Combinatorial analysis of enzymatic bottlenecks of l-tyrosine pathway by p-coumaric acid production in Saccharomyces cerevisiae

Improvement of Chalcone Synthase Activity and High-Efficiency Fermentative Production of (2S)-Naringenin via In Vivo Biosensor-Guided Directed Evolution

Chalcone synthase (CHS) catalyzes the rate-limiting step of (2S)-naringenin (the essential flavonoid skeleton) biosynthesis. Improving the activity of the CHS by protein engineering enhances (2S)-naringenin production by microbial fermentation and can facilitate the production of valuable flavonoids. A (2S)-naringenin biosensor based on the TtgR operon was constructed in Escherichia coli and its detection range was expanded by promoter optimization to 0-300 mg/L, the widest range for (2S)-naringenin reported. The high-throughput screening scheme for CHS was established based on this biosensor. A mutant, SjCHS1S208N with a 2.34-fold increase in catalytic activity, was discovered by directed evolution and saturation mutagenesis. A pathway for de novo biosynthesis of (2S)-naringenin by SjCHS1S208N was constructed in Saccharomyces cerevisiae, combined with CHS precursor pathway optimization, increasing the (2S)-naringenin titer by 65.34% compared with the original strain. Fed-batch fermentation increased the titer of (2S)-naringenin to 2513 ± 105 mg/L, the highest reported so far. These findings will facilitate efficient flavonoid biosynthesis and further modification of the CHS in the future.

A comprehensive review and comparison of L-tryptophan biosynthesis in Saccharomyces cerevisiae and Escherichia coli

L-tryptophan and its derivatives are widely used in the chemical, pharmaceutical, food, and feed industries. Microbial fermentation is the most commonly used method to produce L-tryptophan, which calls for an effective cell factory. The mechanism of L-tryptophan biosynthesis in Escherichia coli, the widely used producer of L-tryptophan, is well understood. Saccharomyces cerevisiae also plays a significant role in the industrial production of biochemicals. Because of its robustness and safety, S. cerevisiae is favored for producing pharmaceuticals and food-grade biochemicals. However, the biosynthesis of L-tryptophan in S. cerevisiae has been rarely summarized. The synthetic pathways and engineering strategies of L-tryptophan in E. coli and S. cerevisiae have been reviewed and compared in this review. Furthermore, the information presented in this review pertains to the existing understanding of how L-tryptophan affects S. cerevisiae's stress fitness, which could aid in developing a novel plan to produce more resilient industrial yeast and E. coli cell factories.

Microbial synthesis of the plant natural product precursor p-coumaric acid with Corynebacterium glutamicum

BACKGROUND

Phenylpropanoids such as p-coumaric acid represent important precursors for the synthesis of a broad range of plant secondary metabolites including stilbenoids, flavonoids, and lignans, which are of pharmacological interest due to their health-promoting properties. Although extraction from plant material or chemical synthesis is possible, microbial synthesis of p-coumaric acid from glucose has the advantage of being less expensive and more resource efficient. In this study, Corynebacterium glutamicum was engineered for the production of the plant polyphenol precursor p-coumaric acid from glucose.

RESULTS

Heterologous expression of the tyrosine ammonia-lyase encoding gene from Flavobacterium johnsoniae enabled the conversion of endogenously provided tyrosine to p-coumaric acid. Product consumption was avoided by abolishing essential reactions of the phenylpropanoid degradation pathway. Accumulation of anthranilate as a major byproduct was eliminated by reducing the activity of anthranilate synthase through targeted mutagenesis to avoid tryptophan auxotrophy. Subsequently, the carbon flux into the shikimate pathway was increased, phenylalanine biosynthesis was reduced, and phosphoenolpyruvate availability was improved to boost p-coumaric acid accumulation. A maximum titer of 661 mg/L p-coumaric acid (4 mM) in defined mineral medium was reached. Finally, the production strain was utilized in co-cultivations with a C. glutamicum strain previously engineered for the conversion of p-coumaric acid into the polyphenol resveratrol. These co-cultivations enabled the synthesis of 31.2 mg/L (0.14 mM) resveratrol from glucose without any p-coumaric acid supplementation.

