Patent Publication Number: US-7897392-B2

Title: PUFA polyketide synthase systems and uses thereof

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. patent application Ser. No. 11/689,453, filed Mar. 21, 2007, which is a divisional of U.S. patent application Ser. No. 10/965,017, filed Oct. 13, 2004, now U.S. Pat. No. 7,217,856, which is a continuation-in-part of U.S. patent application Ser. No. 10/810,352, filed Mar. 26, 2004, now U.S. Pat. No. 7,211,418, which claims priority under 35 U.S.C. §119(e) from U.S. Provisional Application Ser. No. 60/457,979, filed Mar. 26, 2003. U.S. application Ser. No. 10/810,352, supra, is also a continuation-in-part of U.S. patent application Ser. No. 10/124,800, filed Apr. 16, 2002, now U.S. Pat. No. 7,247,461, which claims the benefit of priority under 35 U.S.C. §119(e) to: U.S. Provisional Application Ser. No. 60/284,066, filed Apr. 16, 2001; U.S. Provisional Application Ser. No. 60/298,796, filed Jun. 15, 2001; and U.S. Provisional Application Ser. No. 60/323,269, filed Sep. 18, 2001. U.S. patent application Ser. No. 10/124,800, supra, is also a continuation-in-part of U.S. application Ser. No. 09/231,899, filed Jan. 14, 1999, now U.S. Pat. No. 6,566,583. Each of the above-identified patent applications is incorporated herein by reference in its entirety for all purposes. 
     This application does not claim the benefit of priority from U.S. application Ser. No. 09/090,793, filed Jun. 4, 1998, now U.S. Pat. No. 6,140,486, although U.S. application Ser. No. 09/090,793 is incorporated herein by reference in its entirety. 
    
    
     REFERENCE TO SEQUENCE LISTING 
     This application contains a Sequence Listing submitted as an electronic text file named “Sequence_Listing.txt”, having a size in bytes of 373 kb, and created on 12 Oct. 2004. The information contained in this electronic file is hereby incorporated by reference in its entirety pursuant to 37 CFR §1.52(e)(5). 
     FIELD OF THE INVENTION 
     This invention relates to polyunsaturated fatty acid (PUFA) polyketide synthase (PKS) systems from bacterial microorganisms. More particularly, this invention relates to nucleic acids encoding PUFA PKS systems, to proteins and domains thereof that comprise PUFA PKS systems, to genetically modified organisms comprising such PUFA PKS systems, and to methods of making and using the PUFA PKS systems disclosed herein. This invention also relates to genetically modified plants and microorganisms and methods to efficiently produce lipids enriched in various polyunsaturated fatty acids (PUFAs) by manipulation of a PUFA polyketide synthase (PKS) system. 
     BACKGROUND OF THE INVENTION 
     Polyketide synthase (PKS) systems are generally known in the art as enzyme complexes related to fatty acid synthase (FAS) systems, but which are often highly modified to produce specialized products that typically show little resemblance to fatty acids. It has now been shown, however, that polyketide synthase systems exist in marine bacteria and certain microalgae that are capable of synthesizing polyunsaturated fatty acids (PUFAs) from acetyl-CoA and malonyl-CoA. The PKS pathways for PUFA synthesis in  Shewanella  and another marine bacteria,  Vibrio marinus , are described in detail in U.S. Pat. No. 6,140,486. The PKS pathways for PUFA synthesis in the eukaryotic Thraustochytrid,  Schizochytrium  is described in detail in U.S. Pat. No. 6,566,583. The PKS pathways for PUFA synthesis in eukaryotes such as members of Thraustochytriales, including the complete structural description of the PUFA PKS pathway in  Schizochytrium  and the identification of the PUFA PKS pathway in  Thraustochytrium , including details regarding uses of these pathways, are described in detail in U.S. Patent Application Publication No. 20020194641, published Dec. 19, 2002 (corresponding to U.S. patent application Ser. No. 10/124,800, filed Apr. 16, 2002). U.S. patent application Ser. No. 10/810,352, filed Mar. 24, 2004, discloses the complete structural description of the PUFA PKS pathway in  Thraustochytrium , and further detail regarding the production of eicosapentaenoic acid (C20:5, ω-3) (EPA) and other PUFAs using such systems. 
     Researchers have attempted to exploit polyketide synthase (PKS) systems that have been traditionally described in the literature as falling into one of three basic types, typically referred to as: Type I (modular or iterative), Type II, and Type III. For purposes of clarity, it is noted that the Type I modular PKS system has previously also been referred to as simply a “modular” PKS system, and the Type I iterative PKS system has previously also been referred to simply as a “Type I” PKS system. The Type II system is characterized by separable proteins, each of which carries out a distinct enzymatic reaction. The enzymes work in concert to produce the end product and each individual enzyme of the system typically participates several times in the production of the end product. This type of system operates in a manner analogous to the fatty acid synthase (FAS) systems found in plants and bacteria. Type I iterative PKS systems are similar to the Type II system in that the enzymes are used in an iterative fashion to produce the end product. The Type I iterative differs from Type II in that enzymatic activities, instead of being associated with separable proteins, occur as domains of larger proteins. This system is analogous to the Type I FAS systems found in animals and fungi. 
     In contrast to the Type II systems, in Type I modular PKS systems, each enzyme domain is used only once in the production of the end product. The domains are found in very large proteins and the product of each reaction is passed on to another domain in the PKS protein. Additionally, in the PKS systems described above, if a carbon-carbon double bond is incorporated into the end product, it is usually in the trans configuration. 
     Type III systems have been more recently discovered and belong to the plant chalcone synthase family of condensing enzymes. Type III PKSs are distinct from type I and type II PKS systems and utilize free CoA substrates in iterative condensation reactions to usually produce a heterocyclic end product. 
     Polyunsaturated fatty acids (PUFAs) are critical components of membrane lipids in most eukaryotes (Lauritzen et al.,  Prog. Lipid Res.  40 1 (2001); McConn et al.,  Plant J.  15, 521 (1998)) and are precursors of certain hormones and signaling molecules (Heller et al.,  Drugs  55, 487 (1998); Creelman et al.,  Annu. Rev. Plant Physiol. Plant Mol. Biol.  48, 355 (1997)). Known pathways of PUFA synthesis involve the processing of saturated 16:0 or 18:0 fatty acids (the abbreviation X:Y indicates an acyl group containing X carbon atoms and Y double bonds (usually cis in PUFAs); double-bond positions of PUFAs are indicated relative to the methyl carbon of the fatty acid chain (e.g., ω3 or ω6) with systematic methylene interruption of the double bonds) derived from fatty acid synthase (FAS) by elongation and aerobic desaturation reactions (Sprecher,  Curr. Opin. Clin. Nutr. Metab. Care  2, 135 (1999); Parker-Barnes et al.,  Proc. Natl. Acad. Sci. USA  97, 8284 (2000); Shanklin et al.,  Annu. Rev. Plant Physiol. Plant Nol. Biol.  49, 611 (1998)). Starting from acetyl-CoA, the synthesis of docosahexaenoic acid (DHA) requires approximately 30 distinct enzyme activities and nearly 70 reactions including the four repetitive steps of the fatty acid synthesis cycle. Polyketide synthases (PKSs) carry out some of the same reactions as FAS (Hopwood et al.,  Annu. Rev. Genet.  24, 37 (1990); Bentley et al.,  Annu. Rev. Microbiol.  53, 411 (1999)) and use the same small protein (or domain), acyl carrier protein (ACP), as a covalent attachment site for the growing carbon chain. However, in these enzyme systems, the complete cycle of reduction, dehydration and reduction seen in FAS is often abbreviated so that a highly derivatized carbon chain is produced, typically containing many keto- and hydroxy-groups as well as carbon-carbon double bonds typically in the trans configuration. The linear products of PKSs are often cyclized to form complex biochemicals that include antibiotics and many other secondary products (Hopwood et al., (1990) supra; Bentley et al., (1999), supra; Keating et al.,  Curr. Opin. Chem. Biol.  3, 598 (1999)). 
     Very long chain PUFAs such as docosahexaenoic acid (DHA; 22:6ω3) and eicosapentaenoic acid (EPA; 20:5ω3) have been reported from several species of marine bacteria, including  Shewanella  sp (Nichols et al.,  Curr. Op. Biotechnol.  10, 240 (1999); Yazawa,  Lipids  31, S (1996); DeLong et al.,  Appl. Environ. Microbiol.  51, 730 (1986)). Analysis of a genomic fragment (cloned as plasmid pEPA) from  Shewanella  sp. strain SCRC2738 led to the identification of five open reading frames (Orfs), totaling 20 Kb, that are necessary and sufficient for EPA production in  E. coli  (Yazawa, (1996), supra). Several of the predicted protein domains were homologues of FAS enzymes, while other regions showed no homology to proteins of known function. At least 11 regions within the five Orfs were identifiable as putative enzyme domains (See Metz et al.,  Science  293:290-293 (2001)). When compared with sequences in the gene databases, seven of these were more strongly related to PKS proteins than to FAS proteins. Included in this group were domains putatively encoding malonyl-CoA:ACP acyltransferase (MAT), β-ketoacyl-ACP synthase (KS), β-ketoacyl-ACP reductase (KR), acyltransferase (AT), phosphopantetheine transferase, chain length (or chain initiation) factor (CLF) and a highly unusual cluster of six ACP domains (i.e., the presence of more than two clustered ACP domains had not previously been reported in PKS or FAS sequences). It is likely that the PKS pathway for PUFA synthesis that has been identified in  Shewanella  is widespread in marine bacteria. Genes with high homology to the  Shewanella  gene cluster have been identified in  Photobacterium profundum  (Allen et al.,  Appli. Environ. Microbiol.  65:1710 (1999)) and in  Moritella marina  ( Vibrio marinus ) (see U.S. Pat. No. 6,140,486, ibid., and Tanaka et al.,  Biotechnol. Lett.  21:939 (1999)). 
     Polyunsaturated fatty acids (PUFAs) are considered to be useful for nutritional, pharmaceutical, industrial, and other purposes. The current supply of PUFAs from natural sources and from chemical synthesis is not sufficient for commercial needs. A major current source for PUFAs is from marine fish; however, fish stocks are declining, and this may not be a sustainable resource. Additionally, contamination, from both heavy metals and toxic organic molecules, is a serious issue with oil derived from marine fish. Vegetable oils derived from oil seed crops are relatively inexpensive and do not have the contamination issues associated with fish oils. However, the PUFAs found in commercially developed plant oils are typically limited to linoleic acid (eighteen carbons with 2 double bonds, in the delta 9 and 12 positions—18:2 delta 9,12) and linolenic acid (18:3 delta 9,12,15). In the conventional pathway for PUFA synthesis, medium chain-length saturated fatty acids (products of a fatty acid synthase (FAS) system) are modified by a series of elongation and desaturation reactions. Because a number of separate desaturase and elongase enzymes are required for fatty acid synthesis from linoleic and linolenic acids to produce the more saturated and longer chain PUFAs, engineering plant host cells for the expression of PUFAs such as EPA and docosahexaenoic acid (DHA) may require expression of several separate enzymes to achieve synthesis. Additionally, for production of useable quantities of such PUFAs, additional engineering efforts may be required, for example, engineering the down regulation of enzymes that compete for substrate, engineering of higher enzyme activities such as by mutagenesis or targeting of enzymes to plastid organelles. Therefore it is of interest to obtain genetic material involved in PUFA biosynthesis from species that naturally produce these fatty acids and to express the isolated material alone or in combination in a heterologous system which can be manipulated to allow production of commercial quantities of PUFAs. 
     The discovery of a PUFA PKS system in marine bacteria such as  Shewanella  and  Vibrio marinus  (see U.S. Pat. No. 6,140,486, ibid.), discussed above, provided a resource for new methods of commercial PUFA production. However, the marine bacteria containing PUFA PKS systems that have been identified to date have limitations which may ultimately restrict their usefulness on a commercial level. In particular, although U.S. Pat. No. 6,140,486 discloses that these marine bacteria PUFA PKS systems can be used to genetically modify plants, the marine bacteria naturally live and grow in cold marine environments and the enzyme systems of these bacteria do not function well above 22° C. and may optimally function at much lower temperatures. In contrast, many crop plants, which are attractive targets for genetic manipulation using the PUFA PKS system, have normal growth conditions at temperatures above 22° C. and ranging to higher than 40° C. Therefore, the PUFA PKS systems from these marine bacteria are not predicted to be readily adaptable to plant expression under normal growth conditions. 
     With regard to the production of eicosapentaenoic acid (EPA) in particular, researchers have tried to produce EPA with microbes by growing them in both photosynthetic and heterotrophic cultures. They have also used both classical and directed genetic approaches in attempts to increase the productively of the organisms under culture conditions. Other researchers have attempted to produce EPA in oil-seed crop plants by introduction of genes encoding various desaturase and elongase enzymes. 
     Researchers have attempted to use cultures of red microalgae (Monodus), diatoms (e.g.  Phaeodactylum ), other microalgae and fungi (e.g.  Mortierella  cultivated at low temperatures). However, in all cases, productivity was low compared to existing commercial microbial production systems for other long chain PUFAs such as DHA. In many cases, the EPA occurred primarily in the phospholipids (PL) rather than the triacylglycerols (TAG) form. Since productivity of microalgae under heterotrophic growth conditions can be much higher than under phototrophic conditions, researchers have attempted, and achieved, trophic conversion by introduction of genes encoding specific sugar transporters. However, even with the newly acquired heterotrophic capability, productivity in terms of oil remained relatively low. 
     As discussed above, several marine bacteria have been shown to produce PUFAs (EPA as well as DHA). However, these bacteria do not produce significant quantities of TAG, and the EPA is found primarily in the PL membrane form. The levels of EPA produced by these particular bacteria as well as their growth characteristics (discussed above) limit their utility for commercial production of EPA. 
     There have been many efforts to produce EPA in oil-seed crop plants by modification of the endogenously-produced fatty acids. Genetic modification of these plants with various individual genes for fatty acid elongases and desaturases has produced leaves or seeds containing significant levels of EPA but also containing significant levels of mixed shorter-chain and less unsaturated PUFAs (Qi et al.,  Nature Biotech.  22:739 (2004); PCT Publication No. WO 04/071467; Abbadi et al.,  Plant Cell  16:1 (2004)). In contrast, the known EPA-producing PUFA PKS systems as described herein yield a PUFA profile that is essentially pure EPA. 
     Therefore, there is a need in the art for other PUFA PKS systems having greater flexibility for commercial use, and for a biological system that efficiently produces quantities of lipids (e.g., PL and TAG) enriched in desired PUFAs, such as EPA, in a commercially useful production process. 
     SUMMARY OF THE INVENTION 
     One embodiment of the present invention generally relates to isolated nucleic acid molecules encoding PUFA PKS proteins and domains from  Shewanella japonica  or  Shewanella olleyana , and biologically active homologues and fragments thereof. In one aspect, the invention includes an isolated nucleic acid molecule comprising a nucleic acid sequence selected from: (a) a nucleic acid sequence encoding an amino acid sequence selected from the group consisting of: SEQ ID NO:2, SEQ ID NO:3, SEQ ID NO:4, SEQ ID NO:5, SEQ ID NO:6, SEQ ID NO:8, SEQ ID NO:9, SEQ ID NO:10, SEQ ID NO:11, and SEQ ID NO:12; (b) a nucleic acid sequence encoding a fragment of any of the amino acid sequences of (a) having at least one biological activity selected from the group consisting of enoyl-ACP reductase (ER) activity; acyl carrier protein (ACP) activity; β-ketoacyl-ACP synthase (KS) activity; acyltransferase (AT) activity; β-ketoacyl-ACP reductase (KR) activity; FabA-like β-hydroxyacyl-ACP dehydrase (DH) activity; non-FabA-like dehydrase activity; chain length factor (CLF) activity; malonyl-CoA:ACP acyltransferase (MAT) activity; and 4′-phosphopantetheinyl transferase (PPTase) activity; (c) a nucleic acid sequence encoding an amino acid sequence that is at least about 65% identical, and more preferably at least about 75% identical, and more preferably at least about 85% identical, and more preferably at least about 95% identical, to SEQ ID NO:2 or SEQ ID NO:8 and has at least one biological activity selected from the group consisting of: KS activity, MAT activity, KR activity, ACP activity, and non-FabA-like dehydrase activity; (d) a nucleic acid sequence encoding an amino acid sequence that is at least about 60% identical, and more preferably at least about 70% identical, and more preferably at least about 80% identical, and more preferably at least about 90% identical, to SEQ ID NO:3 or SEQ ID NO:9 and has AT biological activity; (e) a nucleic acid sequence encoding an amino acid sequence that is at least about 70% identical and more preferably at least about 80% identical, and more preferably at least about 90% identical, and more preferably at least about 95% identical, to SEQ ID NO:4 or SEQ ID NO:10 and has at least one biological activity selected from the group consisting of KS activity, CLF activity and DH activity; (f) a nucleic acid sequence encoding an amino acid sequence that is at least about 60% identical, and more preferably at least about 70% identical, and more preferably at least about 80% identical, and more preferably at least about 90% identical, to SEQ ID NO:6 or SEQ ID NO:12 and has PPTase biological activity; (g) a nucleic acid sequence encoding an amino acid sequence that is at least about 85% identical, and more preferably at least about 95% identical, and more preferably at least about 96% identical, and more preferably at least about 97% identical, to SEQ ID NO:11, or at least about 95% identical, and more preferably at least about 96% identical, and more preferably at least about 97% identical, and more preferably at least about 98% identical, to SEQ ID NO:5, and has ER biological activity. 
     In one aspect, the fragment set forth in (b) above is selected from: 
     (a) a fragment of SEQ ID NO:2 from about position 29 to about position 513 of SEQ ID NO:2, wherein the domain has KS biological activity; 
     (b) a fragment of SEQ ID NO:2 from about position 625 to about position 943 of SEQ ID NO:2, wherein the domain has MAT biological activity; 
     (c) a fragment of SEQ ID NO:2 from about position 1264 to about position 1889 of SEQ ID NO:2, and subdomains thereof, wherein the domain or subdomain thereof has ACP biological activity; 
     (d) a fragment of SEQ ID NO:2 from about position 2264 to about position 2398 of SEQ ID NO:2, wherein the domain has KR biological activity; 
     (e) a fragment of SEQ ID NO:2 comprising from about position 2504 to about position 2516 of SEQ ID NO:2, wherein the fragment has non-FabA-like dehydrase biological activity; 
     (f) a fragment of SEQ ID NO:3 from about position 378 to about position 684 of SEQ ID NO:3, wherein the domain has AT biological activity; 
     (g) a fragment of SEQ ID NO:4 from about position 5 to about position 483 of SEQ ID NO:4, wherein the domain has KS biological activity; 
     (h) a fragment of SEQ ID NO:4 from about position 489 to about position 771 of SEQ ID NO:4, wherein the domain has CLF biological activity; 
     (i) a fragment of SEQ ID NO:4 from about position 1428 to about position 1570 of SEQ ID NO:4, wherein the domain has DH biological activity; 
     (j) a fragment of SEQ ID NO:4 from about position 1881 to about position 2019 of SEQ ID NO:4, wherein the domain has DH biological activity; 
     (k) a fragment of SEQ ID NO:5 from about position 84 to about position 497 of SEQ ID NO:5, wherein the domain has ER biological activity; 
     (l) a fragment of SEQ ID NO:6 from about position 40 to about position 186 of SEQ ID NO:6, wherein the domain has PPTase biological activity; 
     (m) a fragment of SEQ ID NO:8 from about position 29 to about position 513 of SEQ ID NO:8, wherein the domain has KS biological activity; 
     (n) a fragment of SEQ ID NO:8 from about position 625 to about position 943 of SEQ ID NO:8, wherein the domain has MAT biological activity; 
     (o) a fragment of SEQ ID NO:8 from about position 1275 to about position 1872 of SEQ ID NO:8, and subdomains thereof, wherein the domain or subdomain thereof has ACP biological activity; 
     (p) a fragment of SEQ ID NO:8 from about position 2240 to about position 2374 of SEQ ID NO:8, wherein the domain has KR biological activity; 
     (q) a fragment of SEQ ID NO:8 comprising from about position 2480-2492 of SEQ ID NO:8, wherein the fragment has non-FabA-like dehydrase activity; 
     (r) a fragment of SEQ ID NO:9 from about position 366 to about position 703 of SEQ ID NO:9, wherein the domain has AT biological activity; 
     (s) a fragment of SEQ ID NO:10 from about position 10 to about position 488 of SEQ ID NO:10, wherein the domain has KS biological activity; 
     (t) a fragment of SEQ ID NO:10 from about position 502 to about position 750 of SEQ ID NO:10, wherein the domain has CLF biological activity; 
     (u) a fragment of SEQ ID NO:10 from about position 1431 to about position 1573 of SEQ ID NO:10, wherein the domain has DH biological activity; 
     (v) a fragment of SEQ ID NO:10 from about position 1882 to about position 2020 of SEQ ID NO:10, wherein the domain has DH biological activity; 
     (w) a fragment of SEQ ID NO:11 from about position 84 to about position 497 of SEQ ID NO:1, wherein the domain has ER biological activity; and 
     (x) a fragment of SEQ ID NO:12 from about position 29 to about position 177 of SEQ ID NO:12, wherein the domain has PPTase biological activity. 
     Also included in the present invention are nucleic acid molecules consisting essentially of a nucleic acid sequence that is fully complementary to any of the above-identified the nucleic acid molecules. One aspect of the invention further relates to a recombinant nucleic acid molecule comprising any of the above-identified nucleic acid molecules, operatively linked to at least one expression control sequence. Another aspect of the invention relates to a recombinant cell transfected with any of the such recombinant nucleic acid molecules. 
     Another embodiment of the invention relates to a genetically modified plant or a part of the plant, wherein the plant has been genetically modified to recombinantly express a PKS system comprising at least one biologically active protein or domain thereof of a polyunsaturated fatty acid (PUFA) polyketide synthase (PKS) system, wherein the protein or domain is encoded by any of the above-described nucleic acid molecules. In one aspect, the genetically modified plant or part of a plant, as a result of the genetic modification, produces one or more polyunsaturated fatty acids selected from the group consisting of: DHA (docosahexaenoic acid (C22:6, ω-3)), ARA (eicosatetraenoic acid or arachidonic acid (C20:4, n-6)), DPA (docosapentaenoic acid (C22:5, ω-6 or ω-3)), and/or EPA (eicosapentaenoic acid (C20:5, ω-3). In particularly preferred embodiment, the plant or part of a plant produces DHA, EPA, EPA and DHA, ARA and DHA, or ARA and EPA. Genetically modified plants can include, crop plants, and any dicotyledonous plant or monocotyledonous plant. Preferred plants include, but are not limited to, canola, soybean, rapeseed, linseed, corn, safflower, sunflower and tobacco. 
