Mitochondrial Citrate Transporters CtpA and YhmA Are Required for Extracellular Citric Acid Accumulation and Contribute to Cytosolic Acetyl Coenzyme A Generation in Aspergillus luchuensis mut. kawachii

Aspergillus awamori: potential antioxidant, anti-inflammatory, and anti-apoptotic activities in acetic acid-induced ulcerative colitis in rats

Ulcerative colitis (UC) is a chronic colonic inflammation with a significant health hazard. Aspergillus awamori (A. awamori) is a microorganism with various bioactive compounds with natural antioxidant and anti-inflammatory properties. The present work aimed to elucidate the protective and therapeutic effects of varying concentrations of A. awamori against acetic acid (AA)-induced ulcerative colitis (UC) in rats. Nine groups of albino male rats were established: a control negative group (G1), a control positive group (G2,AA), and preventive protocol groups (including G3A, G4A, and G5A) that received 100 mg, 50 mg, and 25 mg/kg b.w, respectively, of A. awamori orally and daily from the 1st day of the experiment and for 7 consecutive days. Then, they were subjected to one dose of AA intrarectally on day 8th. G3B, G4B, and G5B were termed as curative protocol groups that received one dose of AA on day 8th and then administered 100 mg, 50 mg, and 25 mg/kg b.w. of A. awamori, respectively, on day 9th and continued receiving these doses daily until day 16th. Rats in the AA group exhibited marked histopathological alterations of the distal colon, with an exaggeration of the DAI. In addition, a remarkable increase in oxidative stress was represented by the elevation of MDA and NO levels with a decline in SOD and GPx activities. In addition, upregulation of TNF-α, IL-6, and IL-1β mRNA expressions and downregulation of Muc2 and Nrf2 levels were detected. Unambiguously, a remarkable anti-inflammatory effect was noticed either in A. awamori prevented or treated groups expounded by reducing and regulating TNF-α, IL-6, and IL-1β with improved pathological lesion scoring. The Muc2, Nrf2, and bcl-2 gene levels were upregulated and restored also. In summary, the findings in this work reveal that A. awamori supplementation successfully alleviated the UC induced by AA, which had a better effect when administered before colitis induction.

Deficiency of β-xylosidase activity in Aspergillus luchuensis mut. kawachii IFO 4308

In this study, we investigated a deleterious mutation in the β-xylosidase gene, xylA (AkxylA), in Aspergillus luchuensis mut. kawachii IFO 4308 by constructing an AkxylA disruptant and complementation strains of AkxylA and xylA derived from A. luchuensis RIB2604 (AlxylA), which does not harbor the mutation in xylA. Only the AlxylA complementation strain exhibited significantly higher growth and substantial β-xylosidase activity in medium containing xylan, accompanied by an increase in XylA expression. This resulted in lower xylobiose and higher xylose concentrations in the mash of barley shochu. These findings suggest that the mutation in xylA affects xylose levels during the fermentation process. Because the mutation in xylA was identified not only in the genome of strain IFO 4308 but also the genomes of other industrial strains of A. luchuensis and A. luchuensis mut. kawachii, these findings enhance our understanding of the genetic factors that affect the fermentation characteristics.

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.

Aspergillus oryzae as a Cell Factory: Research and Applications in Industrial Production

Aspergillus oryzae, a biosafe strain widely utilized in bioproduction and fermentation technology, exhibits a robust hydrolytic enzyme secretion system. Therefore, it is frequently employed as a cell factory for industrial enzyme production. Moreover, A. oryzae has the ability to synthesize various secondary metabolites, such as kojic acid and L-malic acid. Nevertheless, the complex secretion system and protein expression regulation mechanism of A. oryzae pose challenges for expressing numerous heterologous products. By leveraging synthetic biology and novel genetic engineering techniques, A. oryzae has emerged as an ideal candidate for constructing cell factories. In this review, we provide an overview of the latest advancements in the application of A. oryzae-based cell factories in industrial production. These studies suggest that metabolic engineering and optimization of protein expression regulation are key elements in realizing the widespread industrial application of A. oryzae cell factories. It is anticipated that this review will pave the way for more effective approaches and research avenues in the future implementation of A. oryzae cell factories in industrial production.

