Characterization of genes encoding ω-6 desaturase PoFAD2 and PoFAD6, and ω-3 desaturase PoFAD3 for ALA accumulation in developing seeds of oil crop Paeonia ostii var. lishizhenii

Paeonia ostii var. lishizhenii has emerged as a valuable oil-producing crop with splendid characteristic of high α-linolenic acid (C18:3, ALA) content in its seed oil for healthy food supplement, but the molecular mechanism for seed ALA accumulation remains enigmatic. In our previous report, a PoSAD gene encoding stearoyl-ACP desaturase had been cloned and functional charactered for the first desaturation procedure involved in ALA biosynthesis pathway in P. ostii var. lishizhenii endosperms, while other participants have not been identified to date. In this study, full-length cDNAs of PoFAD2 (1489 bp), PoFAD6 (1638 bp), and PoFAD3 (1709 bp) were isolated based on our recent transcriptome sequencing data. Bioinformatic analyses revealed that the PoFADs were closest to their counterparts from Paeoniaceae species P. ludlowii, P. rockii, and P. suffruticosa in phylogenetic tree, which shared highly conserved histidine boxes (HXXXH, HXXHH, and HXXHH), exhibiting typical characters of membrane-bound desaturases in higher plants. Additionally, the PoFAD2 and PoFAD3 were specifically expressed and highly associated with LA and ALA accumulation in developing endosperms, whereas PoFAD6 expression has no significantly difference during whole seed developing stages. The catalytic function of these PoFADs were further analyzed by heterologous expression in Saccharomyces cerevisiae and Arabidopsis thaliana. The results showed that PoFAD2 and PoFAD6 could catalyze linoleic acid (C18:2) synthesis, while PoFAD3 had ability to produce ALA. This study functional identified three PoFAD genes, which indicates their critical roles in ALA biosynthesis pathway in P. ostii var. lishizhenii, and is of great theoretical and practical meaning on breeding and cultivating new tree peony varieties to promote human health and nutrition supplement.

Dose-dependent production of linoleic acid analogues in food derived Lactobacillus plantarum K25 and in silico characterization of relevant reactions

The objective of this study was to assess and scrutinize the competency of probiotic L. plantarum K25 to produce linoleic acid analogues in the medium supplemented with different concentrations of linoleic acid, ranging from 1% to 10%, in a dose dependent manner. The analogues produced were identified and quantitated by GC-MS and in silico studies were done to confirm enzymatic reactions involved in its conversion. The results showed that L. plantarum K25 could convert linoleic acid at different concentrations to 9 different fatty acid analogues at concentrations ranging from 0.01 to 17.24 mg/L. Among these metabolites, formation of an essential fatty acid, the linolenic acid, in media supplemented with 9% linoleic acid, is being reported for the first time. Putative candidate enzymes involved in biotransformation of linoleic acid into linoleic acid analogues were identified in the whole genome of L. plantarum K25, which was sequenced previously. In silico studies confirmed that many enzymes, including linoleate isomerase and dehydrogenase, may be involved in biotransformation of linoleic acid into linoleic acid analogues. Both enzymes could effectively bind the linoleic acid molecule, mainly by forming hydrogen bonding between the acidic groups of linoleic acid and the proline residues at the active sites of the enzymes, validating putative reaction partners.

Sorghum MSD3 Encodes an ω-3 Fatty Acid Desaturase that Increases Grain Number by Reducing Jasmonic Acid Levels

Grain number per panicle is an important component of grain yield in sorghum (Sorghum bicolor (L.)) and other cereal crops. Previously, we reported that mutations in multi-seeded 1 (MSD1) and MSD2 genes result in a two-fold increase in grain number per panicle due to the restoration of the fertility of the pedicellate spikelets, which invariably abort in natural sorghum accessions. Here, we report the identification of another gene, MSD3, which is also involved in the regulation of grain numbers in sorghum. Four bulked F₂ populations from crosses between BTx623 and each of the independent msd mutants p6, p14, p21, and p24 were sequenced to 20× coverage of the whole genome on a HiSeq 2000 system. Bioinformatic analyses of the sequence data showed that one gene, Sorbi_3001G407600, harbored homozygous mutations in all four populations. This gene encodes a plastidial ω-3 fatty acid desaturase that catalyzes the conversion of linoleic acid (18:2) to linolenic acid (18:3), a substrate for jasmonic acid (JA) biosynthesis. The msd3 mutants had reduced levels of linolenic acid in both leaves and developing panicles that in turn decreased the levels of JA. Furthermore, the msd3 panicle phenotype was reversed by treatment with methyl-JA (MeJA). Our characterization of MSD1, MSD2, and now MSD3 demonstrates that JA-regulated processes are critical to the msd phenotype. The identification of the MSD3 gene reveals a new target that could be manipulated to increase grain number per panicle in sorghum, and potentially other cereal crops, through the genomic editing of MSD3 functional orthologs.

Isolation of a novel strain of Monoraphidium sp. and characterization of its potential for α-linolenic acid and biodiesel production

α-Linolenic acid (ALA) is an essential fatty acid which cannot be synthesized de novo in mammals and must be ingested regularly in the diet. In this study, a microalgal strain named HDMA-11 was isolated from Lake Ming, China, and was found to accumulate a high ALA content (39.2% of total lipids). Phylogenetic neighbor-joining analysis indicated that HDMA-11 belongs to the genus Monoraphidium (Selenastraceae, Sphaeropleales) and its 18S ribosomal DNA sequence seemed to be a new molecular record of a Monoraphidium species. The fatty acid profiles, biomass productivity and lipid content of HDMA-11 were also investigated in autotrophic conditions. The high levels of polyunsaturated fatty acids in HDMA-11, especially ALA, make it suitable as a source of nutritional supplementation for human health. Furthermore, HDMA-11 exhibited good properties for biodiesel production, characterized by high lipid content (28.5% of dry weight), moderate biomass productivity (31.5 mg L^-1 day^-1) and a promising lipid profile.