     Yet another embodiment of the invention relates to a genetically modified microorganism, wherein the microorganism has been genetically modified to recombinantly express any of the above-described isolated nucleic acid molecules. In one aspect, the microorganism, as a result of the genetic modification, produces a polyunsaturated fatty acid selected from the group consisting of: DHA (docosahexaenoic acid (C22:6, ω-3)), ARA (eicosatetraenoic acid or arachidonic acid (C20:4, n-6)), DPA (docosapentaenoic acid (C22:5, ω-6 or ω-3)), and/or EPA (eicosapentaenoic acid (C20:5, ω-3). In a particularly preferred embodiment, the microorganism, as a result of the genetic modification, produces DHA, EPA, EPA and DHA, ARA and DHA or ARA and EPA. In one aspect, the microorganism is a Thraustochytrid, including, but not limited to,  Schizochytrium  and  Thraustochytrium . In one aspect, the microorganism is a bacterium. 
     In one aspect, the above-described genetically modified plant or microorganism is genetically modified to recombinantly express a nucleic acid molecule encoding at least one amino acid sequence selected from: (a) an amino acid sequence selected from the group consisting of: SEQ ID NO:2, SEQ ID NO:3, SEQ ID NO:4, SEQ ID NO:5, SEQ ID NO:6, SEQ ID NO:8, SEQ ID NO:9, SEQ ID NO:10, SEQ ID NO:11, and SEQ ID NO:12; and (b) a fragment of any of the amino acid sequences of (a) having at least one biological activity selected from the group consisting of enoyl-ACP reductase (ER) activity; acyl carrier protein (ACP) activity; β-ketoacyl-ACP synthase (KS) activity; acyltransferase (AT) activity; β-ketoacyl-ACP reductase (KR) activity; FabA-like β-hydroxyacyl-ACP dehydrase (DH) activity; non-FabA-like dehydrase activity; chain length factor (CLF) activity; malonyl-CoA:ACP acyltransferase (MAT) activity; and 4′-phosphopantetheinyl transferase (PPTase) activity. In one aspect, the plant is genetically modified to recombinantly express a nucleic acid molecule encoding at least one amino acid sequence selected from: SEQ ID NO:2, SEQ ID NO:3, SEQ ID NO:4, SEQ ID NO:5, and/or SEQ ID NO:6. In another aspect, the plant or microorganism is genetically modified to recombinantly express at least one nucleic acid molecule encoding SEQ ID NO:2, SEQ ID NO:3, SEQ ID NO:4, SEQ ID NO:5, and SEQ ID NO:6. In yet another aspect, the plant or microorganism is genetically modified to recombinantly express a nucleic acid molecule encoding at least one amino acid sequence selected from: SEQ ID NO:8, SEQ ID NO:9, SEQ ID NO:10, SEQ ID NO:11, and/or SEQ ID NO:12. In yet another aspect, the plant or microorganism is genetically modified to recombinantly express at least one nucleic acid molecule encoding SEQ ID NO:8, SEQ ID NO:9, SEQ ID NO:10, SEQ ID NO:11, and SEQ ID NO:12. In another aspect, the plant or microorganism is genetically modified to recombinantly express at least one nucleic acid molecule encoding any of the fragments previously described above. 
     In one aspect of the genetically modified plant or part of a plant or microorganism embodiments of the invention, the plant or microorganism is additionally genetically modified to express at least one biologically active protein or domain of a polyunsaturated fatty acid (PUFA) polyketide synthase (PKS) system from a Thraustochytrid, including, but not limited to,  Schizochytrium  and  Thraustochytrium . In one aspect, such a protein or domain comprises an amino acid sequence selected from: (a) SEQ ID NO:14, SEQ ID NO:16, and SEQ ID NO:18; and (b) a fragment of any of the amino acid sequences of (a) having at least one biological activity selected from the group consisting of enoyl-ACP reductase (ER) activity; acyl carrier protein (ACP) activity; β-ketoacyl-ACP synthase (KS) activity; acyltransferase (AT) activity; 1-ketoacyl-ACP reductase (KR) activity; FabA-like β-hydroxyacyl-ACP dehydrase (DH) activity; non-FabA-like dehydrase activity; chain length factor (CLF) activity; malonyl-CoA:ACP acyltransferase (MAT) activity; and 4′-phosphopantetheinyl transferase (PPTase) activity. In another aspect, the protein or domain comprises an amino acid sequence selected from: (a) SEQ ID NO:20, SEQ ID NO:22, and SEQ ID NO:24; and (b) a fragment of any of the amino acid sequences of (a) having at least one biological activity selected from the group consisting of enoyl-ACP reductase (ER) activity; acyl carrier protein (ACP) activity; β-ketoacyl-ACP synthase (KS) activity; acyltransferase (AT) activity; β-ketoacyl-ACP reductase (KR) activity; FabA-like β-hydroxyacyl-ACP dehydrase (DH) activity; non-FabA-like dehydrase activity; chain length factor (CLF) activity; malonyl-CoA:ACP acyltransferase (MAT) activity; and 4′-phosphopantetheinyl transferase (PPTase) activity. 
     In one aspect of the embodiment of the invention related to the genetically modified microorganism, the microorganism comprises an endogenous PUFA PKS system. In this aspect, the endogenous PUFA PKS system can be modified by substitution of another isolated nucleic acid molecule encoding at least one domain of a different PKS system for a nucleic acid sequence encoding at least one domain of the endogenous PUFA PKS system. A different PKS system includes, but is not limited to, a non-bacterial PUFA PKS system, a bacterial PUFA PKS system, a type I modular PKS system, a type I iterative PKS system, a type II PKS system, and a type III PKS system. In another aspect, the endogenous PUFA PKS system has been genetically modified by substitution of any of the above-described isolated nucleic acid molecules of the invention for a nucleic acid sequence encoding at least one domain of the endogenous PUFA PKS system. In another aspect, the microorganism has been genetically modified to recombinantly express a nucleic acid molecule encoding a chain length factor, or a chain length factor plus a β-ketoacyl-ACP synthase (KS) domain, that directs the synthesis of C20 units. In another aspect, the endogenous PUFA PKS system has been modified in a domain or domains selected from the group consisting of a domain encoding FabA-like β-hydroxy acyl-ACP dehydrase (DH) domain and a domain encoding β-ketoacyl-ACP synthase (KS), wherein the modification alters the ratio of long chain fatty acids produced by the PUFA PKS system as compared to in the absence of the modification. Such a modification can include substituting a DH domain that does not possess isomerization activity for a FabA-like β-hydroxy acyl-ACP dehydrase (DH) in the endogenous PUFA PKS system. Such a modification can also include a deletion of all or a part of the domain, a substitution of a homologous domain from a different organism for the domain, and a mutation of the domain. In one aspect, the endogenous PUFA PKS system has been modified in an enoyl-ACP reductase (ER) domain, wherein the modification results in the production of a different compound as compared to in the absence of the modification. In this aspect, such a modification can include a deletion of all or a part of the ER domain, a substitution of an ER domain from a different organism for the ER domain, and a mutation of the ER domain. 
     Another embodiment of the present invention relates to a method to produce a bioactive molecule that is produced by a polyketide synthase system, comprising growing under conditions effective to produce the bioactive molecule, a genetically modified plant as described above. 
     Another embodiment of the present invention relates to a method to produce a bioactive molecule that is produced by a polyketide synthase system, comprising culturing under conditions effective to produce the bioactive molecule, a genetically modified microorganism as described above. 
     In either of the two embodiments directly above, in one aspect, the genetic modification changes at least one product produced by the endogenous PKS system, as compared to a wild-type organism. In another aspect, the organism produces a polyunsaturated fatty acid (PUFA) profile that differs from the naturally occurring organism without a genetic modification. In one aspect, the bioactive molecule is selected from: an anti-inflammatory formulation, a chemotherapeutic agent, an active excipient, an osteoporosis drug, an anti-depressant, an anti-convulsant, an anti- Heliobactor pylori  drug, a drug for treatment of neurodegenerative disease, a drug for treatment of degenerative liver disease, an antibiotic, and a cholesterol lowering formulation. In another aspect, the bioactive molecule is an antibiotic. In another aspect, the bioactive molecule is a polyunsaturated fatty acid (PUFA). In yet another aspect, the bioactive molecule is a molecule including carbon-carbon double bonds in the cis configuration. In another aspect, the bioactive molecule is a molecule including a double bond at every third carbon. 
     Another embodiment of the present invention relates to a method to produce a plant that has a polyunsaturated fatty acid (PUFA) profile that differs from the naturally occurring plant, comprising genetically modifying cells of the plant to express a PKS system comprising at least one recombinant nucleic acid molecule of the present invention described above. 
     Another embodiment of the present invention relates to a method to produce a recombinant microbe, comprising genetically modifying microbial cells to express at least one recombinant nucleic acid molecule of the present invention described above. 
     Yet another embodiment of the present invention relates to a method to modify an endproduct to contain at least one fatty acid, comprising adding to the endproduct an oil produced by a recombinant host cell that expresses at least one recombinant nucleic acid molecule of the present invention as described above. For example, the endproduct can include, but is not limited to, a dietary supplement, a food product, a pharmaceutical formulation, a humanized animal milk, and an infant formula. 
     Yet another embodiment of the present invention relates to a method to produce a humanized animal milk, comprising genetically modifying milk-producing cells of a milk-producing animal with at least one recombinant nucleic acid molecule of the present invention as described above. 
     Another embodiment of the present invention relates to a recombinant host cell which has been modified to express a recombinant bacterial polyunsaturated fatty acid (PUFA) polyketide synthase (PKS) system, wherein the PUFA PKS catalyzes both iterative and non-iterative enzymatic reactions, and wherein the PUFA PKS system comprises: (a) at least one enoyl ACP-reductase (ER) domain; (b) at least six acyl carrier protein (ACP) domains; (c) at least two β-keto acyl-ACP synthase (KS) domains; (d) at least one acyltransferase (AT) domain; (e) at least one ketoreductase (KR) domain; (f) at least two FabA-like β-hydroxy acyl-ACP dehydrase (DH) domains; (g) at least one chain length factor (CLF) domain; (h) at least one malonyl-CoA:ACP acyltransferase (MAT) domain; and (i) at least one 4′-phosphopantetheinyl transferase (PPTase) domain. The PUFA PKS system produces PUFAs at temperatures of at least about 25° C. In one aspect, the PUFA PKS system comprises: (a) one enoyl ACP-reductase (ER) domain; (b) six acyl carrier protein (ACP) domains; (c) two β-keto acyl-ACP synthase (KS) domains; (d) one acyltransferase (AT) domain; (e) one ketoreductase (KR) domain; (f) two FabA-like β-hydroxy acyl-ACP dehydrase (DH) domains; (g) one chain length factor (CLF) domain; (h) one malonyl-CoA:ACP acyltransferase (MAT) domain; and (i) one 4′-phosphopantetheinyl transferase (PPTase) domain. In one aspect, the PUFA PKS system is a PUFA PKS system from a marine bacterium selected from the group consisting of  Shewanella japonica  and  Shewanella olleyana.    
     Yet another embodiment of the present invention relates to a genetically modified organism comprising at least one protein or domain of a bacterial polyunsaturated fatty acid (PUFA) polyketide synthase (PKS) system, wherein the bacterial PUFA PKS system catalyzes both iterative and non-iterative enzymatic reactions, wherein the bacterial PUFA PKS system produces PUFAs at temperatures of at least about 25° C., and wherein the bacterial PUFA PKS system comprises: (a) at least one enoyl ACP-reductase (ER) domain; (b) at least six acyl carrier protein (ACP) domains; (c) at least two β-keto acyl-ACP synthase (KS) domains; (d) at least one acyltransferase (AT) domain; (e) at least one ketoreductase (KR) domain; (f) at least two FabA-like β-hydroxy acyl-ACP dehydrase (DH) domains; (g) at least one chain length factor (CLF) domain; (h) at least one malonyl-CoA:ACP acyltransferase (MAT) domain; and (i) at least one 4′-phosphopantetheinyl transferase (PPTase) domain. The genetic modification affects the activity of the PUFA PKS system. In one aspect, the organism is modified to recombinantly express at least one protein or domain of the bacterial PUFA PKS system. In another aspect, the organism is modified to recombinantly express the bacterial PUFA PKS system. The organism can include a plant or a microorganism. In one aspect, the bacterial PUFA PKS system is a PUFA PKS system from a marine bacterium selected from the group consisting of  Shewanella japonica  and  Shewanella olleyana . In another aspect, the organism expresses at least one additional protein or domain from a second, different PKS system. 
     Another embodiment of the present invention relates to an isolated recombinant nucleic acid molecule encoding at least one protein or functional domain of a bacterial (PUFA) polyketide synthase (PKS) system, wherein the bacterial PUFA PKS system catalyzes both iterative and non-iterative enzymatic reactions, wherein the bacterial PUFA PKS system produces PUFAs at temperatures of at least about 25° C., and wherein the bacterial PUFA PKS system comprises: (a) at least one enoyl ACP-reductase (ER) domain; (b) at least six acyl carrier protein (ACP) domains; (c) at least two β-keto acyl-ACP synthase (KS) domains; (d) at least one acyltransferase (AT) domain; (e) at least one ketoreductase (KR) domain; (f) at least two FabA-like β-hydroxy acyl-ACP dehydrase (DH) domains; (g) at least one chain length factor (CLF) domain; (h) at least one malonyl-CoA:ACP acyltransferase (MAT) domain; and (i) at least one 4′-phosphopantetheinyl transferase (PPTase) domain. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES OF THE INVENTION 
         FIG. 1  is a schematic drawing illustrating the open reading frame (ORF) architecture of EPA production clusters from  Shewanella  sp. SCRC-2738,  Shewanella japonica , and  Shewanella olleyana.    
         FIG. 2  is a schematic drawing illustrating the domain architecture of the EPA production gene clusters from  Shewanella  sp. SCRC-2738,  Shewanella japonica  and  Shewanella olleyana.    
         FIG. 3A  is a sequence alignment showing the overlap between the end of pfaB ORF and the start of pfaC ORF (nucleotides 21101-21150 of SEQ ID NO:1, including the complementary strand, is shown) and their corresponding amino acid translation (pfaB: positions 751-759 of SEQ ID NO:3; pfaC: positions 1-9 of SEQ ID NO:4) from  Shewanella japonica  (cosmid 3F3). 
         FIG. 3B  is a sequence alignment showing the overlap between the end of pfaB ORF and the start of pfaC ORF (nucleotides 27943-28008 of SEQ ID NO:7, including the complementary strand, is shown) and their corresponding amino acid translation (pfaB: positions 735-742 of SEQ ID NO:9; pfaC: positions 1-9 of SEQ ID NO:10) from  Shewanella olleyana  (cosmid 9A10). 
         FIG. 4  is a sequence alignment showing the N-terminal end of the pfaE ORFs (Sja_pfaE: positions 1-70 of SEQ ID NO:6; Sol_pfaE: positions 1-59 of SEQ ID NO:12) versus the annotated start of orf2 from  Shewanella  sp. SCRC-2738 (orf2_ATG: SEQ ID NO:61) and the experimentally functional start of orf2 from  Shewanella  sp. SCRC-2738 (WO 98/55625) (orf2_TTG: SEQ ID NO:62). 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention generally relates to polyunsaturated fatty acid (PUFA) polyketide synthase (PKS) systems from a subset of marine bacteria that naturally produce EPA and grow well at temperatures up to about 30° C. and possibly higher (e.g., up to 35° C. or beyond), to genetically modified organisms comprising such PUFA PKS systems, to methods of making and using such systems for the production of products of interest, including bioactive molecules and particularly, PUFAs, such as DHA, DPA and EPA. 
     As used herein, a PUFA PKS system (which may also be referred to as a PUFA synthase system) generally has the following identifying features: (1) it produces PUFAs as a natural product of the system; and (2) it comprises several multifunctional proteins assembled into a complex that conducts both iterative processing of the fatty acid chain as well non-iterative processing, including trans-cis isomerization and enoyl reduction reactions in selected cycles. Reference to a PUFA PKS system refers collectively to all of the genes and their encoded products that work in a complex to produce PUFAs in an organism. Therefore, the PUFA PKS system refers specifically to a PKS system for which the natural products are PUFAs. 
     More specifically, first, a PUFA PKS system that forms the basis of this invention produces polyunsaturated fatty acids (PUFAs) as products (i.e., an organism that endogenously (naturally) contains such a PKS system makes PUFAs using this system). The PUFAs referred to herein are preferably polyunsaturated fatty acids with a carbon chain length of at least 16 carbons, and more preferably at least 18 carbons, and more preferably at least 20 carbons, and more preferably 22 or more carbons, with at least 3 or more double bonds, and preferably 4 or more, and more preferably 5 or more, and even more preferably 6 or more double bonds, wherein all double bonds are in the cis configuration. It is an object of the present invention to find or create via genetic manipulation or manipulation of the endproduct, PKS systems which produce polyunsaturated fatty acids of desired chain length and with desired numbers of double bonds. Examples of PUFAs include, but are not limited to, DHA (docosahexaenoic acid (C22:6, ω-3)), ARA (eicosatetraenoic acid or arachidonic acid (C20:4, n-6)), DPA (docosapentaenoic acid (C22:5, ω-6 or ω-3)), and EPA (eicosapentaenoic acid (C20:5, ω-3)). 
     Second, the PUFA PKS system described herein incorporates both iterative and non-iterative reactions, which generally distinguish the system from previously described PKS systems (e.g., type I modular or iterative, type II or type III). More particularly, the PUFA PKS system described herein contains domains that appear to function during each cycle as well as those which appear to function during only some of the cycles. A key aspect of this functionality may be related to the domains showing homology to the bacterial Fab-A enzymes. For example, the Fab-A enzyme of  E. coli  has been shown to possess two enzymatic activities. It possesses a dehydration activity in which a water molecule (H 2 O) is abstracted from a carbon chain containing a hydroxy group, leaving a trans double bond in that carbon chain. In addition, it has an isomerase activity in which the trans double bond is converted to the cis configuration. This isomerization is accomplished in conjunction with a migration of the double bond position to adjacent carbons. In PKS (and FAS) systems, the main carbon chain is extended in 2 carbon increments. One can therefore predict the number of extension reactions required to produce the PUFA products of these PKS systems. For example, to produce DHA (C22:6, all cis) requires 10 extension reactions. Since there are only 6 double bonds in the end product, it means that during some of the reaction cycles, a double bond is retained (as a cis isomer), and in others, the double bond is reduced prior to the next extension. 
     Before the discovery of a PUFA PKS system in marine bacteria (see U.S. Pat. No. 6,140,486), PKS systems were not known to possess this combination of iterative and selective enzymatic reactions, and they were not thought of as being able to produce carbon-carbon double bonds in the cis configuration. However, the PUFA PKS system described by the present invention has the capacity to introduce cis double bonds and the capacity to vary the reaction sequence in the cycle. 
     The present inventors propose to use these features of the PUFA PKS system to produce a range of bioactive molecules that could not be produced by the previously described (Type I iterative or modular, Type II, or Type III) PKS systems. These bioactive molecules include, but are not limited to, polyunsaturated fatty acids (PUFAs), antibiotics or other bioactive compounds, many of which will be discussed below. For example, using the knowledge of the PUFA PKS gene structures described herein, any of a number of methods can be used to alter the PUFA PKS genes, or combine portions of these genes with other synthesis systems, including other PKS systems, such that new products are produced. The inherent ability of this particular type of system to do both iterative and selective reactions will enable this system to yield products that would not be found if similar methods were applied to other types of PKS systems. 
     In U.S. patent application Ser. No. 10/810,352, supra, the present inventors identified two exemplary marine bacteria (e.g.  Shewanella olleyana  and  Shewanella japonica ) that are particularly suitable for use as sources of PUFA PKS genes, because they have the surprising characteristic of being able to produce PUFAs (e.g., EPA) and grow at temperatures up to about 30° C., in contrast to previously described PUFA PKS-containing marine bacteria, including other species and strains within  Shewanella , which typically produce PUFAs and grow at much lower temperatures. The inventors have now cloned and sequenced the full-length genomic sequence of all of the PUFA PKS open reading frames (Orfs) in each of  Shewanella olleyana  (Australian Collection of Antarctic Microorganisms (ACAM) strain number 644; Skerratt et al.,  Int. J. Syst. Evol. Microbiol.  52, 2101 (2002)) and  Shewanella japonica  (American Type Culture Collection (ATCC) strain number BAA-316; Ivanova et al.,  Int. J. Syst. Evol. Microbiol.  51, 1027 (2001)), and have identified the domains comprising the PUFA PKS system in these special marine bacteria. Therefore, the present invention solves the above-mentioned problem of providing additional PUFA PKS systems that have the flexibility for commercial use. 
     The PUFA PKS systems of the present invention can also be used as a tool in a strategy to solve the above-identified problem for production of commercially valuable lipids enriched in a desired PUFA, such as EPA, by the present inventors&#39; development of genetically modified microorganisms and methods for efficiently producing lipids enriched in PUFAs in one or more of their various forms (e.g., triacylglycerols (TAG) and phospholipids (PL)) by manipulation of the polyketide synthase-like system that produces PUFAs in eukaryotes, including members of the order Thraustochytriales such as  Schizochytrium  and  Thraustochytrium . Specifically, and by way of example, the present inventors describe herein a strain of  Schizochytrium  that has previously been optimized for commercial production of oils enriched in PUFA, primarily docosahexaenoic acid (DHA; C22:6 n-3) and docosapentaenoic acid (DPA; C22:5 n-6), and that will now be genetically modified such that EPA (C20:5 n-3) production (or other PUFA production) replaces the DHA production, without sacrificing the oil productivity characteristics of the organism. One can use the marine bacterial PUFA PKS genes from the marine bacteria described in the present invention in one embodiment to produce such a genetically modified microorganism. This is only one example of the technology encompassed by the invention, as the concepts of the invention can readily be applied to other production organisms and other desired PUFAs as described in detail below. 
     As used herein, the term “lipid” includes phospholipids; free fatty acids; esters of fatty acids; triacylglycerols; diacylglycerides; phosphatides; sterols and sterol esters; carotenoids; xanthophylls (e.g., oxycarotenoids); hydrocarbons; and other lipids known to one of ordinary skill in the art. The terms “polyunsaturated fatty acid” and “PUFA” include not only the free fatty acid form, but other forms as well, such as the TAG form and the PL form. 
     In one embodiment, a PUFA PKS system according to the present invention comprises at least the following biologically active domains: (a) at least one enoyl-ACP reductase (ER) domain; (b) at least six acyl carrier protein (ACP) domains; (c) at least two β-ketoacyl-ACP synthase (KS) domains; (d) at least one acyltransferase (AT) domain; (e) at least one β-ketoacyl-ACP reductase (KR) domain; (f) at least two FabA-like β-hydroxyacyl-ACP dehydrase (DH) domains; (g) at least one chain length factor (CLF) domain; and (h) at least one malonyl-CoA:ACP acyltransferase (MAT) domain. A PUFA PKS system also comprises at least one 4′-phosphopantetheinyl transferase (PPTase) domain, and such domain can be considered to be a part of the PUFA PKS system or an accessory domain or protein to the PUFA PKS system. In one embodiment a PUFA PKS system according to the present invention also comprises at least one region containing a dehydratase (DH) conserved active site motif. The functions of these domains and motifs are generally individually known in the art and will be described in detail below with regard to the PUFA PKS system of the present invention. The domains of the present invention may be found as a single protein (i.e., the domain and protein are synonymous) or as one of two or more (multiple) domains in a single protein. The domain architecture of the PUFA PKS systems in these  Shewanella  species is described in more detail below and is illustrated in  FIG. 2 . 
