Mucor rouxii delta9-desaturase gene is transcriptionally regulated during cell growth and by low temperature

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.

Membrane fatty acid desaturase: biosynthesis, mechanism, and architecture

Fatty acid desaturase catalyzes the desaturation reactions by inserting double bonds into the fatty acyl chain, producing unsaturated fatty acids, which play a vital part in the synthesis of polyunsaturated fatty acids. Though soluble fatty acid desaturases have been described extensively in advanced organisms, there are very limited studies of membrane fatty acid desaturases due to their difficulties in producing a sufficient amount of recombinant desaturases. However, the advancement of technology has shown substantial progress towards the development of elucidating crystal structures of membrane fatty acid desaturase, thus, allowing modification of structure to be manipulated. Understanding the structure, mechanism, and biosynthesis of fatty acid desaturase lay a foundation for the potential production of various strategies associated with alteration and modifications of polyunsaturated fatty acids. This manuscript presents the current state of knowledge and understanding about the structure, mechanisms, and biosynthesis of fatty acid desaturase. In addition, the role of unsaturated fatty acid desaturases in health and diseases is also encompassed. This will be useful in understanding the molecular basis and structural protein of fatty acid desaturase that are significant for the advancement of therapeutic strategies associated with the improvement of health status. KEY POINTS: • Current state of knowledge and understanding about the biosynthesis, mechanisms, and structure of fatty acid desaturase. • The role of unsaturated fatty acid desaturase. • The molecular basis and structural protein elucidated the crystal structure of fatty acid desaturase.

Membrane-bound Δ12 fatty acid desaturase (FAD12); From Brassica napus to E. coli expression system

Fatty acid desaturase catalyzes the desaturation reactions by insertion of double bonds into the fatty acyl chain, producing unsaturated fatty acids. Though soluble fatty acid desaturases have been studied widely in advanced organisms, there are very limited studies of membrane fatty acid desaturases due to the difficulty of generating recombinant desaturase. Brassica napus is a rapeseed, which possesses a range of different membrane-bound desaturases capable of producing fatty acids including Δ3, Δ4, Δ8, Δ9, Δ12, and Δ15 fatty acids. The 1155 bp open reading frame of Δ12 fatty acid desaturase (FAD12) from Brassica napus codes for 383 amino acid residues with a molecular weight of 44 kDa. It was expressed in Escherichia coli at 37 °C in soluble and insoluble forms when induced with 0.5 mM IPTG. Soluble FAD12 has been purified using Ni²⁺-Sepharose affinity chromatography with a total protein yield of 0.728 mg/mL. Gas chromatography-mass spectrometry (GC-MS) analysis revealed that desaturase activity of FAD12 could produce linoleic acid from oleic acid at a retention time of 17.6 with a conversion rate of 47%. Characterization of purified FAD12 revealed the optimal temperature of FAD12 was 50 °C with 2 mM preferred substrate concentration of oleic acid. Analysis of circular dichroism (CD) showed FAD12 was made up of 47.3% and 0.9% of alpha-helix and β-sheet secondary structures. The predicted Tm value was 50.2 °C.

Characterization of three Δ9-fatty acid desaturases with distinct substrate specificity from an oleaginous fungus Cunninghamella echinulata

In oleaginous fungus Cunninghamella echinulata, Δ9-fatty acid desaturase introduces the first double bond into a saturated fatty acid. Three distinct genes, designated as d9dma, d9dmb and d9dmc, all encoding putative Δ9-fatty acid desaturases were isolated from this strain. The predicted proteins showed 79-87 % identity to other fungal Δ9-fatty acid desaturases. They all contain three conserved histidine boxes, C-terminal cytochrome b 5 fusion and four transmembrane domains characteristic of Δ9-desaturase. Each putative Δ9-desaturase gene from C. echinulata was able to complement the ole1 mutation in Saccharomyces cerevisiae L8-14C through heterologous expression. Analysis of the fatty acid composition of the transgenic yeast revealed that the conversion rates of 16:0 and 18:0 by D9DMA were obviously higher than those of D9DMB and D9DMC. In addition, D9DMA, D9DMB and D9DMC all had a substrate preference for 18:0 compared with 16:0. Of interest, D9DMA could saturate 12:0, 14:0, 16:0, 17:0, 18:0 and 20:0, while D9DMB saturated 14:0, 16:0, 17:0, 18:0 and 20:0. We also noticed that the transcriptional level of d9dma in C. echinulata was stimulated by cell growth but not by decline in temperature. In contrast, expression of d9dmb and d9dmc was regulated by neither cell growth nor decline in temperature in this strain.