Content of Minerals and Fatty Acids and Their Correlation with Phytochemical Compounds and Antioxidant Activity of Leguminous Seeds

The aim of the study was to determine the mineral composition and fatty acid profile in the seeds of selected Fabaceae species and cultivars and to assess their correlations with phytochemicals and antioxidant activity. The Andean lupine was characterised by a particularly high level of Mg and K as well as Cu, Zn, and Fe (P < 0.05). There were various correlations (P < 0.05) between the total phenols and tannins and these elements. The highest contribution of α-linolenic acid (ALA, 18:3, n-3) in total fatty acids was noted in the lentil (13.8 in 100 g^-1 fat), common bean (11.9 in 100 g^-1 fat), and pea seeds (10.4 in 100 g^-1 fat) (P = 0.028). In turn, the white lupine contained the highest content of ALA-0.67 g 100 g^-1 seeds; its lowest level was determined in the broad bean-0.03 g 100 g^-1 seeds. The seeds exhibited a high proportion of hypocholesterolemic fatty acids (on average 86%). The 2,2-diphenyl-1-picrylhydrazyl antiradical activity was positively correlated with UFA and PUFA (P < 0.05). This indicates great protective potential of legume seeds for prevention and treatment of diet-dependent diseases.

RNA Sequencing and Coexpression Analysis Reveal Key Genes Involved in α-Linolenic Acid Biosynthesis in Perilla frutescens Seed

Perilla frutescen is used as traditional food and medicine in East Asia. Its seeds contain high levels of α-linolenic acid (ALA), which is important for health, but is scarce in our daily meals. Previous reports on RNA-seq of perilla seed had identified fatty acid (FA) and triacylglycerol (TAG) synthesis genes, but the underlying mechanism of ALA biosynthesis and its regulation still need to be further explored. So we conducted Illumina RNA-sequencing in seven temporal developmental stages of perilla seeds. Sequencing generated a total of 127 million clean reads, containing 15.88 Gb of valid data. The de novo assembly of sequence reads yielded 64,156 unigenes with an average length of 777 bp. A total of 39,760 unigenes were annotated and 11,693 unigenes were found to be differentially expressed in all samples. According to Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis, 486 unigenes were annotated in the “lipid metabolism” pathway. Of these, 150 unigenes were found to be involved in fatty acid (FA) biosynthesis and triacylglycerol (TAG) assembly in perilla seeds. A coexpression analysis showed that a total of 104 genes were highly coexpressed (r > 0.95). The coexpression network could be divided into two main subnetworks showing over expression in the medium or earlier and late phases, respectively. In order to identify the putative regulatory genes, a transcription factor (TF) analysis was performed. This led to the identification of 45 gene families, mainly including the AP2-EREBP, bHLH, MYB, and NAC families, etc. After coexpression analysis of TFs with highly expression of FAD2 and FAD3 genes, 162 TFs were found to be significantly associated with two FAD genes (r > 0.95). Those TFs were predicted to be the key regulatory factors in ALA biosynthesis in perilla seed. The qRT-PCR analysis also verified the relevance of expression pattern between two FAD genes and partial candidate TFs. Although it has been reported that some TFs are involved in seed development, more direct evidence is still needed to verify their function. However, these findings can provide clues to reveal the possible molecular mechanisms of ALA biosynthesis and its regulation in perilla seed.

Simultaneous silencing of five lipoxygenase genes increases the contents of α-linolenic and linoleic acids in tomato (Solanum lycopersicum L.) fruits

α-Linolenic and linoleic acids are essential fatty acids (EFAs) for humans and required for maintenance of optimal health, but they cannot be synthesized by the human body and must be obtained from dietary sources. Using TomloxC fragment, TomloxD fragment, and partial TomloxA sequence that is highly identical with TomloxB and TomloxE, a RNAi expression vector was constructed. The construct was used to transform tomato cotyledon explants with the Agrobacterium-mediated co-cultivation method. The real-time reverse transcription polymerase chain reaction analysis showed that the expression of TomloxA, TomloxB, TomloxC, TomloxD, and TomloxE in transgenic tomato plants was drastically repressed, which led to a marked decrease in the levels of lipoxygenase activity. Finally, higher accumulations of the endogenous α-linolenic and linoleic acids were detected in the transgenic tomato fruits, which were 1.65-3.99 and 2.91-4.98 times that of the non-transformed tomato fruits, respectively.

Development of low-linolenic acid Brassica oleracea lines through seed mutagenesis and molecular characterization of mutants

Designing the fatty acid composition of Brassica napus L. seed oil for specific applications would extend the value of this crop. A mutation in Fatty Acid Desaturase 3 (FAD3), which encodes the desaturase responsible for catalyzing the formation of α-linolenic acid (ALA; 18:3 (cisΔ9,12,15)), in a diploid Brassica species would potentially result in useful germplasm for creating an amphidiploid displaying low ALA content in the seed oil. For this, seeds of B. oleracea (CC), one of the progenitor species of B. napus, were treated with ethyl-methane-sulfonate to induce mutations in genes encoding enzymes involved in fatty acid biosynthesis. Seeds from 1,430 M2 plants were analyzed, from which M3 seed families with 5.7-6.9 % ALA were obtained. Progeny testing and selection for low ALA content were carried out in M3-M7 generations, from which mutant lines with <2.0 % ALA were obtained. Molecular analysis revealed that the mutation was due to a single nucleotide substitution from G to A in exon 3 of FAD3, which corresponds to an amino acid residue substitution from glutamic acid to lysine. No obvious differences in the expression of the FAD3 gene were detected between wild type and mutant lines; however, evaluation of the performance of recombinant Δ-15 desaturase from mutant lines in yeast indicated reduced production of ALA. The novelty of this mutation can be inferred from the position of the point mutation in the C-genome FAD3 gene when compared to the position of mutations reported previously by other researchers. This B. oleracea mutant line has the potential to be used for the development of low-ALA B. napus and B. carinata oilseed crops.

