Biosynthesis of C18 polyunsaturated fatty acids in microsomal membrane preparations from the filamentous fungus Mucor circinelloides

Critical applications of Mucor circinelloides within a biorefinery context

The establishment of an efficient and feasible biorefinery model depends on, among other factors, particularly the selection of the most appropriate microorganism. Mucor circinelloides is a dimorphic fungus species able to produce a wide variety of hydrolytic enzymes, lipids prone to biodiesel production, carotenoids, ethanol, and biomass with significant nutritional value. M. circinelloides also has been selected as a model species for genetic modification by being the first filamentous oleaginous species to have its genome fully characterized, as well as being a species characterized as a potential bioremediation agent. Considering the potential of replacing several nonrenewable feedstocks is widely dependent on fossil fuels, the exploitation of microbial processes and products is a desirable solution for promoting a green and sustainable future. Here, we introduce and thoroughly describe the recent and critical applications of this remarkable fungus within the context of developing a fungal-based biorefinery.

Modifying the lipid content and composition of plant seeds: engineering the production of LC-PUFA

Omega-3 fatty acids are characterized by a double bond at the third carbon atom from the end of the carbon chain. Latterly, long chain polyunsaturated omega-3 fatty acids such as eicosapentaenoic acid (EPA; 20:5Δ5,8,11,14,17) and docosahexanoic acid (DHA; 22:6 Δ4,7,10,13,16,19), which typically only enter the human diet via the consumption of oily fish, have attracted much attention. The health benefits of the omega-3 LC-PUFAs EPA and DHA are now well established. Given the desire for a sustainable supply of omega-LC-PUFA, efforts have focused on enhancing the composition of vegetable oils to include these important fatty acids. Specifically, EPA and DHA have been the focus of much study, with the ultimate goal of producing a terrestrial plant-based source of these so-called fish oils. Over the last decade, many genes encoding the primary LC-PUFA biosynthetic activities have been identified and characterized. This has allowed the reconstitution of the LC-PUFA biosynthetic pathway in oilseed crops, producing transgenic plants engineered to accumulate omega-3 LC-PUFA to levels similar to that found in fish oil. In this review, we will describe the most recent developments in this field and the challenges of overwriting endogenous seed lipid metabolism to maximize the accumulation of these important fatty acids.

Lipid accumulation by pelletized culture of Mucor circinelloides on corn stover hydrolysate

Microbial oil accumulated by fungal cells is a potential feedstock for biodiesel production, and lignocellulosic materials can serve as the carbon source to support the fungal growth. The dilute acid pretreatment of corn stover can effectively break down its lignin structure, and this process generates a hydrolysate containing mostly xylose at very dilute concentration and numerous by-products that may significantly inhibit the cell growth. This study utilized corn stover hydrolysate as the culture media for the growth of Mucor circinelloides. The results showed that Mucor cells formed pellets during the cell growth, which facilitates the cell harvest from dilute solution. The results also showed that the inhibitory effect of furfural, 5-hydroxymethylfurfural (HMF), and acetic acid could be avoided if their concentration was low. In fact, all these by-products may be assimilated as carbon sources for the fungal growth. The results proved the feasibility to reuse the cultural broth water for acid pretreatment and then use for subsequent cell cultivation. The results will have a direct impact on the overall water usage of the process.

Metabolic engineering of the omega-3 long chain polyunsaturated fatty acid biosynthetic pathway into transgenic plants

