Ustilago maydis produces itaconic acid via the unusual intermediate trans-aconitate

Phosphate limitation enhances malic acid production on nitrogen-rich molasses with Ustilago trichophora

BACKGROUND

An important step in replacing petrochemical products with sustainable, cost-effective alternatives is the use of feedstocks other than, e.g., pure glucose in the fermentative production of platform chemicals. Ustilaginaceae offer the advantages of a wide substrate spectrum and naturally produce a versatile range of value-added compounds under nitrogen limitation. A promising candidate is the dicarboxylic acid malic acid, which may be applied as an acidulant in the food industry, a chelating agent in pharmaceuticals, or in biobased polymer production. However, fermentable residue streams from the food and agricultural industry with high nitrogen content, e.g., sugar beet molasses, are unsuited for processes with Ustilaginaceae, as they result in low product yields due to high biomass and low product formation.

RESULTS

This study uncovers challenges in evaluating complex feedstock applicability for microbial production processes, highlighting the role of secondary substrate limitations, internal storage molecules, and incomplete assimilation of these substrates. A microliter-scale screening method with online monitoring of microbial respiration was developed using malic acid production with Ustilago trichophora on molasses as an application example. Investigation into nitrogen, phosphate, sulphate, and magnesium limitations on a defined minimal medium demonstrated successful malic acid production under nitrogen and phosphate limitation. Furthermore, a reduction of nitrogen and phosphate in the elemental composition of U. trichophora was revealed under the respective secondary substrate limitation. These adaptive changes in combination with the intricate metabolic response hinder mathematical prediction of product formation and make the presented screening methodology for complex feedstocks imperative. In the next step, the screening was transferred to a molasses-based complex medium. It was determined that the organism assimilated only 25% and 50% of the elemental nitrogen and phosphorus present in molasses, respectively. Due to the overall low content of bioavailable phosphorus in molasses, the replacement of the state-of-the-art nitrogen limitation was shown to increase malic acid production by 65%.

CONCLUSION

The identification of phosphate as a superior secondary substrate limitation for enhanced malic acid production opens up new opportunities for the effective utilization of molasses as a more sustainable and cost-effective substrate than, e.g., pure glucose for biobased platform chemical production.

Overexpression of the DHA1 family, ChlH and ChlK, leads to enhanced dicarboxylic acids production in koji fungi, Aspergillus luchuensis mut. kawachii and Aspergillus oryzae

The white koji fungus Aspergillus luchuensis mut. kawachii secretes substantial amounts of citric acid through the expression of the citric acid exporter CexA, a member of the DHA1 family. In this study, we aimed to characterize 11 CexA homologs (Chl proteins) encoded in the genome of A. luchuensis mut. kawachii to identify novel transporters useful for organic acid production. We constructed overexpression strains of chl genes using a cexA disruptant of the A. luchuensis mut. kawachii as the host strain, which prevented excessive secretion of citric acid into the culture supernatant. Subsequently, we evaluated the effects of overexpression of chl on producing organic acids by analyzing the culture supernatant. All overexpression strains did not exhibit significant citric acid accumulation in the culture supernatant, indicating that Chl proteins are not responsible for citric acid export. Furthermore, the ChlH overexpression strain displayed an accumulation of 2-oxoglutaric and fumaric acids in the culture supernatant, while the ChlK overexpression strain exhibited the accumulation of 2-oxoglutaric, malic and succinic acids. Notably, the ChlH and ChlK overexpression led to a substantial increase in the production of 2-oxoglutaric acid, reaching approximately 25 mM and 50 mM, respectively. Furthermore, ChlH and ChlK overexpression also significantly increased the secretory production of dicarboxylic acids, including 2-oxoglutaric acid, in the yellow koji fungus, Aspergillus oryzae. Our study demonstrates that overexpression of DHA1 family gene results in enhanced secretion of organic acids in koji fungi of the genus Aspergillus.

DoE-based medium optimization for improved biosurfactant production with Aureobasidium pullulans

Polyol lipids (a.k.a. liamocins) produced by the polyextremotolerant, yeast-like fungus Aureobasidium pullulans are amphiphilic molecules with high potential to serve as biosurfactants. So far, cultivations of A. pullulans have been performed in media with complex components, which complicates further process optimization due to their undefined composition. In this study, we developed and optimized a minimal medium, focusing on biosurfactant production. Firstly, we replaced yeast extract and peptone in the best-performing polyol lipid production medium to date with a vitamin solution, a trace-element solution, and a nitrogen source. We employed a design of experiments approach with a factor screening using a two-level-factorial design, followed by a central composite design. The polyol lipid titer was increased by 56% to 48 g L-1, and the space-time yield from 0.13 to 0.20 g L-1 h-1 in microtiter plate cultivations. This was followed by a successful transfer to a 1 L bioreactor, reaching a polyol lipid concentration of 41 g L-1. The final minimal medium allows the investigation of alternative carbon sources and the metabolic pathways involved, to pinpoint targets for genetic modifications. The results are discussed in the context of the industrial applicability of this robust and versatile fungus.

Itaconate Production from Crude Substrates with U. maydis: Scale-up of an Industrially Relevant Bioprocess

BACKGROUND

Industrial by-products accrue in most agricultural or food-related production processes, but additional value chains have already been established for many of them. Crude glycerol has a 60% lower market value than commercial glucose, as large quantities are produced in the biodiesel industry, but its valorisation is still underutilized. Due to its high carbon content and the natural ability of many microorganisms to metabolise it, microbial upcycling is a suitable option for this waste product.

RESULTS

In this work, the use of crude glycerol for the production of the value-added compound itaconate is demonstrated using the smut fungus Ustilago maydis. Starting with a highly engineered strain, itaconate production from an industrial glycerol waste stream was quickly established on a small scale, and the resulting yields were already competitive with processes using commercial sugars. Adaptive laboratory evolution resulted in an evolved strain with a 72% increased growth rate on glycerol. In the subsequent development and optimisation of a fed-batch process on a 1.5-2 L scale, the use of molasses, a side stream of sugar beet processing, eliminated the need for other expensive media components such as nitrogen or vitamins for biomass growth. The optimised process was scaled up to 150 L, achieving an overall titre of 72 g L- 1, a yield of 0.34 g g- 1, and a productivity of 0.54 g L- 1 h- 1.

CONCLUSIONS

Pilot-scale itaconate production from the complementary waste streams molasses and glycerol has been successfully established. In addition to achieving competitive performance indicators, the proposed dual feedstock strategy offers lower process costs and carbon footprint for the production of bio-based itaconate.

