Engineering a functional 1-deoxy-D-xylulose 5-phosphate (DXP) pathway in Saccharomyces cerevisiae

Harnessing iron‑sulfur enzymes for synthetic biology

Reactions catalysed by iron-sulfur (Fe-S) enzymes appear in a variety of biosynthetic pathways that produce valuable natural products. Harnessing these biosynthetic pathways by expression in microbial cell factories grown on an industrial scale would yield enormous economic and environmental benefits. However, Fe-S enzymes often become bottlenecks that limits the productivity of engineered pathways. As a consequence, achieving the production metrics required for industrial application remains a distant goal for Fe-S enzyme-dependent pathways. Here, we identify and review three core challenges in harnessing Fe-S enzyme activity, which all stem from the properties of Fe-S clusters: 1) limited Fe-S cluster supply within the host cell, 2) Fe-S cluster instability, and 3) lack of specialized reducing cofactor proteins often required for Fe-S enzyme activity, such as enzyme-specific flavodoxins and ferredoxins. We highlight successful methods developed for a variety of Fe-S enzymes and electron carriers for overcoming these difficulties. We use heterologous nitrogenase expression as a grand case study demonstrating how each of these challenges can be addressed. We predict that recent breakthroughs in protein structure prediction and design will prove well-suited to addressing each of these challenges. A reliable toolkit for harnessing Fe-S enzymes in engineered metabolic pathways will accelerate the development of industry-ready Fe-S enzyme-dependent biosynthesis pathways.

The non-mevalonate pathway requires a delicate balance of intermediates to maximize terpene production

Terpenes are valuable industrial chemicals whose demands are increasingly being met by bioengineering microbes such as E. coli. Although the bioengineering efforts commonly involve installing the mevalonate (MVA) pathway in E. coli for terpene production, the less studied methylerythritol phosphate (MEP) pathway is a more attractive target due to its higher energy efficiency and theoretical yield, despite its tight regulation. In this study, we integrated an additional copy of the entire MEP pathway into the E. coli genome for stable, marker-free terpene production. The genomically integrated strain produced more monoterpene geraniol than a plasmid-based system. The pathway genes' transcription was modulated using different promoters to produce geraniol as the reporter of the pathway flux. Pathway genes, including dxs, idi, and ispDF, expressed from a medium-strength promoter, led to the highest geraniol production. Quantifying the MEP pathway intermediates revealed that the highest geraniol producers had high levels of isopentenyl pyrophosphate (IPP) and dimethylallyl pyrophosphate (DMAPP), but moderate levels of the pathway intermediates upstream of these two building blocks. A principal component analysis demonstrated that 1-deoxy-D-xylulose 5-phosphate (DXP), the product of the first enzyme of the pathway, was critical for determining the geraniol titer, whereas MEP, the product of DXP reductoisomerase (Dxr or IspC), was the least essential. This work shows that an intricate balance of the MEP pathway intermediates determines the terpene yield in engineered E. coli. The genetically stable and intermediate-balanced strains created in this study will serve as a chassis for producing various terpenes. KEY POINTS: • Genome-integrated MEP pathway afforded higher strain stability • Genome-integrated MEP pathway produced more terpene than the plasmid-based system • High monoterpene production requires a fine balance of MEP pathway intermediates.

Engineering yeast for the production of plant terpenoids using synthetic biology approaches

Covering: 2011-2022The low amounts of terpenoids produced in plants and the difficulty in synthesizing these complex structures have stimulated the production of terpenoid compounds in microbial hosts by metabolic engineering and synthetic biology approaches. Advances in engineering yeast for terpenoid production will be covered in this review focusing on four directions: (1) manipulation of host metabolism, (2) rewiring and reconstructing metabolic pathways, (3) engineering the catalytic activity, substrate selectivity and product specificity of biosynthetic enzymes, and (4) localizing terpenoid production via enzymatic fusions and scaffolds, or subcellular compartmentalization.

Construction and Optimization of Nonclassical Isoprenoid Biosynthetic Pathways in Yeast Peroxisomes for (+)-Valencene Production

Isoprenoids are a kind of natural product with various activities, but their plant extraction suffers low concentration. The rapid development of synthetic biology offers a sustainable route for supply of high-value-added natural products by engineering microorganisms. However, the complexity of cellular metabolism makes engineering endogenous isoprenoid biosynthetic pathways with metabolic interaction difficult. Here, for the first time, we constructed and optimized three types of isoprenoid pathways (the Haloarchaea-type, Thermoplasma-type, and isoprenoid alcohol pathway) in yeast peroxisomes for the synthesis of sesquiterpene (+)-valencene. In yeast, the Haloarchaea-type MVA pathway is more effective than the classical MVA pathway. MVK and IPK were determined to be the rate-limiting steps of the Haloarchaea-type MVA pathway, and the production of 869 mg/L (+)-valencene under fed-batch fermentation in shake flasks was realized. This work expands isoprenoid synthesis in eukaryotes and provides a more efficient pathway for isoprenoid synthesis.