     In another embodiment, the PUFA PKS system comprises at least the following biologically active domains: (a) at least one enoyl-ACP reductase (ER) domain; (b) multiple acyl carrier protein (ACP) domain(s) (at least from one to four, and preferably at least five, and more preferably at least six, and even more preferably seven, eight, nine, or more than nine); (c) at least two β-ketoacyl-ACP synthase (KS) domains; (d) at least one acyltransferase (AT) domain; (e) at least one β-ketoacyl-ACP reductase (KR) domain; (f) at least two FabA-like β-hydroxyacyl-ACP dehydrase (DH) domains; (g) at least one chain length factor (CLF) domain; (h) at least one malonyl-CoA:ACP acyltransferase (MAT) domain; and (i) at least one 4′-phosphopantetheinyl transferase (PPTase) domain. In one embodiment a PUFA PKS system according to the present invention also comprises at least one region containing a dehydratase (DH) conserved active site motif. 
     According to the present invention, a domain or protein having β-ketoacyl-ACP synthase (KS) biological activity (function) is characterized as the enzyme that carries out the initial step of the FAS (and PKS) elongation reaction cycle. The term “β-ketoacyl-ACP synthase” can be used interchangeably with the terms “3-keto acyl-ACP synthase”, “β-keto acyl-ACP synthase”, and “keto-acyl ACP synthase”, and similar derivatives. The acyl group destined for elongation is linked to a cysteine residue at the active site of the enzyme by a thioester bond. In the multi-step reaction, the acyl-enzyme undergoes condensation with malonyl-ACP to form -ketoacyl-ACP, CO 2  and free enzyme. The KS plays a key role in the elongation cycle and in many systems has been shown to possess greater substrate specificity than other enzymes of the reaction cycle. For example,  E. coli  has three distinct KS enzymes—each with its own particular role in the physiology of the organism (Magnuson et al.,  Microbiol. Rev.  57, 522 (1993)). The two KS domains of the PUFA-PKS systems described herein could have distinct roles in the PUFA biosynthetic reaction sequence. 
     As a class of enzymes, KS&#39;s have been well characterized. The sequences of many verified KS genes are known, the active site motifs have been identified and the crystal structures of several have been determined. Proteins (or domains of proteins) can be readily identified as belonging to the KS family of enzymes by homology to known KS sequences. 
     According to the present invention, a domain or protein having malonyl-CoA:ACP acyltransferase (MAT) biological activity (function) is characterized as one that transfers the malonyl moiety from malonyl-CoA to ACP. The term “malonyl-CoA:ACP acyltransferase” can be used interchangeably with “malonyl acyltransferase” and similar derivatives. In addition to the active site motif (GxSxG), these enzymes possess an extended motif (R and Q amino acids in key positions) that identifies them as MAT enzymes (in contrast to the AT domain, discussed below). In some PKS systems (but not the PUFA PKS domain), MAT domains will preferentially load methyl- or ethyl-malonate on to the ACP group (from the corresponding CoA ester), thereby introducing branches into the linear carbon chain. MAT domains can be recognized by their homology to known MAT sequences and by their extended motif structure. 
     According to the present invention, a domain or protein having acyl carrier protein (ACP) biological activity (function) is characterized as being a small polypeptide (typically, 80 to 100 amino acids long), that functions as a carrier for growing fatty acyl chains via a thioester linkage to a covalently bound co-factor of the protein. These polypeptides occur as separate units or as domains within larger proteins. ACPs are converted from inactive apo-forms to functional holo-forms by transfer of the phosphopantetheinyl moiety of CoA to a highly conserved serine residue of the ACP. Acyl groups are attached to ACP by a thioester linkage at the free terminus of the phosphopantetheinyl moiety. ACPs can be identified by labeling with radioactive pantetheine and by sequence homology to known ACPs. The presence of variations of an active site motif (LGIDS*; e.g., see amino acids 1296-1300 of SEQ ID NO:2) is also a signature of an ACP. 
     According to the present invention, a domain or protein having β-ketoacyl-ACP reductase (KR) activity is characterized as one that catalyzes the pyridine-nucleotide-dependent reduction of 3-ketoacyl forms of ACP. The term “β-ketoacyl-ACP reductase” can be used interchangeably with the terms “ketoreductase”, “3-ketoacyl-ACP reductase”, “keto-acyl ACP reductase” and similar derivatives of the term. It is the first reductive step in the de novo fatty acid biosynthesis elongation cycle and a reaction often performed in polyketide biosynthesis. Significant sequence similarity is observed with one family of enoyl-ACP reductases (ER), the other reductase of FAS (but not the ER family present in the PUFA PKS system), and the short-chain alcohol dehydrogenase family. Pfam analysis of this PUFA PKS region may reveal the homology to the short-chain alcohol dehydrogenase family in the core region. Blast analysis of the same region may reveal matches in the core area to known KR enzymes as well as an extended region of homology to domains from the other characterized PUFA PKS systems. 
     According to the present invention, a domain or protein is referred to as a chain length factor (CLF) based on the following rationale. The CLF was originally described as characteristic of Type II (dissociated enzymes) PKS systems and was hypothesized to play a role in determining the number of elongation cycles, and hence the chain length, of the end product. CLF amino acid sequences show homology to KS domains (and are thought to form heterodimers with a KS protein), but they lack the active site cysteine. The role of CLF in PKS systems has been controversial. Evidence (C. Bisang et al.,  Nature  401, 502 (1999)) suggests a role in priming the PKS systems (by providing the initial acyl group to be elongated). In this role, the CLF domain is thought to decarboxylate malonate (as malonyl-ACP), thus forming an acetate group that can be transferred to the KS active site. This acetate therefore acts as the ‘priming’ molecule that can undergo the initial elongation (condensation) reaction. Homologues of the Type II CLF have been identified as ‘loading’ domains in some type I modular PKS systems. However, other recent evidence suggests a genuine role of the CLF domains in determining chain length (Yi et al.,  J. Am. Chem. Soc.  125:12708 (2003). A domain with the sequence features of the CLF is found in all currently identified PUFA PKS systems and in each case is found as part of a multidomain protein. 
     Reference to an “acyltransferase” or “AT” refers to a general class of enzymes that can carry out a number of distinct acyl transfer reactions. The term “acyltransferase” can be used interchangeably with the term “acyl transferase”. The  Schizochytrium  domain shows good homology to a domain present in all of the other PUFA PKS systems currently examined and very weak homology to some acyltransferases whose specific functions have been identified (e.g. to malonyl-CoA:ACP acyltransferase, MAT). In spite of the weak homology to MAT, the AT domain is not believed to function as a MAT because it does not possess an extended motif structure characteristic of such enzymes (see MAT domain description, above). For the purposes of this disclosure, the functions of the AT domain in a PUFA PKS system include, but are not limited to: transfer of the fatty acyl group from the OrfA ACP domain(s) to water (i.e. a thioesterase-releasing the fatty acyl group as a free fatty acid), transfer of a fatty acyl group to an acceptor such as CoA, transfer of the acyl group among the various ACP domains, or transfer of the fatty acyl group to a lipophilic acceptor molecule (e.g. to lysophosphadic acid). 
     According to the present invention, a protein or domain having enoyl-ACP reductase (ER) biological activity reduces the trans-double bond (introduced by the DH activity) in the fatty acyl-ACP, resulting in fully saturating those carbons. The ER domain in the PUFA-PKS shows homology to a newly characterized family of ER enzymes (Heath et al.,  Nature  406, 145 (2000)). According to the present invention, the term “enoyl-ACP reductase” can be used interchangeably with “enoyl reductase”, “enoyl ACP-reductase” and “enoyl acyl-ACP reductase”. Heath and Rock identified this new class of ER enzymes by cloning a gene of interest from  Streptococcus pneumoniae , purifying a protein expressed from that gene, and showing that it had ER activity in an in vitro assay. The bacterial PUFA PKS systems described herein contain one ER domain. 
     According to the present invention, a protein or domain having dehydrase or dehydratase (DH) activity catalyzes a dehydration reaction. As used generally herein, reference to DH activity typically refers to FabA-like β-hydroxyacyl-ACP dehydrase (DH) biological activity. FabA-like β-hydroxyacyl-ACP dehydrase (DH) biological activity removes HOH from a β-ketoacyl-ACP and initially produces a trans double bond in the carbon chain. The term “FabA-like β-hydroxyacyl-ACP dehydrase” can be used interchangeably with the terms “FabA-like β-hydroxy acyl-ACP dehydrase”, “β-hydroxyacyl-ACP dehydrase”, “dehydrase” and similar derivatives. The DH domains of the PUFA PKS systems show homology to bacterial DH enzymes associated with their FAS systems (rather than to the DH domains of other PKS systems). A subset of bacterial DH&#39;s, the FabA-like DH&#39;s, possesses cis-trans isomerase activity (Heath et al.,  J. Biol. Chem.,  271, 27795 (1996)). It is the homology to the FabA-like DH proteins that indicate that one or all of the DH domains described herein is responsible for insertion of the cis double bonds in the PUFA PKS products. 
     A protein of the invention may also have dehydratase activity that is not characterized as FabA-like (e.g., the cis-trans activity described above is associated with FabA-like activity), generally referred to herein as non-FabA-like DH activity, or non-FabA-like β-hydroxyacyl-ACP dehydrase (DH) biological activity. More specifically, a conserved active site motif (˜13 amino acids long: L*xxHxxxGxxxxP; amino acids 2504-2516 of SEQ ID NO:2; *in the motif, L can also be I) is found in dehydratase domains in PKS systems (Donadio S, Katz L. Gene. 1992 Feb. 1; 111(1):51-60). This conserved motif, also referred to herein as a dehydratase (DH) conserved active site motif or DH motif, is found in a similar region of all known PUFA-PKS sequences described to date and in the PUFA PKS sequences described herein (e.g., amino acids 2504-2516 of SEQ ID NO:2, or amino acids 2480-2492 of SEQ ID NO:8), but it is believed that his motif has been previously undetected until the present invention. This conserved motif is within an uncharacterized region of high homology in the PUFA-PKS sequence. The proposed biosynthesis of PUFAs via the PUFA-PKS requires a non-FabA like dehydration, and this motif may be responsible for the reaction. 
     According to the present invention, a domain or protein having 4′-phosphopantetheinyl transferase (PPTase) biological activity (function) is characterized as the enzyme that transfers a 4′-phosphopantetheinyl moiety from Coenzyme A to the acyl carrier protein (ACP). This transfer to an invariant serine reside of the ACP activates the inactive apo-form to the holo-form. In both polyketide and fatty acid synthesis, the phosphopantetheine group forms thioesters with the growing acyl chains. The PPTases are a family of enzymes that have been well characterized in fatty acid synthesis, polyketide synthesis, and non-ribosomal peptide synthesis. The sequences of many PPTases are known, and crystal structures have been determined (e.g., Reuter K, Mofid M R, Marahiel M A, Ficner R. “Crystal structure of the surfactin synthetase-activating enzyme sfp: a prototype of the 4′-phosphopantetheinyl transferase superfamily” EMBO J. 1999 Dec. 1;18(23):6823-31) as well as mutational analysis of amino acid residues important for activity (Mofid M R, Finking R, Essen L O, Marahiel M A. “Structure-based mutational analysis of the 4′-phosphopantetheinyl transferases Sfp from  Bacillus subtilis : carrier protein recognition and reaction mechanism” Biochemistry. 2004 Apr. 13;43(14):4128-36). These invariant and highly conserved amino acids in PPTases are contained within the pfaE ORFs from both  Shewanella  strains described herein. Additionally, the pfaE ORF homolog in  Shewanella  sp. SCRC-2738 orf2 has been shown to be required for activity in the native strain (Yazawa K. “Production of eicosapentaenoic acid from marine bacteria”. Lipids. 1996 Mar;31 Suppl:S297-300) and labeling experiments confirming its PPTase activity (WO 98/55625). 
     The PUFA PKS systems of particular marine bacteria (e.g.,  Shewanella olleyana  and  Shewanella japonica ) that produce PUFAs and grow well at temperatures of up to about 25-30° C., and possibly higher (e.g., 35° C.), are the basis of the present invention, although the present invention does contemplate the use of domains from these bacterial PUFA PKS systems in conjunction with domains from other bacterial and non-bacterial PUFA PKS systems that have been described, for example, in U.S. Pat. Nos. 6,140,486, 6,566,583, U.S. patent application Ser. Nos. 10/124,800, and 10/810,352. More particularly, the PUFA PKS systems of the present invention can be used with other PUFA PKS systems to produce hybrid constructs and genetically modified microorganisms and plants for improved and or modified production of biological products by such microorganisms and plants. For example, according to the present invention, genetically modified organisms can be produced which incorporate non-bacterial PUFA PKS functional domains with bacterial PUFA PKS functional domains (preferably those of the present invention), as well as PKS functional domains or proteins from other PKS systems (type I, type II, type III) or FAS systems. 
     Reference herein to a “non-bacterial PUFA PKS” system is reference to a PUFA PKS system that has been isolated from an organism that is not a bacterium, or is a homologue of, or derived from, a PUFA PKS system from an organism that is not a bacterium, such as a eukaryote or an archaebacterium. Eukaryotes are separated from prokaryotes based on the degree of differentiation of the cells, with eukaryotes having more highly differentiated cells and prokaryotes having less differentiated cells. In general, prokaryotes do not possess a nuclear membrane, do not exhibit mitosis during cell division, have only one chromosome, their cytoplasm contains 70S ribosomes, they do not possess any mitochondria, endoplasmic reticulum, chloroplasts, lysosomes or Golgi apparatus, their flagella (if present) consists of a single fibril. In contrast, eukaryotes have a nuclear membrane, they do exhibit mitosis during cell division, they have many chromosomes, their cytoplasm contains 80S ribosomes, they do possess mitochondria, endoplasmic reticulum, chloroplasts (in algae), lysosomes and Golgi apparatus, and their flagella (if present) consists of many fibrils. In general, bacteria are prokaryotes, while algae, fungi, protist, protozoa and higher plants are eukaryotes. 
     Non-bacterial PUFA PKS systems include those that have been described in the above identified patents and applications, and particularly include any PUFA PKS system isolated or derived from any Thraustochytrid. In U.S. Pat. No. 6,566,583, several cDNA clones from  Schizochytrium  showing homology to  Shewanella  sp. strain SCRC2738 PKS genes were sequenced, and various clones were assembled into nucleic acid sequences representing two partial open reading frames and one complete open reading frame. Further sequencing of cDNA and genomic clones by the present inventors allowed the identification of the full-length genomic sequence of each of OrfA, OrfB and OrfC in  Schizochytrium  and the complete identification of the domains in  Schizochytrium  with homology to those in  Shewanella . These genes are described in detail in U.S. patent application Ser. No. 10/124,800, supra and are described in some detail below. Similarly, U.S. patent application Ser. No. 10/810,352 describes in detail the full-length genomic sequence of the genes encoding the PUFA PKS system in a  Thraustochytrium  (specifically,  Thraustochytrium  sp. 23B (ATCC 20892)) as well as the domains comprising the PUFA PKS system in  Thraustochytrium.    
     According to the present invention, the phrase “open reading frame” is denoted by the abbreviation “Orf”. It is noted that the protein encoded by an open reading frame can also be denoted in all upper case letters as “ORF” and a nucleic acid sequence for an open reading frame can also be denoted in all lower case letters as “orf”, but for the sake of consistency, the spelling “Orf” is preferentially used herein to describe either the nucleic acid sequence or the protein encoded thereby. It will be obvious from the context of the usage of the term whether a protein or nucleic acid sequence is referenced. 
       FIG. 1  shows the architecture of the PUFA PKS (also referred to as “EPA production”) clusters from  Shewanella  sp. SCRC-2738 (“Yazawa” strain; Yazawa K. “Production of eicosapentaenoic acid from marine bacteria” Lipids. 1996 Mar;31 Suppl:S297-300) versus the gene clusters of the present invention from  Shewanella japonica  (cosmid 3F3) and  Shewanella olleyana  (cosmid 9A10).  FIG. 2  shows the domain architecture of the PUFA PKS gene clusters from  Shewanella  sp. SCRC-2738 (“Yazawa” strain) verses that encoded by the gene clusters from  Shewanella japonica  (cosmid 3F3) and  Shewanella olleyana  (cosmid 9A10). The domain structure of each open reading frame is described below. 
       Shewanella japonica  PUFA PKS 
     SEQ ID NO:1 is the nucleotide sequence for  Shewanella japonica  cosmid 3F3 and is found to contain 15 ORFs as detailed in Table 1 (see Example 2). The ORFs related to the PUFA PKS system in this microorganism are characterized as follows. 
     pfaA (nucleotides 10491-18854 of SEQ ID NO:1) encodes PFAS A (SEQ ID NO:2), a PUFA PKS protein harboring the following domains: β-ketoacyl-synthase (KS) (nucleotides 10575-12029 of SEQ ID NO:1, amino acids 29-513 of SEQ ID NO:2); malonyl-CoA: ACP acyltransferase (MAT) (nucleotides 12366-13319 of SEQ ID NO:1, amino acids 625-943 of SEQ ID NO:2); six tandem acyl-carrier proteins (ACP) domains (nucleotides 14280-16157 of SEQ ID NO:1, amino acids 1264-1889 of SEQ ID NO:2); β-ketoacyl-ACP reductase (KR) (nucleotides 17280-17684 of SEQ ID NO:1, amino acids 2264-2398 of SEQ ID NO:2); and a region of the PFAS A protein between amino acids 2399 and 2787 of SEQ ID NO:2 containing a dehydratase (DH) conserved active site motif LxxHxxxGxxxxP (amino acids 2504-2516 of SEQ ID NO:2), referred to herein as DH-motif region. 
     In PFAS A, a KS active site DXAC* is located at amino acids 226-229 of SEQ ID NO:2 with the C* being the site of the acyl attachment. A MAT active site, GHS*XG, is located at amino acids 721-725 of SEQ ID NO:2, with the S* being the acyl binding site. ACP active sites of LGXDS* are located at the following positions: amino acids 1296-1300, amino acids 1402-1406, amino acids 1513-1517, amino acids 1614-1618, amino acids 1728-1732, and amino acids 1843-1847 in SEQ ID NO:2, with the S* being the phosphopantetheine attachment site. Between amino acids 2399 and 2787 of SEQ ID NO:2, the PFAS A also contains the dehydratase (DH) conserved active site motif LxxHxxxGxxxxP (amino acids 2504-2516 of SEQ ID NO:2) referenced above. 
     pfaB (nucleotides 18851-21130 of SEQ ID NO:1) encodes PFAS B (SEQ ID NO:3), a PUFA PKS protein harboring the following domain: acyltransferase (AT) (nucleotides 19982-20902 of SEQ ID NO:1, amino acids 378-684 of SEQ ID NO:3). 
     In PFAS B, an active site GXS*XG motif is located at amino acids 463-467 of SEQ ID NO:3, with the S* being the site of acyl-attachment. 
     pfaC (nucleotides 21127-27186 of SEQ ID NO:1) encodes PFAS C (SEQ ID NO:4), a PUFA PKS protein harboring the following domains: KS (nucleotides 21139-22575 of SEQ ID NO:1, amino acids 5-483 of SEQ ID NO:4); chain length factor (CLF) (nucleotides 22591-23439 of SEQ ID NO:1, amino acids 489-771 of SEQ ID NO:4); and two FabA 3-hydroxyacyl-ACP dehydratases, referred to as DH1 (nucleotides 25408-25836 of SEQ ID NO:1, amino acids 1428-1570 of SEQ ID NO:4) and DH2 (nucleotides 26767-27183 of SEQ ID NO:1, amino acids 1881-2019 of SEQ ID NO:4). 
     In PFAS C, a KS active site DXAC* is located at amino acids 211-214 of SEQ ID NO:4 with the C* being the site of the acyl attachment. 
     pfaD (nucleotides 27197-28825 of SEQ ID NO:1) encodes the PFAS D (SEQ ID NO:5), a PUFA PKS protein harboring the following domain: an enoyl reductase (ER) (nucleotides 27446-28687 of SEQ ID NO:1, amino acids 84-497 of SEQ ID NO:5). 
     pfaE (nucleotides 6150-7061 of SEQ ID NO:1 on the reverse complementary strand) encodes PFAS E (SEQ ID NO:6), a 4′-phosphopantetheinyl transferase (PPTase) with the identified domain (nucleotides 6504-6944 of SEQ ID NO:1, amino acids 40-186 of SEQ ID NO:6). 
       Shewanella olleyana  PUFA PKS 
     SEQ ID NO:7 is the nucleotide sequence for  Shewanella olleyana  cosmid 9A10 and was found to contain 17 ORFs as detailed in Table 2 (see Example 2). The ORFs related to the PUFA PKS system in this microorganism are characterized as follows. 
     pfaA (nucleotides 17437-25743 of SEQ ID NO:7) encodes PFAS A (SEQ ID NO:8), a PUFA PKS protein harboring the following domains: β-ketoacyl-synthase (KS) (nucleotides 17521-18975 of SEQ ID NO:7, amino acids 29-513 of SEQ ID NO:8); malonyl-CoA: ACP acyltransferase (MAT) (nucleotides 19309-20265 of SEQ ID NO:7, amino acids 625-943 of SEQ ID NO:8); six tandem acyl-carrier proteins (ACP) domains (nucleotides 21259-23052 of SEQ ID NO:7, amino acids 1275-1872 of SEQ ID NO:8); β-ketoacyl-ACP reductase (KR) (nucleotides 24154-24558 of SEQ ID NO:7, amino acids 2240-2374 of SEQ ID NO:8); and a region of the PFAS A protein between amino acids 2241 and 2768 of SEQ ID NO:8 containing a dehydratase (DH) conserved active site motif LxxHxxxGxxxxP (amino acids 2480-2492 of SEQ ID NO:8), referred to herein as DH-motif region. 
     In PFAS A, a KS active site DXAC* is located at AA 226-229 of SEQ ID NO:8 with the C* being the site of the acyl attachment. A MAT active site, GHS*XG, is located at amino acids 721-725 of SEQ ID NO:8 with the S* being the acyl binding site. ACP active sites of LGXDS* are located at: amino acids 1307-1311, amino acids 1408-1412, amino acids 1509-1513, amino acids 1617-1621, amino acids 1721-1725, and amino acids 1826-1830 in SEQ ID NO:8, with the S* being the phosphopantetheine attachment site. Between amino acids 2241 and 2768 of SEQ ID NO:8, the PFAS A also contains the dehydratase (DH) conserved active site motif LxxHxxxGxxxxP (amino acids 2480-2492 of SEQ ID NO:8) referenced above. 
     pfaB (nucleotides 25740-27971 of SEQ ID NO:7) encodes PFAS B (SEQ ID NO:9), a PUFA PKS protein harboring the following domain: acyltransferase (AT) (nucleotides 26837-27848 of SEQ ID NO:1, amino acids 366-703 of SEQ ID NO:9). 
