Patent Publication Number: US-11046937-B2

Title: Methods for conversion of the substrate specificity of desaturases

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of Ser. No. 15/105,080, which is a U.S. National Stage application of International Application No. PCT/IB2014/067021, filed Dec. 17, 2014, which claims the benefit of U.S. Provisional Application No. 61/916,825, filed Dec. 17, 2013; the aforementioned applications are hereby incorporated by reference herein in their entirety. 
    
    
     INCORPORATION BY REFERENCE OF MATERIAL SUBMITTED ELECTRONICALLY 
     The Sequence Listing, which is a part of the present disclosure, is submitted concurrently with the specification as a text file. The name of the text file containing the Sequence Listing is “H75716A_Seqlisting.txt”, which was created on May 6, 2019 and is 114,844 bytes in size. The subject matter of the Sequence Listing is incorporated herein in its entirety by reference. 
     The present invention relates to methods for the conversion of the substrate specificity of desaturases. Specifically, the present invention pertains to a method for the conversion of the substrate specificity of a delta (Δ) 5 and/or Δ6 desaturase to the substrate specificity of a Δ4 desaturase, the method comprising: identifying regions and/or amino acid residues which control the substrate specificity of (i) the Δ5 and/or Δ6 desaturase and (ii) the Δ4 desaturase; and replacing in the amino acid sequence of the mentioned Δ5 and/or Δ6 desaturase, the regions and/or amino acid residues which control the substrate specificity of the Δ5 and/or Δ6 desaturase, by the corresponding regions and/or amino acid residues which control the substrate specificity of the Δ4 desaturase, thereby converting the substrate specificity of the Δ5 and/or Δ6 desaturase to the substrate specificity of the Δ4 desaturase. The present invention further concerns a method for the conversion of the substrate specificity of a Δ4 desaturase to the substrate specificity of a Δ5 and/or Δ6 desaturase, the method comprising: identifying regions and/or amino acid residues which control the substrate specificity of (i) the Δ4 desaturase and (ii) the Δ5 and/or Δ6 desaturase; and replacing in the amino acid sequence of the indicated Δ4 desaturase, the regions and/or amino acid residues which control the substrate specificity of the Δ4 desaturase, by the corresponding regions and/or amino acid residues which control the substrate specificity of the Δ5 and/or Δ6 desaturase, thereby converting the substrate specificity of the Δ4 desaturase to the substrate specificity of the Δ5 and/or Δ6 desaturase. In addition, the invention encompasses desaturases with converted substrate specificity. 
     Very long chain polyunsaturated fatty acids such as arachidonic acid (ARA, 20:4 ω6 ), eicosapentaenoic acid (EPA, 20:5 ω3 ), docosapentaenoic acid (DPA, 22:5 ω3 ) and docosahexaenoic acid (DHA, 22:6 ω3 ) are essential components of cell membranes, and are precursors for a group of hormone-like bioactive compounds (eicosanoids and docosanoids) involved in regulation of various physiological activities in animals and humans. The biosynthesis of these fatty acids involves an alternating process of fatty acid desaturation and elongation, mediated by fatty acid desaturases and elongases, respectively. 
     Fatty acid desaturases act by removing two hydrogen atoms from the fatty acid hydrocarbon chain, resulting in the formation of a double bond (Shanklin et al., 2009; Meesapyodsuk and Qiu, 2012). Three conserved histidine boxes, generally containing the consensus sequences H-X[3-4]-H, H-X[2-3]-H-H and H-X[2]-H-H, are found in almost all fatty acid desaturases (Pereira et al., 2003). These three histidine boxes form the active site of desaturases, and are involved in the formation of a di-iron complex (Sperling et al., 2001; Shanklin et al., 2009). By using site-directed mutagenesis, Shanklin and colleagues (1994) showed that each of the eight histidine residues in these boxes is essential for the catalytic function of the stearoyl-CoA Δ9 desaturase from rat liver. In addition, Broun et al. (1998) modified amino acids in close proximity to the histidine boxes and were able to convert an oleate 12-desaturase to a hydroxylase and vice versa. Similarly, amino acid modifications close to the histidine boxes of  Claviceps purpurea  Δ12 and 15 desaturases also affected the catalytic properties and regioselectivity of the enzymes (Meesapyodsuk et al., 2007). 
     Several pathways have been suggested for the formation of very long chain Δ4-desaturated fatty acids such as DHA. One route for DHA synthesis uses 18:3 ω3  as a substrate, which under-goes a Δ6 desaturation and a Δ6 elongation to produce 20:4 ω3 , followed by a Δ5 desaturation step and a Δ5 elongation to form 22:5 ω3  (Napier et al., 2003; Napier and Sayanova, 2005). Traditionally, due to the lack of observed Δ4 desaturase activity in mammals (Zheng et al., 2004), the main route for production of 22:6 ω3  from 22:5 ω3  was thought to proceed via a Δ7 elongation and a Δ6 desaturation, producing 24:6 ω3 , followed by a β-oxidation reaction to remove two carbon units from the fatty acid (Sprecher et al., 1995; Sprecher, 2000). A simpler route for the production of 22:6 ω3  from 22:5 ω3  is via Δ4 desaturation, and it is noteworthy that the first Δ4 desaturase was cloned in 2001 from the marine protist  Thraustochytrium  (Qiu et al., 2001). Since then, several other groups have identified Δ4 desaturases from protists and microalgae (Meyer et al., 2003; Tonon, 2003; Tripodi et al., 2006; Zhou et al., 2007; Ahmann et al., 2011). Recently, the first vertebrate Δ4 desaturase gene was isolated from the marine teleost fish  Siganus canaliculatus  ( S. canaliculatus ) (Li et al., 2010), which suggests that there might be a more straightforward mechanism available for DHA biosynthesis in vertebrates as well. 
     To date, the only fish species with demonstrated Δ5 and Δ6 fatty acid desaturation activity are Atlantic salmon, zebrafish and  Siganus canaliculatus  (Hastings et al., 2001; Hastings et al., 2005; Zheng et al., 2005; Li et al., 2010). Atlantic salmon has separate genes encoding Δ5 and Δ6 desaturation activities, whereas zebrafish and  S. canaliculatus  possess bi-functional Δ5/Δ6 fatty acid desaturase genes. 
     A better understanding of the structure, function and evolution of desaturases, as well as their roles in the biosynthesis of very long chain polyunsaturated fatty acids offers the opportunity to engineer production of these fatty acids, e.g., in transgenic oilseed plants for nutraceutical markets. Polyunsaturated fatty acids (PUFAs) cannot be synthesized de novo by vertebrates and so must be obtained in the diet. To make possible the fortification of food and of feed with polyunsaturated fatty acids, there is therefore a great need for means and methods for the production of these polyunsaturated fatty acids. 
     The object on which the present invention is based is the provision of such means and measures. This object is achieved by the embodiments which are described in the claims and herein below. 
     The present invention thus relates to a method for the conversion of the substrate specificity of a delta (Δ) 5 and/or Δ6 desaturase to the substrate specificity of a Δ4 desaturase, the method comprising:
         a) identifying regions and/or amino acid residues which control the substrate specificity of (i) the Δ5 and/or Δ6 desaturase and (ii) the Δ4 desaturase; and   b) replacing in the amino acid sequence of the Δ5 and/or Δ6 desaturase referred to in step a), the regions and/or amino acid residues which control the substrate specificity of the Δ5 and/or Δ6 desaturase, by the corresponding regions and/or amino acid residues which control the substrate specificity of the Δ4 desaturase, thereby converting the substrate specificity of the Δ5 and/or Δ6 desaturase to the substrate specificity of the Δ4 desaturase.       

     The present invention further pertains to a method for the conversion of the substrate specificity of a Δ4 desaturase to the substrate specificity of a Δ5 and/or Δ6 desaturase, the method comprising:
         a) identifying regions and/or amino acid residues which control the substrate specificity of (i) the Δ4 desaturase and (ii) the Δ5 and/or Δ6 desaturase; and   b) replacing in the amino acid sequence of the Δ4 desaturase referred to in step a), the regions and/or amino acid residues which control the substrate specificity of the Δ4 desaturase, by the corresponding regions and/or amino acid residues which control the substrate specificity of the Δ5 and/or Δ6 desaturase, thereby converting the substrate specificity of the Δ4 desaturase to the substrate specificity of the Δ5 and/or Δ6 desaturase.       