CONCLUSIONS

The utilization of a heterologous tyrosine ammonia-lyase in combination with optimization of the shikimate pathway enabled the efficient production of p-coumaric acid with C. glutamicum. Reducing the carbon flux into the phenylalanine and tryptophan branches was the key to success along with the introduction of feedback-resistant enzyme variants.

Efficient Secretory Production of δ-Tocotrienol by Combining Pathway Modularization and Transportation Engineering

The vitamin E component δ-tocotrienol has shown impressive activities in radioprotection, neuroprotection, and cholesterol reduction. Its production is limited by the low content in plants and difficulty in separation from other tocotrienols. Fermentative production using a microbial cell factory that exclusively produces and secretes δ-tocotrienol is a promising alternative approach. Assembly of the δ-tocotrienol synthetic pathway in Saccharomyces cerevisiae followed by comprehensive pathway engineering led to the production of 73.45 mg/L δ-tocotrienol. Subsequent addition of 2-hydroxypropyl-β-cyclodextrin (CD) and overexpression of the transcription factor PDR1 significantly elevated δ-tocotrienol titer to 241.7 mg/L (63.65 mg/g dry cell weight) in shake flasks, with 30.4% secreted. By properly adding CD and the in situ extractant olive oil, 181.12 mg/L of δ-tocotrienol was collected as an extracellular product, accounting for 85.6% of the total δ-tocotrienol production. This process provides not only a promising δ-tocotrienol cell factory but also insights into yeast engineering toward secretory production of other terpenoids.

Biological valorization of lignin to flavonoids

Lignin is the most affluent natural aromatic biopolymer on the earth, which is the promising renewable source for valuable products to promote the sustainability of biorefinery. Flavonoids are a class of plant polyphenolic secondary metabolites containing the benzene ring structure with various biological activities, which are largely applied in health food, pharmaceutical, and medical fields. Due to the aromatic similarity, microbial conversion of lignin derived aromatics to flavonoids could facilitate flavonoid biosynthesis and promote the lignin valorization. This review thereby prospects a novel valorization route of lignin to high-value natural products and demonstrates the potential advantages of microbial bioconversion of lignin to flavonoids. The biodegradation of lignin polymers is summarized to identify aromatic monomers as momentous precursors for flavonoid synthesis. The biosynthesis pathways of flavonoids in both plants and strains are introduced and compared. After that, the key branch points and important intermediates are clearly discussed in the biosynthesis pathways of flavonoids. Moreover, the most significant enzyme reactions including Claisen condensation, cyclization and hydroxylation are demonstrated in the biosynthesis pathways of flavonoids. Finally, current challenges and potential future strategies are also discussed for transforming lignin into various flavonoids. The holistic microbial conversion routes of lignin to flavonoids could make a sustainable production of flavonoids and improve the feasibility of lignin valorization.

A highly efficient transcriptome-based biosynthesis of non-ethanol chemicals in Crabtree negative Saccharomyces cerevisiae

BACKGROUND

Owing to the Crabtree effect, Saccharomyces cerevisiae produces a large amount of ethanol in the presence of oxygen and excess glucose, leading to a loss of carbon for the biosynthesis of non-ethanol chemicals. In the present study, the potential of a newly constructed Crabtree negative S. cerevisiae, as a chassis cell, was explored for the biosynthesis of various non-ethanol compounds.

RESULTS

To understand the metabolic characteristics of Crabtree negative S. cerevisiae sZJD-28, its transcriptional profile was compared with that of Crabtree positive S. cerevisiae CEN.PK113-11C. The reporter GO term analysis showed that, in sZJD-28, genes associated with translational processes were down-regulated, while those related to carbon metabolism were significantly up-regulated. To verify a potential increase in carbon metabolism for the Crabtree negative strain, the production of non-ethanol chemicals, derived from different metabolic nodes, was then undertaken for both sZJD-28 and CEN.PK113-11C. At the pyruvate node, production of 2,3-butanediol and lactate in sZJD-28-based strains was remarkably higher than that of CEN.PK113-11C-based ones, representing 16.8- and 1.65-fold increase in titer, as well as 4.5-fold and 0.65-fold increase in specific titer (mg/L/OD), respectively. Similarly, for shikimate derived p-coumaric acid, the titer of sZJD-28-based strain was 0.68-fold higher than for CEN.PK113-11C-based one, with a 0.98-fold increase in specific titer. While farnesene and lycopene, two acetoacetyl-CoA derivatives, showed 0.21- and 1.88-fold increases in titer, respectively. From malonyl-CoA, the titer of 3-hydroxypropionate and fatty acids in sZJD-28-based strains were 0.19- and 0.76-fold higher than that of CEN.PK113-11C-based ones, respectively. In fact, yields of products also improved by the same fold due to the absence of residual glucose. Fed-batch fermentation further showed that the titer of free fatty acids in sZJD-28-based strain 28-FFA-E reached 6295.6 mg/L with a highest reported specific titer of 247.7 mg/L/OD in S. cerevisiae.

