Photosynthetic conversion of CO2 to farnesyl diphosphate-derived phytochemicals (amorpha-4,11-diene and squalene) by engineered cyanobacteria

Metabolic engineering for enhanced terpenoid production: Leveraging new horizons with an old technique

Terpenoids are a vast class of plant specialized metabolites (PSMs) manufactured by plants and are involved in their interactions with environment. In addition, they add health benefits to human nutrition and are widely used as pharmaceutically active compounds. However, native plants produce a limited amount of terpenes restricting metabolite yield of terpene-related metabolites. Exponential growth in the plant metabolome data and the requirement of alternative approaches for producing the desired amount of terpenoids, has redirected plant biotechnology research to plant metabolic engineering, which requires in-depth knowledge and precise expertise about dynamic plant metabolic pathways and cellular physiology. Metabolic engineering is an assuring tool for enhancing the concentration of terpenes by adopting specific strategies such as overexpression of the key genes associated with the biosynthesis of targeted metabolites, controlling the modulation of transcription factors, downregulation of competitive pathways (RNAi), co-expression of the biosynthetic pathway genes in heterologous system and other combinatorial approaches. Microorganisms, fast-growing host plants (such as Nicotiana benthamiana), and cell suspension/callus cultures have provided better means for producing valuable terpenoids. Manipulation in the biosynthetic pathways responsible for synthesis of terpenoids can provide opportunities to enhance the content of desired terpenoids and open up new avenues to enhance their production. This review deliberates the worth of metabolic engineering in medicinal plants to resolve issues associated with terpenoid production at a commercial scale. However, to bring the revolution through metabolic engineering, further implementation of genome editing, elucidation of metabolic pathways using omics approaches, system biology approaches, and synthetic biology tactics are essentially needed.

Impact of Carbon Fixation, Distribution and Storage on the Production of Farnesene and Limonene in Synechocystis PCC 6803 and Synechococcus PCC 7002

Terpenes are high-value chemicals which can be produced by engineered cyanobacteria from sustainable resources, solar energy, water and CO2. We previously reported that the euryhaline unicellular cyanobacteria Synechocystis sp. PCC 6803 (S.6803) and Synechococcus sp. PCC 7002 (S.7002) produce farnesene and limonene, respectively, more efficiently than other terpenes. In the present study, we attempted to enhance farnesene production in S.6803 and limonene production in S.7002. Practically, we tested the influence of key cyanobacterial enzymes acting in carbon fixation (RubisCO, PRK, CcmK3 and CcmK4), utilization (CrtE, CrtR and CruF) and storage (PhaA and PhaB) on terpene production in S.6803, and we compared some of the findings with the data obtained in S.7002. We report that the overproduction of RubisCO from S.7002 and PRK from Cyanothece sp. PCC 7425 increased farnesene production in S.6803, but not limonene production in S.7002. The overexpression of the crtE genes (synthesis of terpene precursors) from S.6803 or S.7002 did not increase farnesene production in S.6803. In contrast, the overexpression of the crtE gene from S.6803, but not S.7002, increased farnesene production in S.7002, emphasizing the physiological difference between these two model cyanobacteria. Furthermore, the deletion of the crtR and cruF genes (carotenoid synthesis) and phaAB genes (carbon storage) did not increase the production of farnesene in S.6803. Finally, as a containment strategy of genetically modified strains of S.6803, we report that the deletion of the ccmK3K4 genes (carboxysome for CO2 fixation) did not affect the production of limonene, but decreased the production of farnesene in S.6803.

A Ribulose-5-phosphate Shunt from the Calvin-Benson Cycle to Methylerythritol Phosphate Pathway for Enhancing Photosynthetic Terpenoid Production

Cyanobacteria are attractive hosts for photosynthetic terpenoid production, using CO2 as the sole carbon source. Although the methylerythritol phosphate (MEP) pathway is superior to the mevalonate pathway for cyanobacterial terpenoid synthesis, the first reaction of the MEP pathway, which is catalyzed by 1-deoxy-d-xylulose-5-phosphate (DXP) synthase, involves complex regulation and carbon loss. Here, we constructed a direct route linking ribulose-5-phosphate (Ru5P) in the Calvin-Benson (CB) cycle with DXP in the MEP pathway in a cyanobacterium to increase the terpenoid yield from CO2 and bypass the DXS-targeted regulations. By employing the adaptive laboratory evolution, we identified new RibB variants including RibB 90-92del with a high activity of synthesizing DXP from Ru5P. These RibB variants were introduced into Synechococcus elongatus, resulting in the significantly increased photosynthetic production of isopentenol. The 13C tracer experiments demonstrated a direct carbon flow from Ru5P in the CB cycle to the MEP pathway; thus, this direct route was denoted as the Ru5P shunt. The strain harboring the Ru5P shunt produced 105.2 mg L-1 of isopentenol with an average rate of 17.5 mg L-1 d-1 under continuous light conditions, which is higher than those ever reported for five-carbon alcohol production by photoautotrophic microorganisms. Utilization of the Ru5P shunt in cyanobacterial cells also improved the pinene production, which demonstrates that this shunt can be used to enhance the photosynthetic production of diverse terpenoids.

Two-Phase Fermentation Systems for Microbial Production of Plant-Derived Terpenes

Microbial cell factories, renowned for their economic and environmental benefits, have emerged as a key trend in academic and industrial areas, particularly in the fermentation of natural compounds. Among these, plant-derived terpenes stand out as a significant class of bioactive natural products. The large-scale production of such terpenes, exemplified by artemisinic acid-a crucial precursor to artemisinin-is now feasible through microbial cell factories. In the fermentation of terpenes, two-phase fermentation technology has been widely applied due to its unique advantages. It facilitates in situ product extraction or adsorption, effectively mitigating the detrimental impact of product accumulation on microbial cells, thereby significantly bolstering the efficiency of microbial production of plant-derived terpenes. This paper reviews the latest developments in two-phase fermentation system applications, focusing on microbial fermentation of plant-derived terpenes. It also discusses the mechanisms influencing microbial biosynthesis of terpenes. Moreover, we introduce some new two-phase fermentation techniques, currently unexplored in terpene fermentation, with the aim of providing more thoughts and explorations on the future applications of two-phase fermentation technology. Lastly, we discuss several challenges in the industrial application of two-phase fermentation systems, especially in downstream processing.