Cytosolic iron-sulfur protein assembly system identifies clients by a C-terminal tripeptide

The eukaryotic cytosolic Fe-S protein assembly (CIA) machinery inserts iron-sulfur (Fe-S) clusters into cytosolic and nuclear proteins. In the final maturation step, the Fe-S cluster is transferred to the apo-proteins by the CIA-targeting complex (CTC). However, the molecular recognition determinants of client proteins are unknown. We show that a conserved [LIM]-[DES]-[WF]-COO- tripeptide present at the C-terminus of clients is necessary and sufficient for binding to the CTC in vitro and directing Fe-S cluster delivery in vivo. Remarkably, fusion of this TCR (target complex recognition) signal enables engineering of cluster maturation on a non-native protein via recruitment of the CIA machinery. Our study significantly advances our understanding of Fe-S protein maturation and paves the way for bioengineering applications.

Optimization of extraction conditions for LC-ToF-MS analysis of mevalonate pathway metabolites in engineered E. coli strain via statistical experimental designs

Isoprenoids give rise to many functional products used today such as flavours, fragrances and even pharmaceutical compounds. Mevalonate pathway metabolites are the key intermediates that affect the production yield of isoprenoids. With increasing demand and benefit of isoprenoids, the present study adopts Analytical Quality-by-Design (AQbD) approach to establish an efficacious extraction protocol prior to the determination of mevalonate pathway metabolites in an engineered Escherichia coli model. The statistical experimental design approach, described in this work, has successfully validated an optimised sample preparation method i.e., using acetonitrile: 50 mM ammonium formate (pH 9.5) (7:3) (ACN73) at -20 °C for 10 min without solvent evaporation to retain the targeted mevalonate metabolites in engineered E. coli strain. The study also demonstrates the use of liquid chromatography paired with a Time-of-Flight Mass Spectrometer (LC-ToF-MS) for the quantitative analysis of the mevalonate pathway metabolites in E. coli. The analytical method was validated in accordance with guidelines in Metabolomics Standards Initiative and ICH Q2 (R1) with analyte spike recoveries at 80% and above. In short, the present study overcomes the one-variable-at-a-time (OVAT) limitations in analytical development, minimises metabolite losses and gives better cost and time efficiencies by eliminating the solvent evaporation and swapping process. This work highlights the importance of analytical methods development in microbial metabolomics studies.

Enhanced production of 3,4-dihydroxybutyrate from xylose by engineered yeast via xylonate re-assimilation under alkaline condition

To realize lignocellulose-based bioeconomy, efficient conversion of xylose into valuable chemicals by microbes is necessary. Xylose oxidative pathways that oxidize xylose into xylonate can be more advantageous than conventional xylose assimilation pathways because of fewer reaction steps without loss of carbon and ATP. Moreover, commodity chemicals like 3,4-dihydroxybutyrate and 3-hydroxybutyrolactone can be produced from the intermediates of xylose oxidative pathway. However, successful implementations of xylose oxidative pathway in yeast have been hindered because of the secretion and accumulation of xylonate which is a key intermediate of the pathway, leading to low yield of target product. Here, high-yield production of 3,4-dihydroxybutyrate from xylose by engineered yeast was achieved through genetic and environmental perturbations. Specifically, 3,4-dihydroxybutyrate biosynthetic pathway was established in yeast through deletion of ADH6 and overexpression of yneI. Also, inspired by the mismatch of pH between host strain and key enzyme of XylD, alkaline fermentations (pH ≥ 7.0) were performed to minimize xylonate accumulation. Under the alkaline conditions, xylonate was re-assimilated by engineered yeast and combined product yields of 3,4-dihydroxybutyrate and 3-hydroxybutyrolactone resulted in 0.791 mol/mol-xylose, which is highest compared with previous study. These results shed light on the utility of the xylose oxidative pathway in yeast.

