Porting the synthetic D‐glucaric acid pathway from Escherichia coli to Saccharomyces cerevisiae

An Eclectic Review on Dicarboxylic Acid Production Through Yeast Cell Factories and Its Industrial Prominence

Dicarboxylic acid (DCA) is a multifaceted chemical intermediate, recoursed to produce many industrially important products such as adhesives, plasticizers, lubricants, polymers, etc. To bypass the shortcomings of the chemical methods of synthesis of DCA and to reduce fossil fuel footprints, bio-based synthesis is gaining attention. In pursuit of an eco-friendly sustainable alternative method of DCA production, microbial cell factories, and renewable organic resources are gaining popularity. Among the plethora of microbial communities, yeast is being favored industrially compared to bacterial fermentation due to its hyperosmotic and low pH tolerance and flexibility for gene manipulations. By application of rapidly evolving genetic manipulation techniques, the bio-based DCA production could be made more precise and economical. To bridge the gap between supply and demand of DCA, many strategies are employed to improve the fermentation. This review briefly outlines the advancements in DCA production using yeast cell factories with the exemplification of strain improvement strategies.

Production of D-glucaric acid with phosphoglucose isomerase-deficient Saccharomyces cerevisiae

D-Glucaric acid is a potential biobased platform chemical. Previously mainly Escherichia coli, but also the yeast Saccharomyces cerevisiae, and Pichia pastoris, have been engineered for conversion of D-glucose to D-glucaric acid via myo-inositol. One reason for low yields from the yeast strains is the strong flux towards glycolysis. Thus, to decrease the flux of D-glucose to biomass, and to increase D-glucaric acid yield, the four step D-glucaric acid pathway was introduced into a phosphoglucose isomerase deficient (Pgi1p-deficient) Saccharomyces cerevisiae strain. High D-glucose concentrations are toxic to the Pgi1p-deficient strains, so various feeding strategies and use of polymeric substrates were studied. Uniformly labelled 13C-glucose confirmed conversion of D-glucose to D-glucaric acid. In batch bioreactor cultures with pulsed D-fructose and ethanol provision 1.3 g D-glucaric acid L-1 was produced. The D-glucaric acid titer (0.71 g D-glucaric acid L-1) was lower in nitrogen limited conditions, but the yield, 0.23 g D-glucaric acid [g D-glucose consumed]-1, was among the highest that has so far been reported from yeast. Accumulation of myo-inositol indicated that myo-inositol oxygenase activity was limiting, and that there would be potential to even higher yield. The Pgi1p-deficiency in S. cerevisiae provides an approach that in combination with other reported modifications and bioprocess strategies would promote the development of high yield D-glucaric acid yeast strains.

Efficient Production of Glucaric Acid by Engineered Saccharomyces cerevisiae

Glucaric acid is a valuable chemical with applications in the detergent, polymer, pharmaceutical and food industries. In this study, two key enzymes for glucaric acid biosynthesis, MIOX4 (myo-inositol oxygenase) and Udh (uronate dehydrogenase), were fused and expressed with different peptide linkers. It was found that a strain harboring the fusion protein MIOX4-Udh linked by the peptide (EA3K)3 produced the highest glucaric acid titer and thereby resulted in glucaric acid production that was 5.7-fold higher than that of the free enzymes. Next, the fusion protein MIOX4-Udh linked by (EA3K)3 was integrated into delta sequence sites of the Saccharomyces cerevisiae opi1 mutant, and a strain, GA16, that produced a glucaric acid titer of 4.9 g/L in a shake flask fermentation was identified by a high-throughput screening method using an Escherichia coli glucaric acid biosensor. Strain improvement by further engineering was performed to regulate the metabolic flux of myo-inositol to increase the supply of glucaric acid precursors. The downregulation of ZWF1 and the overexpression of INM1 and ITR1 increased glucaric acid production significantly, and glucaric acid production was increased to 8.49 g/L in the final strain GA-ZII in a shake flask fermentation. Finally, in a 5-L bioreactor, GA-ZII produced a glucaric acid titer of 15.6 g/L through fed-batch fermentation. IMPORTANCE Glucaric acid is a value-added dicarboxylic acid that was synthesized mainly through the oxidation of glucose chemically. Due to the problems of the low selectivity, by-products, and highly polluting waste of this process, producing glucaric acid biologically has attracted great attention. The activity of key enzymes and the intracellular myo-inositol level were both rate-limiting factors for glucaric acid biosynthesis. To increase glucaric acid production, this work improved the activity of the key enzymes in the glucaric acid biosynthetic pathway through the expression of a fusion of Arabidopsis thaliana MIOX4 and Pseudomonas syringae Udh as well as a delta sequence-based integration. Furthermore, intracellular myo-inositol flux was optimized by a series of metabolic strategies to increase the myo-inositol supply, which improved glucaric acid production to a higher level. This study provided a way for constructing a glucaric acid-producing strain with good synthetic performance, making glucaric acid production biologically in yeast cells much more competitive.

