Fusion of farnesyldiphosphate synthase and epi-aristolochene synthase, a sesquiterpene cyclase involved in capsidiol biosynthesis in Nicotiana tabacum

Towards rational control of seed oil composition: dissecting cellular organization and flux control of lipid metabolism

Plant lipids represent a fascinating field of scientific study, in part due to a stark dichotomy in the limited fatty acid (FA) composition of cellular membrane lipids vs the huge diversity of FAs that can accumulate in triacylglycerols (TAGs), the main component of seed storage oils. With few exceptions, the strict chemical, structural, and biophysical roles imposed on membrane lipids since the dawn of life have constrained their FA composition to predominantly lengths of 16-18 carbons and containing 0-3 methylene-interrupted carbon-carbon double bonds in cis-configuration. However, over 450 "unusual" FA structures can be found in seed oils of different plants, and we are just beginning to understand the metabolic mechanisms required to produce and maintain this dichotomy. Here we review the current state of plant lipid research, specifically addressing the knowledge gaps in membrane and storage lipid synthesis from 3 angles: pathway fluxes including newly discovered TAG remodeling, key acyltransferase substrate selectivities, and the possible roles of "metabolons."

Engineering substrate channeling in a bifunctional terpene synthase

Fusicoccadiene synthase from Phomopsis amygdala (PaFS) is a bifunctional terpene synthase. It contains a prenyltransferase (PT) domain that generates geranylgeranyl diphosphate (GGPP) from dimethylallyl diphosphate and three equivalents of isopentenyl diphosphate, and a cyclase domain that converts GGPP into fusicoccadiene, a precursor of the diterpene glycoside Fusicoccin A. The two catalytic domains are connected by a flexible 69-residue linker. The PT domain mediates oligomerization to form predominantly octamers, with cyclase domains randomly splayed out around the PT core. Surprisingly, despite the random positioning of cyclase domains, substrate channeling is operative in catalysis since most of the GGPP generated by the PT remains on the enzyme for cyclization. Here, we demonstrate that covalent linkage of the PT and cyclase domains is not required for GGPP channeling, although covalent linkage may improve channeling efficiency. Moreover, GGPP competition experiments with other diterpene cyclases indicate that the PaFS PT and cyclase domains are preferential partners regardless of whether they are covalently linked or not. The cryoelectron microscopy structure of the 600-kD "linkerless" construct, in which the 69-residue linker is spliced out and replaced with the tripeptide PTQ, reveals that cyclase pairs associate with all four sides of the PT octamer and exhibit fascinating quaternary structural flexibility. These results suggest that optimal substrate channeling is achieved when a cyclase domain associates with the side of the PT octamer, regardless of whether the two domains are covalently linked and regardless of whether this interaction is transient or locked in place.

Engineering of CYP82Y1, a cytochrome P450 monooxygenase: a key enzyme in noscapine biosynthesis in opium poppy

Protein engineering provides a powerful base for the circumvention of challenges tied with characteristics accountable for enzyme functions. CYP82Y1 introduces a hydroxyl group (-OH) into C1 of N-methylcanadine as the substrate to yield 1-hydroxy-N-methylcanadine. This chemical process has been found to be the gateway to noscapine biosynthesis. Owning to the importance of CYP82Y1 in this biosynthetic pathway, it has been selected as a target for enzyme engineering. The insertion of tags to the N- and C-terminal of CYP82Y1 was assessed for their efficiencies for improvement of the physiological performances of CYP82Y1. Although these attempts achieved some positive results, further strategies are required to dramatically enhance the CYP82Y1 activity. Here methods that have been adopted to achieve a functionally improved CYP82Y1 will be reviewed. In addition, the possibility of recruitment of other techniques having not yet been implemented in CYP82Y1 engineering, including the substitution of the residues located in the substrate recognition site, formation of the synthetic fusion proteins, and construction of the artificial lipid-based scaffold will be discussed. Given the fact that the pace of noscapine synthesis is constrained by the CYP82Y1-catalyzing step, the methods proposed here are capable of accelerating the rate of reaction performed by CYP82Y1 through improving its properties, resulting in the enhancement of noscapine accumulation.

Chaga mushroom triterpenoids as adjuncts to minimally invasive cancer therapies: A review

Cancer has become the second leading cause of death in the world. Integrative cancer therapy management is continuously evolving to enhance treatment outcomes. Chaga mushroom (Inonotus obliquus) is a parasitic fungus acclaimed to contain pharmaceutical and nutraceutical value in the fight against cancer. In particular, triterpenoid constituents derived from Chaga mushrooms have been recognized for their anti-cancer activity after distinguished cytotoxicity was repeatedly observed in cancer cells treated in vitro with lipophilic fractions of extract compared to aqueous ones. Studies that investigate the anti-cancer activity of Chaga mushroom triterpenoids are reviewed in this article to determine which cancer cell lines demonstrate the greatest susceptibility to them while highlighting the structure-activity relationships that are involved. Triterpenoid supplementation as an adjunct to cancer treatment may be a viable option as inotodiol and 3-β-22 α-dihydroxylanosta-8, 25-diene-24-one have been shown to exhibit anti-cancer activity similar to that of conventional drugs. Advances in addressing bioavailability challenges are also included in this review as studies include in vivo components.

Biotechnological Approaches for Enhancing Polyhydroxyalkanoates (PHAs) Production: Current and Future Perspectives

The benefits of biotechnology are not limited to genetic engineering, but it also displays its great impact on industrial uses of crops (e.g., biodegradable plastics). Polyhydroxyalkanoates (PHAs) make a diverse class of bio-based and biodegradable polymers naturally synthesized by numerous microorganisms. However, several C3 and C4 plants have also been genetically engineered to produce PHAs. The highest production yield of PHAs was obtained with a well-known C3 plant, Arabidopsis thaliana, upto 40% of the dry weight of the leaf. This review summarizes all biotechnological mechanisms that have been adopted with the goal of increasing PHAs production in bacteria and plant species alike. Moreover, the possibility of using some methods that have not been applied in bioplastic science are discussed with special attention to plants. These include producing PHAs in transgenic hairy roots and cell suspension cultures, making transformed bacteria and plants via transposons, constructing an engineered metabolon, and overexpressing of phaP and the ABC operon concurrently. Taken together, that biotechnology will be highly beneficial for reducing plastic pollution through the implementation of biotechnological strategies is taken for granted.

Metabolic flux enhancement from the translational fusion of terpene synthases is linked to terpene synthase accumulation

The end-to-end fusion of enzymes that catalyse successive steps in a reaction pathway is a metabolic engineering strategy that has been successfully applied in a variety of pathways and is particularly common in terpene bioproduction. Despite its popularity, limited work has been done to interrogate the mechanism of metabolic enhancement from enzyme fusion. We observed a remarkable >110-fold improvement in nerolidol production upon translational fusion of nerolidol synthase (a sesquiterpene synthase) to farnesyl diphosphate synthase. This delivered a titre increase from 29.6 mg/L up to 4.2 g/L nerolidol in a single engineering step. Whole-cell proteomic analysis revealed that nerolidol synthase levels in the fusion strains were greatly elevated compared to the non-fusion control. Similarly, the fusion of nerolidol synthase to non-catalytic domains also produced comparable increases in titre, which coincided with improved enzyme expression. When farnesyl diphosphate synthase was fused to other terpene synthases, we observed more modest improvements in terpene titre (1.9- and 3.8-fold), corresponding with increases of a similar magnitude in terpene synthase levels. Our data demonstrate that increased in vivo enzyme levels - resulting from improved expression and/or improved protein stability - is a major driver of catalytic enhancement from enzyme fusion.

