Computational redesign of taxane-10β-hydroxylase for de novo biosynthesis of a key paclitaxel intermediate

Paclitaxel (Taxol®) is the most popular anticancer diterpenoid predominantly present in Taxus. The core skeleton of paclitaxel is highly modified, but researches on the cytochrome P450s involved in post-modification process remain exceedingly limited. Herein, the taxane-10β-hydroxylase (T10βH) from Taxus cuspidata, which is the third post-modification enzyme that catalyzes the conversion of taxadiene-5α-yl-acetate (T5OAc) to taxadiene-5α-yl-acetoxy-10β-ol (T10OH), was investigated in Escherichia coli by combining computation-assisted protein engineering and metabolic engineering. The variant of T10βH, M3 (I75F/L226K/S345V), exhibited a remarkable 9.5-fold increase in protein expression, accompanied by respective 1.3-fold and 2.1-fold improvements in turnover frequency (TOF) and total turnover number (TTN). Upon integration into the engineered strain, the variant M3 resulted in a substantial enhancement in T10OH production from 0.97 to 2.23 mg/L. Ultimately, the titer of T10OH reached 3.89 mg/L by fed-batch culture in a 5-L bioreactor, representing the highest level reported so far for the microbial de novo synthesis of this key paclitaxel intermediate. This study can serve as a valuable reference for further investigation of other P450s associated with the artificial biosynthesis of paclitaxel and other terpenoids. KEY POINTS: • The T10βH from T. cuspidata was expressed and engineered in E. coli unprecedentedly. • The expression and activity of T10βH were improved through protein engineering. • De novo biosynthesis of T10OH was achieved in E. coli with a titer of 3.89 mg/L.

Optimization of IspSib stability through directed evolution to improve isoprene production

Enzyme stability is often a limiting factor in the microbial production of high-value-added chemicals and commercial enzymes. A previous study by our research group revealed that the unstable isoprene synthase from Ipomoea batatas (IspSib) critically limits isoprene production in engineered Escherichia coli. Directed evolution was, therefore, performed in the present study to improve the thermostability of IspSib. First, a tripartite protein folding system designated as lac'-IspSib-'lac, which could couple the stability of IspSib to antibiotic ampicillin resistance, was successfully constructed for the high-throughput screening of variants. Directed evolution of IspSib was then performed through two rounds of random mutation and site-saturation mutation, which produced three variants with higher stability: IspSibN397V A476V, IspSibN397V A476T, and IspSibN397V A476C. The subsequent in vitro thermostability test confirmed the increased protein stability. The melting temperatures of the screened variants IspSibN397V A476V, IspSibN397V A476T, and IspSibN397V A476C were 45.1 ± 0.9°C, 46.1 ± 0.7°C, and 47.2 ± 0.3°C, respectively, each of which was higher than the melting temperature of wild-type IspSib (41.5 ± 0.4°C). The production of isoprene at the shake-flask fermentation level was increased by 1.94-folds, to 1,335 mg/L, when using IspSibN397V A476T. These findings provide insights into the optimization of the thermostability of terpene synthases, which are key enzymes for isoprenoid production in engineered microorganisms. In addition, the present study would serve as a successful example of improving enzyme stability without requiring detailed structural information or catalytic reaction mechanisms.IMPORTANCEThe poor thermostability of IspSib critically limits isoprene production in engineered Escherichia coli. A tripartite protein folding system designated as lac'-IspSib-'lac, which could couple the stability of IspSib to antibiotic ampicillin resistance, was successfully constructed for the first time. In order to improve the enzyme stability of IspSib, the directed evolution of IspSib was performed through error-PCR, and high-throughput screening was realized using the lac'-IspSib-'lac system. Three positive variants with increased thermostability were obtained. The thermostability test and the melting temperature analysis confirmed the increased stability of the enzyme. The production of isoprene was increased by 1.94-folds, to 1,335 mg/L, using IspSibN397V A476T. The directed evolution process reported here is also applicable to other terpene synthases key to isoprenoid production.

Recent Advances on Key Enzymes of Microbial Origin in the Lycopene Biosynthesis Pathway

Lycopene is a common carotenoid found mainly in ripe red fruits and vegetables that is widely used in the food industry due to its characteristic color and health benefits. Microbial synthesis of lycopene is gradually replacing the traditional methods of plant extraction and chemical synthesis as a more economical and productive manufacturing strategy. The biosynthesis of lycopene is a typical multienzyme cascade reaction, and it is important to understand the characteristics of each key enzyme involved and how they are regulated. In this paper, the catalytic characteristics of the key enzymes involved in the lycopene biosynthesis pathway and related studies are first discussed in detail. Then, the strategies applied to the key enzymes of lycopene synthesis, including fusion proteins, enzyme screening, combinatorial engineering, CRISPR/Cas9-based gene editing, DNA assembly, and scaffolding technologies are purposefully illustrated and compared in terms of both traditional and emerging multienzyme regulatory strategies. Finally, future developments and regulatory options for multienzyme synthesis of lycopene and similar secondary metabolites are also discussed.