Polymers without Petrochemicals: Sustainable Routes to Conventional Monomers

Access to a wide range of plastic materials has been rationalized by the increased demand from growing populations and the development of high-throughput production systems. Plastic materials at low costs with reliable properties have been utilized in many everyday products. Multibillion-dollar companies are established around these plastic materials, and each polymer takes years to optimize, secure intellectual property, comply with the regulatory bodies such as the Registration, Evaluation, Authorisation and Restriction of Chemicals and the Environmental Protection Agency and develop consumer confidence. Therefore, developing a fully sustainable new plastic material with even a slightly different chemical structure is a costly and long process. Hence, the production of the common plastic materials with exactly the same chemical structures that does not require any new registration processes better reflects the reality of how to address the critical future of sustainable plastics. In this review, we have highlighted the very recent examples on the synthesis of common monomers using chemicals from sustainable feedstocks that can be used as a like-for-like substitute to prepare conventional petrochemical-free thermoplastics.

Enzyme activity engineering based on sequence co-evolution analysis

The utility of engineering enzyme activity is expanding with the development of biotechnology. Conventional methods have limited applicability as they require high-throughput screening or three-dimensional structures to direct target residues of activity control. An alternative method uses sequence evolution of natural selection. A repertoire of mutations was selected for fine-tuning enzyme activities to adapt to varying environments during the evolution. Here, we devised a strategy called sequence co-evolutionary analysis to control the efficiency of enzyme reactions (SCANEER), which scans the evolution of protein sequences and direct mutation strategy to improve enzyme activity. We hypothesized that amino acid pairs for various enzyme activity were encoded in the evolutionary history of protein sequences, whereas loss-of-function mutations were avoided since those are depleted during the evolution. SCANEER successfully predicted the enzyme activities of beta-lactamase and aminoglycoside 3'-phosphotransferase. SCANEER was further experimentally validated to control the activities of three different enzymes of great interest in chemical production: cis-aconitate decarboxylase, α-ketoglutaric semialdehyde dehydrogenase, and inositol oxygenase. Activity-enhancing mutations that improve substrate-binding affinity or turnover rate were found at sites distal from known active sites or ligand-binding pockets. We provide SCANEER to control desired enzyme activity through a user-friendly webserver.

Artificial Self-assembling Nanocompartment for Organizing Metabolic Pathways in Yeast

Metabolic pathways are commonly organized by sequestration into discrete cellular compartments. Compartments prevent unfavorable interactions with other pathways and provide local environments conducive to the activity of encapsulated enzymes. Such compartments are also useful synthetic biology tools for examining enzyme/pathway behavior and for metabolic engineering. Here, we expand the intracellular compartmentalization toolbox for budding yeast (Saccharomyces cerevisiae) with Murine polyomavirus virus-like particles (MPyV VLPs). The MPyV system has two components: VP1 which self-assembles into the compartment shell and a short anchor, VP2C, which mediates cargo protein encapsulation via binding to the inner surface of the VP1 shell. Destabilized green fluorescent protein (GFP) fused to VP2C was specifically sorted into VLPs and thereby protected from host-mediated degradation. An engineered VP1 variant displayed improved cargo capture properties and differential subcellular localization compared to wild-type VP1. To demonstrate their ability to function as a metabolic compartment, MPyV VLPs were used to encapsulate myo-inositol oxygenase (MIOX), an unstable and rate-limiting enzyme in d-glucaric acid biosynthesis. Strains with encapsulated MIOX produced ∼20% more d-glucaric acid compared to controls expressing "free" MIOX─despite accumulating dramatically less expressed protein─and also grew to higher cell densities. This is the first demonstration in yeast of an artificial biocatalytic compartment that can participate in a metabolic pathway and establishes the MPyV platform as a promising synthetic biology tool for yeast engineering.

