities, yet the full biosynthetic steps towards the formation of their characteristic chemical features remain unknown. This highlight provides an overview of the recent advances made in understanding the biosynthesis of this fascinating class of compounds, complementing and extending a previous comprehensive review of this topic (K. Scherlach, D. Boettger, N. Remme and C. Hertweck, Nat. Prod. Rep., 2010, 27, 869-886).