Microbial itaconic acid bioproduction towards sustainable development: Insights, challenges, and prospects

Microbial biomanufacturing is a promising approach to produce high-value compounds with low-carbon footprint and significant economic benefits. Among twelve "Top Value-Added Chemicals from Biomass", itaconic acid (IA) stands out as a versatile platform chemical with numerous applications. IA is naturally produced by Aspergillus and Ustilago species through a cascade enzymatic reaction between aconitase (EC 4.2.1.3) and cis-aconitic acid decarboxylase (EC 4.1.1.6). Recently, non-native hosts such as Escherichia coli, Corynebacterium glutamicum, Saccharomyces cerevisiae, and Yarrowia lipolytica have been genetically engineered to produce IA through the introduction of key enzymes. This review provides an up-to-date summary of the progress made in IA bioproduction, from native to engineered hosts, covers in vivo and in vitro approaches, and highlights the prospects of combination tactics. Current challenges and recent endeavors are also addressed to envision comprehensive strategies for renewable IA production in the future towards sustainable development goals (SDGs).

Engineering plant family TPS into cyanobacterial host for terpenoids production

Terpenoids are synthesized naturally by plants as secondary metabolites, and are diverse and complex in structure with multiple applications in bioenergy, food, cosmetics, and medicine. This makes the production of terpenoids such as isoprene, β-phellandrene, farnesene, amorphadiene, and squalene valuable, owing to which their industrial demand cannot be fulfilled exclusively by plant sources. They are synthesized via the Methylerythritol phosphate pathway (MEP) and the Mevalonate pathway (MVA), both existing in plants. The advent of genetic engineering and the latest accomplishments in synthetic biology and metabolic engineering allow microbial synthesis of terpenoids. Cyanobacteria manifest to be the promising hosts for this, utilizing sunlight and CO₂. Cyanobacteria possess MEP pathway to generate precursors for terpenoid synthesis. The terpenoid synthesis can be amplified by overexpressing the MEP pathway and engineering MVA pathway genes. According to the desired terpenoid, terpene synthases unique to the plant kingdom must be incorporated in cyanobacteria. Engineering an organism to be used as a cell factory comes with drawbacks such as hampered cell growth and disturbance in metabolic flux. This review set forth a comparison between MEP and MVA pathways, strategies to overexpress these pathways with their challenges.

Advances in engineering the production of the natural red pigment lycopene: A systematic review from a biotechnology perspective

BACKGROUND

Lycopene is a natural red compound with potent antioxidant activity that can be utilized both as pigment and as a raw material in functional food, and so possesses good commercial prospects. The biosynthetic pathway has already been documented, which provides the foundation for lycopene production using biotechnology.

AIM OF REVIEW

Although lycopene production has begun to take shape, there is still an urgent need to alleviate the yield of lycopene. Progress in this area can provide useful reference for metabolic engineering of lycopene production utilizing multiple approaches. Key scientific concepts of review Using conventional microbial fermentation approaches, biotechnologists have enhanced the yield of lycopene by selecting suitable host strains, utilizing various additives, and optimizing culture conditions. With the development of modern biotechnology, genetic engineering, protein engineering, and metabolic engineering have been applied for lycopene production. Extraction from natural plants is the main way for lycopene production at present. Based on the molecular mechanism of lycopene accumulation, the production of lycopene by plant bioreactor through genetic engineering has a good prospect. Here we summarized common strategies for optimizing lycopene production engineering from a biotechnology perspective, which are mainly carried out by microbial cultivation. We reviewed the challenges and limitations of this approach, summarized the critical aspects, and provided suggestions with the aim of potential future breakthroughs for lycopene production in plants.

Optimizing the downstream MVA pathway using a combination optimization strategy to increase lycopene yield in Escherichia coli

Background

Lycopene is increasing in demand due to its widespread use in the pharmaceutical and food industries. Metabolic engineering and synthetic biology technologies have been widely used to overexpress the heterologous mevalonate pathway and lycopene pathway in Escherichia coli to produce lycopene. However, due to the tedious metabolic pathways and complicated metabolic background, optimizing the lycopene synthetic pathway using reasonable design approaches becomes difficult.

Results

In this study, the heterologous lycopene metabolic pathway was introduced into E. coli and divided into three modules, with mevalonate and DMAPP serving as connecting nodes. The module containing the genes (MVK, PMK, MVD, IDI) of downstream MVA pathway was adjusted by altering the expression strength of the four genes using the ribosome binding sites (RBSs) library with specified strength to improve the inter-module balance. Three RBS libraries containing variably regulated MVK, PMK, MVD, and IDI were constructed based on different plasmid backbones with the variable promoter and replication origin. The RBS library was then transformed into engineered E. coli BL21(DE3) containing pCLES and pTrc-lyc to obtain a lycopene producer library and employed high-throughput screening based on lycopene color to obtain the required metabolic pathway. The shake flask culture of the selected high-yield strain resulted in a lycopene yield of 219.7 mg/g DCW, which was 4.6 times that of the reference strain.

Conclusion

A strain capable of producing 219.7 mg/g DCW with high lycopene metabolic flux was obtained by fine-tuning the expression of the four MVA pathway enzymes and visual selection. These results show that the strategy of optimizing the downstream MVA pathway through RBS library design can be effective, which can improve the metabolic flux and provide a reference for the synthesis of other terpenoids.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12934-022-01843-z.

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.