CONCLUSIONS

Compared with CEN.PK113-11C, the Crabtree negative sZJD-28 strain displayed a significantly different transcriptional profile and obvious advantages in the biosynthesis of non-ethanol chemicals due to redirected carbon and energy sources towards metabolite biosynthesis. The findings, therefore, suggest that a Crabtree negative S. cerevisiae strain could be a promising chassis cell for the biosynthesis of various chemicals.

De novo biosynthesis of p-coumaric acid and caffeic acid from carboxymethyl-cellulose by microbial co-culture strategy

Background

Aromatic compounds, such as p-coumaric acid (p-CA) and caffeic acid, are secondary metabolites of various plants, and are widely used in diet and industry for their biological activities. In addition to expensive and unsustainable methods of plant extraction and chemical synthesis, the strategy for heterologous synthesis of aromatic compounds in microorganisms has received much attention. As the most abundant renewable resource in the world, lignocellulose is an economical and environmentally friendly alternative to edible, high-cost carbon sources such as glucose.

Results

In the present study, carboxymethyl-cellulose (CMC) was utilized as the sole carbon source, and a metabolically engineered Saccharomyces cerevisiae strain SK10-3 was co-cultured with other recombinant S. cerevisiae strains to achieve the bioconversion of value-added products from CMC. By optimizing the inoculation ratio, interval time, and carbon source content, the final titer of p-CA in 30 g/L CMC medium was increased to 71.71 mg/L, which was 155.9-fold higher than that achieved in mono-culture. The de novo biosynthesis of caffeic acid in the CMC medium was also achieved through a three-strain co-cultivation. Caffeic acid production was up to 16.91 mg/L after optimizing the inoculation ratio of these strains.

Conclusion

De novo biosynthesis of p-CA and caffeic acid from lignocellulose through a co-cultivation strategy was achieved for the first time. This study provides favorable support for the biosynthesis of more high value-added products from economical substrates. In addition, the multi-strain co-culture strategy can effectively improve the final titer of the target products, which has high application potential in the field of industrial production.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12934-022-01805-5.

Saccharomyces cerevisiae as host for the recombinant production of polyketides and nonribosomal peptides

As a robust, fast growing and genetically tractable organism, the budding yeast Saccharomyces cerevisiae is one of the most widely used hosts in biotechnology. Its applications range from the manufacturing of vaccines and hormones to bulk chemicals and biofuels. In recent years, major efforts have been undertaken to expand this portfolio to include structurally complex natural products, such as polyketides and nonribosomally synthesized peptides. These compounds often have useful pharmacological properties, which make them valuable drugs for the treatment of infectious diseases, cancer, or autoimmune disorders. In nature, polyketides and nonribosomal peptides are generated by consecutive condensation reactions of short chain acyl-CoAs or amino acids, respectively, with the substrates and reaction intermediates being bound to large, multidomain enzymes. For the reconstitution of these multistep catalytic processes, the enzymatic assembly lines need to be functionally expressed and the required substrates must be supplied in reasonable quantities. Furthermore, the production hosts need to be protected from the toxicity of the biosynthetic products. In this review, we will summarize and evaluate the status quo regarding the heterologous production of polyketides and nonribosomal peptides in S. cerevisiae. Based on a comprehensive literature analysis, prerequisites for a successful pathway reconstitution could be deduced, as well as recurring bottlenecks in this microbial host.