Gamma-linolenic acid production of Mucor rouxii by solid-state fermentation using agricultural by-products

AIMS

This study aims to maximize the yield of gamma-linolenic acid by a filamentous fungus, Mucor rouxii, using low cost production by solid-state fermentation.

METHODS AND RESULTS

We optimized substrate types and culture conditions including inoculum size and temperature. The optimal growth of M. rouxii was found in the cultures inoculated with 5 x 10(5) spores g(-1) substrate. The fungal cells grew well on rice bran and soy bean meal, whereas a lower biomass was found in the cultures grown on polished rice, broken rice and spent malt grain. The GLA content was highly accumulated in rice bran ferment and its maximal content of about 6 g kg(-1) fermented mass was observed in the 5th-day culture grown at 30 degrees C. However, the GLA content in the rice bran ferment was not enhanced by low temperature.

CONCLUSIONS

The GLA production of M. rouxii could be enhanced by optimizing the agricultural by-product substrates and culture condition.

SIGNIFICANCE AND IMPACT OF THE STUDY

Low cost GLA production process was achieved, and fermented product containing GLA can be incorporated into feed additives without further oil extraction to alternate the expensive plant oils.

Filamentous fungi for production of food additives and processing aids

Filamentous fungi are metabolically versatile organisms with a very wide distribution in nature. They exist in association with other species, e.g. as lichens or mycorrhiza, as pathogens of animals and plants or as free-living species. Many are regarded as nature's primary degraders because they secrete a wide variety of hydrolytic enzymes that degrade waste organic materials. Many species produce secondary metabolites such as polyketides or peptides and an increasing range of fungal species is exploited commercially as sources of enzymes and metabolites for food or pharmaceutical applications. The recent availability of fungal genome sequences has provided a major opportunity to explore and further exploit fungi as sources of enzymes and metabolites. In this review chapter we focus on the use of fungi in the production of food additives but take a largely pre-genomic, albeit a mainly molecular, view of the topic.

Ethanol-induced changes in expression profiles of cell growth, fatty acid and desaturase genes of Mucor rouxii

We investigated the response of Mucor rouxii to ethanolic stress conditions. A differential response was found that was dependent on growth phase and ethanol concentration. 5% (v/v) ethanol showed an inhibitory effect on the mycelial growth of all stages. However, the ethanol sensitivity was specifically observed in active growing phases (12 and 21 h-grown cultures), in which the biomass and ratio of unsaturated/saturated fatty acids (UFA/SFA) decreased greatly after ethanol exposure compared to non-ethanol adding culture. With respect to different ethanol concentrations, M. rouxii was tolerant to low ethanol concentrations (about 1-3%, v/v) such that there was not much change in biomass and UFA/SFA ratio, in contrast to the 5% ethanol-added culture. We also showed the molecular basis of this response mechanism, demonstrating that expression of Delta(9)-, Delta(12)- and Delta(6)-desaturase genes, responsible for fatty acid desaturation in M. rouxii, were coordinately down-regulated upon exposure to ethanol stress.