Photosynthetic production of α-farnesene by engineered Synechococcus elongatus UTEX 2973 from carbon dioxide

Cyanobacteria are the prospective biosolar cell factories to produce a range of bioproducts through CO2 sequestration. Farnesene is a sesquiterpene with an array of applications in biofuels, pest management, cosmetics, flavours and fragrances. This is the first time a codon-optimized farnesene synthase (AFS) gene is engineered into the genomic neutral site of Synechococcus elongatus UTEX 2973 for farnesene synthesis through its endogenous methylerythritol phosphate (MEP) pathway, rendering UTEX AFS strain. Similarly, bottleneck gene(s) of the MEP pathway, 1-deoxy-D-xylulose-5-phosphate synthase (dxs) and/or fusion of isopentenyl diphosphate isomerase and farnesyl diphosphate synthase (idispA) were engineered engendering UTEX AFS::dxs, UTEX AFS::idispA and UTEX AFS::dxs::idispA strains. UTEX AFS::dxs::idispA achieves farnesene productivity of 2.57 mg/L/day, the highest among engineered cyanobacterial strains studied so far. It demonstrates farnesene production, which is 31.3-times higher than the UTEX AFS strain. Moreover, the engineered strains show similar productivity over a three-month period, stipulating the genetic stability of the strains.

Bioactive substances of cyanobacteria and microalgae: Sources, metabolism, and anticancer mechanism insights

Cyanobacteria and microalgae contain various phytochemicals, including bioactive components in the form of secondary metabolites, namely flavonoids, phenolic acids, terpenoids, and tannins, with remarkable anticancer effects. This review highlights the recent advances in bioactive compounds, with potential anticancer activity, produced by cyanobacteria and microalgae. Previous in vitro investigations showed that many of these bioactive compounds exhibit potent effects against different human cancer types, such as leukemia and breast cancers. Multiple mechanisms implicated in the antitumor effect of these compounds were elucidated, including their ability to target cellular, subcellular, and molecular checkpoints linked to cancer development and promotion. Recent findings have highlighted various mechanisms of action of bioactive compounds produced by cyanobacteria and microalgae, including induction of autophagy and apoptosis, inhibition of telomerase and protein kinases, as well as modulation of epigenetic modifications. In vivo investigations have demonstrated a potent anti-angiogenesis effect on solid tumors, as well as a reduction in tumor volume. Some of these compounds were examined in clinical investigations for certain types of cancers, making them potent candidates/scaffolds for antitumor drug development.

Metabolic flux phenotyping of secondary metabolism in cyanobacteria

Cyanobacteria generate energy from photosynthesis and produce various secondary metabolites with diverse commercial and pharmaceutical applications. Unique metabolic and regulatory pathways in cyanobacteria present new challenges for researchers to enhance their product yields, titers, and rates. Therefore, further advancements are critically needed to establish cyanobacteria as a preferred bioproduction platform. Metabolic flux analysis (MFA) quantitatively determines the intracellular flows of carbon within complex biochemical networks, which elucidate the control of metabolic pathways by transcriptional, translational, and allosteric regulatory mechanisms. The emerging field of systems metabolic engineering (SME) involves the use of MFA and other omics technologies to guide the rational development of microbial production strains. This review highlights the potential of MFA and SME to optimize the production of cyanobacterial secondary metabolites and discusses the technical challenges that lie ahead.

Bioprocess conditions and regulation factors to optimize squalene production in thraustochytrids

Squalene is a widely distributed natural triterpene, as it is a key precursor in the biosynthesis of all sterols. It is a compound of high commercial value worldwide because it has nutritional, medicinal, pharmaceutical, and cosmetic applications, due to its different biological properties. The main source of extraction has been shark liver oil, which is currently unviable on a larger scale due to the impacts of overexploitation. Secondary sources are mainly vegetable oils, although a limited one, as they allow low productive yields. Due to the diversity of applications that squalene presents and its growing demand, there is an increasing interest in identifying sustainable sources of extraction. Wild species of thraustochytrids, which are heterotrophic protists, have been identified to have the highest squalene content compared to bacteria, yeasts, microalgae, and vegetable sources. Several studies have been carried out to identify the bioprocess conditions and regulation factors, such as the use of eustressors that promote an increase in the production of this triterpene; however, studies focused on optimizing their productive yields are still in its infancy. This review includes the current trends that also comprises the advances in genetic regulations in these microorganisms, with a view to identify the culture conditions that have been favorable in increasing the production of squalene, and the influences that both bioprocess conditions and applied regulation factors partake at a metabolic level.

A systematic overexpression approach reveals native targets to increase squalene production in Synechocystis sp. PCC 6803

Cyanobacteria are a promising platform for the production of the triterpene squalene (C30), a precursor for all plant and animal sterols, and a highly attractive intermediate towards triterpenoids, a large group of secondary plant metabolites. Synechocystis sp. PCC 6803 natively produces squalene from CO₂ through the MEP pathway. Based on the predictions of a constraint-based metabolic model, we took a systematic overexpression approach to quantify native Synechocystis gene's impact on squalene production in a squalene-hopene cyclase gene knock-out strain (Δshc). Our in silico analysis revealed an increased flux through the Calvin-Benson-Bassham cycle in the Δshc mutant compared to the wildtype, including the pentose phosphate pathway, as well as lower glycolysis, while the tricarboxylic acid cycle predicted to be downregulated. Further, all enzymes of the MEP pathway and terpenoid synthesis, as well as enzymes from the central carbon metabolism, Gap2, Tpi and PyrK, were predicted to positively contribute to squalene production upon their overexpression. Each identified target gene was integrated into the genome of Synechocystis Δshc under the control of the rhamnose-inducible promoter P_rha. Squalene production was increased in an inducer concentration dependent manner through the overexpression of most predicted genes, which are genes of the MEP pathway, ispH, ispE, and idi, leading to the greatest improvements. Moreover, we were able to overexpress the native squalene synthase gene (sqs) in Synechocystis Δshc, which reached the highest production titer of 13.72 mg l^-1 reported for squalene in Synechocystis sp. PCC 6803 so far, thereby providing a promising and sustainable platform for triterpene production.