Ctp1 and Yhm2: Two Mitochondrial Citrate Transporters to Support Metabolic Flexibility of Saccharomyces cerevisiae

Differently from higher eukaryotic cells, in the yeast Saccharomyces cerevisiae there are two mitochondrial carrier proteins involved in the transport of citrate: Ctp1 and Yhm2. Very little is known about the physiological role of these proteins. Wild-type and mutant yeast strains deleted in CTP1 and YHM2 were grown in media supplemented with a fermentable (glucose) or a nonfermentable (ethanol) carbon source. To assess changes in Ctp1 and Yhm2 mRNA expression levels, real-time PCR was performed after total RNA extraction. In the wild-type strain, the metabolic switch from the exponential to the stationary phase is associated with an increase in the expression level of the two citrate transporters. In addition, the results obtained in the mutant strains suggest that the presence of a single citrate transporter can partially compensate for the absence of the other. Ctp1 and Yhm2 differently contribute to fermentative and respiratory metabolism. Moreover, the two mitochondrial carriers represent a link between the Krebs cycle and the glyoxylate cycle, which play a key role in the metabolic adaptation strategies of S. cerevisiae.

Expression of heterochromatin protein 1 affects citric acid production in Aspergillus luchuensis mut. kawachii

A putative methyltransferase, LaeA, controls citric acid production through epigenetic regulation of the citrate exporter gene, cexA, in the white koji fungus Aspergillus luchuensis mut. kawachii. In this study, we investigated the role of another epigenetic regulator, heterochromatin protein 1, HepA, in citric acid production. The ΔhepA strain exhibited reduced citric acid production in liquid culture, although to a lesser extent compared to the ΔlaeA strain. In addition, the ΔlaeA ΔhepA strain showed citric acid production similar to the ΔlaeA strain, indicating that HepA plays a role in citric acid production, albeit with a less-significant regulatory effect than LaeA. RNA-seq analysis revealed that the transcriptomic profiles of the ΔhepA and ΔlaeA strains were similar, and the expression level of cexA was reduced in both strains. These findings suggest that the genes regulated by HepA are similar to those regulated by LaeA in A. luchuensis mut. kawachii. However, the reductions in citric acid production and cexA expression observed in the disruptants were mitigated in rice koji, a solid-state culture. Thus, the mechanism by which citric acid production is regulated differs between liquid and solid cultivation. Further investigation is thus needed to understand the regulatory mechanism in koji.

Fungal carboxylate transporters: recent manipulations and applications

Carboxylic acids containing acidic groups with additional keto/hydroxyl-groups or unsaturated bond have displayed great applicability in the food, agricultural, cosmetic, textile, and pharmaceutical industries. The traditional approach for carboxylate production through chemical synthesis is based on petroleum derivatives, resulting in concerns for the environmental complication and energy crisis, and increasing attention has been attracted to the eco-friendly and renewable bio-based synthesis for carboxylate production. The efficient and specific export of target carboxylic acids through the microbial membrane is essential for high productivity, yield, and titer of bio-based carboxylates. Therefore, understanding the characteristics, regulations, and efflux mechanisms of carboxylate transporters will efficiently increase industrial biotechnological production of carboxylic acids. Several transporters from fungi have been reported and used for improved synthesis of target products. The transport activity and substrate specificity are two key issues that need further improvement in the application of carboxylate transporters. This review presents developments in the structural and functional diversity of carboxylate transporters, focusing on the modification and regulation of carboxylate transporters to alter the transport activity and substrate specificity, providing new strategy for transporter engineering in constructing microbial cell factory for carboxylate production. KEY POINTS: • Structures of multiple carboxylate transporters have been predicted. • Carboxylate transporters can efficiently improve production. • Modification engineering of carboxylate transporters will be more popular in the future.