Optimizing microbial networks through metabolic bypasses

Metabolism has long been considered as a relatively stiff set of biochemical reactions. This somewhat outdated and dogmatic view has been challenged over the last years, as multiple studies exposed unprecedented plasticity of metabolism by exploring rational and evolutionary modifications within the metabolic network of cell factories. Of particular importance is the emergence of metabolic bypasses, which consist of enzymatic reaction(s) that support unnatural connections between metabolic nodes. Such novel topologies can be generated through the introduction of heterologous enzymes or by upregulating native enzymes (sometimes relying on promiscuous activities thereof). Altogether, the adoption of bypasses resulted in an expansion in the capacity of the host's metabolic network, which can be harnessed for bioproduction. In this review, we discuss modifications to the canonical architecture of central carbon metabolism derived from such bypasses towards six optimization purposes: stoichiometric gain, overcoming kinetic limitations, solving thermodynamic barriers, circumventing toxic intermediates, uncoupling product synthesis from biomass formation, and altering redox cofactor specificity. The metabolic costs associated with bypass-implementation are likewise discussed, including tailoring their design towards improving bioproduction.

Cellular assays identify barriers impeding iron-sulfur enzyme activity in a non-native prokaryotic host

Iron-sulfur (Fe-S) clusters are ancient and ubiquitous protein cofactors and play irreplaceable roles in many metabolic and regulatory processes. Fe-S clusters are built and distributed to Fe-S enzymes by dedicated protein networks. The core components of these networks are widely conserved and highly versatile. However, Fe-S proteins and enzymes are often inactive outside their native host species. We sought to systematically investigate the compatibility of Fe-S networks with non-native Fe-S enzymes. By using collections of Fe-S enzyme orthologs representative of the entire range of prokaryotic diversity, we uncovered a striking correlation between phylogenetic distance and probability of functional expression. Moreover, coexpression of a heterologous Fe-S biogenesis pathway increases the phylogenetic range of orthologs that can be supported by the foreign host. We also find that Fe-S enzymes that require specific electron carrier proteins are rarely functionally expressed unless their taxon-specific reducing partners are identified and co-expressed. We demonstrate how these principles can be applied to improve the activity of a radical S-adenosyl methionine(rSAM) enzyme from a Streptomyces antibiotic biosynthesis pathway in Escherichia coli. Our results clarify how oxygen sensitivity and incompatibilities with foreign Fe-S and electron transfer networks each impede heterologous activity. In particular, identifying compatible electron transfer proteins and heterologous Fe-S biogenesis pathways may prove essential for engineering functional Fe-S enzyme-dependent pathways.

Alternative metabolic pathways and strategies to high-titre terpenoid production in Escherichia coli

Covering: up to 2021Terpenoids are a diverse group of chemicals used in a wide range of industries. Microbial terpenoid production has the potential to displace traditional manufacturing of these compounds with renewable processes, but further titre improvements are needed to reach cost competitiveness. This review discusses strategies to increase terpenoid titres in Escherichia coli with a focus on alternative metabolic pathways. Alternative pathways can lead to improved titres by providing higher orthogonality to native metabolism that redirects carbon flux, by avoiding toxic intermediates, by bypassing highly-regulated or bottleneck steps, or by being shorter and thus more efficient and easier to manipulate. The canonical 2-C-methyl-D-erythritol 4-phosphate (MEP) and mevalonate (MVA) pathways are engineered to increase titres, sometimes using homologs from different species to address bottlenecks. Further, alternative terpenoid pathways, including additional entry points into the MEP and MVA pathways, archaeal MVA pathways, and new artificial pathways provide new tools to increase titres. Prenyl diphosphate synthases elongate terpenoid chains, and alternative homologs create orthogonal pathways and increase product diversity. Alternative sources of terpenoid synthases and modifying enzymes can also be better suited for E. coli expression. Mining the growing number of bacterial genomes for new bacterial terpenoid synthases and modifying enzymes identifies enzymes that outperform eukaryotic ones and expand microbial terpenoid production diversity. Terpenoid removal from cells is also crucial in production, and so terpenoid recovery and approaches to handle end-product toxicity increase titres. Combined, these strategies are contributing to current efforts to increase microbial terpenoid production towards commercial feasibility.