The current situations and limitations of genetic engineering in cyanobacteria: a mini review

Cyanobacteria are an ancient group of photoautotrophic prokaryotes, and play an essential role in the global carbon cycle. They are also model organisms for studying photosynthesis and circadian regulation, and metabolic engineering and synthetic biology strategies grants light-driven biotechnological applications to cyanobacteria, especially for engineering cyanobacteria cells to achieve an efficient light-driven system for synthesizing any product of interest from renewable feedstocks. However, lower yield limits the potential of industrial application of cyanobacterial synthetic biology, and some key limitations must be overcome to realize the full biotechnological potential of these versatile microorganisms. Although genetic engineering toolkits for cyanobacteria have made some progress, the tools available still lag behind conventional heterotrophic microorganism. Consequently, this study describes the current situations and limitations of genetic engineering in cyanobacteria, and further improvements are proposed to improve the output of targeted products. We believe that cyanobacteria-mediated light-driven platforms towards efficient synthesis of green chemicals could unlock a bright future by developing the tools for strain manipulation and novel chassis organisms with excellent performance for biotechnological applications, which could also accelerate the advancement of bio-manufacturing industries.

Current Metabolic Engineering Strategies for Photosynthetic Bioproduction in Cyanobacteria

Cyanobacteria are photosynthetic microorganisms capable of using solar energy to convert CO₂ and H₂O into O₂ and energy-rich organic compounds, thus enabling sustainable production of a wide range of bio-products. More and more strains of cyanobacteria are identified that show great promise as cell platforms for the generation of bioproducts. However, strain development is still required to optimize their biosynthesis and increase titers for industrial applications. This review describes the most well-known, newest and most promising strains available to the community and gives an overview of current cyanobacterial biotechnology and the latest innovative strategies used for engineering cyanobacteria. We summarize advanced synthetic biology tools for modulating gene expression and their use in metabolic pathway engineering to increase the production of value-added compounds, such as terpenoids, fatty acids and sugars, to provide a go-to source for scientists starting research in cyanobacterial metabolic engineering.

Characterization of a cyanobacterial rep protein with broad-host range and its utilization for expression vectors

Owing to their photosynthetic capabilities, cyanobacteria are regarded as ecologically friendly hosts for production of biomaterials. However, compared to other bacteria, tools for genetic engineering, especially expression vector systems, are limited. In this study, we characterized a Rep protein, exhibiting replication activity in multiple cyanobacteria and established an expression vector using this protein. Our comprehensive screening using a genomic library of Synechocystis sp. PCC 6803 revealed that a certain region encoding a Rep-related protein (here named Cyanobacterial Rep protein A2: CyRepA2) exhibits high autonomous replication activity in a heterologous host cyanobacterium, Synechococcus elongatus PCC 7942. A reporter assay using GFP showed that the expression vector pYS carrying CyRepA2 can be maintained in not only S. 6803 and S. 7942, but also Synechococcus sp. PCC 7002 and Anabaena sp. PCC 7120. In S. 7942, GFP expression in the pYS-based system was tightly regulated by IPTG, achieving 10-fold higher levels than in the chromosome-based system. Furthermore, pYS could be used together with the conventional vector pEX, which was constructed from an endogenous plasmid in S. 7942. The combination of pYS with other vectors is useful for genetic engineering, such as modifying metabolic pathways, and is expected to improve the performance of cyanobacteria as bioproduction chassis.

Metabolic Engineering of the Isopentenol Utilization Pathway Enhanced the Production of Terpenoids in Chlamydomonas reinhardtii

Eukaryotic green microalgae show considerable promise for the sustainable light-driven biosynthesis of high-value fine chemicals, especially terpenoids because of their fast and inexpensive phototrophic growth. Here, the novel isopentenol utilization pathway (IUP) was introduced into Chlamydomonas reinhardtii to enhance the hemiterpene (isopentenyl pyrophosphate, IPP) titers. Then, diphosphate isomerase (IDI) and limonene synthase (MsLS) were further inserted for limonene production. Transgenic algae showed 8.6-fold increase in IPP compared with the wild type, and 23-fold increase in limonene production compared with a single MsLS expressing strain. Following the culture optimization, the highest limonene production reached 117 µg/L, when the strain was cultured in a opt2 medium supplemented with 10 mM isoprenol under a light: dark regimen. This demonstrates that transgenic algae expressing the IUP represent an ideal chassis for the high-value terpenoid production. The IUP will facilitate further the metabolic and enzyme engineering to enhance the terpenoid titers by significantly reducing the number of enzyme steps required for an optimal biosynthesis.

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.

Engineering phototrophic bacteria for the production of terpenoids

With more than 80 000 compounds, terpenoids represent one of the largest classes of secondary metabolites naturally produced by various plants and other organisms. Owing to the tremendous structural diversity, they offer a wide range of properties relevant for biotechnological and pharmaceutical applications. In this context, heterologous terpenoid production in engineered microbial hosts represents an often cost-effective and eco-friendly way to make these valuable compounds industrially available. This review provides an overview of current strategies to employ and engineer oxygenic and anoxygenic phototrophic bacteria as alternative cell factories for sustainable terpenoid production. Besides terpenoid pathway engineering, the effects of different illumination strategies on terpenoid photoproduction are key elements in the latest studies.

Optimal NaCl Medium Enhances Squalene Accumulation in Thraustochytrium sp. ATCC 26185 and Influences the Expression Levels of Key Metabolic Genes

Squalene, a natural lipid of the terpenoid family, is well-recognized for its roles in regulating cholesterol metabolism, preventing tumor development, and improving immunity. For large-scale squalene production, the unicellular marine protists—thraustochytrids—have shown great potential. However, the growth of thraustochytrids is known to be affected by salt stress, which can eventually influence the squalene content. Here, we study the effects of an optimal concentration of NaCl on the squalene content and transcriptome of Thraustochytrium sp. ATCC 26185. Under the optimal culture conditions (glucose, 30 g/L; yeast extract, 2.5 g/L; and NaCl, 5 g/L; 28°C), the strain yielded 67.7 mg squalene/g cell dry weight, which was significantly greater than that (5.37 mg/g) under the unoptimized conditions. NaCl was determined as the most significant (R = 135.24) factor for squalene production among glucose, yeast extract, and NaCl. Further comparative transcriptomics between the ATCC 26185 culture with and without NaCl addition revealed that NaCl (5 g/L) influences the expression of certain key metabolic genes, namely, IDI, FAS-a, FAS-b, ALDH3, GS, and NDUFS4. The differential expression of these genes possibly influenced the acetyl-CoA and glutamate metabolism and resulted in an increased squalene production. Through the integration of bioprocess technology and transcriptomics, this report provides the first evidence of the possible mechanisms underscoring increased squalene production by NaCl.

