CDs-g-C3N4-oleaginous yeast hybrid system: Microbial lipid synthesis and fermentation residual reutilization

The utilization of solar energy and fast-growing heterotrophic microbes for biofuel production has been recognized as a promising approach to achieve carbon neutrality and address energy crisis. In this work, we synthesized different kinds of photocatalysts based on graphitic carbon nitride (g-C3N4). We found that carbon dots modified-graphitic carbon nitride (CDs-g-C3N4) showed the highest photocatalytic activity. Subsequently, we developed a photocatalyst-microbe hybrid (PMH) system by combining CDs-g-C3N4 with an oleaginous yeast strain, Cutaneotrichosporon dermatis ZZ-46. Under visible light irradiation, the lipid yield of this PMH system reached 1.70 g/L at 120 h, representing a 36 % increase compared to the control. The photocatalytic reaction-induced ROS and the reductive photogenerated electrons facilitated ZZ-46 cells to synthesize more lipids. Furthermore, the fermentation residual of this PMH system was reutilized to prepare biochar via pyrolysis. The biochar generated at 550 °C (BC-550) demonstrated exceptional adsorption capabilities, particularly with a 57 % adsorption rate for methylene blue (MB), and maintained its perfect adsorption efficacy even after five regeneration cycles. These results offer promising avenues for addressing energy shortages and environmental contamination.

Microbial lipid synthesis based on visible light-driven oxygen doped-graphitic carbon nitride /oleaginous yeast hybrid system

The use of solar energy and heterotrophic microbes to synthesize microbial lipids is a promising strategy to solve energy crisis and reduce CO2 emissions. In this study, a photocatalyst, oxygen-doped graphitic carbon nitride (O-g-C3N4), was synthesized and combined with an oleaginous yeast strain, Cutaneotrichosporon dermatis ZZ-46, to construct a photocatalyst-microbe hybrid (PMH) system. Under illumination, the lipid yield of the PMH system reached 1.61 g/L after 96 h (87 % higher than that of control). NADPH/NADP+ ratio of ZZ-46 cells in the PMH system increased. Metabolomics results revealed that glutathione generation was increased, and the fatty acid decomposition pathway in ZZ-46 cells was inhibited in the PMH system. This study provides a new approach for the synthesis of microbial lipids based on solar energy and heterotrophic microbes.

Mechanism of enhanced microalgal biomass and lipid accumulation through symbiosis between a highly succinic acid-producing strain of Escherichia coli SUC and Aurantiochytrium sp. SW1

Microalgae, known for rapid growth and lipid richness, hold potential in biofuels and high-value biomolecules. The symbiotic link with bacteria is crucial in large-scale open cultures. This study explores algal-bacterial interactions using a symbiotic model, evaluating acid-resistant Lactic acid bacteria (LAB), stress-resilient Bacillus subtilis and Bacillus licheniformis, and various Escherichia coli strains in the Aurantiochytrium sp. SW1 system. It was observed that E. coli SUC significantly enhanced the growth and lipid production of Aurantiochytrium sp. SW1 by increasing enzyme activity (NAD-IDH, NAD-ME, G6PDH) while maintaining sustained succinic acid release. Optimal co-culture conditions included temperature 28 °C, a 1:10 algae-to-bacteria ratio, and pH 8. Under these conditions, Aurantiochytrium sp. SW1 biomass increased 3.17-fold to 27.83 g/L, and total lipid content increased 2.63-fold to 4.87 g/L. These findings have implications for more efficient microalgal lipid production and large-scale cultivation.