Omega-3 (ω-3) very long chain polyunsaturated fatty acids (VLC-PUFAs) such as eicosapentaenoic acid (EPA; 20:5 Δ5,8,11,14,17) and docosahexaenoic acid (DHA; 22:6 Δ4,7,10,13,16,19) have been shown to have significant roles in human health. Currently the primary dietary source of these fatty acids are marine fish; however, the increasing demand for fish and fish oil (in particular the expansion of the aquaculture industry) is placing enormous pressure on diminishing marine stocks. Such overfishing and concerns related to pollution in the marine environment have directed research towards the development of a viable alternative sustainable source of VLC-PUFAs. As a result, the last decade has seen many genes encoding the primary VLC-PUFA biosynthetic activities identified and characterized. This has allowed the reconstitution of the VLC-PUFA biosynthetic pathway in oilseed crops, producing transgenic plants engineered to accumulate ω-3 VLC-PUFAs at levels approaching those found in native marine organisms. Moreover, as a result of these engineering activities, knowledge of the fundamental processes surrounding acyl exchange and lipid remodelling has progressed. The application of new technologies, for example lipidomics and next-generation sequencing, is providing a better understanding of seed oil biosynthesis and opportunities for increasing the production of unusual fatty acids. Certainly, it is now possible to modify the composition of plant oils successfully, and, in this review, the most recent developments in this field and the challenges of producing VLC-PUFAs in the seed oil of higher plants will be described.

The role of Δ6-desaturase acyl-carrier specificity in the efficient synthesis of long-chain polyunsaturated fatty acids in transgenic plants

The role of acyl-CoA-dependent Δ6-desaturation in the heterologous synthesis of omega-3 long-chain polyunsaturated fatty acids was systematically evaluated in transgenic yeast and Arabidopsis thaliana. The acyl-CoA Δ6-desaturase from the picoalga Ostreococcus tauri and orthologous activities from mouse (Mus musculus) and salmon (Salmo salar) were shown to generate substantial levels of Δ6-desaturated acyl-CoAs, in contrast to the phospholipid-dependent Δ6-desaturases from higher plants that failed to modify this metabolic pool. Transgenic plants expressing the acyl-CoA Δ6-desaturases from either O. tauri or salmon, in conjunction with the two additional activities required for the synthesis of C20 polyunsaturated fatty acids, contained higher levels of eicosapentaenoic acid compared with plants expressing the borage phospholipid-dependent Δ6-desaturase. The use of acyl-CoA-dependent Δ6-desaturases almost completely abolished the accumulation of unwanted biosynthetic intermediates such as γ-linolenic acid in total seed lipids. Expression of acyl-CoA Δ6-desaturases resulted in increased distribution of long-chain polyunsaturated fatty acids in the polar lipids of transgenic plants, reflecting the larger substrate pool available for acylation by enzymes of the Kennedy pathway. Expression of the O. tauriΔ6-desaturase in transgenic Camelina sativa plants also resulted in the accumulation of high levels of Δ6-desaturated fatty acids. This study provides evidence for the efficacy of using acyl-CoA-dependent Δ6-desaturases in the efficient metabolic engineering of transgenic plants with high value traits such as the synthesis of omega-3 LC-PUFAs.

Essential role of ELOVL4 protein in very long chain fatty acid synthesis and retinal function

Very long chain polyunsaturated fatty acid (VLC-PUFA)-containing glycerophospholipids are highly enriched in the retina; however, details regarding the specific synthesis and function of these highly unusual retinal glycerophospholipids are lacking. Elongation of very long chain fatty acids-4 (ELOVL4) has been identified as a fatty acid elongase protein involved in the synthesis of VLC-PUFAs. Mutations in ELOVL4 have also been implicated in an autosomal dominant form of Stargardt disease (STGD3), a type of juvenile macular degeneration. We have generated photoreceptor-specific conditional knock-out mice and used high performance liquid chromatography-mass spectrometry (HPLC-MS) to examine and analyze the fatty acid composition of retinal membrane glycerophosphatidylcholine and glycerophosphatidylethanolamine species. We also used immunofluorescent staining and histology coupled with electrophysiological data to assess retinal morphology and visual response. The conditional knock-out mice showed a significant decrease in retinal glycerophospholipids containing VLC-PUFAs, specifically contained in the sn-1 position of glycerophosphatidylcholine, implicating the role of Elovl4 in their synthesis. Conditional knock-out mice were also found to have abnormal accumulation of lipid droplets and lipofuscin-like granules while demonstrating photoreceptor-specific abnormalities in visual response, indicating the critical role of Elovl4 for proper rod or cone photoreceptor function. Altogether, this study demonstrates the essential role of ELOVL4 in VLC-PUFA synthesis and retinal function.