Diversifying Isoprenoid Platforms via Atypical Carbon Substrates and Non-model Microorganisms

Isoprenoid compounds are biologically ubiquitous, and their characteristic modularity has afforded products ranging from pharmaceuticals to biofuels. Isoprenoid production has been largely successful in Escherichia coli and Saccharomyces cerevisiae with metabolic engineering of the mevalonate (MVA) and methylerythritol phosphate (MEP) pathways coupled with the expression of heterologous terpene synthases. Yet conventional microbial chassis pose several major obstacles to successful commercialization including the affordability of sugar substrates at scale, precursor flux limitations, and intermediate feedback-inhibition. Now, recent studies have challenged typical isoprenoid paradigms by expanding the boundaries of terpene biosynthesis and using non-model organisms including those capable of metabolizing atypical C1 substrates. Conversely, investigations of non-model organisms have historically informed optimization in conventional microbes by tuning heterologous gene expression. Here, we review advances in isoprenoid biosynthesis with specific focus on the synergy between model and non-model organisms that may elevate the commercial viability of isoprenoid platforms by addressing the dichotomy between high titer production and inexpensive substrates.

Towards a Synthetic Biology Toolset for Metallocluster Enzymes in Biosynthetic Pathways: What We Know and What We Need

Microbes are routinely engineered to synthesize high-value chemicals from renewable materials through synthetic biology and metabolic engineering. Microbial biosynthesis often relies on expression of heterologous biosynthetic pathways, i.e., enzymes transplanted from foreign organisms. Metallocluster enzymes are one of the most ubiquitous family of enzymes involved in natural product biosynthesis and are of great biotechnological importance. However, the functional expression of recombinant metallocluster enzymes in live cells is often challenging and represents a major bottleneck. The activity of metallocluster enzymes requires essential supporting pathways, involved in protein maturation, electron supply, and/or enzyme stability. Proper function of these supporting pathways involves specific protein-protein interactions that remain poorly characterized and are often overlooked by traditional synthetic biology approaches. Consequently, engineering approaches that focus on enzymatic expression and carbon flux alone often overlook the particular needs of metallocluster enzymes. This review highlights the biotechnological relevance of metallocluster enzymes and discusses novel synthetic biology strategies to advance their industrial application, with a particular focus on iron-sulfur cluster enzymes. Strategies to enable functional heterologous expression and enhance recombinant metallocluster enzyme activity in industrial hosts include: (1) optimizing specific maturation pathways; (2) improving catalytic stability; and (3) enhancing electron transfer. In addition, we suggest future directions for developing microbial cell factories that rely on metallocluster enzyme catalysis.

Key Enzymes Involved in the Synthesis of Hops Phytochemical Compounds: From Structure, Functions to Applications

Humulus lupulus L. is an essential source of aroma compounds, hop bitter acids, and xanthohumol derivatives mainly exploited as flavourings in beer brewing and with demonstrated potential for the treatment of certain diseases. To acquire a comprehensive understanding of the biosynthesis of these compounds, the primary enzymes involved in the three major pathways of hops' phytochemical composition are herein critically summarized. Hops' phytochemical components impart bitterness, aroma, and antioxidant activity to beers. The biosynthesis pathways have been extensively studied and enzymes play essential roles in the processes. Here, we introduced the enzymes involved in the biosynthesis of hop bitter acids, monoterpenes and xanthohumol derivatives, including the branched-chain aminotransferase (BCAT), branched-chain keto-acid dehydrogenase (BCKDH), carboxyl CoA ligase (CCL), valerophenone synthase (VPS), prenyltransferase (PT), 1-deoxyxylulose-5-phosphate synthase (DXS), 4-hydroxy-3-methylbut-2-enyl diphosphate reductase (HDR), Geranyl diphosphate synthase (GPPS), monoterpene synthase enzymes (MTS), cinnamate 4-hydroxylase (C4H), chalcone synthase (CHS_H1), chalcone isomerase (CHI)-like proteins (CHIL), and O-methyltransferase (OMT1). Furthermore, research advancements of each enzyme in terms of reaction conditions, substrate recognition, enzyme structures, and use in engineered microbes are described in depth. Hence, an extensive review of the key enzymes involved in the phytochemical compounds of hops will provide fundamentals for their applications in beer production.

Biosynthesis and heterologous production of furanocoumarins: perspectives and current challenges

Covering: up to October 2020 Furanocoumarins are plant secondary metabolites used to treat several skin disorders, such as psoriasis and vitiligo, and also with other potential therapeutic activities. Furanocoumarins are extracted from plants where they accumulate in low amounts over long growth periods. In addition, their extraction and purification are difficult in an environmentally unfriendly and expensive process. Hence, new sustainable and greener production schemes able to overcome such limitations ought to be developed. While the heterologous production of simple coumarins has been demonstrated, the biosynthesis of more complex furanocoumarins remains greatly unexplored. Although several important steps of the pathway have been elucidated in the last decade, the complete pathway has not been completely unravelled. In this paper, we review the natural conversion of amino acids into furanocoumarins, as well as the heterologous expression of each enzyme of the pathway. We also explore the challenges that need to be addressed so that their heterologous production can become a viable alternative.