     Although ω3- and ω6-desaturases have been well studied in terms of substrate preference and regiospecificity, relatively little is known about the membrane-bound, “front-end” long chain fatty acid desaturases, such as Δ4-, Δ5- or Δ6 desaturases. For example, the first vertebrate Δ4 desaturase was only recently identified in the marine teleost fish  Siganus canaliculatus  ( S. canaliculatus ), which also possesses a bi-functional Δ5/Δ6 desaturase.  S. canaliculatus  Δ4 and Δ5/Δ6 desaturases are active with both ω3 and ω6 substrates: the Δ4 desaturase acts on 22:5ω3 and 22:4ω6 and the Δ5/Δ6 desaturase on 18:3ω3, 18:2ω6, 20:4ω3, and 20:3ω6 (Li et al., 2010). For instance, the  S. canaliculatus  Δ4 desaturase introduces a double bond at the Δ4 position of DPAω3, thereby producing DHAw3, whereas the  S. canaliculatus  Δ5/Δ6 desaturase introduces a double bond at the Δ6 position of alpha (α)-linolenic acid (ALA, 18:3ω3) to produce stearidonic acid (SDAω3, 18:4ω3). Both enzymes were shown to have a preference for ω3 fatty acids, in the yeast expression system (Li et al., 2010). Although  S. canaliculatus  Δ4 and Δ5/Δ6 desaturases are functionally different in terms of specificity for substrate chain-length and regioselectivity, these enzymes share a high degree of amino acid identity (83%). 
     As demonstrated in the following Examples, the inventors were able to identify specific regions and amino acid residues in the amino acid sequences of the  S. canaliculatus  Δ4 and Δ5/Δ6 desaturases which cause the functional divergence between these two highly similar desaturases. To this end, a series of chimeric Δ4 and Δ5/Δ6 desaturases of  S. canaliculatus  has been designed, in which regions of one enzyme have been replaced by the corresponding regions of the other, based on amino acid sequence comparisons between the Δ4 and Δ5/Δ6 genes. The inventors have then analyzed the effects of these mutations on the substrate specificity and regioselectivity of the enzymes by functional characterization, in yeast. Surprisingly, heterologous expression of the  S. canaliculatus  Δ4 and Δ5/Δ6 desaturase chimeric polypeptides in yeast indicated that the substitution of a four amino acid region was sufficient to convert a Δ5/6 desaturase to an enzyme with only Δ4 desaturase activity, and vice versa. Specifically, the inventors were able to identify a discrete protein region containing “YNYN” corresponding to position 280-283 in the amino acid sequence of the  S. canaliculatus  Δ4 desaturase (SEQ ID NO: 174), and the corresponding amino acid residues “FHYQ” at position 278-281 in the amino acid sequence of the  S. canaliculatus  Δ5/6-desaturase (SEQ ID NO: 172) which is responsible for both the substrate chain-length specificity and the regioselectivity of the  S. canaliculatus  Δ 4 and Δ 5/6 desaturases. Unexpectedly, the four critical amino acids influencing the substrate specificity and regioselectivity of the  S. canaliculatus  desaturases are predicted to be located in the third putative transmembrane domain. In addition, the inventors were able to produce enzymes having both Δ4- and Δ6 desaturase activity, by single or double amino acid substitutions within the mentioned four amino acid region. 
     As further illustrated in the following examples, the methods for the conversion of the substrate specificity of a desaturase provided by the present invention are not only limited to the  S. canaliculatus  desaturases but can be used for the conversion of the substrate specificity of any other Δ4, Δ5 and/or Δ6 desaturase as well, especially of “front-end” desaturases as defined herein. Accordingly, said methods of the invention can be used for rationally engineering the substrate specificity and regioselectivity of any desired Δ4, Δ5 and/or Δ6 desaturase, as appreciated by those skilled in the art. 
     In addition, the invention makes available Δ4, Δ5 and/or Δ6 desaturases with converted substrate specificity, i.e. engineered substrate specificity. Preferably, said desaturases are “front-end” desaturases, as defined elsewhere herein. Such enzymes are particularly appropriate for the production of very long chain polyunsaturated fatty acids (VLCPUFAs), including, for example, DHA, arachidonic acid (ARA), eicosapentaenoic acid (EPA), gamma-linolenic acid (GLA) and stearidonic acid (SDA). These VLCPUFAs have profound effects on the structural function of membranes, and also have important health effects in humans. DHA and ARA are considered crucial in the development of the brain and central nervous system, and deficiencies of DHA and ARA may adversely affect pre- and postnatal development (Crawfort 2000; Rapoport, 2008; Ahmann et al., 2011). VLCPUFAs also induce broad anti-inflammatory responses (Ahmann et al., 2011; Pollak et al., 2012) and consumption of EPA and DHA leads to a reduction in nonfatal and fatal cardiovascular disease events (Thies et al., 2003). Thus, the health benefits provided by these fatty acids make them important products for nutraceutical and pharmaceutical industries. 
     The term “polypeptide” or “protein” as used herein refers to any chain of amino acids regardless of length or post-translational modification such as glycosylation, palmitoylation, myristoylation or phosphorylation. Considerations for choosing a specific polypeptide having an engineered substrate specificity include, but are not limited to, the pH optimum of the polypeptide, whether the polypeptide is a rate-limiting enzyme or a component thereof, whether the polypeptide used is essential for synthesis of a desired PUFA, and/or whether a co-factor is required by the polypeptide. The expressed polypeptide preferably has characteristics that are compatible with the biochemical environment of its location in the host cell. For example, the polypeptide may have to compete for substrate(s). Analysis of the K m  and specific activity or substrate specificity of a polypeptide in question may be considered in determining the suitability of a given polypeptide for modifying PUFA(s) production, level or profile in a given host cell referred to elsewhere herein. A polypeptide used in a particular situation is one which typically can function under the conditions present in the intended host cell, but otherwise may be any polypeptide with an engineered substrate specificity as described herein and/or being capable of modifying the relative production, level or profile of a/the desired PUFA(s) or any other desired characteristics as discussed herein. The substrate may be naturally or recombinantly produced by the host cell or may be exogenously supplied. 
     A “desaturase” as referred to herein is a special type of oxygenase that can remove two hydrogens from a hydrocarbon chain, especially from a fatty acyl chain, catalyzing the formation of a double bond in the substrate. Unlike normal oxygenases which directly transfer molecular oxygen to a substrate, a desaturase uses activated molecular oxygen to abstract hydrogens from the substrate creating a carbon/carbon double bond in a fatty acid and a molecule of water. According to their regioselectivity, desaturases are typically categorized as delta (Δ)x desaturase that introduces a double bond at position “x” referred to from the carboxyl end of a fatty acid; or ωy desaturase that introduces a double bond at position “y” referred to from the methyl end. In addition, desaturases can additionally be labelled as v+z or v−z desaturases. The v+z desaturase introduces a double bond at “z” carbons after the pre-existing double bond v, i.e. towards the methyl end, while the v-z desaturase can introduce a double bond at “z” carbons before the pre-existing double bond v, i.e. towards the carboxyl end. An example of a Δx desaturase is the acyl-ACP Δ9 desaturase from plants, a soluble enzyme introducing a first Δ9 double bond into saturated palmitoyl-ACP or stearoyl-ACP. The membrane-bound ω3 desaturase from nematode  Caenorhabditis elegans  is an example of an ωy desaturase that inserts an ω3 double bond into a polyunsaturated fatty acid. An example of v+3 desaturase is the  Claviceps purpurea  Δ12 desaturase that has a preference for introducing double bonds at the Δ12 position, three carbons after the pre-existing double bond at the ninth position, while the Δ4 desaturase from  Thraustochytrium  is catalytically a v−3 desaturase and can only introduce a double bond at position 4 which is three carbons before the pre-existing double bond at the seventh position. 
     Based on the position of the double bond insertion relative to a pre-existing double bond in a fatty acyl chain, desaturases can also be referred to as “front-end” desaturases or methyl-end desaturases. Unsaturated fatty acids are essential for all living species in which the initial de novo fatty acid synthesis generally results in production of saturated fatty acids with 18 carbons or 16 carbons in length. The first double bond is often inserted at approximately the middle position of a fatty acid chain. Fatty acids with different chain length and double bond position are generated later by various fatty acid modifying enzymes, such as elongases and desaturases. A methyl-end desaturase introduces a double bond between the pre-existing double bond and the methyl-end, while a front-end desaturase inserts a double bond between the pre-existing double bond and the carboxyl end of a fatty acid. Commonly-found membrane-bound ωy and v+z desaturases such as Δ12, Δ15 and ω3 desaturases in plants are examples of methyl-end desaturases, while widely spread v-z desaturases in microorganisms such as Δ4, Δ5, Δ6 and Δ8 desaturases belong to “front-end” desaturases. 
     Although both methyl-end desaturases and “front-end” desaturases are involved in the biosynthesis of very long chain polyunsaturated fatty acids, their occurrence in living species is not identical. The former is widely present in plants and microorganisms, while the latter mostly occur in animals and microorganisms, although certain types of “front-end” desaturases have been identified in a small number of higher plants, such as borage, echium and conifers. Higher animals including humans lack the methyl-end desaturase such as Δ12, Δ15 and ω3 desaturase. Consequently, they cannot synthesize linoleic acid (LA, 18:2d9,12 or synonymously 18:2Δ9,12) and alpha-linolenic acid (ALA, 18:3d9,12,15) from oleic acid (OA, 18:1d9), the two essential fatty acids that have to be acquired from the diet. LA and ALA are precursors for the biosynthesis of very long chain polyunsaturated fatty acids such as arachidonic acid (20:4n-6, ARA), eicosapentaenoic acid (20:5n-3, EPA) and docosahexaenoic acid (22:6n-3, DHA). To synthesize these fatty acids, LA and ALA are desaturated by a first front-end desaturase—the Δ6 desaturase, introducing a Δ6 double bond into the substrates giving gamma-linolenic acid (GLA, 18:3d6,9,12), in the ω6 pathway, and stearidonic acid (SDA, 18:4d6,9,12,15) in the ω3 pathway, respectively. GLA and SDA are elongated by a Δ6 elongase to dihomo-gamma-linolenic acid (DGLA, 20:3d8,11,14) and eicosatetraenoic acid (ETA, 20:4d8,11,14,17), which are then desaturated by a second front-end desaturase—the Δ5 desaturase, giving rise to arachidonic acid (ARA, 20:4d5,8,11,14) and eicosapentaenoic acid (EPA, 20:5d5,8,11,14,17), respectively. EPA is elongated to docosapentaenoic acid (DPA, 22:5d7,10,13,16,19) which is then desaturated by a third front-end desaturase—the Δ4 desaturase, giving docosahexaenoic acid (DHA, 22:6d4,7,10,13,16,19), in the ω3 pathway. However, mammals including humans lack the Δ4 desaturase. Biosynthesis of DHA in mammals takes “the retro-conversion pathway” which involves two rounds of chain elongation of EPA and another Δ6 desaturation on the elongated product, followed by a single 2-carbon chain shortening of the Δ6 desaturated product in the peroxisome, giving DHA. The Δ8 desaturase is another front-end desaturase involved in the biosynthesis of very long chain polyunsaturated fatty acids. This desaturase works on a branching pathway of the biosynthesis, introducing a Δ8 double bond into elongated products LA or ALA, i.e. 20:2d11,14 or 20:3d11,14,17, giving rise to DGLA and ETA, respectively, which can then be desaturated by a Δ5 desaturase, giving ARA and EPA, as described above (see, e.g., Meesapyodsuk and Qiu, 2012). 
     Accordingly, the term “Δ5 desaturase” as used herein denotes a fatty acid desaturase which creates a double bond at the 5 th  position from the carboxyl group of a fatty acid. The term “Δ6 desaturase” means a desaturase which desaturates at the 6 th  position from the carboxyl group of a fatty acid, and the term “Δ4 desaturase” denotes a desaturase which desaturates at the 4 th  position from the carboxyl group of a fatty acid. The term “Δ5 and Δ6 desaturase” as referred to herein refers to a bi-functional delta-5/delta-6 desaturase, i.e. an enzyme which is capable of desaturating both at the 5 th  position and the 6 th  position from the carboxyl group of a fatty acid. The term “Δ4 and Δ6 desaturase” as referred to herein refers to a bi-functional delta-4/delta-6 desaturase, i.e. an enzyme which is capable of desaturating both at the 4 th  position and the 6th position from the carboxyl group of a fatty acid. 
     The term “desaturase activity” or “catalytic activity of a desaturase” as used in the present application refers to the enzymatic activity of a fatty acid desaturase that can catalyze the formation of a double bond between consecutive carbons of one or more fatty acids to produce a mono- or polyunsaturated fatty acid or precursor thereof, including enzymatic activity which desaturates at position 4 (i.e. Δ4 desaturase activity), position 5 (i.e. Δ5 desaturase activity), position 6 (i.e. Δ6 desaturase activity), positions 5 and 6 (i.e. Δ5/Δ6 desaturase activity), or positions 4 and 6 (i.e. Δ4/Δ6 desaturase activity), from the carboxyl group of a fatty acid. The enzymatic or catalytic activity (both terms are interchangeable) of a desaturase can be measured by methods described in the art; see, e.g., Hastings et al., 2001; Hastings et al., 2005; Zheng et al., 2005; Li et al., 2010. 
     The fatty acid desaturase can be derived from an animal, such as a vertebrate or a mammal, a plant, e.g. a higher plant such as borage, echium and conifers, or a microorganism, including prokaryotes, namely bacteria and archaea and various forms of eukaryotes, comprising the protozoa, fungi, algae, and microscopic plants (green algae). Further organisms from which the fatty acid desaturases referred to herein can be derived from are indicated in the following examples. The term “derived from” as used herein, means that the desaturase can be isolated, e.g., from an animal, plant or microorganism, by cloning methods known in the art and also described elsewhere herein and in the following examples. Alternatively, the sequences of the desaturases can be synthesized by chemical methods known in the art. By using sequence comparisons with known desaturase sequences and/or the identification of conserved domains of desaturases such as the His boxes and/or the cytochrome b 5 -like domain, further homolog desaturases can be isolated from any desired species or organism, using the mentioned methods. The desaturase activity can be tested by methods described in the art. The corresponding nucleic acid or amino acid sequences of said isolated or synthesized desaturases can be used to engineer the substrate specificity by the methods of the invention. For instance, chimeric polypeptides of different desaturases can be generated, optionally in combination with mutagenesis methods, in order to convert the substrate specificity of any desired desaturase. 
     The term “substrate” as used herein means one or more fatty acids on which a desaturase acts. Since the fatty acid substrate must fit into the active site of the desaturase before catalysis can occur, only properly designed fatty acids can serve as substrates for a specific desaturase, as known in the art. As set forth above, a fatty acid desaturase introduces double bonds at specific positions in fatty acids of defined chain lengths. Substrates of a respective desaturase in question are well described in the literature (see, e.g., Meesapyodsuk and Qiu, 2007; 2012; Li et al., 2010) and also referred to herein. 
     The term “substrate specificity” as used herein denotes the specificity of a desaturase (for example, a Δ4 desaturase) for one or more fatty acids (for example, DPA) of defined chain lengths (for example, C22) in which the formation of a double bond between consecutive carbons at specific positions (for example, position 4 or delta-4 or d4 as defined herein) is catalyzed in order to produce a mono- or polyunsaturated fatty acid or precursor thereof (for example, DHA. The term “regioselectivity” or “regiospecificity” of a desaturase as used herein means the selective insertion of a double bond at a specific position in the fatty acyl chain and, thus, can be considered as a particular aspect of the substrate specificity. For instance, the “regioselectivity” or “regiospecificity” of a Δ4 desaturase denotes the selective insertion of a double bond at position 4, in the fatty acyl chain. The term “fatty acid substrate specificity” or “specificity” of a desaturase as used herein refers to the type of fatty acid for which a desaturase shows highest activity in in vivo or in vitro enzymatic assays, when each tested fatty acid substrate is investigated individually in different assays. The term “fatty acid substrate selectivity”, or “selectivity” of a desaturase as used herein refers to the type of fatty acid for which a desaturase shows highest activity in in vivo or in vitro enzymatic assays, when all tested fatty acid substrates are investigated together in the same assays. The fatty acid substrate specificity and/or fatty acid substrate selectivity and/or, the fatty acid regioselectivity and/or fatty acid regioselectivity of a desaturase can be determined by methods known in the art (e.g. Domergue et al, 2002) and also shown in the following Examples. For instance, mutagenesis studies such as site-directed mutagenesis can be conducted to determine the structural basis for the substrate and double bond positional specificities displayed by a particular desaturase. Thereafter, functional expression of the mutated desaturase can be carried out, e.g., in yeast, in the presence of exogenously supplied fatty acids. The analysis of the desaturated fatty acids can be carried out, e.g., by gas chromatography. 
     Δ4, Δ5 and/or Δ6 desaturases can be active with both ω3 and ω6 fatty acid substrates, or can show preferred substrate specificity for either the ω3 or the ω6 fatty acid substrates. A Δ4 desaturase can act, e.g., on 22:5ω3 or 22:4ω6, a Δ5 desaturase on 20:4ω3 or 20:3ω6, a Δ6 desaturase on 18:3ω3 or 18:2ω6, and a Δ5/Δ6 desaturase on 20:4ω3, 20:3ω6, 18:3ω3 or 18:2ω6. 
     For example, LA and/or ALA can be desaturated by a Δ6 desaturase, introducing a Δ6 double bond into the substrates giving gamma-linolenic acid (GLA, 18:3d6,9,12), in the ω6 pathway, and stearidonic acid (SDA, 18:4d6,9,12,15) in the ω3 pathway, respectively. GLA and/or SDA can be elongated by a Δ6 elongase to dihomo-gamma-linolenic acid (DGLA, 20:3d8,11,14) in the ω6 pathway and/or eicosatetraenoic acid (ETA, 20:4d8,11,14,17) in the ω3 pathway, respectively. DGLA and/or ETA can then be desaturated by a Δ5 desaturase, giving rise to arachidonic acid (ARA, 20:4d5,8,11,14) in the ω6 pathway, and/or eicosapentaenoic acid (EPA, 20:5d5,8,11,14,17) in the ω3 pathway, respectively. ARA and/or EPA can then be elongated to docosatetraenoic acid (DTA, 22:5d7,10,13,16) in the ω6 pathway and/or omega-3 docosapentaenoic acid (DPA, 22:5d7,10,13,16,19) in the ω3 pathway, respectively. DTA and/or DPA can then be desaturated by a Δ4 desaturase, giving omega-3 docosapentaenoic acid (DPAn-3, 22:5d4,7,10,13,16), in the ω6 pathway, and/or docosahexaenoic acid (DHA, 22:6d4,7,10,13,16,19), in the ω3 pathway, respectively. Of particular interest in the context of the present invention, are polypeptides with converted, i.e. engineered, substrate specificity, which allow the conversion of the respective substrate to produce very long chain polyunsaturated fatty acids, such as gamma-linolenic acid (GLA, 18:3d6,9,12), stearidonic acid (SDA, 18:4d6,9,12,15), arachidonic acid (20:4n-6, ARA), eicosapentaenoic acid (20:5n-3, EPA) and/or docosahexaenoic acid (22:6n-3, DHA). 
     The term “identifying regions and/or amino acid residues which control the substrate specificity of a desaturase” referred to herein means to determine the specific parts or fragments of the amino acid sequence and/or specific amino acid residues of a desaturase which are mediating the specificity of said desaturase for (a) distinct fatty acid substrate(s). The determination of regions and/or amino acid residues which control the substrate specificity of a desaturase can be carried out by methods known in the art and include, for example, the generation of chimeric polypeptides, site-directed mutagenesis, crystal structure analysis, 3-D modeling, FIND technology (Alligator Bioscience) or DNA shuffling in directed evolution experiements, Structure-Based Combinatorial Protein Engineering (SCOPE, WO 2005/118861), staggered extension process, overlap extension PCR (‘sewing’ PCR) etc. For example, for the identification of regions that determine the differences in substrate specificity, a series of chimeric Δ4 and Δ5 and/or Δ6 desaturases can be produced in which regions of one enzyme are substituted by the corresponding regions of the other, based, for instance, on amino acid sequence comparisons between the Δ4 and Δ5 and/or Δ6 desaturases. Then, the effects of these mutations on the substrate specificity and/or catalytic activity of the enzymes can be determined by functional characterization of said chimeric polypeptides, e.g., in yeast, in comparison to the non-mutated enzymes. Upon heterologous expression of the chimeric series, e.g., in yeast, amino acid regions can be identified which are sufficient to convert, e.g., a Δ5 and/or Δ6 desaturase to an enzyme with only Δ4 desaturase activity, and vice versa. In addition, site-directed mutagenesis can be performed to identify specific amino acid residues within the mentioned regions which are relevant for the substrate specificity of a particular desaturase, as shown in the following Examples. 
     Thereafter, regions and/or amino acid residues which control the substrate specificity of, e.g., a Δ5 and/or Δ6 desaturase can be replaced in the amino acid sequence of said Δ5 and/or Δ6 desaturase, by the corresponding regions and/or amino acid residues which control the substrate specificity of a Δ4 desaturase, thereby converting completely or at least partially the substrate specificity of the Δ5 and/or Δ6 desaturase to the substrate specificity of a Δ4 desaturase. For example, a “complete conversion” of the substrate specificity of the Δ5 and/or Δ6 desaturase to the substrate specificity of a Δ4 desaturase means that such an enzyme exhibits only substrate specificity of the Δ4 desaturase, after replacement of the mentioned regions and/or amino acid residues which control the substrate specificity. Said enzyme is then no longer able to desaturate substrates of the Δ5 and/or Δ6 desaturase. Envisaged by the present application is also a “partial conversion” of the substrate specificity of the Δ5 and/or Δ6 desaturase to the substrate specificity of a Δ4 desaturase which means that such an enzyme exhibits both Δ4 desaturase substrate specificity and Δ5 and/or Δ6 desaturase substrate specificity, after replacement of the mentioned regions and/or amino acid residues that control the substrate specificity. 
     Alternatively, the regions and/or amino acid residues which control the substrate specificity of the Δ4 desaturase can be replaced in the amino acid sequence of the Δ4 desaturase, by the corresponding regions and/or amino acid residues which control the substrate specificity of the Δ5 and/or Δ6 desaturase, thereby converting the substrate specificity of the Δ4 desaturase to the substrate specificity of the Δ5 and/or Δ6 desaturase. 
     Alternatively, the regions and/or amino acid residues which control the substrate specificity of the Δ5 desaturase can be replaced in the amino acid sequence of the Δ5 desaturase, by the corresponding regions and/or amino acid residues which control the substrate specificity of the Δ6 desaturase, thereby converting the substrate specificity of the Δ5 desaturase to the substrate specificity of the Δ6 desaturase. Vice versa, the regions and/or amino acid residues which control the substrate specificity of the Δ6 desaturase can be replaced in the amino acid sequence of the Δ6 desaturase, by the corresponding regions and/or amino acid residues which control the substrate specificity of the Δ5 desaturase, thereby converting the substrate specificity of the Δ6 desaturase to the substrate specificity of the Δ5 desaturase. 
     Methods for this conversion of the substrate specificity of a particular Δ4, Δ5 and/or Δ 6 desaturases include conventional cloning methods known in the art, as described, e.g., in Sambrook et al., Molecular cloning. A Laboratory Manual (2001); or Ausubel, Current protocols in molecular biology (2003). Further methods such as “sewing” PCR are described in the following Examples. Accordingly, the term “conversion of substrate specificity” as used herein means altering the substrate specificity of a desaturase for one or more fatty acids of defined chain lengths in which the formation of a double bond between consecutive carbons at specific positions is being catalyzed, to one or more other, i.e. different fatty acids of defined chain lengths in which the formation of a double bond between consecutive carbons at other, i.e. different specific positions, is being catalyzed. The conversion of substrate specificity can concern the change of the substrate specificity of a particular desaturase for a distinct fatty acid substrate, the specific length of a fatty acid and/or the regioselectivity of the desaturase, i.e. the selective insertion of a double bond at a specific position in the fatty acyl chain. For example, a Δ4 desaturase can desaturate the fatty acid substrates 22:5ω3 and 22:4ω6 at position 4, whereas a Δ5 desaturase can act on 20:4ω3 and 20:3ω6 at position 5. Conversion of the substrate specificity of a Δ4 desaturase to the substrate specificity of a Δ5 desaturase means that the Δ4 desaturase is no longer able to desaturate the fatty acid substrates 22:5ω3 and 22:4ω6 at position 4, but instead is capable of desaturating the fatty acid substrates 20:4ω3 and 20:3ω6 at position 5. It is also envisaged by the invention that said Δ4 desaturase with converted substrate specificity is still able to desaturate the fatty acid substrates 22:5ω3 and 22:4ω6 at position 4, but is also capable of desaturating the fatty acid substrates 20:4ω3 and 20:3ω6 at position 5. The conversion of substrate specificity of a desaturase can be performed by methods known in the art, e.g., by designing a series of chimeric polypeptides of the Δ4 desaturase and the Δ5 desaturase, appropriately in combination with site-directed mutagenesis, as described in the following Exampies and in the literature; see, e.g., Cahoon et al., 1997; 1998. 
     In one aspect of the methods of the invention, the desaturase is an integral membrane fatty acid desaturase. In certain aspects, the desaturase is a microsomal fatty acid desaturase. In a specific aspect of the methods of the invention, the Δ4, Δ5 and/or Δ6 desaturase is a “front-end” desaturase which has high regioselectivity and substrate specificity, due to their structure (Meesapyodsuk and Qiu, 2012). For example, for the front-end borage Δ6 and Δ8 desaturases, the transmembrane domains and carboxyl termini were shown to play a major role in determining substrate specificity and regioselectivity, possibly by contributing to the formation of the substrate binding site (Libisch et al., 2000). “Front-end” desaturases characteristically contain a cytochrome b 5 -like domain at the N-terminus of the protein, and introduce a double bond between the carboxyl-end of the fatty acid and a pre-existing double bond. Three conserved histidine boxes, generally containing the consensus sequences H-X[3-4]-H, H-X[2-3]-H-H and H-X[2]-H-H, are found in almost all fatty acid desaturases (Pereira et al., 2003). These three histidine boxes form the active site of desaturases, and are involved in the formation of a di-iron complex (Sperling et al., 2001; Shanklin et al., 2009). In “front-end” desaturases, the first histidine residue in the third histidine box is usually replaced by glutamine. This glutamine residue appears to be essential for enzyme activity, since substituting glutamine in this position in a borage Δ6 desaturase with histidine, resulted in enzymatic activity being abolished (Sayanova et al., 2001). 
     In a more specific aspect of the methods of the invention, the “front-end” desaturase is derived from a plant, animal or microorganism. The animal can be a vertebrate such as a fish, for example  Siganus canaliculatus , Atlantic salmon, or zebrafish; a mammal such as baboon or human; a plant, e.g. an oil crop, borage, echium or conifers; or a microorganism, including prokaryotes, namely bacteria and archaea and various forms of eukaryotes, comprising the protozoa, fungi, algae, and microscopic plants (green algae). 
     For example, the Δ4 desaturases can be derived from  Monosiga brevicollis  (SEQ ID NO: 129 of WO 2012/052468, XP_001749702.1),  Euglena gracilis  (SEQ ID NO: 2 of WO 2004/090123),  Emiliana huxleyi  (SEQ ID NO: 6 of WO 2011/006948),  Pavlova lutheri  (AAQ98793.1),  Pavlova salina  or  Rebecca salina  (AAY15136.1),  Sphaeroforma arctica  (AGN91198.1),  Siganus canaliculatus  (SEQ ID NO: 174),  Thalassiosira pseudonana  (SEQ ID NO: 13 of WO 2005/080578), or  Thraustochytrium  sp. (SEQ ID NO: 2 of WO 2002/026946, AAM09688.1). 
     The Δ6 desaturases can be derived from Atlantic salmon (ADA56788.1, ADA56789.1, NP_001165251.1, NP_001165752.1),  Pythium irregulare  (SEQ ID NO: 8 of WO 2002/026946, AAL13310.1),  Mortierella alpina  (AAL73947.1),  Ostreococcus lucimarinus  (SEQ ID NO: 14 of WO 2008/040787, DAA34893.1),  Ostreococcus tauri  (SEQ ID NO: 90 of WO 2005/083093, XP_003082578.1),  Siganus canaliculatus  (SEQ-ID NO: 172) or  Primula farinosa  (SEQ ID NO: 2 of WO 2003/072784). 
     The Δ5 desaturase can be derived from  Thraustochytrium  sp. (SEQ-ID NO: 4 of WO 2002/026946, AAM09687.1),  Pavlova salina  or  Rebecca salina  (ABL96295.1),  Mortierella alpina AAC 72755.1), Atlantic salmon (AAL82631.2) or  Siganus canaliculatus  (SEQ-ID NO: 172). 
     The Δ5/Δ6 desaturase can be derived from  Siganus canaliculatus  (SEQ-ID NO: 172) or zebrafish ( Danio rerio : Q9DEX7.1). 
     Further “front-end” desaturases which can be used in the methods of the invention are described, e.g., in Meesapyodsuk and Qiu, 2012. 
     The nucleic acid molecules of the mentioned desaturases or of a part thereof can be isolated using molecular-biological standard techniques and the sequence information provided herein. Also, for example a homologous sequence or homologous, conserved sequence regions can be identified at the DNA or amino acid level with the aid of comparative algorithms. They can be used as hybridization probe and standard hybridization techniques (such as, for example, those described in Sambrook et al., Molecular Cloning: A Laboratory Manual. 2nd ed., Cold Spring Harbor Laboratory, Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y., 1989), preferably stringent hybridization conditions, for isolating further nucleic acid sequences which can be used in the methods of the invention. Moreover, nucleic acid molecules of the mentioned desaturases or of a part thereof can be isolated by polymerase chain reaction, where oligonucleotide primers which are used on the basis of this sequence or parts thereof (for example a nucleic acid molecule comprising the complete sequence or part thereof can be isolated by polymerase chain reaction using oligonucleotide primers which have been generated based on this same sequence). For example, mRNA can be isolated from cells (for example by means of the guanidinium thiocyanate extraction method of Chirgwin et al. (1979) Biochemistry 18:5294-5299) and cDNA by means of reverse transcriptase (for example Moloney MLV reverse transcriptase, available from Gibco/BRL, Bethesda, Md., or AMV reverse transcriptase, available from Seikagaku America, Inc., St. Petersburg, Fla.). Synthetic oligonucleotide primers for the amplification by means of polymerase chain reaction can be generated based on one of the sequences shown in the indicated accession numbers, the description, the references cited herein, the Examples or the sequence listing. A nucleic acid can be amplified by standard PCR amplification techniques using cDNA or, alternatively, genomic DNA as template and suitable oligonucleotide primers. The nucleic acid amplified thus can be cloned into a suitable vector and characterized by means of DNA sequence analysis. Oligonucleotides which correspond to a desaturase nucleotide sequence can be generated by standard synthetic methods, for example using an automatic DNA synthesizer. 
     In a further aspect of the methods of the invention, the Δ4, Δ5 and/or Δ6 desaturase have substrate specificity for both ω3 and ω6 substrates, preferably for ω3 substrates. 
     In certain aspects of the methods of the invention, the substrate specificity of the desaturase is as follows: 
     The substrate specificity of the Δ4 desaturase is for 22:5ω3 and 22:4ω6. If the substrate 22:5ω3 is desaturared by the Δ4 desaturase, the corresponding product is 22:6ω3. If the substrate 22:4ω6 is used as a substrate by the Δ4 desaturase, the corresponding product is 22:5ω6. 
     The substrate specificity of the Δ5 desaturase is for 20:4ω3 and 20:3ω6. If the substrate for the Δ5 desaturase is 20:4ω3, the corresponding product is 20:5ω3. If the substrate for the Δ5 desaturase is 20:3ω6, the corresponding product is 20:4ω6. 
     The substrate specificity of the Δ6 desaturase is for 18:3ω3 and 18:2ω6. If the substrate for the Δ6 desaturase is 18:3ω3, the corresponding product is 18:4ω3. If the substrate for the Δ6 desaturase is 18:2ω6, the corresponding product is 18:3ω6. 
     The substrate specificity of the Δ5 and/or Δ6 desaturase is for 20:4ω3, 20:3ω6, 18:3ω3 and 18:2ω6. The corresponding products generated by such a bi-functional desaturase have already been indicated when specifying the respective substrates and products of Δ5 desaturase and Δ6 desaturases. 
     In a specific aspect of the methods of the invention, the Δ4, Δ5 and/or Δ6 desaturase is a fish Δ4, Δ5 and/or Δ6 desaturase, for example, derived from Atlantic salmon, zebrafish  Danio rerio  or  Siganus canaliculatus . In a more specific aspect, the Δ4, Δ5 and/or Δ6 desaturase is a  Siganus canaliculatus  Δ4, Δ5 and/or Δ6 desaturase. The corresponding sequences of the  S. canaliculatus  Δ4 desaturase is shown in GenBank accession no. GU594278.1. Further, the corresponding sequence of the Δ5/Δ6 desaturase of  S. canaliculatus  is shown in GenBank accession no. EF424276.2. The cDNA of the Δ4 desaturase of  S. canaliculatus  is 1831 bp in length (excluding polyA tail) and contains a 1338 bp open-reading frame (ORF). The deduced protein has 445 amino acids and is 82.7% identical to the Δ5/Δ6 desaturase of  S. canaliculatus , and 67.8%, 57.8% and 63.6% identical to  Danio rerio  Δ6/Δ5 (AF309556),  Homo sapiens  Δ5 (AF199596) and Δ6 (AF126799), respectively. The deduced polypeptide sequence of the Δ4 desaturase of  S. canaliculatus  has a number of characteristic features of microsomal fatty acyl desaturases (Fad) proteins, including three histidine boxes, an N-terminal cytochrome b5 domain containing the heme-binding motif, and two transmembrane regions. Phylogenetic analysis of fatty acyl desaturases with a variety of fatty acyl desaturases of other species shows that  S. canaliculatus  desaturases are most closely related to marine teleost Δ6 fatty acyl desaturases, and more distantly from lower eukaryotes Δ4 and Δ5 fatty acyl desaturases; Li et al., 2010. 
     In a still further aspect of the methods of the invention, the regions and/or amino acid residues which control the substrate specificity of the Δ4, Δ5 and/or Δ6 desaturase are localized within a transmembrane domain. Preferably, the transmembrane domain is the third transmembrane domain. 
     Unexpectedly, the regions and/or amino acid residues which control the substrate specificity of the Δ4, Δ5 and/or Δ6 desaturase of  S. canaliculatus  are predicted to be localized in the third putative transmembrane domain. 
     The putative protein topology of a Δ4, Δ5 and/or Δ6 desaturase can be determined by appropriate software known in the art and/or topology prediction methods such as TMHMM, HMMTOP, MEMSAT, TOPPRED, PHD, DAS-TMfilter, Phobius, PredictProtein, SOSUI, TMPred, or TOPCONS membrane topology prediction software. 
     In a specific aspect of the invention, the regions and/or amino acid residues which control the substrate specificity of the Δ4 desaturase comprises the amino acid sequence “YNYN” and the regions and/or amino acid residues which control the substrate specificity of the Δ5 and/or Δ6 desaturase comprise the amino acid sequence “FHYQ”. 
     In specific aspects of the methods of the invention, the amino acid sequences of the Δ5 desaturase and/or Δ6 desaturase and the Δ4 desaturase, have a sequence identity of at least 40%, at least 50%, at least 60%, at least 70%, at least 75%, at least 80%, at least 83%, at least 85%, or even at least 90%. 
     A number of Δ4 and Δ6 desaturases from non-vertebrate sources have previously been identified, including enzymes from  Thraustochytrium  sp.,  P. lutheri, T. pseudonana  and  E. gracilis  (Qiu et al., 2001; Meyer et al., 2003; Tonon et al., 2003; Bowler et al., 2008). However, the amino acid sequence similarity among these “front-end” desaturases tends to be low. For example,  Thraustochytrium  sp. Δ4 (AF 489589) and Δ5 (AF489588) desaturases share only 19% identity, and  Mortierella alpina  Δ5 (AF054824) and Δ6 desaturases (AF110510) have only 23% identity in the amino acid sequence. Thus, in order to localise regions affecting substrate specificity (including regiospecificity) in the amino acid sequences of desaturases, preferably “front-end” desaturases, which show only low sequence similarity, the analysis of the crystal structure and 3-D modeling can be required to localize the critical regions and/or amino acid residues necessary for interaction with and desaturation of the substrate. For understanding the molecular basis for chain-length recognition and positional placement of double bonds into fatty acids, chimeric and/or mutant enzymes can be designed based on amino acid sequence comparisons of homologous desaturases or on three-dimensional information from the crystal structure of a desaturase to be further analysed. With more closely related enzymes, construction of chimeric or mutant proteins can be sufficient, possibly in combination with site-directed mutagenesis, as demonstrated in the following examples. There, it is shown that the substrate and regiospecifities of fatty acid desaturases as exemplified for the  Siganus canaliculatus  Δ4 and Δ5/Δ6 desaturases can be modified by the replacement of specific amino acid residues. 
     The present invention pertains also to a Δ4 desaturase, Δ5 desaturase, Δ6 desaturase, or Δ5/Δ6 desaturase (i.e. bi-functional desaturase) in which the substrate specificity has been converted to the substrate specificity of another desaturase. More specifically, the present invention concerns a Δ5 and/or Δ6 desaturase in which the substrate specificity of the Δ5 and/or Δ6 desaturase has been converted to the substrate specificity of a Δ4 desaturase. In addition, the invention relates to a Δ4 desaturase in which the substrate specificity of the Δ4 desaturase has been converted to the substrate specificity of a Δ5 and/or Δ6 desaturase. For example, in order to generate a Δ4 desaturase in which the substrate specificity of the Δ4 desaturase is converted to the substrate specificity of a Δ5 and/or Δ6 desaturase, the regions and/or amino acid residues which control the substrate specificity of the Δ4 desaturase are being first identified, by methods described elsewhere herein. In addition, the regions and/or amino acid residues which control the substrate specificity of the Δ5 and/or Δ6 desaturase are being determined. Subsequently, the regions and/or amino acid residues which control the substrate specificity of the Δ4 desaturase are replaced by the corresponding regions and/or amino acid residues which control the substrate specificity of the Δ5 and/or Δ6 desaturase, in the amino acid sequence of the Δ4 desaturase. This replacement can be carried out, e.g., by recombinant DNA technology or chemical synthesis. Thereby, the substrate specificity of the Δ4 desaturase can be converted to the substrate specificity of the Δ5 and/or Δ6 desaturase. By using this strategy, it is possible to rationally engineer the substrate specificity and regioselectivity of any desired Δ4, Δ5 and/or Δ6 desaturase, preferably a “front-end” Δ4, Δ5 and/or Δ6 desaturase. 
     The conversion of the substrate specificity of a desaturase in question can be completely or at least partially. For instance, a complete conversion of the substrate specificity of the Δ4 desaturase to the substrate specificity of a Δ5 and/or Δ6 desaturase can mean that such an enzyme exhibits the substrate specificity of the 5 and/or Δ6 desaturase only, after replacement of the corresponding regions and/or amino acid residues which control the substrate specificity. Such an enzyme is then no longer able to desaturate substrates of the Δ4 desaturase and exhibits Δ5 and/or Δ6 desaturase activity merely, i.e. desaturates substrates of the Δ5 and/or Δ6 desaturase. Envisaged by the present application is also a partial conversion of the substrate specificity of the Δ4 desaturase to the substrate specificity of a Δ5 and/or Δ6 desaturase. This means that such an enzyme exhibits both Δ4 desaturase substrate specificity and Δ5 and/or Δ6 desaturase substrate specificity, after replacement of the mentioned regions and/or amino acid residues that control the substrate specificity. 
     In certain aspects, the desaturases with converted substrate specificity of the invention are obtainable or obtained by the methods of the invention. 
     As shown in the following Examples, the present inventors have found that the substitution of a four amino acid region was sufficient to convert a Δ5/6 desaturase to a Δ4 desaturase, and vice versa. A discrete protein region containing the four amino acid residues “YNYN” corresponding to position 280-283 in the amino acid sequence of the  S. canaliculatus  Δ4 desaturase (SEQ ID NO: 174) has been found to control the substrate specificity of said Δ4 desaturase. The corresponding amino acid residues “FHYQ” at position 278-281 in the amino acid sequence of the  S. canaliculatus  Δ5/6-desaturase (SEQ ID NO: 172) regulate both the substrate chain-length specificity and the regioselectivity, i.e. the substrate specificity, of the  S. canaliculatus  Δ 5/6 desaturase. Substitution of the Δ4 desaturase amino acid sequence “YNYN” to the Δ5/Δ6 desaturase sequence “FHYQ” at the same location (Sig-17C; SEQ ID NO: 16;  FIG. 6  left column) changed the substrate specificity from the Δ4 desaturase to the Δ5/Δ6 desaturase, DPA ω3  to ALA (12.5±0.8%). Likewise, altering the  S. canaliculatus  Δ5/Δ6 desaturase sequence from “FHYQ” to the corresponding “YNYN” sequence (Sig-18C; SEQ ID NO: 8;  FIG. 6  right column) resulted in a loss of Δ5/Δ6 desaturase activity but a gain in Δ4 desaturase activity (0.5±0.1%, DPA ω3 ). Thus, the four amino acids that regulate the substrate specificity of  S. canaliculatus  and Δ5/Δ6 desaturases are “YNYN” and the corresponding sequence “FHYQ”; see Example 2. Based on sequence alignments and topology predictions, the corresponding amino acid regions controlling the substrate specificity could be identified, in Δ4, Δ5 and/or Δ6 desaturases of other organisms as well, as illustrated in Example 3. By exchanging the corresponding amino acid regions regulating the substrate specificity shown in said Example, the substrate specificity could also be switched in said Δ4, Δ5 and/or Δ6 desaturases, indicating that the teaching from the  S. canaliculatus  Δ4 and Δ 5/6 desaturases could be readily transferred to other “front-end” desaturases. In particular, this has been exemplified for the Δ6 desaturases of  Ostreococcus tauri  and  Pythium  irregular, and the delta-5 desaturase of  Thraustochytrium  sp. 
     In one aspect, the present invention concerns a Δ5 and/or Δ6 desaturase in which the substrate specificity of the Δ5 and/or Δ6 desaturase has been converted to the substrate specificity of a desaturase, comprising an amino acid sequence selected from the group consisting of:
         a) an amino acid sequence as shown in SEQ ID NO: 2, 4, 6, or 8; and   b) an amino acid sequence at least 50% identical to SEQ ID NO: 2, 4, 6, or 8 which is capable of desaturating docosapentaenoic acid (DPA) at position Δ4.       