Identification of a novel delta9-fatty acid desaturase gene and its promoter from oil-producing fungus Rhizopus arrhizus

Reverse Transcriptional Polymerase Chain Reaction (RT-PCR), Rapid Amplification of the cDNA ends (RACE) and Thermal asymmetric interlaced (TAIL)-PCR were used successfully to clone the open reading frame (1,377 bp) of delta9-fatty acid desaturase gene (named as RAD9) and its promoter region from oil-producing fungi Rhizopus arrhizus. Functional identification of the protein was done by sub-cloning RAD9 into the expression vector pYES2.0 to generate a recombinant plasmid pYRAD9, which was then subsequently transformed into Saccharomyces cerevisiae delta9-fatty acid desaturase mutation strain L8-14C to be expressed under the control of GAL1 promoter. The transformant containing RAD9 named as L8-14C-pYRAD9 could grow on the synthetic minimal medium plate with out oleic acid supplement. Fatty acid analysis also showed that the transformant contained 16:1 and 18:1. This indicated that pYRAD9 could successfully complement the mutation of L8-14C. Computational analysis of the nucleotide sequence of RAD9 promoter showed several basic transcriptional elements including a CAAT box, a GC box, a CACCC box, two TATA boxes and also several putative target-binding sites for transcription factors, which have been reported to be involved in the regulation of lipid metabolism. Preliminary functional analysis of this promoter in S. cerevisiae was done using lacZ report gene.

Spore germination in Mortierella alpina is associated with a transient depletion of arachidonic acid and induction of fatty acid desaturase gene expression

Mortierella alpina is an oleaginous filamentous fungus whose vegetative mycelium is known to accumulate triglyceride oil containing large amounts of arachidonic acid (ARA 20:4, n - 6). We report that the spores of Mortierella alpina also contain a large proportion of ARA, comprising 50% of total fatty acid. Fatty acid desaturase genes were not expressed in dormant spores but were induced during germination, following a significant drop in the level of ARA (down from 50% of total fatty acid to 12%) prior to germ-tube emergence. We propose that ARA serves as a reserve supply of carbon and energy that is utilised during the early stages of spore germination in Mortierella alpina.

Comparative fatty acid profiling ofMucor rouxiiunder different stress conditions

To understand the relationship between fatty acid metabolism and the growth morphology of Mucor rouxii, fatty acid profiling was studied comparatively in cells grown under conditions which included different atmospheric conditions or the addition of phenethyl alcohol (PEA). The significant difference in fatty acid profiles from M. rouxii grown under aerobic or anaerobic conditions was not found to be directly related to morphological growth. Oxygen limitation, which induced the formation of pure multipolar budding yeasts, led to a decrease in long-chain fatty acids-- particularly unsaturated fatty acids-- and an increase in medium-chain saturated fatty acids, a finding which contrasted with the aerobic cultures, including mycelia and PEA-induced bipolar budding cells. High levels of C18 : 1Delta(9) were found in aerobic yeast cultures with additional PEA when compared to that in the aerobically grown mycelia. The identification of unusual fatty acids in Mucor in response to alcoholic and hypoxic stresses - including odd-numbered fatty acids and 7-hydroxy dodecanoic acid (7-OH C12 : 0) in addition to the more common fatty acids - implied that an important role existed for these unusual fatty acids.

Targeted mutagenesis of a fatty acid Delta6-desaturase from Mucor rouxii: role of amino acid residues adjacent to histidine-rich motif II

The amino acid residues serine at position 213 (S213) and lysine at position 218 (K218), which are present in close proximity to the histidine-rich motif II of Mucor rouxii fatty acid Delta(6)-desaturase isoform II, were targeted for studying structure-function relationships using site-directed mutagenesis. The mutants were functionally characterized in a heterologous host, Saccharomyces cerevisiae. Substrate specificity and preference studies revealed that S213 and K218 are involved in substrate recognition. K218 plays a role in substrate preference by involvement in the binding of substrates, particularly C15-C18 monoene fatty acids. Modification of the M. rouxii Delta(6)-desaturase therefore has potential in specifically altering substrate utilization for production of desired fatty acids.

Promoter analysis of Mucor rouxii delta9-desaturase: its implication for transcriptional regulation in Saccharomyces cerevisiae

Promoter study was performed to understand the transcriptional control of delta9-desaturase gene of Mucor rouxii. Several putative cis-elements involved in lipid metabolism were mapped by computational analysis. 5' deletion analysis shows the presence of elements with repressing activity, especially in 122 bp located upstream of the transcription start site. Truncation of these repressor domains showed that the promoter of M. rouxii is functional in Saccharomyces cerevisiae without additional components and is insensitive to nutritional depletion. The promoter also drove effectively the expression of a M. rouxii delta12-desaturase gene, and the linoleic acid content increased with the age of the yeast culture in parallel with the promoter activity. This approach provides a genetic tool for programming heterologous protein production in the yeast.