Metabolic engineering and fermentation optimization strategies for producing organic acids of the tricarboxylic acid cycle by microbial cell factories

The organic acids of the tricarboxylic acid (TCA) pathway are important platform compounds and are widely used in many areas. The high-productivity strains and high-efficient and low-cost fermentation are required to satisfy a huge market size. The high metabolic flux of the TCA pathway endows microorganisms potential to produce high titers of these organic acids. Coupled with metabolic engineering and fermentation optimization, the titer of the organic acids has been significantly improved in recent years. Herein, we discuss and compare the recent advances in synthetic pathway engineering, cofactor engineering, transporter engineering, and fermentation optimization strategies to maximize the biosynthesis of organic acids. Such engineering strategies were mainly based on the TCA pathway and glyoxylate pathway. Furthermore, organic-acid-secretion enhancement and renewable-substrate-based fermentation are often performed to assist the biosynthesis of organic acids. Further strategies are also discussed to construct high-productivity and acid-resistant strains for industrial large-scale production.

Overexpression of Citrate-Malate Carrier Promoted Lipid Accumulation in Oleaginous Filamentous Fungus Mortierella alpina

The mitochondrial citrate-malate carrier is responsible for the transport of citrate and malate between the mitochondria and cytosol, ensuring citrate supply substrate for fatty acid synthesis. In this study, we investigated the overexpression of the citrate-malate carrier coded by three genes (MaCT1/MaCT2/MaTCT) in Mortierella alpina to enhance lipid accumulation. Our results showed that the overexpression of MaCT1, MaCT2, and MaTCT increased the fatty acid content by up to 21.7, 29.5, and 12.8%, respectively, compared with the control strain, but had no effect on the growth. Among them, the MaCT2-overexpressing strain performed the best, and its total fatty acid yield was increased by 51.6% compared to the control. Furthermore, the relative transcription level of MaCT2 indeed increased significantly in the recombinant strains. These findings are beneficial to understanding the citrate transport system and improve the industrial applications of the oleaginous filamentous fungus M. alpina.

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.

Identification and genetic characterization of mitochondrial citrate transporters in Aspergillus niger

Aspergillus niger is a major cell factory for citric acid production, and the process of citrate export from mitochondria to cytoplasm is predicted to be one of rate-limiting steps in citric acid accumulation. Currently, the mitochondrial citrate transporters (Ctps) in A. niger are not fully characterized. Here, six putative Ctp encoding genes (ctpA to ctpF) were identified based on their homology with a mitochondrial citrate transporter ScCtp1 from Saccharomyces cerevisiae. Disruption of individual ctpA to ctpF caused varying degrees of decline in citric acid accumulation at different fermentation stages, whereas a mutant strain S1696 with disruption of all six ctps showed complete loss of citiric acid production. S1696 also exhibited delayed growth, reduced conidia formation, and decreased pigmentogenesis. Exogenous addition of citrate partially restored the conidia formation and pigmentogenesis in S1696 mutant. Reintroduction of individual ctps (ctpA to ctpF) into S1696 at the amyA locus showed that ctpA, ctpB, and ctpD restored the citric acid titers to 88.5, 93.8, and 94.6% of the parent strain, respectively. Additionally, the formation of conidia and pigment production was partially restored after reintroduction of ctpA, ctpB, or ctpD. Overexpression of respective ctpA, ctpB, and ctpD in the parent strain resulted in increases in citric acid accumulation by 32.8, 19.3, and 24.2%, respectively. These results demonstrate that CtpA, CtpB, and CtpD play important roles in citric acid transport across the mitochondrial membrane and function in a redundant manner. Enhancement of citric acid transport process can serve as a target for boosting citric acid accumulation in A. niger.

Multi-omics analyses of the transition to the Crabtree effect in S. cerevisiae reveals a key role for the citric acid shuttle

The Crabtree effect in the yeast, Saccharomyces cerevisiae, has been extensively studied, but only few studies have analyzed the dynamic conditions across the critical specific growth rate where the Crabtree effect sets in. Here, we carried out a multi-omics analysis of S. cerevisiae undergoing a specific growth rate transition from 0.2 h-1 to 0.35 h-1. The extracellular metabolome, the transcriptome and the proteome were analyzed in an 8-hour transition period after the specific growth rate shifted from 0.2 h-1 to 0.35 h-1. The changing trends of both the transcriptome and proteome were analyzed using principal component analysis, which showed that the transcriptome clustered together after 60 min, while the proteome reached steady-state much later. Focusing on central carbon metabolism, we analyzed both the changes in the transcriptome and proteome, and observed an interesting changing pattern in the tricarboxylic acid (TCA) pathway, which indicates an important role for citric acid shuttling across the mitochondrial membrane for α-ketoglutarate accumulation during the transition from respiratory to respiro-fermentative metabolism. This was supported by a change in the oxaloacetate and malate shuttle. Together, our findings shed new light into the onset of the Crabtree effect in S. cerevisiae.