     In another aspect, the amino acid sequence referred to in b) has at least 50%, at least 60% and preferably at least 70%, 80%, or 90%, and more preferably at least 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99% or more sequence identity to the amino acid sequences of SEQ ID NO: 2, 4, 6, or 8 and is capable of desaturating docosapentaenoic acid (DPA) at position Δ4. 
     In a further aspect, the invention relates to a Δ4 desaturase in which the substrate specificity of the Δ4 desaturase has been converted to the substrate specificity of a Δ5 and/or Δ6 desaturase, comprising the amino acid sequence selected form the group consisting of:
         a) an amino acid sequence as shown in SEQ ID NO: 10, 12, 14, or 16; and   b) an amino acid sequence at least 50% identical to SEQ ID NO: 10, 12, 14, or 16, and capable of desaturating alpha-linolenic acid (ALA) at position Δ6.       

     In one aspect, the amino acid sequence referred to in b) has at least 50%, at least 60% and more preferably at least 70%, 80%, or 90%, and most preferably at least 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99% or more sequence identity to the amino acid sequences of SEQ ID NO: 10, 12, 14, or 16, and is capable of desaturating alpha-linolenic acid (ALA) at position Δ6. 
     In a specific aspect, the desaturase with converted substrate specificity is a chimeric polypeptide. Preferably, said chimeric polypeptide is artificial and not found in nature. 
     A Δ4 desaturase in which the substrate specificity is converted to the substrate specificity of a Δ5 and/or Δ6 desaturase, can comprise the complete or partial amino acid sequence of the Δ4 desaturase, except for the regions and/or amino acid residues which control the substrate specificity of the Δ4 desaturase which have been replaced by the corresponding regions and/or amino acid residues which control the substrate specificity of the Δ5 and/or Δ6 desaturase. Alternatively, a Δ5 and/or Δ6 desaturase in which the substrate specificity is converted to the substrate specificity of a Δ4 desaturase, comprises the amino acid sequence of the Δ5 and/or Δ6 desaturase, except for the regions and/or amino acid residues which control the substrate specificity of the Δ5 and/or Δ6 desaturase which have been replaced by the corresponding regions and/or amino acid residues which control the substrate specificity of the Δ4 desaturase. Accordingly, the desaturase with converted substrate specificity is a chimeric polypeptide in that it comprises the above mentioned amino acid sequences of at least two, three or even more different fatty acid desaturases, for example, of a Δ4 desaturase and a Δ5 desaturase, a Δ4 desaturase and a Δ6 desaturase, a Δ5 desaturase and Δ6 desaturase, or a Δ5/Δ6 (bi-functional) desaturase and a Δ4 desaturase. By using this strategy, it is possible to rationally engineer the substrate specificity and regioselectivity of any desired Δ4, Δ5 and/or Δ6 desaturase, preferably a “front-end” Δ4, Δ5 and/or Δ6 desaturase. 
     Desaturases which can be used for the generation of the chimeric polypeptides of the invention are described elsewhere herein. Said fatty acid desaturases can originate from the same organism or genetically different organisms, for example, the fatty acid desaturases can be derived from the same or different species. For instance, the Δ5 and/or Δ6 desaturase and the Δ4 desaturase can originate from the same species, such as  Siganus canaliculatus . Alternatively, the Δ5 and/or Δ6 desaturase and the Δ4 desaturase can be derived from different species, for example, the Δ4 desaturase can be a  Siganus canaliculatus  Δ4 desaturase and the Δ5 and/or Δ6 desaturase can be a zebrafish Δ5 and/or Δ6 desaturase. The amino acid sequence of the desaturase in question can be the complete amino acid sequence (except for the region and/or amino acid residues controlling the substrate specificity) or parts thereof. Parts thereof comprise fragments of at least 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 150, 200, 250 or even more amino acid residues. 
     The region(s) and/or amino acid residue(s) which control(s) the substrate specificity comprises or consists of at least one, two, three, four, five, six, seven, eight, nine, ten, 11, 12, 13, 14, 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 150, 200, 250 or even more amino acid residues. For instance, four amino acid residues regulate the substrate specificity of  S. canaliculatus  Δ4 and Δ5/Δ6 desaturases; see Example 2. As acknowledged by those skilled in the art, said four amino acids can be also comprised in longer fragments. Methods for identifying said region(s) and/or amino acid residue(s) which control(s) the substrate specificity are described elsewhere herein. 
     In order to generate, for example, chimeric polypeptides of a Δ4 desaturase in which the substrate specificity of said Δ4 desaturase is converted to the substrate specificity of a Δ5 and/or Δ6 desaturase, the regions and/or amino acid residues which control the substrate specificity of the Δ4 desaturase are being first identified by methods described elsewhere herein. In addition, the regions and/or amino acid residues which control the substrate specificity of the Δ5 and/or Δ6 desaturase is determined. Subsequently, the regions and/or amino acid residues which control the substrate specificity of the Δ4 desaturase are replaced by the corresponding regions and/or amino acid residues which control the substrate specificity of the Δ5 and/or Δ6 desaturase, in the amino acid sequence of the Δ4 desaturase, e.g., by recombinant DNA technology or chemical synthesis. Thereby, the substrate specificity of the Δ4 desaturase is converted to the substrate specificity of the Δ5 and/or Δ6 desaturase. This process can also be carried out vice versa. By using this strategy, it is possible to rationally engineer the substrate specificity and regioselectivity of any desired Δ4, Δ5 and/or Δ6 desaturase, preferably “front-end” desaturases. 
     In a still further aspect, the invention relates to a chimeric polypeptide having substrate specificity of a Δ4 desaturase and a Δ6 desaturase, comprising an amino acid sequence selected from the group consisting of:
         a) an amino acid sequence as shown in SEQ ID NO: 18, 20, 22, 24 or 26; and   b) an amino acid sequence at least 50% identical to SEQ ID NO: SEQ ID NO: 18, 20, 22, 24 or 26 and comprising the amino acid sequence capable of desaturating docosapentaenoic acid (DPA) at position Δ4 and capable of desaturating alpha-linolenic acid (ALA) at position Δ6.       