Involvement of low molecular mass soluble acyl-CoA-binding protein in seed oil biosynthesis

Acyl-CoA-binding protein (ACBP), a low molecular mass (m) (∼ 10 kDa) soluble protein ubiquitous in eukaryotes, plays an important housekeeping role in lipid metabolism by maintaining the intracellular acyl-CoA pool. ACBP is involved in lipid biosynthesis and transport, gene expression, and membrane biogenesis. In plants, low m ACBP and high m ACBPs participate in response mechanisms to biotic and abiotic factors, acyl-CoA transport in phloem, and biosynthesis of structural and storage lipids. In light of current research on the modification of seed oil, insight into mechanisms of substrate trafficking within lipid biosynthetic pathways is crucial for developing rational strategies for the production of specialty oils with the desired alterations in fatty acid composition. In this review, we summarize our knowledge of plant ACBPs with emphasis on the role of low m ACBP in seed oil biosynthesis, based on in vitro studies and analyses of transgenic plants. Future prospects and possible applications of low m ACBP in seed oil modification are discussed.

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.

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.

Trypanosoma cruzi oleate desaturase: molecular characterization and comparative analysis in other trypanosomatids

Trypanosoma cruzi lipids contain a high content of unsaturated fatty acids, primarily oleic acid (C18:1) and linoleic acid (C18:2). Previous data suggest that this parasite is able to convert oleic acid into linoleic acid; humans are not able to do this. Presently, we show that T. cruzi has a gene with high similarity to the delta12 (omega6)-oleate desaturase from plants. Northern blot analysis of the oleate desaturase gene from T. cruzi (OD(Tc)) indicated that this gene is transcribed in epimastigote, amastigote, and trypomastigote forms. Pulsed-field analysis showed that OD(Tc) is located at distinct chromosomal bands on distinct T. cruzi phylogenetic groups. In addition, the chromoblot analysis demonstrated the presence of homologous OD(Tc) genes in several trypanosomatids; namely, Crithidia fasciculata, Herpetomonas megaseliae, Leptomonas seymouri, Trypanosoma freitasi, Trypanosoma rangeli, Trypanosoma lewisi, Blastocrithidia sp., Leishmania amazonensis, Endotrypanum schaudinni, and Trypanosoma conorhini. The native OD(Tc) activity was detected by metabolic labeling and analysis of total fatty acids from epimastigotes and trypomastigotes of T. cruzi, coanomastigotes of C. fasciculata, and promastigotes of L. amazonensis, H. megaseliae, and L. seymouri. The fact that the enzyme oleate desaturase is not present in humans makes it an ideal molecular target for the development of new chemotherapeutic approaches against Chagas disease.

Identification of bifunctional delta12/omega3 fatty acid desaturases for improving the ratio of omega3 to omega6 fatty acids in microbes and plants

We report the identification of bifunctional Delta12/omega3 desaturases from Fusarium moniliforme, Fusarium graminearum, and Magnaporthe grisea. The bifunctional activity of these desaturases distinguishes them from all known Delta12 or omega3 fatty acid desaturases. The omega3 desaturase activity of these enzymes also shows a broad omega6 fatty acid substrate specificity by their ability to convert linoleic acid (LA), gamma-linolenic acid, di-homo-gamma-linolenic acid, and arachidonic acid to the omega3 fatty acids, alpha-linolenic acid (ALA), stearidonic acid, eicosatetraenoic acid, and eicosapentaenoic acid (EPA), respectively. Phylogenetic analysis suggests that omega3 desaturases arose by independent gene duplication events from a Delta12 desaturase ancestor. Expression of F. moniliforme Delta12/omega3 desaturase resulted in high ALA content in both Yarrowia lipolytica, an oleaginous yeast naturally deficient in omega3 desaturation, and soybean. In soybean, seed-specific expression resulted in 70.9 weight percent of total fatty acid (%TFA) ALA in a transformed seed compared with 10.9%TFA in a null segregant seed and 53.2%TFA in the current best source of ALA, linseed oil. The ALA/LA ratio in transformed seed was 22.3, a 110- and 7-fold improvement over the null segregant seed and linseed oil, respectively. Thus, these desaturases have potential for producing nutritionally desirable omega3 long-chain polyunsaturated fatty acids, such as EPA, with a significantly improved ratio of omega3/omega6 long-chain polyunsaturated fatty acids in both oilseeds and oleaginous microbes.