Identification of a specific exporter that enables high production of aconitic acid in Aspergillus pseudoterreus

Aconitic acid is an unsaturated tricarboxylic acid that is attractive for its potential use in manufacturing biodegradable and biocompatible polymers, plasticizers, and surfactants. Previously Aspergillus pseudoterreus was engineered as a platform to produce aconitic acid by deleting the cadA (cis-aconitic acid decarboxylase) gene in the itaconic acid biosynthetic pathway. In this study, the aconitic acid transporter gene (aexA) was identified using comparative global discovery proteomics analysis between the wild-type and cadA deletion strains. The protein AexA belongs to the Major Facilitator Superfamily (MFS). Deletion of aexA almost abolished aconitic acid secretion, while its overexpression led to a significant increase in aconitic acid production. Transportation of aconitic acid across the plasma membrane is a key limiting step in its production. In vitro, proteoliposome transport assay further validated AexA's function and substrate specificity. This research provides new approaches to efficiently pinpoint and characterize exporters of fungal organic acids and accelerate metabolic engineering to improve secretion capability and lower the cost of bioproduction.

Development of an itaconic acid production process with Ustilaginaceae on alternative feedstocks

BACKGROUND

Currently, Aspergillus terreus is used for the industrial production of itaconic acid. Although, alternative feedstock use in fermentations is crucial for cost-efficient and sustainable itaconic acid production, their utilisation with A. terreus most often requires expensive pretreatment. Ustilaginacea are robust alternatives for itaconic acid production, evading the challenges, including the pretreatment of crude feedstocks regarding reduction of manganese concentration, that A. terreus poses.

RESULTS

In this study, five different Ustilago strains were screened for their growth and production of itaconic acid on defined media. The most promising strains were then used to find a suitable alternative feedstock, based on the local food industry. U. cynodontis ITA Max pH, a highly engineered production strain, was selected to determine the biologically available nitrogen concentration in thick juice and molasses. Based on these findings, thick juice was chosen as feedstock to ensure the necessary nitrogen limitation for itaconic acid production. U. cynodontis ITA Max pH was further characterised regarding osmotolerance and product inhibition and a successful scale-up to a 2 L stirred tank reactor was accomplished. A titer of 106.4 gitaconic acid/L with a theoretical yield of 0.50 gitaconic acid/gsucrose and a space-time yield of 0.72 gitaconic acid/L/h was reached.

CONCLUSIONS

This study demonstrates the utilisation of alternative feedstocks to produce ITA with Ustilaginaceae, without drawbacks in either titer or yield, compared to glucose fermentations.

Recent Progress in Metabolic Engineering of Escherichia coli for the Production of Various C4 and C5-Dicarboxylic Acids

As an alternative to petrochemical synthesis, well-established industrial microbes, such as Escherichia coli, are employed to produce a wide range of chemicals, including dicarboxylic acids (DCAs), which have significant potential in diverse areas including biodegradable polymers. The demand for biodegradable polymers has been steadily rising, prompting the development of efficient production pathways on four- (C4) and five-carbon (C5) DCAs derived from central carbon metabolism to meet the increased demand via the biosynthesis. In this context, E. coli is utilized to produce these DCAs through various metabolic engineering strategies, including the design or selection of metabolic pathways, pathway optimization, and enhancement of catalytic activity. This review aims to highlight the recent advancements in metabolic engineering techniques for the production of C4 and C5 DCAs in E. coli.

Microbial production of trans-aconitic acid

Trans-aconitic acid (TAA) is a promising bio-based chemical with the structure of unsaturated tricarboxylic acid, and also has the potential to be a non-toxic nematicide as a potent inhibitor of aconitase. However, TAA has not been commercialized because the traditional production processes of plant extraction and chemical synthesis cannot achieve large-scale production at a low cost. The availability of TAA is a serious obstacle to its widespread application. In this study, we developed an efficient microbial synthesis and fermentation production process for TAA. An engineered Aspergillus terreus strain producing cis-aconitic acid and TAA was constructed by blocking itaconic acid biosynthesis in the industrial itaconic acid-producing strain. Through heterologous expression of exogenous aconitate isomerase, we further designed a more efficient cell factory to specifically produce TAA. Subsequently, the fermentation process was developed and scaled up step-by-step, achieving a TAA titer of 60 g L-1 at the demonstration scale of a 20 m3 fermenter. Finally, the field evaluation of the produced TAA for control of the root-knot nematodes was performed in a field trial, effectively reducing the damage of the root-knot nematode. Our work provides a commercially viable solution for the green manufacturing of TAA, which will significantly facilitate biopesticide development and promote its widespread application as a bio-based chemical.

Holistic Approach to Process Design and Scale-Up for Itaconic Acid Production from Crude Substrates

Bio-based bulk chemicals such as carboxylic acids continue to struggle to compete with their fossil counterparts on an economic basis. One possibility to improve the economic feasibility is the use of crude substrates in biorefineries. However, impurities in these substrates pose challenges in fermentation and purification, requiring interdisciplinary research. This work demonstrates a holistic approach to biorefinery process development, using itaconic acid production on thick juice based on sugar beets with Ustilago sp. as an example. A conceptual process design with data from artificially prepared solutions and literature data from fermentation on glucose guides the simultaneous development of the upstream and downstream processes up to a 100 L scale. Techno-economic analysis reveals substrate consumption as the main constituent of production costs and therefore, the product yield is the driver of process economics. Aligning pH-adjusting agents in the fermentation and the downstream process is a central lever for product recovery. Experiments show that fermentation can be transferred from glucose to thick juice by changing the feeding profile. In downstream processing, an additional decolorization step is necessary to remove impurities accompanying the crude substrate. Moreover, we observe an increased use of pH-adjusting agents compared to process simulations.