Substrate specificity and preference of Δ6-desaturase ofMucor rouxii

The Delta(6)-fatty acid desaturase is a key enzyme in the synthesis of an important fatty acid, gamma-linolenic acid. We have characterized, by heterologous expression in Saccharomyces cerevisiae, substrate specificity and preference of Delta(6)-desaturase of Mucor rouxii. Fatty acid supplementation was carried out based on the predicted enzyme topology, fatty acid phenotype and the corresponding metabolic pathway in M. rouxii. The enzyme has a broad substrate specificity as based on C15-C18. The result also supported classification of the M. rouxii Delta(6)-desaturase into a front-end desaturase. Interestingly, a relatively rare activity based on odd acyl chains and not described previously in other eukaryotic Delta(6)-desaturases was also observed.

Cloning and functional characterization of a Delta12 fatty acid desaturase gene from the basidiomycete Lentinula edodes

In the basidiomycete Lentinula edodes, a famous edible mushroom (shiitake), the fatty acyl composition of total lipids has previously been shown to change during cell differentiation. In the present study, we succeeded in cloning a gene for a Delta12 fatty acid desaturase from L. edodes. The ORF of this gene (named Le-FAD2) consists of 1308 bp and codes for 435 amino acids. The deduced Le-FAD2 protein shows 40-45% identity to Delta12 fatty acid desaturases from other fungi, and the three histidine clusters typical of the catalytic domain of such enzymes are conserved. Expression of the Le-FAD2 gene in the budding yeast Saccharomyces cerevisiae indicated that its product was able to synthesize linoleic acid (C18:2). Analysis of Le-FAD2 expression in L. edodes revealed that levels of transcription were higher in fruiting body primordia and in mature fruiting bodies, the two differentiated tissues, than in mycelium, and reduction of the growth temperature from 25 to 18 degrees C had no effect on the level of the Le-FAD2 transcript. Thus, although Le-FAD2 expression is correlated with the alteration in the complement of unsaturated fatty acids (UFAs) observed during fruiting body formation, the gene does not respond to a downshift in temperature to 18 degrees C.

Genetic modification of essential fatty acids biosynthesis inHansenula polymorpha

The Delta(6)-desaturase gene isoform II involved in the formation of gamma-linolenic acid (GLA) was identified from Mucor rouxii. To study the possibility of alteration of the synthetic pathway of essential fatty acids in the methylotrophic yeast, Hansenula polymorpha, the cloned gene of M. rouxii under the control of the methanol oxidase (MOX) promoter of H. polymorpha, was used for genetic modification of this yeast. Changes in flux through the n-3 and n-6 pathways in the transgenic yeast were observed. The proportion of GLA varied dramatically depending on the growth temperature and media composition. This can be explained by the effects of either substrate availability or enzymatic activity. In addition to the potential application for manipulating the fatty acid profile, this study provides an attractive model system of H. polymorpha for investigating the deviation of fatty acid metabolism in eukaryotes.

Cold adaptation in budding yeast

We have determined the transcriptional response of the budding yeast Saccharomyces cerevisiae to cold. Yeast cells were exposed to 10 degrees C for different lengths of time, and DNA microarrays were used to characterize the changes in transcript abundance. Two distinct groups of transcriptionally modulated genes were identified and defined as the early cold response and the late cold response. A detailed comparison of the cold response with various environmental stress responses revealed a substantial overlap between environmental stress response genes and late cold response genes. In addition, the accumulation of the carbohydrate reserves trehalose and glycogen is induced during late cold response. These observations suggest that the environmental stress response (ESR) occurs during the late cold response. The transcriptional activators Msn2p and Msn4p are involved in the induction of genes common to many stress responses, and we show that they mediate the stress response pattern observed during the late cold response. In contrast, classical markers of the ESR were absent during the early cold response, and the transcriptional response of the early cold response genes was Msn2p/Msn4p independent. This implies that the cold-specific early response is mediated by a different and as yet uncharacterized regulatory mechanism.