     In PFAS B, an active site GXS*XG motif is located at amino acids 451-455 of SEQ ID NO:9 with the S* being the site of acyl-attachment. 
     pfaC (nucleotides 27968-34030 of SEQ ID NO:7) encodes PFAS C (SEQ ID NO:10), a PUFA PKS protein harboring the following domains: KS (nucleotides 27995-29431 SEQ ID NO:7, amino acids 10-488 SEQ ID NO:10); chain length factor (CLF) (nucleotides 29471-30217 SEQ ID NO:7, amino acids 502-750 SEQ ID NO:10); and two FabA 3-hydroxyacyl-ACP dehydratases, referred to as DH1 (nucleotides 32258-32686 SEQ ID NO:7, amino acids 1431-1573 SEQ ID NO:10), and DH2 (nucleotides 33611-34027 of SEQ ID NO:7, amino acids 1882-2020 of SEQ ID NO:10). 
     In PFAS C, a KS active site DXAC* is located at amino acids 216-219 of SEQ ID NO:10 with the C* being the site of the acyl attachment. 
     pfaD (nucleotides 34041-35669 of SEQ ID NO:7) encodes the PFAS D (SEQ ID NO:11), a PUFA PKS protein harboring the following domain: an enoyl reductase (ER) (nucleotides 34290-35531 of SEQ ID NO:7, amino acids 84-497 of SEQ ID NO:11). 
     pfaE (nucleotides 13027-13899 of SEQ ID NO:7 on the reverse complementary strand) encodes PFAS E (SEQ ID NO:12), a 4′-phosphopantetheinyl transferase (PPTase) with the identified domain (nucleotides 13369-13815 of SEQ ID NO:7, amino acid 29-177 of SEQ ID NO:12). 
     The pfaC ORF from both  Shewanella  strains described above and the pfaE ORF from  Shewanella olleyana  are predicted to have TTG as their start codon. While TTG is a less common start codon in bacteria then ATG and GTG, it has been predicted to be the start codon for 1.1% of  E. coli  genes and 11.2% of  Bacillus subtilis  genes (Hannenhalli SS, Hayes W S, Hatzigeorgiou A G, Fickett J W. “Bacterial start site prediction”. Nucleic Acids Res. 1999 Sep. 1;27(17):3577-82). There are several lines of evidence to annotate these ORFs start with a TTG codon. First, both computational gene finding tools (EasyGene and GeneMark.hmm) predicted the TTG start codon for these three ORFs. Second, translation from the TTG start in these three ORFs conserves the spacing and range of identical and similar protein residues to homologous genes in the GenBank database. Another line of evidence for the TTG start codon in these genes is the predicted ribosome binding sites (RBS). The RBS is approximately 7 to 12 nucleotides upstream of the start codon and is usually purine rich. Table 5 (see Example 2) shows the upstream regions of all the pfa ORFs and possible RBS. Both pfaC ORFs show very high homology to canonical RBS upstream of the TTG start codon. Alternative starting codons and RBS for these three ORFs annotated with the TTG start codon are also shown in Table 5. It is also noted that the pfaE ORFs from the  Shewanella  strains described here are homologous to orf2 from the EPA biosynthetic cluster from  Shewanella  sp. SCRC-2738 (GenBank accession number U73935). Expression of the  Shewanella  sp. SCRC-2738 orf2 from the annotated ATG was shown not to support EPA production in a heterologous expression system (see PCT Publication No. WO 98/55625). When an alternate upstream start codon of TTG was used in the expression, EPA production was seen in a heterologous expression system. The annotated start codons for both pfaE ORFs described here encode similar and identical amino acids to those encoded from the alternate TTG start codon from orf2 of  Shewanella  sp. SCRC-2738 ( FIG. 4 ). This also supports the TTG start annotation for pfaE ORF from  Sh. olleyana . Lastly, the pfaC ORF start codons from both  Shewanella  strains overlap with the pfaB stop codons ( FIG. 3 ). The overlap of ORFs is a common feature in bacterial operons and is thought to be one means for coupling two or more genes at the transcriptional level. 
     One embodiment of the present invention relates to an isolated protein or domain from a bacterial PUFA PKS system described herein, a homologue thereof, and/or a fragment thereof. Also included in the invention are isolated nucleic acid molecules encoding any of the proteins, domains or peptides described herein (discussed in detail below). According to the present invention, an isolated protein or peptide, such as a protein or peptide from a PUFA PKS system, is a protein or a fragment thereof (including a polypeptide or peptide) that has been removed from its natural milieu (i.e., that has been subject to human manipulation) and can include purified proteins, partially purified proteins, recombinantly produced proteins, and synthetically produced proteins, for example. As such, “isolated” does not reflect the extent to which the protein has been purified. Preferably, an isolated protein of the present invention is produced recombinantly. An isolated peptide can be produced synthetically (e.g., chemically, such as by peptide synthesis) or recombinantly. In addition, and by way of example, a “ Shewanella japonica  PUFA PKS protein” refers to a PUFA PKS protein (generally including a homologue of a naturally occurring PUFA PKS protein) from a  Shewanella japonica  microorganism, or to a PUFA PKS protein that has been otherwise produced from the knowledge of the structure (e.g., sequence), and perhaps the function, of a naturally occurring PUFA PKS protein from  Shewanella japonica . In other words, general reference to a  Shewanella japonica  PUFA PKS protein includes any PUFA PKS protein that has substantially similar structure and function of a naturally occurring PUFA PKS protein from  Shewanella japonica  or that is a biologically active (i.e., has biological activity) homologue of a naturally occurring PUFA PKS protein from  Shewanella japonica  as described in detail herein. As such, a  Shewanella japonica  PUFA PKS protein can include purified, partially purified, recombinant, mutated/modified and synthetic proteins. The same description applies to reference to other proteins or peptides described herein, such as the PUFA PKS proteins and domains from  Shewanella olleyana.    
     According to the present invention, the terms “modification” and “mutation” can be used interchangeably, particularly with regard to the modifications/mutations to the primary amino acid sequences of a protein or peptide (or nucleic acid sequences) described herein. The term “modification” can also be used to describe post-translational modifications to a protein or peptide including, but not limited to, methylation, farnesylation, carboxymethylation, geranyl geranylation, glycosylation, phosphorylation, acetylation, myristoylation, prenylation, palmitation, and/or amidation. Modifications can also include, for example, complexing a protein or peptide with another compound. Such modifications can be considered to be mutations, for example, if the modification is different than the post-translational modification that occurs in the natural, wild-type protein or peptide. 
     As used herein, the term “homologue” is used to refer to a protein or peptide which differs from a naturally occurring protein or peptide (i.e., the “prototype” or “wild-type” protein) by one or more minor modifications or mutations to the naturally occurring protein or peptide, but which maintains the overall basic protein and side chain structure of the naturally occurring form (i.e., such that the homologue is identifiable as being related to the wild-type protein). Such changes include, but are not limited to: changes in one or a few amino acid side chains; changes one or a few amino acids, including deletions (e.g., a truncated version of the protein or peptide) insertions and/or substitutions; changes in stereochemistry of one or a few atoms; and/or minor derivatizations, including but not limited to: methylation, farnesylation, geranyl geranylation, glycosylation, carboxymethylation, phosphorylation, acetylation, myristoylation, prenylation, palmitation, and/or amidation. A homologue can have either enhanced, decreased, or substantially similar properties as compared to the naturally occurring protein or peptide. Preferred homologues of a PUFA PKS protein or domain are described in detail below. It is noted that homologues can include synthetically produced homologues, naturally occurring allelic variants of a given protein or domain, or homologous sequences from organisms other than the organism from which the reference sequence was derived. 
     Conservative substitutions typically include substitutions within the following groups: glycine and alanine; valine, isoleucine and leucine; aspartic acid, glutamic acid, asparagine, and glutamine; serine and threonine; lysine and arginine; and phenylalanine and tyrosine. Substitutions may also be made on the basis of conserved hydrophobicity or hydrophilicity (Kyte and Doolittle,  J. Mol. Biol.  157:105 (1982)), or on the basis of the ability to assume similar polypeptide secondary structure (Chou and Fasman,  Adv. Enzymol.  47: 45 (1978)). 
     Homologues can be the result of natural allelic variation or natural mutation. A naturally occurring allelic variant of a nucleic acid encoding a protein is a gene that occurs at essentially the same locus (or loci) in the genome as the gene which encodes such protein, but which, due to natural variations caused by, for example, mutation or recombination, has a similar but not identical sequence. Allelic variants typically encode proteins having similar activity to that of the protein encoded by the gene to which they are being compared. One class of allelic variants can encode the same protein but have different nucleic acid sequences due to the degeneracy of the genetic code. Allelic variants can also comprise alterations in the 5′ or 3′ untranslated regions of the gene (e.g., in regulatory control regions). Allelic variants are well known to those skilled in the art. 
     Homologues can be produced using techniques known in the art for the production of proteins including, but not limited to, direct modifications to the isolated, naturally occurring protein, direct protein synthesis, or modifications to the nucleic acid sequence encoding the protein using, for example, classic or recombinant DNA techniques to effect random or targeted mutagenesis. 
     Modifications or mutations in protein homologues, as compared to the wild-type protein, either increase, decrease, or do not substantially change, the basic biological activity of the homologue as compared to the naturally occurring (wild-type) protein. In general, the biological activity or biological action of a protein refers to any function(s) exhibited or performed by the protein that is ascribed to the naturally occurring form of the protein as measured or observed in vivo (i.e., in the natural physiological environment of the protein) or in vitro (i.e., under laboratory conditions). Biological activities of PUFA PKS systems and the individual proteins/domains that make up a PUFA PKS system have been described in detail elsewhere herein. Modifications of a protein, such as in a homologue, may result in proteins having the same biological activity as the naturally occurring protein, or in proteins having decreased or increased biological activity as compared to the naturally occurring protein. Modifications which result in a decrease in protein expression or a decrease in the activity of the protein, can be referred to as inactivation (complete or partial), down-regulation, or decreased action (or activity) of a protein. Similarly, modifications which result in an increase in protein expression or an increase in the activity of the protein, can be referred to as amplification, overproduction, activation, enhancement, up-regulation or increased action (or activity) of a protein. It is noted that general reference to a homologue having the biological activity of the wild-type protein does not necessarily mean that the homologue has identical biological activity as the wild-type protein, particularly with regard to the level of biological activity. Rather, a homologue can perform the same biological activity as the wild-type protein, but at a reduced or increased level of activity as compared to the wild-type protein. A functional domain of a PUFA PKS system is a domain (i.e., a domain can be a portion of a protein) that is capable of performing a biological function (i.e., has biological activity). 
     Methods of detecting and measuring PUFA PKS protein or domain biological activity include, but are not limited to, measurement of transcription of a PUFA PKS protein or domain, measurement of translation of a PUFA PKS protein or domain, measurement of posttranslational modification of a PUFA PKS protein or domain, measurement of enzymatic activity of a PUFA PKS protein or domain, and/or measurement production of one or more products of a PUFA PKS system (e.g., PUFA production). It is noted that an isolated protein of the present invention (including a homologue) is not necessarily required to have the biological activity of the wild-type protein. For example, a PUFA PKS protein or domain can be a truncated, mutated or inactive protein, for example. Such proteins are useful in screening assays, for example, or for other purposes such as antibody production. In a preferred embodiment, the isolated proteins of the present invention have a biological activity that is similar to that of the wild-type protein (although not necessarily equivalent, as discussed above). 
     Methods to measure protein expression levels generally include, but are not limited to: Western blot, immunoblot, enzyme-linked immunosorbant assay (ELISA), radioimmunoassay (RIA), immunoprecipitation, surface plasmon resonance, chemiluminescence, fluorescent polarization, phosphorescence, immunohistochemical analysis, matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry, microcytometry, microarray, microscopy, fluorescence activated cell sorting (FACS), and flow cytometry, as well as assays based on a property of the protein including but not limited to enzymatic activity or interaction with other protein partners. Binding assays are also well known in the art. For example, a BIAcore machine can be used to determine the binding constant of a complex between two proteins. The dissociation constant for the complex can be determined by monitoring changes in the refractive index with respect to time as buffer is passed over the chip (O&#39;Shannessy et al.  Anal. Biochem.  212:457 (1993); Schuster et al.,  Nature  365:343 (1993)). Other suitable assays for measuring the binding of one protein to another include, for example, immunoassays such as enzyme linked immunoabsorbent assays (ELISA) and radioimmunoassays (RIA); or determination of binding by monitoring the change in the spectroscopic or optical properties of the proteins through fluorescence, UV absorption, circular dichroism, or nuclear magnetic resonance (NMR). 
     In one embodiment, the present invention relates to an isolated protein comprising, consisting essentially of, or consisting of, an amino acid sequence selected from: any one of SEQ ID NOs:2-6 or 8-12, or biologically active domains or fragments thereof. The domains contained within the PUFA PKS proteins represented by SEQ ID NOs:2-6 and 8-12 have been described in detail above. In another embodiment, the present invention relates to an isolated homologue of a protein represented by any one of SEQ ID NOs:2-6 and 8-12. Such a homologue comprises, consists essentially of, or consists of, an amino acid sequence that is at least about 60% identical to any one of SEQ ID NOs:2-6 or 8-12 and has a biological activity of at least one domain that is contained within the corresponding protein represented by SEQ ID NOs:2-6 or 8-12. In a further embodiment, the present invention relates to a homologue of a domain of a PUFA PKS protein represented by any one of SEQ ID NO:2-6 or 8-12, wherein the homologue comprises, consists essentially of, or consists of, an amino acid sequence that is at least about 60% identical to a domain from any one of SEQ ID NOs:2-6 or 8-12, and which has a biological activity of such domain from any one of SEQ ID NOs:2-6 or 8-12. In additional embodiments, any of the above-described homologues is at least about 65% identical, and more preferably at least about 70% identical, and more preferably at least about 75% identical, and more preferably at least about 80% identical, and more preferably at least about 85% identical, and more preferably at least about 90% identical, and more preferably at least about 95% identical, and more preferably at least about 96% identical, and more preferably at least about 97% identical, and more preferably at least about 98% identical, and more preferably at least about 99% identical (or any percentage between 60% and 99%, in whole single percentage increments) to any one of SEQ ID NOs:2-6 or 8-12, or to a domain contained within these sequences. As above, the homologue preferably has a biological activity of the protein or domain from which it is derived or related (i.e., the protein or domain having the reference amino acid sequence). 
     One embodiment of the invention relates to an isolated homologue of a protein represented by SEQ ID NO:2 that comprises, consists essentially of, or consists of, an amino acid sequence that is at least about 65% identical to SEQ ID NO:2 or to a biologically active domain within SEQ ID NO:2 as previously described herein, wherein the homologue has a biological activity of at least one domain that is contained within the corresponding protein represented by SEQ ID NO:2. In additional embodiments, the homologue is at least about 70% identical, and more preferably at least about 75% identical, and more preferably at least about 80% identical, and more preferably at least about 85% identical, and more preferably at least about 90% identical, and more preferably at least about 95% identical, and more preferably at least about 96% identical, and more preferably at least about 97% identical, and more preferably at least about 98% identical, and more preferably at least about 99% identical (or any percentage between 65% and 99%, in whole single percentage increments) to SEQ ID NO:2 or a domain thereof. 
     Another embodiment of the invention relates to an isolated homologue of a protein represented by SEQ ID NO:3 that comprises, consists essentially of, or consists of, an amino acid sequence that is at least about 60% identical to SEQ ID NO:3 or to a biologically active domain within SEQ ID NO:3 as previously described herein, wherein the homologue has a biological activity of at least one domain that is contained within the corresponding protein represented by SEQ ID NO:3. In additional embodiments, the homologue is at least about 65% identical, and more preferably at least about 70% identical, and more preferably at least about 75% identical, and more preferably at least about 80% identical, and more preferably at least about 85% identical, and more preferably at least about 90% identical, and more preferably at least about 95% identical, and more preferably at least about 96% identical, and more preferably at least about 97% identical, and more preferably at least about 98% identical, and more preferably at least about 99% identical (or any percentage between 60% and 99%, in whole single percentage increments) to SEQ ID NO:3 or a domain thereof. 
     Another embodiment of the invention relates to an isolated homologue of a protein represented by SEQ ID NO:4 that comprises, consists essentially of, or consists of, an amino acid sequence that is at least about 70% identical to SEQ ID NO:4 or to a biologically active domain within SEQ ID NO:4 as previously described herein, wherein the homologue has a biological activity of at least one domain that is contained within the corresponding protein represented by SEQ ID NO:4. In additional embodiments, the homologue is at least about 75% identical, and more preferably at least about 80% identical, and more preferably at least about 85% identical, and more preferably at least about 90% identical, and more preferably at least about 95% identical, and more preferably at least about 96% identical, and more preferably at least about 97% identical, and more preferably at least about 98% identical, and more preferably at least about 99% identical (or any percentage between 60% and 99%, in whole single percentage increments) to SEQ ID NO:4 or a domain thereof. 
     Another embodiment of the invention relates to an isolated homologue of a protein represented by SEQ ID NO:5 that comprises, consists essentially of, or consists of, an amino acid sequence that is at least about 95% identical to SEQ ID NO:5 or to a biologically active domain within SEQ ID NO:5 as previously described herein, wherein the homologue has a biological activity of at least one domain that is contained within the corresponding protein represented by SEQ ID NO:5. In additional embodiments, the homologue is at least about 96% identical, and more preferably at least about 97% identical, and more preferably at least about 98% identical, and more preferably at least about 99% identical to SEQ ID NO:5 or a domain thereof. 
     Another embodiment of the invention relates to an isolated homologue of a protein represented by SEQ ID NO:6 that comprises, consists essentially of, or consists of, an amino acid sequence that is at least about 60% identical to SEQ ID NO:6 or to a biologically active domain within SEQ ID NO:6 as previously described herein, wherein the homologue has a biological activity of at least one domain that is contained within the corresponding protein represented by SEQ ID NO:6. In additional embodiments, the homologue is at least about 65% identical, and more preferably at least about 70% identical, and more preferably at least about 75% identical, and more preferably at least about 80% identical, and more preferably at least about 85% identical, and more preferably at least about 90% identical, and more preferably at least about 95% identical, and more preferably at least about 96% identical, and more preferably at least about 97% identical, and more preferably at least about 98% identical, and more preferably at least about 99% identical (or any percentage between 60% and 99%, in whole single percentage increments) to SEQ ID NO:6 or a domain thereof. 
     Another embodiment of the invention relates to an isolated homologue of a protein represented by SEQ ID NO:8 that comprises, consists essentially of, or consists of, an amino acid sequence that is at least about 65% identical to SEQ ID NO:8 or to a biologically active domain within SEQ ID NO:8 as previously described herein, wherein the homologue has a biological activity of at least one domain that is contained within the corresponding protein represented by SEQ ID NO:8. In additional embodiments, the homologue is at least about 70% identical, and more preferably at least about 75% identical, and more preferably at least about 80% identical, and more preferably at least about 85% identical, and more preferably at least about 90% identical, and more preferably at least about 95% identical, and more preferably at least about 96% identical, and more preferably at least about 97% identical, and more preferably at least about 98% identical, and more preferably at least about 99% identical (or any percentage between 60% and 99%, in whole single percentage increments) to SEQ ID NO:8 or a domain thereof. 
     Another embodiment of the invention relates to an isolated homologue of a protein represented by SEQ ID NO:9 that comprises, consists essentially of, or consists of, an amino acid sequence that is at least about 60% identical to SEQ ID NO:9 or to a biologically active domain within SEQ ID NO:9 as previously described herein, wherein the homologue has a biological activity of at least one domain that is contained within the corresponding protein represented by SEQ ID NO:9. In additional embodiments, the homologue is at least about 65% identical, and more preferably at least about 70% identical, and more preferably at least about 75% identical, and more preferably at least about 80% identical, and more preferably at least about 85% identical, and more preferably at least about 90% identical, and more preferably at least about 95% identical, and more preferably at least about 96% identical, and more preferably at least about 97% identical, and more preferably at least about 98% identical, and more preferably at least about 99% identical (or any percentage between 60% and 99%, in whole single percentage increments) to SEQ ID NO:9 or a domain thereof. 
     Another embodiment of the invention relates to an isolated homologue of a protein represented by SEQ ID NO:10 that comprises, consists essentially of, or consists of, an amino acid sequence that is at least about 70% identical to SEQ ID NO:10 or to a biologically active domain within SEQ ID NO:10 as previously described herein, wherein the homologue has a biological activity of at least one domain that is contained within the corresponding protein represented by SEQ ID NO:10. In additional embodiments, the homologue is at least about 75% identical, and more preferably at least about 80% identical, and more preferably at least about 85% identical, and more preferably at least about 90% identical, and more preferably at least about 95% identical, and more preferably at least about 96% identical, and more preferably at least about 97% identical, and more preferably at least about 98% identical, and more preferably at least about 99% identical (or any percentage between 60% and 99%, in whole single percentage increments) to SEQ ID NO:10 or a domain thereof. 
     Another embodiment of the invention relates to an isolated homologue of a protein represented by SEQ ID NO:11 that comprises, consists essentially of, or consists of, an amino acid sequence that is at least about 85% identical to SEQ ID NO:11 or to a biologically active domain within SEQ ID NO:11 as previously described herein, wherein the homologue has a biological activity of at least one domain that is contained within the corresponding protein represented by SEQ ID NO:1. In additional embodiments, the homologue is at least about 90% identical, and more preferably at least about 95% identical, and more preferably at least about 96% identical, and more preferably at least about 97% identical, and more preferably at least about 98% identical, and more preferably at least about 99% identical (or any percentage between 60% and 99%, in whole single percentage increments) to SEQ ID NO:11 or a domain thereof. 
     Another embodiment of the invention relates to an isolated homologue of a protein represented by SEQ ID NO:12 that comprises, consists essentially of, or consists of, an amino acid sequence that is at least about 60% identical to SEQ ID NO: 12 or to a biologically active domain within SEQ ID NO: 12 as previously described herein, wherein the homologue has a biological activity of at least one domain that is contained within the corresponding protein represented by SEQ ID NO: 12. In additional embodiments, the homologue is at least about 65% identical, and more preferably at least about 70% identical, and more preferably at least about 75% identical, and more preferably at least about 80% identical, and more preferably at least about 85% identical, and more preferably at least about 90% identical, and more preferably at least about 95% identical, and more preferably at least about 96% identical, and more preferably at least about 97% identical, and more preferably at least about 98% identical, and more preferably at least about 99% identical (or any percentage between 60% and 99%, in whole single percentage increments) to SEQ ID NO: 12 or a domain thereof. 
     In one aspect of the invention, a PUFA PKS protein or domain encompassed by the present invention, including a homologue of a particular PUFA PKS protein or domain described herein, comprises an amino acid sequence that includes at least about 100 consecutive amino acids of the amino acid sequence chosen from any one of SEQ ID NOs:2-6 or 8-12, wherein the amino acid sequence of the homologue has a biological activity of at least one domain or protein as described herein. In a further aspect, the amino acid sequence of the protein is comprises at least about 200 consecutive amino acids, and more preferably at least about 300 consecutive amino acids, and more preferably at least about 400 consecutive amino acids, and more preferably at least about 500 consecutive amino acids, and more preferably at least about 600 consecutive amino acids, and more preferably at least about 700 consecutive amino acids, and more preferably at least about 800 consecutive amino acids, and more preferably at least about 900 consecutive amino acids, and more preferably at least about 1000 consecutive amino acids of any of SEQ ID NOs:2-6 or 8-12. 