Effect of different metabolic pathways on itaconic acid production in engineered Corynebacterium glutamicum

Itaconic acid (IA), a C5-dicarboxylic acid, is a potential bio-based building block for the polymer industry. There are three pathways for IA production from natural IA producers; however, most of the engineered strains were used for IA production by heterologous expression of cis-aconitate decarboxylase gene (cadA) from Aspergillus terreus. In this study, IA was produced by an engineered Corynebacterium glutamicum ATCC 13032 expressing two different types of genes from two distinct pathways. The first involves the mammalian immunoresponsive gene1 (Irg1) derived from Mus musculus. The second (termed here the trans-pathway) involves two genes from the natural IA producer Ustilago maydis which are aconitate-delta-isomerase (Adi1) and trans-aconitate decarboxylase (Tad1) genes. The constructed strains developing the two distinct IA production pathways: C. glutamicum ATCC 13032 pCH-Irg1_opt and C. glutamicum ATCC 13032 pCH-Tad1_optadi1_opt were used for production of IA from different carbon sources. The results reflect the possibility for IA production from C. glutamicum expressing the trans-pathway (Adi1/Tad1 genes) and cis-pathway (Irg1 gene) other than the well-known cis-pathway that depends mainly on cadA gene from A. terreus. The developed strain expressing trans-pathway from U. maydis; however, proved to be better at IA production with high titers of 12.25, 11.34, and 11.02 g/L, and a molar yield of 0.22, 0.42, and 0.43 mol/mol from glucose, maltose, and sucrose, respectively, via fed-batch fermentation. The present study suggests that trans-pathway is better than cis-pathway for IA production in engineered C. glutamicum.

Microbial itaconic acid bioproduction towards sustainable development: Insights, challenges, and prospects

Microbial biomanufacturing is a promising approach to produce high-value compounds with low-carbon footprint and significant economic benefits. Among twelve "Top Value-Added Chemicals from Biomass", itaconic acid (IA) stands out as a versatile platform chemical with numerous applications. IA is naturally produced by Aspergillus and Ustilago species through a cascade enzymatic reaction between aconitase (EC 4.2.1.3) and cis-aconitic acid decarboxylase (EC 4.1.1.6). Recently, non-native hosts such as Escherichia coli, Corynebacterium glutamicum, Saccharomyces cerevisiae, and Yarrowia lipolytica have been genetically engineered to produce IA through the introduction of key enzymes. This review provides an up-to-date summary of the progress made in IA bioproduction, from native to engineered hosts, covers in vivo and in vitro approaches, and highlights the prospects of combination tactics. Current challenges and recent endeavors are also addressed to envision comprehensive strategies for renewable IA production in the future towards sustainable development goals (SDGs).

Functional Clustering of Metabolically Related Genes Is Conserved across Dikarya

Transcriptional regulation is vital for organismal survival, with many layers and mechanisms collaborating to balance gene expression. One layer of this regulation is genome organization, specifically the clustering of functionally related, co-expressed genes along the chromosomes. Spatial organization allows for position effects to stabilize RNA expression and balance transcription, which can be advantageous for a number of reasons, including reductions in stochastic influences between the gene products. The organization of co-regulated gene families into functional clusters occurs extensively in Ascomycota fungi. However, this is less characterized within the related Basidiomycota fungi despite the many uses and applications for the species within this clade. This review will provide insight into the prevalence, purpose, and significance of the clustering of functionally related genes across Dikarya, including foundational studies from Ascomycetes and the current state of our understanding throughout representative Basidiomycete species.

Structural characterization of the Aspergillus niger citrate transporter CexA uncovers the role of key residues S75, R192 and Q196

The Aspergillus niger CexA transporter belongs to the DHA1 (Drug-H⁺ antiporter) family. CexA homologs are exclusively found in eukaryotic genomes, and CexA is the sole citrate exporter to have been functionally characterized in this family so far. In the present work, we expressed CexA in Saccharomyces cerevisiae, demonstrating its ability to bind isocitric acid, and import citrate at pH 5.5 with low affinity. Citrate uptake was independent of the proton motive force and compatible with a facilitated diffusion mechanism. To unravel the structural features of this transporter, we then targeted 21 CexA residues for site-directed mutagenesis. Residues were identified by a combination of amino acid residue conservation among the DHA1 family, 3D structure prediction, and substrate molecular docking analysis. S. cerevisiae cells expressing this library of CexA mutant alleles were evaluated for their capacity to grow on carboxylic acid-containing media and transport of radiolabeled citrate. We also determined protein subcellular localization by GFP tagging, with seven amino acid substitutions affecting CexA protein expression at the plasma membrane. The substitutions P200A, Y307A, S315A, and R461A displayed loss-of-function phenotypes. The majority of the substitutions affected citrate binding and translocation. The S75 residue had no impact on citrate export but affected its import, as the substitution for alanine increased the affinity of the transporter for citrate. Conversely, expression of CexA mutant alleles in the Yarrowia lipolytica cex1Δ strain revealed the involvement of R192 and Q196 residues in citrate export. Globally, we uncovered a set of relevant amino acid residues involved in CexA expression, export capacity and import affinity.

Recent Advances on the Production of Itaconic Acid via the Fermentation and Metabolic Engineering

Itaconic acid (ITA) is one of the top 12 platform chemicals. The global ITA market is expanding due to the rising demand for bio-based unsaturated polyester resin and its non-toxic qualities. Although bioconversion using microbes is the main approach in the current industrial production of ITA, ecological production of bio-based ITA faces several issues due to: low production efficiency, the difficulty to employ inexpensive raw materials, and high manufacturing costs. As metabolic engineering advances, the engineering of microorganisms offers a novel strategy for the promotion of ITA bio-production. In this review, the most recent developments in the production of ITA through fermentation and metabolic engineering are compiled from a variety of perspectives, including the identification of the ITA synthesis pathway, the metabolic engineering of natural ITA producers, the design and construction of the ITA synthesis pathway in model chassis, and the creation, as well as application, of new metabolic engineering strategies in ITA production. The challenges encountered in the bio-production of ITA in microbial cell factories are discussed, and some suggestions for future study are also proposed, which it is hoped offers insightful views to promote the cost-efficient and sustainable industrial production of ITA.

Characterization and engineering of branched short-chain dicarboxylate metabolism in Pseudomonas reveals resistance to fungal 2-hydroxyparaconate

In recent years branched short-chain dicarboxylates (BSCD) such as itaconic acid gained increasing interest in both medicine and biotechnology. Their use as building blocks for plastics urges for developing microbial upcycling strategies to provide sustainable end-of-life solutions. Furthermore, many BSCD exhibit anti-bacterial properties or exert immunomodulatory effects in macrophages, indicating a medical relevance for this group of molecules. For both of these applications, a detailed understanding of the microbial metabolism of these compounds is essential. In this study, the metabolic pathway of BSCD degradation from Pseudomonas aeruginosa PAO1 was studied in detail by heterologously transferring it to Pseudomonas putida. Heterologous expression of the PA0878-0886 itaconate metabolism gene cluster enabled P. putida KT2440 to metabolize itaconate, (S)- and (R)-methylsuccinate, (S)-citramalate, and mesaconate. The functions of the so far uncharacterized genes PA0879 and PA0881 were revealed and proven to extend the substrate range of the core degradation pathway. Furthermore, the uncharacterized gene PA0880 was discovered to encode a 2-hydroxyparaconate (2-HP) lactonase that catalyzes the cleavage of the itaconate derivative 2-HP to itatartarate. Interestingly, 2-HP was found to inhibit growth of the engineered P. putida on itaconate. All in all, this study extends the substrate range of P. putida to include BSCD for bio-upcycling of high-performance polymers, and also identifies 2-HP as promising candidate for anti-microbial applications.

