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

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.

Overexpression of delta-12 desaturase in the yeast Schwanniomyces occidentalis enhances the production of linoleic acid

The oleaginous yeast Schwanniomyces occidentalis was previously isolated because of its excellent suitability to convert lignocellulosic hydrolysates into triacyl glycerides: it is able to use a broad range of sugars and is able to tolerate high concentrations of lignocellulosic hydrolysate inhibitors. Compared to other oleaginous yeasts S. occidentalis however produces a low content of unsaturated fatty acids. We show here that the linoleic acid content can be significantly improved by (over)expression Δ12-desaturases derived from S. occidentalis and Fusarium moniliforme. Expression was stable for the homologous expression but decreased during heterologous expression. Both homologous and heterologous expression of mCherry-Δ12-desaturase led to a 4-fold increase in linoleic acid from 0.02 g/g biomass to 0.08 g/g biomass resulting in the production of 2.23 g/L and 2.05 g/L of linoleic acid.

Improved γ-linolenic acid production in Mucor circinelloides by homologous overexpressing of delta-12 and delta-6 desaturases

Background

γ-Linolenic acid (GLA) is important because of its nutritional value and medicinal applications. Although the biosynthetic pathways of some plant and microbial GLA have been deciphered, current understanding of the correlation between desaturases and GLA synthesis in oleaginous fungi is incomplete. In previous work, we found that a large amount of oleic acid (OA) had not been converted to linoleic acid (LA) or GLA in Mucor circinelloides CBS 277.49, which may be due to inadequate activities of the delta-12 or delta-6 desaturases, and thus leading to the accumulation of OA and LA. Thus, it is necessary to explore the main contributing factor during the process of GLA biosynthesis in M. circinelloides.

Results

To enhance GLA production in M. circinelloides, homologous overexpression of delta-12 and two delta-6 desaturases (named delta-6-1 and delta-6-2, respectively) were analyzed. When delta-6 desaturase were overexpressed in M. circinelloides, up to 43% GLA was produced in the total fatty acids, and the yield of GLA reached 180 mg/l, which were, respectively, 38 and 33% higher than the control strain.

Conclusion

These findings revealed that delta-6 desaturase (especially for delta-6-1 desaturase) plays an important role in GLA synthesis by M. circinelloides. The strain overexpressing delta-6-1 desaturase may have potential application in microbial GLA production.

Electronic supplementary material

The online version of this article (doi:10.1186/s12934-017-0723-8) contains supplementary material, which is available to authorized users.

Identification and characterization of a novel bifunctional Δ(12)/Δ(15)-fatty acid desaturase gene from Rhodosporidium kratochvilovae

OBJECTIVES

To elucidate the biosynthesis pathway of linoleic acid and α-linolenic acid in Rhodosporidium kratochvilovae YM25235 and investigate the correlation of polyunsaturated fatty acids with its cold adaptation.

RESULTS

A 1341 bp cDNA sequence, designated as RKD12, putatively encoding a Δ(12)-desaturase was isolated from YM25235. Sequence analysis indicated that this sequence comprised a complete ORF encoding 446 amino acids of 50.6 kDa. The encoded amino acid sequence shared higher similarity to known fungal Δ(12)-desaturases that are characteristic of three conserved histidine-rich motifs. RKD12 was further transformed into Saccharomyces cerevisiae INVScl for functional characterization. Fatty acid analysis showed the yeast transformants accumulated two new fatty acids: linoleic acid and α-linolenic acid. Furthermore, mRNA expression level of RKD12 and the content of linoleic acid and α-linolenic acid were increased significantly with the culture temperature downshift from 30 to 15 °C, which might be helpful for the cold adaptation of YM25235.

CONCLUSION

RKD12 is a novel bifunctional ∆(12)/∆(15)-desaturase gene, and the increased RKD12 mRNA expression level and PUFAs content at low temperature might be helpful for the cold adaptation of YM25235.