Biotechnological advances for improving natural pigment production: a state-of-the-art review

In current years, natural pigments are facing a fast-growing global market due to the increase of people's awareness of health and the discovery of novel pharmacological effects of various natural pigments, e.g., carotenoids, flavonoids, and curcuminoids. However, the traditional production approaches are source-dependent and generally subject to the low contents of target pigment compounds. In order to scale-up industrial production, many efforts have been devoted to increasing pigment production from natural producers, via development of both in vitro plant cell/tissue culture systems, as well as optimization of microbial cultivation approaches. Moreover, synthetic biology has opened the door for heterologous biosynthesis of pigments via design and re-construction of novel biological modules as well as biological systems in bio-platforms. In this review, the innovative methods and strategies for optimization and engineering of both native and heterologous producers of natural pigments are comprehensively summarized. Current progress in the production of several representative high-value natural pigments is also presented; and the remaining challenges and future perspectives are discussed.

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

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

Toward improved terpenoids biosynthesis: strategies to enhance the capabilities of cell factories

Terpenoids form the most diversified class of natural products, which have gained application in the pharmaceutical, food, transportation, and fine and bulk chemical industries. Extraction from naturally occurring sources does not meet industrial demands, whereas chemical synthesis is often associated with poor enantio-selectivity, harsh working conditions, and environmental pollutions. Microbial cell factories come as a suitable replacement. However, designing efficient microbial platforms for isoprenoid synthesis is often a challenging task. This has to do with the cytotoxic effects of pathway intermediates and some end products, instability of expressed pathways, as well as high enzyme promiscuity. Also, the low enzymatic activity of some terpene synthases and prenyltransferases, and the lack of an efficient throughput system to screen improved high-performing strains are bottlenecks in strain development. Metabolic engineering and synthetic biology seek to overcome these issues through the provision of effective synthetic tools. This review sought to provide an in-depth description of novel strategies for improving cell factory performance. We focused on improving transcriptional and translational efficiencies through static and dynamic regulatory elements, enzyme engineering and high-throughput screening strategies, cellular function enhancement through chromosomal integration, metabolite tolerance, and modularization of pathways.

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.

Combinatorial transient gene expression strategies to enhance terpenoid production in plants

INTRODUCTION

The monoterpenoid linalool and sesquiterpenoid costunolide are ubiquitous plant components that have been economically exploited for their respective essential oils and pharmaceutical benefits. In general, monoterpenes and sesquiterpenes are produced by the plastid 2-C-methyl-D-erythritol 4-phosphate (MEP) and cytosolic mevalonate (MVA) pathways, respectively. Herein, we investigated the individual and combinatorial potential of MEP and MVA pathway genes in increasing linalool and costunolide production in Nicotiana benthamiana.

METHODS

First, six genes from the MEP (1-deoxy-D-xylulose-5-phosphate synthase, 1-deoxy-D-xylulose 5-phosphate reductoisomerase, 4-diphosphocytidyl-2-C-methyl-D-erythritol kinase, geranyl pyrophosphate synthase, and linalool synthase) and MVA (acetoacetyl-CoA-thiolase, hydroxy-3-methylglutaryl-CoA reductase, farnesyl pyrophosphate synthase, germacrene A synthase, germacrene A oxidase, and costunolide synthase) pathways were separately cloned into the modular cloning (MoClo) golden gateway cassette. Second, the cassettes were transformed individually or in combination into the leaves of N. benthamiana by agroinfiltration.

RESULTS AND DISCUSSION

Five days post infiltration (DPI), all selected genes were transiently 5- to 94-fold overexpressed. Quantification using gas chromatography-Q-orbitrap-mass spectrometry (GC-Q-Orbitrap-MS) determined that the individual and combinatorial expression of MEP genes increased linalool production up to 50-90ng.mg^-1 fresh leaf weight. Likewise, MVA genes increased costunolide production up to 70-90ng.mg^-1 fresh leaf weight. Our findings highlight that the transient expression of MEP and MVA pathway genes (individually or in combination) enhances linalool and costunolide production in plants.

Unraveling the roles of plant specialized metabolites: using synthetic biology to design molecular biosensors

Plants are a rich source of specialized metabolites with a broad range of bioactivities and many applications in human daily life. Over the past decades significant progress has been made in identifying many such metabolites in different plant species and in elucidating their biosynthetic pathways. However, the biological roles of plant specialized metabolites remain elusive and proposed functions lack an identified underlying molecular mechanism. Understanding the roles of specialized metabolites frequently is hampered by their dynamic production and their specific spatiotemporal accumulation within plant tissues and organs throughout a plant's life cycle. In this review, we propose the employment of strategies from the field of Synthetic Biology to construct and optimize genetically encoded biosensors that can detect individual specialized metabolites in a standardized and high-throughput manner. This will help determine the precise localization of specialized metabolites at the tissue and single-cell levels. Such information will be useful in developing complete system-level models of specialized plant metabolism, which ultimately will demonstrate how the biosynthesis of specialized metabolites is integrated with the core processes of plant growth and development.

Metabolic engineering and synthetic biology for isoprenoid production in Escherichia coli and Saccharomyces cerevisiae

Isoprenoids, often called terpenoids, are the most abundant and highly diverse family of natural organic compounds. In plants, they play a distinct role in the form of photosynthetic pigments, hormones, electron carrier, structural components of membrane, and defence. Many isoprenoids have useful applications in the pharmaceutical, nutraceutical, and chemical industries. They are synthesized by various isoprenoid synthase enzymes by several consecutive steps. Recent advancement in metabolic engineering and synthetic biology has enabled the production of these isoprenoids in the heterologous host systems like Escherichia coli and Saccharomyces cerevisiae. Both heterologous systems have been engineered for large-scale production of value-added isoprenoids. This review article will provide the detailed description of various approaches used for engineering of methyl-D-erythritol-4-phosphate (MEP) and mevalonate (MVA) pathway for synthesizing isoprene units (C₅) and ultimate production of diverse isoprenoids. The review particularly highlighted the efforts taken for the production of C₅-C₂₀ isoprenoids by metabolic engineering techniques in E. coli and S. cerevisiae over a decade. The challenges and strategies are also discussed in detail for scale-up and engineering of isoprenoids in the heterologous host systems.Key points• Isoprenoids are beneficial and valuable natural products.• E. coli and S. cerevisiae are the promising host for isoprenoid biosynthesis.• Emerging techniques in synthetic biology enabled the improved production.• Need to expand the catalogue and scale-up of un-engineered isoprenoids. Metabolic engineering and synthetic biology for isoprenoid production in Escherichia coli and Saccharomyces cerevisiae.