Recent advances and perspectives on production of value-added organic acids through metabolic engineering

Organic acids are important consumable materials with a wide range of applications in the food, biopolymer and chemical industries. The global consumer organic acids market is estimated to increase to $36.86 billion by 2026. Conventionally, organic acids are produced from the chemical catalysis process with petrochemicals as raw materials, which posts severe environmental concerns and conflicts with our sustainable development goals. Most of the commonly used organic acids can be produced from various organisms. As a state-of-the-art technology, large-scale fermentative production of important organic acids with genetically-modified microbes has become an alternative to the chemical route to meet the market demand. Despite the fact that bio-based organic acid production from renewable cheap feedstock provides a viable solution, low productivity has impeded their industrial-scale application. With our deeper understanding of strain genetics, physiology and the availability of strain engineering tools, new technologies including synthetic biology, various metabolic engineering strategies, omics-based system biology tools, and high throughput screening methods are gradually established to bridge our knowledge gap. And they were further applied to modify the cellular reaction networks of potential microbial hosts and improve the strain performance, which facilitated the commercialization of consumable organic acids. Here we present the recent advances of metabolic engineering strategies to improve the production of important organic acids including fumaric acid, citric acid, itaconic acid, adipic acid, muconic acid, and we also discuss the current challenges and future perspectives on how we can develop a cost-efficient, green and sustainable process to produce these important chemicals from low-cost feedstocks.

Metabolic Engineering: Methodologies and Applications

Metabolic engineering aims to improve the production of economically valuable molecules through the genetic manipulation of microbial metabolism. While the discipline is a little over 30 years old, advancements in metabolic engineering have given way to industrial-level molecule production benefitting multiple industries such as chemical, agriculture, food, pharmaceutical, and energy industries. This review describes the design, build, test, and learn steps necessary for leading a successful metabolic engineering campaign. Moreover, we highlight major applications of metabolic engineering, including synthesizing chemicals and fuels, broadening substrate utilization, and improving host robustness with a focus on specific case studies. Finally, we conclude with a discussion on perspectives and future challenges related to metabolic engineering.

Metabolic engineering of Schizosaccharomyces pombe for itaconic acid production

The economical production of value-added chemicals from renewable biomass is a promising aspect of producing a sustainable economy. Itaconic acid (IA) is a high value-added compound that is expected to be an alternative to petroleum-based chemicals. In this study, we developed a metabolic engineering strategy for the large-scale production of IA from glucose using the fission yeast Schizosaccharomyces pombe. Heterologous expression of the cis-aconitic acid decarboxylase (CAD) gene from Aspergillus terreus, which encodes cis-aconitate decarboxylase in the cytosol, led to the production of 0.132 g/L of IA. We demonstrated that mitochondrial localization of CAD enhanced the production of IA. To prevent the leakage of carbon flux from the TCA cycle, we generated a strain in which the endogenous malate exporter, citrate lyase, and citrate transporter genes were disrupted. A titer of 1.110 g/L of IA was obtained from a culture of this strain started with 50 g/L of glucose. By culturing the multiple mutant strain at increased cell density, we succeeded in enhancing the IA production to 1.555 g/L. The metabolic engineering strategies presented in this study have the potential to improve the titer of the biosynthesis of derivatives of intermediates of the TCA cycle.

Import and Export of Mannosylerythritol Lipids by Ustilago maydis

Upon nitrogen starvation, the basidiomycete Ustilago maydis, which causes smut disease on corn, secretes amphipathic glycolipids, including mannosylerythritol lipids (MELs). MELs consist of a carbohydrate core whose mannosyl moiety is both acylated with fatty acids of different lengths and acetylated. Here, we report the transport of MELs into and out of the cell depending on the transport protein Mmf1, which belongs to the major facilitator superfamily. Analysis of mmf1 mutants and mutants lacking the acetyltransferase Mat1 revealed that Mmf1 is necessary for the export of acetylated MELs, while MELs without an acetyl group are secreted independently of this transporter. Upon deletion of mmf1, we detected novel MEL species lacking the acyl side chain at C-3′. With the help of feeding experiments, we demonstrate that MELs are taken up by U. maydis in an mmf1-independent manner. This leads to catabolism or rearrangement of acetyl and acyl side groups and subsequent secretion. The catabolism of MELs involves the presence of Mac2, an enzyme required for MEL biosynthesis. In cocultivation experiments, mutual exchange of MELs between different mutants was observed. Thus, we propose a novel function for fungal glycolipids as an external carbon storage.

Synergistic effects on itaconic acid production in engineered Aspergillus niger expressing the two distinct biosynthesis clusters from Aspergillus terreus and Ustilago maydis

BACKGROUND

Itaconic acid (IA) is a versatile platform chemical widely used for the synthesis of various polymers and current methods for IA production based on Aspergillus terreus fermentation are limited in terms of process efficiency and productivity. To construct more efficient IA production strains, A. niger was used as a chassis for engineering IA production by assembling the key components of IA biosynthesis pathways from both A. terreus and Ustilago maydis.

RESULTS

Recombinant A. niger S1596 overexpressing the A. terreus IA biosynthesis genes cadA, mttA, mfsA produced IA of 4.32 g/L, while A. niger S2120 overexpressing the U. maydis IA gene cluster adi1, tad1, mtt1, itp1 achieved IA of 3.02 g/L. Integration of the two IA production pathways led to the construction of A. niger S2083 with IA titers of 5.58 g/L. Increasing cadA copy number in strain S2083 created strain S2209 with titers of 7.99 g/L and deleting ictA to block IA degradation in S2209 created strain S2288 with IA titers of 8.70 g/L. Overexpressing acoA to enhance the supply of IA precursor in strain S2288 generated strain S2444 with IA titers of 9.08 g/L in shake flask.

