Rewiring Yarrowia lipolytica toward triacetic acid lactone for materials generation

Enhanced production of isobutyl and isoamyl acetate using Yarrowia lipolytica

Short-chain esters, particularly isobutyl acetate and isoamyl acetate, hold significant industrial value due to their wide-ranging applications in flavors, fragrances, solvents, and biofuels. In this study, we demonstrated the biosynthesis of acetate esters using Yarrowia lipolytica as a host by feeding alcohols to the yeast culture. Initially, we screened for optimal alcohol acyltransferases for ester biosynthesis in Y. lipolytica. Strains of Y. lipolytica expressing atf1 from Saccharomyces cerevisiae, produced 251 or 613 mg/L of isobutyl acetate or of isoamyl acetate, respectively. We found that introducing additional copies of ATF1 enhanced ester production. Furthermore, by increasing the supply of acetyl-CoA and refining the culture conditions, we achieved high production of isoamyl acetate, reaching titers of 3404 mg/L. We expanded our study to include the synthesis of a range of acetate esters, facilitated by enriching the culture medium with various alcohols. This study underscores the versatility and potential of Y. lipolytica in the industrial production of acetate esters.

Comparative transcriptome analysis reveals the redirection of metabolic flux from cell growth to astaxanthin biosynthesis in Yarrowia lipolytica

Engineering Yarrowia lipolytica to produce astaxanthin provides a promising route. Here, Y. lipolytica M2 producing a titer of 181 mg/L astaxanthin was isolated by iterative atmospheric and room-temperature plasma mutagenesis and diphenylamine-mediated screening. Interestingly, a negative correlation was observed between cell biomass and astaxanthin production. To reveal the underlying mechanism, RNA-seq analysis of transcriptional changes was performed in high producer M2 and reference strain M1, and a total of 1379 differentially expressed genes were obtained. Data analysis revealed that carbon flux was elevated through lipid metabolism, acetyl-CoA and mevalonate supply, but restrained through central carbon metabolism in strain M2. Moreover, upregulation of other pathways such as ATP-binding cassette transporter and thiamine pyrophosphate possibly provided more cofactors for carotenoid hydroxylase and relieved cell membrane stress caused by astaxanthin insertion. These results suggest that balancing cell growth and astaxanthin production may be important to promote efficient biosynthesis of astaxanthin in Y. lipolytica.

Engineering a carbon source-responsive promoter for improved biosynthesis in the non-conventional yeast Kluyveromyces marxianus

Many desired biobased chemicals exhibit a range of toxicity to microbial cell factories, making industry-level biomanufacturing more challenging. Separating microbial growth and production phases is known to be beneficial for improving production of toxic products. Here, we developed a novel synthetic carbon-responsive promoter for use in the rapidly growing, stress-tolerant yeast Kluyveromyces marxianus, by fusing carbon-source responsive elements of the native ICL1 promoter to the strong S. cerevisiae TDH3 or native NC1 promoter cores. Two hybrids, P IT350 and P IN450 , were validated via EGFP fluorescence and demonstrated exceptional strength, partial repression during growth, and late phase activation in glucose- and lactose-based medium, respectively. Expressing the Gerbera hybrida 2-pyrone synthase (2-PS) for synthesis of the polyketide triacetic acid lactone (TAL) under the control of P IN450 increased TAL more than 50% relative to the native NC1 promoter, and additional promoter engineering further increased TAL titer to 1.39 g/L in tube culture. Expression of the Penicillium griseofulvum 6-methylsalicylic acid synthase (6-MSAS) under the control of P IN450 resulted in a 6.6-fold increase in 6-MSA titer to 1.09 g/L and a simultaneous 1.5-fold increase in cell growth. Finally, we used P IN450 to express the Pseudomonas savastanoi IaaM and IaaH proteins and the Salvia pomifera sabinene synthase protein to improve production of the auxin hormone indole-3-acetic acid and the monoterpene sabinene, respectively, both extremely toxic to yeast. The development of carbon-responsive promoters adds to the synthetic biology toolbox and available metabolic engineering strategies for K. marxianus, allowing greater control over heterologous protein expression and improved production of toxic metabolites.

Improvement of Chalcone Synthase Activity and High-Efficiency Fermentative Production of (2S)-Naringenin via In Vivo Biosensor-Guided Directed Evolution

Chalcone synthase (CHS) catalyzes the rate-limiting step of (2S)-naringenin (the essential flavonoid skeleton) biosynthesis. Improving the activity of the CHS by protein engineering enhances (2S)-naringenin production by microbial fermentation and can facilitate the production of valuable flavonoids. A (2S)-naringenin biosensor based on the TtgR operon was constructed in Escherichia coli and its detection range was expanded by promoter optimization to 0-300 mg/L, the widest range for (2S)-naringenin reported. The high-throughput screening scheme for CHS was established based on this biosensor. A mutant, SjCHS1S208N with a 2.34-fold increase in catalytic activity, was discovered by directed evolution and saturation mutagenesis. A pathway for de novo biosynthesis of (2S)-naringenin by SjCHS1S208N was constructed in Saccharomyces cerevisiae, combined with CHS precursor pathway optimization, increasing the (2S)-naringenin titer by 65.34% compared with the original strain. Fed-batch fermentation increased the titer of (2S)-naringenin to 2513 ± 105 mg/L, the highest reported so far. These findings will facilitate efficient flavonoid biosynthesis and further modification of the CHS in the future.

Remodeling the Homologous Recombination Mechanism of Yarrowia lipolytica for High-Level Biosynthesis of Squalene

Squalene is a high-value antioxidant with many commercial applications. The use of microbial cell factories to produce squalene as an alternative to plant and animal extracts could meet increasing market demand. Yarrowia lipolytica is an excellent host for squalene production due to its high levels of acetyl-CoA and a hydrophobic environment. However, the need for precise and complicated gene editing has hindered the industrialization of this strain. Herein, the rapid construction of a strain with high squalene production was achieved by enhancing the homologous recombination efficiency in Y. lipolytica. First, remodeling of the homologous recombination efficiency resulted in a 10-fold increase in the homologous recombination rate. Next, the whole mevalonate pathway was integrated into the chromosome to enhance squalene production. Then, a higher level of squalene accumulation was achieved by increasing the level of acetyl coenzyme A and regulating the downstream steroid synthesis pathway. Finally, the squalene production reached 35 g/L after optimizing the fermentation conditions and performing a fed-batch culture in a 5 L jar fermenter. This is the highest squalene production ever reported to date by de novo biosynthesis without adding any inhibitors, paving a new path toward the industrial production of squalene and its downstream products.