     In one aspect, the amino acid sequence referred to in b) has at least 60% and more preferably at least 70%, 80%, or 90%, and most preferably at least 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99% or more sequence identity to the amino acid sequences of SEQ ID NO: 18, 20, 22, 24 or 26 and is capable of desaturating docosapentaenoic acid (DPA) at position Δ4 and capable of desaturating alpha-linolenic acid (ALA) at position Δ6. 
     Surprisingly, bi-functional chimeric polypeptides having substrate specificity of a Δ4 desaturase and a Δ6 desaturase could be generated by the methods of the invention, as shown in the following Examples and in  FIG. 6  (see chimeric constructs Sig-22C to Sig-26C). Said bi-functional desaturases are able to desaturate the substrates ALA ω3 and DPA ω3 to produce SDA ω3 and DHA ω3, respectively. 
     In a specific aspect, the chimeric polypeptide is able to convert at least the substrates ALA ω3 and DPA ω3. Desaturation of these substrates by the novel desaturases of the invention having substrate specificity of a Δ4 desaturase and a Δ6 desaturase produces SDA ω3 and DHA ω3. 
     The definitions and embodiments presented with respect to the methods of the invention apply mutatis mutandis to the desaturases with converted substrate specificity of the invention, and vice versa. 
     Further, the present invention relates to the nucleic acid sequences encoding the amino acid sequences of the desaturases with converted substrate specificity, provided by the present invention. Further encompassed by the present invention are derivatives of said nucleic acid sequences having at least 50%, 60%, 70%, 75%, 80%, 85%, 90%, 95%, preferably at least 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99% or 100% sequence identity or homology with a nucleic acid sequence encoding the desaturases with converted substrate specificity, of the invention. Such derivatives can comprise also functional variants which can be obtained by deletion, insertion, or substitution of nucleotides from/into the sequence. Said variants, however, maintain the desired (i.e. engineered) substrate specificity of the encoded polypeptide. Such variants can be generated by mutagenesis methods known in the art (see, e.g., Sambrook; Ausubel, loc. cit). 
     “Homologues” means bacterial, fungal, plant or or animal homologues, as indicated elsewhere herein, truncated sequences, single-stranded DNA or RNA of the coding and non-coding DNA sequence and derivatives thereof. In order to determine the percentage of homology (=sequence identity of two amino acid sequences or nucleic acid sequences), the sequences are written one under the other for an optimal comparison (for example, gaps may be introduced into the sequence of a protein or of a nucleic acid in order to generate an optimal alignment with the other protein or the other nucleic acid). Then, the amino acid residues or nucleotides at the corresponding amino acid positions or nucleotide positions are compared. If a position in a sequence is occupied by the same amino acid residue or the same nucleotide as the corresponding position in the other sequence, then the molecules are homologous at this position, i.e. amino acid or nucleic acid “homology” as used in the present context corresponds to amino acid or nucleic acid “identity”. The percentage of homology or sequence identity between the two sequences is a function of the number of positions which the sequences share (i.e. percent homology=number of identical positions/total number of positions×100). The terms “homology” and “sequence identity” are therefore considered as synonymous. 
     The homology or sequence identity is preferably calculated over the entire amino acid or nucleic acid sequence region. The skilled worker has available a series of programs which are based on various algorithms for the comparison of various sequences. Here, the algorithms of Needleman and Wunsch or Smith and Waterman give particularly reliable results. The program PileUp (J. Mol. Evolution., 25, 351-360, 1987, Higgins et al., CABIOS, 5 1989: 151-153) or the programs Gap and BestFit [Needleman and Wunsch (J. Mol. Biol. 48; 443-453 (1970) and Smith and Waterman (Adv. Appl. Math. 2; 482-489 (1981)], which are part of the GCG software packet [Genetics Computer Group, 575 Science Drive, Madison, Wis., USA 53711 (1991)], can be used for the sequence alignment. The sequence homology values which are indicated above as a percentage can be determined over the entire sequence region using the program GAP and the following settings: Gap Weight: 50, Length Weight: 3, Average Match: 10.000 and Average Mismatch: 0.000. These settings can be used as standard settings for the sequence alignments. 
     The nucleic acid sequences of the invention which encodes a desaturase with converted substrate specificity provided by the invention, are advantageously introduced into an expression cassette which makes possible the expression of the nucleic acids in organisms such as microorganisms, animals or plants. 
     Therefore, in another aspect of the invention, there is provided a gene construct comprising a nucleic acid sequence which encodes a desaturase with converted substrate specificity of the invention, operably linked with one or more regulatory sequences. 
     In the expression cassette, the nucleic acid sequence which encodes a desaturase with converted substrate specificity, is linked operably with one or more regulatory sequences, advantageously for enhancing gene expression. These regulatory sequences are intended to make possible the specific expression of the genes and proteins. Depending on the host organism, this may mean, for example, that the gene is expressed and/or overexpressed only after induction has taken place, or else that it is expressed and/or overexpressed immediately. 
     For example, these regulatory sequences take the form of sequences to which inductors or repressors bind, thus controlling the expression of the nucleic acid. In addition to these novel regulatory sequences, or instead of these sequences, the natural regulatory elements of these sequences may still be present before the actual structural genes and, if appropriate, may have been genetically modified in such a way that their natural regulation is eliminated and the expression of the genes is enhanced. However, the expression cassette (=expression construct=gene construct) can also be simpler in construction, that is to say no additional regulatory signals have been inserted before the nucleic acid sequence or its derivatives, and the natural promoter together with its regulation was not removed. Instead, the natural regulatory sequence has been mutated in such a way that regulation no longer takes place and/or gene expression is enhanced. These modified promoters can also be positioned on their own before the natural gene in the form of part-sequences (=promoter with parts of the nucleic acid sequences used in accordance with the invention) in order to enhance the activity. Moreover, the gene construct may advantageously also comprise one or more what are known as enhancer sequences in operable linkage with the promoter, which make possible an enhanced expression of the nucleic acid sequence. Additional advantageous sequences such as further regulatory elements or terminator sequences may also be inserted at the 3′ end of the DNA sequences. The genes may be present in one or more copies of the expression cassette (=gene construct). Preferably, only one copy of the genes is present in each expression cassette. This gene construct or the gene constructs can be expressed together in the host organism. In this context, the gene construct(s) can be inserted in one or more vectors and be present in the cell in free form, or else be inserted in the genome. It is advantageous for the insertion of further genes in the genome when the genes to be expressed are present together in one gene construct. 
     In this context, the regulatory sequences or factors can, as described above, preferably have a positive effect on the gene expression of the genes introduced, thus enhancing it. Thus, an enhancement of the regulatory elements, advantageously at the transcriptional level, may take place by using strong transcription signals such as promoters and/or enhancers. In addition, however, enhanced translation is also possible, for example by improving the stability of the mRNA. 
     The regulatory sequences include, in particular, plant sequences such as promoter and terminator sequences. The constructs can advantageously be stably propagated in microorganisms, in particular in  E. coli  and  Agrobacterium tumefaciens , under selective conditions and make possible the transfer of heterologous DNA into plants or microorganisms. Useful regulatory sequences are present, for example, in promoters such as the cos, tac, trp, tet, trp-tet, Ipp, lac, lpp-lac, laclq, T7, T5, T3, gal, trc, ara, SP6, λ-PR or λ-PL promoter and are advantageously employed in Gram-negative bacteria. Further advantageous regulatory sequences are, for example, present in the Gram-positive promoters amy and SPO2, in the yeast or fungal promoters ADC1, MFα, AC, P-60, CYC1, GAPDH, TEF, rp28, ADH or in the plant promoters CaMV/35S [Franck et al., Cell 21 (1980) 285-294], PRP1 [Ward et al., Plant. Mol. Biol. 22 (1993)], SSU, OCS, lib4, usp, STLS1, B33, nos or in the ubiquitin or phaseolin promoter. Advantageous in this context are also inducible promoters, such as the promoters described in EP-A-0 388 186 (benzenesulfonamide-inducible), Plant J. 2, 1992:397-404 (Gatz et al., tetracycline-inducible), EP-A-0 335 528 (abscissic acid-inducible) or WO 93/21334 (ethanol- or cyclohexenol-inducible) promoters. Further suitable plant promoters are the cytosolic FBPase promoter or the ST-LSI promoter of potato (Stockhaus et al., EMBO J. 8, 1989, 2445), the  glycine max  phosphoribosyl pyrophosphate amidotransferase promoter (Genbank Accession No. U87999) or the node-specific promoter described in EP-A-0 249 676. Especially advantageous promoters are promoters which make possible the expression in tissues which are involved in the biosynthesis of fatty acids. Very especially advantageous are seed-specific promoters, such as the USP promoter as described, but also other promoters such as the LeB4, DC3, phaseolin or napin promoter. Further especially advantageous promoters are seed-specific promoters which can be used for monocotyledonous or dicotyledonous plants and which are described in U.S. Pat. No. 5,608,152 (oilseed rape napin promoter), WO 98/45461 ( Arabidopsis  oleosin promoter), U.S. Pat. No. 5,504,200 ( Phaseolus vulgaris  phaseolin promoter), WO 91/13980 ( Brassica  Bce4 promoter), by Baeumlein et al., Plant J., 2, 2, 1992:233-239 (LeB4 promoter from a legume), these promoters being suitable for dicots. Examples of promoters which are suitable for monocots are the barley lpt-2 or lpt-1 promoter (WO 95/15389 and WO 95/23230), the barley hordein promoter and other suitable promoters described in WO 99/16890. 
     In principle, it is possible to use all natural promoters together with their regulatory sequences, such as those mentioned above. It is also possible and advantageous to use synthetic promoters, either in addition or alone, in particular when they mediate seed-specific expression, such as those described in WO 99/16890. 
     In order to achieve a particular high unsaturated fatty acid content, especially in transgenic plants, the nucleic acid sequences encoding the desaturases with converted substrate specificity of the invention should advantageously be expressed in oil crops, in a seed-specific manner. To this end, seed-specific promoters can be used, or those promoters which are active in the embryo and/or in the endosperm. In principle, seed-specific promoters can be isolated both from dicotyledonous and from monocotyledonous plants. Preferred promoters are listed hereinbelow: USP (=unknown seed protein) and vicilin ( Vicia faba ) [Bäumlein et al., Mol. Gen Genet., 1991, 225(3)], napin (oilseed rape) [U.S. Pat. No. 5,608,152], acyl carrier protein (oilseed rape) [U.S. Pat. No. 5,315,001 and WO 92/18634], oleosin ( Arabidopsis thaliana ) [WO 98/45461 and WO 93/20216], phaseolin ( Phaseolus vulgaris ) [U.S. Pat. No. 5,504,200], Bce4 [WO 91/13980], legumines B4 (LegB4 promoter) [Bäumlein et al., Plant J., 2, 2, 1992], Lpt2 and lpt1 (barley) [WO 95/15389 and WO95/23230], seed-specific promoters from rice, maize and wheat [WO 99/16890], Amy32b, Amy 6-6 and aleurain [U.S. Pat. No. 5,677,474], Bce4 (oilseed rape) [U.S. Pat. No. 5,530,149], glycinin (soybean) [EP 571 741], phosphoenol pyruvate carboxylase (soybean) [JP 06/62870], ADR12-2 (soybean) [WO 98/08962], isocitrate lyase (oilseed rape) [U.S. Pat. No. 5,689,040] or α-amylase (barley) [EP 781 849]. 
     Plant gene expression can also be facilitated via a chemically inducible promoter (see review in Gatz 1997, Annu. Rev. Plant Physiol. Plant Mol. Biol., 48:89-108). Chemically inducible promoters are particularly suitable when it is desired that gene expression should take place in a time-specific manner. Examples of such promoters are a salicylic-acid-inducible promoter (WO 95/19443), a tetracycline-inducible promoter (Gatz et al. (1992) Plant J. 2, 397-404) and an ethanol-inducible promoter. 
     To ensure the stable integration of the genes into the transgenic plant over a plurality of generations, each of the nucleic acids which encode the desaturase with converted substrate specificity, optionally in combination with a Δ12 desaturase, ω3 desaturase, Δ9 elongase, Δ6 desaturase, Δ8 desaturase, Δ6 elongase, Δ5 desaturase, Δ5 elongase and/or M desaturase and which are used in the process should be expressed under the control of a separate promoter, preferably a promoter which differs from the other promoters, since repeating sequence motifs can lead to instability of the T-DNA, or to recombination events. In this context, the expression cassette is advantageously constructed in such a way that a promoter is followed by a suitable cleavage site, advantageously in a polylinker, for insertion of the nucleic acid to be expressed and, if appropriate, a terminator sequence is positioned behind the polylinker. This sequence is repeated several times, preferably three, four or five times, so that up to five genes can be combined in one construct and introduced into the transgenic plant in order to be expressed. Advantageously, the sequence is repeated up to three times. To express the nucleic acid sequences, the latter are inserted behind the promoter via a suitable cleavage site, for example in the polylinker. Advantageously, each nucleic acid sequence has its own promoter and, if appropriate, its own terminator sequence. Such advantageous constructs are disclosed, for example, in DE 101 02 337 or DE 101 02 338. However, it is also possible to insert a plurality of nucleic acid sequences behind a promoter and, if appropriate, before a terminator sequence. Here, the insertion site, or the sequence, of the inserted nucleic acids in the expression cassette is not of critical importance, that is to say a nucleic acid sequence can be inserted at the first or last position in the cassette without its expression being substantially influenced thereby. Advantageously, different promoters such as, for example, the USP, LegB4 or DC3 promoter, and different terminator sequences can be used in the expression cassette. However, it is also possible to use only one type of promoter in the cassette. This, however, may lead to undesired recombination events. 
     As described above, the transcription of the genes which have been introduced should advantageously be terminated by suitable terminator sequences at the 3′ end of the biosynthesis genes which have been introduced (behind the stop codon). An example of a sequence which can be used in this context is the OCS 1 terminator sequence. As is the case with the promoters, different terminator sequences should be used for each gene. 
     The gene construct of the present invention may also comprise biosynthesis genes of the fatty acid or lipid metabolism selected from the group acyl-CoA dehydrogenase(s), acyl-ACP [=acyl carrier protein] desaturase(s), acyl-ACP thioesterase(s), fatty acid acyltransferase(s), acyl-CoA:lysophospholipid acyltransferase(s), fatty acid synthase(s), fatty acid hydroxylase(s), acetyl-coenzyme A carboxylase(s), acyl-coenzyme A oxidase(s), fatty acid desaturase(s), fatty acid acetylenases, lipoxygenases, triacylglycerol lipases, allenoxide synthases, hydroperoxide lyases or fatty acid elongase(s) and desaturase(s) such as M desaturase, Δ5 desaturase, Δ6 desaturase, Δ8 desaturase, Δ9 desaturase, Δ12 desaturase or Δ6 elongase. 
     These additional nucleic acids or genes can be cloned into the expression cassettes, which are then used for transforming plants with the aid of vectors such as  Agrobacterium.    
     Here, the regulatory sequences or factors can, as described above, preferably have a positive effect on, and thus enhance, the expression genes which have been introduced. Thus, enhancement of the regulatory elements can advantageously take place at the transcriptional level by using strong transcription signals such as promoters and/or enhancers. However, an enhanced translation is also possible, for example by improving the stability of the mRNA. In principle, the expression cassettes can be used directly for introduction into the plants or else be introduced into a vector. 
     Therefore, in yet another aspect of the invention, there is provided a vector comprising a nucleic acid or a gene construct encoding the desaturase with converted substrate specificity in any of the aspects of the invention described above. 
     In one embodiment, the vector may be a cloning vector. 
     The nucleic acid sequences encoding the desaturases with converted substrate specificity of the invention may be introduced alone, or preferably, in combination with an expression cassette (nucleic acid construct) into an organism. To introduce the nucleic acids, the latter are advantageously amplified and ligated in the known manner. Preferably, a procedure following the protocol for Pfu DNA polymerase or a Pfu/Taq DNA polymerase mixture is followed. The primers are selected taking into consideration the sequence to be amplified. The primers should advantageously be chosen in such a way that the amplificate comprises the entire codogenic sequence from the start codon to the stop codon. After the amplification, the amplificate is expediently analyzed. For example, a gel-electrophoretic separation can be carried out, which is followed by a quantitative and a qualitative analysis. Thereafter, the amplificate can be purified following a standard protocol (for example Qiagen). An aliquot of the purified amplificate is then available for the subsequent cloning step. 
     Suitable cloning vectors are generally known to the skilled worker. These include, in particular, vectors which are capable of replication in microbial systems, that is to say mainly vectors which ensure efficient cloning in yeasts or fungi and which make possible the stable transformation of plants. Those which must be mentioned in particular are various binary and cointegrated vector systems which are suitable for the T-DNA-mediated transformation. Such vector systems are, as a rule, characterized in that they comprise at least the vir genes required for the  Agrobacterium -mediated transformation and the T-DNA-delimiting sequences (T-DNA border). These vector systems advantageously also comprise further cis-regulatory regions such as promoters and terminator sequences and/or selection markers, by means of which suitably transformed organisms can be identified. While in the case of cointegrated vector systems vir genes and T-DNA sequences are arranged on the same vector, binary systems are based on at least two vectors, one of which bears vir genes, but no T-DNA, while a second one bears T-DNA, but no vir gene. Owing to this fact, the last-mentioned vectors are relatively small, easy to manipulate and to replicate both in  E. coli  and in  Agrobacterium . These binary vectors include vectors from the series pBIB-HYG, pPZP, pBecks, pGreen. In accordance with the invention, Bin19, pB1101, pBinAR, pGPTV and pCAMBIA are used by preference. An overview of the binary vectors and their use is found in Hellens et al, Trends in Plant Science (2000) 5, 446-451. In order to prepare the vectors, the vectors can first be linearized with restriction endonuclease(s) and then modified enzymatically in a suitable manner. Thereafter, the vector is purified, and an aliquot is employed for the cloning step. In the cloning step, the enzymatically cleaved and, if appropriate, purified amplificate is cloned with vector fragments which have been prepared, in a similar manner using ligase. In this context, a particular nucleic acid construct, or vector or plasmid construct, can have one or else more than one codogenic gene segment. The codogenic gene segments in these constructs are preferably linked operably with regulatory sequences. The regulatory sequences include, in particular, plant sequences such as the above-described promoters and terminator sequences. The constructs can advantageously be stably propagated in microorganisms, in particular in  E. coli  and  Agrobacterium tumefaciens , under selective conditions and make possible the transfer of heterologous DNA into plants or microorganisms. 
     The nucleic acids used in the process, the inventive nucleic acids and nucleic acid constructs, can be introduced into organisms such as microorganisms or advantageously plants, advantageously using cloning vectors, and thus be used in the transformation of plants such as those which are published and cited in: Plant Molecular Biology and Biotechnology (CRC Press, Boca Raton, Fla.), Chapter 6/7, p. 71-119 (1993); F. F. White, Vectors for Gene Transfer in Higher Plants; in: Transgenic Plants, Vol. 1, Engineering and Utilization, Ed.: Kung and R. Wu, Academic Press, 1993, 15-38; B. Jenes et al., Techniques for Gene Transfer, in: Transgenic Plants, Vol. 1, Engineering and Utilization, Ed.: Kung and R. Wu, Academic Press (1993), 128-143; Potrykus, Annu. Rev. Plant Physiol. Plant Molec. Biol. 42 (1991), 205-225. Thus, the nucleic acids, the inventive nucleic acids and nucleic acid constructs, and/or vectors used in the process can be used for the recombinant modification of a broad spectrum of organisms, advantageously plants, so that the latter become better and/or more efficient producers of unsaturated fatty acids, as referred to herein. 
     A series of mechanisms exist by which a modification of the desaturase with converted substrate specificity is possible, so that the yield, production and/or production efficiency of the advantageous unsaturated fatty acids in a plant, preferably in an oil crop plant or a microorganism, can be influenced directly owing to this modified desaturase protein. The number or activity of the proteins or genes can be increased, so that greater amounts of the gene products and, ultimately, greater amounts of the unsaturated fatty acids as referred to herein are produced. A de novo synthesis in an organism which has lacked the activity and ability to biosynthesize the fatty acids prior to introduction of the corresponding gene(s) is also possible. This applies analogously to the combination with further desaturases or elongases or further enzymes of the fatty acid and lipid metabolism. The use of various divergent sequences, i.e. sequences which differ at the DNA sequence level, may also be advantageous in this context, or else the use of promoters for gene expression which make possible a different gene expression in the course of time, for example as a function of the degree of maturity of a seed or an oil-storing tissue. 
     Owing to the introduction of a gene or nucleic acid coding for a desaturase with converted substrate specificity into an organism, alone or in combination with other genes in a cell, it is not only possible to increase biosynthesis flux towards the end product, but also to increase, or to create de novo the corresponding triacylglycerol composition. Likewise, the number or activity of other genes which are involved in the import of nutrients which are required for the biosynthesis of one or more fatty acids, oils, polar and/or neutral lipids, can be increased, so that the concentration of these precursors, cofactors or intermediates within the cells or within the storage compartment is increased, whereby the ability of the cells to produce unsaturated fatty acids as described below is enhanced further. By optimizing the activity or increasing the number of one or more genes which are involved in the biosynthesis of these compounds, or by destroying the activity of one or more genes which are involved in the degradation of these compounds, an enhanced yield, production and/or efficiency of production of fatty acid and lipid molecules in organisms, advantageously in plants, is made possible. 
     In an alternative embodiment, the vector may be an expression vector designed to transform an organism in which the nucleic acid is to be expressed and the unsaturated fatty acid as referred to herein is synthesized. 
     These advantageous vectors, preferably expression vectors, comprise the nucleic acids which encode the desaturase with converted substrate specificity. 
     As used in the present context, the term “vector” refers to a nucleic acid molecule which is capable of transporting another nucleic acid to which it is bound. One type of vector is a “plasmid”, a circular double-stranded DNA loop into which additional DNA segments can be ligated. A further type of vector is a viral vector, it being possible for additional DNA segments to be ligated into the viral genome. Certain vectors are capable of autonomous replication in a host cell into which they have been introduced (for example bacterial vectors with bacterial replication origin). Other vectors are advantageously integrated into the genome of a host cell when they are introduced into the host cell, and thus replicate together with the host genome. Moreover, certain vectors can govern the expression of genes with which they are in operable linkage. These vectors are referred to in the present context as “expression vectors”. Usually, expression vectors which are suitable for DNA recombination techniques take the form of plasmids. 
     In the present description, where the term “plasmid” is used, it should be understood that plasmids can be substituted for other types of expression vector, such as viral vectors, which exert similar functions. Furthermore, the term “vector” is also intended to comprise other vectors with which the skilled worker is familiar, such as phages, viruses such as SV40, CMV, TMV, transposons, IS elements, phasmids, phagemids, cosmids, linear or circular DNA. 
     The recombinant expression vectors advantageously used in the process of producing PUFAs comprise the nucleic acids or gene construct of the invention in a form which is suitable for expressing the nucleic acids used in a host cell, which means that the recombinant expression vectors comprise one or more regulatory sequences, selected on the basis of the host cells used for the expression, which regulatory sequence(s) is/are linked operably with the nucleic acid sequence to be expressed. In a recombinant expression vector, “linked operably” means that the nucleotide sequence of interest is bound to the regulatory sequence(s) in such a way that the expression of the nucleotide sequence is possible and they are bound to each other in such a way that both sequences carry out the predicted function which is ascribed to the sequence (for example in an in-vitro transcription/translation system, or in a host cell if the vector is introduced into the host cell). The term “regulatory sequence” is intended to comprise promoters, enhancers and other expression control elements (for example polyadenylation signals). These regulatory sequences are described, for example, in Goeddel: Gene Expression Technology: Methods in Enzymology 185, Academic Press, San Diego, Calif. (1990), or see: Gruber and Crosby, in: Methods in Plant Molecular Biology and Biotechnolgy, CRC Press, Boca Raton, Fla., Ed.: Glick and Thompson, Chapter 7, 89-108, including the references cited therein. Regulatory sequences comprise those which govern the constitutive expression of a nucleotide sequence in many types of host cell and those which govern the direct expression of the nucleotide sequence only in specific host cells under specific conditions. The skilled worker knows that the design of the expression vector can depend on factors such as the choice of host cell to be transformed, the desired expression level of the protein and the like. 
     The recombinant expression vectors used can be designed for the expression of the desaturase with converted substrate specificity of the invention, in prokaryotic or eukaryotic cells. This is advantageous since intermediate steps of the vector construction are frequently carried out in microorganisms for the sake of simplicity. For example, the gene or nucleic acid coding for the desaturase with converted substrate specificity of the invention, can be expressed in bacterial cells, insect cells (using Baculovirus expression vectors), yeast and other fungal cells (see Romanos, M. A., et al. (1992) “Foreign gene expression in yeast: a review”, Yeast 8:423-488; van den Hondel, C. A. M. J. J., et al. (1991) “Heterologous gene expression in filamentous fungi”, in: More Gene Manipulations in Fungi, J. W. Bennet &amp; L. L. Lasure, Ed., pp. 396-428: Academic Press: San Diego; and van den Hondel, C. A. M. J. J., &amp; Punt, P. J. (1991) “Gene transfer systems and vector development for filamentous fungi, in: Applied Molecular Genetics of Fungi, Peberdy, J. F., et al., Ed., pp. 1-28, Cambridge University Press: Cambridge), algae (Falciatore et al., 1999, Marine Biotechnology.1, 3:239-251), ciliates of the types: Holotrichia, Peritrichia, Spirotrichia, Suctoria, Tetrahymena, Paramecium, Colpidium, Glaucoma, Platyophrya, Potomacus, Desaturaseudocohnilembus, Euplotes, Engelmaniella and Stylonychia, in particular of the genus Stylonychia lemnae, using vectors in a transformation method as described in WO 98/01572 and, preferably, in cells of multi-celled plants (see Schmidt, R. and Willmitzer, L. (1988) “High efficiency  Agrobacterium tumefaciens -mediated transformation of  Arabidopsis thaliana  leaf and cotyledon explants” Plant Cell Rep.: 583-586; Plant Molecular Biology and Biotechnology, C Press, Boca Raton, Fla., Chapter 6/7, pp. 71-119 (1993); F. F. White, B. Jenes et al., Techniques for Gene Transfer, in: Transgenic Plants, Vol. 1, Engineering and Utilization, Ed.: Kung and R. Wu, Academic Press (1993), 128-43; Potrykus, Annu. Rev. Plant Physiol. Plant Molec. Biol. 42 (1991), 205-225 (and references cited therein)). Suitable host cells are furthermore discussed in Goeddel, Gene Expression Technology: Methods in Enzymology 185, Academic Press, San Diego, Calif. (1990). As an alternative, the recombinant expression vector can be transcribed and translated in vitro, for example using T7-promoter regulatory sequences and T7-polymerase. 
     In most cases, the expression of proteins in prokaryotes involves the use of vectors comprising constitutive or inducible promoters which govern the expression of fusion or nonfusion proteins. Typical fusion expression vectors are, inter alia, pGEX (Pharmacia Biotech Inc; Smith, D. B., and Johnson, K. S. (1988) Gene 67:31-40), pMAL (New England Biolabs, Beverly, Mass.) and pRIT5 (Pharmacia, Piscataway, N.J.), where glutathione S-transferase (GST), maltose-E binding protein and protein A, respectively, is fused with the recombinant target protein. 
     Examples of suitable inducible nonfusion  E. coli  expression vectors are, inter alia, pTrc (Amann et al. (1988) Gene 69:301-315) and pET 11d (Studier et al., Gene Expression Technology: Methods in Enzymology 185, Academic Press, San Diego, Calif. (1990) 60-89). The target gene expression from the pTrc vector is based on the transcription from a hybrid trp-lac fusion promoter by the host RNA polymerase. The target gene expression from the vector pET 11d is based on the transcription of a T7-gn10-lac fusion promoter, which is mediated by a viral RNA polymerase (T7 gn1), which is coexpressed. This viral polymerase is provided by the host strains BL21 (DE3) or HMS174 (DE3) from a resident A-prophage which harbors a T7 gn1 gene under the transcriptional control of the lacUV 5 promoter. 
     Other vectors which are suitable for prokaryotic organisms are known to the skilled worker, these vectors are, for example in  E. coli  pLG338, pACYC184, the pBR series such as pBR322, the pUC series such as pUC18 or pUC19, the M113mp series, pKC30, pRep4, pHS1, pHS2, pPLc236, pMBL24, pLG200, pUR290, plN-III113-B1, λgt11 or pBdCI, in  Streptomyces  pIJ101, pIJ364, pIJ702 or pIJ361, in  Bacillus  pUB110, pC194 or pBD214, in  Corynebacterium  pSA77 or pAJ667. 
     In a further aspect, the expression vector is a yeast expression vector. Examples for vectors for expression in the yeast  S. cerevisiae  comprise pYeDesaturasec1 (Baldari et al. (1987) Embo J. 6:229-234), pMFa (Kurjan and Herskowitz (1982) Cell 30:933-943), pJRY88 (Schultz et al. (1987) Gene 54:113-123) and pYES2 (Invitrogen Corporation, San Diego, Calif.). Vectors and processes for the construction of vectors which are suitable for use in other fungi, such as the filamentous fungi, comprise those which are described in detail in: van den Hondel, C. A. M. J. J., &amp; Punt, P. J. (1991) “Gene transfer systems and vector development for filamentous fungi, in: Applied Molecular Genetics of fungi, J. F. Peberdy et al., Ed., pp. 1-28, Cambridge University Press: Cambridge, or in: More Gene Manipulations in Fungi [J. W. Bennet &amp; L. L. Lasure, Ed., pp. 396-428: Academic Press: San Diego]. Further suitable yeast vectors are, for example, pAG-1, YEp6, YEp13 or pEMBLYe23. 
     As an alternative, the desaturase with converted substrate specificity of the invention can be expressed in insect cells using Baculovirus vectors. Baculovirus expression vectors which are available for the expression of proteins in cultured insect cells (for example Sf9 cells) comprise the pAc series (Smith et al. (1983) Mol. Cell Biol. 3:2156-2165) and the pVL series (Lucklow and Summers (1989) Virology 170:31-39). 
     The abovementioned vectors are only a small overview over suitable vectors which are possible. Further plasmids are known to the skilled worker and are described, for example, in: Cloning Vectors (Ed. Pouwels, P. H., et al., Elsevier, Amsterdam-New York-Oxford, 1985, ISBN 0 444 904018). For further suitable expression systems for prokaryotic and eukaryotic cells, see the Chapters 16 and 17 in Sambrook, J., Fritsch, E. F., and Maniatis, T., Molecular Cloning: A Laboratory Manual, 2. edition, Cold Spring Harbor Laboratory, Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y., 1989. 
     In a further embodiment of the process, the desaturase with converted substrate specificity of the invention can be expressed in single-celled plant cells (such as algae), see Falciatore et al., 1999, Marine Biotechnology 1 (3):239-251 and references cited therein, and in plant cells from higher plants (for example spermatophytes such as arable crops). Examples of plant expression vectors comprise those which are described in detail in: Becker, D., Kemper, E., Schell, J., and Masterson, R. (1992) “New plant binary vectors with selectable markers located proximal to the left border”, Plant Mol. Biol. 20:1195-1197; and Bevan, M. W. (1984) “Binary  Agrobacterium  vectors for plant transformation”, Nucl. Acids Res. 12:8711-8721; Vectors for Gene Transfer in Higher Plants; in: Transgenic Plants, Vol. 1, Engineering and Utilization, Ed.: Kung and R. Wu, Academic Press, 1993, p. 15-38. 
     A plant expression cassette preferably comprises regulatory sequences which are capable of governing the expression of genes in plant cells and which are linked operably so that each sequence can fulfill its function, such as transcriptional termination, for example polyadenylation signals. Preferred polyadenylation signals are those which are derived from  Agrobacterium tumefaciens  T-DNA, such as gene 3 of the Ti plasmid pTiACH5 (Gielen et al., EMBO J. 3 (1984) 835 et seq.), which is known as octopine synthase, or functional equivalents thereof, but all other terminator sequences which are functionally active in plants are also suitable. 
     Since plant gene expression is very often not limited to the transcriptional level, a plant expression cassette preferably comprises other sequences which are linked operably, such as translation enhancers, for example the overdrive sequence, which enhances the tobacco mosaic virus 5′-untranslated leader sequence, which increases the protein/RNA ratio (Gallie et al., 1987, Nucl. Acids Research 15:8693-8711). 
     As described above, plant gene expression must be linked operably with a suitable promoter which triggers gene expression with the correct timing or in a cell- or tissue-specific manner. Utilizable promoters are constitutive promoters (Benfey et al., EMBO J. 8 (1989) 2195-2202), such as those which are derived from plant viruses, such as 35S CaMV (Franck et al., Cell 21 (1980) 285-294), 19S CaMV (see also U.S. Pat. No. 5,352,605 and WO 84/02913), or plant promoters, such as the promoter of the Rubisco subunit, which is described in U.S. Pat. No. 4,962,028. 
     Other preferred sequences for use in operable linkage in plant gene expression cassettes are targeting sequences, which are required for steering the gene product into its corresponding cell compartment (see a review in Kermode, Crit. Rev. Plant Sci. 15, 4 (1996) 285-423 and references cited therein), for example into the vacuole, into the nucleus, all types of plastids, such as amyloplasts, chloroplasts, chromoplasts, the extracellular space, the mitochondria, the endoplasmid reticulum, elaioplasts, peroxisomes and other compartments of plant cells. 
     As described above, plant gene expression can also be achieved via a chemically inducible promoter (see review in Gatz 1997, Annu. Rev. Plant Physiol. Plant Mol. Biol., 48:89-108). Chemically inducible promoters are particularly suitable when it is desired that the gene expression takes place in a time-specific manner. Examples of such promoters are a salicylicacid-inducible promoter (WO 95/19443), a tetracyclin-inducible promoter (Gatz et al. (1992) Plant J. 2, 397-404) and an ethanol-inducible promoter. 
     Promoters which respond to biotic or abiotic stress conditions are also suitable, for example the pathogen-induced PRP1 gene promoter (Ward et al., Plant. Mol. Biol. 22 (1993) 361-366), the heat-inducible tomato hsp80 promoter (U.S. Pat. No. 5,187,267), the chill-inducible potato alpha-amylase promoter (WO 96/12814) or the wound-inducible pinII promoter (EP-A-0 375 091). 
     Especially preferred are those promoters which bring about the gene expression in tissues and organs in which the biosynthesis of fatty acids, lipids and oils takes place, in seed cells, such as cells of the endosperm and of the developing embryo. Suitable promoters are the oilseed rape napin promoter (U.S. Pat. No. 5,608,152), the  Vicia faba  USP promoter (Baeumlein et al., Mol Gen Genet, 1991, 225 (3):459-67), the  Arabidopsis  oleosin promoter (WO 98/45461), the  Phaseolus vulgaris  phaseolin promoter (U.S. Pat. No. 5,504,200), the  Brassica  Bce4 promoter (WO 91/13980) or the legumine B4 promoter (LeB4; Baeumlein et al., 1992, Plant Journal, 2 (2):233-9), and promoters which bring about the seed-specific expression in monocotyledonous plants such as maize, barley, wheat, rye, rice and the like. Suitable noteworthy promoters are the barley Ipt2 or Ipt1 gene promoter (WO 95/15389 and WO 95/23230) or the promoters from the barley hordein gene, the rice glutelin gene, the rice oryzin gene, the rice prolamine gene, the wheat gliadine gene, the wheat glutelin gene, the maize zeine gene, the oat glutelin gene, the sorghum kasirin gene or the rye secalin gene, which are described in WO 99/16890. 
     Other promoters which are likewise especially suitable are those which bring about a plastidspecific expression, since plastids constitute the compartment in which the precursors and some end products of lipid biosynthesis are synthesized. Suitable promoters, such as the viral RNA polymerase promoter, are described in WO 95/16783 and WO 97/06250, and the clpP promoter from  Arabidopsis , described in WO 99/46394. 
     Vector DNA can be introduced into prokaryotic and eukaryotic cells via conventional transformation or transfection techniques. The terms “transformation” and “transfection”, conjugation and transduction, as used in the present context, are intended to comprise a multiplicity of methods known in the prior art for the introduction of foreign nucleic acid (for example DNA) into a host cell, including calcium phosphate or calcium chloride coprecipitation, DEAE-dextran-mediated transfection, lipofection, natural competence, chemically mediated transfer, electroporation or particle bombardment. Suitable methods for the transformation or transfection of host cells, including plant cells, can be found in Sambrook et al. (Molecular Cloning: A Laboratory Manual., 2nd ed., Cold Spring Harbor Laboratory, Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y., 1989) and other laboratory textbooks such as Methods in Molecular Biology, 1995, Vol. 44,  Agrobacterium  protocols, Ed.: Gartland and Davey, Humana Press, Totowa, N.J. 
     In a further aspect of the invention there is provided a transgenic non human organism comprising at least one nucleic acid, gene construct or vector according to a previous aspect of the invention. 
     The transgenic nonhuman organism may be a microorganism, a nonhuman animal or a plant. 
     Host cells which are suitable in principle for taking up the nucleic acid encoding the desaturase with converted substrate specificity of the invention, the gene product according to the invention or the vector according to the invention are all prokaryotic or eukaryotic organisms. The host organisms which are advantageously used are microorganisms such as fungi or yeasts, or plant cells, preferably plants or parts thereof. Fungi, yeasts or plants are preferably used, especially plants, for example plants such as oil crops, which are high in lipid compounds, such as oilseed rape, evening primrose, hemp, thistle, peanut, canola, linseed, soybean, safflower, sunflower, borage, or plants such as maize, wheat, rye, oats, triticale, rice, barley, cotton, cassava, pepper,  Tagetes , Solanacea plants such as potato, tobacco, eggplant and tomato,  Vicia  species, pea, alfalfa, bushy plants (coffee, cacao, tea),  Salix  species, trees (oil palm, coconut), and perennial grasses and fodder crops. Especially preferred plants according to the invention are oil crops such as soybean, peanut, oilseed rape, canola, linseed, hemp, evening primrose, sunflower, safflower, trees (oil palm, coconut). 
     The abovementioned nucleic acids encoding the desaturases with converted substrate specificity and desaturases with converted substrate specificity provided by the present invention can be used in a process for the modulation of the production of Δ4, Δ5, and/or Δ6 unsaturated fatty acids in transgenic organisms, advantageously in plants, such as maize, wheat, rye, oats, triticale, rice, barley, soybean, peanut, cotton,  Linum  species such as linseed or flax,  Brassica  species such as oilseed rape, canola and turnip rape, pepper, sunflower, borage, evening primrose and  Tagetes , Solanaceae plants such as potato, tobacco, eggplant and tomato,  Vicia  species, pea, cassava, alfalfa, bushy plants (coffee, cacao, tea),  Salix  species, trees (oil palm, coconut) and perennial grasses and fodder crops, either directly (for example when the overexpression or optimization of a fatty acid biosynthesis protein has a direct effect on the yield, production and/or production efficiency of the fatty acid from modified organisms) and/or can have an indirect effect which nevertheless leads to an enhanced yield, production and/or production efficiency of the Δ4, Δ5, and/or Δ6 unsaturated fatty acids or a reduction of undesired compounds (for example when the modulation of the metabolism of lipids and fatty acids, cofactors and enzymes lead to modifications of the yield, production and/or production efficiency or the composition of the desired compounds within the cells, which, in turn, can affect the production of one or more fatty acids as referred to herein). 
     The combination of various precursor molecules and biosynthesis enzymes leads to the production of various fatty acid molecules, which has a decisive effect on lipid composition, since Δ4, Δ5, and/or Δ6 unsaturated fatty acids are not only incorporated into triacylglycerol but also into membrane lipids. 
     Brassicaceae, Boraginaceae, Primulaceae, or Linaceae are particularly suitable for the production of PUFAs, for example stearidonic acid, eicosapentaenoic acid and docosahexaenoic acid. Linseed ( Linum usitatissimum ) is especially advantageously suitable for the production of PUFAs with the nucleic acid sequences encoding desaturases with converted substrate specificity according to the invention, advantageously, as described, in combination with further desaturases and elongases. 
     Lipid synthesis can be divided into two sections: the synthesis of fatty acids and their binding to sn-glycerol-3-phosphate, and the addition or modification of a polar head group. Usual lipids which are used in membranes comprise phospholipids, glycolipids, sphingolipids and phosphoglycerides. Fatty acid synthesis starts with the conversion of acetyl-CoA into malonylCoA by acetyl-CoA carboxylase or into acetyl-ACP by acetyl transacylase. After condensation reaction, these two product molecules together form acetoacetyl-ACP, which is converted via a series of condensation, reduction and dehydratization reactions so that a saturated fatty acid molecule with the desired chain length is obtained. The production of the unsaturated fatty acids from these molecules is catalyzed by specific desaturases, either aerobically by means of molecular oxygen or anaerobically (regarding the fatty acid synthesis in microorganisms, see F. C. Neidhardt et al. (1996)  E. coli  and  Salmonella . ASM Press: Washington, D.C., pp. 612-636 and references cited therein; Lengeler et al. (Ed.) (1999) Biology of Procaryotes. Thieme: Stuttgart, N.Y., and the references therein, and Magnuson, K., et al. (1993) Microbiological Reviews 57:522-542 and the references therein). To undergo the further elongation steps, the resulting phospholipid-bound fatty acids must be returned to the fatty acid CoA ester pool. This is made possible by acyl-CoA:lysophospholipid acyltransferases. Moreover, these enzymes are capable of transferring the elongated fatty acids from the CoA esters back to the phospholipids. If appropriate, this reaction sequence can be followed repeatedly. 
     The synthesized Δ4, Δ5, and/or Δ6 unsaturated fatty acids can be obtained in the form of the free fatty acid or in the form of their esters, for example in the form of their glycerides. 
     The term “glyceride” is understood as meaning glycerol esterified with one, two or three carboxyl radicals (mono-, di- or triglyceride). “Glyceride” is also understood as meaning a mixture of various glycerides. The glyceride or glyceride mixture may comprise further additions, for example free fatty acids, antioxidants, proteins, carbohydrates, vitamins and/or other substances. 
     For the purposes of the invention, a “glyceride” is furthermore understood as meaning glycerol derivatives. In addition to the above-described fatty acid glycerides, these also include glycerophospholipids and glyceroglycolipids. Preferred examples which may be mentioned in this context are the glycerophospholipids such as lecithin (phosphatidylcholine), cardiolipin, phosphatidylglycerol, phosphatidylserine and alkylacylglycerophospholipids. 
     Furthermore, fatty acids must subsequently be translocated to various modification sites and incorporated into the triacylglycerol storage lipid. A further important step in lipid synthesis is the transfer of fatty acids to the polar head groups, for example by glycerol fatty acid acyltransferase (see Frentzen, 1998, Lipid, 100(4-5):161-166). 
     Publications on plant fatty acid biosynthesis and on the desaturation, the lipid metabolism and the membrane transport of lipidic compounds, on beta-oxidation, fatty acid modification and cofactors, triacylglycerol storage and triacylglycerol assembly, including the references therein, see the following papers: Kinney, 1997, Genetic Engeneering, Ed.: J K Setlow, 19:149-166; Ohlrogge and Browse, 1995, Plant Cell 7:957-970; Shanklin and Cahoon, 1998, Annu. Rev. Plant Physiol. Plant Mol. Biol. 49:611-641; Voelker, 1996, Genetic Engeneering, Ed.: J K Setlow, 18:111-13; Gerhardt, 1992, Prog. Lipid R. 31:397-417; Gühnemann-Schäfer &amp; Kindl, 1995, Biochim. Biophys Acta 1256:181-186; Kunau et al., 1995, Prog. Lipid Res. 34:267-342; Stymne et al., 1993, in: Biochemistry and Molecular Biology of Membrane and Storage Lipids of Plants, Ed.: Murata and Somerville, Rockville, American Society of Plant Physiologists, 150-158, Murphy &amp; Ross 1998, Plant Journal. 13(1):1-16. 
     Phospholipids for the purposes of the invention are understood as meaning phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, phosphatidylglycerol and/or phosphatidylinositol, advantageously phosphatidylcholine. The terms “production” or “productivity” are known in the art and comprise the concentration of the fermentation product which is formed within a specific period of time and in a specific fermentation volume (for example kg of product per hour per liter). It also comprises the productivity within a plant cell or a plant, that is to say the content of the desired fatty acids produced in the process relative to the content of all fatty acids in this cell or plant. The term “production efficiency” comprises the time required for obtaining a specific production quantity (for example the time required by the cell to establish a certain throughput rate of a fine chemical). The term yield or product/carbon yield is known in the art and comprises the efficiency of the conversion of the carbon source into the product (i.e. the fine chemical). This is usually expressed for example as kg of product per kg of carbon source. By increasing the yield or production of the compound, the amount of the molecules obtained of this compound, or of the suitable molecules of this compound obtained in a specific culture quantity over a specified period of time is increased. The terms biosynthesis or biosynthetic pathway are known in the art and comprise the synthesis of a compound, preferably an organic compound, by a cell from intermediates, for example in a multi-step and strongly regulated process. The terms “catabolism” or “catabolic pathway” are known in the art and comprise the cleavage of a compound, preferably of an organic compound, by a cell to give catabolites (in more general terms, smaller or less complex molecules), for example in a multi-step and strongly regulated process. The term “metabolism” is known in the art and comprises the totality of the biochemical reactions which take place in an organism. The metabolism of a certain compound (for example the metabolism of a fatty acid) thus comprises the totality of the biosynthetic pathways, modification pathways and catabolic pathways of this compound in the cell which relate to this compound. 
     The invention further relates to a process for the production of a substance which has the structure shown in the general formula I hereinbelow 
                         