Saccharomyces kluyveri FAD3 encodes an ω3 fatty acid desaturase

Fungi, like plants, are capable of producing the 18-carbon polyunsaturated fatty acids linoleic acid andα-linolenic acid. These fatty acids are synthesized by catalytic reactions of Δ12 andω3 fatty acid desaturases. This paper describes the first cloning and functional characterization of a yeastω3 fatty acid desaturase gene. The deduced protein encoded by theSaccharomyces kluyveri FAD3gene (Sk-FAD3) consists of 419 amino acids, and shows 30–60 % identity with Δ12 fatty acid desaturases of several eukaryotic organisms and 29–31 % identity withω3 fatty acid desaturases of animals and plants. DuringSk-FAD3expression inSaccharomyces cerevisiae,α-linolenic acid accumulated only when linoleic acid was added to the culture medium. The disruption ofSk-FAD3led to the disappearance ofα-linolenic acid inS. kluyveri. These findings suggest thatSk-FAD3is the onlyω3 fatty acid desaturase gene in this yeast. Furthermore, transcriptional expression ofSk-FAD3appears to be regulated by low-temperature stress in a manner different from the other fatty acid desaturase genes inS. kluyveri.

A Mucor rouxii mutant with high accumulation of an unusual trans-linoleic acid (9c,12t-C18:2)

Genetic and biochemical approaches reveal the existence of a gamma-linolenic acid biosynthetic pathway in Mucor rouxii. By treatment with ultraviolet light, combined with low temperature cultivation and filtration enrichment, a mutant defective in polyunsaturated fatty acid synthesis was isolated. Genetic analysis and fatty acid supplementation indicate that the defect occurred in the Delta(12)-desaturation resulting in the absence of cis-linoleic acid and gamma-linolenic acid and in the accumulation of monounsaturated fatty acids. In addition, an unusual fatty acid, trans-linoleic acid (9c,12t-C18:2), which has not been reported previously in this fungus, was found to increase in the mutant. The information gained from the mutant was used to develop the hypothetical pathway of fatty acid desaturation in M. rouxii.

Merging of multiple signals regulating delta9 fatty acid desaturase gene transcription in Saccharomyces cerevisiae

Fatty acid desaturation, which requires molecular oxygen (O2) as an electron acceptor, is catalyzed by delta9 fatty acid desaturase, which is encoded by OLE1 in Saccharomyces cerevisiae. Transcription of the OLE1 gene is repressed by unsaturated fatty acids (UFAs) and activated by hypoxia and low temperatures via the endoplasmic reticulum membrane protein Mga2p. We previously reported the isolation of the nfo3-1 (negative factor for OLE1) mutant, which exhibits enhanced expression of OLE1 in the presence of UFA and under aerobic conditions. In this work, we demonstrated that the NFO3 gene is identical to OLE1 and that the nfo3-1 mutation (renamed ole1-101) alters arginine-346, in the vicinity of the conserved histidine-rich motif essential for the catalytic function of the Ole1 protein, to lysine. The ratio of UFAs to total fatty acids in the ole1-101 mutant was 60%, compared to 75% in the wild type, suggesting that the reduction in relative levels of intracellular UFAs activates OLE1 transcription. However, in ole1-101 cells grown in the presence of oleic acid, the level of OLE1 expression remained high, although the relative amount of UFAs in the ole1-101 mutant cells was almost the same as that in wild-type cells growing under the same conditions. By contrast, when cells were grown with linoleic acid, which has a lower melting point than oleic acid, the elevation of the OLE1 expression level due to the ole1-101 mutation was almost completely suppressed. These observations suggest that the ole1-101 cells activate OLE1 transcription by sensing not only the intracellular UFA level, but also membrane fluidity or the nature of the UFA species itself. Furthermore, we found that not only the fatty acid- regulated (FAR) element but also the O2- regulated (O2R) element in the OLE1 promoter was involved in the activation of OLE1 transcription by the ole1-101 mutation, and that the effects of the low-oxygen signal and the ole1-101-generated signal on OLE1 expression were not additive. Taken together, these findings suggest that signals associated with hypoxia, low temperatures and intracellular UFA depletion activate OLE1 transcription by a common pathway.