Metabolic engineering of Saccharomyces cerevisiae for enhanced production of caffeic acid

As a natural phenolic acid product of plant source, caffeic acid displays diverse biological activities and acts as an important precursor for the synthesis of other valuable compounds. Limitations in chemical synthesis or plant extraction of caffeic acid trigger interest in its microbial biosynthesis. Recently, Saccharomyces cerevisiae has been reported for the biosynthesis of caffeic acid via episomal plasmid-mediated expression of pathway genes. However, the production was far from satisfactory and even relied on the addition of precursor. In this study, we first established a controllable and stable caffeic acid pathway by employing a modified GAL regulatory system to control the genome-integrated pathway genes in S. cerevisiae and realized biosynthesis of 222.7 mg/L caffeic acid. Combinatorial engineering strategies including eliminating the tyrosine-induced feedback inhibition, deleting genes involved in competing pathways, and overexpressing rate-limiting enzymes led to about 2.6-fold improvement in the caffeic acid production, reaching up to 569.0 mg/L in shake-flask cultures. To our knowledge, this is the highest ever reported titer of caffeic acid synthesized by engineered yeast. This work showed the prospect for microbial biosynthesis of caffeic acid and laid the foundation for constructing biosynthetic pathways of its derived metabolites. KEY POINTS: Genomic integration of ORgTAL, OHpaB, and HpaC for caffeic acid production in yeast. Feedback inhibition elimination and Aro10 deletion improved caffeic acid production. The highest ever reported titer (569.0 mg/L) of caffeic acid synthesized by yeast.

Engineering of Saccharomyces cerevisiae for anthranilate and methyl anthranilate production

Background

Anthranilate is a platform chemical used by the industry in the synthesis of a broad range of high-value products, such as dyes, perfumes and pharmaceutical compounds. Currently anthranilate is produced via chemical synthesis from non-renewable resources. Biological synthesis would allow the use of renewable carbon sources and avoid accumulation of toxic by-products. Microorganisms produce anthranilate as an intermediate in the tryptophan biosynthetic pathway. Several prokaryotic microorganisms have been engineered to overproduce anthranilate but attempts to engineer eukaryotic microorganisms for anthranilate production are scarce.

Results

We subjected Saccharomyces cerevisiae, a widely used eukaryotic production host organism, to metabolic engineering for anthranilate production. A single gene knockout was sufficient to trigger anthranilate accumulation both in minimal and SCD media and the titer could be further improved by subsequent genomic alterations. The effects of the modifications on anthranilate production depended heavily on the growth medium used. By growing an engineered strain in SCD medium an anthranilate titer of 567.9 mg l⁻¹ was obtained, which is the highest reported with an eukaryotic microorganism. Furthermore, the anthranilate biosynthetic pathway was extended by expression of anthranilic acid methyltransferase 1 from Medicago truncatula. When cultivated in YPD medium, this pathway extension enabled production of the grape flavor compound methyl anthranilate in S. cerevisiae at 414 mg l⁻¹.

Conclusions

In this study we have engineered metabolism of S. cerevisiae for improved anthranilate production. The resulting strains may serve as a basis for development of efficient production host organisms for anthranilate-derived compounds. In order to demonstrate suitability of the engineered S. cerevisiae strains for production of such compounds, we successfully extended the anthranilate biosynthesis pathway to synthesis of methyl anthranilate.

Improving the Level of the Tyrosine Biosynthesis Pathway in Saccharomyces cerevisiae through HTZ1 Knockout and Atmospheric and Room Temperature Plasma (ARTP) Mutagenesis

In recent years, many studies have been conducted on the expression of multiple aromatic compounds by Saccharomyces cerevisiae. The concentration of l-tyrosine, as a precursor of such valuable compounds, is crucial for the biosynthesis of aromatic metabolites. In this study, a novel function of HTZ1 was found to be related to tyrosine biosynthesis, which has not yet been reported. Knockout of this gene could significantly improve the ability of yeast cells to synthesize tyrosine, and its p-coumaric acid (p-CA) titer was approximately 3.9-fold higher than that of the wild-type strain BY4742. Subsequently, this strain was selected for random mutagenesis through an emerging mutagenesis technique, namely, atmospheric and room temperature plasma (ARTP). After two rounds of mutagenesis, five tyrosine high-producing mutants were obtained. The highest production of p-CA was 7.6-fold higher than that of the wild-type strain. Finally, transcriptome data of the htz1Δ strain and the five mutants were analyzed. The genome of mutagenic strains was also resequenced to reveal the mechanism underlying the high titer of tyrosine. This system of target engineering combined with random mutagenesis to screen excellent mutants provides a new basis for synthetic biology.