     In a preferred embodiment of the present invention, an isolated protein or domain of the present invention comprises, consists essentially of, or consists of, an amino acid sequence chosen from: SEQ ID NO:2, SEQ ID NO:3, SEQ ID NO:4, SEQ ID NO:5, SEQ ID NO:6, SEQ ID NO:8, SEQ ID NO:9, SEQ ID NO:10, SEQ ID NO:11, SEQ ID NO:12, or any biologically active fragments or domains thereof. 
     In one embodiment, a biologically active domain of a PUFA PKS system as described herein and referenced above comprises, consists essentially of, or consists of, an amino acid sequence chosen from: (1) from about position 29 to about position 513 of SEQ ID NO:2, wherein the domain has KS biological activity; (2) from about position 625 to about position 943 of SEQ ID NO:2, wherein the domain has MAT biological activity; (3) from about position 1264 to about position 1889 of SEQ ID NO:2, and subdomains thereof, wherein the domain or subdomain thereof has ACP biological activity; (4) from about position 2264 to about position 2398 of SEQ ID NO:2, wherein the domain has KR biological activity; (5) a sequence comprising from about position 2504 to about position 2516 of SEQ ID NO:2, wherein the domain has DH biological activity, and preferably, non-FabA-like DH activity; (6) from about position 378 to about position 684 of SEQ ID NO:3, wherein the domain has AT biological activity; (7) from about position 5 to about position 483 of SEQ ID NO:4, wherein the domain has KS biological activity; (8) from about position 489 to about position 771 of SEQ ID NO:4, wherein the domain has CLF biological activity; (9) from about position 1428 to about position 1570 of SEQ ID NO:4, wherein the domain has DH biological activity, and preferably, FabA-like DH activity; (10) from about position 1881 to about position 2019 of SEQ ID NO:4, wherein the domain has DH biological activity, and preferably, FabA-like DH activity; (11) from about position 84 to about position 497 of SEQ ID NO:5, wherein the domain has ER biological activity; (12) from about position 40 to about position 186 of SEQ ID NO:6, wherein the domain has PPTase biological activity; (13) from about position 29 to about position 513 of SEQ ID NO:8, wherein the domain has KS biological activity; (14) from about position 625 to about position 943 of SEQ ID NO:8, wherein the domain has MAT biological activity; (15) from about position 1275 to about position 1872 of SEQ ID NO:8, and subdomains thereof, wherein the domain or subdomain thereof has ACP biological activity; (16) from about position 2240 to about position 2374 of SEQ ID NO:8, wherein the domain has KR biological activity; (17) a sequence comprising from about position 2480-2492 of SEQ ID NO:8, wherein the sequence has DH biological activity, and preferably, non-FabA-like DH activity; (18) from about position 366 to about position 703 of SEQ ID NO:9, wherein the domain has AT biological activity; (19) from about position 10 to about position 488 of SEQ ID NO:10, wherein the domain has KS biological activity; (20) from about position 502 to about position 750 of SEQ ID NO:10, wherein the domain has CLF biological activity; (21) from about position 1431 to about position 1573 of SEQ ID NO:10, wherein the domain has DH biological activity, and preferably, FabA-like DH activity; (22) from about position 1882 to about position 2020 of SEQ ID NO:10, wherein the domain has DH biological activity, and preferably, FabA-like DH activity; (23) from about position 84 to about position 497 of SEQ ID NO:11, wherein the domain has ER biological activity; or (24) from about position 29 to about position 177 of SEQ ID NO:12, wherein the domain has PPTase biological activity. 
     According to the present invention, the term “contiguous” or “consecutive”, with regard to nucleic acid or amino acid sequences described herein, means to be connected in an unbroken sequence. For example, for a first sequence to comprise 30 contiguous (or consecutive) amino acids of a second sequence, means that the first sequence includes an unbroken sequence of 30 amino acid residues that is 100% identical to an unbroken sequence of 30 amino acid residues in the second sequence. Similarly, for a first sequence to have “100% identity” with a second sequence means that the first sequence exactly matches the second sequence with no gaps between nucleotides or amino acids. 
     As used herein, unless otherwise specified, reference to a percent (%) identity refers to an evaluation of homology which is performed using: (1) a BLAST 2.0 Basic BLAST homology search using blastp for amino acid searches, blastn for nucleic acid searches, and blastX for nucleic acid searches and searches of translated amino acids in all 6 open reading frames, all with standard default parameters, wherein the query sequence is filtered for low complexity regions by default (described in Altschul, S. F., Madden, T. L., Schääffer, A. A., Zhang, J., Zhang, Z., Miller, W. &amp; Lipman, D. J. (1997) “Gapped BLAST and PSI-BLAST: a new generation of protein database search programs.” Nucleic Acids Res. 25:3389, incorporated herein by reference in its entirety); (2) a BLAST 2 alignment (using the parameters described below); (3) and/or PSI-BLAST with the standard default parameters (Position-Specific Iterated BLAST). It is noted that due to some differences in the standard parameters between BLAST 2.0 Basic BLAST and BLAST 2, two specific sequences might be recognized as having significant homology using the BLAST 2 program, whereas a search performed in BLAST 2.0 Basic BLAST using one of the sequences as the query sequence may not identify the second sequence in the top matches. In addition, PSI-BLAST provides an automated, easy-to-use version of a “profile” search, which is a sensitive way to look for sequence homologues. The program first performs a gapped BLAST database search. The PSI-BLAST program uses the information from any significant alignments returned to construct a position-specific score matrix, which replaces the query sequence for the next round of database searching. Therefore, it is to be understood that percent identity can be determined by using any one of these programs. 
     Two specific sequences can be aligned to one another using BLAST 2 sequence as described in Tatusova and Madden, “Blast 2 sequences—a new tool for comparing protein and nucleotide sequences”,  FEMS Microbiol Lett.  174:247 (1999), incorporated herein by reference in its entirety. BLAST 2 sequence alignment is performed in blastp or blastn using the BLAST 2.0 algorithm to perform a Gapped BLAST search (BLAST 2.0) between the two sequences allowing for the introduction of gaps (deletions and insertions) in the resulting alignment. For purposes of clarity herein, a BLAST 2 sequence alignment is performed using the standard default parameters as follows. 
     For blastn, using 0 BLOSUM62 matrix: 
     
         
         
           
             Reward for match=1 
             Penalty for mismatch=−2 
             Open gap (5) and extension gap (2) penalties 
             gap x_dropoff (50) expect (10) word size (11) filter (on)
 
For blastp, using 0 BLOSUM62 matrix:
 
             Open gap (11) and extension gap (1) penalties 
             gap x_dropoff (50) expect (10) word size (3) filter (on). 
           
         
       
    
     According to the present invention, an amino acid sequence that has a biological activity of at least one domain of a PUFA PKS system is an amino acid sequence that has the biological activity of at least one domain of the PUFA PKS system described in detail herein (e.g., a KS domain, an AT domain, a CLF domain, etc.). Therefore, an isolated protein useful in the present invention can include: the translation product of any PUFA PKS open reading frame, any PUFA PKS domain, any biologically active fragment of such a translation product or domain, or any homologue of a naturally occurring PUFA PKS open reading frame product or domain which has biological activity. 
     In another embodiment of the invention, an amino acid sequence having the biological activity of at least one domain of a PUFA PKS system of the present invention includes an amino acid sequence that is sufficiently similar to a naturally occurring PUFA PKS protein or polypeptide that is specifically described herein that a nucleic acid sequence encoding the amino acid sequence is capable of hybridizing under moderate, high, or very high stringency conditions (described below) to (i.e., with) a nucleic acid molecule encoding the naturally occurring PUFA PKS protein or polypeptide (i.e., to the complement of the nucleic acid strand encoding the naturally occurring PUFA PKS protein or polypeptide). Preferably, an amino acid sequence having the biological activity of at least one domain of a PUFA PKS system of the present invention is encoded by a nucleic acid sequence that hybridizes under moderate, high or very high stringency conditions to the complement of a nucleic acid sequence that encodes any of the above-described amino acid sequences for a PUFA PKS protein or domain. Methods to deduce a complementary sequence are known to those skilled in the art. It should be noted that since amino acid sequencing and nucleic acid sequencing technologies are not entirely error-free, the sequences presented herein, at best, represent apparent sequences of PUFA PKS domains and proteins of the present invention. 
     As used herein, hybridization conditions refer to standard hybridization conditions under which nucleic acid molecules are used to identify similar nucleic acid molecules. Such standard conditions are disclosed, for example, in Sambrook et al.,  Molecular Cloning: A Laboratory Manual , Cold Spring Harbor Labs Press (1989). Sambrook et al., ibid., is incorporated by reference herein in its entirety (see specifically, pages 9.31-9.62). In addition, formulae to calculate the appropriate hybridization and wash conditions to achieve hybridization permitting varying degrees of mismatch of nucleotides are disclosed, for example, in Meinkoth et al.,  Anal. Biochem.  138, 267 (1984); Meinkoth et al., ibid., is incorporated by reference herein in its entirety. 
     More particularly, moderate stringency hybridization and washing conditions, as referred to herein, refer to conditions which permit isolation of nucleic acid molecules having at least about 70% nucleic acid sequence identity with the nucleic acid molecule being used to probe in the hybridization reaction (i.e., conditions permitting about 30% or less mismatch of nucleotides). High stringency hybridization and washing conditions, as referred to herein, refer to conditions which permit isolation of nucleic acid molecules having at least about 80% nucleic acid sequence identity with the nucleic acid molecule being used to probe in the hybridization reaction (i.e., conditions permitting about 20% or less mismatch of nucleotides). Very high stringency hybridization and washing conditions, as referred to herein, refer to conditions which permit isolation of nucleic acid molecules having at least about 90% nucleic acid sequence identity with the nucleic acid molecule being used to probe in the hybridization reaction (i.e., conditions permitting about 10% or less mismatch of nucleotides). As discussed above, one of skill in the art can use the formulae in Meinkoth et al., ibid. to calculate the appropriate hybridization and wash conditions to achieve these particular levels of nucleotide mismatch. Such conditions will vary, depending on whether DNA:RNA or DNA:DNA hybrids are being formed. Calculated melting temperatures for DNA:DNA hybrids are 10° C. less than for DNA:RNA hybrids. In particular embodiments, stringent hybridization conditions for DNA:DNA hybrids include hybridization at an ionic strength of 6× SSC (0.9 M Na + ) at a temperature of between about 20° C. and about 35° C. (lower stringency), more preferably, between about 28° C. and about 40° C. (more stringent), and even more preferably, between about 35° C. and about 45° C. (even more stringent), with appropriate wash conditions. In particular embodiments, stringent hybridization conditions for DNA:RNA hybrids include hybridization at an ionic strength of 6× SSC (0.9 M Na + ) at a temperature of between about 30° C. and about 45° C., more preferably, between about 38° C. and about 50° C., and even more preferably, between about 45° C. and about 55° C., with similarly stringent wash conditions. These values are based on calculations of a melting temperature for molecules larger than about 100 nucleotides, 0% formamide and a G+C content of about 40%. Alternatively, T m  can be calculated empirically as set forth in Sambrook et al.,  supra , pages 9.31 to 9.62. In general, the wash conditions should be as stringent as possible, and should be appropriate for the chosen hybridization conditions. For example, hybridization conditions can include a combination of salt and temperature conditions that are approximately 20-25° C. below the calculated T m  of a particular hybrid, and wash conditions typically include a combination of salt and temperature conditions that are approximately 12-20° C. below the calculated T m  of the particular hybrid. One example of hybridization conditions suitable for use with DNA:DNA hybrids includes a 2-24 hour hybridization in 6× SSC (50% formamide) at about 42° C., followed by washing steps that include one or more washes at room temperature in about 2× SSC, followed by additional washes at higher temperatures and lower ionic strength (e.g., at least one wash as about 37° C. in about 0.1×-0.5× SSC, followed by at least one wash at about 68° C. in about 0.1×-0.5× SSC). 
     The present invention also includes a fusion protein that includes any PUFA PKS protein or domain or any homologue or fragment thereof attached to one or more fusion segments. Suitable fusion segments for use with the present invention include, but are not limited to, segments that can: enhance a protein&#39;s stability; provide other desirable biological activity; and/or assist with the purification of the protein (e.g., by affinity chromatography). A suitable fusion segment can be a domain of any size that has the desired function (e.g., imparts increased stability, solubility, biological activity; and/or simplifies purification of a protein). Fusion segments can be joined to amino and/or carboxyl termini of the protein and can be susceptible to cleavage in order to enable straight-forward recovery of the desired protein. Fusion proteins are preferably produced by culturing a recombinant cell transfected with a fusion nucleic acid molecule that encodes a protein including the fusion segment attached to either the carboxyl and/or amino terminal end of the protein of the invention as discussed above. 
     In one embodiment of the present invention, any of the above-described PUFA PKS amino acid sequences, as well as homologues of such sequences, can be produced with from at least one, and up to about 20, additional heterologous amino acids flanking each of the C- and/or N-terminal end of the given amino acid sequence. The resulting protein or polypeptide can be referred to as “consisting essentially of” a given amino acid sequence. According to the present invention, the heterologous amino acids are a sequence of amino acids that are not naturally found (i.e., not found in nature, in vivo) flanking the given amino acid sequence or which would not be encoded by the nucleotides that flank the naturally occurring nucleic acid sequence encoding the given amino acid sequence as it occurs in the gene, if such nucleotides in the naturally occurring sequence were translated using standard codon usage for the organism from which the given amino acid sequence is derived. Similarly, the phrase “consisting essentially of”, when used with reference to a nucleic acid sequence herein, refers to a nucleic acid sequence encoding a given amino acid sequence that can be flanked by from at least one, and up to as many as about 60, additional heterologous nucleotides at each of the 5′ and/or the 3′ end of the nucleic acid sequence encoding the given amino acid sequence. The heterologous nucleotides are not naturally found (i.e., not found in nature, in vivo) flanking the nucleic acid sequence encoding the given amino acid sequence as it occurs in the natural gene. 
     The minimum size of a protein or domain and/or a homologue or fragment thereof of the present invention is, in one aspect, a size sufficient to have the requisite biological activity, or sufficient to serve as an antigen for the generation of an antibody or as a target in an in vitro assay. In one embodiment, a protein of the present invention is at least about 8 amino acids in length (e.g., suitable for an antibody epitope or as a detectable peptide in an assay), or at least about 25 amino acids in length, or at least about 50 amino acids in length, or at least about 100 amino acids in length, or at least about 150 amino acids in length, or at least about 200 amino acids in length, or at least about 250 amino acids in length, or at least about 300 amino acids in length, or at least about 350 amino acids in length, or at least about 400 amino acids in length, or at least about 450 amino acids in length, or at least about 500 amino acids in length, and so on, in any length between 8 amino acids and up to the full length of a protein or domain of the invention or longer, in whole integers (e.g., 8, 9, 10, . . . 25, 26, . . . 500, 501, . . . ). There is no limit, other than a practical limit, on the maximum size of such a protein in that the protein can include a portion of a PUFA PKS protein, domain, or biologically active or useful fragment thereof, or a full-length PUFA PKS protein or domain, plus additional sequence (e.g., a fusion protein sequence), if desired. 
     One embodiment of the present invention relates to isolated nucleic acid molecules comprising, consisting essentially of, or consisting of nucleic acid sequences that encode any of the PUFA PKS proteins or domains described herein, including a homologue or fragment of any of such proteins or domains, as well as nucleic acid sequences that are fully complementary thereto. In accordance with the present invention, an isolated nucleic acid molecule is a nucleic acid molecule that has been removed from its natural milieu (i.e., that has been subject to human manipulation), its natural milieu being the genome or chromosome in which the nucleic acid molecule is found in nature. As such, “isolated” does not necessarily reflect the extent to which the nucleic acid molecule has been purified, but indicates that the molecule does not include an entire genome or an entire chromosome in which the nucleic acid molecule is found in nature. An isolated nucleic acid molecule can include a gene. An isolated nucleic acid molecule that includes a gene is not a fragment of a chromosome that includes such gene, but rather includes the coding region and regulatory regions associated with the gene, but no additional genes that are naturally found on the same chromosome, with the exception of other genes that encode other proteins of the PUFA PKS system as described herein. An isolated nucleic acid molecule can also include a specified nucleic acid sequence flanked by (i.e., at the 5′ and/or the 3′ end of the sequence) additional nucleic acids that do not normally flank the specified nucleic acid sequence in nature (i.e., heterologous sequences). Isolated nucleic acid molecule can include DNA, RNA (e.g., mRNA), or derivatives of either DNA or RNA (e.g., cDNA). Although the phrase “nucleic acid molecule” primarily refers to the physical nucleic acid molecule and the phrase “nucleic acid sequence” primarily refers to the sequence of nucleotides on the nucleic acid molecule, the two phrases can be used interchangeably, especially with respect to a nucleic acid molecule, or a nucleic acid sequence, being capable of encoding a protein or domain of a protein. 
     Preferably, an isolated nucleic acid molecule of the present invention is produced using recombinant DNA technology (e.g., polymerase chain reaction (PCR) amplification, cloning) or chemical synthesis. Isolated nucleic acid molecules include natural nucleic acid molecules and homologues thereof, including, but not limited to, natural allelic variants and modified nucleic acid molecules in which nucleotides have been inserted, deleted, substituted, and/or inverted in such a manner that such modifications provide the desired effect on PUFA PKS system biological activity as described herein. Protein homologues (e.g., proteins encoded by nucleic acid homologues) have been discussed in detail above. 
     A nucleic acid molecule homologue can be produced using a number of methods known to those skilled in the art (see, for example, Sambrook et al.,  Molecular Cloning: A Laboratory Manual , Cold Spring Harbor Labs Press (1989)). For example, nucleic acid molecules can be modified using a variety of techniques including, but not limited to, classic mutagenesis techniques and recombinant DNA techniques, such as site-directed mutagenesis, chemical treatment of a nucleic acid molecule to induce mutations, restriction enzyme cleavage of a nucleic acid fragment, ligation of nucleic acid fragments, PCR amplification and/or mutagenesis of selected regions of a nucleic acid sequence, synthesis of oligonucleotide mixtures and ligation of mixture groups to “build” a mixture of nucleic acid molecules and combinations thereof. Nucleic acid molecule homologues can be selected from a mixture of modified nucleic acids by screening for the function of the protein encoded by the nucleic acid and/or by hybridization with a wild-type gene. 
     The minimum size of a nucleic acid molecule of the present invention is a size sufficient to form a probe or oligonucleotide primer that is capable of forming a stable hybrid (e.g., under moderate, high or very high stringency conditions) with the complementary sequence of a nucleic acid molecule of the present invention, or of a size sufficient to encode an amino acid sequence having a biological activity of at least one domain of a PUFA PKS system according to the present invention. As such, the size of the nucleic acid molecule encoding such a protein can be dependent on nucleic acid composition and percent homology or identity between the nucleic acid molecule and complementary sequence as well as upon hybridization conditions per se (e.g., temperature, salt concentration, and formamide concentration). The minimal size of a nucleic acid molecule that is used as an oligonucleotide primer or as a probe is typically at least about 12 to about 15 nucleotides in length if the nucleic acid molecules are GC-rich and at least about 15 to about 18 bases in length if they are AT-rich. There is no limit, other than a practical limit, on the maximal size of a nucleic acid molecule of the present invention, in that the nucleic acid molecule can include a sequence sufficient to encode a biologically active fragment of a domain of a PUFA PKS system, an entire domain of a PUFA PKS system, several domains within an open reading frame (Orf) of a PUFA PKS system, an entire single- or multi-domain protein of a PUFA PKS system, or more than one protein of a PUFA PKS system. 
     In one embodiment of the present invention, an isolated nucleic acid molecule comprises, consists essentially of, or consists of a nucleic acid sequence encoding any of the above-described amino acid sequences, including any of the amino acid sequences, or homologues thereof, from  Shewanella japonica  or  Shewanella olleyana  described herein. In one aspect, the nucleic acid sequence is selected from the group of: SEQ ID NO:1 or SEQ ID NO:7 or any fragment (segment, portion) of SEQ ID NO:1 or SEQ ID NO:7 that encodes one or more domains or proteins of the PUFA PKS systems described herein. In another aspect, the nucleic acid sequence includes any homologues of SEQ ID NO:1 or SEQ ID NO:7 or any fragment of SEQ ID NO:1 or SEQ ID NO:7 that encodes one or more domains or proteins of the PUFA PKS systems described herein (including sequences that are at least about 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99% identical to such sequences). In yet another aspect, fragments and any complementary sequences of such nucleic acid sequences are encompassed by the invention. 
     Another embodiment of the present invention includes a recombinant nucleic acid molecule comprising a recombinant vector and a nucleic acid sequence encoding protein or peptide having a biological activity of at least one domain (or homologue or fragment thereof) of a PUFA PKS protein as described herein. Such nucleic acid sequences are described in detail above. According to the present invention, a recombinant vector is an engineered (i.e., artificially produced) nucleic acid molecule that is used as a tool for manipulating a nucleic acid sequence of choice and for introducing such a nucleic acid sequence into a host cell. The recombinant vector is therefore suitable for use in cloning, sequencing, and/or otherwise manipulating the nucleic acid sequence of choice, such as by expressing and/or delivering the nucleic acid sequence of choice into a host cell to form a recombinant cell. Such a vector typically contains heterologous nucleic acid sequences, that is nucleic acid sequences that are not naturally found adjacent to nucleic acid sequence to be cloned or delivered, although the vector can also contain regulatory nucleic acid sequences (e.g., promoters, untranslated regions) which are naturally found adjacent to nucleic acid molecules of the present invention or which are useful for expression of the nucleic acid molecules of the present invention (discussed in detail below). The vector can be either RNA or DNA, either prokaryotic or eukaryotic, and typically is a plasmid. The vector can be maintained as an extrachromosomal element (e.g., a plasmid) or it can be integrated into the chromosome of a recombinant organism (e.g., a microbe or a plant). The entire vector can remain in place within a host cell, or under certain conditions, the plasmid DNA can be deleted, leaving behind the nucleic acid molecule of the present invention. The integrated nucleic acid molecule can be under chromosomal promoter control, under native or plasmid promoter control, or under a combination of several promoter controls. Single or multiple copies of the nucleic acid molecule can be integrated into the chromosome. A recombinant vector of the present invention can contain at least one selectable marker. 
     In one embodiment, a recombinant vector used in a recombinant nucleic acid molecule of the present invention is an expression vector. As used herein, the phrase “expression vector” is used to refer to a vector that is suitable for production of an encoded product (e.g., a protein of interest). In this embodiment, a nucleic acid sequence encoding the product to be produced (e.g., a PUFA PKS domain or protein) is inserted into the recombinant vector to produce a recombinant nucleic acid molecule. The nucleic acid sequence encoding the protein to be produced is inserted into the vector in a manner that operatively links the nucleic acid sequence to regulatory sequences in the vector that enable the transcription and translation of the nucleic acid sequence within the recombinant host cell. 