Altered Carbon Partitioning Enhances CO2 to Terpene Conversion in Cyanobacteria

Photosynthetic terpene production represents one of the most carbon and energy-efficient routes for converting CO2 into hydrocarbon. In photosynthetic organisms, metabolic engineering has led to limited success in enhancing terpene productivity, partially due to the low carbon partitioning. In this study, we employed systems biology analysis to reveal the strong competition for carbon substrates between primary metabolism (e.g., sucrose, glycogen, and protein synthesis) and terpene biosynthesis in Synechococcus elongatus PCC 7942. We then engineered key "source" and "sink" enzymes. The "source" limitation was overcome by knocking out either sucrose or glycogen biosynthesis to significantly enhance limonene production via altered carbon partitioning. Moreover, a fusion enzyme complex with geranyl diphosphate synthase (GPPS) and limonene synthase (LS) was designed to further improve pathway kinetics and substrate channeling. The synergy between "source" and "sink" achieved a limonene titer of 21.0 mg/L. Overall, the study demonstrates that balancing carbon flux between primary and secondary metabolism can be an effective approach to enhance terpene bioproduction in cyanobacteria. The design of "source" and "sink" synergy has significant potential in improving natural product yield in photosynthetic species.

Novel computational and experimental approaches for investigating the thermodynamics of metabolic networks

Thermodynamic analysis of metabolic networks has emerged as a useful new tool for pathway design and metabolic engineering. Understanding the relationship between the thermodynamic driving force of biochemical reactions and metabolic flux has generated new insights regarding the design principles of microbial carbon metabolism. This review summarizes the various lessons that can be obtained from the thermodynamic analysis of metabolic pathways, illustrates concepts of computational thermodynamic tools, and highlights recent applications of thermodynamic analysis to pathway design in industrially relevant microbes.

Advances in developing metabolically engineered microbial platforms to produce fourth-generation biofuels and high-value biochemicals

Producing bio-based chemicals is imperative to establish an eco-friendly circular bioeconomy. However, the compromised titer of these biochemicals hampers their commercial implementation. Advances in genetic engineering tools have enabled researchers to develop robust strains producing desired titers of the next-generation biofuels and biochemicals. The native and non-native pathways have been extensively engineered in various host strains via pathway reconstruction and metabolic flux redirection of lipid metabolism and central carbon metabolism to produce myriad biomolecules including alcohols, isoprenoids, hydrocarbons, fatty-acids, and their derivatives. This review has briefly covered the research efforts made during the previous decade to produce advanced biofuels and biochemicals through engineered microbial platforms along with the engineering approaches employed. The efficiency of the various techniques along with their shortcomings is also covered to provide a comprehensive overview of the progress and future directions to achieve higher titer of fourth-generation biofuels and biochemicals while keeping environmental sustainability intact.

Metabolic engineering of microbes for monoterpenoid production

Monoterpenoids are an important class of natural products that are derived from the condensation of two five‑carbon isoprene subunits. They are widely used for flavouring, fragrances, colourants, cosmetics, fuels, chemicals, and pharmaceuticals in various industries. They can also serve as precursors for the production of many industrially important products. Currently, monoterpenoids are produced predominantly through extraction from plant sources. However, the small quantity of monoterpenoids in nature renders this method of isolation non-economically viable. Similarly impractical is the chemical synthesis of these compounds as they suffer from high energy consumption and pollutant discharge. Microbial biosynthesis, however, exists as a potential solution to these hindrances, but the transformation of cells into efficient factories remains a major impediment. Here, we critically review the recent advances in engineering microbes for monoterpenoid production, with an emphasis on categorized strategies, and discuss the challenges and perspectives to offer guidance for future engineering.

Amorpha-4,11-diene synthase: a key enzyme in artemisinin biosynthesis and engineering

Amorpha-4,11-diene synthase (ADS) catalyzes the first committed step in the artemisinin biosynthetic pathway, which is the first catalytic reaction enzymatically and genetically characterized in artemisinin biosynthesis. The advent of ADS in Artemisia annua is considered crucial for the emergence of the specialized artemisinin biosynthetic pathway in the species. Microbial production of amorpha-4,11-diene is a breakthrough in metabolic engineering and synthetic biology. Recently, numerous new techniques have been used in ADS engineering; for example, assessing the substrate promiscuity of ADS to chemoenzymatically produce artemisinin. In this review, we discuss the discovery and catalytic mechanism of ADS, its application in metabolic engineering and synthetic biology, as well as the role of sesquiterpene synthases in the evolutionary origin of artemisinin.

Metabolic engineering strategies for sesquiterpene production in microorganism

Sesquiterpenes are a large variety of terpene natural products, widely existing in plants, fungi, marine organisms, insects, and microbes. Value-added sesquiterpenes are extensively used in industries such as: food, drugs, fragrances, and fuels. With an increase in market demands and the price of sesquiterpenes, the biosynthesis of sesquiterpenes by microbial fermentation methods from renewable feedstocks is acquiring increasing attention. Synthetic biology provides robust tools of sesquiterpene production in microorganisms. This review presents a summary of metabolic engineering strategies on the hosts and pathway engineering for sesquiterpene production. Advances in synthetic biology provide new strategies on the creation of desired hosts for sesquiterpene production. Especially, metabolic engineering strategies for the production of sesquiterpenes such as: amorphadiene, farnesene, bisabolene, and caryophyllene are emphasized in: Escherichia coli, Saccharomyces cerevisiae, and other microorganisms. Challenges and future perspectives of the bioprocess for translating sesquiterpene production into practical industrial work are also discussed.