Rational engineering of Kluyveromyces marxianus to create a chassis for the production of aromatic products

BACKGROUND

The yeast Kluyveromyces marxianus offers unique potential for industrial biotechnology because of useful features like rapid growth, thermotolerance and a wide substrate range. As an emerging alternative platform, K. marxianus requires the development and validation of metabolic engineering strategies to best utilise its metabolism as a basis for bio-based production.

RESULTS

To illustrate the synthetic biology strategies to be followed and showcase its potential, we describe a comprehensive approach to rationally engineer a metabolic pathway in K. marxianus. We use the phenylalanine biosynthetic pathway both as a prototype and because phenylalanine is a precursor for commercially valuable secondary metabolites. First, we modify and overexpress the pathway to be resistant to feedback inhibition so as to overproduce phenylalanine de novo from synthetic minimal medium. Second, we assess native and heterologous means to increase precursor supply to the biosynthetic pathway. Finally, we eliminate branch points and competing reactions in the pathway and rebalance precursors to redirect metabolic flux to a specific product, 2-phenylethanol (2-PE). As a result, we are able to construct robust strains capable of producing over 800 mg L-1 2-PE from minimal medium.

CONCLUSIONS

The strains we constructed are a promising platform for the production of aromatic amino acid-based biochemicals, and our results illustrate challenges with attempting to combine individually beneficial modifications in an integrated platform.

Engineering the oleaginous yeast Yarrowia lipolytica for high-level resveratrol production

Resveratrol is a plant secondary metabolite with multiple health-beneficial properties. Microbial production of resveratrol in model microorganisms requires extensive engineering to reach commercially viable levels. Here, we explored the potential of the non-conventional yeast Yarrowia lipolytica to produce resveratrol and several other shikimate pathway-derived metabolites (p-coumaric acid, cis,cis-muconic acid, and salicylic acid). The Y. lipolytica strain expressing a heterologous pathway produced 52.1 ± 1.2 mg/L resveratrol in a small-scale cultivation. The titer increased to 409.0 ± 1.2 mg/L when the strain was further engineered with feedback-insensitive alleles of the key genes in the shikimate pathway and with five additional copies of the heterologous biosynthetic genes. In controlled fed-batch bioreactor, the strain produced 12.4 ± 0.3 g/L resveratrol, the highest reported titer to date for de novo resveratrol production, with a yield on glucose of 54.4 ± 1.6 mg/g and a productivity of 0.14 ± 0.01 g/L/h. The study showed that Y. lipolytica is an attractive host organism for the production of resveratrol and possibly other shikimate-pathway derived metabolites.

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Oleaginous yeast Y. lipolytica was engineered for production of aromatic compounds.

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High resveratrol production required increased activities of Aro4p and Aro7p.

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Multiple integration of resveratrol biosynthetic genes improved production.

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Fed-batch fermentation enabled de novo production of 12.4 g/L resveratrol.

Optimization of the l-tyrosine metabolic pathway in Saccharomyces cerevisiae by analyzing p-coumaric acid production

In this study, we applied a series of genetic modifications to wild-type S. cerevisiae strain BY4741 to address the bottlenecks in the l-tyrosine pathway. A tyrosine ammonia-lyase (TAL) gene from Rhodobacter capsulatus, which can catalyze conversion of l-tyrosine into p-coumaric acid, was overexpressed to facilitate the analysis of l-tyrosine and test the strain's capability to synthesize heterologous derivatives. First, we enhanced the supply of precursors by overexpressing transaldolase gene TAL1, enolase II gene ENO2, and pentafunctional enzyme gene ARO1 resulting in a 1.55-fold increase in p-coumaric acid production. Second, feedback inhibition of 3-deoxy-d-arabino-heptulosonate-7-phosphate synthase and chorismate mutase was relieved by overexpressing the mutated feedback-resistant ARO4 ^K229L and ARO7 ^G141S , and a 3.61-fold improvement of p-coumaric acid production was obtained. Finally, formation of byproducts was decreased by deleting pyruvate decarboxylase gene PDC5 and phenylpyruvate decarboxylase gene ARO10, and p-coumaric acid production was increased 2.52-fold. The best producer-when TAL1, ENO2, ARO1, ARO4 ^K229L , ARO7 ^G141S , and TAL were overexpressed, and PDC5 and ARO10 were deleted-increased p-coumaric acid production by 14.08-fold (from 1.4 to 19.71 mg L^-1). Our study provided a valuable insight into the optimization of l-tyrosine metabolic pathway.