Making small molecules in plants: A chassis for synthetic biology-based production of plant natural products

Plant natural products have been extensively exploited in food, medicine, flavor, cosmetic, renewable fuel, and other industrial sectors. Synthetic biology has recently emerged as a promising means for the cost-effective and sustainable production of natural products. Compared with engineering microbes for the production of plant natural products, the potential of plants as chassis for producing these compounds is underestimated, largely due to challenges encountered in engineering plants. Knowledge in plant engineering is instrumental for enabling the effective and efficient production of valuable phytochemicals in plants, and also paves the way for a more sustainable future agriculture. In this manuscript, we briefly recap the biosynthesis of plant natural products, focusing primarily on industrially important terpenoids, alkaloids, and phenylpropanoids. We further summarize the plant hosts and strategies that have been used to engineer the production of natural products. The challenges and opportunities of using plant synthetic biology to achieve rapid and scalable production of high-value plant natural products are also discussed.

Another level of complex-ity: The role of metabolic channeling and metabolons in plant terpenoid metabolism

Terpenoids constitute one of the largest and most diverse classes of plant metabolites. While some terpenoids are involved in essential plant processes such as photosynthesis, respiration, growth, and development, others are specialized metabolites playing roles in the interaction of plants with their biotic and abiotic environment. Due to the distinct functions and properties of specific terpenoid compounds, there is a growing interest to introduce or modify their production in plants by metabolic engineering for agricultural, pharmaceutical, or industrial applications. The MVA and MEP pathways and the prenyltransferases providing the general precursors for terpenoid formation, as well as the enzymes of the various downstream metabolic pathways leading to the formation of different groups of terpenoid compounds have been characterized in detail in plants. In contrast, the molecular mechanisms directing the metabolic flux of precursors specifically toward one of several potentially competing terpenoid biosynthetic pathways are still not well understood. The formation of metabolons, multi-protein complexes composed of enzymes catalyzing sequential reactions of a metabolic pathway, provides a promising concept to explain the metabolic channeling that appears to occur in the complex terpenoid biosynthetic network of plants. Here we provide an overview about examples of potential metabolons involved in plant terpenoid metabolism that have been recently characterized and the first attempts to utilize metabolic channeling in terpenoid metabolic engineering. In addition, we discuss the gaps in our current knowledge and in consequence the need for future basic and applied research.

Engineering the Prenyltransferase Domain of a Bifunctional Assembly-Line Terpene Synthase

Copalyl diphosphate (CPP) synthase from Penicillium verruculosum (PvCPS) is a bifunctional diterpene synthase with both prenyltransferase and class II cyclase activities. The prenyltransferase α domain catalyzes the condensation of C₅ dimethylallyl diphosphate with three successively added C₅ isopentenyl diphosphates (IPPs) to form C₂₀ geranylgeranyl diphosphate (GGPP), which then undergoes a class II cyclization reaction at the βγ domain interface to generate CPP. The prenyltransferase α domain mediates oligomerization to form a 648-kD (αβγ)₆ hexamer. In the current study, we explore prenyltransferase structure-function relationships in this oligomeric assembly-line platform with the goal of generating alternative linear isoprenoid products. Specifically, we report steady-state enzyme kinetics, product analysis, and crystal structures of various site-specific variants of the prenyltransferase α domain. Crystal structures of the H786A, F760A, S723Y, S723F, and S723T variants have been determined at resolutions of 2.80, 3.10, 3.15, 2.65, and 2.00 Å, respectively. The substitution of S723 with bulky aromatic amino acids in the S723Y and S723F variants constricts the active site, thereby directing the formation of the shorter C₁₅ isoprenoid, farnesyl diphosphate. While the S723T substitution only subtly alters enzyme kinetics and does not compromise GGPP biosynthesis, the crystal structure of this variant reveals a nonproductive binding mode for IPP that likely accounts for substrate inhibition at high concentrations. Finally, mutagenesis of the catalytic general acid in the class II cyclase domain, D313A, significantly compromises prenyltransferase activity. This result suggests molecular communication between the prenyltransferase and cyclase domains despite their distant connection by a flexible polypeptide linker.

Assembly-Line Catalysis in Bifunctional Terpene Synthases

The magnificent chemodiversity of more than 95 000 terpenoid natural products identified to date largely originates from catalysis by two types of terpene synthases, prenyltransferases and cyclases. Prenyltransferases utilize 5-carbon building blocks in processive chain elongation reactions to generate linear C_5n isoprenoid diphosphates (n ≥ 2), which in turn serve as substrates for terpene cyclases that convert these linear precursors into structurally complex hydrocarbon products containing multiple rings and stereocenters. Terpene cyclization reactions are the most complex organic transformations found in nature in that more than half of the substrate carbon atoms undergo changes in chemical bonding during a multistep reaction sequence proceeding through several carbocation intermediates. Two general classes of cyclases are established on the basis of the chemistry of initial carbocation formation, and structural studies from our laboratory and others show that three fundamental protein folds designated α, β, and γ govern this chemistry. Catalysis by a class I cyclase occurs in an α domain, where a trinuclear metal cluster activates the substrate diphosphate leaving group to generate an allylic cation. Catalysis by a class II cyclase occurs in a β domain or at the interface of β and γ domains, where an aspartic acid protonates the terminal π bond of the substrate to yield a tertiary carbocation. Crystal structures reveal domain architectures of α, αβ, αβγ, βγ, and β.In some terpene synthases, these domains are combined to yield bifunctional enzymes that catalyze successive biosynthetic steps in assembly line fashion. Structurally characterized examples include bacterial geosmin synthase, an αα domain enzyme that catalyzes a class I cyclization reaction of C₁₅ farnesyl diphosphate in one active site and a transannulation-fragmentation reaction in the other to yield C₁₂ geosmin and C₃ acetone products. In comparison, plant abietadiene synthase is an αβγ domain enzyme in which C₂₀ geranylgeranyl diphosphate undergoes tandem class II-class I cyclization reactions to yield the tricyclic product. Recent structural studies from our laboratory show that bifunctional fungal cyclases form oligomeric complexes for assembly line catalysis. Bifunctional (+)-copalyl diphosphate synthase adopts (αβγ)₆ architecture in which the α domain generates geranylgeranyl diphosphate, which then undergoes class II cyclization in the βγ domains to yield the bicyclic product. Bifunctional fusicoccadiene synthase adopts (αα)₆ or (αα)₈ architecture in which one α domain generates geranylgeranyl diphosphate, which then undergoes class I cyclization in the other α domain to yield the tricyclic product. The prenyltransferase α domain mediates oligomerization in these systems. Attached by flexible polypeptide linkers, cyclase domains splay out from oligomeric prenyltransferase cores.In this Account, we review structure-function relationships for these bifunctional terpene synthases, with a focus on the oligomeric systems studied in our laboratory. The observation of substrate channeling for fusicoccadiene synthase suggests a model for dynamic cluster channeling in catalysis by oligomeric assembly line terpenoid synthases. Resulting efficiencies in carbon management suggest that such systems could be particularly attractive for use in synthetic biology approaches to generate high-value terpenoid natural products.

