Expression of a borage desaturase cDNA containing an N-terminal cytochrome b5 domain results in the accumulation of high levels of delta6-desaturated fatty acids in transgenic tobacco

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.

Genetic engineering of plant lipids

Vegetable oils are a major component of human diets, comprising as much as 25% of average caloric intake. Until recently, it was not possible to exert significant control over the chemical composition of vegetable oils derived from different plants. However, the advent of genetic engineering has provided novel opportunities to tailor the composition of plant-derived lipids so that they are optimized with respect to food functionality and human dietary needs. In order to exploit this new capability, it is essential for food scientists and nutritionists to define the lipid compositions that would be most desirable for various purposes.

Cloning of delta12- and delta6-desaturases from Mortierella alpina and recombinant production of gamma-linolenic acid in Saccharomyces cerevisiae

Two cDNA clones with homology to known desaturase genes were isolated from the fungus Mortierella alpina. The open reading frame in one clone encoded 399 amino acids and exhibited delta12-desaturase activity when expressed in Saccharomyces cerevisiae in the presence of endogenous fatty acid substrate oleic acid. The insert in another clone contained an open reading frame encoding 457 amino acids and exhibited delta6-desaturase activity in S. cerevisiae in the presence of exogenous fatty acid substrate linoleic acid. Expression of the delta12-desaturase gene under appropriate media and temperature conditions led to the production of linoleic acid at levels up to 25% of the total fatty acids in yeast. When linoleic acid was provided as an exogenous substrate to the yeast cultures expressing the delta6-desaturase activity, the level of gamma-linolenic acid reached 10% of the total yeast fatty acids. Co-expression of both the delta6- and delta12-desaturase cDNA resulted in the endogenous production of gamma-linolenic acid. The yields of gamma-linolenic acid reached as high as 8% of total fatty acids in yeast.

The Delta8-desaturase of Euglena gracilis: an alternate pathway for synthesis of 20-carbon polyunsaturated fatty acids

Desaturation of fatty acids is an important metabolic process. In mammals, 20-carbon and longer polyunsaturated fatty acids are not only incorporated into cellular membranes in a tissue-specific manner, but also serve as the precursors to synthesis of eicosanoid metabolic regulators. The processes of desaturation and elongation in human liver are well characterized, but an alternate Delta8 desaturation pathway that may be important in certain tissues or in cancer cells is less well examined. The Delta8-desaturase enzyme introduces a double bond at the 8-position in 20-carbon fatty acids that have an existing Delta11 unsaturation. We have isolated the first fatty acid Delta8-desaturase, from the protist Euglena gracilis, in order to explore this alternate pathway. A full-length cDNA was obtained after reverse transcription of mRNA purified from heterotrophically grown Euglena, followed by PCR amplification with primers degenerate to conserved histidine-rich regions of microsomal desaturases. The protein predicted from the cDNA sequence is highly homologous to Delta5 and Delta6 desaturases of Caenhorabditis elegans. When the cDNA was expressed in Saccharomyces cerevisiae, the yeast cultures readily desaturated appropriate 20-carbon fatty acids by inserting an additional double bond at the Delta8-position. The enzyme demonstrated a preference for substrates of metabolic significance, 20:3 Delta11,14,17 and 20:2 Delta11,14. Cloning of a Delta8 fatty acid desaturase offers the opportunity to examine an alternate pathway of long chain fatty acid biosynthesis.

Identification of Delta12-fatty acid desaturase from arachidonic acid-producing mortierella fungus by heterologous expression in the yeast Saccharomyces cerevisiae and the fungus Aspergillus oryzae