Role of AMP Deaminase in Mucor circinelloides: Implications for Nitrogen Utilization and Lipid Biosynthesis

Lipid accumulation in oleaginous organisms is initiated by AMP deaminase (AMPD) after nitrogen depletion because it mediates the concentration of intracellular adenosine monophosphate (AMP). However, the role of AMPD in lipogenesis in the oleaginous fungus Mucor circinelloides is largely unknown. Therefore, we identified the genes (ampd1 and ampd2) encoding AMPD and investigated the role of AMPD in lipid synthesis in this fungus by overexpressing and deleting ampd genes. Deletion of ampd1 and ampd2 caused 21 and 28% increments in lipid contents under N-limited conditions, respectively. These increases were correlated with the activation of enzymes involved in lipogenesis and the alteration of energy balance. Unexpectedly, overexpression of ampd genes affected nitrogen consumption in both N-limited and N-excess media, which resulted in an increase in cell growth and lipid accumulation compared with the control strain when nitrogen was available. Furthermore, the increased lipid accumulation in the ampd-overexpressing mutants in N-excess media was accompanied by enhanced activities of lipid biosynthetic enzymes. These data suggested that nitrogen metabolism and energy metabolism are affected by AMPD, and overexpression of ampd genes induced lipid accumulation under nitrogen-rich conditions by mimicking the nitrogen limitation response. This highlights an intriguing function of AMPD in M. circinelloides.

Simultaneous overexpression of ∆6-, ∆12- and ∆9-desaturases enhanced the production of γ-linolenic acid in Mucor circinelloides WJ11

Mucor circinelloides WJ11, an oleaginous filamentous fungus, produces 36% lipid of its cell dry weight when cultured in a high C/N ratio medium, however, the yield of γ-linolenic acid (GLA) is insufficient to make it competitive with other plant sources. To increase the GLA content in M. circinelloides WJ11, this fungus was engineered by overexpression of its key genes such as Δ6-, Δ12-, and Δ9-desaturases involved in GLA production. Firstly, we tried to overexpress two Δ6-desaturase isozymes to determine which one played important role in GLA synthesis. Secondly, Δ6-and Δ12-desaturase were co-overexpressed to check whether linoleic acid (LA), the precursor for GLA synthesis, is a limiting factor or not. Moreover, we tried to explore the effects of simultaneous overexpression of Δ6-, Δ12-, and Δ9-desaturases on GLA production. Our results showed that overexpression (1 gene) of DES61 promoted higher GLA content (21% of total fatty acids) while co-overexpressing (2 genes) DES61 and DES12 and simultaneous overexpressing (3 genes) DES61, DES12, and DES91 increased the GLA production of engineered strains by 1.5 folds and 1.9 folds compared to the control strain, respectively. This study provided more insights into GLA biosynthesis in oleaginous fungi and laid a foundation for further increase in GLA production into fungus such as M. circinelloides.

Homologous and Heterologous Expression of Delta(12)-Desaturase in Mucor circinelloides Enhanced the Production of Linolenic Acid

Linolenic acid (LA) is gaining more interest within the scientific community. This is because it has a potential medical role in reducing the risk of inflammation, carcinogenesis, atherosclerosis and diabetes and is a valuable nutraceutical for human health. The oleaginous fungus Mucor circinelloides produces a high lipid content (36%), including valuable polyunsaturated fatty acids (PUFAs). However, the critical step in which oleic acid (OA) is converted into LA is not efficient at supplying enough substrates for PUFA synthesis. Hence, we propose a method to increase LA production based on genetic engineering. The overexpression of the Δ12-desaturase gene from M. circinelloides and Mortierella alpina increased the LA content and improved the lipid accumulation (from 14.9% to 21.6% in the Δ12-desaturase gene of the M. circinelloides overexpressing strain (Mc-D12MC) and from 14.9% to 18.7% in the Δ12-desaturase gene of M. alpina overexpressing strain (Mc-D12MA)). Additionally, the up-regulated expression levels of these genes targeted the genes involved in NADPH production, implying that the elevated Δ12-desaturase gene may function as a critical regulator of NADPH and lipid synthesis in M. circinelloides. This study provides the first evidence to support the design of metabolic engineering related to LA and PUFA production in M. circinelloides for potential industrial applications.