Metabolic Engineering of Different Microbial Hosts for Lycopene Production

As a result of the extensive use of lycopene in a variety of fields, especially the dietary supplement and health food industries, the production of lycopene has attracted considerable interest. Lycopene can be obtained through extraction from vegetables and chemical synthesis. Alternatively, the microbial production of lycopene has been extensively researched in recent years. Various types of microbial hosts have been evaluated for their potential to accumulate a high level of lycopene. Metabolic engineering of the hosts and optimization of culture conditions are performed to enhance lycopene production. After years of research, great progress has been made in lycopene production. In this review, strategies used to improve lycopene production in different microbial hosts and the advantages and disadvantages of each microbial host are summarized. In addition, future perspectives of lycopene production in different microbial hosts are discussed.

Corynebacterium glutamicum CrtR and Its Orthologs in Actinobacteria: Conserved Function and Application as Genetically Encoded Biosensor for Detection of Geranylgeranyl Pyrophosphate

Carotenoid biosynthesis in Corynebacteriumglutamicum is controlled by the MarR-type regulator CrtR, which represses transcription of the promoter of the crt operon (PcrtE) and of its own gene (PcrtR). Geranylgeranyl pyrophosphate (GGPP), and to a lesser extent other isoprenoid pyrophosphates, interfere with the binding of CrtR to its target DNA in vitro, suggesting they act as inducers of carotenoid biosynthesis. CrtR homologs are encoded in the genomes of many other actinobacteria. In order to determine if and to what extent the function of CrtR, as a metabolite-dependent transcriptional repressor of carotenoid biosynthesis genes responding to GGPP, is conserved among actinobacteria, five CrtR orthologs were characterized in more detail. EMSA assays showed that the CrtR orthologs from Corynebacteriumcallunae, Acidipropionibacteriumjensenii, Paenarthrobacternicotinovorans, Micrococcusluteus and Pseudarthrobacterchlorophenolicus bound to the intergenic region between their own gene and the divergently oriented gene, and that GGPP inhibited these interactions. In turn, the CrtR protein from C. glutamicum bound to DNA regions upstream of the orthologous crtR genes that contained a 15 bp DNA sequence motif conserved between the tested bacteria. Moreover, the CrtR orthologs functioned in C. glutamicum in vivo at least partially, as they complemented the defects in the pigmentation and expression of a PcrtE_gfpuv transcriptional fusion that were observed in a crtR deletion mutant to varying degrees. Subsequently, the utility of the PcrtE_gfpuv transcriptional fusion and chromosomally encoded CrtR from C. glutamicum as genetically encoded biosensor for GGPP was studied. Combined FACS and LC-MS analysis demonstrated a correlation between the sensor fluorescent signal and the intracellular GGPP concentration, and allowed us to monitor intracellular GGPP concentrations during growth and differentiate between strains engineered to accumulate GGPP at different concentrations.

Metabolic Engineering Escherichia coli for the Production of Lycopene

Lycopene, a potent antioxidant, has been widely used in the fields of pharmaceuticals, nutraceuticals, and cosmetics. However, the production of lycopene extracted from natural sources is far from meeting the demand. Consequently, synthetic biology and metabolic engineering have been employed to develop microbial cell factories for lycopene production. Due to the advantages of rapid growth, complete genetic background, and a reliable genetic operation technique, Escherichia coli has become the preferred host cell for microbial biochemicals production. In this review, the recent advances in biological lycopene production using engineered E. coli strains are summarized: First, modification of the endogenous MEP pathway and introduction of the heterogeneous MVA pathway for lycopene production are outlined. Second, the common challenges and strategies for lycopene biosynthesis are also presented, such as the optimization of other metabolic pathways, modulation of regulatory networks, and optimization of auxiliary carbon sources and the fermentation process. Finally, the future prospects for the improvement of lycopene biosynthesis are also discussed.

Modular engineering for microbial production of carotenoids

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

Developing a highly efficient hydroxytyrosol whole-cell catalyst by de-bottlenecking rate-limiting steps

Hydroxytyrosol is an antioxidant free radical scavenger that is biosynthesized from tyrosine. In metabolic engineering efforts, the use of the mouse tyrosine hydroxylase limits its production. Here, we design an efficient whole-cell catalyst of hydroxytyrosol in Escherichia coli by de-bottlenecking two rate-limiting enzymatic steps. First, we replace the mouse tyrosine hydroxylase by an engineered two-component flavin-dependent monooxygenase HpaBC of E. coli through structure-guided modeling and directed evolution. Next, we elucidate the structure of the Corynebacterium glutamicum VanR regulatory protein complexed with its inducer vanillic acid. By switching its induction specificity from vanillic acid to hydroxytyrosol, VanR is engineered into a hydroxytyrosol biosensor. Then, with this biosensor, we use in vivo-directed evolution to optimize the activity of tyramine oxidase (TYO), the second rate-limiting enzyme in hydroxytyrosol biosynthesis. The final strain reaches a 95% conversion rate of tyrosine. This study demonstrates the effectiveness of sequentially de-bottlenecking rate-limiting steps for whole-cell catalyst development.