Leveraging a Y. lipolytica naringenin chassis for biosynthesis of apigenin and associated glucoside

Flavonoids are a diverse set of natural products with promising bioactivities including anti-inflammatory, anti-cancer, and neuroprotective properties. Previously, the oleaginous host Yarrowia lipolytica has been engineered to produce high titers of the base flavonoid naringenin. Here, we leverage this host along with a set of E. coli bioconversion strains to produce the flavone apigenin and its glycosylated derivative isovitexin, two potential nutraceutical and pharmaceutical candidates. Through downstream strain selection, co-culture optimization, media composition, and mutant isolation, we were able to produce168 mg/L of apigenin, representing a 46% conversion rate of 2-(R/S)-naringenin to apigenin. This apigenin platform was modularly extended to produce isovitexin by addition of a second bioconversion strain. Together, these results demonstrate the promise of microbial production and modular bioconversion to access diversified flavonoids.

Exploiting synthetic biology platforms for enhanced biosynthesis of natural products in Yarrowia lipolytica

With the rapid development of synthetic biology, researchers can design, modify, or even synthesize microorganisms de novo, and microorganisms endowed with unnatural functions can be considered "artificial life" and facilitate the development of functional products. Based on this concept, researchers can solve critical problems related to the insufficient supply of natural products, such as low yields, long production cycles, and cumbersome procedures. Due to its superior performance and unique physiological and biochemical characteristics, Yarrowia lipolytica is a favorable chassis cell used for green biomanufacturing by numerous researchers. This paper mainly reviews the development of synthetic biology techniques for Y. lipolytica and summarizes the recent research progress on the synthesis of natural products in Y. lipolytica. This review will promote the continued innovative development of Y. lipolytica by providing theoretical guidance for research on the biosynthesis of natural products.

First-class - biosynthesis of 6-MSA and bostrycoidin type I polyketides in Yarrowia lipolytica

Fungal polyketides are a large group of secondary metabolites, valuable due to their diverse spectrum of pharmacological activities. Polyketide biosynthesis in filamentous fungi presents some challenges: small yield and low-purity titers. To tackle these issues, we switched to the yeast Yarrowia lipolytica, an easily cultivable heterologous host. As an oleaginous yeast, Y. lipolytica displays a high flux of acetyl- and malonyl-CoA precursors used in lipid synthesis. Likewise, acetyl- and malonyl-CoA are the building blocks of many natural polyketides, and we explored the possibility of redirecting this flux toward polyketide production. Despite its promising prospect, Y. lipolytica has so far only been used for heterologous expression of simple type III polyketide synthases (PKSs) from plants. Therefore, we decided to evaluate the potential of Y. lipolytica by targeting the more complex fungal polyketides synthesized by type I PKSs. We employed a CRISPR-Cas9-mediated genome editing method to achieve markerless gene integration of the genes responsible for bostrycoidin biosynthesis in Fusarium solani (fsr1, fsr2, and fsr3) and 6-methylsalicylic acid (6-MSA) biosynthesis in Aspergillus hancockii (6MSAS). Moreover, we attempted titer optimization through metabolic engineering by overexpressing two enzymes, TGL4 and AOX2, involved in lipid β-oxidation, but we did not observe an effect on polyketide production. With maximum titers of 403 mg/L 6-MSA and 35 mg/L bostrycoidin, the latter being substantially higher than our previous results in Saccharomyces cerevisiae (2.2 mg/L), this work demonstrates the potential of Y. lipolytica as a platform for heterologous production of complex fungal polyketides.

Enhancing 2-Pyrone Synthase Efficiency by High-Throughput Mass-Spectrometric Quantification and In Vitro/In Vivo Catalytic Performance Correlation

Engineering efficient biocatalysts is essential for metabolic engineering to produce valuable bioproducts from renewable resources. However, due to the complexity of cellular metabolic networks, it is challenging to translate success in vitro into high performance in cells. To meet such a challenge, an accurate and efficient quantification method is necessary to screen a large set of mutants from complex cell culture and a careful correlation between the catalysis parameters in vitro and performance in cells is required. In this study, we employed a mass-spectrometry based high-throughput quantitative method to screen new mutants of 2-pyrone synthase (2PS) for triacetic acid lactone (TAL) biosynthesis through directed evolution in E. coli. From the process, we discovered two mutants with the highest improvement (46 fold) in titer and the fastest kcat (44 fold) over the wild type 2PS, respectively, among those reported in the literature. A careful examination of the correlation between intracellular substrate concentration, Michaelis-Menten parameters and TAL titer for these two mutants reveals that a fast reaction rate under limiting intracellular substrate concentrations is important for in-cell biocatalysis. Such properties can be tuned by protein engineering and synthetic biology to adopt these engineered proteins for the maximum activities in different intracellular environments.

Metabolic engineering of Yarrowia lipolytica for high-level production of squalene

Squalene is an important triterpene with a wide range of applications. Given the growing market demand for squalene, the development of microbial cell factories capable of squalene production is considered a sustainable method. This study aimed to investigate the squalene production potential of Yarrowia lipolytica. First, HMG-CoA reductase from Saccharomyces cerevisiae and squalene synthase from Y. lipolytica was co-overexpressed in Y. lipolytica. Second, by enhancing the supply of NADPH in the squalene synthesis pathway, the production of squalene in Y. lipolytica was effectively increased. Furthermore, by constructing an isoprenol utilization pathway and overexpressing YlDGA1, the strain YLSQ9, capable of producing 868.1 mg/L squalene, was obtained. Finally, by optimizing the fermentation conditions, the highest squalene concentration of 1628.2 mg/L (81.0 mg/g DCW) in Y. lipolytica to date was achieved. This study demonstrated the potential for achieving high squalene production using Y. lipolytica.

Building Yarrowia lipolytica Cell Factories for Advanced Biomanufacturing: Challenges and Solutions

Microbial cell factories have shown great potential for industrial production with the benefit of being environmentally friendly and sustainable. Yarrowia lipolytica is a promising and superior non-model host for biomanufacturing due to its cumulated advantages compared to model microorganisms, such as high fluxes of metabolic precursors (acetyl-CoA and malonyl-CoA) and its naturally hydrophobic microenvironment. However, although diverse compounds have been synthesized in Y. lipolytica cell factories, most of the relevant studies have not reached the level of industrialization and commercialization due to a number of remaining challenges, including unbalanced metabolic flux, conflict between cell growth and product synthesis, and cytotoxic effects. Here, various metabolic engineering strategies for solving the challenges are summarized, which is developing fast and extremely conducive to rational design and reconstruction of robust Y. lipolytica cell factories for advanced biomanufacturing. Finally, future engineering efforts for enhancing the production efficiency of this platform strain are highlighted.