Mining methods and typical structural mechanisms of terpene cyclases

Terpenoids, formed by cyclization and/or permutation of isoprenes, are the most diverse and abundant class of natural products with a broad range of significant functions. One family of the critical enzymes involved in terpenoid biosynthesis is terpene cyclases (TCs), also known as terpene synthases (TSs), which are responsible for forming the ring structure as a backbone of functionally diverse terpenoids. With the recent advances in biotechnology, the researches on terpene cyclases have gradually shifted from the genomic mining of novel enzyme resources to the analysis of their structures and mechanisms. In this review, we summarize both the new methods for genomic mining and the structural mechanisms of some typical terpene cyclases, which are helpful for the discovery, engineering and application of more and new TCs.

Structural insight on assembly-line catalysis in terpene biosynthesis

Fusicoccadiene synthase from Phomopsis amygdali (PaFS) is a unique bifunctional terpenoid synthase that catalyzes the first two steps in the biosynthesis of the diterpene glycoside Fusicoccin A, a mediator of 14-3-3 protein interactions. The prenyltransferase domain of PaFS generates geranylgeranyl diphosphate, which the cyclase domain then utilizes to generate fusicoccadiene, the tricyclic hydrocarbon skeleton of Fusicoccin A. Here, we use cryo-electron microscopy to show that the structure of full-length PaFS consists of a central octameric core of prenyltransferase domains, with the eight cyclase domains radiating outward via flexible linker segments in variable splayed-out positions. Cryo-electron microscopy and chemical crosslinking experiments additionally show that compact conformations can be achieved in which cyclase domains are more closely associated with the prenyltransferase core. This structural analysis provides a framework for understanding substrate channeling, since most of the geranylgeranyl diphosphate generated by the prenyltransferase domains remains on the enzyme for cyclization to form fusicoccadiene.

Enhancing Sesquiterpenoid Production from Methane via Synergy of the Methylerythritol Phosphate Pathway and a Short-Cut Route to 1-Deoxy-D-xylulose 5-Phosphate in Methanotrophic Bacteria

Sesquiterpenoids are one of the most diverse classes of isoprenoids which exhibit numerous potentials in industrial biotechnology. The methanotrophs-based methane bioconversion is a promising approach for sustainable production of chemicals and fuels from methane. With intrinsic high carbon flux though the ribulose monophosphate cycle in Methylotuvimicrobium alcaliphilum 20Z, we demonstrated here that employing a short-cut route from ribulose 5-phosphate to 1-deoxy-d-xylulose 5-phosphate (DXP) could enable a more efficient isoprenoid production via the methylerythritol 4-phosphate (MEP) pathway, using α-humulene as a model compound. An additional 2.8-fold increase in α-humulene production yield was achieved by the fusion of the nDXP enzyme and DXP reductase. Additionally, we utilized these engineering strategies for the production of another sesquiterpenoid, α-bisabolene. The synergy of the nDXP and MEP pathways improved the α-bisabolene titer up to 12.24 ± 0.43 mg/gDCW, twofold greater than that of the initial strain. This study expanded the suite of sesquiterpenoids that can be produced from methane and demonstrated the synergistic uses of the nDXP and MEP pathways for improving sesquiterpenoid production in methanotrophic bacteria.

Unlocking the biosynthesis of sesquiterpenoids from methane via the methylerythritol phosphate pathway in methanotrophic bacteria, using α-humulene as a model compound

Isoprenoids are an abundant and diverse class of natural products with various applications in the pharmaceutical, cosmetics and biofuel industries. A methanotroph-based biorefinery is an attractive scenario for the production of a variety of value-added compounds from methane, because methane is a promising alternative feedstock for industrial biomanufacturing. In this study, we metabolically engineered Methylotuvimicrobium alcaliphilum 20Z for de novo synthesis of a sesquiterpenoid from methane, using α-humulene as a model compound, via optimization of the native methylerythritol phosphate (MEP) pathway. Expression of codon-optimized α-humulene synthase from Zingiber zerumbet in M. alcaliphilum 20Z resulted in an initial yield of 0.04 mg/g dry cell weight. Overexpressing key enzymes (IspA, IspG, and Dxs) for debottlenecking of the MEP pathway increased α-humulene production 5.2-fold compared with the initial strain. Subsequently, redirecting the carbon flux through the Embden-Meyerhof-Parnas pathway resulted in an additional 3-fold increase in α-humulene production. Additionally, a genome-scale model using flux scanning based on enforced objective flux method was used to identify potential overexpression targets to increase flux towards isoprenoid production. Several target reactions from cofactor synthesis pathways were probed and evaluated for their effects on α-humulene synthesis, resulting in α-humulene yield up to 0.75 mg/g DCW with 18.8-fold enhancement from initial yield. This study first demonstrates production of a sesquiterpenoid from methane using methanotrophs as the biocatalyst and proposes potential strategies to enhance production of sesquiterpenoid and related isoprenoid products in engineered methanotrophic bacteria.

Transient expression and purification of β-caryophyllene synthase in Nicotiana benthamiana to produce β-caryophyllene in vitro

The sesquiterpene β-caryophyllene is an ubiquitous component in many plants that has commercially been used as an aroma in cosmetics and perfumes. Recent studies have shown its potential use as a therapeutic agent and biofuel. Currently, β-caryophyllene is isolated from large amounts of plant material. Molecular farming based on the Nicotiana benthamiana transient expression system may be used for a more sustainable production of β-caryophyllene. In this study, a full-length cDNA of a new duplicated β-caryophyllene synthase from Artemisia annua (AaCPS1) was isolated and functionally characterized. In order to produce β-caryophyllene in vitro, the AaCPS1 was cloned into a plant viral-based vector pEAQ-HT. Subsequently, the plasmid was transferred into the Agrobacterium and agroinfiltrated into N. benthamiana leaves. The AaCPS1 expression was analyzed by quantitative PCR at different time points after agroinfiltration. The highest level of transcripts was observed at 9 days post infiltration (dpi). The AaCPS1 protein was extracted from the leaves at 9 dpi and purified by cobalt–nitrilotriacetate (Co-NTA) affinity chromatography using histidine tag with a yield of 89 mg kg⁻¹ fresh weight of leaves. The protein expression of AaCPS1 was also confirmed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and western blot analyses. AaCPS1 protein uses farnesyl diphosphate (FPP) as a substrate to produce β-caryophyllene. Product identification and determination of the activity of purified AaCPS1 were done by gas chromatography–mass spectrometry (GC–MS). GC–MS results revealed that the AaCPS1 produced maximum 26.5 ± 1 mg of β-caryophyllene per kilogram fresh weight of leaves after assaying with FPP for 6 h. Using AaCPS1 as a proof of concept, we demonstrate that N. benthamiana can be considered as an expression system for production of plant proteins that catalyze the formation of valuable chemicals for industrial applications.