Role of Cytosolic Malic Enzyme in Oleaginicity of High-Lipid-Producing Fungal Strain Mucor circinelloides WJ11

Mucor circinelloides, an oleaginous filamentous fungus, is gaining popularity due to its ability to synthesize significant amounts of lipids containing γ-linolenic acid (GLA) that have important health benefits. Malic enzyme (ME), which serves as the main source of NADPH in some fungi, has been found to regulate lipid accumulation in oleaginous fungi. In the present study, the role of two cytosolic ME genes, cmalA and cmalB, in the lipid accumulation of the M. circinelloides high-lipid-producing strain WJ11, was evaluated. Strains overexpressing cmalA and cmalB showed a 9.8- and 6.4-fold rise in specific ME activity, respectively, and an elevation of the lipid content by 23.2% and 5.8%, respectively, suggesting that these genes are involved in lipid biosynthesis. Due to increased lipid accumulation, overall GLA content in biomass was observed to be elevated by 11.42% and 16.85% in cmalA and cmalB overexpressing strains, respectively. Our study gives an important insight into different studies exploring the role of the cmalA gene, while we have for the first time investigated the role of the cmalB gene in the M. circinelloides WJ11 strain.

Mucor circinelloides: a model organism for oleaginous fungi and its potential applications in bioactive lipid production

Microbial oils have gained massive attention because of their significant role in industrial applications. Currently plants and animals are the chief sources of medically and nutritionally important fatty acids. However, the ever-increasing global demand for polyunsaturated fatty acids (PUFAs) cannot be met by the existing sources. Therefore microbes, especially fungi, represent an important alternative source of microbial oils being investigated. Mucor circinelloides—an oleaginous filamentous fungus, came to the forefront because of its high efficiency in synthesizing and accumulating lipids, like γ-linolenic acid (GLA) in high quantity. Recently, mycelium of M. circinelloides has acquired substantial attraction towards it as it has been suggested as a convenient raw material source for the generation of biodiesel via lipid transformation. Although M. circinelloides accumulates lipids naturally, metabolic engineering is found to be important for substantial increase in their yields. Both modifications of existing pathways and re-formation of biosynthetic pathways in M. circinelloides have shown the potential to improve lipid levels. In this review, recent advances in various important metabolic aspects of M. circinelloides have been discussed. Furthermore, the potential applications of M. circinelloides in the fields of antioxidants, nutraceuticals, bioremediation, ethanol production, and carotenoids like beta carotene and astaxanthin having significant nutritional value are also deliberated.

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.

Heterologous Expression of Two Malate Transporters From an Oleaginous Fungus Mucor circinelloides Improved the Lipid Accumulation in Mucor lusitanicus

The fungus, Mucor lusitanicus, is of great interest for microbial lipids, because of its ability to accumulate intracellular lipid using various carbon sources. The biosynthesis of fatty acid requires the reducing power NADPH, and acetyl-CoA, which is produced by the cleavage of citrate in cytosol. In this study, we employed different strategies to increase lipid accumulation in the low lipid-producing fungi via metabolic engineering technology. Hence, we constructed the engineered strain of M. lusitanicus CBS 277.49 by using malate transporter (mt) and 2-oxoglutarate: malate antiporter (sodit) from M. circinelloides WJ11. In comparison with the control strain, the lipid content of the overexpressed strains of mt and sodit genes were increased by 24.6 and 33.8%, respectively. These results showed that mt and sodit can affect the distribution of malate in mitochondria and cytosol, provide the substrates for the synthesis of citrate in the mitochondria, and accelerate the transfer of citrate from mitochondria to cytosol, which could play a significant regulatory role in fatty acid synthesis leading to lipids over accumulation.

Progress of metabolic engineering for the production of eicosapentaenoic acid

Eicosapentaenoic Acid (EPA) is an essential ω-3 polyunsaturated fatty acid for human health. Currently, high-quality EPA production is largely dependent on the extraction of fish oil, but this unsustainable approach cannot meet its rising market demand. Biotechnological approaches for EPA production from microorganisms have received increasing attention due to their suitability for large-scale production and independence of the seasonal or climate restrictions. This review summarizes recent research on different microorganisms capable of producing EPA, such as microalgae, bacteria, and fungi, and introduces the different EPA biosynthesis pathways. Notably, some novel engineering strategies have been applied to endow and improve the abilities of microorganisms to synthesize EPA, including the construction and optimization of the EPA biosynthesis pathway, an increase in the acetyl-CoA pool supply, the increase of NADPH and the inhibition of competing pathways. This review aims to provide an updated summary of EPA production.