In vivo characterization of the first acyl-CoA Delta6-desaturase from a member of the plant kingdom, the microalga Ostreococcus tauri

Genomic DNA of Ostreococcus tauri, a fully sequenced marine unicellular alga from the phytoplankton, was used to amplify a gene coding for a typical front-end desaturase involved in polyunsaturated fatty acid biosynthesis. Heterologous expression in Saccharomyces cerevisiae revealed very high desaturation activity with Delta6-regioselectivity. Short-time kinetic experiments showed that the desaturase product was detected in the acyl-CoA pool 5 min after addition of the exogenous substrate to the yeast medium and long before its appearance in the total fatty acids. When this desaturase was co-expressed with the acyl-CoA Delta6-elongase from Physcomitrella patens and the lipid-linked Delta5-desaturase from Phaeodactylum tricornutum, high proportions of arachidonic or eicosapentaenoic acid were obtained, because nearly all of the Delta6-desaturated products were elongated. Furthermore, the product/educt ratios calculated in each glycerolipid for the Delta6-desaturase or for the acyl-CoA Delta6-elongase were in about the same range, whereas this ratio showed a very uneven profile in the case of the lipid-linked Delta5-desaturase. Finally, a sequence-based comparison of all the functionally characterized Delta6-desaturases showed that this enzyme was not related to any previously described sequence. Altogether, our data suggest that this desaturase from O. tauri is an acyl-CoA Delta6-desaturase, the first one cloned from a photosynthetically active organism.

A novel fungal omega3-desaturase with wide substrate specificity from arachidonic acid-producing Mortierella alpina 1S-4

A filamentous fungus, Mortierella alpina 1S-4, is capable of producing not only arachidonic acid (AA; 20:4n-6) but also eicosapentaenoic acid (EPA; 20:5n-3) below a cultural temperature of 20 degrees C. Here, we describe the isolation and characterization of a gene (maw3) that encodes a novel omega3-desaturase from M. alpina 1S-4. Based on the conserved sequence information for M. alpina 1S-4 Delta12-desaturase and Saccharomyces kluyveri omega3-desaturase, the omega3-desaturase gene from M. alpina 1S-4 was cloned. Homology analysis of protein databases revealed that the amino acid sequence showed 51% identity, at the highest, with M. alpina 1S-4 Delta12-desaturase, whereas it exhibited 36% identity with Sac. kluyveri omega3-desaturase. The cloned cDNA was confirmed to encode the omega3-desaturase by its expression in the yeast Sac. cerevisiae. Analysis of the fatty acid composition of the yeast transformant demonstrated that 18-carbon and 20-carbon n-3 polyunsaturated fatty acids (PUFAs) were accumulated through conversion of exogenous 18-carbon and 20-carbon n-6 PUFAs. The substrate specificity of the M. alpina 1S-4 omega3-desaturase differs from those of the known fungal omega3-desaturases from Sac. kluyveri and Saprolegnia diclina. Plant, cyanobacterial and Sac. kluyveri omega3-desaturases desaturate 18-carbon n-6 PUFAs, Spr. diclina omega3-desaturase desaturates 20-carbon n-6 PUFAs and Caenorhabditis elegans omega3-desaturase prefers 18-carbon n-6 PUFAs as substrates rather than 20-carbon n-6 PUFAs. The substrate specificity of M. alpina 1S-4 omega3-desaturase is rather similar to that of C. elegans omega3-desaturase, but the M. alpina omega3-desaturase can more effectively convert AA into EPA when expressed in yeast. The M. alpina 1S-4 omega3-desaturase is the first known fungal desaturase that uses both 18-carbon and 20-carbon n-6 PUFAs as substrates.