Nicotiana attenuata α-DIOXYGENASE1 through its production of 2-hydroxylinolenic acid is required for intact plant defense expression against attack from Manduca sexta larvae

Nicotiana attenuata α-DIOXYGENASE1 (α-DOX1) is an oxylipin-forming gene elicited during herbivory by fatty acid amino acid conjugates (FACs) contained in oral secretions of Manduca sexta. To understand the roles of Naα-DOX1 and its major product, 2-hydroxylinolenic acid (2-hydroxylinolenic acid), in N. attenuata's anti-herbivore defenses, we used a transgenic line specifically silenced in Naα-DOX1 expression (ir-α-dox1) and monitored 2-HOT production in M. sexta-damaged tissues and its role in influencing the production of direct defense compounds and resistance to this insect. Attack by M. sexta larvae amplified 2-HOT formation at the feeding sites; a reaction probably facilitated by Naα-DOX1's high pH optimum which allows 2-HOT formation to occur in the more alkaline conditions at the feeding sites or potentially in the insect mouth parts after the leaf tissue is ingested. Manduca sexta larvae performed better on ir-α-dox1 plants than on wild-type (WT) plants as a result of attenuated herbivory-specific JA and 2-HOT bursts as well as JA-inducible well-established defenses (nicotine, caffeoylputrescine and trypsin proteinase inhibitors). Repeated applications of 2-HOT to wounds before insect feeding partly amplified JA-controlled defenses and restored the resistance of ir-α-dox1 plants. We conclude that 2-HOT, produced by attack-activated α-DOX1 activity, participates in defense activation during insect feeding.

Combinations of mutant FAD2 and FAD3 genes to produce high oleic acid and low linolenic acid soybean oil

High oleic acid soybeans were produced by combining mutant FAD2-1A and FAD2-1B genes. Despite having a high oleic acid content, the linolenic acid content of these soybeans was in the range of 4-6 %, which may be high enough to cause oxidative instability of the oil. Therefore, a study was conducted to incorporate one or two mutant FAD3 genes into the high oleic acid background to further reduce the linolenic acid content. As a result, soybean lines with high oleic acid and low linolenic acid (HOLL) content were produced using different sources of mutant FAD2-1A genes. While oleic acid content of these HOLL lines was stable across two testing environments, the reduction of linolenic acid content varied depending on the number of mutant FAD3 genes combined with mutant FAD2-1 genes, on the severity of mutation in the FAD2-1A gene, and on the testing environment. Combination of two mutant FAD2-1 genes and one mutant FAD3 gene resulted in less than 2 % linolenic acid content in Portageville, Missouri (MO) while four mutant genes were needed to achieve the same linolenic acid in Columbia, MO. This study generated non-transgenic soybeans with the highest oleic acid content and lowest linolenic acid content reported to date, offering a unique alternative to produce a fatty acid profile similar to olive oil.

Conjugated linoleic and linolenic acid production kinetics by bifidobacteria differ among strains

There is great interest in conjugated linoleic acid (CLA) and conjugated linolenic acid (CLNA) isomers because of their supposed health-promoting properties. Therefore, the differences in production kinetics of CLA and CLNA isomers from linoleic acid (LA) and α-linolenic acid (α-LNA), respectively, by bifidobacteria were investigated. Laboratory fermentations, supplemented with LA or α-LNA in the fermentation medium, were performed with Bifidobacterium bifidum LMG 10645, Bifidobacterium breve LMG 11040, B. breve LMG 11084, B. breve LMG 11613, B. breve LMG 13194, and Bifidobacterium pseudolongum subsp. pseudolongum LMG 11595. Conversion of LA and α-LNA to CLA and CLNA isomers, respectively, started immediately upon addition of the substrate fatty acids. During the active growth phase, the c9, t11-CLA isomer and the putative c9, t11, c15-CLNA isomer were formed. Further fermentation resulted in a reduction in the concentration of c9, t11-CLA and c9, t11, c15-CLNA and the subsequent production of the t9, t11-CLA isomer and the putative t9, t11, c15-CLNA isomer, respectively. Modelling of the growth and metabolite data indicated differences in production kinetics among the strains. Some strains displayed a high specific conversion of LA and α-LNA despite poor growth, whereas other strains grew well but displayed lower conversion. Production of specific CLA and CLNA isomers by bifidobacteria holds potential for the production of functional foods and could contribute to their probiotic properties.

Analysis of the subcellular localisation of lipoxygenase in legume and actinorhizal nodules

Plant lipoxygenases (LOXs; EC 1.13.11.12) catalyse the oxygenation of polyunsaturated fatty acids, linoleic (18:2) and α-linolenic acid (18:3(n-3)) and are involved in processes such as stress responses and development. Depending on the regio-specificity of a LOX, the incorporation of molecular oxygen leads to formation of 9- or 13-fatty acid hydroperoxides, which are used by LOX itself as well as by members of at least six different enzyme families to form a series of biologically active molecules, collectively called oxylipins. The best characterised oxylipins are the jasmonates: jasmonic acid (JA) and its isoleucine conjugate that are signalling compounds in vegetative and propagative plant development. In several types of nitrogen-fixing root nodules, LOX expression and/or activity is induced during nodule development. Allene oxide cyclase (AOC), a committed enzyme of the JA biosynthetic pathway, has been shown to localise to plastids of nodules of one legume and two actinorhizal plants, Medicago truncatula, Datisca glomerata and Casuarina glauca, respectively. Using an antibody that recognises several types of LOX interspecifically, LOX protein levels were compared in roots and nodules of these plants, showing no significant differences and no obvious nodule-specific isoforms. A comparison of the cell-specific localisation of LOXs and AOC led to the conclusion that (i) only cytosolic LOXs were detected although it is generally assumed that the (13S)-hydroperoxy α-linolenic acid for JA biosynthesis is produced in the plastids, and (ii) in cells of the nodule vascular tissue that contain AOC, no LOX protein could be detected.

Changes in apolar metabolites during in vitro organogenesis of Pancratium maritimum

Calli, shoot-clumps and regenerated plants were initiated from young fruits of Pancratium maritimum L. Their genetic stability was monitored by flow cytometry before chemical studies. Apolar metabolites (alkaloids extracted at pH > 7, free fatty acids and fatty alcohols, sterols etc.) were qualitatively and quantitatively analyzed by GC-MS. The results clearly demonstrated that alkaloid synthesis in P. maritimum is closely related with tissue differentiation. The highest amounts of alkaloids and presence of homolycorine and tazettine type compounds (end products of the biosynthetic pathway of the Amaryllidaceae alkaloids) were found in highly differentiated tissues. Galanthamine accumulated in the leaves of plantlets. The amount of hordenine, a protoalkaloid, is related with the ability of tissues to synthesize alkaloids. Saturated fatty acids were found in considerably higher levels in undifferentiated callus cultures and partially differentiated shoot-clumps than in regenerated plants. Mono- and dienoic fatty acids were found at higher levels in non-photosynthesizing tissues - calli, and in vitro and intact bulbs, while α-linolenic acid (trienoic acid) was found in higher amounts in the photosynthesizing leaves of shoot-clumps and regenerated plants than in bulbs and calli. Fatty alcohols were found mainly in leaves, while sterols tended to accumulate in photosynthesizing and undifferentiated tissues.