De novo assembly of the Mylia taylorii transcriptome and identification of sesquiterpene synthases

Mylia taylorii is an ancient nonseed land plant that accumulates various sesquiterpenes with insecticidal and antibacterial activities. Recently, microbial-type sesquiterpene synthases (STSs) with atypical aspartate-rich metal ion binding motifs have been identified in some liverworts. Here, transcriptome analysis of M. taylorii was performed to identify M. taylorii sesquiterpene synthases (MtSTSs) that are potentially involved in sesquiterpene biosynthesis and diversity. A total of 255,669 unigenes were obtained with an average length of 963 bp in the transcriptome data of M. taylorii, among which 148,093 (57.92%) unigenes had BLAST results. Forty-eight unigenes were related to the sesquiterpene backbone biosynthesis according to KEGG annotation. In addition, MtSTS1, MtSTS2 and MtSTS3 identified from putative MtSTSs display sesquiterpene catalytic activities on the basis of functional characterizations in yeast. Interestingly, MtSTSs exhibit a noncanonical metal ion binding motif and the structural composition of a single α-domain, which are features of microbial STSs instead of archetypical plant STSs. This study revealed new microbial-type STS members of nonseed plants, and functionally identified that MtSTSs may contribute to the investigation of the biosynthesis and biological role of sesquiterpenes in M. taylorii.

Streamlining Natural Products Biomanufacturing With Omics and Machine Learning Driven Microbial Engineering

Increasing demands for the supply of biopharmaceuticals have propelled the advancement of metabolic engineering and synthetic biology strategies for biomanufacturing of bioactive natural products. Using metabolically engineered microbes as the bioproduction hosts, a variety of natural products including terpenes, flavonoids, alkaloids, and cannabinoids have been synthesized through the construction and expression of known and newly found biosynthetic genes primarily from model and non-model plants. The employment of omics technology and machine learning (ML) platforms as high throughput analytical tools has been increasingly leveraged in promoting data-guided optimization of targeted biosynthetic pathways and enhancement of the microbial production capacity, thereby representing a critical debottlenecking approach in improving and streamlining natural products biomanufacturing. To this end, this mini review summarizes recent efforts that utilize omics platforms and ML tools in strain optimization and prototyping and discusses the beneficial uses of omics-enabled discovery of plant biosynthetic genes in the production of complex plant-based natural products by bioengineered microbes.

Overcoming Challenges in Expressing Iron-Sulfur Enzymes in Yeast

Iron-sulfur clusters are metal cofactors that are present in all domains of life. Many enzymes that require these cofactors have biotechnological importance, because they can be used to uncover catabolic routes to new sugar substrates or can be a critical part of pathways to produce chemicals and biofuels. However, the expression of these iron-sulfur enzymes of bacterial origin in yeast at high levels is a significant bottleneck. Intermediates upstream of the enzyme accumulate, because the activity of these enzymes is either low or completely absent. In this review, we examine possible explanations for this limitation, discuss potential genetic interventions in the yeast host that can increase iron-sulfur enzyme activity, and suggest future directions for creating more efficient yeast hosts capable of high iron-sulfur enzyme expression.

Metabolic Engineering Strategies for Sustainable Terpenoid Flavor and Fragrance Synthesis

Terpenoids derived from plant material are widely applied in the flavor and fragrance industry. Traditional extraction methods are unsustainable, but microbial synthesis offers a promising solution to attain efficient production of natural-identical terpenoids. Overproduction of terpenoids in microbes requires careful balancing of the synthesis pathway constituents within the constraints of host cell metabolism. Advances in metabolic engineering have greatly facilitated overcoming the challenges of achieving high titers, rates, and yields (TRYs). The review summarizes recent development in the molecular biology toolbox to achieve high TRYs for terpenoid biosynthesis, mainly in the two industrial platform microorganisms: Escherichia coli and Saccharomyces cerevisiae. The biosynthetic pathways, including alternative pathway designs, are briefly introduced, followed by recently developed methodologies used for pathway, genome, and strain optimization. Integrated applications of these tools are important to achieve high "TRYs" of terpenoid production and pave the way for translating laboratory research into successful commercial manufacturing.