     In another embodiment, a recombinant vector used in a recombinant nucleic acid molecule of the present invention is a targeting vector. As used herein, the phrase “targeting vector” is used to refer to a vector that is used to deliver a particular nucleic acid molecule into a recombinant host cell, wherein the nucleic acid molecule is used to delete, inactivate, or replace an endogenous gene or portion of a gene within the host cell or microorganism (i.e., used for targeted gene disruption or knock-out technology). Such a vector may also be known in the art as a “knock-out” vector. In one aspect of this embodiment, a portion of the vector, but more typically, the nucleic acid molecule inserted into the vector (i.e., the insert), has a nucleic acid sequence that is homologous to a nucleic acid sequence of a target gene in the host cell (i.e., a gene which is targeted to be deleted or inactivated). The nucleic acid sequence of the vector insert is designed to associate with the target gene such that the target gene and the insert may undergo homologous recombination, whereby the endogenous target gene is deleted, inactivated, attenuated (i.e., by at least a portion of the endogenous target gene being mutated or deleted), or replaced. The use of this type of recombinant vector to replace an endogenous  Schizochytrium  gene, for example, with a recombinant gene is described in the Examples section, and the general technique for genetic transformation of Thraustochytrids is described in detail in U.S. patent application Ser. No. 10/124,807, published as U.S. Patent Application Publication No. 20030166207, published Sep. 4, 2003. Genetic transformation techniques for plants are well-known in the art. It is an embodiment of the present invention that the marine bacterial genes described herein can be used to transform plants or microorganisms such as Thraustochytrids to improve and/or alter (modify, change) the PUFA PKS production capabilities of such plants or microorganisms. 
     Typically, a recombinant nucleic acid molecule includes at least one nucleic acid molecule of the present invention operatively linked to one or more expression control sequences. As used herein, the phrase “recombinant molecule” or “recombinant nucleic acid molecule” primarily refers to a nucleic acid molecule or nucleic acid sequence operatively linked to a expression control sequence, but can be used interchangeably with the phrase “nucleic acid molecule”, when such nucleic acid molecule is a recombinant molecule as discussed herein. According to the present invention, the phrase “operatively linked” refers to linking a nucleic acid molecule to an expression control sequence (e.g., a transcription control sequence and/or a translation control sequence) in a manner such that the molecule can be expressed when transfected (i.e., transformed, transduced, transfected, conjugated or conduced) into a host cell. Transcription control sequences are sequences that control the initiation, elongation, or termination of transcription. Particularly important transcription control sequences are those that control transcription initiation, such as promoter, enhancer, operator and repressor sequences. Suitable transcription control sequences include any transcription control sequence that can function in a host cell or organism into which the recombinant nucleic acid molecule is to be introduced. 
     Recombinant nucleic acid molecules of the present invention can also contain additional regulatory sequences, such as translation regulatory sequences, origins of replication, and other regulatory sequences that are compatible with the recombinant cell. In one embodiment, a recombinant molecule of the present invention, including those that are integrated into the host cell chromosome, also contains secretory signals (i.e., signal segment nucleic acid sequences) to enable an expressed protein to be secreted from the cell that produces the protein. Suitable signal segments include a signal segment that is naturally associated with the protein to be expressed or any heterologous signal segment capable of directing the secretion of the protein according to the present invention. In another embodiment, a recombinant molecule of the present invention comprises a leader sequence to enable an expressed protein to be delivered to and inserted into the membrane of a host cell. Suitable leader sequences include a leader sequence that is naturally associated with the protein, or any heterologous leader sequence capable of directing the delivery and insertion of the protein to the membrane of a cell. 
     One or more recombinant molecules of the present invention can be used to produce an encoded product (e.g., a PUFA PKS domain, protein, or system) of the present invention. In one embodiment, an encoded product is produced by expressing a nucleic acid molecule as described herein under conditions effective to produce the protein. A preferred method to produce an encoded protein is by transfecting a host cell with one or more recombinant molecules to form a recombinant cell. Suitable host cells to transfect include, but are not limited to, any bacterial, fungal (e.g., yeast), insect, plant or animal cell that can be transfected. In one embodiment of the invention, a preferred host cell is a Thraustochytrid host cell (described in detail below) or a plant host cell. Host cells can be either untransfected cells or cells that are already transfected with at least one other recombinant nucleic acid molecule. 
     According to the present invention, the term “transfection” is used to refer to any method by which an exogenous nucleic acid molecule (i.e., a recombinant nucleic acid molecule) can be inserted into a cell. The term “transformation” can be used interchangeably with the term “transfection” when such term is used to refer to the introduction of nucleic acid molecules into microbial cells, such as algae, bacteria and yeast, or into plant cells. In microbial and plant systems, the term “transformation” is used to describe an inherited change due to the acquisition of exogenous nucleic acids by the microorganism or plant and is essentially synonymous with the term “transfection.” However, in animal cells, transformation has acquired a second meaning which can refer to changes in the growth properties of cells in culture after they become cancerous, for example. Therefore, to avoid confusion, the term “transfection” is preferably used with regard to the introduction of exogenous nucleic acids into animal cells, and the term “transfection” will be used herein to generally encompass transfection of animal cells, and transformation of microbial cells or plant cells, to the extent that the terms pertain to the introduction of exogenous nucleic acids into a cell. Therefore, transfection techniques include, but are not limited to, transformation, particle bombardment, diffusion, active transport, bath sonication, electroporation, microinjection, lipofection, adsorption, infection and protoplast fusion. 
     It will be appreciated by one skilled in the art that use of recombinant DNA technologies can improve control of expression of transfected nucleic acid molecules by manipulating, for example, the number of copies of the nucleic acid molecules within the host cell, the efficiency with which those nucleic acid molecules are transcribed, the efficiency with which the resultant transcripts are translated, and the efficiency of post-translational modifications. Additionally, the promoter sequence might be genetically engineered to improve the level of expression as compared to the native promoter. Recombinant techniques useful for controlling the expression of nucleic acid molecules include, but are not limited to, integration of the nucleic acid molecules into one or more host cell chromosomes, addition of vector stability sequences to plasmids, substitutions or modifications of transcription control signals (e.g., promoters, operators, enhancers), substitutions or modifications of translational control signals (e.g., ribosome binding sites, Shine-Dalgamo sequences), modification of nucleic acid molecules to correspond to the codon usage of the host cell, and deletion of sequences that destabilize transcripts. 
     General discussion above with regard to recombinant nucleic acid molecules and transfection of host cells is intended to be applied to any recombinant nucleic acid molecule discussed herein, including those encoding any amino acid sequence having a biological activity of at least one domain from a PUFA PKS system, those encoding amino acid sequences from other PKS systems, and those encoding other proteins or domains. 
     Polyunsaturated fatty acids (PUFAs) are essential membrane components in higher eukaryotes and the precursors of many lipid-derived signaling molecules. The PUFA PKS system of the present invention uses pathways for PUFA synthesis that do not require desaturation and elongation of saturated fatty acids. The pathways catalyzed by PUFA PKS systems are distinct from previously recognized PKS systems in both structure and mechanism. Generation of cis double bonds is suggested to involve position-specific isomerases; these enzymes are believed to be useful in the production of new families of antibiotics. 
     To produce significantly high yields of one or more desired polyunsaturated fatty acids or other bioactive molecules, an organism, preferably a microorganism or a plant, can be genetically modified to alter the activity and particularly, the end product, of the PUFA PKS system in the microorganism or plant or to introduce a PUFA PKS system into the microorganism or plant. 
     Therefore, one embodiment of the present invention relates to a genetically modified microorganism, wherein the microorganism expresses a PKS system comprising at least one biologically active domain of a polyunsaturated fatty acid (PUFA) polyketide synthase (PKS) system as described herein (e.g., at least one domain or protein, or biologically active fragment or homologue thereof, of a PUFA PKS system from  Shewanella japonica  or  Shewanella olleyana ). The genetic modification of the microorganism affects the activity of the PKS system in the organism. The domain of the PUFA PKS system can include any of the domains, including homologues thereof, for the marine bacterial PUFA PKS systems as described above, and can also include any domain of a PUFA PKS system from any other bacterial or non-bacterial microorganism, including any eukaryotic microorganism, and particularly including any Thraustochytrid microorganism or any domain of a PUFA PKS system from a microorganism identified by a screening method as described in U.S. patent application Ser. No. 10/124,800, supra. Briefly, the screening process described in U.S. patent application Ser. No. 10/124,800 includes the steps of: (a) selecting a microorganism that produces at least one PUFA; and, (b) identifying a microorganism from (a) that has an ability to produce increased PUFAs under dissolved oxygen conditions of less than about 5% of saturation in the fermentation medium, as compared to production of PUFAs by the microorganism under dissolved oxygen conditions of greater than about 5% of saturation, and preferably about 10%, and more preferably about 15%, and more preferably about 20% of saturation in the fermentation medium. Proteins, domains, and homologues thereof for other bacterial PUFA PKS systems are described in U.S. Pat. No. 6,140,486, supra, incorporated by reference in its entirety. Proteins, domains, and homologues thereof for Thraustochytrid PUFA PKS systems are described in detail in U.S. Pat. No. 6,566,583, supra; U.S. patent application Ser. No. 10/124,800, supra; and U.S. patent application Ser. No. 10/810,352, supra, each of which is incorporated herein by reference in its entirety. 
     In one aspect of the invention, a genetically modified organism can endogenously contain and express a PUFA PKS system, and the genetic modification can be a genetic modification of one or more of the functional domains of the endogenous PUFA PKS system, whereby the modification has some effect on the activity of the PUFA PKS system. For example, the  Shewanella japonica  or  Shewanella olleyana  species described herein may be genetically modified by modifying an endogenous PUFA PKS gene or genes that results in some alteration (change, modification) of the PUFA PKS function in that microorganism. 
     In another aspect of the invention, a genetically modified organism can endogenously contain and express a PUFA PKS system, and the genetic modification can be an introduction of at least one exogenous nucleic acid sequence (e.g., a recombinant nucleic acid molecule), wherein the exogenous nucleic acid sequence encodes at least one biologically active domain or protein from a second PKS system (including a PUFA PKS system or another type of PKS system) and/or a protein that affects the activity of the PUFA PKS system. In this aspect of the invention, the organism can also have at least one modification to a gene or genes comprising its endogenous PUFA PKS system. 
     In yet another aspect of the invention, the genetically modified organism does not necessarily endogenously (naturally) contain a PUFA PKS system, but is genetically modified to introduce at least one recombinant nucleic acid molecule encoding an amino acid sequence having the biological activity of at least one domain of a PUFA PKS system. Preferably, the organism is genetically modified to introduce more than one recombinant nucleic acid molecule which together encode the requisite components of a PUFA PKS system for production of a PUFA PKS system product (bioactive molecule, such as a PUFA or antibiotic), or to introduce a recombinant nucleic acid molecule encoding multiple domains comprising the requisite components of a PUFA PKS system for production of a PUFA PKS product. Various embodiments associated with each of these aspects will be discussed in greater detail below. 
     It is to be understood that a genetic modification of a PUFA PKS system or an organism comprising a PUFA PKS system can involve the modification and/or utilization of at least one domain of a PUFA PKS system (including a portion of a domain), more than one or several domains of a PUFA PKS system (including adjacent domains, non-contiguous domains, or domains on different proteins in the PUFA PKS system), entire proteins of the PUFA PKS system, and the entire PUFA PKS system (e.g., all of the proteins encoded by the PUFA PKS genes) or even more than one PUFA PKS system (e.g., one from an organism that naturally produces DHA and one from an organism that naturally produces EPA). As such, modifications can include, but are not limited to: a small modification to a single domain of an endogenous PUFA PKS system; substitution of, deletion of or addition to one or more domains or proteins of an endogenous PUFA PKS system; introduction of one or more domains or proteins from a recombinant PUFA PKS system; introduction of a second PUFA PKS system in an organism with an endogenous PUFA PKS system; replacement of the entire PUFA PKS system in an organism with the PUFA PKS system from a different organism; or introduction of one, two, or more entire PUFA PKS systems to an organism that does not endogenously have a PUFA PKS system. One of skill in the art will understand that any genetic modification to a PUFA PKS system is encompassed by the invention. 
     As used herein, a genetically modified microorganism can include a genetically modified bacterium, protist, microalgae, fungus, or other microbe, and particularly, any of the genera of the order Thraustochytriales (e.g., a Thraustochytrid), including any microorganism in the families Thraustochytriaceae and Labyrinthulaceae described herein (e.g.,  Schizochytrium, Thraustochytrium, Japonochytrium, Labyrinthula, Labyrinthuloides , etc.). Such a genetically modified microorganism has a genome which is modified (i.e., mutated or changed) from its normal (i.e., wild-type or naturally occurring) form such that the desired result is achieved (i.e., increased or modified PUFA PKS activity and/or production of a desired product using the PKS system). Genetic modification of a microorganism can be accomplished using classical strain development and/or molecular genetic techniques. Such techniques known in the art and are generally disclosed for microorganisms, for example, in Sambrook et al., 1989 , Molecular Cloning: A Laboratory Manual , Cold Spring Harbor Labs Press. The reference Sambrook et al., ibid., is incorporated by reference herein in its entirety. A genetically modified microorganism can include a microorganism in which nucleic acid molecules have been inserted, deleted or modified (i.e., mutated; e.g., by insertion, deletion, substitution, and/or inversion of nucleotides), in such a manner that such modifications provide the desired effect within the microorganism. 
     Examples of suitable host microorganisms for genetic modification include, but are not limited to, yeast including  Saccharomyces cerevisiae, Saccharomyces carlsbergensis , or other yeast such as  Candida, Kluyveromyces , or other fungi, for example, filamentous fungi such as  Aspergillus, Neurospora, Penicillium , etc. Bacterial cells also may be used as hosts. These include, but are not limited to,  Escherichia coli , which can be useful in fermentation processes. Alternatively, and only by way of example, a host such as a  Lactobacillus  species or  Bacillus  species can be used as a host. 
     Particularly preferred host cells for use in the present invention include microorganisms from a genus including, but not limited to:  Thraustochytrium, Japonochytrium, Aplanochytrium, Elina  and  Schizochytrium  within the Thraustochytriaceae, and  Labyrinthula, Labyrinthuloides , and  Labyrinthomyxa  within the Labyrinthulaceae. Preferred species within these genera include, but are not limited to: any species within  Labyrinthula , including  Labyrinthula  sp.,  Labyrinthula algeriensis, Labyrinthula cienkowskii, Labyrinthula chattonii, Labyrinthula coenocystis, Labyrinthula macrocystis, Labyrinthula macrocystis atlantica, Labyrinthula macrocystis macrocystis, Labyrinthula magnifica, Labyrinthula minuta, Labyrinthula roscoffensis, Labyrinthula valkanovii, Labyrinthula vitellina, Labyrinthula vitellina pacifica, Labyrinthula vitellina vitellina, Labyrinthula zopfii ; any  Labyrinthuloides  species, including  Labyrinthuloides  sp.,  Labyrinthuloides minuta, Labyrinthuloides schizochytrops ; any  Labyrinthomyxa  species, including  Labyrinthomyxa  sp.,  Labyrinthomyxa pohlia, Labyrinthomyxa sauvageaui , any  Aplanochytrium  species, including  Aplanochytrium  sp. and  Aplanochytrium kerguelensis ; any  Elina  species, including  Elina  sp.,  Elina marisalba, Elina sinorifica ; any  Japonochytrium  species, including  Japonochytrium  sp.,  Japonochytrium marinum ; any  Schizochytrium  species, including  Schizochytrium  sp.,  Schizochytrium aggregatum, Schizochytrium limacinum, Schizochytrium minutum, Schizochytrium octosporum ; and any  Thraustochytrium  species, including  Thraustochytrium  sp.,  Thraustochytrium aggregatum, Thraustochytrium arudimentale, Thraustochytrium aureum, Thraustochytrium benthicola, Thraustochytrium globosum, Thraustochytrium kinnei, Thraustochytrium motivum, Thraustochytrium pachydermum, Thraustochytrium proliferum, Thraustochytrium roseum, Thraustochytrium striatum, Ulkenia  sp.,  Ulkenia minuta, Ulkenia profunda, Ulkenia radiate, Ulkenia sarkariana , and  Ulkenia visurgensis . Particularly preferred species within these genera include, but are not limited to: any  Schizochytrium  species, including  Schizochytrium aggregatum, Schizochytrium limacinum, Schizochytrium minutum ; or any  Thraustochytrium  species (including former  Ulkenia  species such as  U. visurgensis, U. amoeboida, U. sarkariana, U. profunda, U. radiata, U. minuta  and  Ulkenia  sp. BP-5601), and including  Thraustochytrium striatum, Thraustochytrium aureum, Thraustochytrium roseum ; and any  Japonochytrium  species. Particularly preferred strains of Thraustochytriales include, but are not limited to:  Schizochytrium  sp. (S31) (ATCC 20888);  Schizochytrium  sp. (S8) (ATCC 20889);  Schizochytrium  sp. (LC-RM) (ATCC 18915);  Schizochytrium  sp. (SR21);  Schizochytrium aggregatum  (Goldstein et Belsky) (ATCC 28209);  Schizochytrium limacinum  (Honda et Yokochi) (IFO 32693);  Thraustochytrium  sp. (23B) (ATCC 20891);  Thraustochytrium striatum  (Schneider) (ATCC 24473);  Thraustochytrium aureum  (Goldstein) (ATCC 34304);  Thraustochytrium roseum  (Goldstein) (ATCC 28210); and  Japonochytrium  sp. (L1) (ATCC 28207). 
     According to the present invention, the terms/phrases “Thraustochytrid”, “Thraustochytriales microorganism” and “microorganism of the order Thraustochytriales” can be used interchangeably and refer to any members of the order Thraustochytriales, which includes both the family Thraustochytriaceae and the family Labyrinthulaceae. The terms “Labyrinthulid” and “Labyrinthulaceae” are used herein to specifically refer to members of the family Labyrinthulaceae. To specifically reference Thraustochytrids that are members of the family Thraustochytriaceae, the term “Thraustochytriaceae” is used herein. Thus, for the present invention, members of the Labyrinthulids are considered to be included in the Thraustochytrids. 
     Developments have resulted in frequent revision of the taxonomy of the Thraustochytrids. Taxonomic theorists generally place Thraustochytrids with the algae or algae-like protists. However, because of taxonomic uncertainty, it would be best for the purposes of the present invention to consider the strains described in the present invention as Thraustochytrids to include the following organisms: Order: Thraustochytriales; Family: Thraustochytriaceae (Genera:  Thraustochytrium, Schizochytrium, Japonochytrium, Aplanochytrium , or  Elina ) or Labyrinthulaceae (Genera  Labyrinthula, Labyrinthuloides , or  Labyrinthomyxa ). Also, the following genera are sometimes included in either family Thraustochytriaceae or Labyrinthulaceae:  Althornia, Corallochytrium, Diplophyrys , and  Pyrrhosorus ), and for the purposes of this invention are encompassed by reference to a Thraustochytrid or a member of the order Thraustochytriales. It is recognized that at the time of this invention, revision in the taxonomy of Thraustochytrids places the genus  Labyrinthuloides  in the family of Labyrinthulaceae and confirms the placement of the two families Thraustochytriaceae and Labyrinthulaceae within the Stramenopile lineage. It is noted that the Labyrinthulaceae are sometimes commonly called labyrinthulids or labyrinthula, or labyrinthuloides and the Thraustochytriaceae are commonly called thraustochytrids, although, as discussed above, for the purposes of clarity of this invention, reference to Thraustochytrids encompasses any member of the order Thraustochytriales and/or includes members of both Thraustochytriaceae and Labyrinthulaceae. Recent taxonomic changes are summarized below. 
     Strains of certain unicellular microorganisms disclosed herein are members of the order Thraustochytriales. Thraustochytrids are marine eukaryotes with an evolving taxonomic history. Problems with the taxonomic placement of the Thraustochytrids have been reviewed by Moss (in “ The Biology of Marine Fungi” , Cambridge University Press p. 105 (1986)), Bahnweb and Jackle (ibid. p. 131) and Chamberlain and Moss (BioSystems 21:341 (1988)). 
     For convenience purposes, the Thraustochytrids were first placed by taxonomists with other colorless zoosporic eukaryotes in the Phycomycetes (algae-like fungi). The name Phycomycetes, however, was eventually dropped from taxonomic status, and the Thraustochytrids were retained in the Oomycetes (the biflagellate zoosporic fungi). It was initially assumed that the Oomycetes were related to the heterokont algae, and eventually a wide range of ultrastructural and biochemical studies, summarized by Barr (Barr.  Biosystems  14:359 (1981)) supported this assumption. The Oomycetes were in fact accepted by Leedale (Leedale.  Taxon  23:261 (1974)) and other phycologists as part of the heterokont algae. However, as a matter of convenience resulting from their heterotrophic nature, the Oomycetes and Thraustochytrids have been largely studied by mycologists (scientists who study fungi) rather than phycologists (scientists who study algae). 
     From another taxonomic perspective, evolutionary biologists have developed two general schools of thought as to how eukaryotes evolved. One theory proposes an exogenous origin of membrane-bound organelles through a series of endosymbioses (Margulis, 1970 , Origin of Eukaryotic Cells . Yale University Press, New Haven); e.g., mitochondria were derived from bacterial endosymbionts, chloroplasts from cyanophytes, and flagella from spirochaetes. The other theory suggests a gradual evolution of the membrane-bound organelles from the non-membrane-bounded systems of the prokaryote ancestor via an autogenous process (Cavalier-Smith, 1975 , Nature  (Lond.) 256:462-468). Both groups of evolutionary biologists however, have removed the Oomycetes and Thraustochytrids from the fungi and place them either with the chromophyte algae in the kingdom Chromophyta (Cavalier-Smith  BioSystems  14:461 (1981)) (this kingdom has been more recently expanded to include other protists and members of this kingdom are now called Stramenopiles) or with all algae in the kingdom Protoctista (Margulis and Sagen.  Biosystems  18:141 (1985)). 
     With the development of electron microscopy, studies on the ultrastructure of the zoospores of two genera of Thraustochytrids,  Thraustochytrium  and  Schizochytrium , (Perkins, 1976, pp. 279-312 in “ Recent Advances in Aquatic Mycology”  (ed. E. B. G. Jones), John Wiley &amp; Sons, New York; Kazama.  Can. J. Bot.  58:2434 (1980); Barr, 1981 , Biosystems  14:359-370) have provided good evidence that the Thraustochytriaceae are only distantly related to the Oomycetes. Additionally, genetic data representing a correspondence analysis (a form of multivariate statistics) of 5-S ribosomal RNA sequences indicate that Thraustochytriales are clearly a unique group of eukaryotes, completely separate from the fungi, and most closely related to the red and brown algae, and to members of the Oomycetes (Mannella et al.  Mol. Evol.  24:228 (1987)). Most taxonomists have agreed to remove the Thraustochytrids from the Oomycetes (Bartnicki-Garcia. p. 389 in “ Evolutionary Biology of the Fungi”  (eds. Rayner, A. D. M., Brasier, C. M. &amp; Moore, D.), Cambridge University Press, Cambridge). 
     In summary, employing the taxonomic system of Cavalier-Smith (Cavalier-Smith.  BioSystems  14:461 (1981); Cavalier-Smith.  Microbiol. Rev.  57:953 (1993)), the Thraustochytrids are classified with the chromophyte algae in the kingdom Chromophyta (Stramenopiles). This taxonomic placement has been more recently reaffirmed by Cavalier-Smith et al. using the 18s rRNA signatures of the Heterokonta to demonstrate that Thraustochytrids are chromists not Fungi (Cavalier-Smith et al.  Phil. Tran. Roy. Soc. London Series BioSciences  346:387 (1994)). This places the Thraustochytrids in a completely different kingdom from the fungi, which are all placed in the kingdom Eufungi. 