Based on the sequence information for the omega3-desaturase genes (from Brassica napus and Caenorhabditis elegans), which are involved in the desaturation of linoleic acid (Delta9, Delta12-18 : 2) to alpha-linolenic acid (Delta9, Delta12, Delta15-18 : 3), a cDNA was cloned from the filamentous fungal strain, Mortierella alpina 1S-4, which is used industrially to produce arachidonic acid. Homology analysis with protein databases revealed that the amino acid sequence showed 43.7% identity as the highest match with the microsomal omega6-desaturase (from Glycine max, soybean), whereas it exhibited 38.9% identity with the microsomal omega3-desaturase (from soybean). The evolutionary implications of these enzymes will be discussed. The cloned cDNA was confirmed to encode a Delta12-desaturase, which was involved in the desaturation of oleic acid (Delta9-18 : 1) to linoleic acid, by its expression in both the yeast Saccharomyces cerevisiae and the fungus Aspergillus oryzae. Analysis of the fatty acid composition of yeast and fungus transformants demonstrated that linoleic acid (which was not contained in the control strain of S. cerevisiae) was accumulated in the yeast transformant and that the fungal transformant contained a large amount of linoleic acid (71.9%). Genomic Southern blot analysis of the transformants with the Mortierella Delta12-desaturase gene as a probe confirmed integration of this gene into the genome of A. oryzae. The M. alpina 1S-4 Delta12-desaturase is the first example of a cloned nonplant Delta12-desaturase.

Temporal and spatial gene expression of cytochrome B5 during flower and fruit development in olives

We report the characterisation of two cytochrome b5 genes and their spatial and temporal patterns of expression during development in olive, Olea europaea. A PCR-generated probe, based on a tobacco cytochrome b5 sequence, was used to isolate two full-length cDNA clones (cytochrome b5-15 and cytochrome b5-38) from a library derived from 13 WAF olive fruits. The cDNAs encoded proteins of 17.0 and 17.7 kDa, which contained all the characteristic motifs of cytochromes b5 from other organisms and exhibited 63% identity and 85% similarity with each other. The olive cytochrome b5-15 cDNA was then used as a probe for more detailed analysis. Southern blotting revealed a gene family of at least 4-6 members while northern blotting and in situ hybridisation showed a highly specific pattern of gene expression. Very low levels of cytochrome b5 mRNA were detected in tissues characterised by high rates of lipid accumulation, such as young expanding leaves, maturing seeds and ripening mesocarp. The cytochrome b5 genes were not induced at 6 degrees C and their response to ABA was relatively slow compared with fatty acid desaturase genes. In contrast, high levels of cytochrome b5 gene expression were found in young fruits at the pattern formation (globular/heart) stage of embryogenesis and in vascular and transmitting tissues of male and female reproductive organs. The data are consistent with a major role for cytochrome b5 in developmental processes related to plant reproduction in addition to being an electron donor to microsomal desaturases.

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

Unsaturated fatty acids are essential lipid components of Mucor rouxii. Gamma-linolenic acid (GLA) is synthesized via the desaturase enzymes: delta9-desaturase catalyzes mono-unsaturated fatty acids that are utilized as substrate for GLA biosynthesis. We cloned and characterized a M. rouxii gene highly homologous to delta9-desaturase genes. This sequence encodes for a protein of 452 amino acids and contains two introns of 60 and 61 nucleotides. Delta9-desaturase of M. rouxii is expressed during cell growth when cells are subjected to temperature shifts. At 30 degrees C, the mRNA level of late log phase is about 6.4-fold higher than that of early log phase. A shift from 30 to 15 degrees C induced transcription of delta9-desaturase gene in both early and late log phases. However, the pattern of increased transcription by cold induction varied depending on growth conditions: transcription of late log phase is higher than that of early log phase. These results indicate that cell growth and low temperature influence the expression of delta9-desaturase gene and fatty acid composition of M. rouxii.

Production of novel oils in plants

We have now isolated the great majority of genes encoding enzymes of storage oil biosynthesis in plants. In the past two years, particular progress has been made with acyltransferases, ketoacyl-acyl carrier protein synthetases and with desaturases and their relatives. In some cases, these enzymes have been reengineered to create novel products. Nevertheless, the single or multiple insertion of such transgenes into oil crops has not always led to the desired phenotype. We are only now beginning to appreciate some of the complexities of storage and membrane lipid formation, such as acyl group remodelling and the turnover of unusual fatty acids. This understanding will be vital for future attempts at the rational engineering of transgenic oil crops. In parallel with this, the domestication of plants already synthesising useful fatty acids should be considered as a real alternative to the transgenic approach to producing novel oil crops.