Computational Analysis of Dynamic Light Exposure of Unicellular Algal Cells in a Flat-Panel Photobioreactor to Support Light-Induced CO2 Bioprocess Development

Cyanobacterial cell factories trace a vibrant pathway to climate change neutrality and sustainable development owing to their ability to turn carbon dioxide-rich waste into a broad portfolio of renewable compounds, which are deemed valuable in green chemistry cross-sectorial applications. Cell factory design requires to define the optimal operational and cultivation conditions. The paramount parameter in biomass cultivation in photobioreactors is the light intensity since it impacts cellular physiology and productivity. Our modeling framework provides a basis for the predictive control of light-limited, light-saturated, and light-inhibited growth of the Synechocystis sp. PCC 6803 model organism in a flat-panel photobioreactor. The model here presented couples computational fluid dynamics, light transmission, kinetic modeling, and the reconstruction of single cell trajectories in differently irradiated areas of the photobioreactor to relate key physiological parameters to the multi-faceted processes occurring in the cultivation environment. Furthermore, our analysis highlights the need for properly constraining the model with decisive qualitative and quantitative data related to light calibration and light measurements both at the inlet and outlet of the photobioreactor in order to boost the accuracy and extrapolation capabilities of the model.

Heterologous expression and metabolic engineering tools for improving terpenoids production

Terpenoids, also referred to as isoprenoids, are the largest group of natural compounds which have contributed significantly to the pharmaceutical industry. The challenges in producing bioactive terpenoids from their original host or by organic synthesis methods spurred the endeavors of producing terpenoids in heterologous host. Modern advances utilizing synthetic biology and biological engineering tools have provided a variety of pharmaceutical terpenoids in large-scale and with diversified structures. In this review, we will summarize the progress in production of typical terpenoids skeletons using heterologous expression method assisted by metabolic engineering techniques, with the purpose of enlightening further efforts in developing advanced cell factories for producing terpenoid based pharmaceuticals.

Side Products of Recombinant Amorpha-4,11-diene Synthase and Their Effect on Microbial Artemisinin Production

Amorpha-4,11-diene synthase (ADS) is the first committed enzyme in the biosynthesis of artemisinin. Artemisinin production by biobased fermentation is considered a reliable alternative pathway. Heterologously expressed ADS has been established to generate several minor products, including structural analogues of amorpha-4,11-diene, but their fate in fermentation is still unknown. Here, using chiral analysis, we found that ADS produces one of the analogues, amorpha-4-en-11-ol, as a pair of epimers. Labeling experiments revealed that ADS mutants yielded amorphene-type sesquiterpenes, indicating the co-occurrence of initial 1,6 and 1,10 cyclization of farnesyl diphosphate in a single enzyme. Interestingly, the immediate downstream oxidase CYP71AV1 had very low affinity to the side products of the recombinant ADS, including amorpha-4-en-7-ol, which is structurally similar to amorpha-4,11-diene. Our data uncover the complex catalytic mechanism of recombinant ADS and reveal a potential negative effect of the side products of recombinant ADS on the production of the artemisinin precursor in microbes.

Biocontainment of Engineered Synechococcus elongatus PCC 7942 for Photosynthetic Production of α-Farnesene from CO2

Biocontainment systems have been developed to mitigate the concerns regarding biosafety and environmental risk because of the possible escape of genetically modified organisms into the environment following large-scale outdoor cultivation. Here, we present a biocontainment system entailing genetically modified Synechococcus elongatus PCC 7942, also engineered for α-farnesene production using a de-evolutionary strategy. In this approach, the gene cluster encoding the β-carboxysome and the associated carbon concentrating mechanism (CCM) were deleted in the α-farnesene-producing cyanobacteria, resulting in no cell growth and no α-farnesene production at ambient CO₂ concentrations (100% air bubbling). However, cell growth and α-farnesene production were detected in the CCM-deficient strains at high CO₂ concentrations (5% CO₂ [v/v], 10% CO₂ [v/v]), albeit at levels lower than those of the parental control. To overcome this limitation, the overexpression of carbonic anhydrase and bicarbonate transporter genes in the CCM-deficient strains restored cell growth and the production level of α-farnesene (5.0 ± 0.6 mg/L) to that of the parental control. The production of α-farnesene in the later strains strictly depended on CO₂ concentration in the photobioreactor and did not rely on a chemical induction process. Thus, next generation bio-solar cell factories could be promoted with the suggested biocontainment system.

Current Status and Future Strategies to Increase Secondary Metabolite Production from Cyanobacteria

Cyanobacteria, given their ability to produce various secondary metabolites utilizing solar energy and carbon dioxide, are a potential platform for sustainable production of biochemicals. Until now, conventional metabolic engineering approaches have been applied to various cyanobacterial species for enhanced production of industrially valued compounds, including secondary metabolites and non-natural biochemicals. However, the shortage of understanding of cyanobacterial metabolic and regulatory networks for atmospheric carbon fixation to biochemical production and the lack of available engineering tools limit the potential of cyanobacteria for industrial applications. Recently, to overcome the limitations, synthetic biology tools and systems biology approaches such as genome-scale modeling based on diverse omics data have been applied to cyanobacteria. This review covers the synthetic and systems biology approaches for advanced metabolic engineering of cyanobacteria.

Biosynthesis of the Calorie-Free Sweetener Precursor ent-Kaurenoic Acid from CO2 Using Engineered Cyanobacteria

To supply the sustainable calorie-free sweetener stevioside, synthetic photosynthetic bacteria were developed to produce ent-kaurenoic acid as a precursor of stevioside directly from CO₂. By the use of a combinatorial and modular approach for gene expression, including a cytochrome P450 and the corresponding reductase, engineered Synechoccous elongatus PCC 7942 as a model cyanobacterium enabled the biosynthesis of ent-kaurenoic acid at 2.9 ± 0.01 mg L^-1 from CO₂. We found that the order of genes for expression was critical, producing ent-kaurenoic acid by balancing gene expressions and accumulation of the toxic intermediate in a cell. The engineered bacteria allowed the complete biosynthesis of ent-kaurenoic acid, and it will be used for stevioside biosynthesis from CO₂ as a controlled fermentation.