wherein the variables and substituents are as follows:
         R 1 =hydroxyl, coenzyme A (thioester), lysophosphatidylcholine, lysophosphatidylethanolamine, lysophosphatidylglycerol, lysodiphosphatidylglycerol, lysophosphatidylserine, lysophosphatidylinositol, sphingo base or a radical of the formula II       

     
       
         
         
             
             
         
       
         
         
           
             R 2 =hydrogen, phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, phosphatidylinositol or a saturated or unsaturated C 2  to C 24 -alkylcarbonyl, 
           
         
       
    
     R 3 =hydrogen, a saturated or unsaturated C 2  to C 24 -alkylcarbonyl, or R 2  and R 3  independently of one another are a radical of the formula Ia: 
     
       
         
         
             
             
         
       
         
         
           
             in which 
             n=2, 3, 4, 5, 6, 7 or 9, m=2, 3, 4, 5 or 6 and p=0 or 3;
 
and wherein the process comprises the cultivation of (i) a host cell expressing at least one desaturase with converted substrate specificity of the invention or (ii) of a transgenic, nonhuman organism expressing at least one desaturase with converted substrate specificity of the invention under conditions which permit the biosynthesis of the substance. Preferably, the abovementioned substance is provided in an amount of at least 1% by weight based on the total lipid content in the host cell or the transgenic organism. Further, the host cell or transgenic, nonhuman organism used in this process of the invention, can express two, three, four, five, six or more desaturases with converted substrate specificity of the invention.
 
           
         
       
    
     Preferred alkyl radicals R 2  which may be mentioned are substituted or unsubstituted, saturated or unsaturated C 2 -C 24 -alkylcarbonyl chains such as ethylcarbonyl, n-propylcarbonyl, n-butylcarbonyl, n-pentylcarbonyl, n-hexylcarbonyl, n-heptylcarbonyl, n-octylcarbonyl, n-nonylcarbonyl, n-decylcarbonyl, n-undecylcarbonyl, n-dodecylcarbonyl, n-tridecylcarbonyl, n-tetradecylcarbonyl, n-pentadecylcarbonyl, n-hexadecylcarbonyl, n-heptadecylcarbonyl, n-octadecylcarbonyl, n-nonadecylcarbonyl, n-eicosylcarbonyl, n-docosanylcarbonyl or n-tetracosanylcarbonyl, which comprise one or more double bonds. Saturated or unsaturated C 10 -C 22 -alkylcarbonyl radicals such as n-decylcarbonyl, n-undecylcarbonyl, n-dodecylcarbonyl, n-tridecylcarbonyl, n-tetradecylcarbonyl, n-pentadecylcarbonyl, n-hexadecylcarbonyl, n-heptadecylcarbonyl, n-octadecylcarbonyl, n-nonadecylcarbonyl, n-eicosylcarbonyl, n-docosanylcarbonyl or n-tetracosanylcarbonyl, which comprise one or more double bonds are preferred. Especially preferred are saturated and/or unsaturated C 10 -C 22 -alkylcarbonyl radicals such as C 10 -alkylcarbonyl, C 11 -alkylcarbonyl, C 12 -alkylcarbonyl, C 13 -alkylcarbonyl, C 14 -alkylcarbonyl, C 16 -alkylcarbonyl, C 18 -alkylcarbonyl, C 20 -alkylcarbonyl or C 22 -alkylcarbonyl radicals which comprise one or more double bonds. Very especially preferred are saturated or unsaturated C 16 -C 22 -alkylcarbonyl radicals such as C 16 -alkylcarbonyl, C 18 -alkylcarbonyl, C 20 -alkylcarbonyl or C 22 -alkylcarbonyl radicals which comprise one or more double bonds. These preferred radicals can comprise two, three, four, five or six double bonds. The especially preferred radicals with 20 or 22 carbon atoms in the fatty acid chain comprise up to six double bonds, preferably three, four, five or six double bonds, especially preferably five or six double bonds. All the abovementioned radicals are derived from the corresponding fatty acids. 
     Preferred alkyl radicals R 3  which may be mentioned are substituted or unsubstituted, saturated or unsaturated C 2 -C 24 -alkylcarbonyl chains such as ethylcarbonyl, n-propylcarbonyl, n-butylcarbonyl, n-pentylcarbonyl, n-hexylcarbonyl, n-heptylcarbonyl, n-octylcarbonyl, n-nonylcarbonyl, n-decylcarbonyl, n-undecylcarbonyl, n-dodecylcarbonyl, n-tridecylcarbonyl, n-tetradecylcarbonyl, n-pentadecylcarbonyl, n-hexadecylcarbonyl, n-heptadecylcarbonyl, n-octadecylcarbonyl, n-nonadecylcarbonyl, n-eicosylcarbonyl, n-docosanylcarbonyl or n-tetracosanylcarbonyl, which comprise one or more double bonds. Saturated or unsaturated C 10 -C 22 -alkylcarbonyl radicals such as n-decylcarbonyl, n-undecylcarbonyl, n-dodecylcarbonyl, n-tridecylcarbonyl, n-tetradecylcarbonyl, n-pentadecylcarbonyl, n-hexadecylcarbonyl, n-heptadecylcarbonyl, n-octadecylcarbonyl, n-nonadecylcarbonyl, n-eicosylcarbonyl, n-docosanylcarbonyl or n-tetracosanylcarbonyl, which comprise one or more double bonds are preferred. Especially preferred are saturated and/or unsaturated C 10 -C 22 -alkylcarbonyl radicals such as C 10 -alkylcarbonyl, C 11 -alkylcarbonyl, C 12 -alkylcarbonyl, C 13 -alkylcarbonyl, C 14 -alkylcarbonyl, C 16 -alkylcarbonyl, C 18 -alkylcarbonyl, C 20 -alkylcarbonyl or C 22 -alkylcarbonyl radicals which comprise one or more double bonds. Very especially preferred are saturated or unsaturated C 16 -C 22 -alkylcarbonyl radicals such as C 16 -alkylcarbonyl, C 18 -alkylcarbonyl, C 20 -alkylcarbonyl or C 22 -alkylcarbonyl radicals which comprise one or more double bonds. These preferred radicals can comprise two, three, four, five or six double bonds. The especially preferred radicals with 20 or 22 carbon atoms in the fatty acid chain comprise up to six double bonds, preferably three, four, five or six double bonds, especially preferably five or six double bonds. All the abovementioned radicals are derived from the corresponding fatty acids. 
     The abovementioned radicals of R 1 , R 2  and R 3  can be substituted by hydroxyl and/or epoxy groups and/or can comprise triple bonds. 
     The polyunsaturated fatty acids produced in the process according to the invention advantageously comprise at least two, advantageously three, four, five or six, double bonds. The fatty acids especially advantageously comprise four, five or six double bonds. Fatty acids produced in the process advantageously have 18, 20 or 22 C atoms in the fatty acid chain; the fatty acids preferably comprise 20 or 22 carbon atoms in the fatty acid chain. Saturated fatty acids are advantageously reacted to a minor degree, or not at all, with the nucleic acids used in the process. To a minor degree is to be understood as meaning that the saturated fatty acids are reacted with less than 5% of the activity, advantageously less than 3%, especially advantageously with less than 2%, very especially preferably with less than 1, 0.5, 0.25 or 0.125% in comparison with polyunsaturated fatty acids. These fatty acids which have been produced can be produced in the process as a single product or be present in a fatty acid mixture. 
     The radicals R 2  or R 3  in the general formulae II may be identical or non-identical, R2 and R3 preferably being a saturated or unsaturated C 18 -C 22 -alkylcarbonyl, especially preferably an unsaturated C 18 -, C 20 - or C 22 -alkylcarbonyl with at least two double bonds. 
     The polyunsaturated fatty acids produced in the process are advantageously bound in membrane lipids and/or triacylglycerides, but may also occur in the organisms as free fatty acids or else bound in the form of other fatty acid esters. In this context, they may be present as “pure products” or else advantageously in the form of mixtures of various fatty acids or mixtures of different glycerides. The various fatty acids which are bound in the triacylglycerides can be derived from short-chain fatty acids with 4 to 6 C atoms, medium-chain fatty acids with 8 to 12 C atoms or long-chain fatty acids with 14 to 24 C atoms; preferred are long-chain fatty acids, more preferably long-chain fatty acids LCPUFAs of C 18 -, C 20 - and/or C 22 -fatty acids. 
     The process according to the invention advantageously yields fatty acid esters with polyunsaturated C 18 -, C 20 - and/or C 22 -fatty acid molecules with at least two double bonds in the fatty acid ester, advantageously with at least three, four, five or six double bonds in the fatty acid ester, especially advantageously with at least five or six double bonds in the fatty acid ester and advantageously leads to the synthesis of linoleic acid (=LA, C18:2 Δ9,12 ), γ-linolenic acid (=GLA, C18:3 Δ6,9,12 ), stearidonic acid (=SDA, C18:4 Δ6,9,12,15 ), dihomo-γ-linolenic acid (=DGLA, 20:3 Δ8,11,14 ), ω3-eicosatetraenoic acid (=ETA, C20:4 Δ5,8,11,14 ), arachidonic acid (ARA, C20:4 Δ5,8,11,14 ), eicosapentaenoic acid (EPA, C20:5 Δ5,8,11,14,17 ), ω6-docosapentaenoic acid (C22:5 Δ7,10,13,16 ), ω6-docosatetraenoic acid (C22:4 Δ7,10,13,16 ), ω3-docosapentaenoic acid (=DPA, C22:5 Δ7,10,13,16,19 ), docosahexaenoic acid (=DHA, C22:6 Δ4,7,10,13,16,19 ) or mixtures of these, preferably ARA, EPA and/or DHA. ω3-Fatty acids such as EPA and/or DHA are very especially preferably produced. 
     The fatty acid esters with polyunsaturated C 18 -, C 20 - and/or C 22 -fatty acid molecules can be isolated in the form of an oil or lipid, for example in the form of compounds such as sphingolipids, phosphoglycerides, lipids, glycolipids such as glycosphingolipids, phospholipids such as phosphatidylethanolamine, phosphatidylcholine, phosphatidylserine, phosphatidylglycerol, phosphatidylinositol or diphosphatidylglycerol, monoacylglycerides, diacylglycerides, triacylglycerides or other fatty acid esters such as the acyl-coenzyme A esters which comprise the polyunsaturated fatty acids with at least two, three, four, five or six, preferably five or six double bonds, from the organisms which have been used for the preparation of the fatty acid esters; advantageously, they are isolated in the form of their diacylglycerides, triacylglycerides and/or in the form of phosphatidylcholine, especially preferably in the form of the triacylglycerides. In addition to these esters, the polyunsaturated fatty acids are also present in the organisms, advantageously the plants, as free fatty acids or bound in other compounds. As a rule, the various abovementioned compounds (fatty acid esters and free fatty acids) are present in the organisms with an approximate distribution of 80 to 90% by weight of triglycerides, 2 to 5% by weight of diglycerides, 5 to 10% by weight of monoglycerides, 1 to 5% by weight of free fatty acids, 2 to 8% by weight of phospholipids, the total of the various compounds amounting to 100% by weight. 
     The process according to the invention yields the LCPUFAs produced in a content of at least 3% by weight, advantageously at least 5% by weight, preferably at least 8% by weight, especially preferably at least 10% by weight, most preferably at least 15% by weight, based on the total fatty acids in the transgenic organisms, advantageously in a transgenic plant. In this context, it is advantageous to convert C 18 - and/or C 20 -fatty acids which are present in the host organisms to at least 10%, advantageously to at least 20%, especially advantageously to at least 30%, most advantageously to at least 40% to give the corresponding products such as DPA or DHA, to mention just two examples. The fatty acids are advantageously produced in bound form. These unsaturated fatty acids can, with the aid of the nucleic acids used in the process according to the invention, be positioned at the sn1, sn2 and/or sn3 position of the advantageously produced triglycerides. Since a plurality of reaction steps are performed by the starting compounds linoleic acid (C18:2) and linolenic acid (C18:3) in the process according to the invention, the end products of the process such as, for example, arachidonic acid (ARA), eicosapentaenoic acid (EPA), ω6-docosapentaenoic acid or DHA are not obtained as absolutely pure products; minor traces of the precursors are always present in the end product. If, for example, both linoleic acid and linolenic acid are present in the starting organism and the starting plant, the end products such as ARA, EPA or DHA are present as mixtures. The precursors should advantageously not amount to more than 20% by weight, preferably not to more than 15% by weight, especially preferably not to more than 10% by weight, most preferably not to more than 5% by weight, based on the amount of the end product in question. Advantageously, only ARA, EPA or only DHA, bound or as free acids, are produced as end products in a transgenic plant in the process according to the invention. If the compounds ARA, EPA and DHA are produced simultaneously, they are advantageously produced in a ratio of at least 3:2:1 (EPA:ARA:DHA). 
     Fatty acid esters or fatty acid mixtures produced by the process according to the invention advantageously comprise 6 to 15% of palmitic acid, 1 to 6% of stearic acid, 7-85% of oleic acid, 0.5 to 8% of vaccenic acid, 0.1 to 1% of arachic acid, 7 to 25% of saturated fatty acids, 8 to 85% of monounsaturated fatty acids and 60 to 85% of polyunsaturated fatty acids, in each case based on 100% and on the total fatty acid content of the organisms. Advantageous polyunsaturated fatty acid which is present in the fatty acid esters or fatty acid mixtures is preferably eicosapentaenoic acid. Moreover, the fatty acid esters or fatty acid mixtures which have been produced by the process of the invention advantageously comprise fatty acids selected from the group of the fatty acids erucic acid (13-docosaenoic acid), sterculic acid (9,10-methyleneoctadec-9-enoic acid), malvalic acid (8,9-methyleneheptadec-8-enoic acid), chaulmoogric acid (cyclopentenedodecanoic acid), furan fatty acid (9,12-epoxyoctadeca-9,11-dienoic acid), vernolic acid (9,10-epoxyoctadec-12-enoic acid), tarinic acid (6-octadecynoic acid), 6-nonadecynoic acid, santalbic acid (t11-octadecen-9-ynoic acid), 6,9-octadecenynoic acid, pyrulic acid (t10-heptadecen-8-ynoic acid), crepenynic acid (9-octadecen-12-ynoic acid), 13,14-dihydrooropheic acid, octadecen-13-ene-9,11-diynoic acid, petroselinic acid (cis-6-octadecenoic acid), 9c,12t-octadecadienoic acid, calendulic acid (8t10t12c-octadecatrienoic acid), catalpic acid (9t11t13c-octadecatrienoic acid), eleostearic acid (9c11t13t-octadecatrienoic acid), jacaric acid (8c10t12c-octadecatrienoic acid), punicic acid (9c11t13c-octadecatrienoic acid), parinaric acid (9c11t13t15c-octadecatetraenoic acid), pinolenic acid (all-cis-5,9,12-octadecatrienoic acid), laballenic acid (5,6-octadecadienallenic acid), ricinoleic acid (12-hydroxyoleic acid) and/or coriolic acid (13-hydroxy-9c,11t-octadecadienoic acid). The abovementioned fatty acids are, as a rule, advantageously only found in traces in the fatty acid esters or fatty acid mixtures produced by the process according to the invention, that is to say that, based on the total fatty acids, they occur to less than 30%, preferably to less than 25%, 24%, 23%, 22% or 21%, especially preferably to less than 20%, 15%, 10%, 9%, 8%, 7%, 6% or 5%, very especially preferably to less than 4%, 3%, 2% or 1%. The fatty acid esters or fatty acid mixtures produced by the process according to the invention advantageously comprise less than 0.1%, based on the total fatty acids, or no butyric acid, no cholesterol, no clupanodonic acid (=docosapentaenoic acid, C22:5 Δ4,8,12,15,21 ) and no nisinic acid (tetracosahexaenoic acid, C23:6 Δ3,8,12,15,18,21 ). 
     Chemically pure polyunsaturated fatty acids or fatty acid compositions can also be prepared by the processes described above. To this end, the fatty acids or the fatty acid compositions are isolated from the organism, such as the microorganisms or the plants or the culture medium in or on which the organisms have been grown, or from the organism and the culture medium, in a known manner, for example via extraction, distillation, crystallization, chromatography or combinations of these methods. These chemically pure fatty acids or fatty acid compositions are advantageous for applications in the food industry sector, the cosmetic industry sector and especially the pharmacological industry sector. 
     Vegetable oils rich in mono- or polyunsaturated fatty acids are important in human nutrition and can be used as renewable sources of industrial chemicals. The ability to manipulate carbon chain lengths and double bond positions of mono- or polyunsaturated fatty acids by the desaturases with converted substrate specificity of the invention offers advantageously a way of altering the physical properties (e.g., melting points) and commercial uses of conventional plant oils. Accordingly, the present invention provides methods and compositions for creation of transgenic organisms, such as transgenic plants, with improved or altered PUFA content. PUFA as used herein means polyunsaturated fatty acids which are also referred to as PUFAs, LCPUFA or LCPUFAs (polyunsaturated fatty acids, PUFA, long-chain polyunsaturated fatty acids, LCPUFA). The modification of the fatty acid content of an organism such as a plant presents many advantages including improved nutrition and health benefits. Modification of the fatty acid content can be used to achieve beneficial levels of profiles of desired PUFAs in plant, plant parts and plant products including plant seed oils. For example, when the desired PUFAs are produced in the seed tissue of a plant, the oil may be isolated from the seeds typically resulting in an oil high in desired PUFAs or an oil having a desired fatty acid content or profile, which may in turn be used to provide beneficial characteristics in food stuffs and other products. The desaturases having engineered substrate specificity of a Δ4, Δ5 and/or Δ6 desaturase generated by the methods of the invention and/or provided by the present invention can be advantageously used for the modification of the PUFA content of a cell, for example, modification of the PUFA content of a plant cell or plant cells. 
     The content of all the references, patent applications, patents and published patent applications cited in the present patent application is hereby incorporated by reference to the respective specific disclosure. 
    