Cloning and characterization of a maize cytochrome-b5 reductase with Fe3+-chelate reduction capability

We previously purified an NADH-dependent Fe3+-chelate reductase (NFR) from maize roots with biochemical features of a cytochrome-b5 reductase (b5R) [Sparla, Bagnaresi, Scagliarini and Trost (1997) FEBS Lett. 414, 571-575]. We have now cloned a maize root cDNA that, on the basis of sequence information, calculated parameters and functional assay, codes for NFR. Maize NFR has 66% and 65% similarity to mammal and yeast b5R respectively. It has a deduced molecular mass of 31.17 kDa and a pI of 8.53. An uncharged region is observed at its N-terminus but no myristoylation consensus site is present. Taken together, these results, coupled with previous biochemical evidence, prove that NFR belongs to the b5R class and document b5R from a plant at the molecular level for the first time. We have also identified a putative Arabidopsis thaliana NFR gene. Its organization (nine exons) closely resembles mammalian b5Rs. Several NFR isoforms are expected to exist in maize. They are probably not produced by alternative translational mechanisms as occur in mammals, because of specific constraints observed in the maize NFR cDNA sequence. In contrast with yeast and mammals, tissue-specific and various subcellular localizations of maize b5R isoforms could result from differential expression of the various members of a multigene family. The first molecular characterization of a plant b5R indicates an overall remarkable evolutionary conservation for these versatile reductase systems. In addition, the well-characterized Fe3+-chelate reduction capabilities of NFR, in addition to known Fe3+-haemoglobin reduction roles for mammal b5R isoforms, suggest further and more generalized roles for the b5R class in endocellular iron reduction.

Molecular cloning and functional characterization of rat delta-6 fatty acid desaturase

Mammalian cDNA fragments putatively encoding amino acid sequences characteristic of the fatty acid desaturase were obtained using expressed sequence tag (EST) sequence informations. These fragments were subsequently used to screen a rat liver cDNA library, yielding a 1573-bp clone. Expression of DNA fragment containing either of two possible open reading frames (nucleotide numbers 97-1431 and 148-1431) of the isolated clone in yeast led to the accumulation of gamma-linolenic acid in the presence of exogenous linoleic acid. In this system, the addition of alpha-linolenic acid also resulted in the accumulation of its Delta-6 desaturated product whereas dihomo-gamma-linolenic acid failed to be a substrate. These results indicate that the protein encoded by the rat cDNA is Delta-6 fatty acid desaturase, and the first 17 amino acids corresponding to the coding region 97-147 of the clone are not required to function in yeast.

Isolation and characterization of a Delta 5-fatty acid desaturase from Caenorhabditis elegans

Arachidonic acid and eicosapentaenoic acid are important precursors for the production of prostaglandins and other hormone-like eicosanoid molecules. These fatty acids are synthesized by animals by elongating and desaturating precursor fatty acids such as linoleic acid (18:2Delta9,12) and alpha-linolenic acid (18:3Delta9, 12,15). We have identified a Delta5 fatty acid desaturase gene (fat-4) from the nematode Caenorhabditis elegans. We have expressed this gene product in Saccharomyces cerevisiae and demonstrate that it readily converts di-homo-gamma-linolenic acid (20:3Delta8,11,14) to arachidonic acid (20:4Delta5,8,11,14). The FAT-4 Delta5-desaturase also acts on a number of other substrates, including fatty acids that do not contain a double bond at the Delta8 position.