Acyl carriers used as substrates by the desaturases and elongases involved in very long-chain polyunsaturated fatty acids biosynthesis reconstituted in yeast

The health benefits attributed to very long-chain polyunsaturated fatty acids and the long term goal to produce them in transgenic oilseed crops have led to the cloning of all the genes coding for the desaturases and elongases involved in their biosynthesis. The encoded activities have been confirmed in vivo by heterologous expression, but very little is known about the actual acyl substrates involved in these pathways. Using a Delta 6-elongase and front-end desaturases from different organisms, we have reconstituted in Saccharomyces cerevisiae the biosynthesis of arachidonic acid from exogenously supplied linoleic acid in order to identify these acyl carriers. Acyl-CoA measurements strongly suggest that the elongation step involved in polyunsaturated fatty acids biosynthesis is taking place within the acyl-CoA pool. In contrast, detailed analyses of lipids revealed that the two desaturation steps (Delta 5 and Delta 6) occur predominantly at the sn-2 position of phosphatidylcholine when using Delta 5- and Delta 6-desaturases from lower plants, fungi, worms, and algae. The specificity of these Delta 6-desaturases for the fatty acid acylated at this particular position as well as a limiting re-equilibration with the acyl-CoA pool result in the accumulation of gamma-linolenic acid at the sn-2 position of phosphatidylcholine and prevent efficient arachidonic acid biosynthesis in yeast. We confirm by using a similar experimental approach that, in contrast, the human Delta 6-desaturase uses linoleoyl-CoA as substrate, which results in high efficiency of the subsequent elongation step. In addition, we report that Delta 12-desaturases have no specificity toward the lipid polar headgroup or the sn-position.

Biosynthesis of docosahexaenoic acid in Euglena gracilis: biochemical and molecular evidence for the involvement of a Delta4-fatty acyl group desaturase

Docosahexaenoic acid (DHA) can be synthesized via alternative routes from which only the omega3/omega6-pathways involve the action of a Delta4-fatty acid desaturase. We examined the suitability of Euglena gracilis, Thraustochytrium sp., Schizochytrium sp., and Crypthecodinium cohnii to serve as sources for cloning a cDNA encoding a Delta4-fatty acid desaturase. For this purpose we carried out in vivo labeling studies with radiolabeled C22 polyunsaturated fatty acid substrates. Schizochytrium sp. was unable to convert exogenously supplied [2-(14)C]-docosapentaenoic acid (DPA, 22:5(Delta)(7,10,13,16,19)) to DHA, while E. gracilis and Thraustochytrium sp. carried out this desaturation very efficiently. Hydrogenation and alpha-oxidation of the labeled DHA isolated from these two organisms showed that it was the result of direct Delta4-desaturation and not of substrate breakdown and resynthesis. To clone the desaturase gene, a cDNA library of E. gracilis was subjected to mass sequencing. A full-length clone with highest homology to the Delta4-desaturase of Thraustochytrium sp. was isolated, and its function was verified by heterologous expression in yeast. The desaturase efficiently converted DPA to DHA. Analysis of the substrate specificity demonstrated that the enzyme activity was not limited to C22 fatty acids, since it also efficiently desaturated C16 fatty acids. The enzyme showed strict Delta4-regioselectivity and required the presence of a Delta7-double bond in the substrate. Positional analysis of phosphatidylcholine revealed that the proportion of the Delta4-desaturated products was up to 20 times higher in the sn-2 position than in the sn-1 position.