Consolidated bioprocessing of lignocellulose for production of glucaric acid by an artificial microbial consortium

BACKGROUND

The biomanufacturing of D-glucaric acid has attracted increasing interest because it is one of the top value-added chemicals produced from biomass. Saccharomyces cerevisiae is regarded as an excellent host for D-glucaric acid production.

RESULTS

The opi1 gene was knocked out because of its negative regulation on myo-inositol synthesis, which is the limiting step of D-glucaric acid production by S. cerevisiae. We then constructed the biosynthesis pathway of D-glucaric acid in S. cerevisiae INVSc1 opi1Δ and obtained two engineered strains, LGA-1 and LGA-C, producing record-breaking titers of D-glucaric acid: 9.53 ± 0.46 g/L and 11.21 ± 0.63 g/L D-glucaric acid from 30 g/L glucose and 10.8 g/L myo-inositol in fed-batch fermentation mode, respectively. However, LGA-1 was preferable because of its genetic stability and its superior performance in practical applications. There have been no reports on D-glucaric acid production from lignocellulose. Therefore, the biorefinery processes, including separated hydrolysis and fermentation (SHF), simultaneous saccharification and fermentation (SSF) and consolidated bioprocessing (CBP) were investigated and compared. CBP using an artificial microbial consortium composed of Trichoderma reesei (T. reesei) Rut-C30 and S. cerevisiae LGA-1 was found to have relatively high D-glucaric acid titers and yields after 7 d of fermentation, 0.54 ± 0.12 g/L D-glucaric acid from 15 g/L Avicel and 0.45 ± 0.06 g/L D-glucaric acid from 15 g/L steam-exploded corn stover (SECS), respectively. In an attempt to design the microbial consortium for more efficient CBP, the team consisting of T. reesei Rut-C30 and S. cerevisiae LGA-1 was found to be the best, with excellent work distribution and collaboration.

CONCLUSIONS

Two engineered S. cerevisiae strains, LGA-1 and LGA-C, with high titers of D-glucaric acid were obtained. This indicated that S. cerevisiae INVSc1 is an excellent host for D-glucaric acid production. Lignocellulose is a preferable substrate over myo-inositol. SHF, SSF, and CBP were studied, and CBP using an artificial microbial consortium of T. reesei Rut-C30 and S. cerevisiae LGA-1 was found to be promising because of its relatively high titer and yield. T. reesei Rut-C30 and S. cerevisiae LGA-1were proven to be the best teammates for CBP. Further work should be done to improve the efficiency of this microbial consortium for D-glucaric acid production from lignocellulose.

Effect of magnesium ions on glucaric acid production in the engineered Saccharomyces cerevisiae

Glucaric acid has been successfully produced in Escherichia coli and fungus. Here, we first analyzed the effects of different metal ions on glucaric acid production in the engineered Saccharomyces cerevisiae Bga-3 strain harboring the glucaric acid synthesis pathway. We found that magnesium ions could promote the growth rate of yeast cells, and thus, increase the glucaric acid production by elevating the glucose and myo-inositol utilization of Bga-3 strain. RNA-Seq transcriptome analysis results showed that the upregulation of genes involved in the gluconeogenesis pathway, as well as the downregulation of genes associated with the glycolysis pathway and pentose phosphate pathway in response to MgCl₂ were all benefit for the enhancement of the glucose-6-phosphate flux, which was the precursor for myo-inositol and glucaric acid. In addition, we found that MgCl₂ could also increase the activity of MIOX4, which was also crucial for glucaric acid synthesis. At last, a final glucaric acid titer of 10.6 g/L, the highest reported titer, was achieved in the fed-batch fermentation using a 5-L bioreactor by adding 100 mM MgCl₂. Our findings will provide a new way of promoting the production of other chemicals in the engineered yeast cells.

Microbial engineering for the production of C2-C6 organic acids

Covering: up to the end of 2020Organic acids, as building block compounds, have been widely used in food, pharmaceutical, plastic, and chemical industries. Until now, chemical synthesis is still the primary method for industrial-scale organic acid production. However, this process encounters some inevitable challenges, such as depletable petroleum resources, harsh reaction conditions and complex downstream processes. To solve these problems, microbial cell factories provide a promising approach for achieving the sustainable production of organic acids. However, some key metabolites in central carbon metabolism are strictly regulated by the network of cellular metabolism, resulting in the low productivity of organic acids. Thus, multiple metabolic engineering strategies have been developed to reprogram microbial cell factories to produce organic acids, including monocarboxylic acids, hydroxy carboxylic acids, amino carboxylic acids, dicarboxylic acids and monomeric units for polymers. These strategies mainly center on improving the catalytic efficiency of the enzymes to increase the conversion rate, balancing the multi-gene biosynthetic pathways to reduce the byproduct formation, strengthening the metabolic flux to promote the product biosynthesis, optimizing the metabolic network to adapt the environmental conditions and enhancing substrate utilization to broaden the substrate spectrum. Here, we describe the recent advances in producing C₂-C₆ organic acids by metabolic engineering strategies. In addition, we provide new insights as to when, what and how these strategies should be taken. Future challenges are also discussed in further advancing microbial engineering and establishing efficient biorefineries.