Delta 9-fatty acid desaturase from arachidonic acid-producing fungus. Unique gene sequence and its heterologous expression in a fungus, Aspergillus

Based on the sequence information for delta 9-desaturase genes (from rat, mouse and yeast), which are involved in the desaturation of palmitic acid and stearic acid to palmitoleic acid and oleic acid, respectively, the corresponding cDNA and genomic gene were cloned from the fungal strain, Mortierella alpina 1S-4, which industrially produces arachidonic acid. There was a cytochrome b5-like domain linked to the carboxyl terminus of this Mortierella desaturase, as also seen in the yeast delta 9-desaturase. The Mortierella delta 9-desaturase genomic gene had only one intron, in which a novel phenomenon was observed: there was a GC-end at the 5'-terminus instead of a GT-end that is, in general, found in introns of eukaryotic genes. The full-length cDNA clone was expressed under the control of an amyB promoter in a filamentous fungus, Aspergillus oryzae, resulting in drastic changes in the fatty acid composition in the transformant cells; the contents of palmitoleic acid (16:1) and oleic acid (18:1) increased significantly, with accompanying decreases in palmitic acid (16:0) and stearic acid (18:0). These changes were controlled by the addition of maltose as a carbon source to the medium. Also, the expression of the gene caused a significant change in the lipid composition in the Aspergillus transformant. Genomic Southern blot analysis of the transformant with the Mortierella delta 9-desaturase gene as a probe confirmed the integration of this gene into the genome of A. oryzae.

Cloning, expression, and nutritional regulation of the mammalian Delta-6 desaturase

Arachidonic acid (20:4(n-6)) and docosahexaenoic acid (22:6(n-3)) have a variety of physiological functions that include being the major component of membrane phospholipid in brain and retina, substrates for eicosanoid production, and regulators of nuclear transcription factors. The rate-limiting step in the production of 20:4(n-6) and 22:6(n-3) is the desaturation of 18:2(n-6) and 18:3(n-3) by Delta-6 desaturase. In this report, we describe the cloning, characterization, and expression of a mammalian Delta-6 desaturase. The open reading frames for mouse and human Delta-6 desaturase each encode a 444-amino acid peptide, and the two peptides share an 87% amino acid homology. The amino acid sequence predicts that the peptide contains two membrane-spanning domains as well as a cytochrome b5-like domain that is characteristic of nonmammalian Delta-6 desaturases. Expression of the open reading frame in rat hepatocytes and Chinese hamster ovary cells instilled in these cells the ability to convert 18:2(n-6) and 18:3(n-3) to their respective products, 18:3(n-6) and 18:4(n-3). When mice were fed a diet containing 10% fat, hepatic enzymatic activity and mRNA abundance for hepatic Delta-6 desaturase in mice fed corn oil were 70 and 50% lower than in mice fed triolein. Finally, Northern analysis revealed that the brain contained an amount of Delta-6 desaturase mRNA that was several times greater than that found in other tissues including the liver, lung, heart, and skeletal muscle. The RNA abundance data indicate that prior conclusions regarding the low level of Delta-6 desaturase expression in nonhepatic tissues may need to be reevaluated.

Functional identification of a fatty acid delta5 desaturase gene from Caenorhabditis elegans

We have identified a cDNA from the nematode worm Caenorhabditis elegans that encodes a fatty acid delta5 desaturase. Saccharomyces cerevisiae expressing the full-length cDNA was able to convert di-homo-gamma-linolenic acid to arachidonic acid, thus confirming delta5 desaturation. The 1341 bp delta5 desaturase sequence contained an N-terminal cytochrome b5 domain and was located within a kilobase of the C. elegans delta6 desaturase on chromosome IV. With an amino acid identity of 45% it is possible that one of these genes arose from the other by gene duplication. This is the first example of a delta5 desaturase gene isolated from an animal.