Deletion of Plasma Membrane Malate Transporters Increased Lipid Accumulation in the Oleaginous Fungus Mucor circinelloides WJ11

Malate as an important intermediate metabolite, its subcellular location, and concentration have a significant impact on fungal lipid metabolism. Previous studies showed that the mitochondrial malate transporter plays an important role in lipid accumulation in Mucor circinelloides by manipulating intracellular malate concentration. However, the role of plasma membrane malate transporters in oleaginous fungi remains unexplored. Therefore, in this work, two plasma membrane malate transporters "2-oxoglutarate:malate antiporters" (named SoDIT-a and SoDIT-b) of M. circinelloides WJ11 were deleted, and the consequences in growth capacity, lipid accumulation, and metabolism were analyzed. The results showed that deletion of sodit-a or/and sodit-b reduced the extracellular malate, confirming that the products of both genes participate in malate transportation. In parallel, the lipid contents in mutants increased approximately 10-40% higher than that in the control strain, suggesting that the defect in plasma membrane malate transport results in an increase of malate available for lipid biosynthesis. Furthermore, transcriptional analysis showed that the expression levels of multiple key genes involved in the lipid biosynthesis were also increased in the knockout mutants. To the best of our knowledge, this is the first report that demonstrated the association between plasma membrane malate transporters and lipid accumulation in M. circinelloides.

Recent advances in biotechnological production of polyunsaturated fatty acids by Yarrowia lipolytica

Owing to the important physiological functions, polyunsaturated fatty acids (PUFAs) play a vital role in protecting human health, such as preventing cancer, cardiovascular disease, and diabetes. Specifically, Yarrowia lipolytica has been identified as the most popular non-conventional oleaginous yeast, which can accumulate the abundant intracellular lipids, indicating that has great potential as an industrial host for production of PUFAs. Notably, some novel engineering strategies have been applied to endow and improve the abilities of Y. lipolytica to synthesize PUFAs, including construction and optimization of PUFAs biosynthetic pathways, improvement of preucrsors acetyl-coA and NADPH supply, inhibition of competing pathways, knockout of β-oxidation pathways, regulation of oxidative stress defense pathways, and regulation of genes involved in upstream lipid metabolism. Besides, some bypass approaches, such as strain mating, evolutionary engineering, and computational model based on omics, also have been proposed to improve the performance of engineering strains. Generally, in this review, we summarized the recent advances in engineering strategies and bypass approaches for improving PUFAs production by Y. lipolytica. In addition, we further summarized the latest efforts of CRISPR/Cas genome editing technology in Y. lipolytica, which is aimed to provide its potential applications in PUFAs production.

Role of Snf-β in lipid accumulation in the high lipid-producing fungus Mucor circinelloides WJ11

Background

Mucor circinelloides WJ11 is a high-lipid producing strain and an excellent producer of γ-linolenic acid (GLA) which is crucial for human health. We have previously identified genes that encode for AMP-activated protein kinase (AMPK) complex in M. circinelloides which is an important regulator for lipid accumulation. Comparative transcriptional analysis between the high and low lipid-producing strains of M. circinelloides showed a direct correlation in the transcriptional level of AMPK genes with lipid metabolism. Thus, the role of Snf-β, which encodes for β subunit of AMPK complex, in lipid accumulation of the WJ11 strain was evaluated in the present study.