    
     
       The Figures show: 
         FIG. 1 : Alignment of deduced amino acid sequences of  S. canaliculatus  Δ4 desaturase (GenBank accession number: GU594278.1; SEQ ID NO: 174) and Δ5/Δ6 desaturase (GenBank accession number: EF424276.2; SEQ ID NO: 172) generated by VectorNTI software. Non-identical residues are highlighted in grey. The cytochrome b 5 -like domain is underlined by a waved line. The four amino acid residues that determine the functional divergence between the two genes are shown in bold. Histidine boxes are underlined by dotted lines and the putative transmembrane domains are underlined with solid lines. 
         FIG. 2 : Putative protein topology of  S. canaliculatus  Δ4 and Δ5/Δ6 desaturases, based on TOPCONS membrane topology prediction software. The membrane topology was generated by restraining N- and C-terminal regions to the inside (cytoplasmic) area. H denotes histidine box. Stars indicate the approximate location of the four amino acid residues involved in substrate chain-length specificity and regiospecificity. 
         FIG. 3 : Gas chromatograms of yeast cells containing an empty pYES2.1/V5-His-TOPO vector. A, no fatty acid supplied; B, DPA supplied exogenously; C, ALA supplied exogenously. 
         FIG. 4 : Gas chromatograms of yeast cells expressing  S. canaliculatus  wild-type and mutated desaturases. A,  S. canaliculatus  Δ4 desaturase (SEQ ID NO: 174); B,  S. canaliculatus  Δ5/Δ6 desaturase (SEQ ID NO: 172); C, chimeric  S. canaliculatus  desaturase Sig-3C; D, chimeric  S. canaliculatus  desaturase Sig-4C; E, chimeric  S. canaliculatus  desaturase Sig-5C (SEQ ID NO: 10); F, chimeric  S. canaliculatus  desaturase Sig-6C (SEQ ID NO: 2); G, chimeric  S. canaliculatus  desaturase Sig-17C (SEQ ID NO: 16) H, chimeric  S. canaliculatus  desaturase Sig-18C (SEQ ID NO: 8). Top and bottom panels show results from cultures supplied with DPA and ALA respectively. 
         FIGS. 5 and 6 : Schematic representation of  S. canaliculatus  Δ4 and Δ5/Δ6 chimeric gene constructs with associated conversion percentages. Substitution of domains within enzymes.  FIG. 5 :  S. canaliculatus  Δ4 sequence (SEQ ID NO: 174) is represented as a white bar (left) and  S. canaliculatus  Δ5/Δ6 sequence (SEQ ID NO: 172) is represented as a black bar (right). 
         FIG. 6 : Substitutions within a critical eight amino acid region. The amino acid residues shown in each chimeric gene represent the start and end of the domain substitution. The conversion percentages are the mean of three individual assays ±SD.  S. canaliculatus  Δ4 sequence (SEQ ID NO: 174) is represented as a white bar and  S. canaliculatus  Δ5/Δ6 sequence (SEQ ID NO: 172) is represented as a black bar. The bold amino acid residues denote the residues which were changed. The conversion rates are the mean of three individual assays ±SD. 
         FIG. 7 : Amino acid substitutions introduced into the region of Δ4 and Δ5/Δ6 desaturases critical for substrate specificity. Residue numbering is based on the Δ4 and Δ5/Δ6 desaturase amino acid sequences “YNYN” and “FHYQ”, respectively. Residues from the Δ4 desaturase were substituted into the corresponding position of the Δ5/Δ6 sequence, and vice versa. 
         FIG. 8 : Primers used in this study for constructing the chimeric proteins of  S. canaliculatus  Δ4 and Δ5/Δ6 desaturases and site-directed mutagenesis. F: forward primer; R: reverse primer. 
         FIG. 9 : Allocation of the nucleic acid sequences and amino acid sequences of the chimeric  S. canaliculatus  proteins to the corresponding SEQ ID NOs. 
         FIG. 10 : Alignment of the amino acid sequences of the chimeric  S. canaliculatus  proteins indicated in  FIG. 9 . 
     
    
    
     The invention will now be illustrated by the following Examples which shall, however, not be construed as limiting the scope of the invention. 
     EXAMPLES 
     Example 1 
     Growth and induction media for yeast expression were purchased from MP Biologicals (Solon, Ohio). Fatty acid (DPA ω3  and ALA) substrates were purchased from Nu-Chek Prep Inc (Elysian, Minn.). Phusion high fidelity DNA polymerase was from New England Biolabs (Ipswich, Mass.). Taq polymerase was from Invitrogen (Burlington, ON). All HPLC grade solvents were purchased from EMD Inc. (Mississauga, ON) and Fisher Scientific (Ottawa, ON). All chemicals if not mentioned otherwise, were purchased from Sigma-Aldrich (Oakville, ON). 
     1.1  S. canaliculatus  Δ4 and Δ5/Δ6 Desaturases 
     Sequences encoding the  S. canaliculatus  Δ4 desaturase (GenBank accession number: GU594278.1; nucleic acid sequence shown in SEQ ID NO: 173 and amino acid sequence shown in SEQ ID NO: 174) and the Δ5/Δ6 desaturase (GenBank accession number: EF424276.2; nucleic acid sequence shown in SEQ ID NO: 171 and amino acid sequence shown in SEQ ID NO: 172) were synthesized by Life Technologies Corporation (Burlington, ON), based on the GenBank database sequences. Synthetic genes were cloned into pYES2.1/V5-His-TOPO (Invitrogen) and sequenced, prior to being used in further experiments. 
     1.2 Construction of Vectors 
     Chimeric enzymes were constructed from Δ4 and Δ5/Δ6 fatty acid desaturases by overlap extension PCR (‘sewing’ PCR), targeting regions in the open reading frame (ORF) of the desaturases. The positions for substitutions were rationally chosen by aligning  S. canaliculatus  Δ4 and Δ5/Δ6 desaturases to locate the areas with differences in amino acid composition. After regions of interest were identified, overlapping synthetic primers were designed so that fragments from different portions of the genes could be amplified then ‘sewn’ together to form a chimera gene or gene containing a specific site-directed mutation. The initial chimeric genes were constructed using the synthesized  S. canaliculatus  Δ4 and Δ5/Δ6 desaturases (Life Technologies) as the DNA template for PCR. Subsequent chimeric genes were constructed using either the  S. canaliculatus  Δ4 and Δ5/Δ6 desaturases or chimeric genes constructed in this study. The overlap extension PCR was performed using Phusion high fidelity DNA polymerase, a proofreading polymerase which produces blunt-ended DNA products. PCR parameters were as follows: initial denaturation for 2 min at 98° C., followed by 30 cycles of 98° C. denaturation for 30 s, 60° C. annealing for 30 s, and 72° C. extension for 90 s. After amplification, PCR products were subjected to electrophoresis on a 0.8% agarose gel. The DNA band with the appropriate size was cut from the gel and purified using an EZ-10 Spin Column DNA Gel Extraction Kit (Bio Basic Inc., Markham, ON), according to the manufacturer&#39;s instructions. Before cloning the purified DNA product into the pYES2.1/V5-His-TOPO yeast expression vector (Invitrogen), an additional post-amplification step using Taq polymerase was performed. This step added 3′ A-overhangs to the amplified DNA, which is necessary for TA cloning into the pYES2.1/V5-His-TOPO vector. The addition of a 3′ A-overhang was performed by adding 20 μL of gel purified DNA, 2.5 pL 10×PCR buffer minus Mg 2+ , 1.5 μL of 50 mM MgCl 2 , 0.2 mM dNTP and 2.5 units of Taq polymerase, followed by incubation at 72° C. for 10 min. The amplified DNA fragment was cloned into the pYES2.1/V5-His-TOPO vector according to the manufacturer&#39;s instructions, and the plasmid DNA was isolated using an EZ-10 spin column plasmid DNA miniprep kit (Bio Basic Inc.). After sequencing, plasmids were transformed into the  S. cerevisiae  yeast strain INVSc1 (Invitrogen) using the EasyComp transformation kit (Invitrogen). 
     1.3 Expression in Yeast 
     For functional expression of the desaturases, yeast cultures were grown overnight at 30° C. in DOB-URA (yeast synthetic complete media devoid of uracil) containing 2% glucose. The OD600 of the overnight yeast cultures were measured and standardized between samples. The samples were washed with DOB-URA containing 2% galactose and expression was induced with the same media. Expression was carried out for 3 days at 20° C. in the presence of exogenousiy supplied fatty acids (250 μM DPA ω3  or ALA depending on the yeast construct). 
     1.4 Fatty Acid Analysis 
     After 3 days of growth in the presence of exogenousiy supplied fatty acids, the yeast cells were collected by centrifugation at 2,800 rpm for 3 min and washed once with induction buffer (2% galactose) and washed again with sterile distilled water. Two mL of 3N-methanolic HCl was added to each sample and the samples were heated at 80° C. for 40 min, cooled at room temperature, and the hexane phase containing the fatty acid methyl esters was partitioned from the aqueous phase by the addition of 1 mL 0.9% NaCl and 2 mL hexane, followed by centrifugation. The hexane phase was transferred to glass vials and air-dried under a gentle stream of liquid nitrogen gas and was resuspended in 100 μL of hexane before being analyzed by gas chromatography (GC) from Agilent Technologies Inc. (model: 6890N, Integrated peaks calculated by Agilent ChemStation Software). Desaturation percent was calculated as:
 
100/(product/(substrate+product)).
 
     Example 2 
     2.1  S. canaliculatus  Δ4 and Δ5/Δ6 Desaturases have Similar Sequences and Structures 
     The deduced proteins of the Δ4 desaturase (SEQ ID NO: 174) and Δ5/Δ6 desaturase (SEQ ID NO: 174) genes of  S. canaliculatus  have 445 and 443 amino acids respectively, with the two amino acid gap located in the N-terminal region of the alignment ( FIG. 1 ). Based on membrane topology analysis (TOPCONS; restraining N- and C-terminal inside cytoplasmic area;  FIG. 2 ), both the  S. canaliculatus  Δ4 and Δ5/Δ6 genes have four predicted transmembrane domains with two linker regions. Both desaturases have three conserved histidine boxes, and an N-terminal cytochrome b 5 -like domain, which are the characteristic features of a front-end microsomal desaturase (Li et al., 2010). Due to the high similarity in amino acid sequence between the two desaturases, the inventors were able to rationally design primers to produce chimeras based on aligning the open reading frame (ORF) of the  S. canaliculatus  genes ( FIG. 1 ). Specific corresponding regions were selected and exchanged between the two desaturases to form chimeric proteins. Regions that were quite highly conserved, as well as less conserved regions were substituted. 
       FIG. 8  lists the primers and corresponding SEQ ID NOs. used for constructing the chimeras and for performing site-directed mutagenesis to identify the roles of specific amino acid residues. This set of primers allowed the inventors to perform substitutions of domains distributed in the hydrophobic transmembrane domains, C-terminal and N-terminal regions and linkers between transmembrane domains, based on the topology analysis shown in  FIG. 2 . 
     2.2 A Four Amino Acid Region within the  S. canaliculatus  Δ4 and Δ5/Δ6 Desaturases Controls Substrate Length and Regiospecificity 
     Both  S. canaliculatus  desaturases show a preference for ω3 fatty acids (Li et al., 2010). Therefore, the ω3 fatty acids DPA and ALA, which are substrates for the native Δ4- and Δ5/Δ6-desaturases respectively, were used throughout this study. As a negative control,  S. ceriviseae  INVSc1 cells were transformed with the empty pYES2.1/V5-His-TOPO vector. The fatty acids observed in the control included 14:0, 16:0, 16:1ω7, 18:0 and 18:1ω9 and 26:0, but neither Δ4 nor DPA ω3  activity was observed ( FIG. 3 ). Expression of wild-type  S. canaliculatus  Δ4 (SEQ ID NO: 174) and Δ5/Δ6 desaturases (SEQ ID NO: 172) in yeast showed desaturase activities of 12.0±0.5% (DPA ω3 ) and 22.7±2.5% (ALA), respectively ( FIGS. 4 and 5 ; Table 1). The Δ5/Δ6 desaturase did not show activity with DPA ω3 , nor did the Δ4 desaturase show activity with ALA ( FIGS. 4 and 5 ; Table 1). 
     When the  S. canaliculatus  Δ4 desaturase sequence from the amino acid sequence “MPRH” onward, representing approximately the last one-tenth of the enzyme, was replaced with the Δ5/Δ6 sequence, the enzyme maintained the same substrate specificity but the conversion level dropped by approximately 40% (Sig-1C;  FIG. 5  left column). The reverse construct (Sig-2C;  FIG. 5  right column) maintained the substrate specificity of the Δ5/Δ6 enzyme but fatty acid conversion was reduced by approximately 30%. Since the last tenth of the enzymes do not appear to contribute to substrate specificity, chimeric proteins with domain swapping from the “PVYG” amino acid location onward, representing approximately the last third of the enzyme, were constructed (Sig-3C;  FIG. 5  left column, and Sig-4C;  FIG. 5  right column). No change in substrate specificities was observed between the two chimeras but the enzyme activities of Sig-3C and 4C were reduced by approximately 30% and 45%, respectively, compared to wild-type (see also Table 1). 
     The regions from “LIPV” onward, representing approximately the last 40% of the coding regions, were exchanged in the corresponding chimeric constructs Sig-5C (SEQ ID NO: 10;  FIG. 5  left column) and Sig-6C (SEQ ID NO: 2;  FIG. 5  right column). The switch from Δ4 to Δ5/Δ6 sequence at “LIPV” caused Sig-5C to lose Δ4 activity but gain Δ6 activity, showing a conversion level of approximately 23% with ALA (Sig-5C; SEQ ID NO: 10;  FIG. 5  left column; Table 1). Conversely, the reverse construct lost 46 activity but gained activity with DPA (Sig-6C; SEQ ID NO: 2;  FIG. 5  right column; Table 1). Thus, the region responsible for substrate specificity appears to be located between the “LIPV” and “PVYG” regions (amino acid residues position: Δ4, 275-316; Δ5/Δ6, 273-314) of these enzymes, as shown in  FIG. 3A . 
     To confirm that the residues between “LIPV” to “PVYG” alone were responsible for substrate specificity, a pair of chimeric genes (Sig-7C; SEQ ID NO: 12;  FIG. 5  left column, and Sig-8C; SEQ ID NO: 4;  FIG. 5  right column) substituting only this region were constructed by PCR. 
     The gene containing the Δ5/Δ6 desaturase sequence in this area and the Δ4 desaturase sequence throughout the rest of the construct desaturated only ALA (11.5±1.2%) while the reverse construct had activity only with DPA (3.8±0.2%), demonstrating that the region between amino acids “LIPV” and “PVYG” controls substrate specificity. This area includes part of the 3 rd  and 4 th  predicted transmembrane domains along with the linker region ( FIG. 2 ). 
     To further dissect this region, chimeric genes were constructed that divided the region between “LIPV” and “PVYG” of the  S. canaliculatus  Δ4 and Δ5/Δ6 desaturase into 3 fractions (end of linker to 4 th  transmembrane domain: “WAMT/WCLS” to “PVYG”, linker region between 3 rd  and 4 th  transmembrane domain: “TMI/IMI” to “WAMT/WCLS” and 3 rd  transmembrane domain to start of linker region: “LIPVF” to “TMI/IMI”). 
     When expressed in yeast cells, Sig-9C, which is an  S. canaliculatus  Δ4 desaturase gene with the region encoding the amino acids from “WCLS” to “PVYG” substituted with the corresponding Δ5/Δ6 desaturase sequence ( FIG. 5  left column), converted 4.0±0.5% of DPA ω3  to DHA ω3 , while the reverse construct (Sig-10C;  FIG. 5  right column) converted ALA to SDA at a level of 14.9±1.4%, indicating that the 4th transmembrane domain is not relevant in substrate specificity but has an effect on the catalytic activities of the enzymes. 
     Moreover, Sig-11C ( FIG. 5  left column) and its reverse gene construct (Sig-12C;  FIG. 5  right column) which have sequence exchanges between the linker region of 3 rd  and 4th transmembrane domains (sequence from “TMI/IMI” to “WAMT/WCLS”), also retained their original substrate specificity although enzyme activity was reduced to about half of the respective wild-type genes ( FIG. 5 ). These data suggest that the linker region between 3 rd  and 4th transmembrane domain (“TMI/IMI” to “WAMT/WCLS”) on the  S. canaliculatus  Δ4 and Δ5/Δ6 desaturases are not the critical regions for substrate specificity although sequence switching in these regions greatly reduces catalytic activities. 
     However, when the region between the 3 rd  transmembrane domain and the start of linker region involving the sequence from “LIPV” to “IMI/TMI” (Sig-13C; SEQ ID NO: 14;  FIG. 5  left column, and Sig-14C; SEQ ID NO: 6;  FIG. 5  right column) were substituted, there was a shift in substrate specificity. The mutated  S. canaliculatus  Δ4 (Sig-13C; SEQ ID NO: 14;  FIG. 5  left column) desaturates only ALA (13.1±1.8%) but no longer desaturates DPA ω3 . Therefore this mutation allowed the gene to gain Δ5/Δ6 activity but completely abolished the Δ4 activity. In contrast, the reverse construct (Sig-14C; SEQ ID NO: 6;  FIG. 5  right column) gained Δ4 activity as demonstrated by DPA ω3  desaturation (3.4±0.2%) but completely lost its original Δ5/Δ6 activity. Thus, a region that appears to be within the 3 rd  transmembrane domain critically affects substrate specificity. 
     To examine whether the eight amino acids ( S. canaliculatus  Δ4 desaturase: LIPVF YNY - NIMMT MI;  S. canaliculatus Δ5/Δ6 desaturase: LIPVF FHYQLLKI MI) between the “LIPVF” to “TMI/IMI” region were all required for substrate specificity or if only specific amino acids were critical, the inventors made constructs with alterations in this region by dividing the 8 amino acid region into two portions. Altering the “IMMT” amino acid sequence of the  S. canaliculatus  Δ4 desaturase gene to “LLKI” (Sig-15C;  FIG. 6  left column) did not alter the substrate specificity of the Δ4 desaturase gene, as it desaturated DPA ω3 (2.9±0.3%) to generate DHA ω3  and had no activity on ALA. Similarly, the reverse gene construct with alteration of the  S. canaliculatus  Δ5/Δ6 desaturase (Sig-16C;  FIG. 6  right column) from “LLKI” to “IMMT” (corresponding residues in  S. canaliculatus  Δ4 desaturase) did not affect substrate specificity (16.3±1.1%, ALA). However, substitution of the Δ4 desaturase amino acid sequence “YNYN” to the Δ5/Δ6 desaturase sequence “FHYQ” at the same location (Sig-17C; SEQ ID NO: 16;  FIG. 6  left column; Table 1) changed the substrate specificity from DPA ω3  to ALA (12.5±0.8%). Likewise, altering the  S. canaliculatus  Δ5/Δ6 desaturase sequence from “FHYQ” to the corresponding “YNYN” sequence (Sig-18C; SEQ ID NO: 8;  FIG. 6  right column; Table 1) resulted in a loss of Δ5/Δ6 desaturase activity but a gain in Δ4 desaturase activity (0.5±0.1%, DPA ω3 ). Thus, the four amino acids that regulate the substrate specificity of  S. canaliculatus  Δ4 and Δ5/Δ6 desaturases are “YNYN” and the corresponding sequence “FHYQ”. 
                     TABLE 1                  Fatty acid desaturation by yeast expressing  S. canaliculatus  Δ 4-desaturase,        Δ5/6-desaturase and chimera constructs.       Desaturation of substrates                                             DPA   DHA   Conversion   ALA   SDA   Conversion           (mol %)   (mol %)   %   (mol %)   (mol %)   %                                                 Δ4   5.20 ±    0.72 ±    12.12   12.01 ±    0   0       Desaturase   0.34   0.01        2.44               Δ5/6   6.88 ±    0   0    9.88 ±    2.94 ±    22.96       Desaturase   0.95            1.13   0.66           Sig-3C   7.81 ±    0.72 ±    8.48   14.49 ±    0   0           0.62   0.08        3.15               Sig-4C   8.07 ±    0   0   13.11 ±    1.87 ±   12.48           1.30            0.77   0.31           Sig-5C   8.45 ±    0   0   12.59 ±    3.81 ±   23.24           1.45            1.26   0.34           Sig-6C   6.97 ±    0.41 ±    5.49   12.19 ±    0   0           1.47   0.06        2.65               Sig-17C   6.56 ±    0   0    9.53 ±    1.37 ±    12.55           1.07            0.65   0.06           Sig-18C   7.97 ±    0.04 ±    0.50   11.36 ±    0   0           1.37   0.00        1.59                    
2.3 Substitution of Amino Acids within the Four Amino Acid Region Produces Enzymes with Both Δ4- and Δ6-Desaturase Activities
 