CONCLUSION

Recombinant A. niger overexpressing the U. maydis IA biosynthesis pathway was capable of IA accumulation. Combined expression of the two IA biosynthesis pathways from A. terreus and U. maydis in A. niger resulted in much higher IA titers. Furthermore, increasing cadA copy number, deleting ictA to block IA degradation and overexpressing acoA to enhance IA precursor supply all showed beneficial effects on IA accumulation.

Engineering Yarrowia lipolytica to Produce Itaconic Acid From Waste Cooking Oil

Itaconic acid (IA) is a high-value organic acid with a plethora of industrial applications. In this study, we seek to develop a microbial cell factory that could utilize waste cooking oil (WCO) as raw material for circular and cost-effective production of the abovementioned biochemical. Specifically, we expressed cis-aconitic acid decarboxylase (CAD) gene from Aspergillus terreus in either the cytosol or peroxisome of Yarrowia lipolytica and assayed for production of IA on WCO. To further improve production yield, the 10 genes involved in the production pathway of acetyl-CoA, an intermediate metabolite necessary for the synthesis of cis-aconitic acid, were individually overexpressed and investigated for their impact on IA production. To minimize off-target flux channeling, we had also knocked out genes related to competing pathways in the peroxisome. Impressively, IA titer up to 54.55 g/L was achieved in our engineered Y. lipolytica in a 5 L bioreactor using WCO as the sole carbon source.

Seventeen Ustilaginaceae High-Quality Genome Sequences Allow Phylogenomic Analysis and Provide Insights into Secondary Metabolite Synthesis

The family of Ustilaginaceae belongs to the order of Basidiomycetes. Despite their plant pathogenicity causing, e.g., corn smut disease, they are also known as natural producers of value-added chemicals such as extracellular glycolipids, organic acids, and polyols. Here, we present 17 high-quality draft genome sequences (N50 > 1 Mb) combining third-generation nanopore and second-generation Illumina sequencing. The data were analyzed with taxonomical genome-based bioinformatics methods such as Percentage of Conserved Proteins (POCP), Average Nucleotide Identity (ANI), and Average Amino Acid Identity (AAI) analyses indicating that a reclassification of the Ustilaginaceae family might be required. Further, conserved core genes were determined to calculate a phylogenomic core genome tree of the Ustilaginaceae that also supported the results of the other phylogenomic analysis. In addition, to genomic comparisons, secondary metabolite clusters (e.g., itaconic acid, mannosylerythritol lipids, and ustilagic acid) of biotechnological interest were analyzed, whereas the sheer number of clusters did not differ much between species.

State of the Art on the Microbial Production of Industrially Relevant Organic Acids

The industrial relevance of organic acids is high; because of their chemical properties, they can be used as building blocks as well as single-molecule agents with a huge annual market. Organic acid chemical platforms can derive from fossil sources by petrochemical refining processes, but most of them also represent natural metabolites produced by many cells. They are the products, by-products or co-products of many primary metabolic processes of microbial cells. Thanks to the potential of microbial cell factories and to the development of industrial biotechnology, from the last decades of the previous century, the microbial-based production of these molecules has started to approach the market. This was possible because of a joint effort of microbial biotechnologists and biochemical and process engineers that boosted natural production up to the titer, yield and productivity needed to be industrially competitive. More recently, the possibility to utilize renewable residual biomasses as feedstock not only for biofuels, but also for organic acids production is further augmenting the sustainability of their production, in a logic of circular bioeconomy. In this review, we briefly present the latest updates regarding the production of some industrially relevant organic acids (citric fumaric, itaconic, lactic and succinic acid), discussing the challenges and possible future developments of successful production.

Aconitic Acid Recovery from Renewable Feedstock and Review of Chemical and Biological Applications

Aconitic acid (propene-1,2,3-tricarboxylic acid) is the most prevalent 6-carbon organic acid that accumulates in sugarcane and sweet sorghum. As a top value-added chemical, aconitic acid may function as a chemical precursor or intermediate for high-value downstream industrial and biological applications. These downstream applications include use as a bio-based plasticizer, cross-linker, and the formation of valuable and multi-functional polyesters that have also been used in tissue engineering. Aconitic acid also plays various biological roles within cells as an intermediate in the tricarboxylic acid cycle and in conferring unique survival advantages to some plants as an antifeedant, antifungal, and means of storing fixed pools of carbon. Aconitic acid has also been reported as a fermentation inhibitor, anti-inflammatory, and a potential nematicide. Since aconitic acid can be sustainably sourced from renewable, inexpensive sources such as sugarcane, molasses, and sweet sorghum syrup, there is enormous potential to provide multiple streams of additional income to the sugar industry through downstream industrial and biological applications that we discuss in this review.

Production of mannosylerythritol lipids: biosynthesis, multi-omics approaches, and commercial exploitation

Compilation of resources regarding MEL biosynthesis, key production parameters; available omics resources and current commercial applications, for smut fungi known to produce MELs.

John PC, Kumar N. Bayesian Inference for Integrating Yarrowia lipolytica Multiomics Datasets with Metabolic Modeling

Optimizing the metabolism of microbial cell factories for yields and titers is a critical step for economically viable production of bioproducts and biofuels. In this process, tuning the expression of individual enzymes to obtain the desired pathway flux is a challenging step, in which data from separate multiomics techniques must be integrated with existing biological knowledge to determine where changes should be made. Following a design-build-test-learn strategy, building on recent advances in Bayesian metabolic control analysis, we identify key enzymes in the oleaginous yeast Yarrowia lipolytica that correlate with the production of itaconate by integrating a metabolic model with multiomics measurements. To this extent, we quantify the uncertainty for a variety of key parameters, known as flux control coefficients (FCCs), needed to improve the bioproduction of target metabolites and statistically obtain key correlations between the measured enzymes and boundary flux. Based on the top five significant FCCs and five correlated enzymes, our results show phosphoglycerate mutase, acetyl-CoA synthetase (ACSm), carbonic anhydrase (HCO3E), pyrophosphatase (PPAm), and homoserine dehydrogenase (HSDxi) enzymes in rate-limiting reactions that can lead to increased itaconic acid production.