     Currently, there are 71 distinct groups of eukaryotic organisms (Patterson. Am. Nat. 154:S96 (1999)) and within these groups four major lineages have been identified with some confidence: (1) Alveolates, (2) Stramenopiles, (3) a Land Plant-green algae-Rhodophyte_Glaucophyte (“plant”) clade and (4) an Opisthokont clade (Fungi and Animals). Formerly these four major lineages would have been labeled Kingdoms but use of the “kingdom” concept is no longer considered useful by some researchers. 
     As noted by Armstrong, Stramenopile refers to three-parted tubular hairs, and most members of this lineage have flagella bearing such hairs. Motile cells of the Stramenopiles (unicellular organisms, sperm, zoospores) are asymmetrical having two laterally inserted flagella, one long, bearing three-parted tubular hairs that reverse the thrust of the flagellum, and one short and smooth. Formerly, when the group was less broad, the Stramenopiles were called Kingdom Chromista or the heterokont (=different flagella) algae because those groups consisted of the Brown Algae or Phaeophytes, along with the yellow-green Algae, Golden-brown Algae, Eustigmatophytes and Diatoms. Subsequently some heterotrophic, fungal-like organisms, the water molds, and labyrinthulids (slime net amoebas), were found to possess similar motile cells, so a group name referring to photosynthetic pigments or algae became inappropriate. Currently, two of the families within the Stramenopile lineage are the Labyrinthulaceae and the Thraustochytriaceae. Historically, there have been numerous classification strategies for these unique microorganisms and they are often classified under the same order (i.e., Thraustochytriales). Relationships of the members in these groups are still developing. Porter and Leander have developed data based on 18S small subunit ribosomal DNA indicating the thraustochytrid-labyrinthulid clade in monophyletic. However, the clade is supported by two branches; the first contains three species of  Thraustochytrium  and  Ulkenia profunda , and the second includes three species of  Labyrinthula , two species of  Labyrinthuloides  and  Schizochytrium aggregatum.    
     The taxonomic placement of the Thraustochytrids as used in the present invention is therefore summarized below:
     Kingdom: Chromophyta (Stramenopiles)   Phylum: Heterokonta   Order: Thraustochytriales (Thraustochytrids)   Family: Thraustochytriaceae or Labyrinthulaceae   Genera:  Thraustochytrium, Schizochytrium, Japonochytrium, Aplanochytrium, Elina, Labyrinthula, Labyrinthuloides , or  Labyrinthulomyxa      

     Some early taxonomists separated a few original members of the genus  Thraustochytrium  (those with an amoeboid life stage) into a separate genus called  Ulkenia . However it is now known that most, if not all, Thraustochytrids (including  Thraustochytrium  and  Schizochytrium ), exhibit amoeboid stages and as such,  Ulkenia  is not considered by some to be a valid genus. As used herein, the genus  Thraustochytrium  will include  Ulkenia.    
     Despite the uncertainty of taxonomic placement within higher classifications of Phylum and Kingdom, the Thraustochytrids remain a distinctive and characteristic grouping whose members remain classifiable within the order Thraustochytriales. 
     Another embodiment of the present invention relates to a genetically modified plant, wherein the plant has been genetically modified to recombinantly express a PKS system comprising at least one biologically active domain or protein of a polyunsaturated fatty acid (PUFA) polyketide synthase (PKS) system as described herein. The domain of the PUFA PKS system can include any of the domains, including homologues thereof, for PUFA PKS systems as described above (e.g., for  Shewanella japonica  and/or  Shewanella olleyana ), and can also include any domain of a PUFA PKS system from any bacterial or non-bacterial microorganism (including any eukaryotic microorganism and any Thraustochytrid microorganism, such as  Schizochytrium  and/or  Thraustochytrium ) or any domain of a PUFA PKS system from a microorganism identified by a screening method as described in U.S. patent application Ser. No. 10/124,800, supra. The plant can also be further modified with at least one domain or biologically active fragment thereof of another PKS system, including, but not limited to, Type I PKS systems (iterative or modular), Type II PKS systems, and/or Type III PKS systems. The modification of the plant can involve the modification and/or utilization of at least one domain of a PUFA PKS system (including a portion of a domain), more than one or several domains of a PUFA PKS system (including adjacent domains, non-contiguous domains, or domains on different proteins in the PUFA PKS system), entire proteins of the PUFA PKS system, and the entire PUFA PKS system (e.g., all of the proteins encoded by the PUFA PKS genes) or even more than one PUFA PKS system (e.g., one from an organism that naturally produces DHA and one from an organism that naturally produces EPA). 
     As used herein, a genetically modified plant can include any genetically modified plant including higher plants and particularly, any consumable plants or plants useful for producing a desired bioactive molecule of the present invention. “Plant parts”, as used herein, include any parts of a plant, including, but not limited to, seeds, pollen, embryos, flowers, fruits, shoots, leaves, roots, stems, explants, etc. A genetically modified plant has a genome which is modified (i.e., mutated or changed) from its normal (i.e., wild-type or naturally occurring) form such that the desired result is achieved (i.e., increased or modified PUFA PKS activity and/or production of a desired product using the PKS system). Genetic modification of a plant can be accomplished using classical strain development and/or molecular genetic techniques. Methods for producing a transgenic plant, wherein a recombinant nucleic acid molecule encoding a desired amino acid sequence is incorporated into the genome of the plant, are known in the art. A preferred plant to genetically modify according to the present invention is preferably a plant suitable for consumption by animals, including humans. 
     Preferred plants to genetically modify according to the present invention (i.e., plant host cells) include, but are not limited to any higher plants, including both dicotyledonous and monocotyledonous plants, and particularly consumable plants, including crop plants and especially plants used for their oils. Such plants can include, for example: canola, soybeans, rapeseed, linseed, corn, safflowers, sunflowers and tobacco. Other preferred plants include those plants that are known to produce compounds used as pharmaceutical agents, flavoring agents, nutraceutical agents, functional food ingredients or cosmetically active agents or plants that are genetically engineered to produce these compounds/agents. 
     According to the present invention, a genetically modified microorganism or plant includes a microorganism or plant that has been modified using recombinant technology or by classical mutagenesis and screening techniques. As used herein, genetic modifications that result in a decrease in gene expression, in the function of the gene, or in the function of the gene product (i.e., the protein encoded by the gene) can be referred to as inactivation (complete or partial), deletion, interruption, blockage or down-regulation of a gene. For example, a genetic modification in a gene which results in a decrease in the function of the protein encoded by such gene, can be the result of a complete deletion of the gene (i.e., the gene does not exist, and therefore the protein does not exist), a mutation in the gene which results in incomplete or no translation of the protein (e.g., the protein is not expressed), or a mutation in the gene which decreases or abolishes the natural function of the protein (e.g., a protein is expressed which has decreased or no enzymatic activity or action). Genetic modifications that result in an increase in gene expression or function can be referred to as amplification, overproduction, overexpression, activation, enhancement, addition, or up-regulation of a gene. 
     The genetic modification of a microorganism or plant according to the present invention preferably affects the activity of the PKS system expressed by the microorganism or plant, whether the PKS system is endogenous and genetically modified, endogenous with the introduction of recombinant nucleic acid molecules into the organism (with the option of modifying the endogenous system or not), or provided completely by recombinant technology. To alter the PUFA production profile of a PUFA PKS system or organism expressing such system includes causing any detectable or measurable change in the production of any one or more PUFAs (or other bioactive molecule produced by the PUFA PKS system) by the host microorganism or plant as compared to in the absence of the genetic modification (i.e., as compared to the unmodified, wild-type microorganism or plant or the microorganism or plant that is unmodified at least with respect to PUFA synthesis—i.e., the organism might have other modifications not related to PUFA synthesis). To affect the activity of a PKS system includes any genetic modification that causes any detectable or measurable change or modification in the PKS system expressed by the organism as compared to in the absence of the genetic modification. A detectable change or modification in the PKS system can include, but is not limited to: a change or modification (introduction of, increase or decrease) of the expression and/or biological activity of any one or more of the domains in a modified PUFA PKS system as compared to the endogenous PUFA PKS system in the absence of genetic modification; the introduction of PKS system activity (i.e., the organism did not contain a PKS system or a PUFA PKS system prior to the genetic modification) into an organism such that the organism now has measurable/detectable PKS system activity, such as production of a product of a PUFA PKS system; the introduction into the organism of a functional domain from a different PKS system than the PKS system endogenously expressed by the organism such that the PKS system activity is modified (e.g., a bacterial PUFA PKS domain as described herein is introduced into an organism that endogenously expresses a non-bacterial PUFA PKS system, such as a Thraustochytrid); a change in the amount of a bioactive molecule (e.g., a PUFA) produced by the PKS system (e.g., the system produces more (increased amount) or less (decreased amount) of a given product as compared to in the absence of the genetic modification); a change in the type of a bioactive molecule (e.g., a change in the type of PUFA) produced by the PKS system (e.g., the system produces an additional or different PUFA, a new or different product, or a variant of a PUFA or other product that is naturally produced by the system); and/or a change in the ratio of multiple bioactive molecules produced by the PKS system (e.g., the system produces a different ratio of one PUFA to another PUFA, produces a completely different lipid profile as compared to in the absence of the genetic modification, or places various PUFAs in different positions in a triacylglycerol as compared to the natural configuration). Such a genetic modification includes any type of genetic modification and specifically includes modifications made by recombinant technology and/or by classical mutagenesis. 
     It should be noted that reference to increasing the activity of a functional domain or protein in a PUFA PKS system refers to any genetic modification in the organism containing the domain or protein (or into which the domain or protein is to be introduced) which results in increased functionality of the domain or protein system and can include higher activity of the domain or protein (e.g., specific activity or in vivo enzymatic activity), reduced inhibition or degradation of the domain or protein system, and overexpression of the domain or protein. For example, gene copy number can be increased, expression levels can be increased by use of a promoter that gives higher levels of expression than that of the native promoter, or a gene can be altered by genetic engineering or classical mutagenesis to increase the activity of the domain or protein encoded by the gene. 
     Similarly, reference to decreasing the activity of a functional domain or protein in a PUFA PKS system refers to any genetic modification in the organism containing such domain or protein (or into which the domain or protein is to be introduced) which results in decreased functionality of the domain or protein and includes decreased activity of the domain or protein, increased inhibition or degradation of the domain or protein and a reduction or elimination of expression of the domain or protein. For example, the action of domain or protein of the present invention can be decreased by blocking or reducing the production of the domain or protein, “knocking out” the gene or portion thereof encoding the domain or protein, reducing domain or protein activity, or inhibiting the activity of the domain or protein. Blocking or reducing the production of a domain or protein can include placing the gene encoding the domain or protein under the control of a promoter that requires the presence of an inducing compound in the growth medium. By establishing conditions such that the inducer becomes depleted from the medium, the expression of the gene encoding the domain or protein (and therefore, of protein synthesis) could be turned off. The present inventors demonstrate the ability to delete (knock out) targeted genes in a Thraustochytrid microorganism in the Examples section. Blocking or reducing the activity of domain or protein could also include using an excision technology approach similar to that described in U.S. Pat. No. 4,743,546, incorporated herein by reference. To use this approach, the gene encoding the protein of interest is cloned between specific genetic sequences that allow specific, controlled excision of the gene from the genome. Excision could be prompted by, for example, a shift in the cultivation temperature of the culture, as in U.S. Pat. No. 4,743,546, or by some other physical or nutritional signal. 
     In one embodiment of the present invention, the endogenous PUFA PKS system of a microorganism is genetically modified by, for example, classical mutagenesis and selection techniques and/or molecular genetic techniques, include genetic engineering techniques. Genetic engineering techniques can include, for example, using a targeting recombinant vector to delete a portion of an endogenous gene (demonstrated in the Examples) or to replace a portion of an endogenous gene with a heterologous sequence (demonstrated in the Examples). Examples of heterologous sequences that could be introduced into a host genome include sequences encoding at least one functional PUFA PKS domain or protein from another PKS system or even an entire PUFA PKS system (e.g., all genes associated with the PUFA PKS system). A heterologous sequence can also include a sequence encoding a modified functional domain (a homologue) of a natural domain from a PUFA PKS system. Other heterologous sequences that can be introduced into the host genome include a sequence encoding a protein or functional domain that is not a domain of a PKS system per se, but which will affect the activity of the endogenous PKS system. For example, one could introduce into the host genome a nucleic acid molecule encoding a phosphopantetheinyl transferase. Specific modifications that could be made to an endogenous PUFA PKS system are discussed in detail herein. 
     With regard to the production of genetically modified plants, methods for the genetic engineering of plants are also well known in the art. For instance, numerous methods for plant transformation have been developed, including biological and physical transformation protocols. See, for example, Miki et al., “Procedures for Introducing Foreign DNA into Plants” in  Methods in Plant Molecular Biology and Biotechnology , Glick, B. R. and Thompson, J. E. Eds. (CRC Press, Inc., Boca Raton, 1993) pp. 67-88. In addition, vectors and in vitro culture methods for plant cell or tissue transformation and regeneration of plants are available. See, for example, Gruber et al., “Vectors for Plant Transformation” in  Methods in Plant Molecular Biology and Biotechnology , Glick, B. R. and Thompson, J. E. Eds. (CRC Press, Inc., Boca Raton, 1993) pp. 89-119. 
     The most widely utilized method for introducing an expression vector into plants is based on the natural transformation system of  Agrobacterium . See, for example, Horsch et al.,  Science  227:1229 (1985).  A. tumefaciens  and  A. rhizogenes  are plant pathogenic soil bacteria which genetically transform plant cells. The Ti and Ri plasmids of  A. tumefaciens  and  A. rhizogenes , respectively, carry genes responsible for genetic transformation of the plant. See, for example, Kado, C. I.,  Crit. Rev. Plant. Sci.  10:1 (1991). Descriptions of  Agrobacterium  vector systems and methods for  Agrobacterium -mediated gene transfer are provided by numerous references, including Gruber et al., supra, Miki et al., supra, Moloney et al.,  Plant Cell Reports  8:238 (1989), and U.S. Pat. Nos. 4,940,838 and 5,464,763. 
     Another generally applicable method of plant transformation is microprojectile-mediated transformation wherein DNA is carried on the surface of microprojectiles. The expression vector is introduced into plant tissues with a biolistic device that accelerates the microprojectiles to speeds sufficient to penetrate plant cell walls and membranes. Sanford et al.,  Part. Sci. Technol.  5:27 (1987), Sanford, J. C.,  Trends Biotech.  6:299 (1988), Sanford, J. C.,  Physiol. Plant  79:206 (1990), Klein et al.,  Biotechnology  10:268 (1992). 
     Another method for physical delivery of DNA to plants is sonication of target cells. Zhang et al.,  Bio/Technology  9:996 (1991). Alternatively, liposome or spheroplast fusion have been used to introduce expression vectors into plants. Deshayes et al.,  EMBO J.,  4:2731 (1985), Christou et al.,  Proc Natl. Acad. Sci. USA  84:3962 (1987). Direct uptake of DNA into protoplasts using CaCl 2  precipitation, polyvinyl alcohol or poly-L-ornithine have also been reported. Hain et al.,  Mol. Gen. Genet.  199:161 (1985) and Draper et al.,  Plant Cell Physiol.  23:451 (1982). Electroporation of protoplasts and whole cells and tissues have also been described. Donn et al., In Abstracts of VIIth International Congress on Plant Cell and Tissue Culture IAPTC, A2-38, p. 53 (1990); D&#39;Halluin et al.,  Plant Cell  4:1495-1505 (1992) and Spencer et al.,  Plant Mol. Biol.  24:51-61 (1994). 
     In one aspect of this embodiment of the invention, the genetic modification of an organism (microorganism or plant) can include: (1) the introduction into the host of a recombinant nucleic acid molecule encoding an amino acid sequence having a biological activity of at least one domain of a PUFA PKS system; and/or (2) the introduction into the host of a recombinant nucleic acid molecule encoding at least one protein or functional domain that affects the activity of a PUFA PKS system. The host can include: (1) a host cell that does not express any PKS system, wherein all functional domains of a PKS system are introduced into the host cell, and wherein at least one functional domain is from a PUFA PKS system as described herein; (2) a host cell that expresses a PKS system (endogenous or recombinant) having at least one functional domain of a PUFA PKS system described herein; and (3) a host cell that expresses a PKS system (endogenous or recombinant) which does not necessarily include a domain function from a PUFA PKS system described herein (in this case, the recombinant nucleic acid molecule introduced to the host cell includes a nucleic acid sequence encoding at least one functional domain of the PUFA PKS system described herein). In other words, the present invention intends to encompass any genetically modified organism (e.g., microorganism or plant), wherein the organism comprises (either endogenously or introduced by recombinant modification) at least one domain from a PUFA PKS system described herein (e.g., from or derived from  Shewanella japonica  or  Shewanella olleyana ), wherein the genetic modification has a measurable effect on the PUFA PKS activity in the host cell. 
     The present invention relates particularly to the use of PUFA PKS systems and portions thereof from the marine bacteria described herein to genetically modify microorganisms and plants to affect the production of PUFA PKS products by the microorganisms and plants. As discussed above, the bacteria that are useful in the embodiments of the present invention can grow at, and have PUFA PKS systems that are capable of producing PUFAs at (e.g., enzymes and proteins that function well at), temperatures approximating or exceeding about 20° C., preferably approximating or exceeding about 25° C. and even more preferably approximating or exceeding about 30° C. (or any temperature between 20° C. and 30° C. or higher, in whole degree increments, e.g., 21° C., 22° C., 23° C. . . . ). In a preferred embodiment, such bacteria produce PUFAs at such temperatures. As described previously herein, the marine bacteria, other  Shewanella  sp. (e.g., strain SCRC2738) and  Vibrio marinus , described in U.S. Pat. No. 6,140,486, do not produce PUFAs (or produce substantially less or no detectable PUFAs) and do not grow well, if at all, at higher temperatures (e.g., temperatures at or above 20° C.), which limits the usefulness of PUFA PKS systems derived from these bacteria, particularly in plant applications under field conditions. 
     In one embodiment of the present invention, one can identify additional bacteria that have a PUFA PKS system and the ability to grow and produce PUFAs at high temperatures. For example, inhibitors of eukaryotic growth such as nystatin (antifungal) or cycloheximide (inhibitor of eukaryotic protein synthesis) can be added to agar plates used to culture/select initial strains from water samples/soil samples collected from the types of habitats/niches such as marine or estuarian habits, or any other habitat where such bacteria can be found. This process would help select for enrichment of bacterial strains without (or minimal) contamination of eukaryotic strains. This selection process, in combination with culturing the plates at elevated temperatures (e.g. 20-30° C. or 25-30° C.), and then selecting strains that produce at least one PUFA would initially identify candidate bacterial strains with a PUFA PKS system that is operative at elevated temperatures (as opposed to those bacterial strains in the prior art which only exhibit PUFA production at temperatures less than about 20° C. and more preferably below about 5° C.). To evaluate PUFA PKS function at higher temperatures for genes from any bacterial source, one can produce cell-free extracts and test for PUFA production at various temperatures, followed by selection of microorganisms that contain PUFA PKS genes that have enzymatic/biological activity at higher temperature ranges (e.g., 15° C., 20° C., 25° C., or 30° C. or even higher). The present inventors have identified two exemplary bacteria (e.g.  Shewanella olleyana  and  Shewanella japonica ; see Examples) that are particularly suitable as sources of PUFA PKS genes, and others can be readily identified or are known to comprise PUFA PKS genes and may be useful in an embodiment of the present invention (e.g.,  Shewanella gelidimarina ). 
     Using the PUFA PKS systems from the particular marine bacteria described herein, as well as previously described non-bacterial PUFA PKS systems that, for example, make use of PUFA PKS genes from Thraustochytrid and other eukaryotic PUFA PKS systems, gene mixing can be used to extend the range of PUFA products to include EPA, DHA, ARA, GLA, SDA and others (described in detail below), as well as to produce a wide variety of bioactive molecules, including antibiotics, other pharmaceutical compounds, and other desirable products. The method to obtain these bioactive molecules includes not only the mixing of genes from various organisms but also various methods of genetically modifying the PUFA PKS genes disclosed herein. Knowledge of the genetic basis and domain structure of the bacterial PUFA PKS system of the present invention provides a basis for designing novel genetically modified organisms which produce a variety of bioactive molecules. In particular, the use of the bacterial PUFA PKS genes described herein extends that ability to produce modified PUFA PKS systems that function and produce high levels of product at higher temperatures than would be possible using the PUFA PKS genes from previously described marine bacteria. Although mixing and modification of any PKS domains and related genes are contemplated by the present inventors, by way of example, various possible manipulations of the PUFA-PKS system are discussed below with regard to genetic modification and bioactive molecule production. 
     Particularly useful PUFA PKS genes and proteins to use in conjunction with the marine bacterial PUFA PKS genes described above include the PUFA PKS genes from Thraustochytrids, such as those that have been identified in  Schizochytrium  and  Thraustochytrium . Such genes are especially useful for modification, targeting, introduction into a host cell and/or otherwise for the gene mixing and modification discussed above, in combination with various genes, portions thereof and homologues thereof from the marine bacterial genes described herein. These are described in detail in U.S. patent application Ser. No. 10/810,352, supra ( Thraustochytrium ), in U.S. patent application Ser. No. 10/124,800, supra ( Schizochytrium ), and in U.S. Pat. No. 6,566,583, supra ( Schizochytrium ). The PUFA PKS genes in both  Schizochytrium  and  Thraustochytrium  are organized into three multi-domain-encoding open reading frames, referred to herein as OrfA, OrfB and OrfC. 
     The complete nucleotide sequence for  Schizochytrium  OrfA is represented herein as SEQ ID NO:13. OrfA is a 8730 nucleotide sequence (not including the stop codon) which encodes a 2910 amino acid sequence, represented herein as SEQ ID NO:14. Within OrfA are twelve domains: (a) one β-ketoacyl-ACP synthase (KS) domain (represented by about position 1 to about position 500 of SEQ ID NO:14); (b) one malonyl-CoA:ACP acyltransferase (MAT) domain (represented by about position 575 to about position 1000 of SEQ ID NO:14); (c) nine acyl carrier protein (ACP) domains (represented by about position 1095 to about 2096 of SEQ ID NO:14; and the locations of the active site serine residues (i.e., the pantetheine binding site) for each of the nine ACP domains, with respect to the amino acid sequence of SEQ ID NO:14, are as follows: ACP1=S 1157 ; ACP2=S 1266 ; ACP3=S 1377 ; ACP4=S 1488 ; ACP5=S 1604 ; ACP6=S 1715 ; ACP7=S 1819 ; ACP8=S 1930 ; and ACP9=S 2034 ); and (d) one β-ketoacyl-ACP reductase (KR) domain (represented by about position 2200 to about position 2910 of SEQ ID NO:14). 
