Production of Carminic Acid by Metabolically Engineered Escherichia coli

Modular Cloning Tools for Streptomyces spp. and Application to the De Novo Biosynthesis of Flavokermesic Acid

The filamentous Streptomyces are among the most prolific producers of bioactive natural products and are thus attractive chassis for the heterologous expression of native and designed biosynthetic pathways. Although suitable Streptomyces hosts exist, including genetically engineered cluster-free mutants, the approach is currently limited by the relative paucity of synthetic biology tools facilitating the de novo assembly of multicomponent gene clusters. Here, we report a modular system (MoClo) for Streptomyces including a set of adapted vectors and genetic elements, which allow for the construction of complete genetic circuits. Critical functional validation of each of the elements was obtained using the previously reported β-glucuronidase (GusA) reporter system. Furthermore, we provide proof-of-principle for the toolbox inS. albus, demonstrating the efficient assembly of a biosynthetic pathway to flavokermesic acid (FK), an advanced precursor of the commercially valuable carminic acid.

An efficient C-glycoside production platform enabled by rationally tuning the chemoselectivity of glycosyltransferases

Despite the broad potential applications of C-glycosides, facile synthetic methods remain scarce. Transforming glycosyltransferases with promiscuous or natural O-specific chemoselectivity to C-glycosyltransferases is challenging. Here, we employ rational directed evolution of the glycosyltransferase MiCGT to generate MiCGT-QDP and MiCGT-ATD mutants which either enhance C-glycosylation or switch to O-glycosylation, respectively. Structural analysis and computational simulations reveal that substrate binding mode govern C-/O-glycosylation selectivity. Notably, directed evolution and mechanism analysis pinpoint the crucial residues dictating the binding mode, enabling the rational design of four enzymes with superior non-inherent chemoselectivity, despite limited sequence homology. Moreover, our best mutants undergo testing with 34 substrates, demonstrating superb chemoselectivities, regioselectivities, and activities. Remarkably, three C-glycosides and an O-glycoside are produced on a gram scale, demonstrating practical utility. This work establishes a highly selective platform for diverse glycosides, and offers a practical strategy for creating various types of glycosylation platforms to access pharmaceutically and medicinally interesting products.

Glycoside Phosphorylase Catalyzed Cellulose and β-1,3-Glucan Synthesis Using Chromophoric Glycosyl Acceptors

Glycoside phosphorylases are enzymes that are frequently used for polysaccharide synthesis. Some of these enzymes have broad substrate specificity, enabling the synthesis of reducing-end-functionalized glucan chains. Here, we explore the potential of glycoside phosphorylases in synthesizing chromophore-conjugated polysaccharides using commercially available chromophoric model compounds as glycosyl acceptors. Specifically, we report cellulose and β-1,3-glucan synthesis using 2-nitrophenyl β-d-glucopyranoside, 4-nitrophenyl β-d-glucopyranoside, and 2-methoxy-4-(2-nitrovinyl)phenyl β-d-glucopyranoside with Clostridium thermocellum cellodextrin phosphorylase and Thermosipho africanus β-1,3-glucan phosphorylase as catalysts. We demonstrate activity for both enzymes with all assayed chromophoric acceptors and report the crystallization-driven precipitation and detailed structural characterization of the synthesized polysaccharides, i.e., their molar mass distributions and various structural parameters, such as morphology, fibril diameter, lamellar thickness, and crystal form. Our results provide insights for the studies of chromophore-conjugated low molecular weight polysaccharides, glycoside phosphorylases, and the hierarchical assembly of crystalline cellulose and β-1,3-glucan.

Enzymatic glycosylation of aloesone performed by plant UDP-dependent glycosyltransferases