Identification of Delta5-desaturase from Mortierella alpina by heterologous expression in Bakers' yeast and canola

A DNA fragment with homology to Delta6-desaturases from borage and cyanobacteria was isolated after polymerase chain reaction amplification of Mortierella alpina cDNA with oligonucleotide primers corresponding to the conserved regions of known Delta6-desaturase genes. This fragment was used as a probe to isolate a cDNA clone with an open reading frame encoding 446 amino acids from a M. alpina library. Expression of this open reading frame from an inducible promoter in Saccharomyces cerevisiae in the presence of various substrates revealed that the recombinant product had Delta5-desaturase activity. The effects of growth and induction conditions as well as host strain on activity of the recombinant Delta5-desaturase in S. cerevisiae were evaluated. Expression of the M. alpina Delta5-desaturase cDNA in transgenic canola seeds resulted in the production of taxoleic acid (Delta5,9-18:2) and pinolenic acid (Delta5,9,12-18:3), which are the Delta5-desaturation products of oleic and linoleic acids, respectively.

A sphingolipid desaturase from higher plants. Identification of a new cytochrome b5 fusion protein

A recently cloned cDNA from sunflower codes for a fusion protein composed of an N-terminal cytochrome b5 and a domain similar to membrane-bound acyl lipid desaturases. For a functional identification, homologous cDNAs from Brassica napus and Arabidopsis thaliana were expressed in Saccharomyces cerevisiae, and sphingolipid long chain bases were analyzed. The expression of the heterologous enzyme results in significant proportions of new Delta8, 9-cis/trans-phytosphingenines that accompany the residual C18-phytosphinganine predominating in wild-type yeast cells. These results represent the first identification of a gene coding for a sphingolipid desaturase and for a stereounselective desaturase showing trans-activity from any organism. Furthermore, this fusion protein is a new member of the cytochrome b5 superfamily. The formation of the two regioisomeric phytosphingenines in the transformed yeast sheds new light on the factors controlling regioselectivity.

Structure and expression of fatty acid desaturases

Fatty acid desaturases are enzymes that introduce double bonds into fatty acyl chains. They are present in all groups of organisms, i.e., bacteria, fungi, plants and animals, and play a key role in the maintenance of the proper structure and functioning of biological membranes. The desaturases are characterized by the presence of three conserved histidine tracks which are presumed to compose the Fe-binding active centers of the enzymes. Recent findings on the structure and expression of different types of fatty acid desaturase in cyanobacteria, plants and animals are reviewed in this article. Roles of individual desaturases in temperature acclimation and principles of regulation of the desaturase genes are discussed.

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.

Identification of a novel delta 6-acyl-group desaturase by targeted gene disruption in Physcomitrella patens

The moss Physcomitrella patens contains high levels of arachidonic acid. For its synthesis from linoleic acid by desaturation and elongation, novel delta 5- and delta 6-desaturases are required. To isolate one of these, PCR-based cloning was used, and resulted in the isolation of a full-length cDNA coding for a putatively new desaturase. The deduced amino acid sequence has three domains: a N-terminal segment of about 100 amino acids, with no similarity to any sequence in the data banks, followed by a cytochrome b5-related region and a C-terminal sequence with low similarity (27% identify) to acyl-lipid desaturases. To elucidate the function of this protein, we disrupted its gene by transforming P. patens with the corresponding linear genomic sequence, into which a positive selection marker had been inserted. The molecular analysis of five transformed lines showed that the selection cartridge had been inserted into the corresponding genomic locus of all five lines. The gene disruption resulted in a dramatic alteration of the fatty acid pattern in the knockout plants. The large increase in linoleic acid and the concomitant disappearance of gamma-linolenic and arachidonic acid in all knockout lines suggested that the new cDNA coded for a delta 6-desaturase. This was confirmed by expression of the cDNA in yeast and analysis of the resultant fatty acids by GC-MS. Only the transformed yeast cells were able to introduce a further double bond into the delta 6-position of unsaturated fatty acids. To our knowledge, this is the first report of a successful gene disruption in a multicellular plant resulting in a specific biochemical phenotype.