Functional replacement of isoprenoid pathways in Rhodobacter sphaeroides

Advances in synthetic biology and metabolic engineering have proven the potential of introducing metabolic by-passes within cell factories. These pathways can provide a more efficient alternative to endogenous counterparts due to their insensitivity to host's regulatory mechanisms. In this work, we replaced the endogenous essential 2-C-methyl-D-erythritol 4-phosphate (MEP) pathway for isoprenoid biosynthesis in the industrially relevant bacterium Rhodobacter sphaeroides by an orthogonal metabolic route. The native 2-C-methyl-D-erythritol 4-phosphate (MEP) pathway was successfully replaced by a heterologous mevalonate (MVA) pathway from a related bacterium. The functional replacement was confirmed by analysis of the reporter molecule amorpha-4,11-diene after cultivation with [4-13 C]glucose. The engineered R. sphaeroides strain relying exclusively on the MVA pathway was completely functional in conditions for sesquiterpene production and, upon increased expression of the MVA enzymes, it reached even higher sesquiterpene yields than the control strain coexpressing both MEP and MVA modules. This work represents an example where substitution of an essential biochemical pathway by an alternative, heterologous pathway leads to enhanced biosynthetic performance.

Modular engineering for microbial production of carotenoids

There is an increasing demand for carotenoids due to their applications in the food, flavor, pharmaceutical and feed industries, however, the extraction and synthesis of these compounds can be expensive and technically challenging. Microbial production of carotenoids provides an attractive alternative to the negative environmental impacts and cost of chemical synthesis or direct extraction from plants. Metabolic engineering and synthetic biology approaches have been widely utilized to reconstruct and optimize pathways for carotenoid overproduction in microorganisms. This review summarizes the current advances in microbial engineering for carotenoid production and divides the carotenoid biosynthesis building blocks into four distinct metabolic modules: 1) central carbon metabolism, 2) cofactor metabolism, 3) isoprene supplement metabolism and 4) carotenoid biosynthesis. These four modules focus on redirecting carbon flux and optimizing cofactor supplements for isoprene precursors needed for carotenoid synthesis. Future perspectives are also discussed to provide insights into microbial engineering principles for overproduction of carotenoids.

Plant Aromatic Prenyltransferases: Tools for Microbial Cell Factories

In plants, prenylation of aromatic compounds, such as (iso)flavonoids and stilbenoids, by membrane-bound prenyltransferases (PTs), is an essential step in the biosynthesis of many bioactive compounds. Prenylated aromatic compounds have various health-beneficial properties that are interesting for industrial applications, but their exploitation is limited due to their low abundance in nature. Harnessing plant aromatic PTs for prenylation in microbial cell factories may be a sustainable and economically viable alternative. Limitations in prenylated aromatic compound production have been identified, including availability of prenyl donor substrate. In this review, we summarize the current knowledge about plant aromatic PTs and discuss promising strategies towards the optimized production of prenylated aromatic compounds by microbial cell factories.

Advanced Strategies for Production of Natural Products in Yeast

Natural products account for more than 50% of all small-molecule pharmaceutical agents currently in clinical use. However, low availability often becomes problematic when a bioactive natural product is promising to become a pharmaceutical or leading compound. Advances in synthetic biology and metabolic engineering provide a feasible solution for sustainable supply of these compounds. In this review, we have summarized current progress in engineering yeast cell factories for production of natural products, including terpenoids, alkaloids, and phenylpropanoids. We then discuss advanced strategies in metabolic engineering at three different dimensions, including point, line, and plane (corresponding to the individual enzymes and cofactors, metabolic pathways, and the global cellular network). In particular, we comprehensively discuss how to engineer cofactor biosynthesis for enhancing the biosynthesis efficiency, other than the enzyme activity. Finally, current challenges and perspective are also discussed for future engineering direction.

Metabolic flux ratio analysis by parallel 13C labeling of isoprenoid biosynthesis in Rhodobacter sphaeroides