Current Progress in Production of Building-Block Organic Acids by Consolidated Bioprocessing of Lignocellulose

Several organic acids have been indicated among the top value chemicals from biomass. Lignocellulose is among the most attractive feedstocks for biorefining processes owing to its high abundance and low cost. However, its highly complex nature and recalcitrance to biodegradation hinder development of cost-competitive fermentation processes. Here, current progress in development of single-pot fermentation (i.e., consolidated bioprocessing, CBP) of lignocellulosic biomass to high value organic acids will be examined, based on the potential of this approach to dramatically reduce process costs. Different strategies for CBP development will be considered such as: (i) design of microbial consortia consisting of (hemi)cellulolytic and valuable-compound producing strains; (ii) engineering of microorganisms that combine biomass-degrading and high-value compound-producing properties in a single strain. The present review will mainly focus on production of organic acids with application as building block chemicals (e.g., adipic, cis,cis-muconic, fumaric, itaconic, lactic, malic, and succinic acid) since polymer synthesis constitutes the largest sector in the chemical industry. Current research advances will be illustrated together with challenges and perspectives for future investigations. In addition, attention will be dedicated to development of acid tolerant microorganisms, an essential feature for improving titer and productivity of fermentative production of acids.

TCA Cycle Replenishing Pathways in Photosynthetic Purple Non-Sulfur Bacteria Growing with Acetate

Purple non-sulfur bacteria (PNSB) are anoxygenic photosynthetic bacteria harnessing simple organic acids as electron donors. PNSB produce a-aminolevulinic acid, polyhydroxyalcanoates, bacteriochlorophylls a and b, ubiquinones, and other valuable compounds. They are highly promising producers of molecular hydrogen. PNSB can be cultivated in organic waste waters, such as wastes after fermentation. In most cases, wastes mainly contain acetic acid. Therefore, understanding the anaplerotic pathways in PNSB is crucial for their potential application as producers of biofuels. The present review addresses the recent data on presence and diversity of anaplerotic pathways in PNSB and describes different classifications of these pathways.

Membrane transporters in the bioproduction of organic acids: state of the art and future perspectives for industrial applications

Organic acids such as monocarboxylic acids, dicarboxylic acids or even more complex molecules such as sugar acids, have displayed great applicability in the industry as these compounds are used as platform chemicals for polymer, food, agricultural and pharmaceutical sectors. Chemical synthesis of these compounds from petroleum derivatives is currently their major source of production. However, increasing environmental concerns have prompted the production of organic acids by microorganisms. The current trend is the exploitation of industrial biowastes to sustain microbial cell growth and valorize biomass conversion into organic acids. One of the major bottlenecks for the efficient and cost-effective bioproduction is the export of organic acids through the microbial plasma membrane. Membrane transporter proteins are crucial elements for the optimization of substrate import and final product export. Several transporters have been expressed in organic acid-producing species, resulting in increased final product titers in the extracellular medium and higher productivity levels. In this review, the state of the art of plasma membrane transport of organic acids is presented, along with the implications for industrial biotechnology.

Perspectives for the application of Ustilaginaceae as biotech cell factories

Basidiomycetes fungi of the family Ustilaginaceae are mainly known as plant pathogens causing smut disease on crops and grasses. However, they are also natural producers of value-added substances like glycolipids, organic acids, polyols, and harbor secretory enzymes with promising hydrolytic activities. These attributes recently evoked increasing interest in their biotechnological exploitation. The corn smut fungus Ustilago maydis is the best characterized member of the Ustilaginaceae. After decades of research in the fields of genetics and plant pathology, a broad method portfolio and detailed knowledge on its biology and biochemistry are available. As a consequence, U. maydis has developed into a versatile model organism not only for fundamental research but also for applied biotechnology. Novel genetic, synthetic biology, and process development approaches have been implemented to engineer yields and product specificity as well as for the expansion of the repertoire of produced substances. Furthermore, research on U. maydis also substantially promoted the interest in other members of the Ustilaginaceae, for which the available tools can be adapted. Here, we review the latest developments in applied research on Ustilaginaceae towards their establishment as future biotech cell factories.

Production of itaconic acid from alkali pretreated lignin by dynamic two stage bioconversion

Expanding the portfolio of products that can be made from lignin will be critical to enabling a viable bio-based economy. Here, we engineer Pseudomonas putida for high-yield production of the tricarboxylic acid cycle-derived building block chemical, itaconic acid, from model aromatic compounds and aromatics derived from lignin. We develop a nitrogen starvation-detecting biosensor for dynamic two-stage bioproduction in which itaconic acid is produced during a non-growth associated production phase. Through the use of two distinct itaconic acid production pathways, the tuning of TCA cycle gene expression, deletion of competing pathways, and dynamic regulation, we achieve an overall maximum yield of 56% (mol/mol) and titer of 1.3 g/L from p-coumarate, and 1.4 g/L titer from monomeric aromatic compounds produced from alkali-treated lignin. This work illustrates a proof-of-principle that using dynamic metabolic control to reroute carbon after it enters central metabolism enables production of valuable chemicals from lignin at high yields by relieving the burden of constitutively expressing toxic heterologous pathways.

Aspergillus terreus as an industrial filamentous fungus for pharmaceutical biotechnology

Aspergillus terreus is an important Aspergillus species, which has been applied in the industrial production of the bio-based chemical itaconic acid and the lipid-lowering drug lovastatin. The excellent fermentation capability has been demonstrated in these industrial applications. The genomic information revealed that the outstanding capacity of natural product synthesis by A. terreus remains to be further explored. With advances of the genome mining strategy, the products of several cryptic biosynthetic gene clusters have been discovered recently. In addition, a series of metabolic engineering studies have been performed in the industrial strains of lovastatin and itaconic acid to further improve the production processes. This review presents the current progress and the future outlook in the field of A. terreus biotechnology.

Engineering microorganisms for the biosynthesis of dicarboxylic acids

Dicarboxylic acids (DCAs) are important commodity chemicals which have been widely applied in polymer, food and pharmaceutical industries. Biosynthesis of DCAs from renewable carbon sources represents a promising alternative to chemical synthesis. Over the years, the recombinant strains have been constructed to produce an increasing number of DCAs. In this review, recent advances on the microbial synthesis of various DCAs have been summarized and categorized into three groups: the tricarboxylic acid cycle-derived, lysine metabolism-related, and aromatic compounds degradation-derived DCAs. We focused mainly on the metabolic engineering and synthetic biology strategies for improving the production efficiency, including metabolic flux analysis, fine-tuning of gene expression, cofactor balancing, metabolic compartmentalization, dynamic regulation and co-culture to regulate the production at multiple levels. The current challenges and perspectives have also been discussed.