Results

The results showed that lipid content of cell dry weight in Snf-β knockout strain was increased by 32 % (from 19 to 25 %). However, in Snf-β overexpressing strain, lipid content of cell dry weight was decreased about 25 % (from 19 to 14.2 %) compared to the control strain. Total fatty acid analysis revealed that the expression of the Snf-β gene did not significantly affect the fatty acid composition of the strains. However, GLA content in biomass was increased from 2.5 % in control strain to 3.3 % in Snf-β knockout strain due to increased lipid accumulation and decreased to 1.83 % in Snf-β overexpressing strain. AMPK is known to inactivate acetyl-CoA carboxylase (ACC) which catalyzes the rate-limiting step in lipid synthesis. Snf-β manipulation also altered the expression level of the ACC1 gene which may indicate that Snf-β control lipid metabolism by regulating ACC1 gene.

Conclusions

Our results suggested that Snf-β gene plays an important role in regulating lipid accumulation in M. circinelloides WJ11. Moreover, it will be interesting to evaluate the potential of other key subunits of AMPK related to lipid metabolism. Better insight can show us the way to manipulate these subunits effectively for upscaling the lipid production. Up to our knowledge, it is the first study to investigate the role of Snf-β in lipid accumulation in M. circinelloides.

The Potential of Single-Cell Oils Derived From Filamentous Fungi as Alternative Feedstock Sources for Biodiesel Production

Microbial lipids, also known as single-cell oils (SCOs), are highly attractive feedstocks for biodiesel production due to their fast production rates, minimal labor requirements, independence from seasonal and climatic changes, and ease of scale-up for industrial processing. Among the SCO producers, the less explored filamentous fungi (molds) exhibit desirable features such as a repertoire of hydrolyzing enzymes and a unique pellet morphology that facilitates downstream harvesting. Although several oleaginous filamentous fungi have been identified and explored for SCO production, high production costs and technical difficulties still make the process less attractive compared to conventional lipid sources for biodiesel production. This review aims to highlight the ability of filamentous fungi to hydrolyze various organic wastes for SCO production and explore current strategies to enhance the efficiency and cost-effectiveness of the SCO production and recovery process. The review also highlights the mechanisms and components governing lipogenic pathways, which can inform the rational designs of processing conditions and metabolic engineering efforts for increasing the quality and accumulation of lipids in filamentous fungi. Furthermore, we describe other process integration strategies such as the co-production with hydrogen using advanced fermentation processes as a step toward a biorefinery process. These innovative approaches allow for integrating upstream and downstream processing units, thus resulting in an efficient and cost-effective method of simultaneous SCO production and utilization for biodiesel production.

Role of g6pdh and leuB on Lipid Accumulation in Mucor circinelloides

Mucor circinelloides is a valuable oleaginous filamentous fungus rich in γ-linolenic acid (GLA, 18:3; n-6), which is beneficial for human health. Our previous comparative proteomic analysis between high lipid-producing M. circinelloides WJ11 and low lipid-producing M. circinelloides CBS 277.49 indicated that glucose 6-phosphate dehydrogenase (G6PDH) and β-isopropylmalate dehydrogenase (IPMDH) were closely involved in lipid accumulation. Transcription analysis suggested that in the strain WJ11, g6pdh1 and g6pdh2, which encode G6PDH, and leuB, which encodes IPMDH, could be the key genes regulating lipid accumulation. To further analyze the effects of these three genes (i.e., g6pdh1, g6pdh2, and leuB) on lipid accumulation, we respectively overexpressed these genes from M. circinelloides WJ11 in defective CBS 277.49 strains in this study. The results showed that overexpression of g6pdh1 and g6pdh2 genes from strain WJ11 increased the fatty acid content of cell dry weight by 23-38 and 41-47%, respectively, compared with the control strain. Furthermore, overexpression of the leuB gene from strain WJ11 increased the fatty acid content of cell dry weight by up to 67-73%. These results suggest that g6pdh1, g6pdh2, and especially leuB genes play important roles in regulating fatty acid synthesis in M. circinelloides.