The white koji fungus Aspergillus luchuensis mut. kawachii

The white koji fungus, Aspergillus luchuensis mut. kawachii, is used in the production of shochu, a traditional Japanese distilled spirit. White koji fungus plays an important role in the shochu production process by supplying amylolytic enzymes such as α-amylase and glucoamylase. These enzymes convert starch contained in primary ingredients such as rice, barley, buckwheat, and sweet potato into glucose, which is subsequently utilized by the yeast Saccharomyces cerevisiae to produce ethanol. White koji fungus also secretes large amounts of citric acid, which lowers the pH of the shochu mash, thereby preventing the growth of undesired microbes and enabling stable production of shochu in relatively warm regions of Japan. This review describes the historical background, research tools, and recent advances in studies of the mechanism of citric acid production by white koji fungus.

Tricarboxylate Citrate Transporter of an Oleaginous Fungus Mucor circinelloides WJ11: From Function to Structure and Role in Lipid Production

The citrate transporter protein (CTP) plays an important role in citrate efflux from the mitochondrial matrix to cytosol that has great importance in oleaginous fungi. The cytoplasmic citrate produced after citrate efflux serves as the primary carbon source for the triacylglycerol and cholesterol biosynthetic pathways. Because of the CTP's importance, our laboratory has extensively studied its structure/function relationships in Mucor circinelloides to comprehend its molecular mechanism. In the present study, the tricarboxylate citrate transporter (Tct) of M. circinelloides WJ11 has been cloned, overexpressed, purified, kinetically, and structurally characterized. The Tct protein of WJ11 was expressed in Escherichia coli, isolated, and functionally reconstituted in a liposomal system for kinetic studies. Our results showed that Tct has a high affinity for citrate with Km 0.018 mM. Furthermore, the tct overexpression and knockout plasmids were created and transformed into M. circinelloides WJ11. The mitochondria of the tct-overexpressing transformant of M. circinelloides WJ11 showed a 49% increase in citrate efflux, whereas the mitochondria of the tct-knockout transformant showed a 39% decrease in citrate efflux compared to the mitochondria of wild-type WJ11. To elucidate the structure-function relationship of this biologically important transporter a 3D model of the mitochondrial Tct protein was constructed using homology modeling. The overall structure of the protein is V-shaped and its 3D structure is dimeric. The transport stability of the structure was also assessed by molecular dynamics simulation studies. The activity domain was identified to form hydrogen bond and stacking interaction with citrate and malate upon docking. Tricarboxylate citrate transporter has shown high binding energy of −4.87 kcal/mol to citric acid, while −3.80 kcal/mol to malic acid. This is the first report of unraveling the structural characteristics of WJ11 mitochondrial Tct protein and understanding the approach of the transporting toward its substrate. In conclusion, the present findings support our efforts to combine functional and structural data to better understand the Tct of M. circinelloides at the molecular level and its role in lipid accumulation.

Mitochondrial Carriers and Substrates Transport Network: A Lesson from Saccharomyces cerevisiae

The yeast Saccharomyces cerevisiae is one of the most widely used model organisms for investigating various aspects of basic cellular functions that are conserved in human cells. This organism, as well as human cells, can modulate its metabolism in response to specific growth conditions, different environmental changes, and nutrient depletion. This adaptation results in a metabolic reprogramming of specific metabolic pathways. Mitochondrial carriers play a fundamental role in cellular metabolism, connecting mitochondrial with cytosolic reactions. By transporting substrates across the inner membrane of mitochondria, they contribute to many processes that are central to cellular function. The genome of Saccharomyces cerevisiae encodes 35 members of the mitochondrial carrier family, most of which have been functionally characterized. The aim of this review is to describe the role of the so far identified yeast mitochondrial carriers in cell metabolism, attempting to show the functional connections between substrates transport and specific metabolic pathways, such as oxidative phosphorylation, lipid metabolism, gluconeogenesis, and amino acids synthesis. Analysis of the literature reveals that these proteins transport substrates involved in the same metabolic pathway with a high degree of flexibility and coordination. The understanding of the role of mitochondrial carriers in yeast biology and metabolism could be useful for clarifying unexplored aspects related to the mitochondrial carrier network. Such knowledge will hopefully help in obtaining more insight into the molecular basis of human diseases.