     After determining that this four amino acid region was responsible for substrate specificity, substitution of individual amino acids was performed ( FIG. 7 ). An amino acid substitution at position 4 ( FIG. 7 ) of the critical region showed no effect on substrate specificity in either the Δ4 (Sig-19C;  FIG. 6  left column) or Δ5/Δ6 desaturases (Sig-20C;  FIG. 6  right column). However, decreases in substrate conversion levels (10.0±0.4%, DPA ω3  for Sig-19C and 15.6±0.9%, ALA for Sig-20C) were observed. When a single amino acid was substituted at position 2 (N to H or vice versa), the substrate specificity of the point-mutated Δ4 desaturase (Sig-21C;  FIG. 6  left column) remained the same (DPA ω3 ) although enzyme activity was reduced to 3.5±0.2%, which is about a 71% reduction in activity compared to wild-type. The Δ5/Δ6 desaturases with an amino acid substitution at position 2 in the critical region (Sig-22C; SEQ ID NO: 22;  FIG. 6  right column) not only desaturated ALA (13.6±5.0%) but also produced DHA ω3  at a low level (0.5±0.04%). The Sig-23C (Δ4 desaturase; SEQ ID NO: 18;  FIG. 6  left column) and Sig-24C (Δ5/Δ6 desaturase; SEQ ID NO: 24;  FIG. 6  right column) with amino acid substitutions in the corresponding residues at position 1 of the critical region were able to desaturate both DPA ω3  and ALA. Since positions 1 and 2 seem to play a more important role in the substrate specificity, the inventors rationally engineered desaturases with two amino acid substitutions in this region (Sig-25C; SEQ ID NO: 20;  FIG. 6  left column, and Sig-26C SEQ ID NO: 26;  FIG. 6  right column). Interestingly, site-directed mutation of both of these amino acids concurrently generates enzymes with desaturation of DPA ω3  and ALA (Sig-25C; SEQ ID NO: 20;  FIG. 6  left column, and Sig-26C SEQ ID NO: 26;  FIG. 6  right column). Further, mutations including the amino acids at position 1, 2 and 4 (Sig-17C; SEQ ID NO: 16;  FIG. 6  left column, and Sig-18C; SEQ ID NO: 8;  FIG. 6  right column) restored specificity, even though single mutations at the 4 th  position did not directly affect substrate specificity as demonstrated by yeast expression (Sig-19C;  FIG. 6  left column, and Sig-20C;  FIG. 6  right column). This suggests that some type of interaction between these three amino acids is required for the substrate specificity of the enzymes studied here. 
     2.4 Four Amino Acid Residues Determine the Substrate Chain-Length and Regioselectivity of  Siganus canaliculatus  Δ4 and Δ5/6 Desaturases 
     The unusually high similarity in amino acid composition (83%) between  S. canaliculatus  Δ4 and Δ5/Δ6 desaturases gave the present inventors an advantage in locating individual regions or residues involved in substrate specificity and regioselectivity. They identified a discrete protein region containing just four amino acid residues (“YNYN”, position 280-283 on  S. canaliculatus  Δ4 desaturase and the corresponding amino acids “FHYQ” at position 278-281 on the  S. canaliculatus  Δ5/Δ6 desaturase) which is responsible for both the substrate chain-length specificity and the regioselectivity of  S. canaliculatus  Δ4 and Δ5/Δ6 desaturases ( FIG. 6 ). The absence of a crystal structure for any membrane bound desaturase made it difficult to structurally interpret the alteration in regioselectivity and substrate specificity observed here. However, the  S. canaliculatus  Δ4 and Δ5/Δ6 desaturases are membrane bound enzymes and, based on previous studies the histidine boxes, N- and C-termini of the enzymes have been predicted to be located on the cytoplasmic side of the membrane. Given these constraints, the membrane protein topology prediction software (TOPCONS; Bernsel et al., 2009) predicts each of these two  S. canaliculatus  desaturases have four transmembrane domains (amino acid residues 132-152, 159-179, 265-285, 303-323 for Δ4 desaturase and corresponding residues for Δ5/Δ6 desaturase) linked by two exoplasmic regions consisting of six and seventeen amino acids ( FIG. 1 ). It is expected that these highly similar desaturases would share a common membrane topology and presumably a common structural folding. 
     The four critical amino acid residues influencing the substrate specificity and regioselectivity of the  S. canaliculatus  desaturases are predicted to be located in the third putative transmembrane domain ( FIG. 2 ). This four amino acid residue block is at some distance from the histidine-rich active site, which suggests that no direct interaction between the mutated amino acid residues and the substrate is required for the observed alterations in regiospecificity and substrate specificity (Cahoon et al., 1997). However, these four amino acid residues may form part of a hydrophobic substrate binding pocket which places constraints on the chain length of the substrate. Influences due to modification of the hydrophobic substrate binding pocket were observed in lipoxygenases and soluble fatty acid desaturases (Cahoon et al., 1997; Borngraber et al., 1999) and the authors suggest that the size and geometry of this hydrophobic binding pocket are responsible for the substrate specificity and insertion of double bond position. The amino acid residues “YNYN” in the critical region of the  S. canaliculatus  Δ4 desaturase are somewhat less bulky than the respective “FHYQ” residues in  S. canaliculatus  Δ5/Δ6 desaturase, possibly allowing the hydrophobic substrate binding pocket to accommodate a larger substrate such as DPA ω3  (22:5 ω3 ), whereas the presence of bulkier residues may put a greater constraint on the substrate binding pocket to favour the smaller substrate ALA (18:2). Alternatively, the presence of a histidine residue in proximity to aromatic amino acids may result in π-π stacking interactions between aromatic amino acids which could influence packing effects and thus affect the substrate selection (Liao et al., 2013). 
     The inventors have also generated novel desaturases with Δ4 and Δ6 activities (Sig-22C/SEQ ID NO: 22; Sig-23C/SEQ ID NO: 18; Sig-24C/SEQ ID NO: 24; Sig-25C/SEQ ID NO: 20; Sig-26C/SEQ ID NO: 26;  FIG. 6 ) by altering only one or two amino acids at positions 1 and 2 of the critical region ( FIG. 7 ). The dual enzyme activities of the mutants were unexpected, and mutating both residues increases enzyme activity by 2.4 to 3.3 fold compared to mutating only the amino acid residue at position 1. A single amino acid residue mutation at position 4 ( FIG. 7 ), results in a lower catalytic ability in both Δ4 and Δ5/Δ6 desaturases but does not affect the substrate or double bond position specificities. However, when this mutation was combined with double amino acid mutations at positions 1 and 2 ( FIG. 7 ), the Δ4 desaturase was converted to an enzyme with Δ5/Δ6 desaturase activity and substrate specificity, and vice versa. This indicates that the substrate specificity and regioselectivity of the  S. canaliculatus  desaturases cannot be confined to a single amino acid residue. In fact, some degree of interaction between all four amino acid residues (Δ4: “YNYN” and Δ5/Δ6: “FHYQ”) appears to be responsible for the substrate specificity and regioselectivity of the enzymes. Interestingly, similar results were obtained by Yuan et al. (1995). A single amino acid change (T231 K) in a thioesterase by itself did not affect substrate specificity but when this mutation was added to an enzyme with two mutations (M197R/R199H) which possessed equal preference for 12:0 and 14:0, the triple mutant was active only on 14:0. Here, the single mutation at position 4 ( FIG. 7 ) shows a non-additive combinatory effect on the double mutant at positions 1 and 2, which suggests that some type of interaction is occurring between these amino acids (Sandberg and Terwillinger, 1993; Yuan et al., 1995). 
     Example 3 
     Based on the results obtained in Example 2 and further sequence alignments and topology predictions, the corresponding amino acid regions controlling the substrate specificity could be identified, in Δ4, Δ5 and/or Δ6 desaturases of other organisms as well. By exchanging the corresponding amino acid regions regulating the substrate specificity, the substrate specificity can also be switched in said Δ4, Δ5 and/or Δ6 desaturases, indicating that the teaching from the  S. canaliculatus  Δ4 and Δ 5/6 desaturases can be readily transferred to other “front-end” desaturases. In particular, this is exemplified for the Δ6 desaturases of  Ostreococcus tauri  and  Pythium irregulare  and the delta-5 desaturase of  Thraustochytrium  sp., in the following. Accession numbers and/or sequence identity numbers of Δ4, Δ5 and/or Δ6 desaturases referred to in this Example have already been indicated in the description. 
     3.1  Ostreococcus tauri  Δ6 Desaturase 
     The following data is based on the alignment of Δ4 desaturases ( Euglena gracilis, Emiliana huxleyi, Pavlova lutheri, Pavlova salina, Siganus arctica, Siganus canaliculatus ) with Δ6 desaturases ( Ostreococcus lucimarinus, Ostreococcus tauri, Siganus canaliculatus ). 
     3.1.1 In the  O. tauri  Δ6 desaturase, the amino acid residues (AA) “MFFL” (position 283-286) correspond to the four important amino acid residues identified in the studies presented in Example 2 ( S. canaliculatus  Δ4-desaturase: “YNYN” and Δ5/6-desaturase: “FHYQ”)]. The amino acid residues at position 1, 2 and 4 were shown to have a significant influence on substrate specificity in  S. canaliculatus . The  O. lucimarinus Δ 6 desaturase has the same amino acid residues at positions 1, 2 and 4 (“FWMFFLH”) as the  O. tauri Δ 6-desaturase. The corresponding amino acid residues on the  P. salina  and  P. lutheri  Δ4-desaturase are “FKLLFLD”. 
     If this important region from the  O. tauri Δ 6 desaturase is aligned with the  P. salina  and  P. lutheri  Δ4 desaturases, the differences in the amino acid residues are:  O. tauri Δ 6 desaturase, “FWMFFLH” and  P. lutheri/P. salina Δ 4 desaturase, “FKLLFLD”. So performing amino acid residue substitutions based on these differences is feasible (Betts and Russell, 2003). 
     Hydrophobicity: The “W” and “F” amino acid residues in the  O. tauri Δ 6 desaturase are aromatic amino acids which are relatively non-polar and hydrophobic. Changing from “WMF” to “KLL” will reduce the hydrophobicity of the corresponding region. Similarly, from the study with the  Siganus canaliculatus Δ 4 desaturase and Δ5/6 desaturase presented in Example 2, the inventors knew that the Δ5/Δ6 desaturase contains more aromatic amino acid residues in the four amino acid region, and is thus more hydrophobic than the Δ4 desaturase. 
     All mutations based on the above are demonstrated below: 
     
       
         
           
               
               
            
               
                   
                   O .  tauri  Δ6 desaturase: 
               
               
                   
                 SEQ ID NO: 71 
               
            
           
           
               
               
               
               
            
               
                   
                 281 
                 F WMF FL H   
                 287/ 
               
               
                   
                   
               
            
           
           
               
               
            
               
                   
                   P .  lutheri  and  P .  salina  Δ4 desaturase: 
               
               
                   
                 SEQ ID NO: 72 
               
            
           
           
               
               
               
               
            
               
                   
                 256/259 
                 F KLL FL D   
                 262/265/ 
               
               
                   
                   
               
            
           
           
               
               
            
               
                   
                   O .  tauri  Δ6 desaturase: 
               
               
                   
                 SEQ ID NO: 73 
               
            
           
           
               
               
               
               
            
               
                   
                 278 
                 VLLFWMFFLHPSKALK 
                 293/ 
               
               
                   
                   
               
            
           
           
               
               
            
               
                   
                 Mutation 1: 
               
               
                   
                 SEQ ID NO: 74 
               
            
           
           
               
               
               
               
            
               
                   
                 278 
                 VLLF KLL FLHPSKALK 
                 293/ 
               
               
                   
                   
               
            
           
           
               
               
            
               
                   
                 Mutation 2: 
               
               
                   
                 SEQ ID NO: 75 
               
            
           
           
               
               
               
               
            
               
                   
                 278 
                 VLLF KLL FL D PSKALK 
                 293/ 
               
               
                   
                   
               
            
           
           
               
               
            
               
                   
                 Mutation 3: 
               
               
                   
                 SEQ ID NO: 76 
               
            
           
           
               
               
               
               
            
               
                   
                 278 
                 VLLF KL FFLHPSKALK 
                 293/ 
               
               
                   
                   
               
            
           
           
               
               
            
               
                   
                 Mutation 4: 
               
               
                   
                 SEQ ID NO: 77 
               
            
           
           
               
               
               
               
            
               
                   
                 278 
                 VLLF KL FFL D PSKALK 
                 293/ 
               
               
                   
                   
               
            
           
           
               
               
            
               
                   
                 Mutation 5: 
               
               
                   
                 SEQ ID NO: 78 
               
            
           
           
               
               
               
               
            
               
                   
                 278 
                 VLLF K M L FLHPSKALK 
                 293/ 
               
               
                   
                   
               
            
           
           
               
               
            
               
                   
                 Mutation 6: 
               
               
                   
                 SEQ ID NO: 79 
               
            
           
           
               
               
               
               
            
               
                   
                 278 
                 VLLF K M L FL D PSKALK 
                 293/ 
               
               
                   
                   
               
            
           
           
               
               
            
               
                   
                 Mutation 7: 
               
               
                   
                 SEQ ID NO: 80 
               
            
           
           
               
               
               
               
            
               
                   
                 278 
                 VLLFW L FFLHPSKALK 
                 293/ 
               
               
                   
                   
               
            
           
           
               
               
            
               
                   
                 Mutation 8: 
               
               
                   
                 SEQ ID NO: 81 
               
            
           
           
               
               
               
               
            
               
                   
                 278 
                 VLLFW L FFL D PSKALK 
                 293/ 
               
               
                   
                   
               
            
           
           
               
               
            
               
                   
                 Mutation 9: 
               
               
                   
                 SEQ ID NO: 82 
               
            
           
           
               
               
               
               
            
               
                   
                 278 
                 VLLFWM L FLHPSKALK 
                 293/ 
               
               
                   
                   
               
            
           
           
               
               
            
               
                   
                 Mutation 10: 
               
               
                   
                 SEQ ID NO: 83 
               
            
           
           
               
               
               
               
            
               
                   
                 278 
                 VLLFWM L FL D PSKALK 
                 293/ 
               
               
                   
                   
               
            
           
           
               
               
            
               
                   
                 Mutation 11: 
               
               
                   
                 SEQ ID NO: 84 
               
            
           
           
               
               
               
               
            
               
                   
                 278 
                 VLLFW LL FLHPSKALK 
                 293/ 
               
               
                   
                   
               
            
           
           
               
               
            
               
                   
                 Mutation 12: 
               
               
                   
                 SEQ ID NO: 85 
               
            
           
           
               
               
               
               
            
               
                   
                 278 
                 VLLFW LL FL D PSKALK 
                 293/ 
               
               
                   
                   
               
            
           
           
               
               
            
               
                   
                 Mutation 13: 
               
               
                   
                 SEQ ID NO: 86 
               
            
           
           
               
               
               
               
            
               
                   
                 278 
                 VLLF K MFFLHPSKALK 
                 293/ 
               
               
                   
                   
               
            
           
           
               
               
            
               
                   
                 Mutation 14: 
               
               
                   
                 SEQ ID NO: 87 
               
            
           
           
               
               
               
               
            
               
                   
                 278 
                 VLLFWMFFL D PSKALK 
                 293/ 
               
            
           
         
       
     
     3.1.2 The 3 rd  transmembrane (TM) domain of  S. canaliculatus  Δ4 and Δ5/Δ6 desaturases contains the region of interest, as demonstrated in Example 2. A topology prediction (TOPCONS) was used to identify the 3 rd  transmembrane domain of the  O. tauri  Δ6 desaturase from positions 300-320. Alignment of this region with  P. salina, S. artica  or  P. lutheri  Δ4 desaturases identifies a region of homology which can be used to replace the  O. tauri  3 rd  transmembrane region; i.e. the bold  O. tauri  Δ6 desaturase region can be replaced by the bold region of Δ4 desaturase genes which could result in a change of the substrate specificity of the Δ6 desaturase to the substrate specificity of a Δ4 desaturase. Tequence identity numbers only refer to sections in bold lettering. 
     
       
         
           
               
               
            
               
                   O .  tauri  Δ6 desaturase: 
                   
               
               
                 SEQ ID NO: 88 
                   
               
            
           
           
               
               
               
               
            
               
                 290 
                 KALKGGKYEE LVWMLAAHVIRTWTIKAVTGF TAMQSYGLFL 
                 330/ 
                   
               
               
                   
               
            
           
           
               
               
            
               
                   P .  lutheri  Δ4 desaturase: 
                   
               
               
                 SEQ ID NO: 89 
                   
               
            
           
           
               
               
               
               
            
               
                 274 
                 EKISPLARALFAPA VACKLGFWARFVALPLWLQ PTVHTALCIC 
                 316/ 
                   
               
               
                   
               
            
           
           
               
               
            
               
                   S .  artica  Δ4 desaturase: 
                   
               
               
                 SEQ ID NO: 90 
                   
               
            
           
           
               
               
               
               
            
               
                 277 
                 EKLPESYR-KERNI AIGLRVFFFIRKFVVPFALH FSWYTLLCTY 
                 319/ 
                   
               
               
                   
               
            
           
           
               
               
            
               
                   P .  salina  Δ4 desaturase: 
                   
               
               
                 SEQ ID NO: 91 
                   
               
            
           
           
               
               
               
               
            
               
                 280 
                 SKLAGYLFMPS LLLKLTFWARFVALPLYLA PSVHTAVCIA 
                 319/ 
                   
               
            
           
         
       
     
     3.1.3 The 3 rd  transmembrane domain of  O. tauri  Δ6 desaturase (position 300-320, in bold) can be replaced with the corresponding Δ4 desaturase 3 rd  transmembrane domain (in bold) based on TOPCONS topology prediction software. 
     
       
         
           
               
            
               
                   O .  tauri  Δ6 desaturase: (sequence identity 
               
               
                 numbers only refer to sections in bold lettering) 
               
               
                 SEQ ID NO: 88 
               
            
           
           
               
               
               
            
               
                 290 
                 KALKGGKYEE LVWMLAAHVIRTWTIKAVTGF TAMQSYGLFL 
                 330/ 
               
               
                   
               
            
           
           
               
            
               
                 a) Substitute with  P .  lutheri  Δ4 desaturase (281- 
               
               
                 301): 
               
               
                 SEQ ID NO: 92 
               
               
                   RALFAPAVACKLGFWARFVAL / 
               
               
                   
               
               
                 b) Substitute with  P .  salina  Δ4 desaturase (291- 
               
               
                 311): 
               
               
                 SEQ ID NO: 93 
               
               
                   LLLKLTFWARFVALPLYLAPS / 
               
               
                   
               
               
                 c) Substitute with  S .  arctica  Δ4 desaturase (293- 
               
               
                 313): 
               
               
                 SEQ ID NO: 94 
               
               
                   LRVFFFIRKFVVPFALHFSWY / 
               
            
           
         
       
     
     3.1.4 Substitute the entire corresponding “LI PV” to “PVYG” region on  O. tauri  Δ6 desaturase (position 278-321) with the respective Δ4 desaturase domain, based on sequences alignments. This region was shown to be important in the  S. canaliculatus  desaturase sequences, as shown in Example 2. 
     
       
         
           
               
               
            
               
                   
                   O .  tauri  Δ6 desaturase (position 278-321): 
               
               
                   
                 SEQ ID NO: 95 
               
               
                   
                 VLLFWMFFLHPSKALKGGKYEELVWMLAAHVIRTWTIKAVTGFT/ 
               
            
           
         
       
     
     Substitute with: 
     
       
         
           
               
               
            
               
                   
                 a)  P .  lutheri  Δ4 desaturase (265-307): 
               
               
                   
                 SEQ ID NO: 96 
               
               
                   
                 ELLAWRWEGEKISPLARALFAPAVACKLGFWARFVALPLWLQP/ 
               
               
                   
                   
               
               
                   
                 b)  P .  salina  Δ4 desaturase (268-310): 
               
               
                   
                 SEQ ID NO: 97 
               
               
                   
                 ELVMWRWEGEPISKLAGYLFMPSLLLKLTFWARFVALPLYLAP/ 
               
               
                   
                   
               
               
                   
                 c)  S .  artica  Δ4 desaturase (268-310): 
               
               
                   
                 SEQ ID NO: 98 
               
               
                   
                 DLIEMKYKGEKLPESYRKERNIAIGLRVFFFIRKFVVPFALHF/ 
               
            
           
         
       
     
     3.1.5 The four amino acid region of the  S. canaliculatus  Δ4 and Δ5/Δ6 desaturases identified in Example 2 are located at the end of 3 rd  transmembrane domain close to the exoplasmic face of the membrane (positions 280-283 and 278-281, respectively). Thus, based on topology predictions, the comparable amino acid residues on the 3 rd  transmembrane domain of the  O. tauri  Δ6 desaturase are “KAVT” (position 315-318). Substituting said amino acid residues with the Δ4 desaturase amino acid residues from the same topological location can influence the substrate specificity and regioselectivity. 
       O. tauri  (position 315-318) “KAVT”/SEQ ID NO: 99 replace with: 
       P. lutheri  Δ4 desaturase (position 296-299): “ARFV”/SEQ ID NO: 100 
     or  P. salina  Δ4 desaturase (position 306-309): “LYLA”/SEQ ID NO: 101 
     or  S. artica  Δ4 desaturase (position 308-311): “LHFS”/SEQ ID NO: 102 
     3.2  Thraustochytrium  sp. Δ5 Desaturase 
     The following data is based on the sequence alignment of the Δ4 desaturase ( Pavlova lutheri, Pavlova saline, Siganus artica, Thraustochytrium  sp.,  Siganus canaliculatus ) with the Δ5 desaturase ( Thraustochytrium  sp.,  Pavlova saline, Siganus canaliculatus ). 
     3.2.1 In the  Thraustochytrium  sp. Δ5 desaturase, the amino acid residues corresponding to the region identified in  S. canaliculatus  are “TLYL” (position 270-273), with positions 2, 3, and 4 conserved with the  P. saline  Δ5 desaturase. For conversion to a Δ4 desaturase, substitute with the bold region (sequence identity numbers only refer to sections in bold lettering): 
                      Thraustochytrium  sp. Δ5 desaturase:       SEQ ID NO: 103                         265   IGLGW  HPR   276/                           P .  lutheri  Δ4 desaturase:       SEQ ID NO: 104                         265   ELLAW  EKI   276/                           P .  salina  Δ4 desaturase:       SEQ ID NO: 105                         268   ELVMW  EPI   279/                           S .  artica  Δ4 desaturase:       SEQ ID NO: 106                         268   DLIEM  EKL   279/            
a) Substitute one amino acid residue at a time. Bold: substituted amino acid residue/SEQ ID NO.
 
     Replace with “RWEG” (corresponding region on  P. saline  and  P. lutheri  Δ4 desaturase) 
                    Mutation 1:  Thraustochytrium  sp. Δ5 desaturase:       SEQ ID NO: 107                         265   IGLGW R LYLHPR   276/                         Mutation 2:  Thraustochytrium  sp. Δ5 desaturase:       SEQ ID NO: 108                         265   IGLGW RW YLHPR   276/                         Mutation 3:  Thraustochytrium  sp. Δ5 desaturase:       SEQ ID NO: 109                         265   IGLGW RWE LHPR   276/                         Mutation 4:  Thraustochytrium  sp. Δ5 desaturase:       SEQ ID NO: 110                         265   IGLGW RW Y G HPR   276/                         Mutation 5:  Thraustochytrium  sp. Δ5 desaturase:       SEQ ID NO: 111                         265   IGLGW R LY G HPR   276/                         Mutation 6:  Thraustochytrium  sp. Δ5 desaturase:       SEQ ID NO: 112                         265   IGLGW R L E LHPR   276/            
b) Replace with “KYKG” (corresponding region on  S. artica  Δ4 desaturase). Bold: substituted amino acid residue/SEQ ID NO.
 