Characteristics of high alpha-linolenic acid accumulation in seed oils

Modern diets are often deficient in omega-3 fatty acids and additional dietary sources of omega-3 fatty acids are useful. In order to investigate the molecular basis of the high accumulation of the omega-3 fatty acid, alpha-linolenic acid (18:3), in three different plants, flax (Linum usitatissimum), Dracocephalum moldavica, and Perilla frutescens omega-3 desaturase activity, transcript levels, and 18:3 in-vivo synthesis were examined. The 18:3 content was found to be higher at the later developmental stage of D. moldavica (68%) compared with P. frutescens (59%) and flax (45%) cotyledons. The 18:3 and 18:2 contents in both PC and TAG were determined during various stages of seed development for all three plants in addition to soybean (Glycine max). Northern blot analysis data of three different stages of D. moldavica, flax, and P. frutescens compared with moderately low 18:3 producers, soybean (Glycine max), and Arabidopsis thaliana and Brassica napus, (8-10% 18:3) at a stage of zygotic embryo development of high triglyceride synthesis showed that omega-3 desaturase mRNA levels were higher in all three high 18:3 producers, flax, D. moldavica and P. frutescens. This indicates that the high level of alpha-linolenic acid in TAG may be largely controlled by the level of omega-3 desaturase gene expression. However, the PC versus TAG fatty acid composition data suggested that along with omega-3 desaturase other enzymes also play a role in 18:3 accumulation in TAG, and the high accumulators have a selective transfer of alpha-linolenic acid into TAG.

Characteristics of high alpha-linolenic acid accumulation in seed oils

Modern diets are often deficient in omega-3 fatty acids and additional dietary sources of omega-3 fatty acids are useful. In order to investigate the molecular basis of the high accumulation of the omega-3 fatty acid, alpha-linolenic acid (18:3), in three different plants, flax (Linum usitatissimum), Dracocephalum moldavica, and Perilla frutescens omega-3 desaturase activity, transcript levels, and 18:3 in-vivo synthesis were examined. The 18:3 content was found to be higher at the later developmental stage of D. moldavica (68%) compared with P. frutescens (59%) and flax (45%) cotyledons. The 18:3 and 18:2 contents in both PC and TAG were determined during various stages of seed development for all three plants in addition to soybean (Glycine max). Northern blot analysis data of three different stages of D. moldavica, flax, and P. frutescens compared with moderately low 18:3 producers, soybean (Glycine max), and Arabidopsis thaliana and Brassica napus, (8-10% 18:3) at a stage of zygotic embryo development of high triglyceride synthesis showed that omega-3 desaturase mRNA levels were higher in all three high 18:3 producers, flax, D. moldavica and P. frutescens. This indicates that the high level of alpha-linolenic acid in TAG may be largely controlled by the level of omega-3 desaturase gene expression. However, the PC versus TAG fatty acid composition data suggested that along with omega-3 desaturase other enzymes also play a role in 18:3 accumulation in TAG, and the high accumulators have a selective transfer of alpha-linolenic acid into TAG.

Metabolomic alterations in elicitor treated Silybum marianum suspension cultures monitored by nuclear magnetic resonance spectroscopy

A comprehensive metabolomic profiling of Silybum marianum (L.) Gaernt cell cultures elicited with yeast extract or methyl jasmonate for the production of silymarin was carried out using one- and two-dimensional nuclear magnetic resonance spectroscopy. With these techniques we were able to detect both temporal quantitative variations in the metabolite pool in yeast extract-elicited cultures and qualitative differences in cultures treated with the two types of elicitors. Yeast extract and methyl jasmonate caused a metabolic reprogramming that affected amino acid and carbohydrate metabolism; upon elicitation sucrose decreased and glucose levels increased, these changes being dependent on "de novo" protein synthesis. Also dependent on protein synthesis were the increase seen in alanine and glutamine in elicited cultures. Yeast extract differentially acted on threonine and valine metabolism and promoted accumulation of choline and alpha-linolenic acid in cells thus suggesting its action on membranes and the involvement of the octadecanoid pathway in the induction of silymarin in S. marianum cultures. Phenylpropanoid metabolism was altered by elicitation but, depending on elicitor, different phenylpropanoid profile was produced. The results obtained in this study will permit in the future to identify candidate components of the signalling pathway involved in the stimulation of the constitutive pathway of silymarin.

Metabolic pathway that produces essential fatty acids from polymethylene-interrupted polyunsaturated fatty acids in animal cells

Sciadonic acid (20:3 Delta-5,11,14) and juniperonic acid (20:4 Delta-5,11,14,17) are polyunsaturated fatty acids (PUFAs) that lack the Delta-8 double bond of arachidonic acid (20:4 Delta-5,8,11,14) and eicosapentaenoic acid (20:5 Delta-5,8,11,14,17), respectively. Here, we demonstrate that these conifer oil-derived PUFAs are metabolized to essential fatty acids in animal cells. When Swiss 3T3 cells were cultured with sciadonic acid, linoleic acid (18:2 Delta-9,12) accumulated in the cells to an extent dependent on the concentration of sciadonic acid. At the same time, a small amount of 16:2 Delta-7,10 appeared in the cellular lipids. Both 16:2 Delta-7,10 and linoleic acid accumulated in sciadonic acid-supplemented CHO cells, but not in peroxisome-deficient CHO cells. We confirmed that 16:2 Delta-7,10 was effectively elongated to linoleic acid in rat liver microsomes. These results indicate that sciadonic acid was partially degraded to 16:2 Delta-7,10 by two cycles of beta-oxidation in peroxisomes, then elongated to linoleic acid in microsomes. Supplementation of Swiss 3T3 cells with juniperonic acid, an n-3 analogue of sciadonic acid, induced accumulation of alpha-linolenic acid (18:3 Delta-9,12,15) in cellular lipids, suggesting that juniperonic acid was metabolized in a similar manner to sciadonic acid. This PUFA remodeling is thought to be a process that converts unsuitable fatty acids into essential fatty acids required by animals.