Higher-order oligomerization of a chimeric αβγ bifunctional diterpene synthase with prenyltransferase and class II cyclase activities is concentration-dependent

The unusual diterpene (C₂₀) synthase copalyl diphosphate synthase from Penicillium verruculosum (PvCPS) is the first bifunctional terpene synthase identified with both prenyltransferase and class II cyclase activities in a single polypeptide chain with αβγ domain architecture. The C-terminal prenyltransferase α domain generates geranylgeranyl diphosphate which is then cyclized to form copalyl diphosphate at the N-terminal βγ domain interface. We now demonstrate that PvCPS exists as a hexamer at high concentrations - a unique quaternary structure for known αβγ terpene synthases. Hexamer assembly is corroborated by a 2.41 Å-resolution crystal structure of the α domain prenyltransferase obtained from limited proteolysis of full-length PvCPS, as well as the ab initio model of full-length PvCPS derived from small-angle X-ray scattering data. Hexamerization of the prenyltransferase α domain appears to drive the hexamerization of full-length PvCPS. The PvCPS hexamer dissociates into lower-order species at lower concentrations, as evidenced by size-exclusion chromatography in-line with multiangle light scattering, sedimentation velocity analytical ultracentrifugation, and native polyacrylamide gel electrophoresis experiments, suggesting that oligomerization is concentration dependent. Even so, PvCPS oligomer assembly does not affect prenyltransferase activity in vitro.

Enhancing epi-cedrol production in Escherichia coli by fusion expression of farnesyl pyrophosphate synthase and epi-cedrol synthase

Terpene synthase catalyses acyclic diphosphate farnesyl diphosphate into desired sesquiterpenes. In this study, a fusion enzyme was constructed by linking Santalum album farnesyl pyrophosphate synthase (SaFPPS) individually with terpene synthase and Artemisia annua Epi-cedrol synthase (AaECS). The stop codon at the N-terminus of SaFPPS was removed and replaced by a short peptide (GSGGS) to introduce a linker between the two open reading frames. This fusion clone was expressed in Escherichia coli Rosseta DE3 cells. The fusion enzyme FPPS-ECS produced sesquiterpene 8-epi-cedrol from substrates isopentenyl pyrophosphate and dimethylallyl pyrophosphate through sequential reactions. The K _m values for FPPS-ECS for isopentyl diphosphate was 4.71 µM. The fusion enzyme carried out the efficient conversion of IPP to epi-cedrol, in comparison to single enzymes SaFPPS and AaECS when combined together in enzyme assay over time. Further, the recombinant E. coli BL21 strain harbouring fusion plasmid successfully produced epi-cedrol in fermentation medium. The strain having fusion plasmid (pET32a-FPPS-ECS) produced 1.084 ± 0.09 mg/L epi-cedrol, while the strain harbouring mixed plasmid (pRSETB-FPPS and pET28a-ECS) showed 1.002 ± 0.07 mg/L titre in fermentation medium by overexpression and MEP pathway utilization. Structural analysis was done by I-TASSER server and docking was done by AutoDock Vina software, which suggested that secondary structure of the N- C terminal domain and their relative positions to functional domains of the fusion enzyme was greatly significant to the catalytic properties of the fusion enzymatic complex than individual enzymes.

Chimeric Terpene Synthases Possessing both Terpene Cyclization and Prenyltransfer Activities

Prenyltransferase (PT) and terpene synthase (TPS) are key enzymes in the formation of the basic carbon skeletons of terpenoids. The PTs determine the prenyl carbon chain length, whereas TPSs generate the structural complexity of the molecular scaffolds, forming various ring structures. Normally, PTs and TPSs are separate, independent enzymes. However, in 2007, a chimeric enzyme, in which the PT was fused with the TPS, was found in a fungus. Recent studies have revealed that such chimeric TPSs are widely distributed in fungi and function in the biosyntheses of various terpene natural products, including sesterterpenes, which are a relatively rare group of terpenoids. This review summarizes the accumulated knowledge of these recently discovered, unique, chimeric TPSs.

Structural and Chemical Biology of Terpenoid Cyclases

The year 2017 marks the twentieth anniversary of terpenoid cyclase structural biology: a trio of terpenoid cyclase structures reported together in 1997 were the first to set the foundation for understanding the enzymes largely responsible for the exquisite chemodiversity of more than 80000 terpenoid natural products. Terpenoid cyclases catalyze the most complex chemical reactions in biology, in that more than half of the substrate carbon atoms undergo changes in bonding and hybridization during a single enzyme-catalyzed cyclization reaction. The past two decades have witnessed structural, functional, and computational studies illuminating the modes of substrate activation that initiate the cyclization cascade, the management and manipulation of high-energy carbocation intermediates that propagate the cyclization cascade, and the chemical strategies that terminate the cyclization cascade. The role of the terpenoid cyclase as a template for catalysis is paramount to its function, and protein engineering can be used to reprogram the cyclization cascade to generate alternative and commercially important products. Here, I review key advances in terpenoid cyclase structural and chemical biology, focusing mainly on terpenoid cyclases and related prenyltransferases for which X-ray crystal structures have informed and advanced our understanding of enzyme structure and function.

Effect of flexible linker length on the activity of fusion protein 4-coumaroyl-CoA ligase::stilbene synthase

In order to elucidate the effect of flexible linker length on the catalytic efficiency of fusion proteins, two short flexible peptide linkers of various lengths were fused between Arabidopsis thaliana 4-coumaroyl-CoA ligase (4CL) and Polygonum cuspidatum stilbene synthase (STS) to generate fusion proteins 4CL-(GSG)_n-STS (n ≤ 5) and 4CL-(GGGGS)_n-STS (n ≤ 4). The fusion proteins were expressed in both Escherichia coli and Saccharomyces cerevisiae, and their bioactivities were tested in vitro and in vivo using purified proteins and engineered strains, respectively. The catalytic efficiency of the fusions decreased gradually with the increase of GSG or GGGGS repeats. In both engineered S. cerevisiae and E. coli in vivo experiments, the capacity of resveratrol production decreased gradually with increasing linker length. In silico analysis showed that the prediction of homology models of fusion proteins was consistent with the in vitro and in vivo results.

Unearthing a sesterterpene biosynthetic repertoire in the Brassicaceae through genome mining reveals convergent evolution

Sesterterpenoids are a rare terpene class harboring untapped chemodiversity and bioactivities. Their structural diversity originates primarily from the scaffold-generating sesterterpene synthases (STSs). In fungi, all six known STSs are bifunctional, containing C-terminal trans-prenyltransferase (PT) and N-terminal terpene synthase (TPS) domains. In plants, two colocalized PT and TPS gene pairs from Arabidopsis thaliana were recently reported to synthesize sesterterpenes. However, the landscape of PT and TPS genes in plant genomes is unclear. Here, using a customized algorithm for systematically searching plant genomes, we reveal a suite of physically colocalized pairs of PT and TPS genes for the biosynthesis of a large sesterterpene repertoire in the wider Brassicaceae. Transient expression of seven TPSs from A. thaliana, Capsella rubella, and Brassica oleracea in Nicotiana benthamiana yielded fungal-type sesterterpenes with tri-, tetra-, and pentacyclic scaffolds, and notably (-)-ent-quiannulatene, an enantiomer of the fungal metabolite (+)-quiannulatene. Protein and structural modeling analysis identified an amino acid site implicated in structural diversification. Mutation of this site in one STS (AtTPS19) resulted in premature termination of carbocation intermediates and accumulation of bi-, tri-, and tetracyclic sesterterpenes, revealing the cyclization path for the pentacyclic sesterterpene (-)-retigeranin B. These structural and mechanistic insights, together with phylogenetic analysis, suggest convergent evolution of plant and fungal STSs, and also indicate that the colocalized PT-TPS gene pairs in the Brassicaceae may have originated from a common ancestral gene pair present before speciation. Our findings further provide opportunities for rapid discovery and production of sesterterpenes through metabolic and protein engineering.