Morphology and physiology of the dimorphic fungus Mucor circinelloides (syn. M. racemosus) during anaerobic growth

The dimorphic Mucor circinelloides requires an anaerobic atmosphere and the presence of 30% CO2 to grow as a multipolar budding yeast, otherwise hyphal growth predominates. Establishing other means to control the morphology would be a distinct advantage in the development of a fermentation process for this organism for the production of heterologous proteins. Thus, conditions suppressing polarised growth while at the same time abolishing the CO2 requirement were investigated in submerged cultivations. It was found that supplementing cultures with mixtures of ergosterol and Tween 80 resulted in yeast-like growth under 100% N2. Their impact on growth and morphological development was assessed at a range of concentrations. Maximum biomass levels and the specific growth rate decreased at elevated levels of ergosterol and Tween 80. Possible effects of carbon dioxide and the added fatty acid/sterol mixture on supporting yeast growth by influencing the fluidity of the plasma membrane or affecting polarised growth are discussed.

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.

A third fatty acid delta9-desaturase from Mortierella alpina with a different substrate specificity to ole1p and ole2p

A third gene (Delta9-3) encoding a fatty acid Delta9-desaturase was isolated from the oil-producing fungus Mortierella alpina. The predicted protein of 512 aa shared 53% sequence identity with the two fatty acid Delta9-desaturases, ole1p and ole2p, already described in this organism and contained three histidine boxes, four putative transmembrane domains and a C-terminal cytochrome b(5) fusion that are typical of most fungal membrane-bound fatty acid desaturases. However, unlike the M. alpina ole1 and ole2 genes, the Delta9-3 ORF failed to complement the Saccharomyces cerevisiae ole1 mutation. GC-MS analysis of fatty-acid-supplemented ole1 yeast transformants containing the Delta9-3 gene indicated that this enzyme had negligible activity with endogenous palmitic acid (16:0) as substrate and moderate activity (30-65% desaturation) with endogenous stearic acid (18:0). Yeast transformants overexpressing any one of the three M. alpina fatty acid Delta9-desaturase genes or the S. cerevisiae OLE1 gene produced low amounts of hexacosenoic acid [26:1(n-9)], a fatty acid that is not normally present in yeast cells. It follows that these Delta9-desaturases may also display low n-9 desaturation activity with very long-chain saturated fatty acid substrates. Conversely, high levels of desaturase in the endoplasmic reticulum membrane of these yeast transformants may increase the availability of suitable monounsaturated substrates for fatty acid elongation.

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.

Two fatty acid delta9-desaturase genes, ole1 and ole2, from Mortierella alpina complement the yeast ole1 mutation

Genes encoding two distinct fatty acid delta9-desaturases were isolated from strains of the oleaginous fungus Mortierella alpina. Two genomic sequences, delta9-1 and delta9-2, each containing a single intron, were cloned from strain CBS 528.72 while one cDNA clone, LM9, was isolated from strain CBS 210.32. The delta9-1 gene encoded a protein of 445 aa which shared 99% identity with the LM9 gene product. These proteins also showed 40-60% identity to the delta9-desaturases (Ole1p) of other fungi and contained the three conserved histidine boxes, C-terminal cytochrome b5 fusion and transmembrane domains characteristic of endoplasmic reticulum membrane-bound delta9-desaturases. LM9 and delta9-1 are therefore considered to represent the same gene (ole1). The ole1 gene was transcriptionally active in all M. alpina strains tested and its function was confirmed by complementation of the Saccharomyces cerevisiae ole1 mutation. Fatty acid analysis of yeast transformants expressing the CBS 210.32 ole1 gene showed an elevated level of oleic acid (18:1) compared to palmitoleic acid (16:1), the major fatty acid component of wild-type S. cerevisiae. This indicated that the M. alpina delta9-desaturase had a substrate preference for stearic acid (18:0) rather than palmitic acid (16:0). Genomic clone delta9-2 (ole2) also encoded a protein of 445 aa which had 86% identity to the delta9-1 and LM9 proteins and whose ORF also complemented the yeast ole1 mutation. The transcript from this gene could only be detected in one of the six M. alpina strains tested, suggesting that its expression may be strain-specific or induced under certain physiological conditions.