Whole-cell catalyst-based hydroxytyrosol production is low. Here, the authors increase the efficiency of its production in E. coli by de-bottlenecking two enzymatic steps catalyzed by monooxygenase and tyramine oxidase using structure-based enzyme redesign or in vivo-directed evolution with the aid of a newly developed biosensor.

Enhanced Production of Pinene by Using a Cell-Free System with Modular Cocatalysis

α-Pinene is an important monoterpene that is widely used as a pharmaceutical product, biofuel, and so forth. We first established a cell-free system with modular cocatalysis for the production of pinene from glucose. After optimization of the compositions of the cell-free reaction mixture using the Plackett-Burman experimental design and the path of steepest ascent, the production of pinene increased by 57%. It was found that ammonium acetate, NAD⁺, and NADPH are the three most important parameters for the production of pinene. Mix-and-match experiments showed that the simultaneous addition of the lysate of Escherichia coli overexpressing native 4-hydroxy-3-methylbut-2-enyl diphosphate reductase, SufBCD Fe-S cluster assembly protein, isopentenyl-diphosphate isomerase, and Pinus taeda pinene synthase improved the production of pinene. Increasing the enzyme concentration of the extract further enhanced the production of pinene to 1256.31 ± 46.12 mg/L with a productivity of 104.7 mg/L h, almost 1.2-fold faster than any system reported thus far. This study demonstrates that a cell-free system is a powerful and robust platform for biomanufacture.

Dynamic and tunable metabolite control for robust minimal-equipment assessment of serum zinc

Bacterial biosensors can enable programmable, selective chemical production, but difficulties incorporating metabolic pathways into complex sensor circuits have limited their development and applications. Here we overcome these challenges and present the development of fast-responding, tunable sensor cells that produce different pigmented metabolites based on extracellular concentrations of zinc (a critical micronutrient). We create a library of dual-input synthetic promoters that decouple cell growth from zinc-specific metabolite production, enabling visible cell coloration within 4 h. Using additional transcriptional and metabolic control methods, we shift the response thresholds by an order of magnitude to measure clinically relevant zinc concentrations. The resulting sensor cells report zinc concentrations in individual donor serum samples; we demonstrate that they can provide results in a minimal-equipment fashion, serving as the basis for a field-deployable assay for zinc deficiency. The presented advances are likely generalizable to the creation of other types of sensors and diagnostics.

Tightly controlling cell output is challenging, which has limited development and applications of bacterial sensors. Here the authors develop tunable, fast-responding sensors to control production of metabolic pigments and use them to assess zinc deficiency in a low-cost, minimal equipment fashion.

Emerging molecular biology tools and strategies for engineering natural product biosynthesis

Natural products and their related derivatives play a significant role in drug discovery and have been the inspiration for the design of numerous synthetic bioactive compounds. With recent advances in molecular biology, numerous engineering tools and strategies were established to accelerate natural product synthesis in both academic and industrial settings. However, many obstacles in natural product biosynthesis still exist. For example, the native pathways are not appropriate for research or production; the key enzymes do not have enough activity; the native hosts are not suitable for high-level production. Emerging molecular biology tools and strategies have been developed to not only improve natural product titers but also generate novel bioactive compounds. In this review, we will discuss these emerging molecular biology tools and strategies at three main levels: enzyme level, pathway level, and genome level, and highlight their applications in natural product discovery and development.

Genomic and transcriptional changes in response to pinene tolerance and overproduction in evolved Escherichia coli

α-Pinene is an important monoterpene, which is widely used as a flavoring agent and in fragrances, pharmaceuticals and biofuels. Although an evolved strain Escherichia coli YZFP, which had higher tolerance to pinene and titer, has been successfully used to produce high levels of pinene, the pinene titer is much lower than that of hemiterpene (isoprene) and sesquiterpenes (farnesene) to date. Moreover, the overall cellular physiological and metabolic changes caused by higher tolerance to pinene and overproduction of pinene remains unclear. To reveal the mechanism of Escherichia coli YZFP with the higher tolerance to pinene and titer, a comparative genomics and transcriptional level analyses combining with CRISPR activation (CRISPRa) and interference (CRISPRi) were carried out. The results show that the tolerance to pinene and the overproduction of pinene in E. coli may be associated with: 1) the mutations of the DXP pathway genes, the rpoA and some membrane protein genes, and their upregulations of transcription levels; and 2) the mutations of some genes and their downregulation of transcriptional levels. These comparative omics analyses provided some genetic modification strategies to further improve pinene production. Overexpression of the mutated cbpA, tabA, pitA, rpoA, sufBCDS, mutS, ispH, oppF, dusB, dnaK, dxs, dxr and flgFGH genes further improved pinene production. This study also demonstrated that combining comparative omics analysis with CRISPRa and CRISPRi is an efficient technology to quickly find a new metabolic engineering strategy.

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A genomics and transcriptional level analyses combining with CRISPRa and CRISPRi was carried out.