Identification of a trichothecene production inhibitor by chemical array and library screening using trichodiene synthase as a target protein

Trichothecene mycotoxins often accumulate in apparently normal grains of cereal crops. In an effort to develop an agricultural chemical to reduce trichothecene contamination, we screened trichothecene production inhibitors from the compounds on the chemical arrays. By using the trichodiene (TDN) synthase tagged with hexahistidine (rTRI5) as a target protein, 32 hit compounds were obtained from chemical library of the RIKEN Natural Product Depository (NPDepo) by chemical array screening. At 10μgmL^-1, none of the 32 chemicals inhibited trichothecene production by Fusarium graminearum in liquid culture. Against the purified rTRI5 enzyme, however, NPD10133 [progesterone 3-(O-carboxymethyl)oxime amide-bonded to phenylalanine] showed weak inhibitory activity at 10μgmL^-1 (18.7μM). For the screening of chemicals inhibiting trichothecene accumulation in liquid culture, 20 analogs of NPD10133 selected from the NPDepo chemical library were assayed. At 10μM, only NPD352 [testosterone 3-(O-carboxymethyl)oxime amide-bonded to phenylalanine methyl ester] inhibited rTRI5 activity and trichothecene production. Kinetic analysis suggested that the enzyme inhibition was of a mixed-type. The identification of NPD352 as a TDN synthase inhibitor lays the foundation for the development of a more potent inhibitor via systematic introduction of wide structural diversity on the gonane skeleton and amino acid residues.

Multi-domain terpenoid cyclase architecture and prospects for proximity in bifunctional catalysis

Crystal structures of terpenoid cyclases reveal assemblies of three basic domains designated α, β, and γ. While the biosynthesis of cyclic monoterpenes (C₁₀) and sesquiterpenes (C₁₅) most often involves enzymes with α or αβ domain architecture, the biosynthesis of cyclic diterpenes (C₂₀), sesterterpenes (C₂₅), and triterpenes (C₃₀) can involve enzymes with α, αα, βγ, or αβγ domain architecture. Indeed, some enzymes of terpenoid biosynthesis are bifunctional, with distinct active sites that catalyze sequential reactions. Interestingly, some of these enzymes oligomerize to form dimers, tetramers, and hexamers. Not only can such assemblies enable enzyme regulation by allostery, but they can also provide a modest enhancement of terpenoid product flux through proximity channeling or cluster channeling. The mixing and matching of functional terpenoid cyclase domains through tertiary and/or quaternary structure may also comprise an evolutionary strategy for facile product diversification.

Structure and Function of Fusicoccadiene Synthase, a Hexameric Bifunctional Diterpene Synthase

Fusicoccin A is a diterpene glucoside phytotoxin generated by the fungal pathogen Phomopsis amygdali that causes the plant disease constriction canker, first discovered in New Jersey peach orchards in the 1930s. Fusicoccin A is also an emerging new lead in cancer chemotherapy. The hydrocarbon precursor of fusicoccin A is the tricyclic diterpene fusicoccadiene, which is generated by a bifunctional terpenoid synthase. Here, we report X-ray crystal structures of the individual catalytic domains of fusicoccadiene synthase: the C-terminal domain is a chain elongation enzyme that generates geranylgeranyl diphosphate, and the N-terminal domain catalyzes the cyclization of geranylgeranyl diphosphate to form fusicoccadiene. Crystal structures of each domain complexed with bisphosphonate substrate analogues suggest that three metal ions and three positively charged amino acid side chains trigger substrate ionization in each active site. While in vitro incubations reveal that the cyclase domain can utilize farnesyl diphosphate and geranyl diphosphate as surrogate substrates, these shorter isoprenoid diphosphates are mainly converted into acyclic alcohol or hydrocarbon products. Gel filtration chromatography and analytical ultracentrifugation experiments indicate that full-length fusicoccadiene synthase adopts hexameric quaternary structure, and small-angle X-ray scattering data yield a well-defined molecular envelope illustrating a plausible model for hexamer assembly.

In Vivo Validation of In Silico Predicted Metabolic Engineering Strategies in Yeast: Disruption of α-Ketoglutarate Dehydrogenase and Expression of ATP-Citrate Lyase for Terpenoid Production

Background

Engineering of the central carbon metabolism of Saccharomyces cerevisiae to redirect metabolic flux towards cytosolic acetyl-CoA has become a central topic in yeast biotechnology. A cell factory with increased flux into acetyl-CoA can be used for heterologous production of terpenoids for pharmaceuticals, biofuels, fragrances, or other acetyl-CoA derived compounds. In a previous study, we identified promising metabolic engineering targets in S. cerevisiae using an in silico stoichiometric metabolic network analysis. Here, we validate selected in silico strategies in vivo.

Results

Patchoulol was produced by yeast via a heterologous patchoulol synthase of Pogostemon cablin. To increase the metabolic flux from acetyl-CoA towards patchoulol, a truncated HMG-CoA reductase was overexpressed and farnesyl diphosphate synthase was fused with patchoulol synthase. The highest increase in production could be achieved by modifying the carbon source; sesquiterpenoid titer increased from glucose to ethanol by a factor of 8.4. Two strategies predicted in silico were chosen for validation in this work. Disruption of α-ketoglutarate dehydrogenase gene (KGD1) was predicted to redirect the metabolic flux via the pyruvate dehydrogenase bypass towards acetyl-CoA. The metabolic flux was redirected as predicted, however, the effect was dependent on cultivation conditions and the flux was interrupted at the level of acetate. High amounts of acetate were produced. As an alternative pathway to synthesize cytosolic acetyl-CoA, ATP-citrate lyase was expressed as a polycistronic construct, however, in vivo performance of the enzyme needs to be optimized to increase terpenoid production.

Conclusions

Stoichiometric metabolic network analysis can be used successfully as a metabolic prediction tool. However, this study highlights that kinetics, regulation and cultivation conditions may interfere, resulting in poor in vivo performance. Main sites of regulation need to be released and improved enzymes are essential to meet the required activities for an increased product formation in vivo.