Construction and functional analysis of fatty acid desaturase gene disruptants in Candida albicans

Polyunsaturated fatty acids (PUFAs), including linoleic acid (C18 : 2) and alpha-linolenic acid (C18 : 3), are major components of membranes. PUFAs are produced from monounsaturated fatty acids by several fatty acid desaturases (FADs) in many fungi, but Saccharomyces cerevisiae, Schizosaccharomyces pombe and humans do not have these enzymes. Although the fungal pathogen Candida albicans produces C18 : 2 and C18 : 3, the enzymes that synthesize them have not yet been investigated. In this report, two ORFs, CaFAD2 and CaFAD3, were identified based on their homology to other yeast FADs, and CaFAD2 and CaFAD3 gene disruptants were constructed. Cafad2Delta and Cafad3Delta lost their ability to produce C18 : 2 and C18 : 3, respectively. Furthermore, S. cerevisiae cells expressing CaFad2p converted palmitoleic acid (C16 : 1) and C18 : 1 to hexadecadienoic acid (C16 : 2) and C18 : 2, respectively, and CaFad3p-expressing cells converted C18 : 2 to C18 : 3. These results strongly supported that CaFAD2 encodes the Delta12 FAD and that CaFAD3 encodes the omega3 FAD. However, phenotypic analysis demonstrated that the presence of these PUFAs did not affect the virulence to mice, or morphogenesis in the culture media used to induce morphological change of C. albicans.

Biological significance of essential fatty acids

Essential fatty acids (EFAs)--linoleic acid (LA) and alpha-linolenic acid (ALA) are critical for human survival. EFAs are readily available in the diet. But, to derive their full benefit, EFAs need to be metabolized to their respective long-chain metabolites. EFAs not only form precursors to respective prostaglandins (PGs), thromboxanes (TXs), and leukotrienes (LTs), but also give rise to lipoxins (LXs), resolvins, isoprostanes, and hydroxy- and hydroperoxyeicosatetraenoates. Certain PGs, TXs, and LTs have pro-inflammatory actions whereas LXs and resolvins are anti-inflammatory in nature. Furthermore, EFAs and their long-chain metabolites modulate the activities of angiotensin converting and HMG-CoA reductase enzymes, enhance acetylcholine levels in the brain, increase the synthesis of endothelial nitric oxide, augment diuresis, and enhance insulin action. Thus, EFAs and their metabolites may function as endogenous ACE and HMG-CoA reductase inhibitors, nitric oxide enhancers, beta-blockers, diuretics, anti-hypertensive, and anti-atherosclerotic molecules. In addition, EFAs and their long-chain metabolites react with nitric oxide (NO) to yield respective nitroalkene derivatives that exert cell-signaling actions via ligation and activation of peroxisome proliferator-activated receptors (PPARs). Thus, EFAs and their derivatives have varied biological actions that may have relevance to their involvement in several physiological and pathological processes.

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.

[Functional expression of an omega-3 fatty acid desaturase gene from Glycine max in Saccharomyces cerevisiae]

Alpha-linolenic acid(ALA, C18:3delta9,12,15 ) is an essential fatty acid which has many sanitary functions to human. However, its contents in diets are often not enough. In plants, omega-3 fatty acid desaturases(FAD) catalyze linoleic acid(LA, C18:2delta9,12) into ALA. The seed oil of Glycine max contains high level of ALA. To investigate the functions of Glycine max omega-3FAD, the cDNA of GmFAD3 C was amplified by RT-PCR from immature seeds, then cloned into the shuttle expression vector p416 to generate the recombinant vector p4GFAD3C. The resulting vector was transformed into Saccharomyces cerevisiae K601 throuth LiAc method. The positive clones were screened on the CM(Ura-) medium and identified by PCR, and then cultured in CM (Ura-) liquid medium with exogenous LA in 20 degrees C for three days. The intracellular fatty acid composition of the engineering strain Kp416 and Kp4GFAD3C was analyzed by gas chromatography (GC). A novel peak in strain Kp4GFAD3C was detected,which was not detectable in control, Comparison of the retention times of the newly yielded peak with that of authentic standard indicated that the fatty acid is ALA. The content of ALA reached to 3.1% of the total fatty acid in recombinant strain, the content of LA correspondingly decreased from 22% to 16.2% by contrast. It was suggested that the protein encoded by GmFAD3 C can specifically catalyze 18 carbon PUFA substrate of LA into ALA by taking off hydrogen atoms at delta15 location. In this study, we expressed a Glycine max omega-3 fatty acid desaturase gene in S. cerevisiae; An efficient and economical yeast expressing system(K601-p416 system) which is suitable for the expression of FAD was built.

In vitro biosynthesis of volicitin in Spodoptera litura

Volicitin [N-(17-hydroxylinolenoyl)-L-glutamine] and N-linolenoyl-L-glutamine, originally identified in the regurgitant of Spodoptera exigua, induce damaged corn leaves to release volatile compounds which enable parasitic wasps to locate host caterpillars. Here we demonstrate the in vitro biosynthesis of volicitin for the first time by using gut tissues of Spodoptera litura larvae, as well as N-linolenoyl-L-glutamine. When crop, midgut tissues, peritrophic membrane and gut contents isolated from S. litura were incubated with sodium linolenate and L-[alpha-15N] glutamine, not only 15N-labeled N-linolenoyl-L-glutamine but 15N-labeled volicitin was detected mainly in the midgut incubation by LCMS and LCMSMS analysis. In contrast, there were negligible amounts of the newly biosynthesized compounds in the gut content incubation. Furthermore, the microsomal fraction obtained from the gut tissues clearly showed specific incorporation of glutamine. This substrate selectivity accounts for the exclusive uptake of glutamine by fatty acid amides (FAAs) in the noctuid caterpillars, even though glutamine was not a major component in the regurgitant. Additionally, intensive chemical analyses revealed that more than 20% of glutamine in hemolymph was present as conjugates in gut contents. These results suggest that FAA compounds are actively synthesized by caterpillar tissues and might play important physiological role(s) in glutamine metabolism.