     
       
         
           
               
               
            
               
                   
                   Thraustochytrium  sp. Δ5 desaturase: 
               
               
                   
                 SEQ ID NO: 113 
               
            
           
           
               
               
               
               
            
               
                   
                 265 
                 IGLGWTLYLHPR 
                 276/ 
               
               
                   
                   
               
            
           
           
               
               
            
               
                   
                 Mutation 1: 
               
               
                   
                 SEQ ID NO: 114 
               
            
           
           
               
               
               
               
            
               
                   
                 265 
                 IGLGW K LYLHPR 
                 276/ 
               
               
                   
                   
               
            
           
           
               
               
            
               
                   
                 Mutation 2: 
               
               
                   
                 SEQ ID NO: 115 
               
            
           
           
               
               
               
               
            
               
                   
                 265 
                 IGLGW KY YLHPR 
                 276/ 
               
               
                   
                   
               
            
           
           
               
               
            
               
                   
                 Mutation 3: 
               
               
                   
                 SEQ ID NO: 116 
               
            
           
           
               
               
               
               
            
               
                   
                 265 
                 IGLGW KYK LHPR 
                 276/ 
               
               
                   
                   
               
            
           
           
               
               
            
               
                   
                 Mutation 4: 
               
               
                   
                 SEQ ID NO: 117 
               
            
           
           
               
               
               
               
            
               
                   
                 265 
                 IGLGW K L K LHPR 
                 276/ 
               
               
                   
                   
               
            
           
           
               
               
            
               
                   
                 Mutation 5: 
               
               
                   
                 SEQ ID NO: 118 
               
            
           
           
               
               
               
               
            
               
                   
                 265 
                 IGLGW K LY G HPR 
                 276/ 
               
               
                   
                   
               
            
           
           
               
               
            
               
                   
                 Mutation 6: 
               
               
                   
                 SEQ ID NO: 119 
               
            
           
           
               
               
               
               
            
               
                   
                 265 
                 IGLGW K L KG HPR 
                 276/ 
               
               
                   
                   
               
            
           
           
               
               
            
               
                   
                 Mutation 7: 
               
               
                   
                 SEQ ID NO: 120 
               
            
           
           
               
               
               
               
            
               
                   
                 265 
                 IGLGWT Y YLHPR 
                 276/ 
               
               
                   
                   
               
            
           
           
               
               
            
               
                   
                 Mutation 8: 
               
               
                   
                 SEQ ID NO: 121 
               
            
           
           
               
               
               
               
            
               
                   
                 265 
                 IGLGWT YK LHPR 
                 276/ 
               
               
                   
                   
               
            
           
           
               
               
            
               
                   
                 Mutation 9: 
               
               
                   
                 SEQ ID NO: 122 
               
            
           
           
               
               
               
               
            
               
                   
                 265 
                 IGLGWT YKG HPR 
                 276/ 
               
               
                   
                   
               
            
           
           
               
               
            
               
                   
                 Mutation 10: 
               
               
                   
                 SEQ ID NO: 123 
               
            
           
           
               
               
               
               
            
               
                   
                 265 
                 IGLGWTL K LHPR 
                 276/ 
               
               
                   
                   
               
            
           
           
               
               
            
               
                   
                 Mutation 11: 
               
               
                   
                 SEQ ID NO: 124 
               
            
           
           
               
               
               
               
            
               
                   
                 265 
                 IGLGWTL KG HPR 
                 276/ 
               
            
           
         
       
     
     3.2.2 Based on sequence alignments, there are five amino acid residues near the important region that are conserved between  P. lutheri  and  P. saline  Δ4 desaturases. Replacing these 5 amino acid residues can result in a change in desaturase specificity (sequence identity numbers only refer to sections in bold lettering): 
     
       
         
           
               
            
               
                   Thraustochytrium  sp. Δ5 desaturase: 
               
               
                 SEQ ID NO: 125 
               
            
           
           
               
               
               
            
               
                 265 
                 IGLGW TLYLH PR 
                 276/ 
               
               
                   
               
            
           
           
               
            
               
                   P .  lutheri  44 desaturase: 
               
               
                 SEQ ID NO: 126 
               
            
           
           
               
               
               
            
               
                 265 
                 ELLAW RWEGE KI 
                 276/ 
               
               
                   
               
            
           
           
               
            
               
                   P .  salina  44 desaturase: 
               
               
                 SEQ ID NO: 127 
               
            
           
           
               
               
               
            
               
                 268 
                 ELVMW RWEGE PI 
                 279/ 
               
               
                   
               
            
           
           
               
            
               
                 After substitution,  Thraustochytrium  sp. Δ5 
               
               
                 desaturase: 
               
               
                 SEQ ID NO: 128 
               
            
           
           
               
               
               
            
               
                 265 
                 IGLGW RWEGE PR 
                 276/ 
               
            
           
         
       
     
     3.2.3 Based on sequence alignments, the region corresponding from “LIPV” to “PVYG” identifled in Example 2 is from position 265-307, in the amino acid sequence of the  Thraustochytrium  sp. Δ5 desaturase. Substitution of amino acid residues from the  P. lutheri  Δ4 desaturase is shown below. 
     
       
         
           
               
               
            
               
                   Thraustochytrium  sp. Δ5 desaturase: 
                   
               
               
                 SEQ ID NO: 129 
                   
               
            
           
           
               
               
               
               
            
               
                 265 
                 
                   IGLGWTLYLHPRYMLRTKRHMEFVWIFARYIGWFSLMGALGYS 
                 
                 307/ 
                   
               
               
                   
               
            
           
           
               
               
            
               
                   P .  lutheri  Δ4 desaturase: 
                   
               
               
                 SEQ ID NO: 130 
                   
               
            
           
           
               
               
               
               
            
               
                 265 
                 
                   ELLAWRWEGEKISPLARALFAPAVACKLGFWARFVALPLWLQP 
                 
                 307/ 
                   
               
            
           
         
       
     
     3.2.4 Based on protein topology prediction software (TOPCONS), the  Thraustochytrium  sp. Δ5 desaturase has four transmembrane domains and two hydrophobic stretches. The 3 rd  transmembrane domain of  Thraustochytrium  sp. Δ5 desaturase is located from position 287-307. Substitution of the 3 rd  transmembrane domain with the corresponding domain based on homology is shown below: 
     
       
         
           
               
            
               
                   Thraustochytrium  sp. Δ5 desaturase, 3 rd   
               
               
                 transmembrane domain: 
               
               
                 SEQ ID NO: 131 
               
            
           
           
               
               
               
            
               
                 287 
                 FVWIFARYIGWFSLMGALGYS 
                 307/ 
               
            
           
         
       
     
     Replace with: 
     
       
         
           
               
            
               
                   P .  lutheri  Δ4 desaturase: 
               
               
                 SEQ ID NO: 132 
               
            
           
           
               
               
               
            
               
                 287 
                 AVACKLGFWARFVALPLWLQP 
                 307/ 
               
            
           
           
               
            
               
                 or 
               
               
                   
               
               
                   P .  salina  Δ4 desaturase: 
               
               
                 SEQ ID NO: 133 
               
            
           
           
               
               
               
            
               
                 290 
                 SLLLKLTFWARFVALPLYLAP 
                 310/ 
               
            
           
           
               
            
               
                 or 
               
               
                   
               
               
                   S .  artica  Δ4 desaturase: 
               
               
                 SEQ ID NO: 134 
               
            
           
           
               
               
               
            
               
                 290 
                 AIGLRVFFFIRKFVVPFALHF 
                 310/ 
               
            
           
         
       
     
     Based strictly on topology predictions (TOPCONS), the inventors could substitute the 3rd transmembrane domain of the  Thraustochytrium  sp. Δ5 desaturase with the 3rd transmembrane domain of other Δ4 desaturases: 
     
       
         
           
               
               
            
               
                   
                   Thraustochytrium  sp. Δ5 desaturase, 3 rd   
               
               
                   
                 transmembrane domain: 
               
               
                   
                 SEQ ID NO: 131 
               
            
           
           
               
               
               
               
            
               
                   
                 287 
                 FVWIFARYIGWFSLMGALGYS 
                 307/ 
               
            
           
         
       
     
     Replace with: 
     
       
         
           
               
               
            
               
                   
                   P .  lutheri  Δ4 desaturase, 3rd TM: 
               
               
                   
                 SEQ ID NO: 135 
               
            
           
           
               
               
               
               
            
               
                   
                 281 
                 RALFAPAVACKLGFWARFVAL 
                 301/ 
               
            
           
           
               
               
            
               
                   
                 or 
               
               
                   
                   
               
               
                   
                   P .  Salina  Δ4 desaturase, 3rd TM: 
               
               
                   
                 SEQ ID NO: 136 
               
            
           
           
               
               
               
               
            
               
                   
                 291 
                 LLLKLTFWARFVALPLYLAPS 
                 311/ 
               
            
           
           
               
               
            
               
                   
                 or 
               
               
                   
                   
               
               
                   
                   S .  arctica  Δ4 desaturase, 3rd TM: 
               
               
                   
                 SEQ ID NO: 137 
               
            
           
           
               
               
               
               
            
               
                   
                 293 
                 LRVFFFIRKFVVPFALHFSWY 
                 313/ 
               
            
           
         
       
     
     3.2.5 Based on the studies shown in Example 2, the four amino acid residues which influence the substrate specificity are located at the end of the 3rd transmembrane domain close to the exoplasmic face. The four amino acid residues are located in the last 6 amino acid residues in the 3 rd  transmembrane domain of  S. canaliculatus Δ4 and Δ5/Δ6 desaturases. In  Thraustochytrium  sp. Δ5 desaturase, position 302-305 represents the same location based on topology prediction (starts at last 6 amino acid residues in the 3rd transmembrane domain). 
     a) Substitution of amino acid residues corresponding to  Thraustochytrium  sp. Δ5 desaturase position 302-305 (“GALG”/SEQ ID NO: 138) based on sequence alignment as follows: 
                            i)  P .  lutheri  Δ4 desaturase:           SEQ ID NO: 139           302-305 ″PLWL″/                       ii)  P .  salina  Δ4 desaturase:           SEQ ID NO: 140           305-308 ″PLYL″/                       iii)  S .  artica  Δ4 desaturase:           SEQ ID NO: 141           305-308 ″PFAL″/            
b) Substitution of amino acid residues corresponding to  Thraustochytrium  sp. Δ5 desaturase 302-305 (“GALG”/SEQ ID NO: 138) based on topology prediction, as follows:
 
                            i)  P .  lutheri  Δ4 desaturase:           SEQ ID NO: 142           296-299 ″ARFV″/                       ii)  P .  salina  Δ4 desaturase:           SEQ ID NO: 143           306-309 ″LYLA″/                       iii)  S .  artica  Δ4 desaturase:           SEQ ID NO: 144           308-311 ″LHFS″/            
3.3  Pythium irregulare  Δ6 Desaturase
 
     The following data is based on sequence alignments of the  Pythium irregulare  Δ6 desaturase with other Δ6 desaturases ( Siganus canaliculatus .)  Primula farinosa ) and Δ4 desaturases ( Emiliana huxleyi, Monosiga brevicollis, Pavlova lutheri, Pavlova salina, Siganus artica, Thalassiosira pseudonana, Siganus canaliculatus, Thraustochytrium  sp.). 
     3.3.1 The amino acid residues corresponding to the region identified in  S. canaliculatus  in Example 2, in the  P. irregulare  Δ6 desaturase are “AQSF” (position 284-287). These residues are conserved between several Δ6 desaturases ( P. irregulare, P. farinosa ). Based on sequence alignment, these amino acid residues could be substituted with amino acid residues from Δ4 desaturases (sequence identity numbers only refer to sections in bold lettering): 
     
       
         
           
               
               
            
               
                   
                   P .  irregulare  Δ6 desaturase: 
               
               
                   
                 SEQ ID NO: 145 
               
            
           
           
               
               
               
               
            
               
                   
                 280 
                 LSWL AQSF FYV 
                 290/ 
               
            
           
         
       
     
     Replace with amino acid residues from: 
     
       
         
           
               
               
            
               
                   
                   P .  salina  Δ4 desaturase: 
               
               
                   
                 SEQ ID NO: 146 
               
            
           
           
               
               
               
               
            
               
                   
                 259 
                 FKLL FLDI SEL 
                 269/ 
               
               
                   
                   
               
            
           
           
               
               
            
               
                   
                   P .  lutheri  Δ4 desaturase: 
               
               
                   
                 SEQ ID NO: 147 
               
            
           
           
               
               
               
               
            
               
                   
                 256 
                 FKLL FLDA LEL 
                 266/ 
               
               
                   
                   
               
            
           
           
               
               
            
               
                   
                   S .  artica  Δ4 desaturase: 
               
               
                   
                 SEQ ID NO: 148 
               
            
           
           
               
               
               
               
            
               
                   
                 259 
                 FQWV FLGL HDL 
                 269/  
               
               
                   
                   
               
            
           
           
               
               
            
               
                   
                   M .  brevicollis  Δ4 desaturase: 
               
               
                   
                 SEQ ID NO: 149 
               
            
           
           
               
               
               
               
            
               
                   
                 241 
                 AKVI IGDW YNL 
                 251/ 
               
               
                   
                   
               
            
           
           
               
               
            
               
                   
                   E .  huxleyi  Δ4 desaturase: 
               
               
                   
                 SEQ ID NO: 150 
               
            
           
           
               
               
               
               
            
               
                   
                 234 
                 FVFA FTIR KYA 
                 244/ 
               
               
                   
                   
               
            
           
           
               
               
            
               
                   
                   T .  pseudonana  Δ4 desaturase: 
               
               
                   
                 SEQ ID NO: 151 
               
            
           
           
               
               
               
               
            
               
                   
                 315 
                 LAKV FQQD FEV 
                 325/ 
               
               
                   
                   
               
            
           
           
               
               
            
               
                   
                 Thraustochytrium sp. Δ4 desaturase: 
               
               
                   
                 SEQ ID NO: 152 
               
            
           
           
               
               
               
               
            
               
                   
                 290 
                 INKV VTQD VGV 
                 300/ 
               
            
           
         
       
     
     Substitution of single amino acid residues could also be performed as described above. 
     3.3.2 Based on topology prediction (TOPCONS), the  P. irregulare  Δ6 desaturase contains four transmembrane domains and two hydrophobic stretches; the 3rd transmembrane domain is located between position 310-330. From Example 2 it is known, that the amino acid residues of interest are in the third transmembrane domain. Based on sequence alignment, the inventors could substitute the 3rd transmembrane domain of the  P. irregulare  Δ6 desaturase with the corresponding region from Δ4 desaturases. 
     
       
         
           
               
               
            
               
                   
                 3rd TM  P .  irregulare  Δ6 desaturase: 
               
               
                   
                 SEQ ID NO: 153 
               
            
           
           
               
               
               
               
            
               
                   
                 310 
                 AGLIVHYIWQLAIPYFCNMSL 
                 330/ 
               
            
           
         
       
     
     Replace with: 
     
       
         
           
               
               
            
               
                   
                   P .  lutheri  Δ4 desaturase: 
               
               
                   
                 SEQ ID NO: 154 
               
            
           
           
               
               
               
               
            
               
                   
                 288 
                 VACKLGFWARFVALPLWLQPT 
                 308/ 
               
            
           
           
               
               
            
               
                   
                 or 
               
               
                   
                   
               
               
                   
                   P .  salina  Δ4 desaturase: 
               
               
                   
                 SEQ ID NO: 155 
               
            
           
           
               
               
               
               
            
               
                   
                 291 
                 LLLKLTFWARFVALPLYLAPS 
                 311/ 
               
            
           
           
               
               
            
               
                   
                 or 
               
               
                   
                   
               
               
                   
                   S .  artica  Δ4 desaturase: 
               
               
                   
                 SEQ ID NO: 156 
               
            
           
           
               
               
               
               
            
               
                   
                 291 
                 IGLRVFFFIRKFVVPFALHFS 
                 311/ 
               
            
           
           
               
               
            
               
                   
                 or 
               
               
                   
                   
               
               
                   
                   Thraustochytrium  sp. Δ4 desaturase: 
               
               
                   
                 SEQ ID NO: 157 
               
            
           
           
               
               
               
               
            
               
                   
                 325 
                 WIMKALTVLYMVALPCYMQGP 
                 345/ 
               
            
           
           
               
               
            
               
                   
                 or 
               
               
                   
                   
               
               
                   
                   M .  brevicollis  Δ4 desaturase: 
               
               
                   
                 SEQ ID NO: 158 
               
            
           
           
               
               
               
               
            
               
                   
                 274 
                 VLARICWLVRLVAIPVYLHGW 
                 294/ 
               
            
           
         
       
     
     3.3.3 Substitute the 3rd transmembrane domain of the  P. irregulare  Δ6 desaturase with the 3rd transmembrane domain of other Δ4 desaturase based on topology predictions: 
     
       
         
           
               
               
            
               
                   
                 3rd TM  P .  irregulare  Δ6 desaturase: 
               
               
                   
                 (SEQ ID NO: 153) 
               
            
           
           
               
               
               
               
            
               
                   
                 310 
                 AGLIVHYIWQLAIPYFCNMSL 
                 330/ 
               
            
           
         
       
     
     Replace with: 
     
       
         
           
               
            
               
                   P .  lutheri  Δ4 desaturase, 3rd TM: 
               
               
                 SEQ ID NO: 159 
               
            
           
           
               
               
               
            
               
                 281 
                 RALFAPAVACKLGFWARFVAL 
                 301/ 
               
            
           
           
               
            
               
                 or 
               
               
                   
               
               
                   P .  salina  Δ4 desaturase, 3rd TM: 
               
               
                 SEQ ID NO: 160 
               
            
           
           
               
               
               
            
               
                 291 
                 LLLKLTFWARFVALPLYLAPS 
                 311/ 
               
            
           
           
               
            
               
                 or 
               
               
                   
               
               
                   S .  arctica  Δ4 desaturase, 3rd TM: 
               
               
                 SEQ ID NO: 161 
               
            
           
           
               
               
               
            
               
                 293 
                 LRVFFFIRKFVVPFALHFSWY 
                 313/ 
               
            
           
         
       
     
     3.3.4 Based on the studies shown in Example 2, the four amino acid residues influencing the substrate specificity are located at the end of 3 rd  transmembrane domain close to the exoplasmic face of the membrane. In the  P. irregulare  Δ6 desaturase, position 325-328 (“FCNM”) represents the same location, based on topology predictions. This region can be replaced as outlined below: 
     a) Substitute with amino acid residues corresponding to  P. irregulare  Δ6 desaturase position 325-328 (“FCNM”/SEQ ID NO: 162), based on sequence alignment: 
                            i)  P .  lutheri  Δ4 desaturase:           SEQ ID NO: 163           303-306 ″LWLQ″/                       ii)  P .  salina  Δ4 desaturase:           SEQ ID NO: 164           306-309 ″LYLA″/                       iii)  S .  artica  Δ4 desaturase:           SEQ ID NO: 165           306-309 ″FALH″/                       iv)  Thraustochytrium  sp. Δ4 desaturase:           SEQ ID NO: 166           340-343 ″CYMQ″/            
b) Substitute with amino acid residues corresponding to  P. irregulare  Δ6 desaturase position 325-328 (“FCNM”/SEQ ID NO: 162), based on topology prediction:
 
     
       
         
           
               
               
            
               
                   
                 i)  P .  lutheri  Δ4 desaturase: 
               
               
                   
                 SEQ ID NO: 167 
               
               
                   
                 296-299 ″ARFV″/ 
               
               
                   
                   
               
               
                   
                 ii)  P .  salina  Δ4 desaturase: 
               
               
                   
                 SEQ ID NO: 168 
               
               
                   
                 306-309 ″LYLA″/ 
               
               
                   
                   
               
               
                   
                 iii)  S .  artica  Δ4 desaturase: 
               
               
                   
                 SEQ ID NO: 169 
               
               
                   
                 308-311 ″LHFS″/ 
               
               
                   
                   
               
               
                   
                 iv)  Thraustochytrium  sp. Δ4 desaturase: 
               
               
                   
                 SEQ ID NO: 170 
               
               
                   
                 337-340 ″ALPC″/  
               
            
           
         
       
     
     REFERENCES 
     
         
         1. Ahmann, K., Heilmann, M., and Feussner, I. (2011) Identification of a Δ4-desaturase from the microalga  Ostreococcus lucimarinus, Eur. J. Lipid Sci. Technol.  113, 832-840. 
         2. Bernsel, A., Viklund, H., Hennerdal, A., and Elofsson, A. (2009) TOPCONS: consensus prediction of membrane protein topology, Nucleic Acids Res. 37 (Web Server Issue), W465-468. 
         3. Bowler, C., Allen, A. E., Badger, J. H., and et al. (2008) The  Phaeodactylum  genome reveals the evolutionary history of diatom genomes,  Nature  456, 239-244. 
         4. Broadwater, J. A., Whittle, E., and Shanklin, J. (2002) Desaturation and hydroxylation. Residues 148 and 324 of  Arabidopsis  FAD2, in addition to substrate chain length, exert a major influence in partitioning of catalytic specificity,  J. Biol. Chem.  277, 15613-15620. 
         5. Broun, P., Shanklin, J., Whittle, E., and Somerville, C. (1998) Catalytic plasticity of fatty acid modification enzymes underlying chemical diversity of plant lipids,  Science  282, 1315-1317. 
         6. Borngräber, S., Browner, M., Gillmor, S., Gerth, C., Anton, M., Fletterick, R., and Kuhn, H. (1999) Shape and specificity in mammalian 15-lipoxygenase active site,  J. Biol Chem.  274, 37345-37350. 
         7. Cahoon, E. B., Lindqvist, Y., Schneider, G., and Shanklin, J. (1997) Redesign of soluble fatty acid desaturases from plants for altered substrate specificity and double bond position,  Proc. Natl. Acad. Sci. U.S.A.  94, 4872-4877. 
         8. Cahoon, E. B., Shah, S., Shanklin, J., and Browse, J. (1998) A determinant of substrate specificity predicted from the acyl-acyl carrier protein desaturase of developing Cat&#39;s Claw seed,  Plant Physiol.  117, 593-598. 
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         33. Sayanova, O., Shewry, P. R., and Napier, J. A. (1999) Histidine-41 of the cytochrome b 5  domain of the borage Δ6 fatty acid desaturase is essential for enzyme activity,  Plant Physiol.  121, 641-646. 
         34. Sayanova, O., Beaudoin, F., Libisch, B., Castel, A., Shewry, P. R., and Napier, J. A. (2001) Mutagenesis and heterologous expression in yeast of a plant Δ6-fatty acid desaturase,  J. Exp. Bot.  52, 1581-1585. 
         35. Sayanova, O., Haslam, R., Qi, B., Lazarus, C. M., Napier, J. A. (2006) The alternative pathway C20 Delta8-desaturase from the non-photosynthetic organism  Acanthamoeba castellanii  is an atypical cytochrome b 5 -fusion desaturase,  FEBS Lett  580, 1946-1952. 
         36. Schenkman, J. B., and Jansson, I. (2003) The many roles of cytochrome b 5   , Pharmacol Ther.  97, 129-152. 
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         38. Shanklin, J., Guy, J. E., Mishra, G., and Lindqvist, Y. (2009) Desaturases: emerging models for understanding functional diversification of di-iron-containing enzymes,  J. Biol. Chem.  284, 18559-18563. 
         39. Sperling, P., and Heinz, E. (2001) Desaturases fused to their electron donor.  Eur. J. Lipid Sci. Technol.  103, 158-180. 
         40. Sprecher, H., Luthria, D. L., Mohammed, B. S., and Baykousheva, S. P. (1995) Reevaluation of the pathways for the biosynthesis of polyunsaturated fatty acids,  J. Lipid Res.  36, 2471-2477. 
         41. Sprecher, H. (2000) Metabolism of highly unsaturated n-3 and n-6 fatty acids,  Biochim. Biopys. Acta  1486, 219-231. 
         42. Thies, F., Garry, J. M. C., Yaqoob, P., Rerkasem, K., Williams, J., Shearman, C. P., Gallagher, P. J., Calder, P. C., and Grimble, R. F. (2003) Association of n-3 polyunsaturated fatty acids with stability of atherosclerotic plaques: a randomised controlled trial.  Lancet  361, 477-485. 
         43. Tocher, D. R., and Ghioni, C. (1999) Fatty acid metabolism in marine fish: low activity of fatty acyl Δ5 desaturation in gilthead sea bream ( Sparus aurata ) cells,  Lipids,  34, 433-440. 
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