Aloesone is a bioactive natural product and biosynthetic precursor of rare glucosides found in rhubarb and some aloe plants including Aloe vera. This study aimed to investigate biocatalytic aloesone glycosylation and more than 400 uridine diphosphate-dependent glycosyltransferase (UGT) candidates, including multifunctional and promiscuous enzymes from a variety of plant species were assayed. As a result, 137 selective aloesone UGTs were discovered, including four from the natural producer rhubarb. Rhubarb UGT72B49 was further studied and its catalytic constants (kcat = 0.00092 ± 0.00003 s-1, KM = 30 ± 2.5 μM) as well as temperature and pH optima (50 °C and pH 7, respectively) were determined. We further aimed to find an efficient aloesone glycosylating enzyme with potential application for biocatalytic production of the glucoside. We discovered UGT71C1 from Arabidopsis thaliana as an efficient aloesone UGT showing a 167-fold higher catalytic efficiency compared to that of UGT72B49. Interestingly, sequence analysis of all the 137 newly identified aloesone UGTs showed that they belong to different phylogenetic groups, with the highest representation in groups B, D, E, F and L. Finally, our study indicates that aloesone C-glycosylation is highly specific and rare, since it was not possible to achieve in an efficient manner with any of the 422 UGTs assayed, including multifunctional GTs and 28 known C-UGTs.

The crane fly glycosylated triketide δ-lactone cornicinine elicits akinete differentiation of the cyanobiont in aquatic Azolla fern symbioses

The restriction of plant-symbiont dinitrogen fixation by an insect semiochemical had not been previously described. Here we report on a glycosylated triketide δ-lactone from Nephrotoma cornicina crane flies, cornicinine, that causes chlorosis in the floating-fern symbioses from the genus Azolla. Only the glycosylated trans-A form of chemically synthesized cornicinine was active: 500 nM cornicinine in the growth medium turned all cyanobacterial filaments from Nostoc azollae inside the host leaf-cavities into akinetes typically secreting CTB-bacteriocins. Cornicinine further inhibited akinete germination in Azolla sporelings, precluding re-establishment of the symbiosis during sexual reproduction. It did not impact development of the plant Arabidopsis thaliana or several free-living cyanobacteria from the genera Anabaena or Nostoc but affected the fern host without cyanobiont. Fern-host mRNA sequencing from isolated leaf cavities confirmed high NH4-assimilation and proanthocyanidin biosynthesis in this trichome-rich tissue. After cornicinine treatment, it revealed activation of Cullin-RING ubiquitin-ligase-pathways, known to mediate metabolite signaling and plant elicitation consistent with the chlorosis phenotype, and increased JA-oxidase, sulfate transport and exosome formation. The work begins to uncover molecular mechanisms of cyanobiont differentiation in a seed-free plant symbiosis important for wetland ecology or circular crop-production today, that once caused massive CO2 draw-down during the Eocene geological past.

Diverse Combinatorial Biosynthesis Strategies for C-H Functionalization of Anthracyclinones

Streptomyces spp. are "nature's antibiotic factories" that produce valuable bioactive metabolites, such as the cytotoxic anthracycline polyketides. While the anthracyclines have hundreds of natural and chemically synthesized analogues, much of the chemical diversity stems from enzymatic modifications to the saccharide chains and, to a lesser extent, from alterations to the core scaffold. Previous work has resulted in the generation of a BioBricks synthetic biology toolbox in Streptomyces coelicolor M1152ΔmatAB that could produce aklavinone, 9-epi-aklavinone, auramycinone, and nogalamycinone. In this work, we extended the platform to generate oxidatively modified analogues via two crucial strategies. (i) We swapped the ketoreductase and first-ring cyclase enzymes for the aromatase cyclase from the mithramycin biosynthetic pathway in our polyketide synthase (PKS) cassettes to generate 2-hydroxylated analogues. (ii) Next, we engineered several multioxygenase cassettes to catalyze 11-hydroxylation, 1-hydroxylation, 10-hydroxylation, 10-decarboxylation, and 4-hydroxyl regioisomerization. We also developed improved plasmid vectors and S. coelicolor M1152ΔmatAB expression hosts to produce anthracyclinones. This work sets the stage for the combinatorial biosynthesis of bespoke anthracyclines using recombinant Streptomyces spp. hosts.