Fortification of foods with omega-3 polyunsaturated fatty acids

A $600 million nutritional supplements market growing at 30% every year attests to consumer awareness of, and interests in, health benefits attributed to these supplements. For over 80 years the importance of polyunsaturated fatty acid (PUFA) consumption for human health has been established. The FDA recently approved the use of ω-3 PUFAs in supplements. Additionally, the market for ω-3 PUFA ingredients grew by 24.3% last year, which affirms their popularity and public awareness of their benefits. PUFAs are essential for normal human growth; however, only minor quantities of the beneficial ω-3 PUFAs eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) are synthesized by human metabolism. Rather PUFAs are obtained via dietary or nutritional supplementation and modified into other beneficial metabolites. A vast literature base is available on the health benefits and biological roles of ω-3 PUFAs and their metabolism; however, information on their dietary sources and palatability of foods incorporated with ω-3 PUFAs is limited. DHA and EPA are added to many foods that are commercially available, such as infant and pet formulae, and they are also supplemented in animal feed to incorporate them in consumer dairy, meat, and poultry products. The chief sources of EPA and DHA are fish oils or purified preparations from microalgae, which when added to foods, impart a fishy flavor that is considered unacceptable. This fishy flavor is completely eliminated by extensively purifying preparations of n-3 PUFA sources. While n-3 PUFA lipid autoxidation is considered the main cause of fishy flavor, the individual oxidation products identified thus far, such as unsaturated carbonyls, do not appear to contribute to fishy flavor or odor. Alternatively, various compound classes such as free fatty acids and volatile sulfur compounds are known to impart fishy flavor to foods. Identification of the causative compounds to reduce and eventually eliminate fishy flavor is important for consumer acceptance of PUFA-fortified foods.

Cloning and functional analysis of HpFAD2 and HpFAD3 genes encoding Δ12- and Δ15-fatty acid desaturases in Hansenula polymorpha

Two fatty acid desaturase genes have been cloned: HpFAD2 and HpFAD3 encode Hansenula polymorpha Δ12-fatty acid desaturase (HpFad2) and Δ15-fatty acid desaturase (HpFad3), which are responsible for the production of linoleic acid (LA, C18:2, Δ9, Δ12) and α-linolenic acid (ALA, αC18:3, Δ9, Δ12, Δ15), respectively. The open reading frame of the HpFAD2 and HpFAD3 genes is 1215bp and 1239bp, encoding 405 and 413 amino acids, respectively. The putative amino acid sequences of HpFad2 and HpFad3 share more than 60% similarity and three conserved histidine-box motifs with other known yeast Fad homologs. Hpfad2Δ disruptant cannot produce C18:2 and αC18:3, while the deletion of HpFAD3 only causes the absence of αC18:3. Heterologous expression of either the HpFAD2 or the HpFAD3 gene in Saccharomyces cerevisiae resulted in the presence of C18:2 and αC18:3 when the C18:2 precursor was added. Taken together, these observations indicate that HpFAD2 and HpFAD3 indeed encode Δ12- and Δ15-fatty acid desaturases that function as the only ones responsible for desaturation of oleic acid (C18:1) and linoleic acid (C18:2), respectively, in H. polymorpha. Because a Fatty Acid Regulated (FAR) region and a Low Oxygen Response Element (LORE), which are responsible for regulation of a Δ9-fatty acid desaturase gene (ScOLE1) in S. cerevisiae, are present in the upstream regions of both genes, we investigated whether the transcriptional levels of HpFAD2 and HpFAD3 are affected by supplementation with nutrient unsaturated fatty acids or by low oxygen conditions. Whereas both genes were up-regulated under low oxygen conditions, only HpFAD3 transcription was repressed by an excess of C18:1, C18:2 and C18:3, while the HpFAD2 transcript level did not significantly change. These observations indicate that HpFAD2 expression is not controlled at the transcriptional level by fatty acids even though it contains a FAR-like region. This study indicates that HpFAD2 may be regulated by post-transcriptional mechanisms, whereas HpFAD3 may be mainly controlled at a transcriptional level.