Reconfiguration of the reductive TCA cycle enables high-level succinic acid production by Yarrowia lipolytica

Succinic acid (SA) is an important C4-dicarboxylic acid. Microbial production of SA at low pH results in low purification costs and hence good overall process economics. However, redox imbalances limited SA biosynthesis from glucose via the reductive tricarboxylic acid (TCA) cycle in yeast. Here, we engineer the strictly aerobic yeast Yarrowia lipolytica for efficient SA production without pH control. Introduction of the reductive TCA cycle into the cytosol of a succinate dehydrogenase-disrupted yeast strain causes arrested cell growth. Although adaptive laboratory evolution restores cell growth, limited NADH supply restricts SA production. Reconfiguration of the reductive SA biosynthesis pathway in the mitochondria through coupling the oxidative and reductive TCA cycle for NADH regeneration results in improved SA production. In pilot-scale fermentation, the engineered strain produces 111.9 g/L SA with a yield of 0.79 g/g glucose within 62 h. This study paves the way for industrial production of biobased SA.

Efficient Production of Triacetic Acid Lactone from Lignocellulose Hydrolysate by Metabolically Engineered Yarrowia lipolytica

Lignocellulose is a promising renewable feedstock for the bioproduction of high-value biochemicals. The poorly expressed xylose catabolic pathway was the bottleneck in the efficient utilization of the lignocellulose feedstock in yeast. Herein, multiple genetic and process engineering strategies were explored to debottleneck the conversion of xylose to the platform chemical triacetic acid lactone (TAL) in Yarrowia lipolytica. We identified that xylose assimilation generating more cofactor NADPH was favorable for the TAL synthesis. pH control improved the expression of acetyl-CoA carboxylase and generated more precursor malonyl-CoA. Combined with the suppression of the lipid synthesis pathway, 5.03 and 4.18 g/L TAL were produced from pure xylose and xylose-rich wheat straw hydrolysate, respectively. Our work removed the bottleneck of the xylose assimilation pathway and effectively upgraded wheat straw hydrolysate to TAL, which enabled us to build a sustainable oleaginous yeast cell factory to cost-efficiently produce green chemicals from low-cost lignocellulose by Y. lipolytica.

Mining natural products for advanced biofuels and sustainable bioproducts

Recently, there has been growing interest in the sustainable production of biofuels and bioproducts derived from renewable sources. Natural products, the largest and more structurally diverse group of metabolites, hold significant promise as sources for such bio-based products. However, there are two primary challenges in harnessing natural products' potential: precise mining of biosynthetic gene clusters (BGCs) that can be used as scaffolds or bioparts and their functional expression for biofuel and bioproduct manufacture. In this review, we explore recent advances in the development of bioinformatic tools for BGC mining and the manipulation of various hosts for natural product-based biofuels and bioproducts manufacture. Moreover, we discuss potential strategies for expanding the chemical diversity of biofuels and bioproducts and enhancing their overall yield.

A heterologous expression platform in Aspergillus nidulans for the elucidation of cryptic secondary metabolism biosynthetic gene clusters: discovery of the Aspergillus fumigatus sartorypyrone biosynthetic pathway

Aspergillus fumigatus is a serious human pathogen causing life-threatening Aspergillosis in immunocompromised patients. Secondary metabolites (SMs) play an important role in pathogenesis, but the products of many SM biosynthetic gene clusters (BGCs) remain unknown. In this study, we have developed a heterologous expression platform in Aspergillus nidulans, using a newly created genetic dereplication strain, to express a previously unknown BGC from A. fumigatus and determine its products. The BGC produces sartorypyrones, and we have named it the spy BGC. Analysis of targeted gene deletions by HRESIMS, NMR, and microcrystal electron diffraction (MicroED) enabled us to identify 12 products from the spy BGC. Seven of the compounds have not been isolated previously. We also individually expressed the polyketide synthase (PKS) gene spyA and demonstrated that it produces the polyketide triacetic acid lactone (TAL), a potentially important biorenewable platform chemical. Our data have allowed us to propose a biosynthetic pathway for sartorypyrones and related natural products. This work highlights the potential of using the A. nidulans heterologous expression platform to uncover cryptic BGCs from A. fumigatus and other species, despite the complexity of their secondary metabolomes.

Triacetic acid lactone production using 2-pyrone synthase expressing Yarrowia lipolytica via targeted gene deletion

An environmentally sustainable world can be realized by using microorganisms to produce value-added materials from renewable biomass. Triacetic acid lactone (TAL) is a high-value-added compound that is used as a precursor of various organic compounds such as food additives and pharmaceuticals. In this study, we used metabolic engineering to produce TAL from glucose using an oleaginous yeast Yarrowia lipolytica. We first introduced TAL-producing gene 2-pyrone synthase into Y. lipolytica, which enabled TAL production. Next, we increased TAL production by engineering acetyl-CoA and malonyl-CoA biosynthesis pathways by redirecting carbon flux to glycolysis. Finally, we optimized the carbon and nitrogen ratios in the medium, culminating in the production of 4078 mg/L TAL. The strategy presented in this study had the potential to improve the titer and yield of polyketide biosynthesis.

Direct Utilization of Peroxisomal Acetyl-CoA for the Synthesis of Polyketide Compounds in Saccharomyces cerevisiae

Polyketides are a class of natural products with many applications but are mainly appealing as pharmaceuticals. Heterologous production of polyketides in the yeast Saccharomyces cerevisiae has been widely explored because of the many merits of this model eukaryotic microorganism. Although acetyl-CoA and malonyl-CoA, the precursors for polyketide synthesis, are distributed in several yeast subcellular organelles, only cytosolic synthesis of polyketides has been pursued in previous studies. In this study, we investigate polyketide synthesis by directly using acetyl-CoA in the peroxisomes of yeast strain CEN.PK2-1D. We first demonstrate that the polyketide flaviolin can be synthesized in this organelle upon peroxisomal colocalization of native acetyl-CoA carboxylase and 1,3,6,8-tetrahydroxynaphthalene synthase (a type III polyketide synthase). Next, using the synthesis of the polyketide triacetic acid lactone as an example, we show that (1) a new peroxisome targeting sequence, pPTS1, is more effective than the previously reported ePTS1 for peroxisomal polyketide synthesis; (2) engineering peroxisome proliferation is effective to boost polyketide production; and (3) peroxisomes provide an additional acetyl-CoA reservoir and extra space to accommodate enzymes so that utilizing the peroxisomal pathway plus the cytosolic pathway produces more polyketide than the cytosolic pathway alone. This research lays the groundwork for more efficient heterologous polyketide biosynthesis using acetyl-CoA pools in subcellular organelles.

Gaining access to acetyl-CoA by peroxisomal surface display

Synthetic biology is constantly making progress for producing compounds on demand. Recently, Yocum and collaborators have developed an outstanding approach based on the anchoring of biosynthetic enzymes to the peroxisomal membrane. This allowed access to an untapped resource of acetyl-CoA and stimulated the synthesis of a valuable polyketide.