New opportunities and perspectives on biosynthesis and bioactivities of secondary metabolites from Aloe vera

Historically, the genus Aloe has been an indispensable part of both traditional and modern medicine. Decades of intensive research have unveiled the major bioactive secondary metabolites of this plant. Recent pandemic outbreaks have revitalized curiosity in aloe metabolites, as they have proven pharmacokinetic profiles and repurposable chemical space. However, the structural complexity of these metabolites has hindered scientific advances in the chemical synthesis of these compounds. Multi-omics research interventions have transformed aloe research by providing insights into the biosynthesis of many of these compounds, for example, aloesone, aloenin, noreugenin, aloin, saponins, and carotenoids. Here, we summarize the biological activities of major aloe secondary metabolites with a focus on their mechanism of action. We also highlight the recent advances in decoding the aloe metabolite biosynthetic pathways and enzymatic machinery linked with these pathways. Proof-of-concept studies on in vitro, whole-cell, and microbial synthesis of aloe compounds have also been briefed. Research initiatives on the structural modification of various aloe metabolites to expand their chemical space and activity are detailed. Further, the technological limitations, patent status, and prospects of aloe secondary metabolites in biomedicine have been discussed.

Precision fermentation for improving the quality, flavor, safety, and sustainability of foods

Precision fermentation involves the rewiring of metabolic pathways in generally recognized as safe microorganisms, fermentation scale-up, and downstream processing to produce food ingredients from abundant and inexpensive substrates. Using precise genome editing of food-fermenting microorganisms, precision fermentation can also produce fermented foods with more desirable properties. These genetic tools allow for the manipulation of flavors and nutritional content in fermented foods, the economic production of functional food ingredients, and the sustainable production of otherwise-costly macronutrients. By introducing the metabolic designs, genetic modifications, and resulting products of engineered microorganisms developed through academic and industrial research, this review aims to provide insights into the potentials and challenges of precision fermentation for the economic, safe, and sustainable production of foods.

Extraction pathways and purification strategies towards carminic acid as natural-based food colorant: A comprehensive review

As a current trend of fabricating healthier products, food manufacturing companies seek for natural-based food colorant aiming to replace the synthetic ones, which apart from meeting sensorial and organoleptic aspects, they can also act as health promoters offering additional added value. Carminic acid is a natural based food colorant typically found in several insect taxa. However, there are current approaches which pursue the production of this natural pigment via biotechnological synthesis. To date, this colorant has been intensively applied in the manufacture of several food items. Unfortunately, one of the main limitations deals with the establishment of the right protocol of extraction and purification of this component since there is no report analyzing the main extraction techniques for obtaining carminic acid. Therefore, this review, for the first time, comprehensively analyzes the ongoing strategies and protocols proposed by scientists towards either extraction or purification of carminic acid from its origin source, and from biotechnological systems. Emphasis has been focused on the main findings dealing with extraction techniques and the relevant insights in the field. A detailed discussion is provided on the advantages and drawbacks of the reported extraction and purification methods, main solvents used and their key interactions with target molecules.

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.

Targeted and high-throughput gene knockdown in diverse bacteria using synthetic sRNAs

Synthetic sRNAs allow knockdown of target genes at translational level, but have been restricted to a limited number of bacteria. Here, we report the development of a broad-host-range synthetic sRNA (BHR-sRNA) platform employing the RoxS scaffold and the Hfq chaperone from Bacillus subtilis. BHR-sRNA is tested in 16 bacterial species including commensal, probiotic, pathogenic, and industrial bacteria, with >50% of target gene knockdown achieved in 12 bacterial species. For medical applications, virulence factors in Staphylococcus epidermidis and Klebsiella pneumoniae are knocked down to mitigate their virulence-associated phenotypes. For metabolic engineering applications, high performance Corynebacterium glutamicum strains capable of producing valerolactam (bulk chemical) and methyl anthranilate (fine chemical) are developed by combinatorial knockdown of target genes. A genome-scale sRNA library covering 2959 C. glutamicum genes is constructed for high-throughput colorimetric screening of indigoidine (natural colorant) overproducers. The BHR-sRNA platform will expedite engineering of diverse bacteria of both industrial and medical interest.

Metabolic engineering for sustainability and health

Bio-based production of chemicals and materials has attracted much attention due to the urgent need to establish sustainability and enhance human health. Metabolic engineering (ME) allows purposeful modification of cellular metabolic, regulatory, and signaling networks to achieve enhanced production of desired chemicals and degradation of environmentally harmful chemicals. ME has significantly progressed over the past 30 years through further integration of the strategies of synthetic biology, systems biology, evolutionary engineering, and data science aided by artificial intelligence. Here we review the field of ME from its emergence to the current state-of-the-art, highlighting its contribution to sustainable production of chemicals, health, and the environment through representative examples. Future challenges of ME and perspectives are also discussed.