Mitochondrial Citrate Transport System in the Fungus Mucor circinelloides: Identification, Phylogenetic Analysis, and Expression Profiling During Growth and Lipid Accumulation

The mitochondrial citrate transport system, composed of citrate and malate transporters (MTs), can regulate the citrate efflux from mitochondria to cytosol, and then citrate is cleaved into OAA and acetyl-CoA which can be used for fatty acid (FA) biosynthesis. However, in the fungus Mucor circinelloides the molecular mechanism of citrate efflux from the mitochondria by this system and its role in FA synthesis is unclear. In the present study, we have analyzed the genome of high lipid-producing strain WJ11 and the low lipid-producing strain CBS 277.49 to find the potential genes involving in this system. Five potential genes are present in the genome of WJ11. These genes encode one citrate transport protein (CT), one tricarboxylate carrier (TCT), one MT, and two 2-oxoglutarate:malate antiporters (SoDIT-a and SoDIT-b). However, the genome of CBS 277.49 contains the same set of genes, except for the presence of just one SoDIT. The proteins from WJ11 had similar properties as their counterparts in CBS 277.49. Moreover, phylogenetic analyses revealed the evolutionary relationship of these proteins and illuminated their typical motifs related to potential functions. Additionally, the expression of these genes was analyzed to predict the possible functions in lipid metabolism in M. circinelloides. This is the first study to report the in silico analysis of structures and functions of the mitochondrial citrate transport system in M. circinelloides. This work showed a new strategy for research for the selection of candidate genes for further detailed functional investigation of the mitochondrial citrate transport system in lipid accumulation.

Novel Dual-Functional Enzyme Lip10 Catalyzes Lipase and Acyltransferase Activities in the Oleaginous Fungus Mucor circinelloides

Lipases or triacylglycerol (TAG) lipases belong to the α/β-hydrolases superfamily, which are enzymes capable of catalyzing the hydrolysis of the ester bond between fatty acids and glycerol. Interestingly, some lipases have been found to not only possess hydrolysis activity but also acyltransferase activity in yeasts and microalgae. Our present study reported a novel dual-functional Mucor circinelloides lipase Lip10 with a slight lipolysis activity but a noteworthy phospholipid/diacylglycerol acyltransferase (PDAT) activity. The purified Lip10 mutants prefer to utilize phosphatidyl serine to form TAG over phosphatidyl ethanolamine and phosphatidylcholine. Site-directed mutagenesis indicated that the histidine residue in the acyltransferase motif H-(X)₄-D is indispensable for the PDAT activity of Lip10. Overexpression of the acyltransferase motif of Lip10 promoted cell growth by 12% and increased lipid production by 14% compared to the control, whilst overexpression of the lipase motif induced lipid degradation in M. circinelloides.

Multiomics analysis reveals a distinct mechanism of oleaginousness in the emerging model alga Chromochloris zofingiensis

Chromochloris zofingiensis, featured due to its capability to simultaneously synthesize triacylglycerol (TAG) and astaxanthin, is emerging as a leading candidate alga for production uses. To better understand the oleaginous mechanism of this alga, we conducted a multiomics analysis by systematically integrating time-resolved transcriptomes, lipidomes and metabolomes in response to nitrogen deprivation. The data analysis unraveled the distinct mechanism of TAG accumulation, which involved coordinated stimulation of multiple biological processes including supply of energy and reductants, carbon reallocation from protein and starch, and 'pushing' and 'pulling' carbon to TAG synthesis. Unlike the model alga Chlamydomonas, de novo fatty acid synthesis in C. zofingiensis was promoted, together with enhanced turnover of both glycolipids and phospholipids, supporting the drastic need of acyls for TAG assembly. Moreover, genomewide analysis identified many key functional enzymes and transcription factors that had engineering potential for TAG modulation. Two genes encoding glycerol-3-phosphate acyltransferase (GPAT), the first committed enzyme for TAG assembly, were found in the C. zofingiensis genome; in vivo functional characterization revealed that extrachloroplastic GPAT instead of chloroplastic GPAT played a central role in TAG synthesis. These findings illuminate distinct oleaginousness mechanisms in C. zofingiensis and pave the way towards rational manipulation of this alga to becone an emerging model for trait improvements.