Production of the antidepressant orcinol glucoside in Yarrowia lipolytica with yields over 6,400-fold higher than plant extraction

Orcinol glucoside (OG), mainly found in the rhizome of the traditional Chinese herb Curculigo orchioides Gaertn, is noted for its antidepressant effects. In this study, an efficient screening pipeline was established for identifying the highly active orcinol synthase (ORS) and UDP-dependent glycosyltransferase (UGT) involved in the biosynthesis of OG by combining transcriptome analysis, structure-based virtual screening, and in vitro enzyme activity assays. By enhancing the downstream pathway, metabolic engineering and fermentation optimization, the OG production in Yarrowia lipolytica was improved 100-fold, resulting in a final yield of 43.46 g/L (0.84 g/g DCW), which is almost 6,400-fold higher than the extraction yield from C. orchioides roots. This study provides a reference for rapid identification of functional genes and high-yield production of natural products.

Metabolic Engineering and Synthetic Biology Approaches for the Heterologous Production of Aromatic Polyketides

Polyketides are a diverse set of natural products with versatile applications as pharmaceuticals, nutraceuticals, and cosmetics, to name a few. Of several types of polyketides, aromatic polyketides comprising type II and III polyketides contain many chemicals important for human health such as antibiotics and anticancer agents. Most aromatic polyketides are produced from soil bacteria or plants, which are difficult to engineer and grow slowly in industrial settings. To this end, metabolic engineering and synthetic biology have been employed to efficiently engineer heterologous model microorganisms for enhanced production of important aromatic polyketides. In this review, we discuss the recent advancement in metabolic engineering and synthetic biology strategies for the production of type II and type III polyketides in model microorganisms. Future challenges and prospects of aromatic polyketide biosynthesis by synthetic biology and enzyme engineering approaches are also discussed.

Temporal sorting of microdroplets can identify productivity differences of itaconic acid from libraries of Yarrowia lipolytica

Microdroplet screening of microorganisms can improve the rate of strain selection and characterization within the canonical design-build-test paradigm. However, a full analysis of the microdroplet environment and how well these conditions translate to culturing conditions and techniques is lacking in the field. Quantification of three different biosensor/analyte combinations at 12 hour timepoints reveals the potential for extended dose-response ranges as compared to traditional in vitro conditions. Using these dynamics, we present an application and analysis of microfluidic droplet screening utilizing whole-cell biosensors, ultimately identifying an altered productivity profile of itaconic acid in a Yarrowia lipolytica-based piggyBac transposon library. Specifically, we demonstrate that the timepoint for microdroplet selection can influence the outcome of the selection and thus shift the identified strain productivity and final titer. In this case, strains selected at earlier timepoints showed increased early productivity in flask scale, with the converse true as well. Differences in response indicate microdroplet assays require tailored development to more accurately sort for phenotypes that are scalable to larger incubation volumes. Likewise, these results further highlight that screening conditions are critical parameters for success in high-throughput applications.

Metabolic Engineering of Pichia pastoris for the Production of Triacetic Acid Lactone

Triacetic acid lactone (TAL) is a promising renewable platform polyketide with broad biotechnological applications. In this study, we constructed an engineered Pichia pastoris strain for the production of TAL. We first introduced a heterologous TAL biosynthetic pathway by integrating the 2-pyrone synthase encoding gene from Gerbera hybrida (Gh2PS). We then removed the rate-limiting step of TAL synthesis by introducing the posttranslational regulation-free acetyl-CoA carboxylase mutant encoding gene from S. cerevisiae (ScACC1*) and increasing the copy number of Gh2PS. Finally, to enhance intracellular acetyl-CoA supply, we focused on the introduction of the phosphoketolase/phosphotransacetylase pathway (PK pathway). To direct more carbon flux towards the PK pathway for acetyl-CoA generation, we combined it with a heterologous xylose utilization pathway or endogenous methanol utilization pathway. The combination of the PK pathway with the xylose utilization pathway resulted in the production of 825.6 mg/L TAL in minimal medium with xylose as the sole carbon source, with a TAL yield of 0.041 g/g xylose. This is the first report on TAL biosynthesis in P. pastoris and its direct synthesis from methanol. The present study suggests potential applications in improving the intracellular pool of acetyl-CoA and provides a basis for the construction of efficient cell factories for the production of acetyl-CoA derived compounds.

A review of synthetic biology tools in Yarrowia lipolytica

Yarrowia lipolytica is a non-conventional oleaginous yeast with great potential for industrial production. Y. lipolytica has a high propensity for flux through tricarboxylic acid cycle intermediates. Therefore, this host is currently being developed as a workhorse, and is rapidly emerging in biotechnology fields, especially for industrial chemical production, whole-cell bioconversion, and the treatment and recycling of industrial waste. In recent studies, Y. lipolytica has been rewritten and introduced with non-native metabolites of certain compounds of interest owing to the advancement in synthetic biology tools. In this review, we collate recent progress to present a detailed and insightful summary of the major developments in synthetic biology tools and techniques for Y. lipolytica, including promoters, terminators, selection markers, autonomously replicating sequences, DNA assembly techniques, genome editing techniques, and subcellular organelle engineering. This comprehensive overview would be a useful resource for future genetic engineering studies to improve the yield of desired metabolic products in Y. lipolytica.

Global metabolic rewiring of the nonconventional yeast Ogataea polymorpha for biosynthesis of the sesquiterpenoid β-elemene

Bioproduction of natural products via microbial cell factories is a promising alternative to traditional plant extraction. Recently, nonconventional microorganisms have emerged as attractive chassis hosts for biomanufacturing. One such microorganism, Ogataea polymorpha is an industrial yeast used for protein expression with numerous advantages, such as thermal-tolerance, a wide substrate spectrum and high-density fermentation. Here, we systematically rewired the cellular metabolism of O. polymorpha to achieve high-level production of the sesquiterpenoid β-elemene by optimizing the mevalonate pathway, enhancing the supply of NADPH and acetyl-CoA, and downregulating competitive pathways. The engineered strain produced 509 mg/L and 4.7 g/L of β-elemene under batch and fed-batch fermentation, respectively. Therefore, this study identified the potential industrial application of O. polymorpha as a good microbial platform for producing sesquiterpenoids.

De novo biosynthesis of carminic acid in Saccharomyces cerevisiae

Carminic acid is a natural red dye extracted from the insect Dactylopius coccus. Due to its ideal dying effect and high safety, it is widely used in food and cosmetics industries. Previous study showed that introduction of polyketide synthase (OKS) from Aloe arborescens, cyclase (ZhuI) and aromatase (ZhuJ) from Streptomyces sp. R1128, and C-glucosyltransferase (UGT2) from D. coccus into Aspergillus nidulans could achieve trace amounts of de novo production. These four genes were introduced into Saccharomyces cerevisiae, but carminic acid was not detected. Analysis of the genome of A. nidulans revealed that 4'-phosphopantetheinyl transferase (NpgA) and monooxygenase (AptC) are essential for de novo biosynthesis of carminic acid in S. cerevisiae. Additionally, endogenous hydroxylase (Cat5) from S. cerevisiae was found to be responsible for hydroxylation of flavokermesic acid to kermesic acid. Therefore, all enzymes and their functions in the biosynthesis of carminic acid were explored and reconstructed in S. cerevisiae. Through systematic pathway engineering, including regulating enzyme expression, enhancing precursor supply, and modifying the β-oxidation pathway, the carminic acid titer in a 5 L bioreactor reached 7580.9 μg/L, the highest yet reported for a microorganism. Heterologous reconstruction of the carminic acid biosynthetic pathway in S. cerevisiae has great potential for de novo biosynthesis of anthraquinone dye.