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.

Biosynthesis and regulation of microbial polyunsaturated fatty acid production

Growing interest in polyunsaturated fatty acid (PUFA) applications in various fields coupled with their significance in health and dietary requirements has focused attention on the provision of suitable sources of these compounds. Isolation of highly efficient oleaginous microorganisms has led to the development of fermentation technologies as an alternative to agricultural and animal processes. Particularly active in PUFA synthesis are the Zygomycetes fungi and certain microalgae. Emphasis is placed on increasing the product value by employing new biotechnological strategies (e.g. mutation techniques, molecular engineering and biotransformations) which allow the regulation of microbial PUFA formation with satisfactory yield in order to be competitive with other sources. Comparative successes in fungal PUFA production demonstrate microbial potential to synthesize high-value oils and provide the main stimulus for their applications.

Biosynthesis of triacylglycerol in the filamentous fungus Mucor circinelloides

Lipid metabolism was studied in 2-d-old liquid cultures of Mucor circinelloides grown at 25 degrees C. Under these conditions, oil accumulated to 0.5 g l-1 with a gamma-linolenic acid content (gamma 18:3) of 60 mg l-1. The major labelled lipids in cultures incubated with [14C]acetate were triacylglycerol (TAG), phosphatidylcholine (PC) and phosphatidylethanolamine (PE). The proportion of label declined in the phospholipids and increased in TAG with time. [14C]18:1 and [14C]18:2 rapidly appeared in PC and PE and later accumulated in [14C]gamma 18:3. TAG-synthesizing capacity was greatest in the microsomal membrane fraction, which accumulated high levels of phosphatidic acid in the presence of glycerol 3-phosphate and acyl-CoA substrates at pH 7.0. Further metabolism of phosphatidic acid to diacylglycerol and TAG was achieved by increasing the pH to 8.0. Lysophosphatidic acid: acyl-CoA acyltransferase (LPAAT) activity was particularly high and may have accounted for the rapid accumulation of phosphatidic acid in the membranes. The glycerol-3-phosphate: acyl-CoA acyltransferase (GPAAT) and LPAAT were non-specific for a range of saturated and unsaturated species of acyl-CoA although the GPAAT showed a marked selectivity for palmitoyl-CoA and the LPAAT for oleoyl- and linoleoyl-CoA. gamma-Linolenic acid was detected at all three positions of sn-TAG and was particularly enriched at the sn-3 position. The preparation of active in vitro systems (microsomal membranes) capable of the complete biosynthetic pathway for TAG assembly may be valuable in understanding the assembly of oils in future transgenic applications.

Isolation of a Delta5-fatty acid desaturase gene from Mortierella alpina

Arachidonic acid (C20:4 Delta5,8,11,14) is a polyunsaturated fatty acid synthesized by the Delta5-fatty acid desaturation of di-homo-gamma-linolenic acid (C20:3 Delta8,11,14). In mammals, it is known to be a precursor of the prostaglandins and the leukotrienes but it is also accumulated by the filamentous fungus Mortierella alpina. We have isolated a cDNA encoding the Delta5-fatty acid desaturase from M. alpina via a polymerase chain reaction-based strategy using primers designed to the conserved histidine box regions of microsomal desaturases, and confirmed its function by expression in the yeast Saccharomyces cerevisiae. Analysis of the lipids from the transformed yeast demonstrated the accumulation of arachidonic acid. The M. alpina Delta5-desaturase is the first example of a cloned Delta5-desaturase, and differs from other fungal desaturases previously characterized by the presence of an N-terminal domain related to cytochrome b5.