Methanotrophs as a reservoir for bioactive secondary metabolites: Pitfalls, insights and promises

Methanotrophs are potent natural producers of several bioactive secondary metabolites (SMs) including isoprenoids, polymers, peptides, and vitamins. Cryptic biosynthetic gene clusters identified from these microbes via genome mining hinted at the vast and hidden SM biosynthetic potential of these microbes. Central carbon metabolism in methanotrophs offers rare pathway intermediate pools that could be further diversified using advanced synthetic biology tools to produce valuable SMs; for example, plant polyketides, rare carotenoids, and fatty acid-derived SMs. Recent advances in pathway reconstruction and production of isoprenoids, squalene, ectoine, polyhydroxyalkanoate copolymer, cadaverine, indigo, and shinorine serve as proof-of-concept. This review provides theoretical guidance for developing methanotrophs as microbial chassis for high-value SMs. We summarize the distinct secondary metabolic potentials of type I and type II methanotrophs, with specific attention to products relevant to biomedical applications. This review also includes native and non-native SMs from methanotrophs, their therapeutic potential, strategies to induce silent biosynthetic gene clusters, and challenges.

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.

Reshaping the 2-Pyrone Synthase Active Site for Chemoselective Biosynthesis of Polyketides

Engineering enzymes with novel reactivity and applying them in metabolic pathways to produce valuable products are quite challenging due to the intrinsic complexity of metabolic networks and the need for high in vivo catalytic efficiency. Triacetic acid lactone (TAL), naturally generated by 2-pyrone synthase (2PS), is a platform molecule that can be produced via microbial fermentation and further converted into value-added products. However, these conversions require extra synthetic steps under harsh conditions. We herein report a biocatalytic system for direct generation of TAL derivatives under mild conditions with controlled chemoselectivity by rationally engineering the 2PS active site and then rewiring the biocatalytic pathway in the metabolic network of E. coli to produce high-value products, such as kavalactone precursors, with yields up to 17 mg/L culture. Computer modeling indicates sterics and hydrogen-bond interactions play key roles in tuning the selectivity, efficiency and yield.

Microbial Metabolites Annotation by Mass Spectrometry-Based Metabolomics

Since the discovery of penicillin, microbial metabolites have been extensively investigated for drug discovery purposes. In the last decades, microbial derived compounds have gained increasing attention in different fields from pharmacognosy to industry and agriculture. Microbial metabolites in microbiomes present specific functions and can be associated with the maintenance of the natural ecosystems. These metabolites may exhibit a broad range of biological activities of great interest to human purposes. Samples from either microbial isolated cultures or microbiomes consist of complex mixtures of metabolites and their analysis are not a simple process. Mass spectrometry-based metabolomics encompass a set of analytical methods that have brought several improvements to the microbial natural products field. This analytical tool allows the comprehensively detection of metabolites, and therefore, the access of the chemical profile from those biological samples. These analyses generate thousands of mass spectra which is challenging to analyse. In this context, bioinformatic metabolomics tools have been successfully employed to accelerate and facilitate the investigation of specialized microbial metabolites. Herein, we describe metabolomics tools used to provide chemical information for the metabolites, and furthermore, we discuss how they can improve investigation of microbial cultures and interactions.

An interactive metabolic map of bio-based chemicals

Metabolic engineering for the bio-based production of chemicals requires thorough understanding of metabolic reactions including enzymes, cofactors, reactants, and products. Here we present an interactive bio-based chemicals map that visualizes compounds, enzymes, and reaction pathways together with strategies for the production of chemicals by biological, chemical, and combined methods.