Cell-based and cell-free biocatalysis for the production of D-glucaric acid

D-Glucaric acid (GA) is a value-added chemical produced from biomass, and has potential applications as a versatile platform chemical, food additive, metal sequestering agent, and therapeutic agent. Marketed GA is currently produced chemically, but increasing demand is driving the search for eco-friendlier and more efficient production approaches. Cell-based production of GA represents an alternative strategy for GA production. A series of synthetic pathways for GA have been ported into Escherichia coli, Saccharomyces cerevisiae and Pichia pastoris, respectively, and these engineered cells show the ability to synthesize GA de novo. Optimization of the GA metabolic pathways in host cells has leapt forward, and the titer and yield have increased rapidly. Meanwhile, cell-free multi-enzyme catalysis, in which the desired pathway is constructed in vitro from enzymes and cofactors involved in GA biosynthesis, has also realized efficient GA bioconversion. This review presents an overview of studies of the development of cell-based GA production, followed by a brief discussion of potential applications of biosensors that respond to GA in these biosynthesis routes.

Sequence-based bioprospecting of myo-inositol oxygenase (Miox) reveals new homologues that increase glucaric acid production in Saccharomyces cerevisiae

myo-Inositol oxygenase (Miox) is a rate-limiting enzyme for glucaric acid production via microbial fermentation. The enzyme converts myo-inositol to glucuronate, which is further converted to glucaric acid, a natural compound with industrial uses that range from detergents to pharmaceutical synthesis to polymeric materials. More than 2,000 Miox sequences are available in the Uniprot database but only thirteen are classified as reviewed in Swiss-Prot (August 2019). In this study, sequence similarity networks were used to identify new homologues to be expressed in Saccharomyces cerevisiae for glucaric acid production. The expression of four homologues did not lead to product formation. Some of these enzymes may have a defective "dynamic lid" - a structural feature important to close the reaction site - which might explain the lack of activity. Thirty-one selected Miox sequences did allow for product formation, of which twenty-five were characterized for the first time. Expression of Talaromyces marneffei Miox led to the accumulation of 1.76 ± 0.33 g glucaric acid/L from 20 g glucose/L and 10 g/L myo-inositol. Specific glucaric acid titer with TmMiox increased 44 % compared to the often-used Arabidopsis thaliana variant AtMiox4 (0.258 vs. 0.179 g glucaric acid/g biomass). AtMiox4 activity decreased from 12.47 to 0.40 nmol/min/mg protein when cells exited exponential phase during growth on glucose, highlighting the importance of future research on Miox stability in order to further improve microbial production of glucaric acid.

Enhancement of glucaric acid production in Saccharomyces cerevisiae by expressing Vitreoscilla hemoglobin

OBJECTIVE

To enhance the glucaric acid (GA) production in Saccharomyces cerevisiae, the Vitreoscilla hemoglobin was employed to reinforce cellular oxygen supplement. Additionally, the pH-free fermentation strategy was engaged to lower the cost brought by base feeding during the acid-accumulated and long-period glucaric acid production.

RESULTS

Recombinant yeast Bga-4 was constructed harboring Vitreoscilla hemoglobin on the basis of previous Bga-3. Higher glucose uptake rate, growth rate, and ethanol reuse rate were achieved in Bga-4 in shake-flask fermentation than those in Bga-3. Furthermore, the fed-batch fermentation in a 5-L bioreactor was performed without pH control, resulting in a final glucaric acid titer of 6.38 g/L.

CONCLUSIONS

Both the GA titer and biomass were enhanced along with the efficiency of ethanol re-utilization in the presence of VHb. Moreover, the absence of base feeding for long-period fermentation reduced production cost, which is meaningful for industrial applications.