Identification and functional characterization of two Δ12-fatty acid desaturases associated with essential linoleic acid biosynthesis in Physcomitrella patens

Two Δ12-desaturases associated with the primary steps of long-chain polyunsaturated fatty acid (LC-PUFA) biosynthesis were successfully cloned from Physcomitrella patens and their functions identified. The open reading frames (ORFs) of PpFAD2-1 and PpFAD2-2 consisted of 1,128 bp and code for 375 amino acids. Their deduced polypeptides showed 62–64 % identity to microsomal Δ12-desaturases from other higher plants, and each contained the three histidine clusters typical of the catalytic domains of such enzymes. Yeast cells transformed with plasmid constructs containing PpFAD2-1 or PpFAD2-2 produced an appreciable amount of hexadecadienoic (16:2 Δ9,12) and linoleic acids (18:2 Δ9,12), not normally present in wild-type yeast cells, indicating that the genes encoded functional Δ12-desaturase enzymes. In addition, reduction of the growth temperature from 30 to 15 °C resulted in increased accumulation of unsaturated fatty acid products.

Molecular cloning of a Pinguiochrysis pyriformis oleate-specific microsomal Δ12-fatty acid desaturase and functional analysis in yeasts and thraustochytrids

We isolated a putative desaturase gene from a marine alga, Pinguiochrysis pyriformis MBIC 10872, which is capable of accumulating eicosapentaenoic acid (C20:5(Δ5,8,11,14,17)). The gene possessed an open reading frame of 1,314 bp encoding a putative 437 amino acid residues showing high sequence identity (37-48%) with fungal and nematode Δ12-fatty acid desaturases. Yeast cells transformed with the gene converted endogenous oleic acid (C18:1(Δ9)) to linoleic acid (C18:2(Δ9,12)). However, no double bonds were introduced into other endogenous fatty acids or exogenously added fatty acids. Flag-tagged enzyme was recovered in the micosome fraction when expressed in yeast cells. To express the gene in thraustochytrids, a construct driven by the thraustochytrid-derived ubiquitin promoter was used. Interestingly, exogenously added oleic acid was converted to linoleic acid in the gene transformants but not mock transformants of Aurantiochytrium limacinum mh0186. These results clearly indicate that the gene encodes a microsomal Δ12-fatty acid desaturase and was expressed functionally in not only yeasts but also thraustochytrids. This is the first report describing the heterozygous expression of a fatty acid desaturase in thraustochytrids, and could facilitate a genetic approach towards fatty acid synthesis in thraustochytrids which are expected to be an alternative source of polyunsaturated fatty acids.

Characterization of a Delta12-fatty acid desaturase gene from Ceriporiopsis subvermispora, a selective lignin-degrading fungus

Ceriporiopsis subvermispora, a white-rot fungus, is characterized as one of the best biopulping fungi because it can degrade lignin selectively without serious damage to cellulose. We previously demonstrated that during the early stage of wood decay, this fungus produces large amounts of linoleic acid (18:2n-6) and degrades lignin by manganese peroxidase-catalyzed lipid peroxidation. In this study, we cloned a Delta12-fatty acid desaturase gene absolutely essential for the biosynthesis of linoleic acid as the main substrate for lipid peroxidation. This gene designated Cs-fad2 encodes a protein with three histidine-rich domains and four membrane-spanning domains characteristic of other Delta12-fatty acid desaturases. Moreover, we heterologously expressed Cs-fad2 in Saccharomyces cerevisiae lacking Delta12-fatty acid desaturase, and detected the de novo biosynthesis of linoleic acid by gas chromatography-mass spectrometry analysis. We also investigated transcription of Cs-fad2 under various conditions. The transcription was activated and repressed in the presence of a lignin fragment and exogenous fatty acids, respectively. These results may shed light on the molecular relationship between fatty acid metabolism and selective lignin degradation in C. subvermispora.

Atypical biosynthetic properties of a Delta 12/nu+3 desaturase from the model basidiomycete Phanerochaete chrysosporium

The model white-rot basidiomycete Phanerochaete chrysosporium contains a single integral membrane Delta(12)-desaturase FAD2 related to the endoplasmic reticular plant FAD2 enzymes. The fungal fad2-like gene was cloned and distinguished itself from plant homologs by the presence of four introns and a significantly larger coding region. The coding sequence exhibits ca. 35% sequence identity to plant homologs, with the highest sequence conservation found in the putative catalytic and major structural domains. In vivo activity of the heterologously expressed enzyme favors C(18) substrates with nu+3 regioselectivity, where the site of desaturation is three carbons carboxy-distal to the reference position of a preexisting double bond (nu). Linoleate accumulated to levels in excess of 12% of the total fatty acids upon heterologous expression of P. chrysosporium FAD2 in Saccharomyces cerevisiae. In contrast to the behavior of the plant FAD2 enzymes, this oleate desaturase does not 12-hydroxylate lipids and is the first example whose activity increases at higher temperatures (30 degrees C versus 15 degrees C). Thus, while maintaining the hallmark activity of the fatty acyl Delta(12)-desaturase family, the basidiomycete fad2 genes appear to have evolved substantially from an ancestral desaturase.