The heterologous production of terpenes by the thermophile Parageobacillus thermoglucosidasius in a consolidated bioprocess using waste bread

Parageobacillus thermoglucosidasius is a genetically tractable thermophile that grows rapidly at elevated temperatures, with a doubling time at 65 °C comparable to the shortest doubling times of Escherichia coli. It is capable of using a wide variety of substrates, including carbohydrate oligomers, and has been developed for the industrial production of ethanol. In this study, P. thermoglucosidasius NCIMB11955 has been engineered to produce the sesquiterpene τ-muurolol by introduction of a heterologous mevalonate pathway constructed using genes from several thermophilic archaea together with a recently characterised thermostable τ-muurolol synthase. P. thermoglucosidasius naturally uses the methylerythritol phosphate pathway for production of the terpene precursor, isopentenyl pyrophosphate, while archaea use a version of the mevalonate pathway. By introducing the orthogonal archaeal pathway it was possible to increase the flux through to sesquiterpene biosynthesis. Construction of such a large metabolic pathway created problems with genetic vector introduction and stability, so recombinant plasmids were introduced by conjugation, and a thermostable serine integrase system was developed for integration of large pathways onto the chromosome. Finally, by making the heterologous pathway maltose-inducible we demonstrate that the new strain is capable of using waste bread directly as an autoinduction carbon source for the production of terpenes in a consolidated bioprocess.

Current understanding of the cyanobacterial CRISPR-Cas systems and development of the synthetic CRISPR-Cas systems for cyanobacteria

Cyanobacteria are photosynthetic microorganisms that are capable of converting CO2 to value-added chemicals. Engineering of cyanobacteria with synthetic biology tools, including the CRISPR-Cas system, has allowed an opportunity for biological CO2 utilization. Here, we described natural CRISPR-Cas systems for understanding cyanobacterial genomics and synthetic CRISPR-Cas systems for metabolic engineering applications. The natural CRISPR-Cas systems in cyanobacteria have been identified as Class 1, with type I and III, and some Class 2, with type V, as an adaptive immune system against viral invasion. As synthetic tools, CRISPR-Cas9 and -Cas12a have been successfully established in cyanobacteria to delete a target gene without a selection marker. Deactivated Cas9 and Cas12a have also been used to repress genes for metabolic engineering. In addition, a perspective on how advanced CRISPR-Cas systems and a pool of the guide RNAs can be advantageous for precise genome engineering and understanding of unknown functions was discussed for advanced engineering of cyanobacteria.

CRISPRi-dCas12a: A dCas12a-Mediated CRISPR Interference for Repression of Multiple Genes and Metabolic Engineering in Cyanobacteria

In cyanobacteria, metabolic engineering using synthetic biology tools is limited to build a biosolar cell factory that converts CO₂ to value-added chemicals, as repression of essential genes has not been achieved. In this study, we developed a dCas12a-mediated CRISPR interference system (CRISPRi-dCas12a) in cyanobacteria that effectively blocked the transcriptional initiation by means of a CRISPR-RNA (crRNA) and 19-nt direct repeat, resulting in 53-94% gene repression. The repression of multiple genes in a single crRNA array was also successfully achieved without a loss in repression strength. In addition, as a demonstration of the dCas12a-mediated CRISPRi for metabolic engineering, photosynthetic squalene production was improved by repressing the essential genes of either acnB encoding for aconitase or cpcB₂ encoding for phycocyanin β-subunit in Synechococcus elongatus PCC 7942. The ability to regulate gene repression will promote the construction of biosolar cell factories to produce value-added chemicals.

Scalable Cultivation of Engineered Cyanobacteria for Squalene Production from Industrial Flue Gas in a Closed Photobioreactor

Economically feasible photosynthetic cultivation of microalgal and cyanobacterial strains is crucial for the biological conversion of CO₂ and potential CO₂ mitigation to challenge global warming. To overcome the economic barriers, the production of value-added chemicals was desired by compensating for the overall processing cost. Here, we engineered cyanobacteria for photosynthetic squalene production and cultivated them in a scalable photobioreactor using industrial flue gas. First, an inducer-free gene expression system was developed for the cyanobacteria to lower production const. Then, the recombinant cyanobacteria were cultivated in a closed photobioreactor (100 L) using flue gas (5% CO₂) as the sole carbon source under natural sunlight as the only energy source. Seasonal light intensities and temperatures were analyzed along with cyanobacterial cell growth and squalene production in August and October 2019. As a result, the effective irradiation hours were the most critical factor for the large-scale cultivation of cyanobacteria. Thus, an automated photobioprocess system will be developed based on the regional light sources.

Metabolic rewiring of synthetic pyruvate dehydrogenase bypasses for acetone production in cyanobacteria

Designing synthetic pathways for efficient CO₂ fixation and conversion is essential for sustainable chemical production. Here we have designed a synthetic acetate-acetyl-CoA/malonyl-CoA (AAM) bypass to overcome an enzymatic activity of pyruvate dehydrogenase complex. This synthetic pathway utilizes acetate assimilation and carbon rearrangements using a methyl malonyl-CoA carboxyltransferase. We demonstrated direct conversion of CO₂ into acetyl-CoA-derived acetone as an example in photosynthetic Synechococcus elongatus PCC 7942 by increasing the acetyl-CoA pools. The engineered cyanobacterial strain with the AAM-bypass produced 0.41 g/L of acetone at 0.71 m/day of molar productivity. This work clearly shows that the synthetic pyruvate dehydrogenase bypass (AAM-bypass) is a key factor for the high-level production of an acetyl-CoA-derived chemical in photosynthetic organisms.