Genetic Modification of Mucor circinelloides to Construct Stearidonic Acid Producing Cell Factory

Stearidonic acid (SDA; 18:4, n-3) is the delta 15-desaturase product of gamma linolenic acid (GLA; 18:3, n-6) and delta 6-desaturase product of alpha linolenic acid (ALA; 18:3, n-3). Construction of engineered oleaginous microbes have been attracting significant interest in producing SDA because of its nutritional value and pharmaceutical applications. Mucor circinelloides is a GLA producing filamentous fungus, which can be a useful tool to produce SDA. This study has, therefore, overexpressed the delta-15 desaturase (D15D) gene from Mortierella alpina in this fungus to construct a SDA-producing cell factory. To produce SDA in M. circinelloides, the homologous overexpression of D15D gene was analyzed. When the gene was overexpressed in M. circinelloides CBS 277.49, up to 5.0% SDA was accumulated in this strain. According to current knowledge, this is the first study describing the construction of a SDA-producing cell factory by overexpression of D15D gene in oleaginous fungus M. circinelloides. A new scope for further research has been established by this work to improve SDA production in this fungus, specifically in its high lipid-producing strain, WJ11.

A novel malic enzyme gene, Mime2, from Mortierella isabellina M6-22 contributes to lipid accumulation

OBJECTIVE

This study was aimed at cloning and characterizing a novel malic enzyme (ME) gene of Mortierella isabellina M6-22 and identifying its relation with lipid accumulation.

METHODS

Mime2 was cloned from strain M6-22. Plasmid pET32aMIME2 was constructed to express ME of MIME2 in Escherichia coli BL21. After purification, the optimal pH and temperature of MIME2, as well as K_m and V_max for NADP⁺ were determined. The effects of EDTA or metal ions (Mn²⁺, Mg²⁺, Co²⁺, Cu²⁺, Ca²⁺, or Zn²⁺) on the enzymatic activity of MIME2 were evaluated. Besides, plasmid pRHMIME2 was created to express MIME2 in Rhodosporidium kratochvilovae YM25235, and its cell lipid content was measured by the acid-heating method. The optimal pH and temperature of MIME2 are 5.8 and 30 °C, respectively.

RESULTS

The act ivity of MIME2 was significantly increased by Mg²⁺, Ca²⁺, or Mn²⁺ at 0.5 mM but inhibited by Cu²⁺ or Zn²⁺ (p < 0.05). The optimal enzymatic activity of MIME2 is 177.46 U/mg, and the K_m and V_max for NADP⁺ are 0.703 mM and 156.25 μg/min, respectively. Besides, Mime2 transformation significantly increased the cell lipid content in strain YM25235 (3.15 ± 0.24 vs. 2.17 ± 0.31 g/L, p < 0.01).

CONCLUSIONS

The novel ME gene Mime2 isolated from strain M6-22 contributes to lipid accumulation in strain YM25235.

Dual Functions of Lip6 and Its Regulation of Lipid Metabolism in the Oleaginous Fungus Mucor circinelloides

Although multiple roles of lipases have been reported in yeasts and microalgae, the functions of lipases have not been studied in oleaginous filamentous fungi. Lipase Lip6 has been reported in the oleaginous filamentous fungus Mucor circinelloides with the consensus lipase motif GXSXG and the typical acyltransferase motif of H-(X)₄-D. To demonstrate that Lip6 might play dual roles as a lipase and an acyltransferase, we performed site-directed mutagenesis in the lipase motif and the acyltransferase motif of Lip6. Mutation in the lipase motif increased cell biomass by 12%-18% and promoted lipid accumulation by 9%-24%, while mutation in the acyltransferase motif induced lipid degradation. In vitro, purified Lip6 had a slight lipase activity but had a stronger phospholipid:DAG acyltransferase activity. Enzyme activity assays in vivo and phospholipid synthesis pathway analysis suggested that phosphatidyl serine and phosphatidyl ethanolamine can be the supplier of a fatty acyl moiety to form TAG in M. circinelloides.