Engineering Ustilago maydis for production of tailor-made mannosylerythritol lipids

Mannosylerythritol lipids (MELs) are surface active glycolipids secreted by various fungi. MELs can be used as biosurfactants and are a biodegradable resource for the production of detergents or pharmaceuticals. Different fungal species synthesize a unique mixture of MELs differing in acetyl- and acyl-groups attached to the sugar moiety. Here, we report the construction of a toolbox for production of glycolipids with predictable fatty acid side chains in the basidiomycete Ustilago maydis. Genes coding for acyl-transferases involved in MEL production (Mac1 and Mac2) from different fungal species were combined to obtain altered MEL variants with distinct physical properties and altered antimicrobial activity. We also demonstrate that a U. maydis paralog of the acyltransferase Mac2 with a different substrate specificity can be employed for the biosynthesis of modified MEL variants. In summary, our data showcase how the fungal repertoire of Mac enzymes can be used to engineer tailor-made MELs according to specific biotechnological or pharmaceutical requirements.

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Biosynthetic enzymes for MELs from distinct fungal species retain their substrate specificity if expressed in U. maydis.

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The combination of acyltransferases from different fungi leads to the production of unique MEL variants.

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Novel MELs show altered physical properties and antimicrobial activity.

Ustilaginaceae Biocatalyst for Co-Metabolism of CO2-Derived Substrates toward Carbon-Neutral Itaconate Production

The family Ustilaginaceae (belonging to the smut fungi) are known for their plant pathogenicity. Despite the fact that these plant diseases cause agricultural yield reduction, smut fungi attracted special attention in the field of industrial biotechnology. Ustilaginaceae show a versatile product spectrum such as organic acids (e.g., itaconate, malate, succinate), polyols (e.g., erythritol, mannitol), and extracellular glycolipids, which are considered value-added chemicals with potential applications in the pharmaceutical, food, and chemical industries. This study focused on itaconate as a platform chemical for the production of resins, plastics, adhesives, and biofuels. During this work, 72 different Ustilaginaceae strains from 36 species were investigated for their ability to (co-) consume the CO₂-derived substrates acetate and formate, potentially contributing toward a carbon-neutral itaconate production. The fungal growth and product spectrum with special interest in itaconate was characterized. Ustilago maydis MB215 and Ustilago rabenhorstiana NBRC 8995 were identified as promising candidates for acetate metabolization whereas Ustilago cynodontis NBRC 7530 was identified as a potential production host using formate as a co-substrate enhancing the itaconate production. Selected strains with the best itaconate production were characterized in more detail in controlled-batch bioreactor experiments confirming the co-substrate utilization. Thus, a proof-of-principle study was performed resulting in the identification and characterization of three promising Ustilaginaceae biocatalyst candidates for carbon-neutral itaconate production contributing to the biotechnological relevance of Ustilaginaceae.

An Unconventional Melanin Biosynthesis Pathway in Ustilago maydis

Ustilago maydis is a phytopathogenic fungus responsible for corn smut disease. Although it is a very well-established model organism for the study of plant-microbe interactions, its potential to produce specialized metabolites, which might contribute to this interaction, has not been studied in detail. By analyzing the U. maydis genome, we identified a biosynthetic gene cluster whose activation led to the production of a black melanin pigment. Single deletion mutants of the cluster genes revealed that five encoded enzymes are required for the accumulation of the black pigment, including three polyketide synthases (pks3, pks4, and pks5), a cytochrome P450 monooxygenase (cyp4), and a protein with similarity to versicolorin B synthase (vbs1). Metabolic profiles of deletion mutants in this gene cluster suggested that Pks3 and Pks4 act in concert as heterodimers to generate orsellinic acid (OA), which is reduced to the corresponding aldehyde by Pks5. The OA-aldehyde can then react with triacetic acid lactone (TAL), also derived from Pks3/Pks4 heterodimers to form larger molecules, including novel coumarin derivatives. Our findings suggest that U. maydis synthesizes a novel type of melanin based on coumarin and pyran-2-one intermediates, while most fungal melanins are derived from 1,8-dihydroxynaphthalene (DHN) or l-3,4-dihydroxyphenylalanine (l-DOPA). Along with these observations, this work also provides insight into the mechanisms of polyketide synthases in this filamentous fungus.IMPORTANCE The fungus Ustilago maydis represents one of the major threats to maize plants since it is responsible for corn smut disease, which generates considerable economical losses around the world. Therefore, contributing to a better understanding of the biochemistry of defense mechanisms used by U. maydis to protect itself against harsh environments, such as the synthesis of melanin, could provide improved biological tools for tackling the problem and protect the crops. In addition, the fact that this fungus synthesizes melanin in an unconventional way, requiring more than one polyketide synthase for producing melanin precursors, gives a different perspective on the complexity of these multidomain enzymes and their evolution in the fungal kingdom.

High level production of itaconic acid at low pH by Ustilago maydis with fed-batch fermentation

The metabolically engineered plant pathogen Ustilago maydis MB215 Δcyp3 P_etefria1 has been cultivated to produce more than 80 g/L itaconate in 16 L scale pH and temperature controlled fermentation, in fed-batch mode with two successive feedings. The effect of pH as well as successive rounds of feeding has been quantified via elemental balances. Volumetric itaconic acid productivity gradually decreased with successive glucose feedings with increasing itaconic titers, with nearly constant product yield. Extracellular pH was decreased from 6 down to 3.5 and the fermentation was characterized in specific uptake, production, and growth rates. Notable is that the biomass composition changes significantly from growth phase to itaconic acid production phase, carbon content increases from 42% to around 62%. Despite the gradual decrease in itaconic acid levels with decreasing pH (nearly 50% decrease in itaconic acid at pH 3.5, compared to pH 6), significant itaconate production is still observed at pH 4 (around 63 g/L). Biomass yield remained nearly constant until pH 4. Taken together, these results strongly illustrate the potential of engineered Ustilago maydis in itaconate production at commercial levels.

An Optimized Ustilago maydis for Itaconic Acid Production at Maximal Theoretical Yield

Ustilago maydis, a member of the Ustilaginaceae family, is a promising host for the production of several metabolites including itaconic acid. This dicarboxylate has great potential as a bio-based building block in the polymer industry, and is of special interest for pharmaceutical applications. Several itaconate overproducing Ustilago strains have been generated by metabolic and morphology engineering. This yielded stabilized unicellular morphology through fuz7 deletion, reduction of by-product formation through deletion of genes responsible for itaconate oxidation and (glyco)lipid production, and the overexpression of the regulator of the itaconate cluster ria1 and the mitochondrial tricarboxylate transporter encoded by mttA from Aspergillus terreus. In this study, itaconate production was further optimized by consolidating these different optimizations into one strain. The combined modifications resulted in itaconic acid production at theoretical maximal yield, which was achieved under biotechnologically relevant fed-batch fermentations with continuous feed.

