Biosynthesis of pinene in purple non-sulfur photosynthetic bacteria

Application of valencene and prospects for its production in engineered microorganisms

Valencene, a sesquiterpene with the odor of sweet and fresh citrus, is widely used in the food, beverage, flavor and fragrance industry. Valencene is traditionally obtained from citrus fruits, which possess low concentrations of this compound. In the past decades, the great market demand for valencene has attracted considerable attention from researchers to develop novel microbial cell factories for more efficient and sustainable production modes. This review initially discusses the biosynthesis of valencene in plants, and summarizes the current knowledge of the key enzyme valencene synthase in detail. In particular, we highlight the heterologous production of valencene in different hosts including bacteria, fungi, microalgae and plants, and focus on describing the engineering strategies used to improve valencene production. Finally, we propose potential engineering directions aiming to further increase the production of valencene in microorganisms.

Recent Advances and Multiple Strategies of Monoterpenoid Overproduction in Saccharomyces cerevisiae and Yarrowia lipolytica

Monoterpenoids are an important subclass of terpenoids that play important roles in the energy, cosmetics, pharmaceuticals, and fragrances fields. With the development of biotechnology, microbial synthesis of monoterpenoids has received great attention. Yeasts such Saccharomyces cerevisiae and Yarrowia lipolytica are emerging as potential hosts for monoterpenoids production because of unique advantages including rapid growth cycles, mature gene editing tools, and clear genetic background. Recently, advancements in metabolic engineering and fermentation engineering have significantly enhanced the accumulation of monoterpenoids in cell factories. First, this review introduces the biosynthetic pathway of monoterpenoids and comprehensively summarizes the latest production strategies, which encompass enhancing precursor flux, modulating the expression of rate-limited enzymes, suppressing competitive pathway flux, mitigating cytotoxicity, optimizing substrate utilization, and refining the fermentation process. Subsequently, this review introduces four representative monoterpenoids. Finally, we outline the future prospects for efficient construction cell factories tailored for the production of monoterpenoids and other terpenoids.

Recent developments in enzymatic and microbial biosynthesis of flavor and fragrance molecules

Flavors and fragrances are an important class of specialty chemicals for which interest in biomanufacturing has risen during recent years. These naturally occurring compounds are often amenable to biosynthesis using purified enzyme catalysts or metabolically engineered microbial cells in fermentation processes. In this review, we provide a brief overview of the categories of molecules that have received the greatest interest, both academically and industrially, by examining scholarly publications as well as patent literature. Overall, we seek to highlight innovations in the key reaction steps and microbial hosts used in flavor and fragrance manufacturing.

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.

Multidimensional optimization for accelerating light-powered biocatalysis in Rhodopseudomonas palustris

BACKGROUND

Whole-cell biocatalysis has been exploited to convert a variety of substrates into high-value bulk or chiral fine chemicals. However, the traditional whole-cell biocatalysis typically utilizes the heterotrophic microbes as the biocatalyst, which requires carbohydrates to power the cofactor (ATP, NAD (P)H) regeneration.

RESULTS

In this study, we sought to harness purple non-sulfur photosynthetic bacterium (PNSB) as the biocatalyst to achieve light-driven cofactor regeneration for cascade biocatalysis. We substantially improved the performance of Rhodopseudomonas palustris-based biocatalysis using a highly active and conditional expression system, blocking the side-reactions, controlling the feeding strategy, and attenuating the light shading effect. Under light-anaerobic conditions, we found that 50 mM ferulic acid could be completely converted to vanillyl alcohol using the recombinant strain with 100% efficiency, and > 99.9% conversion of 50 mM p-coumaric acid to p-hydroxybenzyl alcohol was similarly achieved. Moreover, we examined the isoprenol utilization pathway for pinene synthesis and 92% conversion of 30 mM isoprenol to pinene was obtained.

CONCLUSIONS

Taken together, these results suggested that R. palustris could be a promising host for light-powered biotransformation, which offers an efficient approach for synthesizing value-added chemicals in a green and sustainable manner.

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.

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.