Engineering a Carotenoid-Overproducing Strain of Azospirillum brasilense for Heterologous Production of Geraniol and Amorphadiene

Escherichia coli and Saccharomyces cerevisiae have been used extensively for heterologous production of a variety of secondary metabolites. Neither has an endogenous high-flux isoprenoid pathway, required for the production of terpenoids. Azospirillum brasilense, a nonphotosynthetic GRAS (generally recognized as safe) bacterium, produces carotenoids in the presence of light. The carotenoid production increases multifold upon inactivating a gene encoding an anti-sigma factor (ChrR1). We used this A. brasilense mutant (Car-1) as a host for the heterologous production of two high-value phytochemicals, geraniol and amorphadiene. Cloned genes (crtE1 and crtE2) of A. brasilense encoding native geranylgeranyl pyrophosphate synthases (GGPPS), when overexpressed and purified, did not produce geranyl pyrophosphate (GPP) in vitro Therefore, we cloned codon-optimized copies of the Catharanthus roseus genes encoding GPP synthase (GPPS) and geraniol synthase (GES) to show the endogenous intermediates of the carotenoid biosynthetic pathway in the Car-1 strain were utilized for the heterologous production of geraniol in A. brasilense Similarly, cloning and expression of a codon-optimized copy of the amorphadiene synthase (ads) gene from Artemisia annua also led to the heterologous production of amorphadiene in Car-1. Geraniol or amorphadiene content was estimated using gas chromatography-mass spectrometry (GC-MS) and GC. These results demonstrate that Car-1 is a promising host for metabolic engineering, as the naturally available endogenous pool of the intermediates of the carotenoid biosynthetic pathway of A. brasilense can be effectively utilized for the heterologous production of high-value phytochemicals.IMPORTANCE To date, the major host organisms used for the heterologous production of terpenoids, i.e., E. coli and S. cerevisiae, do not have high-flux isoprenoid pathways and involve tedious metabolic engineering to increase the precursor pool. Since carotenoid-producing bacteria carry endogenous high-flux isoprenoid pathways, we used a carotenoid-producing mutant of A. brasilense as a host to show its suitability for the heterologous production of geraniol and amorphadiene as a proof-of-concept. The advantages of using A. brasilense as a model system include (i) dispensability of carotenoids and (ii) the possibility of overproducing carotenoids through a single mutation to exploit high carbon flux for terpenoid production.

High density cultivation for efficient sesquiterpenoid biosynthesis in Synechocystis sp. PCC 6803

Cyanobacteria and microalgae are attractive photoautotrophic host systems for climate-friendly production of fuels and other value-added biochemicals. However, for economic applications further development and implementation of efficient and sustainable cultivation strategies are essential. Here, we present a comparative study on cyanobacterial sesquiterpenoid biosynthesis in Synechocystis sp. PCC 6803 using a commercial lab-scale High Density Cultivation (HDC) platform in the presence of dodecane as in-situ extractant. Operating in a two-step semi-batch mode over a period of eight days, volumetric yields of (E)-α-bisabolene were more than two orders of magnitude higher than previously reported for cyanobacteria, with final titers of 179.4 ± 20.7 mg * L⁻¹. Likewise, yields of the sesquiterpene alcohols (−)-patchoulol and (−)-α-bisabolol were many times higher than under reference conditions, with final titers of 17.3 ± 1.85 mg * L⁻¹ and 96.3 ± 2.2 mg * L⁻¹, respectively. While specific productivity was compromised particularly for (E)-α-bisabolene in the HDC system during phases of high biomass accumulation rates, volumetric productivity enhancements during linear growth at high densities were more pronounced for (E)-α-bisabolene than for the hydroxylated terpenoids. Together, this study provides additional insights into cell density-related process characteristics, introducing HDC as highly efficient strategy for phototrophic terpenoid production in cyanobacteria.

Introduction of a green algal squalene synthase enhances squalene accumulation in a strain of Synechocystis sp. PCC 6803

Squalene is a triterpene which is produced as a precursor for a wide range of terpenoid compounds in many organisms. It has commercial use in food and cosmetics but could also be used as a feedstock for production of chemicals and fuels, if generated sustainably on a large scale. We have engineered a cyanobacterium, Synechocystis sp. PCC 6803, for production of squalene from CO₂. In this organism, squalene is produced via the methylerythritol-phosphate (MEP) pathway for terpenoid biosynthesis, and consumed by the enzyme squalene hopene cyclase (Shc) for generation of hopanoids. The gene encoding Shc in Synechocystis was inactivated (Δshc) by insertion of a gene encoding a squalene synthase from the green alga Botryococcus braunii, under control of an inducible promoter. We could demonstrate elevated squalene generation in cells where the algal enzyme was induced. Heterologous overexpression of genes upstream in the MEP pathway further enhanced the production of squalene, to a level three times higher than the Δshc background strain. During growth in flat panel bioreactors, a squalene titer of 5.1 ​mg/L of culture was reached.

Single-Stage Astaxanthin Production Enhances the Nonmevalonate Pathway and Photosynthetic Central Metabolism in Synechococcus sp. PCC 7002

The natural pigment astaxanthin is widely used in aquaculture, pharmaceutical, nutraceutical, and cosmetic industries due to superior antioxidant properties. The green alga Haematococcus pluvialis is currently used for commercial production of astaxanthin pigment. However, slow growing H. pluvialis requires a complex two-stage stress-induced process with high light intensity leading to increased contamination risks. In contrast, the fast-growing euryhaline cyanobacterium Synechococcus sp. PCC 7002 (Synechococcus 7002) is able to reach high density under stress-free phototrophic conditions, and is therefore a promising metabolic engineering platform for astaxanthin production. In the present study, genes encoding β-carotene hydroxylase and β-carotene ketolase, from the marine bacterium Brevundimonas sp. SD212, are integrated into the endogenous plasmid of Synechococcus 7002, and then expressed to biosynthesize astaxanthin. Although Synechococcus 7002 does not inherently produce astaxanthin, the recombinant ZW strain yields 3 mg/g dry cell weight astaxanthin from CO₂ as the sole carbon source, with significantly higher astaxanthin content than previous cyanobacteria reports. Synechococcus 7002 astaxanthin productivity reached 3.35 mg/L/day after just 2 days in a continuous autotrophic process, which is comparable to the best H. pluvialis astaxanthin productivities when factoring in growth times. Metabolomics analysis reveals increases in fractions of hexose-, pentose-, and triose phosphates along with intermediates involved in the nonmevalonate pathway. Dynamic metabolomics analysis of ¹³C labeled metabolites clearly indicates flux enhancements in the Calvin cycle and glycolysis resulting from the overexpression of astaxanthin biosynthetic genes. This study suggests that cyanobacteria may enhance central metabolism as well as the nonmevalonate pathway in an attempt to replenish depleted pigments such as β-carotene and zeaxanthin.