Consolidated bioprocessing of cellulose to itaconic acid by a co-culture of Trichoderma reesei and Ustilago maydis

BACKGROUND

Itaconic acid is a bio-derived platform chemical with uses ranging from polymer synthesis to biofuel production. The efficient conversion of cellulosic waste streams into itaconic acid could thus enable the sustainable production of a variety of substitutes for fossil oil based products. However, the realization of such a process is currently hindered by an expensive conversion of cellulose into fermentable sugars. Here, we present the stepwise development of a fully consolidated bioprocess (CBP), which is capable of directly converting recalcitrant cellulose into itaconic acid without the need for separate cellulose hydrolysis including the application of commercial cellulases. The process is based on a synthetic microbial consortium of the cellulase producer Trichoderma reesei and the itaconic acid producing yeast Ustilago maydis. A method for process monitoring was developed to estimate cellulose consumption, itaconic acid formation as well as the actual itaconic acid production yield online during co-cultivation.

RESULTS

The efficiency of the process was compared to a simultaneous saccharification and fermentation setup (SSF). Because of the additional substrate consumption of T. reesei in the CBP, the itaconic acid yield was significantly lower in the CBP than in the SSF. In order to increase yield and productivity of itaconic acid in the CBP, the population dynamics was manipulated by varying the inoculation delay between T. reesei and U. maydis. Surprisingly, neither inoculation delay nor inoculation density significantly affected the population development or the CBP performance. Instead, the substrate availability was the most important parameter. U. maydis was only able to grow and to produce itaconic acid when the cellulose concentration and thus, the sugar supply rate, was high. Finally, the metabolic processes during fed-batch CBP were analyzed in depth by online respiration measurements. Thereby, substrate availability was again identified as key factor also controlling itaconic acid yield. In summary, an itaconic acid titer of 34 g/L with a total productivity of up to 0.07 g/L/h and a yield of 0.16 g/g could be reached during fed-batch cultivation.

CONCLUSION

This study demonstrates the feasibility of consortium-based CBP for itaconic acid production and also lays the fundamentals for the development and improvement of similar microbial consortia for cellulose-based organic acid production.

Innovations in CAZyme gene diversity and its modification for biorefinery applications

For sustainable growth, concept of biorefineries as recourse to the "fossil derived" energy source is important. Here, the Carbohydrate Active enZymes (CAZymes) play decisive role in generation of biofuels and related sugar-based products utilizing lignocellulose as a carbon source. Given their industrial significance, extensive studies on the evolution of CAZymes have been carried out. Various bacterial and fungal organisms have been scrutinized for the development of CAZymes, where advance techniques for strain enhancement such as CRISPR and analysis of specific expression systems have been deployed. Specific Omic-based techniques along with protein engineering have been adopted to unearth novel CAZymes and improve applicability of existing enzymes. In-Silico computational research and functional annotation of new CAZymes to synergy experiments are being carried out to devise cocktails of enzymes for use in biorefineries. Thus, with the establishment of these technologies, increased diversity of CAZymes with broad span of functions and applications is seen.

Transporter engineering in microbial cell factories: the ins, the outs, and the in-betweens

Engineering the transport of small molecules is an effective approach to improve the performance of microbial cell factories. Transporter engineering can improve the utilization of low-cost alternative substrates, reduce the loss of pathway intermediates, and increase the titer and production rate of the target product. However, transporters are not commonly engineered in strain development programs because the functions of most of the transport proteins are not known. In the recent years, a variety of methods have been developed for identification of transporters for specific substrates and for characterizing transport mechanisms. This review presents recent examples of successful transport engineering for cell factories and discusses the methods for transporter identification and characterization.

Uncovering biosynthetic relationships between antifungal nonadrides and octadrides

Maleidrides are a class of bioactive secondary metabolites unique to filamentous fungi, which contain one or more maleic anhydrides fused to a 7-, 8- or 9- membered carbocycle (named heptadrides, octadrides and nonadrides respectively). Herein structural and biosynthetic studies on the antifungal octadride, zopfiellin, and nonadrides scytalidin, deoxyscytalidin and castaneiolide are described. A combination of genome sequencing, bioinformatic analyses, gene disruptions, biotransformations, isotopic feeding studies, NMR and X-ray crystallography revealed that they share a common biosynthetic pathway, diverging only after the nonadride deoxyscytalidin. 5-Hydroxylation of deoxyscytalidin occurs prior to ring contraction in the zopfiellin pathway of Diffractella curvata. In Scytalidium album, 6-hydroxylation - confirmed as being catalysed by the α-ketoglutarate dependent oxidoreductase ScyL2 - converts deoxyscytalidin to scytalidin, in the final step in the scytalidin pathway. Feeding scytalidin to a zopfiellin PKS knockout strain led to the production of the nonadride castaneiolide and two novel ring-open maleidrides.

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.

Metabolic engineering of an industrial Aspergillus niger strain for itaconic acid production

Itaconic acid is a value-added organic acid that is widely applied in industrial production. It can be converted from citric acid by some microorganisms including Aspergillus terreus and Aspergillus niger. Because of high citric acid production (more than 200 g/L), A. niger strains may be developed into powerful itaconic acid-producing microbial cell factories. In this study, industrial citric acid-producing strain A. niger YX-1217, capable of producing 180.0-200.0 g/L, was modified to produce itaconic acid by metabolic engineering. A key gene cadA encoding aconitase was expressed in A. niger YX-1217 under the control of three different promoters. Analyses showed that the PglaA promoter resulted in higher levels of gene expression than the PpkiA and PgpdA promoters. Moreover, the synthesis pathway of itaconic acid was extended by introducing the acoA gene, and the cadA gene, encoding aconitate decarboxylase, into A. niger YX-1217 under the function of the two rigid short-peptide linkers L₁ or L₂. The resulting recombinant strains L-1 and L-2 were induced to produce itaconic acid in fed-batch fermentations under three-stage control of agitation speed. After fermentation for 104 h, itaconic acid concentrations in the recombinant strain L-2 culture reached 7.2 g/L, which represented a 71.4% increase in itaconic acid concentration compared with strain Z-17 that only expresses cadA. Therefore, co-expression of acoA and cadA resulted in an extension of the citric acid metabolic pathway to the itaconic acid metabolic pathway, thereby increasing the production of itaconic acid by A. niger.