De Novo Biosynthesis of Caffeic Acid from Glucose by Engineered Saccharomyces cerevisiae

Caffeic acid is a plant phenolic compound possessing extensive pharmacological activities. Here, we identified that p-coumaric acid 3-hydroxylase from Arabidopsis thaliana was capable of hydroxylating p-coumaric acid to form caffeic acid in Saccharomyces cerevisiae. Then, we introduced a combined caffeic acid biosynthetic pathway into S. cerevisiae and obtained 0.183 mg L^-1 caffeic acid from glucose. Next we improved the tyrosine biosynthesis in S. cerevisiae by blocking the pathway flux to aromatic alcohols and eliminating the tyrosine-induced feedback inhibition resulting in caffeic acid production of 2.780 mg L^-1. Finally, the medium was optimized, and the highest caffeic acid production obtained was 11.432 mg L^-1 in YPD medium containing 4% glucose. This study opens a route to produce caffeic acid from glucose in S. cerevisiae and establishes a platform for the biosynthesis of caffeic acid derived metabolites.

Bioprocess Optimization for the Production of Aromatic Compounds With Metabolically Engineered Hosts: Recent Developments and Future Challenges

The most common route to produce aromatic chemicals - organic compounds containing at least one benzene ring in their structure - is chemical synthesis. These processes, usually starting from an extracted fossil oil molecule such as benzene, toluene, or xylene, are highly environmentally unfriendly due to the use of non-renewable raw materials, high energy consumption and the usual production of toxic by-products. An alternative way to produce aromatic compounds is extraction from plants. These extractions typically have a low yield and a high purification cost. This motivates the search for alternative platforms to produce aromatic compounds through low-cost and environmentally friendly processes. Microorganisms are able to synthesize aromatic amino acids through the shikimate pathway. The construction of microbial cell factories able to produce the desired molecule from renewable feedstock becomes a promising alternative. This review article focuses on the recent advances in microbial production of aromatic products, with a special emphasis on metabolic engineering strategies, as well as bioprocess optimization. The recent combination of these two techniques has resulted in the development of several alternative processes to produce phenylpropanoids, aromatic alcohols, phenolic aldehydes, and others. Chemical species that were unavailable for human consumption due to the high cost and/or high environmental impact of their production, have now become accessible.

Comparative transcriptome analysis of genomic region deletion strain with enhanced L-tyrosine production in Saccharomyces cerevisiae

OBJECTIVE

To determine the effect of large genomic region deletion in a Saccharomyces cerevisiae strain on tyrosine yield and to identify new genetic modification targets through transcriptome analysis.

RESULTS

TAL was used to produce p-coumaric acid (p-CA) from tyrosine to quantity tyrosine yield. S. cerevisiae mutant strain NK14 with deletion of a 23.8 kb genomic region was identified to have p-CA production of 10.3 mg L^- 1, while the wild-type strain BY4741 had p-CA production of 1.06 mg L^- 1. Analysis of growth patterns and stress tolerance showed that the deletion did not affect the growth phenotype of NK14. Transcriptome analysis suggested that, compared to BY4741, genes related to glycolysis (ENO2, TKL1) and the tyrosine pathway (ARO1, ARO2, ARO4, ARO7, TYR1) were upregulated in NK14 at different levels. Besides genes related to the tyrosine biosynthetic pathway, amino acid transporters (AVT6, VBA5, THI72) and transcription factor (ARO80) also showed changes in transcription levels.

CONCLUSIONS

We developed a strain with improved tyrosine yield and identified new genetic modification candidates for tyrosine production.