In vitro biosynthesis of volicitin in Spodoptera litura

Volicitin [N-(17-hydroxylinolenoyl)-L-glutamine] and N-linolenoyl-L-glutamine, originally identified in the regurgitant of Spodoptera exigua, induce damaged corn leaves to release volatile compounds which enable parasitic wasps to locate host caterpillars. Here we demonstrate the in vitro biosynthesis of volicitin for the first time by using gut tissues of Spodoptera litura larvae, as well as N-linolenoyl-L-glutamine. When crop, midgut tissues, peritrophic membrane and gut contents isolated from S. litura were incubated with sodium linolenate and L-[alpha-15N] glutamine, not only 15N-labeled N-linolenoyl-L-glutamine but 15N-labeled volicitin was detected mainly in the midgut incubation by LCMS and LCMSMS analysis. In contrast, there were negligible amounts of the newly biosynthesized compounds in the gut content incubation. Furthermore, the microsomal fraction obtained from the gut tissues clearly showed specific incorporation of glutamine. This substrate selectivity accounts for the exclusive uptake of glutamine by fatty acid amides (FAAs) in the noctuid caterpillars, even though glutamine was not a major component in the regurgitant. Additionally, intensive chemical analyses revealed that more than 20% of glutamine in hemolymph was present as conjugates in gut contents. These results suggest that FAA compounds are actively synthesized by caterpillar tissues and might play important physiological role(s) in glutamine metabolism.

Expression of the Isochrysis C18-delta9 polyunsaturated fatty acid specific elongase component alters Arabidopsis glycerolipid profiles

A cDNA isolated from the prymnesiophyte micro-alga Isochrysis galbana, designated IgASE1, encodes a fatty acid elongating component that is specific for linoleic acid (C18:2n-6) and alpha-linolenic acid (C18:3n-3). Constitutive expression of IgASE1 in Arabidopsis resulted in the accumulation of eicosadienoic acid (EDA; C20:2n-6) and eicosatrienoic acid (ETrA; C20:3n-3) in all tissues examined, with no visible effects on plant morphology. Positional analysis of the various lipid classes indicated that these novel fatty acids were largely excluded from the sn-2 position of chloroplast galactolipids and seed triacylglycerol, whereas they were enriched in the same position in phosphatidylcholine. EDA and ETrA are precursors of arachidonic acid (C20:4n-6), eicosapentaenoic acid (C20:5n-3), and docosahexaenoic acid (C22:6n-3) synthesized via the so-called omega6 Delta8 desaturase and omega3 Delta8 desaturase biosynthetic pathways, respectively. The synthesis of significant quantities of EDA and ETrA in a higher plant is therefore a key step in the production of very long chain polyunsaturated fatty acid in oil-seed species. The results are further discussed in terms of prokaryotic and eukaryotic pathways of lipid synthesis in plants.

Isolation and characterization of delta(6)-desaturase, an ELO-like enzyme and delta(5)-desaturase from the liverwort Marchantia polymorpha and production of arachidonic and eicosapentaenoic acids in the methylotrophic yeast Pichia pastoris

The liverwort Marchantia polymorpha contains high proportions of arachidonic and eicosapentaenoic acids. In general, these C20 polyunsaturated fatty acids (PUFA) are synthesized from linoleic and alpha -linolenic acids, respectively, by a series of reactions catalyzed by Delta(6)-desaturase, an ELO-like enzyme involved in Delta(6) elongation and Delta(5)-desaturase. Here we report the isolation and characterization of the cDNAs, MpDES6, MpELO1 and MpDES5, coding for the respective enzymes from M. polymorpha. Co-expression of the MpDES6, MpELO1 and MpDES5 cDNAs resulted in the accumulation of arachidonic and eicosapentaenoic acids in the methylotrophic yeast Pichia pastoris. Interestingly, Delta(6) desaturation by the expression of the MpDES6 cDNA appears to occur both in glycerolipids and the acyl-CoA pool, although other lower-plant Delta(6)-desaturases are known to have a strong preference for glycerolipids.

Mechanistic characterization of omega-3 desaturation in the green alga Chlorella vulgaris

alpha-Linolenic acid (ALA, 9(Z),12(Z),15(Z)-octadecatrienoic acid) derivatives are important plant lipids which play a critical key role in cold tolerance. The final steps of ALA biosynthesis feature a series of regio- and stereoselective dehydrogenation reactions which are catalyzed by a set of enzymes known as fatty acid desaturases. In conjunction with ongoing research into the structural biology of these remarkable catalysts, we have examined the mechanism of double bond introduction at C15,16 as it occurs in a model photosynthetic organism, Chlorella vulgaris. The individual deuterium kinetic isotope effects associated with the C-H bond cleavages at C-15 and C-16 of a thialinoleoyl analogue were measured via competition experiments using appropriately deuterium-labelled 7-thia substrates. A large kinetic isotope effect (KIE) (k(H)/k(D)=10.2+/-2.8) was observed for the C-H bond-breaking step at C-15 while the C-H bond cleavage at C-16 was found to be relatively insensitive to deuterium substitution (k(H)/k(D)=0.8+/-0.2). These results point to C-15 as the site of initial oxidation in omega-3 desaturation and imply that the Chlorella and corresponding plant systems share a common active site architecture.

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.