Recent advances in the study of fatty acid desaturases from animals and lower eukaryotes

The biosynthesis of polyunsaturated fatty acids (PUFAs) in different organisms can involve a variety of pathways, catalyzed by a complex series of desaturation and elongation steps. A range of different desaturases have been identified to date, capable of introducing double bonds at various locations on the fatty acyl chain. Some recently identified novel desaturases include a delta4 desaturase from marine fungi, and a bi-functional delta5/delta6 desaturase from zebrafish. Using molecular genetics approaches, these desaturase genes have been isolated, identified, and expressed in variety of heterologous hosts. Results from these studies will help increase our understanding of the biochemistry of desaturases and the regulation of PUFA biosynthesis. This is of significance because PUFAs play critical roles in multiple aspects of membrane physiology and signaling mechanisms which impact human health and development.

Comprehensive expression analysis of time-dependent genetic responses in yeast cells to low temperature

We performed genome-wide expression analysis to determine genetic responses in Saccharomyces cerevisiae to a low temperature environment using a cDNA microarray. Approximately 25% of the genes in the yeast genome were found to be involved in the response of yeast to low temperature. This finding of a large number of genes being involved in the response to low temperature enabled us to give a functional interpretation to the genetic responses to the stimulus. Functional and clustering analyses of temporal changes in gene expression revealed that global states of the expressions of up-regulated genes could be characterized as having three phases (the early, middle, and late phases). In each phase, genes related to rRNA synthesis, ribosomal proteins, or several stress responses are time-dependently up-regulated, respectively. Through these phases, yeast cells may improve reduced efficiency of translation and enhance cell protection mechanisms to survive under a low temperature condition. Furthermore, these time-dependent regulations of these genes would be controlled by the cAMP-protein kinase A pathway. The results of our study provide a global description of transcriptional response for adaptation to low temperature in yeast cells.

Differential expression of desaturases and changes in fatty acid composition during sporangiospore germination and development in Mucor rouxii

Polyunsaturated fatty acids (PUFAs), namely, oleic (C18:1), linoleic (C18:2), and gamma-linolenic acid (C18:3), constituted the majority in the total fatty acid content (44%) of sporangiospores of Mucor rouxii. At 30 degrees C, the germination begins within 1h at which time spore swelling occurs, followed by germ tube emergence within 3-4h. Throughout germination, an increase in gamma-linolenic acid (GLA) was observed and its content was highest at germ tube emergence. It took longer for sporangiospores of M. rouxii to germinate at sub-optimal temperatures (15 and 35 degrees C). However, the content of GLA was higher at the germ tube initiation than at the mycelial stage at all temperatures, suggesting the association of GLA and germination of sporangiospores. This finding was substantially confirmed by differential expression of delta9-, delta12-, and delta6-desaturase genes measured during spore germination. The expression of three desaturase genes parallels the pattern of GLA synthesis. By using RT-PCR techniques to follow gene expression, we found that mRNA of delta12- and delta6-desaturase genes were translated as soon as the spores were introduced into a fresh medium while the mRNA of delta9-desaturase gene could not be detected until 2h after introduction. A sharp increase in mRNA of delta6-desaturase genes correlated well with an increase in GLA content at germ tube emergence (4h). These results demonstrated that changes in fatty acid composition of sporangiospore of M. rouxii and differential expression of desaturase genes occurred during germination, and that extensive changes in GLA synthesis associated with some events in germination process.