Metabolic engineering for increased isoprenoid production often benefits from the simultaneous expression of the two naturally available isoprenoid metabolic routes, namely the 2-methyl-D-erythritol 4-phosphate (MEP) pathway and the mevalonate (MVA) pathway. Quantification of the contribution of these pathways to the overall isoprenoid production can help to obtain a better understanding of the metabolism within a microbial cell factory. Such type of investigation can benefit from ¹³C metabolic flux ratio studies. Here, we designed a method based on parallel labeling experiments (PLEs), using [1-¹³C]- and [4-¹³C]glucose as tracers to quantify the metabolic flux ratios in the glycolytic and isoprenoid pathways. By just analyzing a reporter isoprenoid molecule and employing only four equations, we could describe the metabolism involved from substrate catabolism to product formation. These equations infer ¹³C atom incorporation into the universal isoprenoid building blocks, isopentenyl-pyrophosphate (IPP) and dimethylallyl-pyrophosphate (DMAPP). Therefore, this renders the method applicable to the study of any of isoprenoid of interest. As proof of principle, we applied it to study amorpha-4,11-diene biosynthesis in the bacterium Rhodobacter sphaeroides. We confirmed that in this species the Entner-Doudoroff pathway is the major pathway for glucose catabolism, while the Embden-Meyerhof-Parnas pathway contributes to a lesser extent. Additionally, we demonstrated that co-expression of the MEP and MVA pathways caused a mutual enhancement of their metabolic flux capacity. Surprisingly, we also observed that the isoprenoid flux ratio remains constant under exponential growth conditions, independently from the expression level of the MVA pathway. Apart from proposing and applying a tool for studying isoprenoid biosynthesis within a microbial cell factory, our work reveals important insights from the co-expression of MEP and MVA pathways, including the existence of a yet unclear interaction between them.

Metabolic Engineering of the MEP Pathway in Bacillus subtilis for Increased Biosynthesis of Menaquinone-7

Vitamin K is essential for blood coagulation and plays important roles in bone and cardiovascular health. Menaquinone-7 (MK-7) is one form of vitamin K that is especially useful due to its long half-life in the circulation. MK-7 is difficult to make via organic synthesis, and is thus commonly produced by fermentation. This study aimed to genetically modify Bacillus subtilis cultures to increase their MK-7 yield and reduce production costs. We constructed 12 different strains of B. subtilis 168 by overexpressing different combinations of the rate-limiting enzymes Dxs, Dxr, Idi, and MenA. We observed an 11-fold enhancement of production in the best-performing strain, resulting in 50 mg/L MK-7. Metabolite analysis revealed new bottlenecks in the pathway at IspG and IspH, which suggest avenues for further optimization. This work highlights the usefulness of Bacillus subtilis for industrial production of high value compounds.

Identifying and engineering the ideal microbial terpenoid production host

More than 70,000 different terpenoid structures are known so far; many of them offer highly interesting applications as pharmaceuticals, flavors and fragrances, or biofuels. Extraction of these compounds from their natural sources or chemical synthesis is-in many cases-technically challenging with low or moderate yields while wasting valuable resources. Microbial production of terpenoids offers a sustainable and environment-friendly alternative starting from simple carbon sources and, frequently, safeguards high product specificity. Here, we provide an overview on employing recombinant bacteria and yeasts for heterologous de novo production of terpenoids. Currently, Escherichia coli and Saccharomyces cerevisiae are the two best-established production hosts for terpenoids. An increasing number of studies have been successful in engineering alternative microorganisms for terpenoid biosynthesis, which we intend to highlight in this review. Moreover, we discuss the specific engineering challenges as well as recent advances for microbial production of different classes of terpenoids. Rationalizing the current stages of development for different terpenoid production hosts as well as future prospects shall provide a valuable decision basis for the selection and engineering of the cell factory(ies) for industrial production of terpenoid target molecules.

Engineered Reversal of Function in Glycolytic Yeast Promoters

Promoters are key components of cell factory design, allowing precise expression of genes in a heterologous pathway. Several commonly used promoters in yeast cell factories belong to glycolytic genes, highly expressed in actively growing yeast when glucose is used as a carbon source. However, their expression can be suboptimal when alternate carbon sources are used, or if there is a need to decouple growth from production. Hence, there is a need for alternate promoters for different carbon sources and production schemes. In this work, we demonstrate a reversal of regulatory function in two glycolytic yeast promoters by replacing glycolytic regulatory elements with ones induced by the diauxic shift. We observe a shift in induction from glucose-rich to glucose-poor medium without loss of regulatory activity, and strong ethanol induction. Applications of these promoters were validated for expression of the vanillin biosynthetic pathway, reaching production of vanillin comparable to pathway designs using strong constitutive promoters.

Biosynthesis of Plant Triterpenoid Saponins in Microbial Cell Factories

Triterpenoid saponins are triterpenoid glycoside compounds which have been widely used in pharmaceutical, agricultural, and food industries. Traditionally, they are extracted from plants, which is time-consuming and environmentally unfriendly. Recently, de novo synthesis of triterpenoid saponins in microbial cell factories was realized, which provides a promising and green approach to alter the traditional supply way. However, the complex biosynthetic pathway and the poor suitability between the endogenous and heterogeneous pathways tremendously limit the yield of triterpenoid saponins. We introduce the biosynthetic pathways of triterpenoid saponins first, and we then summarize the microbial cell factories developed to produce these compounds. Further, we discuss the strategies applied to enhance the production. This paper systematically illustrates the biosynthesis of plant triterpenoid saponins in microbial cell factories.