Membrane Traffic in Aspergillus oryzae and Related Filamentous Fungi

The industrially important filamentous fungus Aspergillus oryzae, known as the yellow Koji mold and also designated the Japanese National fungus, has been investigated for understanding the intracellular membrane trafficking machinery due to the great ability of valuable enzyme production. The underlying molecular mechanisms of the secretory pathway delineate the main secretion route from the hyphal tip via the vesicle cluster Spitzenkörper, but also there is a growing body of evidence that septum-directed and unconventional secretion occurs in A. oryzae hyphal cells. Moreover, not only the secretory pathway but also the endocytic pathway is crucial for protein secretion, especially having a role in apical endocytic recycling. As a hallmark of multicellular filamentous fungal cells, endocytic organelles early endosome and vacuole are quite dynamic: the former exhibits constant long-range motility through the hyphal cells and the latter displays pleiomorphic structures in each hyphal region. These characteristics are thought to have physiological roles, such as supporting protein secretion and transporting nutrients. This review summarizes molecular and physiological mechanisms of membrane traffic, i.e., secretory and endocytic pathways, in A. oryzae and related filamentous fungi and describes the further potential for industrial applications.

Making Traditional Japanese Distilled Liquor, Shochu and Awamori, and the Contribution of White and Black Koji Fungi

The traditional Japanese single distilled liquor, which uses koji and yeast with designated ingredients, is called "honkaku shochu." It is made using local agricultural products and has several types, including barley shochu, sweet potato shochu, rice shochu, and buckwheat shochu. In the case of honkaku shochu, black koji fungus (Aspergillus luchuensis) or white koji fungus (Aspergillus luchuensis mut. kawachii) is used to (1) saccharify the starch contained in the ingredients, (2) produce citric acid to prevent microbial spoilage, and (3) give the liquor its unique flavor. In order to make delicious shochu, when cultivating koji fungus during the shochu production process, we use a unique temperature control method to ensure that these three important elements, which greatly affect the taste of the produced liquor, are balanced without any excess or deficiency. This review describes in detail the production method of honkaku shochu, a distilled spirit unique to Japan and whose market is expected to expand worldwide, with special attention paid to the koji fungi cultivation step. Furthermore, we describe the history of the koji fungi used today in the production of shochu, and we provide a thorough explanation of the characteristics of each koji fungi. We also report the latest research progress on this topic.

Rapid and marker-free gene replacement in citric acid-producing Aspergillus tubingensis (A. niger) WU-2223L by the CRISPR/Cas9 system-based genome editing technique using DNA fragments encoding sgRNAs

Strains belonging to Aspergillus section Nigri, including Aspergillus niger, are used for industrial production of citric acid from carbohydrates such as molasses and starch. The objective of this study was to construct the genome editing system that could enable rapid and efficient gene replacement in citric acid-producing fungi for genetic breeding. Using the citric acid-hyperproducer A. tubingensis (formerly A. niger) WU-2223L as a model strain, we developed a CRISPR/Cas9 system-based genome editing technique involving co-transformation of Cas9 and the DNA fragment encoding single guide RNA (sgRNA). Using this system, ATP-sulfurylase gene (sC) knock-out strain derived from WU-2223L was generated; the knock-out efficiency was 29 transformants when 5 μg Cas9 was added to 5 × 105 protoplasts. In the gene replacement method based on this system, a DNA fragment encoding sgRNAs that target both the gene of interest and marker gene was used, and replacement of nitrate reductase gene (niaD) using sC gene as a marker gene was attempted. More than 90% of the sC-knock-out transformants exhibited replaced niaD, indicating efficient gene replacement. Moreover, one-step marker rescue of the sC marker gene was accomplished by excising the knock-in donor via intramolecular homologous recombination, enabling marker-free genome editing and drastically shortening the gene replacement period by circumventing the transformation procedure to recover the sC gene. Thus, we succeeded in constructing a CRISPR/Cas9 system-based rapid and marker-free gene replacement system for the citric acid-hyperproducer strain WU-2223L.