Mga2p is a putative sensor for low temperature and oxygen to induce OLE1 transcription in Saccharomyces cerevisiae

Various low-temperature-inducible genes such as fatty acid desaturase genes are essential for all living organisms to acclimate to low temperature. However, a low-temperature signal transduction pathway has not been identified in eukaryotes. In yeast Saccharomyces cerevisiae, the Delta9 fatty acid desaturase gene OLE1 is activated by ubiquitin/proteasome-dependent processing of two homologous endoplasmic reticulum membrane proteins, Spt23p and Mga2p. We found that OLE1 transcription was transiently activated with resultant increases in the degree of unsaturation of total fatty acids when culture temperature was downshifted from 30 degrees C to 10 degrees C. This activation was greatly depressed in Deltamga2 cells. Although Mga2p is essential for hypoxic activation of OLE1 transcription, and its hypoxic functions are repressed by unsaturated fatty acids (UFAs), low-temperature activation of the OLE1 gene was not repressed by UFAs. These observations suggest that low-temperature and hypoxic signal transduction pathways share some components, and Mga2p is the first identified eukaryotic sensor for low temperature and oxygen.

Genetic Connection between Fatty Acid Metabolism and Sporulation in Aspergillus nidulans

In the Ascomycete fungus Aspergillus nidulans, the ratio of conidia (asexual spores) to ascospores (sexual spores) is affected by linoleic acid moieties including endogenous sporogenic factors called psi factors. Deletion of odeA (Delta odeA), encoding a Delta-12 desaturase that converts oleic acid to linoleic acid, resulted in a strain depleted of polyunsaturated fatty acids (18:2 and 18:3) but increased in oleic acid (18:1) and total percent fatty acid content. Linoleic acid-derived psi factors were absent in this strain but oleic acid-derived psi factors were increased relative to wild type. The Delta odeA strain was reduced in conidial production and mycelial growth; these effects were most noticeable when cultures were grown at 26 degrees C in the dark. Under these environmental conditions, the Delta odeA strain was delayed in ascospore production but produced more ascospores than wild type over time. This suggests a role for oleic acid-derived psi factors in affecting the asexual to sexual spore ratio in A. nidulans. Fatty acid composition and spore development were also affected by veA, a gene previously shown to control light driven conidial and ascospore development. Taken together our results indicate an interaction between veA and odeA alleles for fatty acid metabolism and spore development in A. nidulans.

delta(6)-desaturase of Mucor rouxii with high similarity to plant delta(6)-desaturase and its heterologous expression in Saccharomyces cerevisiae

Gamma-linolenic acid (GLA, gamma-C18:3) is an essential fatty acid that plays a vital role in biological structures and cellular functions. Based on available sequence information and using polymerase chain reaction (PCR) technique, we cloned from the fungus Mucor rouxii the entire coding sequence of a delta(6)-desaturase enzyme, which is responsible for the transformation of linoleic acid into GLA. The deduced amino acid sequence of M. rouxii gene showed the highest homology with the plant delta(6)-desaturase. It comprises the characteristics of membrane-bound desaturases, including histidine-rich motifs and hydrophobic regions. A cytochrome b(5)-like domain was observed at the N-terminus. In addition to three conserved histidine-rich motifs, we found an additional histidine-rich motif, HKHHSH, downstream of the cytochrome b(5)-like domain, which is not present in previously cloned delta(6)-desaturase genes. Heterologous expression of the M. rouxii cDNA in Saccharomyces cerevisiae resulted in the synthesis and accumulation of GLA.

Heterologous expression of Mucor rouxii delta(12)-desaturase gene in Saccharomyces cerevisiae

In this study we present the cloning and functional characterization of a gene whose product is responsible for Delta(12)-desaturase activity and is involved in the metabolic pathway of gamma-linolenic acid (GLA) synthesis of Mucor rouxii. A cDNA encoding for Delta(12)-desaturase of M. rouxii was obtained using the combination of reverse transcription-polymerase chain reaction (RT-PCR) and rapid amplification cDNA ends (RACE) techniques. The 1188 [corrected] bp code for an open reading frame of 396 amino acid residues. The deduced amino acid sequence of the cloned cDNA comprises three conserved histidine regions and two hydrophobic domains and showed similarity with microsomal omega-3 and omega-6 desaturases of plants. Expression of this open reading frame in Saccharomyces cerevisiae resulted in the accumulation of linoleic acid (C18:2), suggesting that this gene encodes for a membrane-bound desaturase, Delta(12)-desaturase, of M. rouxii that is functional in yeast.