Triacylglycerol and phospholipid fatty acids of the silverleaf whitefly: composition and biosynthesis

The identification and composition of the fatty acids of the major lipid classes (triacylglycerols and phospholipids) within Bemisia argentifolii Bellows and Perring (Homoptera: Aleyrodidae) nymphs were determined. Comparisons were made to fatty acids from the internal lipids of B. argentifolii adults. The fatty acids, as ester derivatives, were analyzed by capillary gas chromatography (CGC) and CGC-mass spectrometry (MS). All lipid classes contained variable distributions of eight fatty acids: the saturated fatty acids, myristic acid (14:0), palmitic acid (16:0), stearic acid (18:0), arachidic acid (20:0); the monounsaturated fatty acids, palmitoleic acid (16:1), oleic acid (18:1); the polyunsaturated fatty acids, linoleic acid (18:2), linolenic acid (18:3). Fourth instar nymphs had 5-10 times the quantities of fatty acids as compared to third instar nymphs and 1-3 times the quantities from adults. The fatty acid quantity differences between fourth and third instar nymphs were related to their size and weight differences. The percentage compositions for fatty acids from each lipid class were the same for the pooled groups of third and fourth instar nymphs. For nymphs and adults, triacylglycerols were the major source of fatty acids, with 18:1 and 16:0 acids as major components and the majority of the polyunsaturated fatty acids, 18:2 and 18:3 were present in the two phospholipid fractions, phosphatidylethanolamine and phosphatidylcholine. Evidence was obtained that whiteflies indeed synthesize linoleic acid and linolenic acid de novo: radiolabel from [2-(14)C] acetate was incorporated into 18:2 and 18:3 fatty acids of B. argentifolii adults and CGC-MS of pyrrolidide derivatives established double bonds in the Delta(9,12) and Delta(9,12,15) positions, respectively.

Production of linolenic acid by Mortierella isabellina grown on octadecanol

The production of linolenic acid in mycelial lipids reached 0.31 mg/ml of culture broth when Mortierella isabellina was cultivated in a medium consisting of 2% octadecanol, 1% yeast extract, and 25 mmol/L of Mg2+ at 23 degrees C for 5 days. Cultivation conditions were studied, and the results showed that (i) a suitable concentration of Mg2+ in the medium caused an increase in mycelial mass as well as linolenic acid production; (ii) when incubated at 23 degrees C, maximal linolenic acid productivity was reached, although a higher content of the acid in total fatty acids was found at the lower temperature; (iii) the effect of substrate concentration on linolenic acid yield showed that the latter increased with concentration of substrate, and maximal linolenic acid yield was obtained with concentrations of 2% octadecanol and 1% yeast extract.

Production of gamma-linolenic acid by Mortierella isabellina grown on hexadecanol

AIMS

To optimize the production of linolenic acid by Mortierella isabellina grown on hexadecanol.

METHODS AND RESULTS

Effects of culture conditions such as culture time, pH of medium, hexadecanol concentration, incubation temperature and ageing of mycelia on production of linolenic acid were studied. The production of gamma-linolenic acid reached 2.44 mg ml-1 (271 mg g-1 dry cells) when Mortierella isabellina was cultivated in a medium consisting of 2% hexadecanol and 1% yeast extract at 23 degrees C for 120 h and then the mycelia, after removal of medium by suction filtration, were allowed to stand for a further 15 d at 5 degrees C.

CONCLUSION

Ageing of mycelia and incubation temperature showed predominant effects on the increased linolenic acid production.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study highlights effective conditions for increasing linolenic acid production by Mortierella isabellina grown on hexadecanol.

[Expression of delta 6-fatty acid desaturase gene from Mortierella alpina in Saccharomyces cerevisiae]

delta 6-fatty acid desaturase is the rate-limiting enzyme of the desaturation of linoleic acid in the production of an essential fatty acid, gamma-linolenic acid. The 1.4 kb fragment in plasmid pTMACL6 encoding delta 6-fatty acid desaturase from Mortieralla alpina ATCC16266 was subcloned into the yeast-E. coli shuttle vector pYES2.0, thus an expression recombinant plasmid pYMAD6 containing target gene was constructed and obtained in the SC-Ura media. The pYMAD6 was introduced into defective mutant INCSc1 of Saccharomyces cerevisiae by LiAc method. When linoleic acid was provided as an exogenous substrate to the yeast cultures expressing delta 6-fatty acid desaturase activity under appropriate media and temperature condition, the level of gamma-linolenic acid reached 31.6% of the total yeast fatty acids by GC-MS detecting, which is the highest report of delta 6-fatty-acid desaturase gene in S. cerevisiae.

Essential fatty acid metabolism and its modification in atopic eczema

Research from the 1930s to the 1950s established that a deficit of n-6 essential fatty acids (EFAs) leads to an inflammatory skin condition in both animals and humans. In a common inherited skin condition, atopic dermatitis (eczema), there was evidence of low blood EFA concentrations and of a therapeutic response to exceptionally high doses of linoleic acid. More recently, it has been established that there is no deficit of linoleic acid in atopic eczema. Concentrations of linoleic acid instead tend to be elevated in blood, milk, and adipose tissue of patients with atopic eczema, whereas concentrations of linoleic acid metabolites are substantially reduced. This suggests reduced conversion of linoleic acid to gamma-linolenic acid (GLA). In most but not all studies, administration of GLA has been found to improve the clinically assessed skin condition, the objectively assessed skin roughness, and the elevated blood catecholamine concentrations of patients with atopic eczema. Atopic eczema may be a minor inherited abnormality of EFA metabolism.

Light-induced expression of fatty acid desaturase genes

In cyanobacterial cells, fatty acid desaturation is one of the crucial steps in the acclimation processes to low-temperature conditions. The expression of all the four acyl lipid desaturase genes of Synechocystis PCC 6803 was studied as a function of temperature and separately as a function of light. We used cells grown at 25 degreesC in light-activated heterotrophic growth conditions. In these cells, the production of alpha-linolenic acid and 18:4 fatty acids was negligible and the synthesis of gamma-linolenic acid was remarkably suppressed compared with those of the cells grown photoautotrophically. The cells grown in the light in the presence of glucose showed no difference in fatty acid composition compared with cells grown photoautotrophically. The level of desC mRNA for delta9 desaturase was not affected by either the temperature or the light. It was constitutively expressed at 25 degreesC with and without illumination. The level of desB transcripts was negligible in the dark-grown cells and was enhanced about 10-fold by exposure of the cells to light. The maximum level of expression occurred within 15 min. The level of desA and desD mRNAs was higher in dark-grown cells than that of desB mRNA for omega3 desaturase. However, the induction of both desA and desD mRNAs for delta12 and delta6 desaturases, respectively, was enhanced by light about 10-fold. Rifampicin, chloramphenicol, and 3-(3, 4-dichlorophenyl)-1,1-dimethylurea completely blocked the induction of the expression of desA, desB, and desD. Consequently, we suggest the regulatory role of light via photosynthetic processes in the induction of the expression of acyl lipid desaturases.