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.

Heterologous production of dihomo-gamma-linolenic acid in Saccharomyces cerevisiae

To make dihomo-gamma-linolenic acid (DGLA) (20:3n-6) in Saccharomyces cerevisiae, we introduced Kluyveromyces lactis Delta12 fatty acid desaturase, rat Delta6 fatty acid desaturase, and rat elongase genes. Because Fad2p is able to convert the endogenous oleic acid to linoleic acid, this allowed DGLA biosynthesis without the need to supply exogenous fatty acids on the media. Medium composition, cultivation temperature, and incubation time were examined to improve the yield of DGLA. Fatty acid content was increased by changing the medium from a standard synthetic dropout medium to a nitrogen-limited minimal medium (NSD). Production of DGLA was higher in the cells grown at 15 degrees C than in those grown at 20 degrees C, and no DGLA production was observed in the cells grown at 30 degrees C. In NSD at 15 degrees C, fatty acid content increased up until day 7 and decreased after day 10. When the cells were grown in NSD for 7 days at 15 degrees C, the yield of DGLA reached 2.19 microg/mg of cells (dry weight) and the composition of DGLA to total fatty acids was 2.74%. To our knowledge, this is the first report describing the production of polyunsaturated fatty acids in S. cerevisiae without supplying the exogenous fatty acids.

Cloning and molecular characterization of Δ12-fatty acid desaturase gene from Mortierella isabellina

AIM: To clone Δ¹² -fatty acid desaturase gene of Mortierella isabellina and to functionally characterize this gene in vitro and in vivo.

METHODS: Reverse transcriptional polymerase chain reaction (RT-PCR) was used to clone the open reading frame of Δ¹²-fatty acid desaturase gene (D12D) of Mortierella isabellina. Plasmids pEMICL12 and pYMICL12 were constructed with it. pEMICL12 was transformed into Escherichia coli (E.coli) strain BL21 using CaCl₂ method for expression after induction with IPTG. pTMICL12 was transformed into Saccharomyces cerevisiae strain INVSc1 using lithium acetate method for expression under the induction of galactose. Northern blotting method was used to investigate the effect of temperature on the transcriptional level of this gene in S.cerevisiae strain INVSc1.

RESULTS: Recombinant plasmids pEMICL12 and pTMICL12 were successfully constructed and transformed into E.coli and S.cerevisiae separately with appropriate method. After induction with IPTG and galactose, it was found that expression of Δ¹²-fatty acid desaturase genes in E.coli and S. cerevisiae under appropriate conditions led to the production of active Δ¹²-fatty acid desaturase, which could convert 17.876% and 17.604% of oleic acid respectively to linoleic acid by GC-MS detection in vitro and in vivo.

CONCLUSION: Cloning and expression of M.isabellina D12D gene in E.coli and S.cerevisiae is successfully completed.

Isolation of Δ12 and ω3-fatty acid desaturase genes from the yeastKluyveromyces lactis and their heterologous expression to produce linoleic and α-linolenic acids inSaccharomyces cerevisiae

Two clones with homology to known fatty acid desaturase genes were isolated from the yeast Kluyveromyces lactis. The first gene, which we designate KlFAD2, consists of 411 amino acids with an overall identity of 73.0% to FAD2 from Saccharomyces kluyveri. It exhibited Delta12 fatty acid desaturase activity when expressed in S. cerevisiae under the control of ADH1 promoter and produced endogenous linoleic acid. The second clone, which we designate KlFAD3, consists of 415 amino acids with an overall identity of 79.3% to FAD3 from S. kluyveri. It exhibited omega3 fatty acid desaturase activity in S. cerevisiae when expressed under the control of ADH1 promoter in the presence of the exogenous substrate linoleic acid and produced alpha-linolenic acid. Co-expression of KlFAD2 and KlFAD3 resulted in the endogenous production of both linoleic and alpha-linolenic acids. The yield of alpha-linolenic acid reached 0.8% of total fatty acids and its production was not increased by adding exogenous oleic acid; alpha-linolenic acid reached 8.7% when exogenous linoleic acid was available.

