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

Perspectives of cyanobacterial cell factories

Cyanobacteria are prokaryotic photosynthetic microorganisms that can generate, in addition to biomass, useful chemicals and proteins/enzymes, essentially from sunlight, carbon dioxide, and water. Selected aspects of cyanobacterial production (isoprenoids and high-value proteins) and scale-up methods suitable for product generation and downstream processing are addressed in this review. The work focuses on the challenge and promise of specialty chemicals and proteins production, with isoprenoid products and biopharma proteins as study cases, and the challenges encountered in the expression of recombinant proteins/enzymes, which underline the essence of synthetic biology with these microorganisms. Progress and the current state-of-the-art in these targeted topics are emphasized.

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.

Expression of PnSS Promotes Squalene and Oleanolic Acid (OA) Accumulation in Aralia elata via Methyl Jasmonate (MeJA) Induction

Aralia elata is an important herb due to the abundance of pentacyclic triterpenoid saponins whose important precursors are squalene and OA. Here, we found that MeJA treatment promoted both precursors accumulation, especially the latter, in transgenic A. elata, overexpressing a squalene synthase gene from Panax notoginseng(PnSS). In this study, Rhizobium-mediated transformation was used to express the PnSS gene. Gene expression analysis and high-performance liquid chromatography (HPLC) were used to identify the effect of MeJA on squalene and OA accumulation. The PnSS gene was isolated and expressed in A. elata. Transgenic lines showed a very high expression of the PnSS gene and farnesyl diphosphate synthase gene (AeFPS) and a slightly higher squalene content than the wild-type, but endogenous squalene synthase (AeSS), squalene epoxidase (AeSE), and β-amyrin synthase (Aeβ-AS) gene were decreased as well as OA content. Following one day of MeJA treatment, the expression levels of PeSS, AeSS, and AeSE genes increased significantly. On day 3, the maximum content of both products reached 17.34 and 0.70 mg·g^-1, which increased 1.39- and 4.90-fold than in the same lines without treatment. Transgenic lines expressing PnSS gene had a limited capability to promote squalene and OA accumulation. MeJA strongly activated their biosynthesis pathways, leading to enhance yield.

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.

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.

Exploring cyanobacterial diversity for sustainable biotechnology

Cyanobacteria are an evolutionarily ancient and diverse group of microorganisms. Their genetic diversity has 
allowed them to occupy and play vital roles in a wide range of ecological niches, from desert soil crusts to tropical oceans. Owing to bioprospecting efforts and the development of new platform technologies enabling their study and manipulation, our knowledge of cyanobacterial metabolism is rapidly expanding. This review explores our current understanding of the genetic and metabolic features of cyanobacteria, from the more established cyanobacterial model strains to the newly isolated/described species, particularly the fast-growing, highly productive, and genetically amenable strains, as promising chassis for renewable biotechnology. It also discusses emerging technologies for their study and manipulation, enabling researchers to harness the astounding diversity of the cyanobacterial genomic and metabolic treasure trove towards the establishment of a sustainable bioeconomy.

Microbial genetic engineering approach to replace shark livering for squalene

Squalene is generally sourced from the liver oil of deep sea sharks (Squalus spp.), in which it accounts for 40-70% of liver mass. To meet the growing demand for squalene because of its beneficial effects for human health, three to six million deep sea sharks are slaughtered each year, profoundly endangering marine ecosystems. To overcome this unsustainable practice, microbial sources of squalene might offer a viable alternative to plant- or animal-based squalene, although only a few microorganisms have been found that are capable of synthesizing up to 30% squalene of dry biomass by native biosynthetic pathways. These squalene biosynthetic pathways, on the other hand, can be genetically manipulated to transform microorganisms into 'cellular factories' for squalene overproduction.

Enhancing Squalene Production in Saccharomyces cerevisiae by Metabolic Engineering and Random Mutagenesis

Squalene is an important polyunsaturated triterpene with wide applications in the food, cosmetics, and pharmaceutical industries. Currently, the main method for squalene production is extraction from oil-producing plants, but the scale is limited. The microbial fermentation with Saccharomyces cerevisiae still needs improvement to be economically viable. This study aimed to improve squalene production by metabolic engineering and random mutagenesis. First, the mevalonate (MVA) pathway was enhanced, by integrating tHMG1 and IDI1 into multi-copy site Ty2. Subsequently, the ACL gene from Yarrowia lipolytica, encoding citrate lyase was introduced and the β-oxidation pathway was enhanced with multiple copies of key genes. In addition, a high throughput screening strategy based on Nile red staining was established for high squalene-producer screening. After treatment with ARTP mutagenesis, a higher-producing mutant was obtained, with squalene production enhanced by 18.4%. A two-stage fermentation of this mutant in a 5 L bioreactor produced 8.2 g/L of squalene. These findings may facilitate the development of industrial squalene production by fermentation and potentially, other terpenoids.

Expression of phenylalanine ammonia lyases in Synechocystis sp. PCC 6803 and subsequent improvements of sustainable production of phenylpropanoids

Background

Phenylpropanoids represent a diverse class of industrially important secondary metabolites, synthesized in plants from phenylalanine and tyrosine. Cyanobacteria have a great potential for sustainable production of phenylpropanoids directly from CO₂, due to their photosynthetic lifestyle with a fast growth compared to plants and the ease of generating genetically engineered strains. This study focuses on photosynthetic production of the starting compounds of the phenylpropanoid pathway, trans-cinnamic acid and p-coumaric acid, in the unicellular cyanobacterium Synechocystis sp. PCC 6803 (Synechocystis).