Evolving tolerance of Yarrowia lipolytica to hydrothermal liquefaction aqueous phase waste

Hydrothermal liquefaction (HTL) is an emerging method for thermochemical conversion of wet organic waste and biomass into renewable biocrude. HTL also produces an aqueous phase (HTL-AP) side stream containing 2-4% light organic compounds that require treatment. Although anaerobic digestion (AD) of HTL-AP has shown promise, lengthy time periods were required for AD microbial communities to adapt to metabolic inhibitors in HTL-AP. An alternative for HTL-AP valorization was recently demonstrated using two engineered strains of Yarrowia lipolytica, E26 and Diploid TAL, for the overproduction of lipids and the polyketide triacetic acid lactone (TAL) respectively. These strains tolerated up to 10% HTL-AP (v/v) in defined media and up to 25% (v/v) HTL-AP in rich media. In this work, adaptive laboratory evolution (ALE) of these strains increased the bulk population tolerance for HTL-AP to up to 30% (v/v) in defined media and up to 35% (v/v) for individual isolates in rich media. The predominate organic acids within HTL-AP (acetic, butyric, and propionic) were rapidly consumed by the evolved Y. lipolytica strains. A TAL-producing isolate (strain 144-3) achieved a nearly 3-fold increase in TAL titer over the parent strain while simultaneously reducing the chemical oxygen demand (COD) of HTL-AP containing media. Fermentation with HTL-AP as the sole nutrient source demonstrated direct conversion of waste into TAL at 10% theoretical yield. Potential genetic mutations of evolved TAL production strains that could be imparting tolerance were explored. This work advances the potential of Y. lipolytica to biologically treat and simultaneously extract value from HTL wastewater. KEY POINTS: • Adaptive evolution of two Y. lipolytica strains enhanced their tolerance to waste. • Y. lipolytica reduces chemical oxygen demand in media containing waste. • Y. lipolytica can produce triacetic acid lactone directly from wastewater.

Abolishing storage lipids induces protein misfolding and stress responses in Yarrowia lipolytica

Yarrowia lipolytica naturally saves excess carbon as storage lipids. Engineering efforts allow redirecting the high precursor flux required for lipid synthesis toward added-value chemicals such as polyketides, flavonoids, and terpenoids. To redirect precursor flux from storage lipids to other products, four genes involved in triacylglycerol and sterol ester synthesis (DGA1, DGA2, LRO1, and ARE1) can be deleted. To elucidate the effect of the deletions on cell physiology and regulation, we performed chemostat cultivations under carbon and nitrogen limitations, followed by transcriptome analysis. We found that storage lipid-free cells show an enrichment of the unfolded protein response, and several biological processes related to protein refolding and degradation are enriched. Additionally, storage lipid-free cells show an altered lipid class distribution with an abundance of potentially cytotoxic free fatty acids under nitrogen limitation. Our findings not only highlight the importance of lipid metabolism on cell physiology and proteostasis, but can also aid the development of improved chassy strains of Y. lipolytica for commodity chemical production.

Engineering of non-model eukaryotes for bioenergy and biochemical production

The prospect of leveraging naturally occurring phenotypes to overcome bottlenecks constraining the bioeconomy has marshalled increased exploration of nonconventional organisms. This review discusses the status of non-model eukaryotic species in bioproduction, the evaluation criteria for effectively matching a candidate host to a biosynthetic process, and the genetic engineering tools needed for host domestication. We present breakthroughs in genome editing and heterologous pathway design, delving into innovative spatiotemporal modulation strategies that potentiate more refined engineering capabilities. We cover current understanding of genetic instability and its ramifications for industrial scale-up, highlighting key factors and possible remedies. Finally, we propose future opportunities to expand the current collection of available hosts and provide guidance to benefit the broader bioeconomy.

Advanced one-pot deconstruction and valorization of lignocellulosic biomass into triacetic acid lactone using Rhodosporidium toruloides

BACKGROUND

Rhodosporidium toruloides is capable of co-utilization of complex carbon sources and robust growth from lignocellulosic hydrolysates. This oleaginous yeast is therefore an attractive host for heterologous production of valuable bioproducts at high titers from low-cost, deconstructed biomass in an economically and environmentally sustainable manner. Here we demonstrate this by engineering R. toruloides to produce the polyketide triacetic acid lactone (TAL) directly from unfiltered hydrolysate deconstructed from biomass with minimal unit process operations.

RESULTS

Introduction of the 2-pyrone synthase gene into R. toruloides enabled the organism to produce 2.4 g/L TAL from simple media or 2.0 g/L from hydrolysate produced from sorghum biomass. Both of these titers are on par with titers from other better-studied microbial hosts after they had been heavily engineered. We next demonstrate that filtered hydrolysates produced from ensiled sorghum are superior to those derived from dried sorghum for TAL production, likely due to the substantial organic acids produced during ensiling. We also demonstrate that the organic acids found in ensiled biomass can be used for direct synthesis of ionic liquids within the biomass pretreatment process, enabling consolidation of unit operations of in-situ ionic liquid synthesis, pretreatment, saccharification, and fermentation into a one-pot, separations-free process. Finally, we demonstrate this consolidation in a 2 L bioreactor using unfiltered hydrolysate, producing 3.9 g/L TAL.

CONCLUSION

Many steps involved in deconstructing biomass into fermentable substrate can be combined into a distinct operation, and directly fed to cultures of engineered R. toruloides cultures for subsequent valorization into gram per liter titers of TAL in a cost-effective manner.

Persulfate-promoted synthesis of biphenyl compounds in water from biomass-derived triacetic acid lactone

A metal-free, persulfate-promoted route in water to access substituted biphenyl compounds is described. In our approach, the biomass-derived triacetic acid lactone (TAL) and phenylacetylenes are employed to generate targeted products in high yields and regioselectivity. The present method contributes to the green synthesis initiative to harness renewable biomass-derived compounds in environmentally friendly methodologies.