Structural Studies of Geosmin Synthase, a Bifunctional Sesquiterpene Synthase with αα Domain Architecture That Catalyzes a Unique Cyclization-Fragmentation Reaction Sequence

Geosmin synthase from Streptomyces coelicolor (ScGS) catalyzes an unusual, metal-dependent terpenoid cyclization and fragmentation reaction sequence. Two distinct active sites are required for catalysis: the N-terminal domain catalyzes the ionization and cyclization of farnesyl diphosphate to form germacradienol and inorganic pyrophosphate (PPi), and the C-terminal domain catalyzes the protonation, cyclization, and fragmentation of germacradienol to form geosmin and acetone through a retro-Prins reaction. A unique αα domain architecture is predicted for ScGS based on amino acid sequence: each domain contains the metal-binding motifs typical of a class I terpenoid cyclase, and each domain requires Mg(2+) for catalysis. Here, we report the X-ray crystal structure of the unliganded N-terminal domain of ScGS and the structure of its complex with three Mg(2+) ions and alendronate. These structures highlight conformational changes required for active site closure and catalysis. Although neither full-length ScGS nor constructs of the C-terminal domain could be crystallized, homology models of the C-terminal domain were constructed on the basis of ∼36% sequence identity with the N-terminal domain. Small-angle X-ray scattering experiments yield low-resolution molecular envelopes into which the N-terminal domain crystal structure and the C-terminal domain homology model were fit, suggesting possible αα domain architectures as frameworks for bifunctional catalysis.

Stereochemical inversion of (S)-reticuline by a cytochrome P450 fusion in opium poppy

The gateway to morphine biosynthesis in opium poppy (Papaver somniferum) is the stereochemical inversion of (S)-reticuline since the enzyme yielding the first committed intermediate salutaridine is specific for (R)-reticuline. A fusion between a cytochrome P450 (CYP) and an aldo-keto reductase (AKR) catalyzes the S-to-R epimerization of reticuline via 1,2-dehydroreticuline. The reticuline epimerase (REPI) fusion was detected in opium poppy and in Papaver bracteatum, which accumulates thebaine. In contrast, orthologs encoding independent CYP and AKR enzymes catalyzing the respective synthesis and reduction of 1,2-dehydroreticuline were isolated from Papaver rhoeas, which does not accumulate morphinan alkaloids. An ancestral relationship between these enzymes is supported by a conservation of introns in the gene fusions and independent orthologs. Suppression of REPI transcripts using virus-induced gene silencing in opium poppy reduced levels of (R)-reticuline and morphinan alkaloids and increased the overall abundance of (S)-reticuline and its O-methylated derivatives. Discovery of REPI completes the isolation of genes responsible for known steps of morphine biosynthesis.

Enhancing Terpene yield from sugars via novel routes to 1-deoxy-d-xylulose 5-phosphate

Terpene synthesis in the majority of bacterial species, together with plant plastids, takes place via the 1-deoxy-d-xylulose 5-phosphate (DXP) pathway. The first step of this pathway involves the condensation of pyruvate and glyceraldehyde 3-phosphate by DXP synthase (Dxs), with one-sixth of the carbon lost as CO2. A hypothetical novel route from a pentose phosphate to DXP (nDXP) could enable a more direct pathway from C5 sugars to terpenes and also circumvent regulatory mechanisms that control Dxs, but there is no enzyme known that can convert a sugar into its 1-deoxy equivalent. Employing a selection for complementation of a dxs deletion in Escherichia coli grown on xylose as the sole carbon source, we uncovered two candidate nDXP genes. Complementation was achieved either via overexpression of the wild-type E. coli yajO gene, annotated as a putative xylose reductase, or via various mutations in the native ribB gene. In vitro analysis performed with purified YajO and mutant RibB proteins revealed that DXP was synthesized in both cases from ribulose 5-phosphate (Ru5P). We demonstrate the utility of these genes for microbial terpene biosynthesis by engineering the DXP pathway in E. coli for production of the sesquiterpene bisabolene, a candidate biodiesel. To further improve flux into the pathway from Ru5P, nDXP enzymes were expressed as fusions to DXP reductase (Dxr), the second enzyme in the DXP pathway. Expression of a Dxr-RibB(G108S) fusion improved bisabolene titers more than 4-fold and alleviated accumulation of intracellular DXP.

Cloning and characterization of AabHLH1, a bHLH transcription factor that positively regulates artemisinin biosynthesis in Artemisia annua

Amorpha-4,11-diene synthase (ADS) and Cyt P450 monooxygenase (CYP71AV1) in Artemisia annua L. are two key enzymes involved in the biosynthesis of artemisinin. The promoters of ADS and CYP71AV1 contain E-box elements, which are putative binding sites for basic helix-loop-helix (bHLH) transcription factors. This study successfully isolated a bHLH transcription factor gene from A. annua, designated as AabHLH1, from a cDNA library of the glandular secretory trichomes (GSTs) in which artemisinin is synthesized and sequestered. AabHLH1 encodes a protein of 650 amino acids containing one putative bHLH domain. AabHLH1 and ADS genes were strongly induced by ABA and the fungal elicitor, chitosan. The transient expression analysis of the AabHLH1-green fluorescent protein (GFP) reporter gene revealed that AabHLH1 was targeted to nuclei. Biochemical analysis demonstrated that the AabHLH1 protein was capable of binding to the E-box cis-elements, present in both ADS and CYP71AV1 promoters, and possessed transactivation activity in yeast. In addition, transient co-transformation of AabHLH1 and CYP71AV1Pro::GUS in A. annua leaves showed a significant activation of the expression of the GUS (β-glucuronidase) gene in transformed A. annua, but mutation of the E-boxes resulted in abolition of activation, suggesting that the E-box is important for the CYP71AV1 promoter activity. Furthermore, transient expression of AabHLH1 in A. annua leaves increased transcript levels of the genes involved in artemisinin biosynthesis, such as ADS, CYP71AV1 and HMGR. These results suggest that AabHLH1 can positively regulate the biosynthesis of artemisinin.

Metabolic engineering of volatile isoprenoids in plants and microbes

The chemical properties and diversity of volatile isoprenoids lends them to a broad variety of biological roles. It also lends them to a host of biotechnological applications, both by taking advantage of their natural functions and by using them as industrial chemicals/chemical feedstocks. Natural functions include roles as insect attractants and repellents, abiotic stress protectants in pathogen defense, etc. Industrial applications include use as pharmaceuticals, flavours, fragrances, fuels, fuel additives, etc. Here we will examine the ways in which researchers have so far found to exploit volatile isoprenoids using biotechnology. Production and/or modification of volatiles using metabolic engineering in both plants and microorganisms are reviewed, including engineering through both mevalonate and methylerythritol diphosphate pathways. Recent advances are illustrated using several case studies (herbivores and bodyguards, isoprene, and monoterpene production in microbes). Systems and synthetic biology tools with particular utility for metabolic engineering are also reviewed. Finally, we discuss the practical realities of various applications in modern biotechnology, explore possible future applications, and examine the challenges of moving these technologies forward so that they can deliver tangible benefits. While this review focuses on volatile isoprenoids, many of the engineering approaches described here are also applicable to non-isoprenoid volatiles and to non-volatile isoprenoids.

A raison d'être for two distinct pathways in the early steps of plant isoprenoid biosynthesis?

When compared to other organisms, plants are atypical with respect to isoprenoid biosynthesis: they utilize two distinct and separately compartmentalized pathways to build up isoprene units. The co-existence of these pathways in the cytosol and in plastids might permit the synthesis of many vital compounds, being essential for a sessile organism. While substrate exchange across membranes has been shown for a variety of plant species, lack of complementation of strong phenotypes, resulting from inactivation of either the cytosolic pathway (growth and development defects) or the plastidial pathway (pigment bleaching), seems to be surprising at first sight. Hundreds of isoprenoids have been analyzed to determine their biosynthetic origins. It can be concluded that in angiosperms, under standard growth conditions, C₂₀-phytyl moieties, C₃₀-triterpenes and C₄₀-carotenoids are made nearly exclusively within compartmentalized pathways, while mixed origins are widespread for other types of isoprenoid-derived molecules. It seems likely that this coexistence is essential for the interaction of plants with their environment. A major purpose of this review is to summarize such observations, especially within an ecological and functional context and with some emphasis on regulation. This latter aspect still requires more work and present conclusions are preliminary, although some general features seem to exist.