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.

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.

Identification and characterization of a novel delta12-fatty acid desaturase gene from Rhizopus arrhizus

Based on the sequence information of Delta12-fatty acid desaturase genes (from Mucor circinelloides, Mortierella alpina, Mucor rouxii and Aspergillus nidulans), which were involved in the conversion from C18:1 to C18:2, a cDNA sequence putatively encoding a Delta12-fatty acid desaturase was isolated from Rhizopus arrhizus using the combination of reverse transcription-polymerase chain reaction (RT-PCR) and rapid amplification of cDNA ends (RACE) methods. Sequence analysis indicated that it had an open reading frame (ORF) of 1170 bp, coding for 389 amino acid residues of 45 kDa, pI of the deduced protein was 7.01. The deduced amino acid sequence of this cloned cDNA showed high identity to those filamentous fungal Delta12-desaturases mentioned above, including three conserved histidine-rich motifs and two hydrophobic domains. Functional identification was done heterologously in Saccharomyces cerevisiae strain INVScl. The result demonstrated that the deduced amino acid sequence exhibited Delta12-fatty acid desaturase activity, suggesting that this gene encoded for a membrane-bound desaturase, Delta12-fatty acid desaturase.

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.

Identification and characterization of a novel Δ6 -fatty acid desaturase gene from Rhizopus arrhizus

A cDNA sequence putatively encoding a Delta(6)-fatty acid desaturase was isolated from Rhizopus arrhizus using reverse transcription polymerase chain reaction and rapid amplification of cDNA ends methods. Sequence analysis indicated that this cDNA sequence had an open reading frame of 1377 bp encoding 458 amino acids of 52 kDa. The deduced amino acid sequence showed high similarity to those of fungal Delta(6)-fatty acid desaturases which comprised the characteristics of membrane-bound desaturases, including three conserved histidine-rich motifs and hydropathy profile. A cytochrome b(5)-like domain was observed at the N-terminus. To elucidate the function of this novel putative desaturase, the coding sequence was expressed heterologously in Saccharomyces cerevisiae strain INVScl. The result demonstrated that the coding product of the sequence exhibited Delta(6)-fatty acid desaturase activity by the accumulation of gamma-linolenic acid.

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.

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.

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.

Molecular cloning of full-length cDNA encoding delta-9 desaturase through PCR strategies and its genomic organization and expression in grass carp (Ctenopharyngodon idella)

Desaturases are enzymes that catalyze double bond formation in fatty acids, which is a critical step in the synthesis of unsaturated fatty acids in organisms. Desaturase cDNA has been cloned from various species. Here we report the cloning of a full-length cDNA of Delta(9)-desaturase from grass carp (Ctenopharyngodon idella), using a combination of PCR techniques: reverse transcription-polymerase chain reaction (RT-PCR) and rapid amplification of cDNA ends (RACE). The resolved cDNA encompasses 2420 bp, containing an open reading frame corresponding to 324 amino acids. The deduced amino acid sequence shares high homology with those of mammalian desaturases. Northern blot and RT-PCR analyses demonstrated a high abundance of the transcript in liver tissue but low abundance in brain tissue. Furthermore, the structure of the gene has been resolved by screening its cognate genomic DNA library. The analysis shows that this gene is composed of six exons and five introns, encompassing a region of 8.5 kb. In particular, the last exon contains a length of the 3' untranslated region as long as 1382 bp. Although the primary sequence and the genomic organization are phylogenetically conserved between fish and mammals, the regulation of the gene expression appears to be divergent among species.

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.

Filamentous fungi as microbial cell factories for food use

The knowledge base that will underpin the more efficient use of filamentous fungi as cell factories in food has increased during the past year in the areas of gene regulation, protein secretion, safety and synthesis of ingredients such as long-chain polyunsaturated fatty acids.