Rewiring carbon metabolism in yeast for high level production of aromatic chemicals

The production of bioactive plant compounds using microbial hosts is considered a safe, cost-competitive and scalable approach to their production. However, microbial production of some compounds like aromatic amino acid (AAA)-derived chemicals, remains an outstanding metabolic engineering challenge. Here we present the construction of a Saccharomyces cerevisiae platform strain able to produce high levels of p-coumaric acid, an AAA-derived precursor for many commercially valuable chemicals. This is achieved through engineering the AAA biosynthesis pathway, introducing a phosphoketalose-based pathway to divert glycolytic flux towards erythrose 4-phosphate formation, and optimizing carbon distribution between glycolysis and the AAA biosynthesis pathway by replacing the promoters of several important genes at key nodes between these two pathways. This results in a maximum p-coumaric acid titer of 12.5 g L⁻¹ and a maximum yield on glucose of 154.9 mg g⁻¹.

Microbial production of aromatic amino acid (AAA)-derived chemicals remains an outstanding metabolic engineering challenge. Here, the authors engineer baker’s yeast for high levels p-coumaric acid production by rewiring the central carbon metabolism and channeling more flux to the AAA biosynthetic pathway.

Rational Engineering of Chorismate-Related Pathways in Saccharomyces cerevisiae for Improving Tyrosol Production

Tyrosol is extensively used in the pharmaceutical industry as an important natural product from plants. In this study, an exogenous pathway involved in catalyzing tyrosine to tyrosol was introduced into Saccharomyces cerevisiae. Furthermore, The pyruvate decarboxylase gene pdc1 was deleted to redirect the flux distribution at the pyruvate node, and a bifunctional NAD⁺-dependent fused chorismate mutase/prephenate dehydrogenase from E. coli (EcTyrA) and its' tyrosine inhibition resistant mutant (EcTyrA^M53I/A354V) were heterologously expression in S. cerevisiae to tuning up the chorismate metabolism effectively directed the metabolic flux toward tyrosol production. Finally, the tyrosol yield of the engineered strain GFT-4 was improved to 126.74 ± 6.70 mg/g DCW at 48 h, increased 440 times compared with that of the control strain GFT-0 (0.28 ± 0.01 mg/g DCW). The new synergetic engineering strategy developed in this study can be further applied to increase the production of high value-added aromatic compounds derived from aromatic amino acid or shikimate in S. cerevisiae.

Metabolic engineering strategies for improvement of ethanol production in cellulolytic Saccharomyces cerevisiae

As a traditional ethanol-producing microorganism, Saccharomyces cerevisiae is an ideal host for consolidated bioprocessing. However, expression of heterologous cellulase increases the metabolic burden in yeast, which results in low cellulase activity and poor cellulose degradation efficiency. In this study, cellulase-expressing yeast strains that could efficiently degrade different cellulosic substrates were created by optimizing cellulase ratios through a POT1-mediated δ-integration strategy. Metabolic engineering strategies, including optimization of codon usage, promoter and signal peptide, were also included in this system. We also confirmed that heterologous cellulase expression in cellulosic yeast induced the unfolded protein response. To enhance protein folding capacity, the endoplasmic reticulum chaperone protein BiP and the disulfide isomerase Pdi1p were overexpressed, and the Golgi membrane protein Ca2+/Mn2+ ATPase Pmr1p was disrupted to decrease the glycosylation of cellulase. The resultant strain, SK18-3, could produce 5.4 g L-1 ethanol with carboxymethyl-cellulose. Strain SK12-50 achieved 4.7 g L-1 ethanol production with phosphoric acid swollen cellulose hydrolysis. When Avicel was used as the substrate, 3.8 g L-1 ethanol (75% of the theoretical maximum yield) was produced in SK13-34. This work will significantly increase our knowledge of how to engineer optimal yeast strains for biofuel production from cellulosic biomass.

Potential Natural Food Preservatives and Their Sustainable Production in Yeast: Terpenoids and Polyphenols

Terpenoids and polyphenols are high-valued plant secondary metabolites. Their high antimicrobial activities demonstrate their huge potential as natural preservatives in the food industry. With the rapid development of metabolic engineering, it has become possible to realize large-scale production of non-native terpenoids and polyphenols by using the generally recognized as safe (GRAS) strain, Saccharomyces cerevisiae, as a cell factory. This review will summarize the major terpenoid and polyphenol compounds with high antimicrobial properties, describe their native metabolic pathways as well as antimicrobial mechanisms, and highlight current progress on their heterologous biosynthesis in S. cerevisiae. Current challenges and perspectives for the sustainable production of terpenoid and polyphenol as natural food preservatives via S. cerevisiae will also be discussed.