DESATURATION AND RELATED MODIFICATIONS OF FATTY ACIDS1

Desaturation of a fatty acid first involves the enzymatic removal of a hydrogen from a methylene group in an acyl chain, a highly energy-demanding step that requires an activated oxygen intermediate. Two types of desaturases have been identified, one soluble and the other membrane-bound, that have different consensus motifs. Database searching for these motifs reveals that these enzymes belong to two distinct multifunctional classes, each of which includes desaturases, hydroxylases, and epoxidases that act on fatty acids or other substrates. The soluble class has a consensus motif consisting of carboxylates and histidines that coordinate an active site diiron cluster. The integral membrane class contains a different consensus motif composed of histidines. Biochemical and structural similarities between the integral membrane enzymes suggest that this class also uses a diiron cluster for catalysis. Soluble and membrane enzymes have been successfully re-engineered for substrate specificity and reaction outcome. It is anticipated that rational design of these enzymes will result in new and desired activities that may form the basis for improved oil crops.

Identification of a Caenorhabditis elegans Delta6-fatty-acid-desaturase by heterologous expression in Saccharomyces cerevisiae

We identified a cDNA expressed sequence tag from an animal (the nematode worm Caenorhabditis elegans) that showed weak similarity to a higher-plant microsomal Delta6-desaturase. A full-length cDNA clone was isolated and expressed in the yeast Saccharomyces cerevisiae. This demonstrated that the protein encoded by the C. elegans cDNA was that of a fatty acid Delta6-desaturase, as determined by the accumulation of gamma-linolenic acid. The C. elegans Delta6-desaturase contained an N-terminalcytochrome b5 domain, indicating that it had a similar structure to that of the higher-plant Delta6-desaturase. The C. elegans Delta6-desaturase mapped to cosmid W08D2, a region of chromosome III. This is the first example of a Delta6-desaturase isolated from an animal and also the first example of an animal desaturase containing a cytochrome b5 domain.

Fah1p, a Saccharomyces cerevisiae cytochrome b5 fusion protein, and its Arabidopsis thaliana homolog that lacks the cytochrome b5 domain both function in the alpha-hydroxylation of sphingolipid-associated very long chain fatty acids

A search of the Saccharomyces cerevisiae genome data base for cytochrome b5-like sequences identified a 1.152-kilobase pair open reading frame, located on chromosome XIII at locus YMR272C (FAH1). That gene encodes a putative 384-amino acid protein with an amino-terminal cytochrome b5 domain. The b5 core domain shows a 52% identity and 70% similarity to that of the yeast microsomal cytochrome b5 and a 35% identity and 54% similarity to the b5 core domain of OLE1, the S. cerevisiae Delta-9 fatty acid desaturase. Expression of the S. cerevisiae FAH1 cytochrome b5 domain in Escherichia coli produces a soluble protein that exhibits the typical oxidized versus reduced differential absorbance spectra of cytochrome b5. Sequence analysis of Fah1p reveals other similarities to Ole1p. Both proteins are predicted to have two hydrophobic domains, each capable of spanning the membrane twice, and both have the HX(2-3)(XH)H motifs that are characteristic of membrane-bound fatty acid desaturases. These similarities to Ole1p suggested that Fah1p played a role in the biosynthesis or modification of fatty acids. Disruption of the FAH1 gene in S. cerevisiae did not give any visible phenotype, and there was no observable difference in content or distribution of the most abundant long chain saturated and unsaturated 14-18-carbon fatty acid species. Northern blot analysis, however, showed that this gene is expressed at much lower levels ( approximately 150-fold) than the OLE1 gene, suggesting that it might act on a smaller subset of fatty acids. Analysis of sphingolipid-derived very long chain fatty acids revealed an approximately 40-fold reduction of alpha-HO 26:0 and a complementary increase in 26:0 in the gene-disrupted fah1Delta strain. GAL1 expression of the S. cerevisiae FAH1 genes in the fah1Delta strain restores alpha-HO 26:0 fatty acids to wild type levels. Also identified are a number of homologs to this gene in other species. Expression of an Arabidopsis thaliana FAH1 gene, which does not contain the cytochrome b5 domain, in the fah1Delta strain produced an approximately 25-fold increase in alpha-HO 26:0 and reduced the levels of its 26-carbon precursor, suggesting that it functions in very long chain fatty acid hydroxylation using an alternate electron transfer mechanism.