Recent progress on bio-based production of dicarboxylic acids in yeast

Dicarboxylic acids are widely used in fine chemical and food industries as well as the monomer for polymerisation of high molecular material. Given the problems of environmental contamination and sustainable development faced by traditional production of dicarboxylic acids based on petrol, new approaches such as bio-based production of dicarboxylic acids drew more attentions. The yeast, Saccharomyces cerevisiae, was regarded as an ideal organism for bio-based production of dicarboxylic acids with high tolerance to acidic and hyperosmotic environments, robust growth using a broad range of substrates, great convenience for genetic manipulation, stable inheritance via sub-cultivation, and food compatibility. In this review, the production of major dicarboxylates via S. cerevisiae was concluded and the challenges and opportunities facing were discussed.Key Points• Summary of current production of major dicarboxylic acids by Saccharomyces cerevisiae.• Discussion of influence factors on four-carbon dicarboxylic acids production by Saccharomyces cerevisiae.• Outlook of potential production of five- and six-carbon dicarboxylic acids by Saccharomyces cerevisiae.

Transcription Factor Engineering for High-Throughput Strain Evolution and Organic Acid Bioproduction: A Review

Metabolic regulation of gene expression for the microbial production of fine chemicals, such as organic acids, is an important research topic in post-genomic metabolic engineering. In particular, the ability of transcription factors (TFs) to respond precisely in time and space to various small molecules, signals and stimuli from the internal and external environment is essential for metabolic pathway engineering and strain development. As a key component, TFs are used to construct many biosensors in vivo using synthetic biology methods, which can be used to monitor the concentration of intracellular metabolites in organic acid production that would otherwise remain “invisible” within the intracellular environment. TF-based biosensors also provide a high-throughput screening method for rapid strain evolution. Furthermore, TFs are important global regulators that control the expression levels of key enzymes in organic acid biosynthesis pathways, therefore determining the outcome of metabolic networks. Here we review recent advances in TF identification, engineering, and applications for metabolic engineering, with an emphasis on metabolite monitoring and high-throughput strain evolution for the organic acid bioproduction.

Production of Hexaric Acids from Biomass

Sugar acids obtained by aldohexose oxidation of both the terminal aldehyde group and the hydroxy group at the other end to carboxyl groups are called hexaric acids (i.e., six-carbon aldaric acids). Because hexaric acids have four secondary hydroxy groups that are stereochemically diverse and two carboxyl groups, various applications of these acids have been studied. Conventionally, hexaric acids have been produced mainly by nitric acid oxidation of aldohexose, but full-scale commercialization has not been realized; there are many problems regarding yield, safety, environmental burden, etc. In recent years, therefore, improvements in hexaric acid production by nitric acid oxidation have been made, while new production methods, including biocatalytic methods, are actively being studied. In this paper, we summarize these production methods in addition to research on the application of hexaric acids.

Efficient Bioconversion of Sucrose to High-Value-Added Glucaric Acid by In Vitro Metabolic Engineering

Glucaric acid (GA) is a major value-added chemicals feedstock and additive, especially in the food, cosmetics, and pharmaceutical industries. The increasing demand for GA is driving the search for a more efficient and less costly production pathway. In this study, a new in vitro multi-enzyme cascade system was developed, which converts sucrose efficiently to GA in a single vessel. The in vitro system, which does not require adenosine triphosphate (ATP) or nicotinamide adenine dinucleotide (NAD⁺ ) supplementation, contains seven enzymes. All enzymes were chosen from the BRENDA and NCBI databases and were expressed efficiently in Escherichia coli BL21(DE3). All seven enzymes were combined in an in vitro cascade system, and the reaction conditions were optimized. Under the optimized conditions, the in vitro seven-enzyme cascade system converted 50 mm sucrose to 34.8 mm GA with high efficiency (75 % of the theoretical yield). This system represents an alternative pathway for more efficient and less costly production of GA, which could be adapted for the synthesis of other value-added chemicals.

Polysaccharide monooxygenase-catalyzed oxidation of cellulose to glucuronic acid-containing cello-oligosaccharides

BACKGROUND

Polysaccharide monooxygenases (PMOs) play an important role in the enzymatic degradation of cellulose. They have been demonstrated to able to C6-oxidize cellulose to produce C6-hexodialdoses. However, the biological function of C6 oxidation of PMOs remains unknown. In particular, it is unclear whether C6-hexodialdoses can be further oxidized to uronic acid (glucuronic acid-containing oligosaccharides).