Role of the mitochondrial citrate-oxoglutarate carrier in lipid accumulation in the oleaginous fungus Mortierella alpina

OBJECTIVES

The transport of citrate from the mitochondria to the cytoplasm is essential during lipid accumulation. This study aimed to explore the role of mitochondrial citrate-oxoglutarate carrier in lipid accumulation in the oleaginous fungus Mortierella alpina.

RESULTS

Homologous MaYHM (the gene encoding the mitochondrial citrate-oxoglutarate carrier) was overexpressed in M. alpina. The fatty acid content of MaYHM-overexpressing recombinant strains was increased by up to 30% compared with the control. Moreover, the intracellular α-ketoglutarate level in recombinant strains was increased by 2.2 fold, together with a 23-35% decrease in NAD⁺-isocitrate dehydrogenase activity compared with the control. The overexpression of MaYHM altered the metabolic flux in the glutamate dehydrogenase shunt and 4-aminobutyric acid shunt during metabolic reprogramming, supplying more carbon to synthesize fatty acids.

CONCLUSIONS

Overexpression of MaYHM resulted in more efflux of citrate from mitochondria to the cytoplasm and enhanced lipid accumulation. These findings provide new perspectives for the improvement of industrial lipid production in M. alpina.

Citrate exporter enhances both extracellular and intracellular citric acid accumulation in the koji fungi Aspergillus luchuensis mut. kawachii and Aspergillus oryzae

Citrate exporter CexA plays a key role in the production of citric acid in fungi; however, its role in intracellular metabolism has remained unclear. In this study, we comparably characterized homologous cexA genes in the white koji fungus Aspergillus luchuensis mut. kawachii and the yellow koji fungus Aspergillus oryzae, which exhibit high and low abilities, respectively, to produce citric acid. Disruption of cexA caused a significant decline of both extracellular and intracellular citric acid accumulation in Aspergillus kawachii, while overexpression of the A. kawachii cexA gene (AkcexA) into A. oryzae significantly enhanced both extracellular and intracellular citric acid accumulation in A. oryzae to a level comparable to that of A. kawachii. In addition, overexpression of two intrinsic cexA homologs (AocexA and AocexB) in A. oryzae also enhanced its extracellular and intracellular citric acid accumulation. Comprehensive analysis of intracellular metabolites from an AkcexA-overexpressing strain of A. oryzae compared with its control strain identified metabolic changes associated with intracellular citric acid accumulation via the glycolytic pathway, pentose phosphate pathway, and tricarboxylic acid cycle. Our results indicate that citric acid export enhances not only extracellular citric acid accumulation but also intracellular metabolic fluxes to generate citric acid.

Identification and importance of mitochondrial citrate carriers and ATP citrate lyase for glycolipid production in Starmerella bombicola

Starmerella bombicola is a non-conventional yeast commercially used as a microbial cell factory for sophorolipid production. Sophorolipids are glycolipid biosurfactants composed of a glucose disaccharide sophorose and a fatty acid. In de novo sophorolipid synthesis, the fatty acid moiety is derived from the fatty acid synthesis (FAS) complex; therefore, the yeast's lipid metabolism plays a crucial role in sophorolipid biosynthesis. As a fatty acid precursor, citric acid is a key primary metabolite that connects carbohydrate and lipid metabolism, and in S. bombicola, it also has a regulatory effect on sophorolipid composition and productivity. We aimed to identify the mitochondrial transporters involved in citrate shuttling and the ATP citrate lyase (Acl), the enzyme that converts citric acid into acetyl-CoA. Subsequently, we studied their role in the citric acid shuttle and glycolipid synthesis and the potential of citrate metabolism as a genetic manipulation target for increased glycolipid synthesis. Bioinformatics analyses predicted 32 mitochondrial carriers of which two were identified as citrate transporters, named SbCtp1 and SbYhm2. Deletion of these mitochondrial carriers led to a lesser sophorolipid yield and a shift in the lactonic/acidic sophorolipid ratio. However, only the knockout of SbYhm2 caused a decrease of citric and an increase of malic acid extracellular concentrations. Additionally, deletion of SbAcl1 had a negative effect on S. bombicola's specific growth rate and sophorolipid synthesis and contributed to extra- and intracellular citric acid accumulation. Unexpectedly, SbAcl1 overexpression also decreased glycolipid production.Key Points• Starmerella bombicola is an industrially relevant microbial cell factory for biosurfactant production.• There are 32 predicted mitochondrial carriers in S. bombicola.• Citrate mitochondrial carriers SbYhm2 and SbCtp1 are essential for glycolipid synthesis in S. bombicola.• Deletion of SbAcl1 negatively affects growth and sophorolipid production in S. bombicola. Graphical abstract.