Biosynthesis of cannabinoid precursor olivetolic acid in genetically engineered Yarrowia lipolytica

Engineering microbes to produce plant-derived natural products provides an alternate solution to obtain bioactive products. Here we report a systematic approach to sequentially identify the rate-limiting steps and improve the biosynthesis of the cannabinoid precursor olivetolic acid (OLA) in Yarrowia lipolytica. We find that Pseudomonas sp LvaE encoding a short-chain acyl-CoA synthetase can efficiently convert hexanoic acid to hexanoyl-CoA. The co-expression of the acetyl-CoA carboxylase, the pyruvate dehydrogenase bypass, the NADPH-generating malic enzyme, as well as the activation of peroxisomal β-oxidation pathway and ATP export pathway are effective strategies to redirect carbon flux toward OLA synthesis. Implementation of these strategies led to an 83-fold increase in OLA titer, reaching 9.18 mg/L of OLA in shake flask culture. This work may serve as a baseline for engineering cannabinoids biosynthesis in oleaginous yeast species.

Lipid production from lignocellulosic biomass using an engineered Yarrowia lipolytica strain

BACKGROUND

The utilization of industrial wastes as feedstock in microbial-based processes is a one of the high-potential approach for the development of sustainable, environmentally beneficial and valuable bioproduction, inter alia, lipids. Rye straw hydrolysate, a possible renewable carbon source for bioconversion, contains a large amount of xylose, inaccessible to the wild-type Yarrowia lipolytica strains. Although these oleaginous yeasts possesses all crucial genes for xylose utilization, it is necessary to induce their metabolic pathway for efficient growth on xylose and mixed sugars from agricultural wastes. Either way, biotechnological production of single cell oils (SCO) from lignocellulosic hydrolysate requires yeast genome modification or adaptation to a suboptimal environment.

RESULTS

The presented Y. lipolytica strain was developed using minimal genome modification-overexpression of endogenous xylitol dehydrogenase (XDH) and xylulose kinase (XK) genes was sufficient to allow yeast to grow on xylose as a sole carbon source. Diacylglycerol acyltransferase (DGA1) expression remained stable and provided lipid overproduction. Obtained an engineered Y. lipolytica strain produced 5.51 g/L biomass and 2.19 g/L lipids from nitrogen-supplemented rye straw hydrolysate, which represents an increase of 64% and an almost 10 times higher level, respectively, compared to the wild type (WT) strain. Glucose and xylose were depleted after 120 h of fermentation. No increase in byproducts such as xylitol was observed.

CONCLUSIONS

Xylose-rich rye straw hydrolysate was exploited efficiently for the benefit of production of lipids. This study indicates that it is possible to fine-tune a newly strain with as minimally genetic changes as possible by adjusting to an unfavorable environment, thus limiting multi-level genome modification. It is documented here the use of Y. lipolytica as a microbial cell factory for lipid synthesis from rye straw hydrolysate as a low-cost feedstock.

Engineered biosynthesis of plant polyketides by type III polyketide synthases in microorganisms

Plant specialized metabolites occupy unique therapeutic niches in human medicine. A large family of plant specialized metabolites, namely plant polyketides, exhibit diverse and remarkable pharmaceutical properties and thereby great biomanufacturing potential. A growing body of studies has focused on plant polyketide synthesis using plant type III polyketide synthases (PKSs), such as flavonoids, stilbenes, benzalacetones, curcuminoids, chromones, acridones, xanthones, and pyrones. Microbial expression of plant type III PKSs and related biosynthetic pathways in workhorse microorganisms, such as Saccharomyces cerevisiae, Escherichia coli, and Yarrowia lipolytica, have led to the complete biosynthesis of multiple plant polyketides, such as flavonoids and stilbenes, from simple carbohydrates using different metabolic engineering approaches. Additionally, advanced biosynthesis techniques led to the biosynthesis of novel and complex plant polyketides synthesized by diversified type III PKSs. This review will summarize efforts in the past 10 years in type III PKS-catalyzed natural product biosynthesis in microorganisms, especially the complete biosynthesis strategies and achievements.

Engineering Yarrowia lipolytica for the sustainable production of β-farnesene from waste oil feedstock

BACKGROUND

β-Farnesene is a sesquiterpene with versatile industrial applications. The production of β-farnesene from waste lipid feedstock is an attractive method for sustainable production and recycling waste oil. Yarrowia lipolytica is an unconventional oleaginous yeast, which can use lipid feedstock and has great potential to synthesize acetyl-CoA-derived chemicals.

RESULTS

In this study, we engineered Y. lipolytica to produce β-farnesene from lipid feedstock. To direct the flux of acetyl-CoA, which is generated from lipid β-oxidation, to β-farnesene synthesis, the mevalonate synthesis pathway was compartmentalized into peroxisomes. β-Farnesene production was then engineered by the protein engineering of β-farnesene synthase and pathway engineering. The regulation of lipid metabolism by enhancing β-oxidation and eliminating intracellular lipid synthesis was further performed to improve the β-farnesene synthesis. As a result, the final β-farnesene production with bio-engineering reached 35.2 g/L and 31.9 g/L using oleic acid and waste cooking oil, respectively, which are the highest β-farnesene titers reported in Y. lipolytica.

CONCLUSIONS

This study demonstrates that engineered Y. lipolytica could realize the sustainable production of value-added acetyl-CoA-derived chemicals from waste lipid feedstock.

Designing Microbial Cell Factories for the Production of Chemicals

The sustainable production of chemicals from renewable, nonedible biomass has emerged as an essential alternative to address pressing environmental issues arising from our heavy dependence on fossil resources. Microbial cell factories are engineered microorganisms harboring biosynthetic pathways streamlined to produce chemicals of interests from renewable carbon sources. The biosynthetic pathways for the production of chemicals can be defined into three categories with reference to the microbial host selected for engineering: native-existing pathways, nonnative-existing pathways, and nonnative-created pathways. Recent trends in leveraging native-existing pathways, discovering nonnative-existing pathways, and designing de novo pathways (as nonnative-created pathways) are discussed in this Perspective. We highlight key approaches and successful case studies that exemplify these concepts. Once these pathways are designed and constructed in the microbial cell factory, systems metabolic engineering strategies can be used to improve the performance of the strain to meet industrial production standards. In the second part of the Perspective, current trends in design tools and strategies for systems metabolic engineering are discussed with an eye toward the future. Finally, we survey current and future challenges that need to be addressed to advance microbial cell factories for the sustainable production of chemicals.

Dual cytoplasmic-peroxisomal engineering for high-yield production of sesquiterpene α-humulene in Yarrowia lipolytica

The sesquiterpene α-humulene is an important plant natural product, which has been used in pharmaceutical industry due to the anti-inflammatory and anticancer activities. Although phytoextraction and chemical synthesis have previously been applied into α-humulene production, the low efficiency and high costs limit the development. In this study, Y. lipolytica was engineered as the robust cell factory for sustainable α-humulene production. First, a chassis with high α-humulene output in the cytoplasm was constructed by integrating α-humulene synthases with high catalytic activity, optimizing the flux of MVA and acetyl-CoA pathways. Subsequently, the strategy of dual cytoplasmic-peroxisomal engineering was adopted in Y. lipolytica, the best strain GQ3006 generated by introducing 31 copies of 12 different genes could produce 2280.3 ± 38.2 mg/L (98.7 ± 4.2 mg/g DCW) α-humulene, a 100-fold improvement relative to the baseline strain. In order to further improve the titer, a novel strategy for downregulation of squalene biosynthesis based on Cu²⁺ -repressible promoters was firstly established, which significantly improved the α-humulene titer by 54.2 % to 3516.6 ± 34.3 mg/L. Finally, the engineered strain could produce 21.7 g/L α-humulene in 5-L bioreactor, 6.8-fold higher than the highest α-humulene titer reported prior to this study. Overall, system metabolic engineering strategies used in this study provide a valuable reference for highly sustainable production of terpenoids in Y. lipolytica. This article is protected by copyright. All rights reserved.