Diversion of flux toward sesquiterpene production in Saccharomyces cerevisiae by fusion of host and heterologous enzymes

The ability to transfer metabolic pathways from the natural producer organisms to the well-characterized cell factory Saccharomyces cerevisiae is well documented. However, as many secondary metabolites are produced by collaborating enzymes assembled in complexes, metabolite production in yeast may be limited by the inability of the heterologous enzymes to collaborate with the native yeast enzymes. This may cause loss of intermediates by diffusion or degradation or due to conversion of the intermediate through competitive pathways. To bypass this problem, we have pursued a strategy in which key enzymes in the pathway are expressed as a physical fusion. As a model system, we have constructed several fusion protein variants in which farnesyl diphosphate synthase (FPPS) of yeast has been coupled to patchoulol synthase (PTS) of plant origin (Pogostemon cablin). Expression of the fusion proteins in S. cerevisiae increased the production of patchoulol, the main sesquiterpene produced by PTS, up to 2-fold. Moreover, we have demonstrated that the fusion strategy can be used in combination with traditional metabolic engineering to further increase the production of patchoulol. This simple test case of synthetic biology demonstrates that engineering the spatial organization of metabolic enzymes around a branch point has great potential for diverting flux toward a desired product.

Upregulation of phytosterol and triterpene biosynthesis in Centella asiatica hairy roots overexpressed ginseng farnesyl diphosphate synthase

Farnesyl diphosphate synthase (FPS) plays an essential role in organ development in plants. However, FPS has not previously been identified as a key regulatory enzyme in triterpene biosynthesis. To elucidate the functions of FPS in triterpene biosynthesis, C. asiatica was transformed with a construct harboring Panax ginseng FPS (PgFPS)-encoding cDNA coupled to the cauliflower mosaic virus 35S promoter. Higher levels of CaDDS (C. asiatica dammarenediol synthase) and CaCYS (C. asiatica cycloartenol synthase) mRNA were detected in all hairy root lines overexpressing when compared with the controls. However, no differences were detected in any expression of the CaSQS (C. asiatica squalene synthase) gene. In particular, the upregulation of CaDDS transcripts suggests that FPS may result in alterations in triterpene biosynthesis capacity. Squalene contents in the T17, T24, and T27 lines were increased to 1.1-, 1.3- and 1.5-fold those in the controls, respectively. The total sterol contents in the T24 line were approximately three times higher than those of the controls. Therefore, these results indicated that FPS performs a regulatory function in phytosterol biosynthesis. To evaluate the contribution of FPS to triterpene biosynthesis, we applied methyl jasmonate as an elicitor of hairy roots expressing PgFPS. The results of HPLC analysis revealed that the content of madecassoside and asiaticoside in the T24 line was transiently increased by 1.15-fold after 14 days of MJ treatment. This result may indicate that FPS performs a role not only in phytosterol regulation, but also in triterpene biosynthesis.

Metabolic engineering of natural products in plants; tools of the trade and challenges for the future

Plant natural products play essential roles in plant survivability and many of them are used as nutrients, colorants, flavors, fragrances, and medicines. Genetic engineering of plants for natural products can help alleviate the demands for limited natural resources. Successes in enhancing production capacities have included manipulating blocks of genes coding for segments of pathways, over-expression of putative rate-limiting steps in pathways, expression of transcription factors regulating the entire metabolic pathways, and the construction of novel branch pathways capable of diverting carbon to the biosynthesis of unique metabolites in unexpected intracellular compartments. Further enhancements are likely if more efficient pathways can be constructed, providing for the efficient channeling of intermediates to final products, and if the means for sequestering natural products in planta can be accomplished.

Fusicoccins are biosynthesized by an unusual chimera diterpene synthase in fungi

Fusicoccins are a class of diterpene glucosides produced by the plant-pathogenic fungus Phomopsis amygdali. As modulators of 14-3-3 proteins, fusicoccins function as potent activators of plasma membrane H(+)-ATPase in plants and also exhibit unique biological activity in animal cells. Despite their well studied biological activities, no genes encoding fusicoccin biosynthetic enzymes have been identified. Cyclic diterpenes are commonly synthesized via cyclization of a C(20) precursor, geranylgeranyl diphosphate (GGDP), which is produced through condensation of the universal C(5) isoprene units dimethylallyl diphosphate and isopentenyl diphosphate by prenyltransferases. We found that (+)-fusicocca-2,10 (14)-diene, a tricyclic hydrocarbon precursor for fusicoccins, is biosynthesized from the C(5) isoprene units by an unusual multifunctional enzyme, P. amygdali fusicoccadiene synthase (PaFS), which shows both prenyltransferase and terpene cyclase activities. The functional analysis of truncated mutants and site-directed mutagenesis demonstrated that PaFS consists of two domains: a terpene cyclase domain at the N terminus and a prenyltransferase domain at the C terminus. These findings suggest that fusicoccadiene can be produced efficiently in the fungus by using the C(5) precursors, irrespective of GGDP availability. In fact, heterologous expression of PaFS alone resulted in the accumulation of fusicocca-2,10 (14)-diene in Escherichia coli cells, whereas no product was detected in E. coli cells expressing Gibberella fujikuroi ent-kaurene synthase, another fungal diterpene cyclase that also uses GGDP as a substrate but does not contain a prenyltransferase domain. Genome walking suggested that fusicoccin biosynthetic enzymes are encoded as a gene cluster near the PaFS gene.

Unusual features of a recombinant apple alpha-farnesene synthase

A recombinant alpha-farnesene synthase from apple (Malus x domestica), expressed in Escherichia coli, showed features not previously reported. Activity was enhanced 5-fold by K(+) and all four isomers of alpha-farnesene, as well as beta-farnesene, were produced from an isomeric mixture of farnesyl diphosphate (FDP). Monoterpenes, linalool, (Z)- and (E)-beta-ocimene and beta-myrcene, were synthesised from geranyl diphosphate (GDP), but at 18% of the optimised rate for alpha-farnesene synthesis from FDP. Addition of K(+) reduced monoterpene synthase activity. The enzyme also produced alpha-farnesene by a reaction involving coupling of GDP and isoprenyl diphosphate but at <1% of the rate with FDP. Mutagenesis of active site aspartate residues removed sesquiterpene, monoterpene and prenyltransferase activities suggesting catalysis through the same active site. Phylogenetic analysis clusters this enzyme with isoprene synthases rather than with other sesquiterpene synthases, suggesting that it has evolved differently from other plant sesquiterpene synthases. This is the first demonstration of a sesquiterpene synthase possessing prenyltransferase activity.