RESULTS

A PMO gene, Hipmo1, was isolated from Humicola insolens and expressed in Pichia pastoris. This PMO (HiPMO1), belonging to the auxiliary activity 9 (AA9) family, was shown to able to cleave cellulose to yield non-oxidized and oxidized cello-oligosaccharides. The enzyme oxidizes C6 positions in cellulose to form glucuronic acid-containing cello-oligosaccharides, followed by hydrolysis with beta-glucosidase and beta-glucuronidase to yield glucose, glucuronic acid, and saccharic acid. This indicates that HiPMO1 can catalyze C6 oxidation of hydroxyl groups of cellulose to carboxylic groups.

CONCLUSIONS

HiPMO1 oxidizes C6 of cellulose to form glucuronic acid-containing cello-oligosaccharides followed by hydrolysis with beta-glucosidase and beta-glucuronidase to yield glucose, glucuronic acid, and saccharic acid, and even possibly by beta-eliminative cleavage to produce unsaturated cello-oligosaccharides. This study provides a new mechanism for cellulose cleavage by C6 oxidation of HiPMO1.

Natural antibacterial agents from arid-region pretreated lignocellulosic biomasses and extracts for the control of lactic acid bacteria in yeast fermentation

Bacterial contamination is one of the major challenges faced by yeast fermentation industries as the contaminating microorganisms produce lactic acid and acetic acid, which reduces the viability of yeast, and hence fermentation yields. The primary bacterial contaminants of yeast fermentations are lactic acid bacteria (LAB). This study aims to identify potential natural antibacterial fractions from raw and pretreated lignocellulosic biomasses found in Abu Dhabi, UAE, in terms of LAB inhibition capacity, allowing growth of the yeast. The analysis was carried out using plating technique. Pretreatment liquid of the mangrove stem Avicennia marina hydrothermally pretreated at 210 °C exhibited the widest inhibition zone with an average diameter of 14.5 mm, followed by the pretreatment liquid of mangrove leaf pretreated at 190 °C, Salicornia bigelovii pretreated at 202 °C and rachis of date palm Phoenix dactylifera pretreated at 200 °C. The compounds responsible for the antibacterial activity will be characterized in further study.

Metabolic engineering of Saccharomyces cerevisiae for efficient production of glucaric acid at high titer

BACKGROUND

Glucaric acid is a high-value-added chemical that can be used in various fields. Because chemical oxidation of glucose to produce glucaric acid is not environmentally friendly, microbial production has attracted increasing interest recently. Biological pathways to synthesize glucaric acid from glucose in both Escherichia coli and Saccharomyces cerevisiae by co-expression of genes encoding myo-inositol-1-phosphate synthase (Ino1), myo-inositol oxygenase (MIOX), and uronate dehydrogenase (Udh) have been constructed. However, low activity and instability of MIOX from Mus musculus was proved to be the bottleneck in this pathway.

RESULTS

A more stable miox4 from Arabidopsis thaliana was chosen in the present study. In addition, high copy delta-sequence integration of miox4 into the S. cerevisiae genome was performed to increase its expression level further. Enzymatic assay and quantitative real-time PCR analysis revealed that delta-sequence-based integrative expression increased MIOX4 activity and stability, thus increasing glucaric acid titer about eight times over that of episomal expression. By fed-batch fermentation supplemented with 60 mM (10.8 g/L) inositol, the multi-copy integrative expression S. cerevisiae strain produced 6 g/L (28.6 mM) glucaric acid from myo-inositol, the highest titer that had been ever reported in S. cerevisiae.

CONCLUSIONS

In this study, glucaric acid titer was increased to 6 g/L in S. cerevisiae by integrating the miox4 gene from A. thaliana and the udh gene from Pseudomonas syringae into the delta sequence of genomes. Delta-sequence-based integrative expression increased both the number of target gene copies and their stabilities. This approach could be used for a wide range of metabolic pathway engineering applications with S. cerevisiae.

Construction and Characterization of Broad-Spectrum Promoters for Synthetic Biology

Characterization of genetic circuits and biosynthetic pathways in different hosts always requires promoter substitution and redesigning. Here, a strong, broad-spectrum promoter, P_bs, for Escherichia coli, Bacillus subtilis, and Saccharomyces cerevisiae was constructed, and it was incorporated into the minimal E. coli-B. subtilis-S. cerevisiae shuttle plasmid pEBS (5.8 kb). By applying a random mutation strategy, three broad-spectrum promoters P_bs1, P_bs2, and P_bs3, with different strengths were generated and characterized. These broad-spectrum promoters will expand the synthetic biology toolbox for E. coli, B. subtilis, and S. cerevisiae.