Metabolic engineering of oleaginous yeast Rhodotorula toruloides for overproduction of triacetic acid lactone

The plant‐sourced polyketide triacetic acid lactone (TAL) has been recognized as a promising platform chemical for the biorefinery industry. However, its practical application was rather limited due to low natural abundance and inefficient cell factories for biosynthesis. Here, we report the metabolic engineering of oleaginous yeast Rhodotorula toruloides for TAL overproduction. We first introduced a 2‐pyrone synthase gene from Gerbera hybrida (GhPS) into R. toruloides and investigated the effects of different carbon sources on TAL production. We then systematically employed a variety of metabolic engineering strategies to increase the flux of acetyl‐CoA by enhancing its biosynthetic pathways and disrupting its competing pathways. We found that overexpression of ATP‐citrate lyase (ACL1) improved TAL production by 45% compared to the GhPS overexpressing strain, and additional overexpression of acetyl‐CoA carboxylase (ACC1) further increased TAL production by 29%. Finally, we characterized the resulting strain I12‐ACL1‐ACC1 using fed‐batch bioreactor fermentation in glucose or oilcane juice medium with acetate supplementation and achieved a titer of 28 or 23 g/L TAL, respectively. This study demonstrates that R. toruloides is a promising host for the production of TAL and other acetyl‐CoA‐derived polyketides from low‐cost carbon sources.

Metabolic engineering of Yarrowia lipolytica for scutellarin production

Scutellarin related drugs have superior therapeutic effects on cerebrovascular and cardiovascular diseases. Here, an optimal biosynthetic pathway for scutellarin was constructed in Yarrowia lipolytica platform due to its excellent metabolic potential. By integrating multi-copies of core genes from different species, the production of scutellarin was increased from 15.11 mg/L to 94.79 mg/L and the ratio of scutellarin to the main by-product was improved about 110-fold in flask condition. Finally, the production of scutellarin was improved 23-fold and reached to 346 mg/L in fed-batch bioreactor, which was the highest reported titer for de novo production of scutellarin in microbes. Our results represent a solid basis for further production of natural products on unconventional yeasts and have a potential of industrial implementation.

Research Progress on the Synthetic Biology of Botanical Biopesticides

The production and large-scale application of traditional chemical pesticides will bring environmental pollution and food safety problems. With the advantages of high safety and environmental friendliness, botanical biopesticides are in line with the development trend of modern agriculture and have gradually become the mainstream of modern pesticide development. However, the traditional production of botanical biopesticides has long been faced with prominent problems, such as limited source and supply, complicated production processes, and excessive consumption of resources. In recent years, the rapid development of synthetic biology will break through these bottlenecks, and many botanical biopesticides are produced using synthetic biology, such as emodin, celangulin, etc. This paper reviews the latest progress and application prospect of synthetic biology in the development of botanical pesticides so as to provide new ideas for the analysis of synthetic pathways and heterologous and efficient production of botanical biopesticides and accelerate the research process of synthetic biology of natural products.

Microbial chassis engineering drives heterologous production of complex secondary metabolites

The cryptic secondary metabolite biosynthetic gene clusters (BGCs) far outnumber currently known secondary metabolites. Heterologous production of secondary metabolite BGCs in suitable chassis facilitates yield improvement and discovery of new-to-nature compounds. The two juxtaposed conventional model microorganisms, Escherichia coli, Saccharomyces cerevisiae, have been harnessed as microbial chassis to produce a bounty of secondary metabolites with the help of certain host engineering. In last decade, engineering non-model microbes to efficiently biosynthesize secondary metabolites has received increasing attention due to their peculiar advantages in metabolic networks and/or biosynthesis. The state-of-the-art synthetic biology tools lead the way in operating genetic manipulation in non-model microorganisms for phenotypic optimization or yields improvement of desired secondary metabolites. In this review, we firstly discuss the pros and cons of several model and non-model microbial chassis, as well as the importance of developing broader non-model microorganisms as alternative programmable heterologous hosts to satisfy the desperate needs of biosynthesis study and industrial production. Then we highlight the lately advances in the synthetic biology tools and engineering strategies for optimization of non-model microbial chassis, in particular, the successful applications for efficient heterologous production of multifarious complex secondary metabolites, e.g., polyketides, nonribosomal peptides, as well as ribosomally synthesized and post-translationally modified peptides. Lastly, emphasis is on the perspectives of chassis cells development to access the ideal cell factory in the artificial intelligence-driven genome era.

Considering Strain Variation and Non-Type Strains for Yeast Metabolic Engineering Applications

A variety of yeast species have been considered ideal hosts for metabolic engineering to produce value-added chemicals, including the model organism Saccharomyces cerevisiae, as well as non-conventional yeasts including Yarrowia lipolytica, Kluyveromyces marxianus, and Pichia pastoris. However, the metabolic capacity of these microbes is not simply dictated or implied by genus or species alone. Within the same species, yeast strains can display distinct variations in their phenotypes and metabolism, which affect the performance of introduced pathways and the production of interesting compounds. Moreover, it is unclear how this metabolic potential corresponds to function upon rewiring these organisms. These reports thus point out a new consideration for successful metabolic engineering, specifically: what are the best strains to utilize and how does one achieve effective metabolic engineering? Understanding such questions will accelerate the host selection and optimization process for generating yeast cell factories. In this review, we survey recent advances in studying yeast strain variations and utilizing non-type strains in pathway production and metabolic engineering applications. Additionally, we highlight the importance of employing portable methods for metabolic rewiring to best access this metabolic diversity. Finally, we conclude by highlighting the importance of considering strain diversity in metabolic engineering applications.