Alternate gene expression profiling of monoterpenes in Hymenocrater longiflorus as a novel pharmaceutical plant under water deficit

Hymenocrater longiflorus (surahalala) is a wild plant species with potential pharmaceutical and ornamental interest. To date, the genomics of this plant is unknown and the gene expression profiling of the genes related to its metabolite has never been studied before. In order to study the responses of in vitro-grown surahalala plants to abiotic stresses and the differential expression of the genes related to its essential oils under exogenous proline application; three levels of PEG600 (0, 10, and 20%) and five levels of proline (0, 5, 10, 15, and 20 µm) were combined in the culture media. Thus, water deficit increased oxidants levels and decreased fresh weight of surahalala tissues, whereas addition of proline up to 15 µm was able to relatively compensate the negative effect of water deficit. Contrarily, high proline level (20 µm) had a negative effect on surahalala plants probably due to the stress simulation (nutrition) under high proline concentration. In addition, the best combination for achieving highest essential oils content was 10 µm proline plus 10% PEG. The expressional profiling of the genes TPS27, L3H, TPS2, TPS1, OMT and GDH3 were successfully carried out and their involvement in 1,8-cineole, carvone, α-pinene, thymol, estragole and β-Citronellol biosynthesis, respectively, was verified. In addition, our results indicated that these genes could also be involved in the synthesis of other metabolites under water deficit condition.

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.

Photosynthetic Conversion of CO2 Into Pinene Using Engineered Synechococcus sp. PCC 7002

Metabolic engineering of cyanobacteria has received much attention as a sustainable strategy to convert CO₂ to various longer carbon chain fuels. Pinene has become increasingly attractive since pinene dimers contain high volumetric energy and have been proposed to act as potential aircraft fuels. However, cyanobacteria cannot directly convert geranyl pyrophosphate into pinene due to the lack of endogenous pinene synthase. Herein, we integrated the gene encoding Abies grandis pinene synthase into the model cyanobacterium Synechococcus sp. PCC 7002 through homologous recombination. The genetically modified cyanobacteria achieved a pinene titer of 1.525 ± 0.l45 mg L^-1 in the lab-scale tube photobioreactor with CO₂ aeration. Specifically, the results showed a mixture of α- and β-pinene (∼33:67 ratio). The ratio of β-pinene in the product was significantly increased compared with that previously reported in the engineered Escherichia coli. Furthermore, we investigated the photoautotrophic growth performances of Synechococcus overlaid with different concentrations of dodecane. The work demonstrates that the engineered Synechococcus is a suitable potential platform for β-pinene production.

Screening and engineering of high-activity promoter elements through transcriptomics and red fluorescent protein visualization in Rhodobacter sphaeroides

The versatile photosynthetic α-proteobacterium Rhodobacter sphaeroides, has recently been extensively engineered as a novel microbial cell factory (MCF) to produce pharmaceuticals, nutraceuticals, commodity chemicals and even hydrogen. However, there are no well-characterized high-activity promoters to modulate gene transcription during the engineering of R. sphaeroides. In this study, several native promoters from R. sphaeroides JDW-710 (JDW-710), an industrial strain producing high levels of co-enzyme Q₁₀ (Q₁₀) were selected on the basis of transcriptomic analysis. These candidate promoters were then characterized by using gusA as a reporter gene. Two native promoters, P_{rsp_7571} and P_{rsp_6124}, showed 620% and 800% higher activity, respectively, than the tac promoter, which has previously been used for gene overexpression in R. sphaeroides. In addition, a P_{rsp_7571}-derived synthetic promoter library with strengths ranging from 54% to 3200% of that of the tac promoter, was created on the basis of visualization of red fluorescent protein (RFP) expression in R. sphaeroides. Finally, as a demonstration, the synthetic pathway of Q₁₀ was modulated by the selected promoter T334* in JDW-710; the Q₁₀ yield in shake-flasks increased 28% and the production reached 226 mg/L. These well-characterized promoters should be highly useful in current synthetic biology platforms for refactoring the biosynthetic pathway in R. sphaeroides-derived MCFs.