LaeA Controls Citric Acid Production through Regulation of the Citrate Exporter-Encoding cexA Gene in Aspergillus luchuensis mut. kawachii

The putative methyltransferase LaeA is a global regulator of metabolic and development processes in filamentous fungi. We characterized the homologous laeA genes of the white koji fungus Aspergillus luchuensis mut. kawachii (A. kawachii) to determine their role in citric acid hyperproduction. The ΔlaeA strain exhibited a significant reduction in citric acid production. Cap analysis gene expression (CAGE) revealed that laeA is required for the expression of a putative citrate exporter-encoding cexA gene, which is critical for citric acid production. Deficient citric acid production by a ΔlaeA strain was rescued by the overexpression of cexA to a level comparable with that of a cexA-overexpressing ΔcexA strain. In addition, chromatin immunoprecipitation coupled with quantitative PCR (ChIP-qPCR) analysis indicated that LaeA regulates the expression of cexA via methylation levels of the histones H3K4 and H3K9. These results indicate that LaeA is involved in citric acid production through epigenetic regulation of cexA in A. kawachii IMPORTANCE A. kawachii has been traditionally used for production of the distilled spirit shochu in Japan. Citric acid produced by A. kawachii plays an important role in preventing microbial contamination during the shochu fermentation process. This study characterized homologous laeA genes; using CAGE, complementation tests, and ChIP-qPCR, it was found that laeA is required for citric acid production through the regulation of cexA in A. kawachii The epigenetic regulation of citric acid production elucidated in this study will be useful for controlling the fermentation processes of shochu.

Fernie A. Plant Mitochondrial Carriers: Molecular Gatekeepers That Help to Regulate Plant Central Carbon Metabolism

The evolution of membrane-bound organelles among eukaryotes led to a highly compartmentalized metabolism. As a compartment of the central carbon metabolism, mitochondria must be connected to the cytosol by molecular gates that facilitate a myriad of cellular processes. Members of the mitochondrial carrier family function to mediate the transport of metabolites across the impermeable inner mitochondrial membrane and, thus, are potentially crucial for metabolic control and regulation. Here, we focus on members of this family that might impact intracellular central plant carbon metabolism. We summarize and review what is currently known about these transporters from in vitro transport assays and in planta physiological functions, whenever available. From the biochemical and molecular data, we hypothesize how these relevant transporters might play a role in the shuttling of organic acids in the various flux modes of the TCA cycle. Furthermore, we also review relevant mitochondrial carriers that may be vital in mitochondrial oxidative phosphorylation. Lastly, we survey novel experimental approaches that could possibly extend and/or complement the widely accepted proteoliposome reconstitution approach.

Metabolic specialization in itaconic acid production: a tale of two fungi

Some of the oldest and most established industrial biotechnology processes involve the fungal production of organic acids. In these fungi, the transport of metabolites between cellular compartments, and their secretion, is a major factor. In this review we exemplify the importance of both mitochondrial and plasma membrane transporters in the case of itaconic acid production in two very different fungal systems, Aspergillus and Ustilago. Homologous and heterologous overexpression of both types of transporters, and biochemical analysis of mitochondrial transporter function, show that these two fungi produce the same compound through very different pathways. The way these fungi respond to itaconate stress, especially at low pH, also differs, although this is still an open field which clearly needs additional research.