Shifting the distribution: modulation of the lipid profile in Yarrowia lipolytica via iron content

Microbial fermentation offers a sustainable source of fuels, commodity chemicals, and pharmaceuticals, yet strain performance is influenced greatly by the growth media selected. Specifically, trace metals (e.g., iron, copper, manganese, zinc, and others) are critical for proper growth and enzymatic function within microorganisms yet are non-standardized across media formulation. In this work, the effect of trace metal supplementation on the lipid production profile of Yarrowia lipolytica was explored using tube scale fermentation followed by biomass and lipid characterization. Addition of iron (II) to the chemically defined Yeast Synthetic Complete (YSC) medium increased final optical density nearly twofold and lipid production threefold, while addition of copper (II) had no impact. Additionally, dose-responsive changes in lipid distribution were observed, with the percent of oleic acid increasing and stearic acid decreasing as initial iron concentration increased. These changes were reversible with subsequent iron-selective chelation. Use of rich Yeast Peptone Dextrose (YPD) medium enabled further increases in the production of two specialty oleochemicals ultimately reaching 63 and 47% of the lipid pool as α-linolenic acid and cyclopropane fatty acid, respectively, compared to YSC medium. Selective removal of iron (II) natively present in YPD medium decreased this oleochemical production, ultimately aligning the lipid profile with that of non-supplemented YSC medium. These results provide further insight into the proposed mechanisms for iron regulation in yeasts especially as these productions strains contain a mutant allele of the iron regulator, mga2. The work presented here also suggests a non-genetic method for control of the lipid profile in Y. lipolytica for use in diverse applications. KEY POINTS: • Iron supplementation increases cell density and lipid titer in Yarrowia lipolytica. • Iron addition reversibly alters lipid portfolio increasing linolenic acid. • Removal of iron from YPD media provides a link to enhanced oleochemical production.

Metabolism and strategies for enhanced supply of acetyl-CoA in Saccharomyces cerevisiae

Acetyl-CoA is a kind of important cofactor that is involved in many metabolic pathways. It serves as the precursor for many interesting commercial products, such as terpenes, flavonoids and anthraquinones. However, the insufficient supply of acetyl-CoA limits biosynthesis of its derived compounds in the intracellular. In this review, we outlined metabolic pathways involved in the catabolism and anabolism of acetyl-CoA, as well as some important derived products. We examined several strategies for the enhanced supply of acetyl-CoA, and provided insight into pathways that generate acetyl-CoA to balance metabolism, which can be harnessed to improve the titer, yield and productivities of interesting products in Saccharomyces cerevisiae and other eukaryotic microorganisms. We believe that peroxisomal fatty acid β-oxidation could be an attractive strategy for enhancing the supply of acetyl-CoA.

Mapping enzyme catalysis with metabolic biosensing

Enzymes are represented across a vast space of protein sequences and structural forms and have activities that far exceed the best chemical catalysts; however, engineering them to have novel or enhanced activity is limited by technologies for sensing product formation. Here, we describe a general and scalable approach for characterizing enzyme activity that uses the metabolism of the host cell as a biosensor by which to infer product formation. Since different products consume different molecules in their synthesis, they perturb host metabolism in unique ways that can be measured by mass spectrometry. This provides a general way by which to sense product formation, to discover unexpected products and map the effects of mutagenesis.

The testing of engineered enzymes represents a bottleneck. Here the authors report a screening method combining microfluidics and mass spectrometry, to map the catalysis of a mutated enzyme, characterise the range of products generated and recover the sequences of variants with desired activities.

Developing fungal heterologous expression platforms to explore and improve the production of natural products from fungal biodiversity

Natural products from fungi represent an important source of biologically active metabolites notably for therapeutic agent development. Genome sequencing revealed that the number of biosynthetic gene clusters (BGCs) in fungi is much larger than expected. Unfortunately, most of them are silent or barely expressed under laboratory culture conditions. Moreover, many fungi in nature are uncultivable or cannot be genetically manipulated, restricting the extraction and identification of bioactive metabolites from these species. Rapid exploration of the tremendous number of cryptic fungal BGCs necessitates the development of heterologous expression platforms, which will facilitate the efficient production of natural products in fungal cell factories. Host selection, BGC assembly methods, promoters used for heterologous gene expression, metabolic engineering strategies and compartmentalization of biosynthetic pathways are key aspects for consideration to develop such a microbial platform. In the present review, we summarize current progress on the above challenges to promote research effort in the relevant fields.

Metabolic Engineering of Non-carotenoid-Producing Yeast Yarrowia lipolytica for the Biosynthesis of Zeaxanthin

Zeaxanthin is vital to human health; thus, its production has received much attention, and it is also an essential precursor for the biosynthesis of other critical carotenoids such as astaxanthin and crocetin. Yarrowia lipolytica is one of the most intensively studied non-conventional yeasts and has been genetically engineered as a cell factory to produce carotenoids such as lycopene and β-carotene. However, zeaxanthin production by Y. lipolytica has not been well investigated. To fill this gap, β-carotene biosynthesis pathway has been first constructed in this study by the expression of genes, including crtE, crtB, crtI, and carRP. Three crtZ genes encoding β-carotene hydroxylase from different organisms were individually introduced into β-carotene-producing Y. lipolytica to evaluate their performance for producing zeaxanthin. The expression of crtZ from the bacterium Pantoea ananatis (formerly Erwinia uredovora, Eu-crtZ) resulted in the highest zeaxanthin titer and content on the basis of dry cell weight (DCW). After verifying the function of Eu-crtZ for producing zeaxanthin, the high-copy-number integration into the ribosomal DNA of Y. lipolytica led to a 4.02-fold increase in the titer of zeaxanthin and a 721% increase in the content of zeaxanthin. The highest zeaxanthin titer achieved 21.98 ± 1.80 mg/L by the strain grown on a yeast extract peptone dextrose (YPD)-rich medium. In contrast, the highest content of DCW reached 3.20 ± 0.11 mg/g using a synthetic yeast nitrogen base (YNB) medium to culture the cells. Over 18.0 g/L of citric acid was detected in the supernatant of the YPD medium at the end of cultivation. Furthermore, the zeaxanthin-producing strains still accumulated a large amount of lycopene and β-carotene. The results demonstrated the potential of a cell factory for zeaxanthin biosynthesis and opened up an avenue to engineer this host for the overproduction of carotenoids.

A Novel Cre/lox-Based Genetic Tool for Repeated, Targeted and Markerless Gene Integration in Yarrowia lipolytica

The unconventional yeast Yarrowia lipolytica is extensively applied in bioproduction fields owing to its excellent metabolite and protein production ability. Nonetheless, utilization of this promising host is still restricted by the limited availability of precise and effective gene integration tools. In this study, a novel and efficient genetic tool was developed for targeted, repeated, and markerless gene integration based on Cre/lox site-specific recombination system. The developed tool required only a single selection marker and could completely excise the unnecessary sequences. A total of three plasmids were created and seven rounds of marker-free gene integration were examined in Y. lipolytica. All the integration efficiencies remained above 90%, and analysis of the protein production and growth characteristics of the engineered strains confirmed that genome modification via the novel genetic tool was feasible. Further work also confirmed that the genetic tool was effective for the integration of other genes, loci, and strains. Thus, this study significantly promotes the application of the Cre/lox system and presents a powerful tool for genome engineering in Y. lipolytica.