The Canadian Society for Nutritional Sciences 1995 Young Scientist Award Lecture. Recent studies on the synthesis, beta-oxidation, and deficiency of linoleate and alpha-linolenate: are essential fatty acids more aptly named indispensable or conditionally dispensable fatty acids?

Recent research on the synthesis, beta-oxidation, and deficiency of linoleate and alpha-linolenate raises questions about whether the term essential fatty acid is outdated. Linoleate and alpha-linolenate can be synthesized from their respective 16-carbon precursors, which are present in the human diet; whether the rate of conversion and dietary supply of the precursors are sufficient depends on the actual requirement for linoleate and alpha-linolenate. Pure deficiency of linoleate (diet excluding linoleate but including alpha-linolenate and oleate) has not been studied until recently, so it is unclear whether the recommended linoleate intake at 2% of energy, as based on classical essential fatty acid deficiency studies, is appropriate or too high. Despite marked whole-body depletion of linoleate and poor conservation of linoleate stores, pure linoleate deficiency has little effect on growth in rats, suggesting its requirement may be less than 2% of energy. Whole-body fatty acid balance studies indicate that the main route of linoleate and alpha-linolenate metabolism is oxidation, which increases sufficiently that accumulation of dietary linoleate and alpha-linolenate may actually be prevented in undernutrition and fasting refeeding. Part of the oxidized carbon from linoleate and alpha-linolenate is recycled and used for de novo synthesis of "non-essential" fatty acids and cholesterol, which in the brain of the suckling rat, can exceed conversion to longer chain polyunsaturates by as much as 10- to 40-fold. Given the capability to synthesize linoleate and alpha-linolenate, the imprecise knowledge of true linoleate requirement, and the absence of clear symptoms of their deficiency in healthy adults, it might be advantageous to consider using the terms indispensable and conditionally dispensable to clarify the conditional nature of the dietary requirement for linoleate and alpha-linolenate.

Synthesis of linoleate and alpha-linolenate by chain elongation in the rat

The objective was to determine whether rats could synthesize longer chain polyunsaturates from hexadecadienoate (16:2n-6) and hexadecatrienoate (16:3n-3). Rats were gavaged with uniformly 13C-labelled hexadecadienoate or hexadecatrienoate, euthanized 24 h later, and total lipids were extracted from liver and carcass. Gas chromatography/combustion/isotope ratio mass spectrometry was used to measure 13C levels in individual liver, carcass, and whole body fatty acids. 13C Enrichment was present in desaturated and chain-elongated polyunsaturates, including linoleate, arachidonate, alpha-linolenate, and docosahexaenoate at 12-13% of the dose of tracer given. 13C Enrichment from hexadecatrienoate was highest in carcass and liver alpha-linolenate, representing 3.5 and 17.9% of the total alpha-linolenate pool, respectively. For linoleate, arachidonate, or docosahexaenoate, the contribution of 13C did not exceed 0.2% of the total body pool. Green leafy vegetables common in the human diet were shown to contain up to 1.2% of total fatty acids as hexadecadienoate and 11.6% as hexadecatrienoate. Hence, humans consuming green vegetables probably synthesize a small proportion of their total body content of linoleate and alpha-linolenate.

Arabidopsis FAD2 gene encodes the enzyme that is essential for polyunsaturated lipid synthesis

The polyunsaturated fatty acids linoleate and alpha-linolenate are important membrane components and are the essential fatty acids of human nutrition. The major enzyme responsible for the synthesis of these compounds is the plant oleate desaturase of the endoplasmic reticulum, and its activity is controlled in Arabidopsis by the fatty acid desaturation 2 (fad2) locus. A fad2 allele was identified in a population of Arabidopsis in which mutations had been created by T-DNA insertions. Genomic DNA flanking the T-DNA was cloned by plasmid rescue and used to isolate cDNA and genomic clones of FAD2. A cDNA containing the entire FAD2 coding sequence was expressed in fad2 mutant plants and shown to complement the mutant fatty acid phenotype. The deduced amino acid sequence from the cDNA showed homology to other plant desaturases, and this confirmed that FAD2 is the structural gene for the desaturase. Gel blot analyses of FAD2 mRNA levels showed that the gene is expressed throughout the plant and suggest that transcript levels are in excess of the amount needed to account for oleate desaturation. Sequence analysis identified histidine-rich motifs that could contribute to an iron binding site in the cytoplasmic domain of the protein. Such a position would facilitate interaction between the desaturase and cytochrome b5, which is the direct source of electrons for the desaturation reaction, but would limit interaction of the active site with the fatty acyl substrate.

Arabidopsis FAD2 gene encodes the enzyme that is essential for polyunsaturated lipid synthesis

The polyunsaturated fatty acids linoleate and alpha-linolenate are important membrane components and are the essential fatty acids of human nutrition. The major enzyme responsible for the synthesis of these compounds is the plant oleate desaturase of the endoplasmic reticulum, and its activity is controlled in Arabidopsis by the fatty acid desaturation 2 (fad2) locus. A fad2 allele was identified in a population of Arabidopsis in which mutations had been created by T-DNA insertions. Genomic DNA flanking the T-DNA was cloned by plasmid rescue and used to isolate cDNA and genomic clones of FAD2. A cDNA containing the entire FAD2 coding sequence was expressed in fad2 mutant plants and shown to complement the mutant fatty acid phenotype. The deduced amino acid sequence from the cDNA showed homology to other plant desaturases, and this confirmed that FAD2 is the structural gene for the desaturase. Gel blot analyses of FAD2 mRNA levels showed that the gene is expressed throughout the plant and suggest that transcript levels are in excess of the amount needed to account for oleate desaturation. Sequence analysis identified histidine-rich motifs that could contribute to an iron binding site in the cytoplasmic domain of the protein. Such a position would facilitate interaction between the desaturase and cytochrome b5, which is the direct source of electrons for the desaturation reaction, but would limit interaction of the active site with the fatty acyl substrate.