Results

A selected set of phenylalanine ammonia lyase (PAL) enzymes from different organisms was overexpressed in Synechocystis, and the productivities of the resulting strains compared. To further improve the titer of target compounds, we evaluated the use of stronger expression cassettes for increasing PAL protein levels, as well as knock-out of the laccase gene slr1573, as this was previously reported to prevent degradation of the target compounds in the cell. Finally, to investigate the effect of growth conditions on the production of trans-cinnamic and p-coumaric acids from Synechocystis, cultivation conditions promoting rapid, high density growth were tested. Comparing the different PALs, the highest specific titer was achieved for the strain AtC, expressing PAL from Arabidopsis thaliana. A subsequent increase of protein level did not improve the productivity. Production of target compounds in strains where the slr1573 laccase had been knocked out was found to be lower compared to strains with wild type background, and the Δslr1573 strains exhibited a strong phenotype of slower growth rate and lower pigment content. Application of a high-density cultivation system for the growth of production strains allowed reaching the highest total titers of trans-cinnamic and p-coumaric acids reported so far, at around 0.8 and 0.4 g L⁻¹, respectively, after 4 days.

Conclusions

Production of trans-cinnamic acid, unlike that of p-coumaric acid, is not limited by the protein level of heterologously expressed PAL in Synechocystis. High density cultivation led to higher titres of both products, while knocking out slr1573 did not have a positive effect on production. This work contributes to capability of exploiting the primary metabolism of cyanobacteria for sustainable production of plant phenylpropanoids.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12934-021-01735-8.

Diversifying Isoprenoid Platforms via Atypical Carbon Substrates and Non-model Microorganisms

Isoprenoid compounds are biologically ubiquitous, and their characteristic modularity has afforded products ranging from pharmaceuticals to biofuels. Isoprenoid production has been largely successful in Escherichia coli and Saccharomyces cerevisiae with metabolic engineering of the mevalonate (MVA) and methylerythritol phosphate (MEP) pathways coupled with the expression of heterologous terpene synthases. Yet conventional microbial chassis pose several major obstacles to successful commercialization including the affordability of sugar substrates at scale, precursor flux limitations, and intermediate feedback-inhibition. Now, recent studies have challenged typical isoprenoid paradigms by expanding the boundaries of terpene biosynthesis and using non-model organisms including those capable of metabolizing atypical C1 substrates. Conversely, investigations of non-model organisms have historically informed optimization in conventional microbes by tuning heterologous gene expression. Here, we review advances in isoprenoid biosynthesis with specific focus on the synergy between model and non-model organisms that may elevate the commercial viability of isoprenoid platforms by addressing the dichotomy between high titer production and inexpensive substrates.

Approaches in the photosynthetic production of sustainable fuels by cyanobacteria using tools of synthetic biology

Cyanobacteria, photosynthetic prokaryotic microorganisms having a simple genetic composition are the prospective photoautotrophic cell factories for the production of a wide range of biofuel molecules. The simple genetic composition of cyanobacteria allows effortless genetic manipulation which leads to increased research endeavors from the synthetic biology approach. Various unicellular model cyanobacterial strains like Synechocystis sp. PCC 6803 and Synechococcus elongatus PCC 7942 have been successfully engineered for biofuels generation. Improved development of synthetic biology tools, genetic modification methods and advancement in transformation techniques to construct a strain that can contain multiple foreign genes in a single operon have vastly expanded the functions that can be used for engineering photosynthetic cyanobacteria for the generation of various biofuel molecules. In this review, recent advancements and approaches in synthetic biology tools used for cyanobacterial genome editing have been discussed. Apart from this, cyanobacterial productions of various fuel molecules like isoprene, limonene, α-farnesene, squalene, alkanes, butanol, and fatty acids, which can be a substitute for petroleum and fossil fuels in the future, have been elaborated.

The beauty of biocatalysis: sustainable synthesis of ingredients in cosmetics

Covering: 2015 up to July 2021The market for cosmetics is consumer driven and the desire for green, sustainable and natural ingredients is increasing. The use of isolated enzymes and whole-cell organisms to synthesise these products is congruent with these values, especially when combined with the use of renewable, recyclable or waste feedstocks. The literature of biocatalysis for the synthesis of ingredients in cosmetics in the past five years is herein reviewed.

Sulfamethoxazole and tetracycline induced alterations in biomass, photosynthesis, lipid productivity, and proteomic expression of Synechocystis sp. PCC 6803

Since antibiotics show hormesis effects in cyanobacteria at the nanogram per liter concentration level, the possibility for two commonly used antibiotics (sulfamethoxazole and tetracycline) to increase lipid productivity in Synechocystis sp. PCC 6803 was assessed in the present study. The two target antibiotics significantly promoted (p < 0.05) the biofuel productivity of Synechocystis sp. PCC 6803 through the increase of both biomass and lipid content. Sulfamethoxazole and tetracycline significantly stimulated (p < 0.05) cyanobacterial growth by upregulating proteins related to cell differentiation, cell division, and gene expression; significantly enhanced (p < 0.05) the photosynthetic activity by upregulating photosynthesis-related proteins; and significantly increased (p < 0.05) the lipid content in cyanobacterial cells by downregulating carbohydrate catabolic proteins and carbohydrate transport proteins. Due to the altered expression pattern of biosynthesis-related proteins, the two antibiotics increased the proportion of monounsaturated fatty acids, while tetracycline reduced the proportions of saturated and polyunsaturated fatty acids. The changes in fatty acid composition may improve the combustion performance of biofuel. This study provided insights into the application of antibiotics in cyanobacteria-based biofuel production. Graphical abstract.