A BioBricks Metabolic Engineering Platform for the Biosynthesis of Anthracyclinones in Streptomyces coelicolor

Actinomycetes produce a variety of clinically indispensable molecules, such as antineoplastic anthracyclines. However, the actinomycetes are hindered in their further development as genetically engineered hosts for the synthesis of new anthracycline analogues due to their slow growth kinetics associated with their mycelial life cycle and the lack of a comprehensive genetic toolbox for combinatorial biosynthesis. In this report, we tackled both issues via the development of the BIOPOLYMER (BIOBricks POLYketide Metabolic EngineeRing) toolbox: a comprehensive synthetic biology toolbox consisting of engineered strains, promoters, vectors, and biosynthetic genes for the synthesis of anthracyclinones. An improved derivative of the production host Streptomyces coelicolor M1152 was created by deleting the matAB gene cluster that specifies extracellular poly-β-1,6-N-acetylglucosamine (PNAG). This resulted in a loss of mycelial aggregation, with improved biomass accumulation and anthracyclinone production. We then leveraged BIOPOLYMER to engineer four distinct anthracyclinone pathways, identifying optimal combinations of promoters, genes, and vectors to produce aklavinone, 9-epi-aklavinone, auramycinone, and nogalamycinone at titers between 15-20 mg/L. Optimization of nogalamycinone production strains resulted in titers of 103 mg/L. We structurally characterized six anthracyclinone products from fermentations, including new compounds 9,10-seco-7-deoxy-nogalamycinone and 4-O-β-d-glucosyl-nogalamycinone. Lastly, we tested the antiproliferative activity of the anthracyclinones in a mammalian cancer cell viability assay, in which nogalamycinone, auramycinone, and aklavinone exhibited moderate cytotoxicity against several cancer cell lines. We envision that BIOPOLYMER will serve as a foundational platform technology for the synthesis of designer anthracycline analogues.

Phenolic C-glycoside synthesis using microbial systems

Plants produce different types of phenolic compounds. The majority of these compounds are glycosylated. Phenolic O-glycosides are also common. Recently, C-glycosylation of phenolic compounds has received attention because of the biological importance of phenolic C-glycosides. To date, three classes of C-glycosyltransferases (CGTs) have been characterized based on the type of sugar acceptor: flavonoid CGT, coumarin CGT, and xanthone CGT. Phylogenetic analysis of glycosyltransferases has revealed that CGTs form a distinct class that is clearly different from that of O-glycosyltransferases. The characterized CGTs have been introduced into microbial systems to synthesize phenolic C-glycosides. Here, we review recent progress in the development of CGTs and their application in the synthesis of phenolic C-glycosides using microbial systems.

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.

Three new polyketide derivatives of the endophytic fungus Aspergillus cristatus 2H1

Three new polyketide derivatives, 2-ethoxycarbonyl-endocrocin (1), 6-methoxy-2-ethoxycarbonyl-endocrocin (2) and pannorin C (3), along with sixteen known compounds (4-19) were isolated from a plant endophytic fungus Aspergillus cristatus 2H1. Their structures were elucidated by 1D/2D NMR and HR-ESI-MS data analysis. Compound 3 showed weak antibacterial activity against Staphylococcus aureus (MIC 20 μg/ml). Compounds 14 and 15 showed effective cytotoxicity on human melanoma A375 cells (IC₅₀ 4.13 μM for 14, 3.39 μM for 15).

Metabolic and cellular engineering for the production of natural products

Increased awareness of the environmental and health concerns of consuming chemically synthesized products has led to a rising demand for natural products that are greener and more sustainable. Despite their importance, however, industrial-scale production of natural products has been challenging due to the low yield and high cost of the bioprocesses. To cope with this problem, systems metabolic engineering has been employed to efficiently produce natural products from renewable biomass. Here, we review the recent systems metabolic engineering strategies employed for enhanced production of value-added natural products, together with accompanying examples. Particular focus is set on systems-level engineering and cell physiology engineering strategies. Future perspectives are also discussed.

Research progress of pathway and genome evolution in microbes

Microbes can produce valuable natural products widely applied in medicine, food and other important fields. Nevertheless, it is usually challenging to achieve ideal industrial yields due to low production rate and poor toxicity tolerance. Evolution is a constant mutation and adaptation process used to improve strain performance. Generally speaking, the synthesis of natural products in microbes is often intricate, involving multiple enzymes or multiple pathways. Individual evolution of a certain enzyme often fails to achieve the desired results, and may lead to new rate-limiting nodes that affect the growth of microbes. Therefore, it is inevitable to evolve the biosynthetic pathways or the whole genome. Here, we reviewed the pathway-level evolution including multi-enzyme evolution, regulatory elements engineering, and computer-aided engineering, as well as the genome-level evolution based on several tools, such as genome shuffling and CRISPR/Cas systems. Finally, we also discussed the major challenges faced by in vivo evolution strategies and proposed some potential solutions.