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The mechanism of the tolerance to pinene and overproduction of pinene was obtained.

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Some target genes difficultly found by rational analysis were identified.

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Combining comparative omics analysis with CRISPRa/i is an efficient technology for metabolic engineering.

Directed Coevolution of β-Carotene Ketolase and Hydroxylase and Its Application in Temperature-Regulated Biosynthesis of Astaxanthin

Because it is an outstanding antioxidant with wide applications, biotechnological production of astaxanthin has attracted increasing research interest. However, the astaxanthin titer achieved to date is still rather low, attributed to the poor efficiency of β-carotene ketolation and hydroxylation, as well as the adverse effect of astaxanthin accumulation on cell growth. To address these problems, we constructed an efficient astaxanthin-producing Saccharomyces cerevisiae strain by combining protein engineering and dynamic metabolic regulation. First, superior mutants of β-carotene ketolase and β-carotene hydroxylase were obtained by directed coevolution to accelerate the conversion of β-carotene to astaxanthin. Subsequently, the Gal4M9-based temperature-responsive regulation system was introduced to separate astaxanthin production from cell growth. Finally, 235 mg/L of (3 S,3' S)-astaxanthin was produced by two-stage, high-density fermentation. This study demonstrates the power of combining directed coevolution and temperature-responsive regulation in astaxanthin biosynthesis and may provide methodological reference for biotechnological production of other value-added chemicals.

Synthesizing ginsenoside Rh2 in Saccharomyces cerevisiae cell factory at high-efficiency

Synthetic biology approach has been frequently applied to produce plant rare bioactive compounds in microbial cell factories by fermentation. However, to reach an ideal manufactural efficiency, it is necessary to optimize the microbial cell factories systemically by boosting sufficient carbon flux to the precursor synthesis and tuning the expression level and efficiency of key bioparts related to the synthetic pathway. We previously developed a yeast cell factory to produce ginsenoside Rh2 from glucose. However, the ginsenoside Rh2 yield was too low for commercialization due to the low supply of the ginsenoside aglycone protopanaxadiol (PPD) and poor performance of the key UDP-glycosyltransferase (UGT) (biopart UGTPg45) in the final step of the biosynthetic pathway. In the present study, we constructed a PPD-producing chassis via modular engineering of the mevalonic acid pathway and optimization of P450 expression levels. The new yeast chassis could produce 529.0 mg/L of PPD in shake flasks and 11.02 g/L in 10 L fed-batch fermentation. Based on this high PPD-producing chassis, we established a series of cell factories to produce ginsenoside Rh2, which we optimized by improving the C3–OH glycosylation efficiency. We increased the copy number of UGTPg45, and engineered its promoter to increase expression levels. In addition, we screened for more efficient and compatible UGT bioparts from other plant species and mutants originating from the direct evolution of UGTPg45. Combining all engineered strategies, we built a yeast cell factory with the greatest ginsenoside Rh2 production reported to date, 179.3 mg/L in shake flasks and 2.25 g/L in 10 L fed-batch fermentation. The results set up a successful example for improving yeast cell factories to produce plant rare natural products, especially the glycosylated ones.

Improvement of isoprene production in Escherichia coli by rational optimization of RBSs and key enzymes screening

Background

As an essential platform chemical mostly used for rubber synthesis, isoprene is produced in industry through chemical methods, derived from petroleum. As an alternative, bio-production of isoprene has attracted much attention in recent years. Previous researches were mostly focused on key enzymes to improve isoprene production. In this research, besides screening of key enzymes, we also paid attention to expression intensity of non-key enzymes.

Results

Firstly, screening of key enzymes, IDI, MK and IspS, from other organisms and then RBS optimization of the key enzymes were carried out. The strain utilized IDI_sa was firstly detected to produce more isoprene than other IDIs. IDI_sa expression was improved after RBS modification, leading to 1610-fold increase of isoprene production. Secondly, RBS sequence optimization was performed to reduce translation initiation rate value of non-key enzymes, ERG19 and MvaE. Decreased ERG19 and MvaE expression and increased isoprene production were detected. The final strain showed 2.6-fold increase in isoprene production relative to the original strain. Furthermore, for the first time, increased key enzyme expression and decreased non-key enzyme expression after RBS sequence optimization were obviously detected through SDS-PAGE analysis.

Conclusions

This study prove that desired enzyme expression and increased isoprene production were obtained after RBS sequence optimization. RBS optimization of genes could be a powerful strategy for metabolic engineering of strain. Moreover, to increase the production of engineered strain, attention should not only be focused on the key enzymes, but also on the non-key enzymes.

Electronic supplementary material

The online version of this article (10.1186/s12934-018-1051-3) contains supplementary material, which is available to authorized users.

Systematic overexpression study to find target enzymes enhancing production of terpenes in Synechocystis PCC 6803, using isoprene as a model compound

Of the two natural metabolic pathways for making terpenoids, biotechnological utilization of the mevalonate (MVA) pathway has enabled commercial production of valuable compounds, while the more recently discovered but stoichiometrically more efficient methylerythritol phosphate (MEP) pathway is underdeveloped. We conducted a study on the overexpression of each enzyme in the MEP pathway in the unicellular cyanobacterium Synechocystis sp. PCC 6803, to identify potential targets for increasing flux towards terpenoid production, using isoprene as a reporter molecule. Results showed that the enzymes Ipi, Dxs and IspD had the biggest impact on isoprene production. By combining and creating operons out of those genes, isoprene production was increased 2-fold compared to the base strain. A genome-scale model was used to identify targets upstream of the MEP pathway that could redirect flux towards terpenoids. A total of ten reactions from the Calvin-Benson-Bassham cycle, lower glycolysis and co-factor synthesis pathways were probed for their effect on isoprene synthesis by co-expressing them with the MEP enzymes, resulting in a 60% increase in production from the best strain. Lastly, we studied two isoprene synthases with the highest reported catalytic rates. Only by expressing them together with Dxs and Ipi could we get stable strains that produced 2.8 mg/g isoprene per dry cell weight, a 40-fold improvement compared to the initial strain.