Combinatorial biosynthesis of medicinal plant secondary metabolites

Combinatorial biosynthesis is a new tool in the generation of novel natural products and for the production of rare and expensive natural products. The basic concept is combining metabolic pathways in different organisms on a genetic level. As a consequence heterologous organisms provide precursors from their own primary and secondary metabolism that are metabolised to the desired secondary product due to the expression of foreign genes. In this review we discuss the possibilities and limitations of combining genes from different organisms and the expression of heterologous genes. Major focuses are fundamentals of the genetic work, used expression systems and latest progress in this field. Combinatorial biosynthesis is discussed for important classes of natural products, including alkaloids (vinblastine, vincristine), terpenoids (artemisinin, paclitaxel) and flavonoids. The role and importance of today's used host organisms is critically described, and the latest approaches discussed to give an outlook for future trends and possibilities.

Production of the artemisinin precursor amorpha-4,11-diene by engineered Saccharomyces cerevisiae

The gene encoding for amorpha-4,11-diene synthase from Artemisia annua was transformed into yeast Saccharomyces cerevisiae in two fundamentally different ways. First, the gene was subcloned into the galactose-inducible, high-copy number yeast expression vector pYeDP60 and used to transform the Saccharomyces cerevisiae strain CEN.PK113-5D. Secondly, amorpha-4,11-diene synthase gene, regulated by the same promoter, was introduced into the yeast genome by homologous recombination. In protein extracts from galactose-induced yeast cells, a higher activity was observed for yeast expressing the enzyme from the plasmid. The genome-transformed yeast grows at the same rate as wild-type yeast while plasmid-carrying yeast grows somewhat slower than the wild-type yeast. The plasmid and genome-transformed yeasts produced 600 and 100 microg/l of the artemisinin precursor amorpha-4,11-diene, respectively, during 16-days' batch cultivation.

Redirection of cytosolic or plastidic isoprenoid precursors elevates terpene production in plants

Terpenes constitute a distinct class of natural products that attract insects, defend against phytopathogenic microbes and combat human diseases. However, like most natural products, they are usually made by plants and microbes in small amounts and as complex mixtures. Chemical synthesis is often costly and inefficient, and may not yield enantiomerically pure terpenes, whereas large-scale microbial production requires expensive feedstocks. We engineered high-level terpene production in tobacco plants by diverting carbon flow from cytosolic or plastidic isopentenyl diphosphate through overexpression in either compartment of an avian farnesyl diphosphate synthase and an appropriate terpene synthase. Isotopic labeling studies suggest little, if any, metabolite exchange between these two subcellular compartments. The strategy increased synthesis of the sesquiterpenes patchoulol and amorpha-4,11-diene more than 1,000-fold, as well as the monoterpene limonene 10-30 fold, and seems equally suited to generating higher levels of other terpenes for research, industrial production or therapeutic applications.

Construction and expression of a new fusion protein, thymosin alpha1-cBLyS, E. coli

A fusion thymosin alpha1-soluble B lymphocyte stimulator (TM alpha1-cBLyS) gene was generated to engineer a bifunctional lymphokine, which was then over-produced in Escherichia coli. The molecular weight of the expressed fusion protein was approximately 28 kDa. After being purified by Ni-NTA affinity column, the fusion protein had full activity of BLyS with a slightly higher immunological action than synthetic TMalpha1. Because TM alpha1 regulates the cellular immune response and cBLyS amplifies the humoral response, this bifunctional lymphokine could be useful in the treatment of various immunodeficiency syndromes and serve as an immunomodulator to enhance the host's response to vaccination.

SIPK signaling controls multiple components of harpin-induced cell death in tobacco

Harpin from Pseudomonas syringae pv. phaseolicola (HrpZ) elicits a rapid cell death response in tobacco plants. Multiple signaling components, including mitogen-activated protein kinase (MAPK), reactive oxygen species (ROS) and salicylic acid (SA), have been reported to be involved in this cell death process, but the interaction between these molecules is poorly understood. Here we show through utilizing plants manipulated in SIPK expression levels that lack of SIPK results in increased sensitivity to harpin with concomitant accumulation of higher levels of ROS. Conversely, SIPK-overexpressing plants show reduced sensitivity to harpin relative to wild-type plants, and display reduced ROS accumulation. Harpin-induced cell death was found to be conditional on the ability of the plant to accumulate SA, whereas harpin induction of MAPK activation and ROS accumulation are not. However, harpin-induced ROS accumulation is required for activation of SIPK and wound-induced protein kinase. Transcriptional profiling revealed that suppression of SIPK signaling also affects early expression of a range of pathogen- and stress-responsive genes during harpin challenge.

Diversifying carotenoid biosynthetic pathways by directed evolution

Microorganisms and plants synthesize a diverse array of natural products, many of which have proven indispensable to human health and well-being. Although many thousands of these have been characterized, the space of possible natural products--those that could be made biosynthetically--remains largely unexplored. For decades, this space has largely been the domain of chemists, who have synthesized scores of natural product analogs and have found many with improved or novel functions. New natural products have also been made in recombinant organisms, via engineered biosynthetic pathways. Recently, methods inspired by natural evolution have begun to be applied to the search for new natural products. These methods force pathways to evolve in convenient laboratory organisms, where the products of new pathways can be identified and characterized in high-throughput screening programs. Carotenoid biosynthetic pathways have served as a convenient experimental system with which to demonstrate these ideas. Researchers have mixed, matched, and mutated carotenoid biosynthetic enzymes and screened libraries of these "evolved" pathways for the emergence of new carotenoid products. This has led to dozens of new pathway products not previously known to be made by the assembled enzymes. These new products include whole families of carotenoids built from backbones not found in nature. This review details the strategies and specific methods that have been employed to generate new carotenoid biosynthetic pathways in the laboratory. The potential application of laboratory evolution to other biosynthetic pathways is also discussed.

Enzymes encoded by the farnesyl diphosphate synthase gene family in the Big Sagebrush Artemisia tridentata ssp. spiciformis

Farnesyl diphosphate synthase catalyzes the sequential head-to-tail condensation of two molecules of isopentenyl diphosphate with dimethylallyl diphosphate. In plants the presence of farnesyl diphosphate synthase isozymes offers the possibility of differential regulation. Three full-length cDNAs encoding putative isoprenoid synthases, FDS-1, FDS-2, and FDS-5, with greater than 89% similarity were isolated from a Big Sagebrush Artemisia tridentata cDNA library using a three-step polymerase chain reaction protocol. One of the open reading frames, FDS-5, encoded a protein with an N-terminal amino acid extension that was identified as a plastidial targeting peptide. Recombinant histidine-tagged versions of three proteins were purified, and their enzymatic properties were characterized. FDS-1 and FDS-2 synthesized farnesyl diphosphate as the final chain elongation product, but their kinetic behavior varied. FDS-1 prefers geranyl diphosphate over dimethylallyl diphosphate as an allylic substrate and is active at acidic pH values compared with FDS-2. In contrast, FDS-5 synthesized two irregular monoterpenoids, chrysanthemyl diphosphate and lavandulyl diphosphate, when incubated with dimethylallyl diphosphate and an additional product, the regular monoterpene geranyl diphosphate, when incubated with isopentenyl diphosphate and dimethylallyl diphosphate. Specific cellular functions are proposed for each of the three enzymes, and a scenario for evolution of isoprenyl synthases in plants is presented.