Comparative Proteome Analysis between High Lipid-Producing Strain Mucor circinelloides WJ11 and Low Lipid-Producing Strain CBS 277.49

Mucor circinelloides is one of few oleaginous fungi that produces a useful oil rich in γ-linolenic acid, but it usually only produces <25% total lipid. Nevertheless, we isolated a new strain WJ11 that can produce up to 36% lipid of cell dry weight. In this study, we have systematically analyzed the global changes in protein levels between the high lipid-producing strain WJ11 and the low lipid-producing strain CBS 277.49 (15%, lipid/cell dry weight) at lipid accumulation phase through comparative proteome analysis. Proteome analysis demonstrated that the branched-chain amino acid and lysine metabolism, glycolytic pathway, and pentose phosphate pathway in WJ11 were up-regulated, while the activities of tricarboxylic acid cycle and branch point enzyme for synthesis of isoprenoids were retarded compared with CBS 277.49. The coordinated regulation at proteome level indicate that more acetyl-CoA and NADPH are provided for fatty acid biosynthesis in WJ11 compared with CBS 277.49.

Lipase genes in Mucor circinelloides: identification, sub-cellular location, phylogenetic analysis and expression profiling during growth and lipid accumulation

Lipases or triacylglycerol hydrolases are widely spread in nature and are particularly common in the microbial world. The filamentous fungus Mucor circinelloides is a potential lipase producer, as it grows well in triacylglycerol-contained culture media. So far only one lipase from M. circinelloides has been characterized, while the majority of lipases remain unknown in this fungus. In the present study, 47 potential lipase genes in M. circinelloides WJ11 and 30 potential lipase genes in M. circinelloides CBS 277.49 were identified by extensive bioinformatics analysis. An overview of these lipases is presented, including several characteristics, sub-cellular location, phylogenetic analysis and expression profiling of the lipase genes during growth and lipid accumulation. All of these proteins contained the consensus sequence for a classical lipase (GXSXG motif) and were divided into four types including α/β-hydrolase_1, α/β-hydrolase_3, class_3 and GDSL lipase (GDSL) based on gene annotations. Phylogenetic analyses revealed that class_3 family and α/β-hydrolase_3 family were the conserved lipase family in M. circinelloides. Additionally, some lipases also contained a typical acyltransferase motif of H-(X) 4-D, and these lipases may play a dual role in lipid metabolism, catalyzing both lipid hydrolysis and transacylation reactions. The differential expression of all lipase genes were confirmed by quantitative real-time PCR, and the expression profiling were analyzed to predict the possible biological roles of these lipase genes in lipid metabolism in M. circinelloides. We preliminarily hypothesized that lipases may be involved in triacylglycerol degradation, phospholipid synthesis and beta-oxidation. Moreover, the results of sub-cellular localization, the presence of signal peptide and transcriptional analyses of lipase genes indicated that four lipase in WJ11 most likely belong to extracellular lipases with a signal peptide. These findings provide a platform for the selection of candidate lipase genes for further detailed functional study.

Bioinformatical analysis and preliminary study of the role of lipase in lipid metabolism in Mucor circinelloides

The filamentous fungusMucor circinelloideshas been widely used as a model organism to investigate the mechanisms of lipid accumulation.

The role of a xylose isomerase pathway in the conversion of xylose to lipid in Mucor circinelloides

The xylose isomerase (XI) pathway, which converts xylose in lignocellulosic materials into intermediate metabolites, is characterized for the first time in M. circinelloides.

Effect of flameless catalytic infrared treatment on rancidity and bioactive compounds in wheat germ oil

FCIR was used to inhibit lipase and lipoxygenase activities of WG to extend its shelf life. FCIR inhibits rapid hydrolytic and oxidative rancidity of WG in minutes. The optimum water activity for inhibiting both hydrolytic and oxidative rancidity of WG is about 0.186.