Metabolic engineering for the production of isoprene and isopentenol by Escherichia coli

The biotechnological production of isoprene and isopentenol has recently been studied. Isoprene, which is currently made mainly from petroleum, is an important platform chemical for synthesizing pesticides, medicines, oil additives, fragrances, and more and is especially important in the rubber production industry. Isopentenols, which have better combustion properties than well-known biofuels (ethanol), have recently received more attention. Supplies of petroleum, the conventional source of isoprene and isopentenols, are unsustainable, and chemical synthesis processes could cause serious environmental problems. As an alternative, the biosynthesis of isoprene and isopentenols in cell factories is more sustainable and environmentally friendly. With a number of advantages over other microorganisms, Escherichia coli is considered to be a powerful workhorse organism for producing these compounds. This review will highlight the recent advances in metabolic engineering for isoprene and isopentenol production, especially using E. coli cell factories.

Metabolomics Analysis of the Toxic Effects of the Production of Lycopene and Its Precursors

Using cells as microbial factories enables highly specific production of chemicals with many advantages over chemical syntheses. A number of exciting new applications of this approach are in the area of precision metabolic engineering, which focuses on improving the specificity of target production. In recent work, we have used precision metabolic engineering to design lycopene-producing Escherichia coli for use as a low-cost diagnostic biosensor. To increase precursor availability and thus the rate of lycopene production, we heterologously expressed the mevalonate pathway. We found that simultaneous induction of these pathways increases lycopene production, but induction of the mevalonate pathway before induction of the lycopene pathway decreases both lycopene production and growth rate. Here, we aim to characterize the metabolic changes the cells may be undergoing during expression of either or both of these heterologous pathways. After establishing an improved method for quenching E. coli for metabolomics analysis, we used two-dimensional gas chromatography coupled to mass spectrometry (GCxGC-MS) to characterize the metabolomic profile of our lycopene-producing strains in growth conditions characteristic of our biosensor application. We found that the metabolic impacts of producing low, non-toxic levels of lycopene are of much smaller magnitude than the typical metabolic changes inherent to batch growth. We then used metabolomics to study differences in metabolism caused by the time of mevalonate pathway induction and the presence of the lycopene biosynthesis genes. We found that overnight induction of the mevalonate pathway was toxic to cells, but that the cells could recover if the lycopene pathway was not also heterologously expressed. The two pathways appeared to have an antagonistic metabolic effect that was clearly reflected in the cells’ metabolic profiles. The metabolites homocysteine and homoserine exhibited particularly interesting behaviors and may be linked to the growth inhibition seen when the mevalonate pathway is induced overnight, suggesting potential future work that may be useful in engineering increased lycopene biosynthesis.

Engineering isoprene synthesis in cyanobacteria

The renewable production of isoprene (Isp) hydrocarbons, to serve as fuel and synthetic chemistry feedstock, has attracted interest in the field recently. Isp (C₅ H₈ ) is naturally produced from sunlight, CO₂ and H₂ O photosynthetically in terrestrial plant chloroplasts via the terpenoid biosynthetic pathway and emitted in the atmosphere as a response to heat stress. Efforts to institute a high capacity continuous and renewable process have included heterologous expression of the Isp synthesis pathway in photosynthetic microorganisms. This review examines the premise and promise emanating from this relatively new research effort. Also examined are the metabolic engineering approaches applied in the quest of renewable Isp hydrocarbons production, the progress achieved so far, and barriers encountered along the way.

Patchoulol Production with Metabolically Engineered Corynebacterium glutamicum

Patchoulol is a sesquiterpene alcohol and an important natural product for the perfume industry. Corynebacterium glutamicum is the prominent host for the fermentative production of amino acids with an average annual production volume of ~6 million tons. Due to its robustness and well established large-scale fermentation, C. glutamicum has been engineered for the production of a number of value-added compounds including terpenoids. Both C40 and C50 carotenoids, including the industrially relevant astaxanthin, and short-chain terpenes such as the sesquiterpene valencene can be produced with this organism. In this study, systematic metabolic engineering enabled construction of a patchoulol producing C. glutamicum strain by applying the following strategies: (i) construction of a farnesyl pyrophosphate-producing platform strain by combining genomic deletions with heterologous expression of ispA from Escherichia coli; (ii) prevention of carotenoid-like byproduct formation; (iii) overproduction of limiting enzymes from the 2-c-methyl-d-erythritol 4-phosphate (MEP)-pathway to increase precursor supply; and (iv) heterologous expression of the plant patchoulol synthase gene PcPS from Pogostemon cablin. Additionally, a proof of principle liter-scale fermentation with a two-phase organic overlay-culture medium system for terpenoid capture was performed. To the best of our knowledge, the patchoulol titers demonstrated here are the highest reported to date with up to 60 mg L^−1 and volumetric productivities of up to 18 mg L^−1 d^−1.