Evolutionary Engineering of Cyanobacteria to Enhance the Production of α-Farnesene from CO2

Photosynthetic cyanobacteria can fix CO₂ and utilize it as the sole carbon source for cell growth and production of biochemicals. Here, we metabolically engineered Synechococcus elongatus PCC 7942 for an enhanced production of α-farnesene by optimizing the ribosome-binding site (RBS) of the codon-optimized farnesene synthase gene. The production of α-farnesene was found to be enhanced in strains with a low translation initiation rate, resulting in α-farnesene production (0.57 mg/(L day)). Using the RBS variants and random mutations, we performed fluorescence-based analysis of cells grown in 96-well culture plates to screen the α-farnesene-producing strains but could not improve the titers of the RBS-optimized strains. However, evolutionary engineering of the RBS-optimized strains resulted in a twofold increase in α-farnesene production (1.2 mg/(L day)) compared to the previous study. Therefore, combining metabolic and evolutionary engineering might be helpful for enhancing the cellular fitness of cyanobacteria for the production of target chemicals.

Corrigendum: Engineering Strategies in Microorganisms for the Enhanced Production of Squalene: Advances, Challenges and Opportunities

[This corrects the article DOI: 10.3389/fbioe.2019.00050.].

Identifying and engineering the ideal microbial terpenoid production host

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

Engineering Strategies in Microorganisms for the Enhanced Production of Squalene: Advances, Challenges and Opportunities

The triterpene squalene is a natural compound that has demonstrated an extraordinary diversity of uses in pharmaceutical, nutraceutical, and personal care industries. Emboldened by this range of uses, novel applications that can gain profit from the benefits of squalene as an additive or supplement are expanding, resulting in its increasing demand. Ever since its discovery, the primary source has been the deep-sea shark liver, although recent declines in their populations and justified animal conservation and protection regulations have encouraged researchers to identify a novel route for squalene biosynthesis. This renewed scientific interest has profited from immense developments in synthetic biology, which now allows fine-tuning of a wider range of plants, fungi, and microorganisms for improved squalene production. There are numerous naturally squalene producing species and strains; although they generally do not make commercially viable yields as primary shark liver sources can deliver. The recent advances made toward improving squalene output from natural and engineered species have inspired this review. Accordingly, it will cover in-depth knowledge offered by the studies of the natural sources, and various engineering-based strategies that have been used to drive the improvements in the pathways toward large-scale production. The wide uses of squalene are also discussed, including the notable developments in anti-cancer applications and in augmenting influenza vaccines for greater efficacy.

Heterologous Production of Squalene from Glucose in Engineered Corynebacterium glutamicum Using Multiplex CRISPR Interference and High-Throughput Fermentation

The sustainable production of squalene has driven the development of microbial cell factories due to the limitation of low-yielding bioprocesses from plants and illegal harvesting shark liver. We report the metabolic engineering of Corynebacterium glutamicum to produce squalene from glucose. Combinatorial metabolic engineering strategies for precursor rebalancing, redox balancing, and blocking the competing pathway for the isopentenyl diphosphate availabilities were applied by repressing the target genes using the CRISPR interference. The best engineered strain using high-throughput fermentation produced squalene from glucose at 5.4 ± 0.3 mg/g dry cell weight (DCW) and 105.3 ± 3.0 mg/L, which was a 5.2-fold increase over the parental strain. In addition, flask cultivation of C. glutamicum overexpressing the dxs and idi genes with squalene synthase gene and repressing the idsA gene resulted in production of squalene at 5.8 ± 0.4 mg/g DCW and 82.8 ± 6.2 mg/L, which was a 3.4-fold increase over the parental strain.

Bio-solar cell factories for photosynthetic isoprenoids production

Photosynthetic production of isoprenoids in cyanobacteria is considered in terms of metabolic engineering and biological importance. Metabolic engineering of photosynthetic bacteria (cyanobacteria) has been performed to construct bio-solar cell factories that convert carbon dioxide to various value-added chemicals. Isoprenoids, which are found in nature and range from essential cell components to defensive molecules, have great value in cosmetics, pharmaceutics, and biofuels. In this review, we summarize the recent engineering of cyanobacteria for photosynthetic isoprenoids production as well as carbon molar basis comparisons with heterotrophic isoprenoids production in engineered Escherichia coli.

Engineering cyanobacteria for production of terpenoids

This review summarizes recent advances in cyanobacterial terpenoid production. The challenges and opportunities of improving terpenoid production by cyanobacteria are discussed. Terpenoids are a diverse group of natural products with a variety of commercial applications. With recent advances in synthetic biology and metabolic engineering, microbial terpenoid synthesis is being viewed as a feasible approach for industrial production. Among different microbial hosts, cyanobacteria have the potential of sustainable production of terpenoids using light and CO₂. Terpene synthases and the precursor pathways have been expressed in cyanobacteria for enhanced production of various terpene hydrocarbons, including isoprene, limonene, β-phellandrene, and farnesene. However, the productivities need to be further improved for commercial production. Many barriers remain to be overcome in order to efficiently convert CO₂ to terpenoids. In this review, we will summarize recent efforts on photosynthetic production of terpenoids and discuss the challenges and opportunities of engineering cyanobacteria for terpenoid bioproduction.

Terpenoid Metabolic Engineering in Photosynthetic Microorganisms

Terpenoids are a group of natural products that have a variety of roles, both essential and non-essential, in metabolism and in biotic and abiotic interactions, as well as commercial applications such as pharmaceuticals, food additives, and chemical feedstocks. Economic viability for commercial applications is commonly not achievable by using natural source organisms or chemical synthesis. Engineered bio-production in suitable heterologous hosts is often required to achieve commercial viability. However, our poor understanding of regulatory mechanisms and other biochemical processes makes obtaining efficient conversion yields from feedstocks challenging. Moreover, production from carbon dioxide via photosynthesis would significantly increase the environmental and potentially the economic credentials of these processes by disintermediating biomass feedstocks. In this paper, we briefly review terpenoid metabolism, outline some recent advances in terpenoid metabolic engineering, and discuss why photosynthetic unicellular organisms-such as algae and cyanobacteria-might be preferred production platforms for the expression of some of the more challenging terpenoid pathways.

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.