Production of phenylpropanoids and flavonolignans from glycerol by metabolically engineered Escherichia coli

Phenylpropanoids are a group of plant natural products with medicinal importance derived from aromatic amino acids. Here, we report the production of two representative phenylpropanoids-coniferyl alcohol (CA) and dihydroquercetin (DHQ)-from glycerol by engineered Escherichia coli. First, an E. coli strain capable of producing 187.7 mg/L of CA from glycerol was constructed by the introduction of hpaBC from E. coli and OMT1, 4CL4, and CCR1 from Arabidopsis thaliana to the p-coumaric acid producer. Next, an E. coli strain capable of producing 239.4 mg/L of DHQ from glycerol was constructed by the introduction of F3H, TT7, and CPR from A. thaliana to the naringenin producer, followed by engineering the signal peptide of a cytochrome P450 TT7. Furthermore, to demonstrate the production of flavonolignans, a group of heterodimeric phenylpropanoids, from glycerol, ascorbate peroxidase 1 from Silybum marianum was employed and engineered to produce 0.04 μg/L of silybin and 1.29 μg/L of isosilybin from glycerol by stepwise culture. Finally, a single strain harboring all the 16 necessary genes was constructed, resulting in 0.12 μg/L of isosilybin production directly from glycerol. The strategies described here will be useful for the production of pharmaceutically important yet complex natural products.

Next-Generation Genetic and Fermentation Technologies for Safe and Sustainable Production of Food Ingredients: Colors and Flavorings

A growing human population is a significant issue in food security owing to the limited land and resources available for agricultural food production. To solve these problems, sustainable food manufacturing processes and the development of alternative foods and ingredients are needed. Metabolic engineering and synthetic biology can help solve the food security issue and satisfy the demand for alternative food production. Bioproduction of food ingredients by microbial fermentation is a promising method to replace current manufacturing processes, such as extraction from natural materials and chemical synthesis, with more ecofriendly and sustainable operations. This review highlights successful examples of bioproduction for food additives by engineered microorganisms, with an emphasis on colorants and flavors that are extensively used in the food industry. Recent strain engineering developments and fermentation strategies for producing selected food colorants and flavors are introduced with discussions on the current status and future perspectives.

Expected final online publication date for the Annual Review of Food Science and Technology, Volume 13 is March 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Biosynthesis of a rosavin natural product in Escherichia coli by glycosyltransferase rational design and artificial pathway construction

Phytochemicals are rich resources for pharmaceutical and nutraceutical agents. A key challenge of accessing these precious compounds can present significant bottlenecks for development. The cinnamyl alcohol disaccharides also known as rosavins are the major bioactive ingredients of the notable medicinal plant Rhodiola rosea L. Cinnamyl-(6'-O-β-xylopyranosyl)-O-β-glucopyranoside (rosavin E) is a natural rosavin analogue with the arabinopyranose unit being replaced by its diastereomer xylose, which was only isolated in minute quantity from R. rosea. Herein, we described the de novo production of rosavin E in Escherichia coli. The 1,6-glucosyltransferase CaUGT3 was engineered into a xylosyltransferase converting cinnamyl alcohol monoglucoside (rosin) into rosavin E by replacing the residue T145 with valine. The enzyme activity was further elevated 2.9 times by adding the mutation N375Q. The synthesis of rosavin E from glucose was achieved with a titer of 92.9 mg/L by combining the variant CaUGT3^T145V/N375Q, the UDP-xylose synthase from Sinorhizobium meliloti 1021 (SmUXS) and enzymes for rosin biosynthesis into a phenylalanine overproducing E. coli strain. The production of rosavin E was further elevated by co-overexpressing UDP-xylose synthase from Arabidopsis thaliana (AtUXS3) and SmUXS, and the titer in a 5 L bioreactor with fed-batch fermentation reached 782.0 mg/L. This work represents an excellent example of producing a natural product with a disaccharide chain by glycosyltransferase engineering and artificial pathway construction.

Hot off the Press

A personal selection of 32 recent papers is presented covering various aspects of current developments in bioorganic chemistry and novel natural products such as neopetrothiazide from a Neopetrosia species.