From molecular engineering to process engineering: development of high-throughput screening methods in enzyme directed evolution

With increasing concerns in sustainable development, biocatalysis has been recognized as a competitive alternative to traditional chemical routes in the past decades. As nature's biocatalysts, enzymes are able to catalyze a broad range of chemical transformations, not only with mild reaction conditions but also with high activity and selectivity. However, the insufficient activity or enantioselectivity of natural enzymes toward non-natural substrates limits their industrial application, while directed evolution provides a potent solution to this problem, thanks to its independence on detailed knowledge about the relationship between sequence, structure, and mechanism/function of the enzymes. A proper high-throughput screening (HTS) method is the key to successful and efficient directed evolution. In recent years, huge varieties of HTS methods have been developed for rapid evaluation of mutant libraries, ranging from in vitro screening to in vivo selection, from indicator addition to multi-enzyme system construction, and from plate screening to computation- or machine-assisted screening. Recently, there is a tendency to integrate directed evolution with metabolic engineering in biosynthesis, using metabolites as HTS indicators, which implies that directed evolution has transformed from molecular engineering to process engineering. This paper aims to provide an overview of HTS methods categorized based on the reaction principles or types by summarizing related studies published in recent years including the work from our group, to discuss assay design strategies and typical examples of HTS methods, and to share our understanding on HTS method development for directed evolution of enzymes involved in specific catalytic reactions or metabolic pathways.

Metabolic engineering and synthetic biology approaches driving isoprenoid production in Escherichia coli

Isoprenoids comprise the largest family of natural organic compounds with many useful applications in the pharmaceutical, nutraceutical, and industrial fields. Rapid developments in metabolic engineering and synthetic biology have facilitated the engineering of isoprenoid biosynthetic pathways in Escherichia coli to induce high levels of production of many different isoprenoids. In this review, the stem pathways for synthesizing isoprene units as well as the branch pathways deriving diverse isoprenoids from the isoprene units have been summarized. The review also highlights the metabolic engineering efforts made for the biosynthesis of hemiterpenoids, monoterpenoids, sesquiterpenoids, diterpenoids, carotenoids, retinoids, and coenzyme Q₁₀ in E. coli. Perspectives and future directions for the synthesis of novel isoprenoids, decoration of isoprenoids using cytochrome P450 enzymes, and secretion or storage of isoprenoids in E. coli have also been included.

Precise control of lycopene production to enable a fast-responding, minimal-equipment biosensor

Pigmented metabolites have great potential for use in biosensors that target low-resource areas, since sensor output can be interpreted without any equipment. However, full repression of pigment production when undesired is challenging, as even small amounts of enzyme can catalyze the production of large, visible amounts of pigment. The red pigment lycopene could be particularly useful because of its position in the multi-pigment carotenoid pathway, but commonly used inducible promoter systems cannot repress lycopene production. In this paper, we designed a system that could fully repress lycopene production in the absence of an inducer and produce visible lycopene within two hours of induction. We engineered Lac, Ara, and T7 systems to be up to 10 times more repressible, but these improved systems could still not fully repress lycopene. Translational modifications proved much more effective in controlling lycopene. By decreasing the strength of the ribosomal binding sites on the crtEBI genes, we enabled full repression of lycopene and production of visible lycopene in 3-4h of induction. Finally, we added the mevalonate pathway enzymes to increase the rate of lycopene production upon induction and demonstrated that supplementation of metabolic precursors could decrease the time to coloration to about 1.5h. In total, this represents over an order of magnitude reduction in response time compared to the previously reported strategy. The approaches used here demonstrate the disconnect between fluorescent and metabolite reporters, help enable the use of lycopene as a reporter, and are likely generalizable to other systems that require precise control of metabolite production.

Systematic engineering of pentose phosphate pathway improves Escherichia coli succinate production

BACKGROUND

Succinate biosynthesis of Escherichia coli is reducing equivalent-dependent and the EMP pathway serves as the primary reducing equivalent source under anaerobic condition. Compared with EMP, pentose phosphate pathway (PPP) is reducing equivalent-conserving but suffers from low efficacy. In this study, the ribosome binding site library and modified multivariate modular metabolic engineering (MMME) approaches are employed to overcome the low efficacy of PPP and thus increase succinate production.

RESULTS

Altering expression levels of different PPP enzymes have distinct effects on succinate production. Specifically, increased expression of five enzymes, i.e., Zwf, Pgl, Gnd, Tkt, and Tal, contributes to increased succinate production, while the increased expression of two enzymes, i.e., Rpe and Rpi, significantly decreases succinate production. Modular engineering strategy is employed to decompose PPP into three modules according to position and function. Engineering of Zwf/Pgl/Gnd and Tkt/Tal modules effectively increases succinate yield and production, while engineering of Rpe/Rpi module decreases. Imbalance of enzymatic reactions in PPP is alleviated using MMME approach. Finally, combinational utilization of engineered PPP and SthA transhydrogenase enables succinate yield up to 1.61 mol/mol glucose, which is 94% of theoretical maximum yield (1.71 mol/mol) and also the highest succinate yield in minimal medium to our knowledge.

CONCLUSIONS

In summary, we systematically engineered the PPP for improving the supply of reducing equivalents and thus succinate production. Besides succinate, these PPP engineering strategies and conclusions can also be applicable to the production of other reducing equivalent-dependent biorenewables.