Breeding and Engineering Trees to Accumulate High Levels of Terpene Metabolites for Plant Defense and Renewable Chemicals

Plants evolved the capacity to synthesize highly diverse sets of secondary metabolites which are important for plant adaptation and health. In forest trees, many classes of compounds, particularly ones related to defense against insects, fungi, and bacteria accumulate to levels that enable their recovery and commercial use. One of the oldest examples is conifer terpenes, but terpenes are important secondary products from other tree species including eucalypts. Because terpenes, latex, and natural gums are synthesized and stored in specialized secretory glands, ducts, and laticifers in mostly pure forms they can be collected from live trees in addition to being extracted during industrial processing of wood. This minireview describes the potential of breeding and genetic engineering approaches to increase the quantities of terpene secondary metabolites to increase the amount of secondary products and thereby increasing the value of planted and managed forest trees. I advance the view that breeding and genetic engineering of metabolic pathways and specialized cell secretory structures can dramatically increase tissue terpene content.

Microbial Platform for Terpenoid Production: Escherichia coli and Yeast

Terpenoids, also called isoprenoids, are a large and highly diverse family of natural products with important medical and industrial properties. However, a limited production of terpenoids from natural resources constrains their use of either bulk commodity products or high valuable products. Microbial production of terpenoids from Escherichia coli and yeasts provides a promising alternative owing to available genetic tools in pathway engineering and genome editing, and a comprehensive understanding of their metabolisms. This review summarizes recent progresses in engineering of industrial model strains, E. coli and yeasts, for terpenoids production. With advances of synthetic biology and systems biology, both strains are expected to present the great potential as a platform of terpenoid synthesis.

The Smell of Synthetic Biology: Engineering Strategies for Aroma Compound Production in Yeast

Yeast—especially Saccharomyces cerevisiae—have long been a preferred workhorse for the production of numerous recombinant proteins and other metabolites. S. cerevisiae is a noteworthy aroma compound producer and has also been exploited to produce foreign bioflavour compounds. In the past few years, important strides have been made in unlocking the key elements in the biochemical pathways involved in the production of many aroma compounds. The expression of these biochemical pathways in yeast often involves the manipulation of the host strain to direct the flux towards certain precursors needed for the production of the given aroma compound. This review highlights recent advances in the bioengineering of yeast—including S. cerevisiae—to produce aroma compounds and bioflavours. To capitalise on recent advances in synthetic yeast genomics, this review presents yeast as a significant producer of bioflavours in a fresh context and proposes new directions for combining engineering and biology principles to improve the yield of targeted aroma compounds.

Exploring D-xylose oxidation in Saccharomyces cerevisiae through the Weimberg pathway

Engineering of the yeast Saccharomyces cerevisiae towards efficient D-xylose assimilation has been a major focus over the last decades since D-xylose is the second most abundant sugar in nature, and its conversion into products could significantly improve process economy in biomass-based processes. Up to now, two different metabolic routes have been introduced via genetic engineering, consisting of either the isomerization or the oxido-reduction of D-xylose to D-xylulose that is further connected to the pentose phosphate pathway and glycolysis. In the present study, cytosolic D-xylose oxidation was investigated instead, through the introduction of the Weimberg pathway from Caulobacter crescentus in S. cerevisiae. This pathway consists of five reaction steps that connect D-xylose to the TCA cycle intermediate α-ketoglutarate. The corresponding genes could be expressed in S. cerevisiae, but no growth was observed on D-xylose indicating that not all the enzymes were functionally active. The accumulation of the Weimberg intermediate D-xylonate suggested that the dehydration step(s) might be limiting, blocking further conversion into α-ketoglutarate. Although four alternative dehydratases both of bacterial and archaeon origins were evaluated, D-xylonate accumulation still occurred. A better understanding of the mechanisms associated with the activity of dehydratases, both at a bacterial and yeast level, appears essential to obtain a fully functional Weimberg pathway in S. cerevisiae.

Synthetic biology strategies toward heterologous phytochemical production

This review summarizes the recent progress in heterologous phytochemical biosynthetic pathway reconstitution in plant, bacteria, and yeast, with a focus on the synthetic biology strategies applied in these engineering efforts.