     The complete nucleotide sequence for  Schizochytrium  OrfB is represented herein as SEQ ID NO:15. OrfB is a 6177 nucleotide sequence (not including the stop codon) which encodes a 2059 amino acid sequence, represented herein as SEQ ID NO:16. Within OrfB are four domains: (a) one β-ketoacyl-ACP synthase (KS) domain (represented by about position 1 to about position 450 of SEQ ID NO:16); (b) one chain length factor (CLF) domain (represented by about position 460 to about position 900 of SEQ ID NO:16); (c) one acyltransferase (AT) domain (represented by about position 901 to about position 1400 of SEQ ID NO:16); and, (d) one enoyl-ACP reductase (ER) domain (represented by about position 1550 to about position 2059 of SEQ ID NO:16). 
     The complete nucleotide sequence for  Schizochytrium  OrfC is represented herein as SEQ ID NO:17. OrfC is a 4509 nucleotide sequence (not including the stop codon) which encodes a 1503 amino acid sequence, represented herein as SEQ ID NO:18. Within OrfC are three domains: (a) two FabA-like β-hydroxyacyl-ACP dehydrase (DH) domains (represented by about position 1 to about position 450 of SEQ ID NO:18; and represented by about position 451 to about position 950 of SEQ ID NO:18); and (b) one enoyl-ACP reductase (ER) domain (represented by about position 1000 to about position 1502 of SEQ ID NO:18). 
     The complete nucleotide sequence for  Thraustochytrium  OrfA is represented herein as SEQ ID NO:19. OrfA is a 8433 nucleotide sequence (not including the stop codon) which encodes a 2811 amino acid sequence, represented herein as SEQ ID NO:20. Within OrfA are 11 domains: (a) one β-ketoacyl-ACP synthase (KS) domain (represented by about position 1 to about position 500 of SEQ ID NO:20); (b) one malonyl-CoA:ACP acyltransferase (MAT) domain (represented by about position 501 to about position 1000 of SEQ ID NO:20); (c) eight acyl carrier protein (ACP) domains (represented by about position 1069 to about 1998 of SEQ ID NO:20; and the locations of the active site serine residues (i.e., the pantetheine binding site) for each of the nine ACP domains, with respect to the amino acid sequence of SEQ ID NO:20, are as follows: 1128 (ACP1), 1244 (ACP2), 1360 (ACP3), 1476 (ACP4), 1592 (ACP5), 1708 (ACP6), 1824 (ACP7) and 1940 (ACP8)); and (d) one β-ketoacyl-ACP reductase (KR) domain (represented by about position 2001 to about position 2811 of SEQ ID NO:20). 
     The complete nucleotide sequence for  Thraustochytrium  OrfB is represented herein as SEQ ID NO:21. OrfB is a 5805 nucleotide sequence (not including the stop codon) which encodes a 1935 amino acid sequence, represented herein as SEQ ID NO:22. Within OrfB are four domains: (a) one β-ketoacyl-ACP synthase (KS) domain (represented by about position 1 to about position 500 of SEQ ID NO:22); (b) one chain length factor (CLF) domain (represented by about position 501 to about position 1000 of SEQ ID NO:22); (c) one acyltransferase (AT) domain (represented by about position 1001 to about position 1500 of SEQ ID NO:22); and, (d) one enoyl-ACP reductase (ER) domain (represented by about position 1501 to about position 1935 of SEQ ID NO:22). 
     The complete nucleotide sequence for  Thraustochytrium  OrfC is represented herein as SEQ ID NO:23. OrfC is a 4410 nucleotide sequence (not including the stop codon) which encodes a 1470 amino acid sequence, represented herein as SEQ ID NO:24. Within Orfc are three domains: (a) two FabA-like β-hydroxyacyl-ACP dehydrase (DH) domains (represented by about position 1 to about position 500 of SEQ ID NO:24; and represented by about position 501 to about position 1000 of SEQ ID NO:24); and (b) one enoyl-ACP reductase (ER) domain (represented by about position 1001 to about position 1470 of SEQ ID NO:24). 
     Accordingly, encompassed by the present invention are methods to genetically modify microbial or plant cells by: genetically modifying at least one nucleic acid sequence in the organism that encodes at least one functional domain or protein (or biologically active fragment or homologue thereof) of a bacterial PUFA PKS system described herein (e.g., from or derived from the  Shewanella japonica  or  Shewanella olleyana  PUFA PKS systems described herein), and/or expressing at least one recombinant nucleic acid molecule comprising a nucleic acid sequence encoding such domain or protein. Various embodiments of such sequences, methods to genetically modify an organism, and specific modifications have been described in detail above. Typically, the method is used to produce a particular genetically modified organism that produces a particular bioactive molecule or molecules. 
     A particularly preferred embodiment of the present invention relates to a genetically modified plant or part of a plant, wherein the plant has been genetically modified using the PUFA PKS genes described herein so that the plant produces a desired product of a PUFA PKS system (e.g., a PUFA or other bioactive molecule). Knowledge of the genetic basis and domain structure of the bacterial PUFA PKS system of the present invention combined with the knowledge of the genetic basis and domain structure for various Thraustochytrid PUFA PKS systems provides a basis for designing novel genetically modified plants which produce a variety of bioactive molecules. For example, one can now design and engineer a novel PUFA PKS construct derived from various combinations of domains from the PUFA PKS systems described herein. Such constructs can first be prepared in microorganisms such as  E. coli , a yeast, or a Thraustochytrid, in order to demonstrate the production of the desired bioactive molecule, for example, followed by isolation of the construct and use of the same to transform plants to impart similar bioactive molecule production properties onto the plants. Plants are not known to endogenously contain a PUFA PKS system, and therefore, the PUFA PKS systems of the present invention represent an opportunity to produce plants with unique fatty acid production capabilities. It is a particularly preferred embodiment of the present invention to genetically engineer plants to produce one or more PUFAs in the same plant, including, EPA, DHA, DPA, ARA, GLA, SDA and others. The present invention offers the ability to create any one of a number of “designer oils” in various ratios and forms. Moreover, the disclosure of the PUFA PKS genes from the particular marine bacteria described herein offer the opportunity to more readily extend the range of PUFA production and successfully produce such PUFAs within temperature ranges used to grow most crop plants. 
     Another embodiment of the present invention relates to a genetically modified Thraustochytrid microorganism, wherein the microorganism has an endogenous polyunsaturated fatty acid (PUFA) polyketide synthase (PKS) system, and wherein the endogenous PUFA PKS system has been genetically modified to alter the expression profile of a polyunsaturated fatty acid (PUFA) by the microorganism as compared to the Thraustochytrid microorganism in the absence of the modification. Thraustochytrid microorganisms useful as host organisms in the present invention endogenously contain and express a PUFA PKS system. The genetic modification based on the present invention includes the introduction into the Thraustochytrid of at least one recombinant nucleic acid sequence encoding a PUFA PKS domain or protein (or homologue or functional fragment thereof) from a bacterial PUFA PKS system described herein. The Thraustochytrid may also contain genetic modifications within its endogenous PUFA PKS genes, including substitutions, additions, deletions, mutations, and including a partial or complete deletion of the Thraustochytrid PUFA PKS genes and replacement with the PUFA PKS genes from the preferred marine bacteria of the present invention. 
     This embodiment of the invention is particularly useful for the production of commercially valuable lipids enriched in a desired PUFA, such as EPA, via the present inventors&#39; development of genetically modified microorganisms and methods for efficiently producing lipids (triacylglycerols (TAG) as well as membrane-associated phospholipids (PL)) enriched in PUFAs. Such microorganisms are also useful as “surrogate” hosts to determine optimum gene combinations for later use in the transformation of plant cells, although other microorganisms, including many bacterial and yeast hosts, for example, can also be used as “surrogate” hosts 
     This particular embodiment of the present invention is derived in part from the following knowledge: (1) utilization of the inherent TAG production capabilities of selected microorganisms, and particularly, of Thraustochytrids, such as the commercially developed  Schizochytrium  strain ATCC 20888; (2) the present inventors&#39; detailed understanding of PUFA PKS biosynthetic pathways (i.e., PUFA PKS systems) in eukaryotes and in particular, in members of the order Thraustochytriales, and in the marine bacteria used in the present invention; and, (3) utilization of a homologous genetic recombination system in  Schizochytrium . Based on the inventors&#39; knowledge of the systems involved, the same general approach may be exploited to produce PUFAs other than EPA. 
     For example, in one embodiment of the invention, the endogenous Thraustochytrid PUFA PKS genes, such as the  Schizochytrium  genes encoding PUFA PKS enzymes that normally produce DHA and DPA, are modified by random or targeted mutagenesis, replaced with genes from other organisms that encode homologous PKS proteins (e.g., from bacteria or other sources), such as the marine bacterial PUFA PKS genes from  Shewanella japonica  or  Shewanella olleyana  described in detail herein, and/or replaced with genetically modified  Schizochytrium, Thraustochytrium  or other Thraustochytrid PUFA PKS genes. As discussed above, combinations of nucleic acid molecules encoding various domains from the marine bacterial and Thraustochytrid or other PKS systems can be “mixed and matched” to create a construct(s) that will result in production of a desired PUFA or other bioactive molecule. The product of the enzymes encoded by these introduced and/or modified genes can be EPA, for example, or it could be some other related molecule, including other PUFAs. One feature of this method is the utilization of endogenous components of Thraustochytrid PUFA synthesis and accumulation machinery that is essential for efficient production and incorporation of the PUFA into PL and TAG, while taking further advantage of the ability of the marine bacterial genes, for example, to produce EPA. In particular, this embodiment of the invention is directed to the modification of the type of PUFA produced by the organism, while retaining the high oil productivity of the parent strain. 
     Although some of the following discussion uses the organism  Schizochytrium  as an exemplary host organism, any Thraustochytrid can be modified according to the present invention, including members of the genera  Thraustochytrium, Labyrinthuloides , and  Japonochytrium . For example,  Thraustochytrium  as described above can also serve as a host organism for genetic modification using the methods described herein, although it is more likely that the  Thraustochytrium  PUFA PKS genes will be used to modify the endogenous PUFA PKS genes of another Thraustochytrid, such as  Schizochytrium . Furthermore, using methods for screening organisms as set forth in U.S. application Ser. No. 10/124,800, supra, one can identify other organisms useful in the present method and all such organisms are encompassed herein. Moreover, PUFA PKS systems can be constructed using the exemplary information provided herein, produced in other microorganisms, such as bacteria or yeast, and transformed into plants cells to produce genetically modified plants. The concepts discussed herein can be applied to various systems as desired. 
     This embodiment of the present invention can be illustrated as follows. By way of example, based on the present inventors&#39; current understanding of PUFA synthesis and accumulation in  Schizochytrium , the overall biochemical process can be divided into three parts. 
     First, the PUFAs that accumulate in  Schizochytrium  oil (DHA and DPA) are the product of a PUFA PKS system as discussed above. The PUFA PKS system in  Schizochytrium  converts malonyl-CoA into the end product PUFA without release of significant amounts of intermediate compounds. In  Schizochytrium  and also in  Thraustochytrium , three genes have previously been identified (Orfs A, B and C; also represented by SEQ ID NOs:13, 15 and 17 in  Schizochytrium  and by SEQ ID NOs:19, 21 and 23 in  Thraustochytrium , respectively) that encode all of the enzymatic domains known to be required for actual synthesis of PUFAs in these organisms. Similar sets of genes (encoding proteins containing homologous sets of enzymatic domains) have been cloned and characterized from several other non-eukaryotic organisms that produce PUFAs, namely, several strains of marine bacteria, and now in the present invention, the present inventors have identified and sequenced PUFA PKS genes in two particularly useful strains of marine bacteria,  Shewanella japonica  and  Shewanella olleyana . The PUFA products of these marine bacteria are EPA. It is an embodiment of the invention that any PUFA PKS gene set or combinations thereof could be envisioned to substitute for the  Schizochytrium  genes described in the example herein, as long as the physiological growth requirements of the production organism (e.g.,  Schizochytrium ) in fermentation conditions were satisfied. In particular, the PUFA-producing bacterial strains described above grow well at relatively high temperatures (e.g., greater than 25° C.) which further indicates that their PUFA PKS gene products will function at standard growth temperatures for  Schizochytrium  (25-30° C.). It will be apparent to those skilled in the art from this disclosure that other currently unstudied or unidentified PUFA-producing bacteria could also contain PUFA PKS genes useful for modification of Thraustochytrids. 
     Second, in addition to the genes that encode the enzymes directly involved in PUFA synthesis, an “accessory” enzyme is required. The gene encodes a phosphopantetheine transferase (PPTase) that activates the acyl-carrier protein (ACP) domains present in the PUFA PKS complex. Activation of the ACP domains by addition of this co-factor is required for the PUFA PKS enzyme complex to function. All of the ACP domains of the PUFA PKS systems identified so far show a high degree of amino acid sequence conservation and, without being bound by theory, the present inventors believe that the PPTase of  Schizochytrium  and other Thraustochytrids will recognize and activate ACP domains from other PUFA PKS systems, and vice versa. This gene is identified and included as part of the PUFA PKS system in the marine bacterial PUFA PKS systems described herein and can be used in the genetic modification scenarios encompassed by the invention. As proof of principle that heterologous PPTases and PUFA PKS genes can function together to produce a PUFA product, the present inventors have demonstrated the use of two different heterologous PPTases with the PUFA PKS genes from  Schizochytrium  to produce a PUFA in a bacterial host cell. 
     Third, in  Schizochytrium  and other Thraustochytrids, the products of the PUFA PKS system are efficiently channeled into both the phospholipids (PL) and triacylglycerols (TAG). The present inventors&#39; data suggest that the PUFA is transferred from the ACP domains of the PKS complex to coenzyme A (CoA). As in other eukaryotic organisms, this acyl-CoA would then serve as the substrate for the various acyl-transferases that form the PL and TAG molecules. In contrast, the data indicate that in bacteria, transfer to CoA does not occur; rather, there is a direct transfer from the ACP domains of the PKS complex to the acyl-transferases that form PL. The enzymatic system in  Schizochytrium  that transfers PUFA from ACP to CoA clearly can recognize both DHA and DPA and therefore, the present inventors believe that it is predictable that any PUFA product of the PUFA PKS system (as attached to the PUFA PKS ACP domains) will serve as a substrate. 
     Therefore, in one embodiment of the present invention, the present inventors propose to alter the genes encoding the components of the PUFA PKS enzyme complex in a Thraustochytrid host (e.g., by introducing at least one recombinant nucleic acid molecule encoding at least one domain or functional portion thereof from a marine bacteria PUFA PKS of the present invention) while utilizing the endogenous PPTase from  Schizochytrium , another Thraustochytrid host, or the PPTase from the marine bacteria of the invention; and PUFA-ACP to PUFA-CoA transferase activity and TAG/PL synthesis systems (or other endogenous PUFA ACP to TAG/PL mechanism. These methods of the present invention are supported by experimental data, some of which are presented in the Examples section in detail. 
     The present inventors and others have previously shown that the PUFA PKS system can be transferred between organisms, and that some parts are interchangeable. More particularly, it has been previously shown that the PUFA PKS pathways of the marine bacteria,  Shewanella  SCR2738 (Yazawa Lipids 31:S297 (1996)) and  Vibrio marinus  (along with the PPTase from  Shewanella ) (U.S. Pat. No. 6,140,486), can be successfully transferred to a heterologous host (i.e., to  E. coli ). Additionally, the degree of structural homology between the subunits of the PUFA PKS enzymes from these two organisms ( Shewanella  SCRC2738 and  Vibrio marinus ) is such that it has been possible to mix and match genes from the two systems (U.S. Pat. No. 6,140,486, supra). The functional domains of all of the PUFA PKS enzymes identified so far show some sequence homology to one another. Similarly, these data indicated that PUFA PKS systems, including those from the marine bacteria, can be transferred to, and will function in,  Schizochytrium  and other Thraustochytrids. 
     The present inventors have now expressed the PUFA PKS genes (Orfs A, B and C) from  Schizochytrium  in an  E. coli  host and have demonstrated that the cells made DHA and DPA in about the same ratio as the endogenous production of these PUFAs in  Schizochytrium  (see Example 3). Therefore, it has been demonstrated that the recombinant  Schizochytrium  PUFA PKS genes encode a functional PUFA synthesis system. Additionally, all or portions of the  Thraustochytrium  23B OrfA and OrfC genes have been shown to function in  Schizochytrium  (see Example 7). Furthermore, the present inventors have also replaced the entire  Schizochytrium  orfC coding sequence completely and exactly by the  Thraustochytrium  23B orfC coding sequence, which resulted in a PUFA production profile in the  Schizochytrium  host that was shifted toward that of  Thraustochytrium  (see Example 8). 
     The present inventors have previously found that PPTases can activate heterologous PUFA PKS ACP domains. Production of DHA in  E. coli  transformed with the PUFA PKS genes from  Vibrio marinus  occurred only when an appropriate PPTase gene (in this case, from  Shewanella  SCRC2738) was also present (see U.S. Pat. No. 6,140,486, supra). This demonstrated that the  Shewanella  PPTase was able to activate the  Vibrio  PUFA PKS ACP domains. Additionally, the present inventors have now demonstrated the activation (pantetheinylation) of ACP domains from  Schizochytrium  Orf A using a PPTase (sfp) from  Bacillus subtilus  (see Example 3). The present inventors have also demonstrated activation (pantetheinylation) of ACP domains from  Schizochytrium  Orf A by a PPTase called Het I from Nostoc (see Example 3). The HetI enzyme was additionally used as the PPTase in the experiments discussed above for the production of DHA and DPA in  E. coli  using the recombinant  Schizochytrium  PUFA PKS genes (Example 3). 
     The data also indicate that DHA-CoA and DPA-CoA may be metabolic intermediates in the  Schizochytrium  TAG and PL synthesis pathway. Published biochemical data suggest that in bacteria, the newly synthesized PUFAs are transferred directly from the PUFA PKS ACP domains to the phospholipid synthesis enzymes. In contrast, the present inventors&#39; data indicate that in  Schizochytrium , a eukaryotic organism, there may be an intermediate between the PUFA on the PUFA PKS ACP domains and the target TAG and PL molecules. The typical carrier of fatty acids in the eukaryotic cytoplasm is CoA. The inventors examined extracts of  Schizochytrium  cells and found significant levels of compounds that co-migrated during HPLC fractionation with authentic standards of DHA-CoA, DPA-CoA, 16:0-CoA and 18:1-CoA. The identity of the putative DHA-CoA and DPA-CoA peaks were confirmed using mass spectroscopy. In contrast, the inventors were not able to detect DHA-CoA in extracts of  Vibrio marinus , again suggesting that a different mechanism exists in bacteria for transfer of the PUFA to its final target (e.g., direct transfer to PL). The data indicate a mechanism likely exists in  Schizochytrium  for transfer of the newly synthesized PUFA to CoA (probably via a direct transfer from the ACP to CoA). Both TAG and PL synthesis enzymes could then access this PUFA-CoA. The observation that both DHA and DPA CoA are produced suggests that the enzymatic transfer machinery may recognize a range of PUFAs. 
     The present inventors have also created knockouts of Orf A, Orf B, and Orf C in  Schizochytrium  (see Example 4). The knockout strategy relies on the homologous recombination that has been demonstrated to occur in  Schizochytrium  (see U.S. patent application Ser. No. 10/124,807, supra). Several strategies can be employed in the design of knockout constructs. The specific strategy used to inactivate these three genes utilized insertion of a Zeocin™ resistance gene coupled to a tubulin promoter (derived from pMON50000, see U.S. patent application Ser. No. 10/124,807) into a cloned portion of the Orf. The new construct containing the interrupted coding region was then used for the transformation of wild type  Schizochytrium  cells via particle bombardment (see U.S. patent application Ser. No. 10/124,807). Bombarded cells were spread on plates containing both Zeocin™ and a supply of PUFA (see below). Colonies that grew on these plates were then streaked onto Zeocin™ plates that were not supplemented with PUFAs. Those colonies that required PUFA supplementation for growth were candidates for having had the PUFA PKS Orf inactivated via homologous recombination. In all three cases, this presumption was confirmed by rescuing the knockout by transforming the cells with a full-length genomic DNA clones of the respective  Schizochytrium  Orfs. Furthermore, in some cases, it was found that in the rescued transformants the Zeocin™ resistance gene had been removed (see Example 6), indicating that the introduced functional gene had integrated into the original site by double homologous recombination (i.e. deleting the resistance marker). One key to the success of this strategy was supplementation of the growth medium with PUFAs. In the present case, an effective means of supplementation was found to be sequestration of the PUFA by mixing with partially methylated beta-cyclodextrin prior to adding to the growth medium (see Example 6). Together, these experiments demonstrate the principle that one of skill in the art, given the guidance provided herein, can inactivate one or more of the PUFA PKS genes in a PUFA PKS-containing microorganism such as  Schizochytrium , and create a PUFA auxotroph which can then be used for further genetic modification (e.g., by introducing other PKS genes) according to the present invention (e.g., to alter the fatty acid profile of the recombinant organism). 
     One element of the genetic modification of the organisms of the present invention is the ability to directly transform a Thraustochytrid genome. In U.S. application Ser. No. 10/124,807, supra, transformation of  Schizochytrium  via single crossover homologous recombination and targeted gene replacement via double crossover homologous recombination were demonstrated. As discussed above, the present inventors have now used this technique for homologous recombination to inactivate Orf A, Orf B and OrfC of the PUFA-PKA system in  Schizochytrium . The resulting mutants are dependent on supplementation of the media with PUFA. Several markers of transformation, promoter elements for high level expression of introduced genes and methods for delivery of exogenous genetic material have been developed and are available. Therefore, the tools are in place for knocking out endogenous PUFA PKS genes in Thraustochytrids and other eukaryotes having similar PUFA PKS systems and replacing them with genes from other organisms, such as the marine bacterial genes described herein and as proposed above. 
     In one approach for production of EPA-rich TAG, the PUFA PKS system of  Schizochytrium  can be altered by the addition of heterologous genes encoding a PUFA PKS system whose product is EPA, such as the genes from  Shewanella japonica  and  Shewanella olleyana  described herein. It is anticipated that the endogenous PPTase will activate the ACP domains of that heterologous PUFA PKS system, but the inventors have also cloned and sequenced the PPTase from the marine bacteria, which could also be introduced into the host. Additionally, it is anticipated that the EPA will be converted to EPA-CoA and will readily be incorporated into  Schizochytrium  TAG and PL membranes. Therefore, in one embodiment, genes encoding a heterologous PUFA PKS system that produce EPA (e.g., from the marine bacteria above) can be introduced into a microorganism that naturally produces DHA (e.g.,  Schizochytrium ) so that the resulting microorganism produces both EPA and DHA. This technology can be further applied to genetically modified plants, for example, by introducing the two different PUFA PKS systems described above into plant cells to produce a plant that produces both EPA and DHA, or whatever combination of PUFAs is desired. 
     In one modification of this approach, techniques can be used to modify the relevant domains of the endogenous  Schizochytrium  system (either by introduction of specific regions of heterologous genes or by mutagenesis of the  Schizochytrium  genes themselves) such that its end product is EPA rather than DHA and DPA, or alternatively, so that the endproduct is both EPA and DHA and/or DPA, or so that the endproduct is EPA and ARA instead of DHA and DPA. This is an exemplary approach, as this technology can be applied to the production of other PUFA end products and to any eukaryotic microorganism that comprises a PUFA PKS system and that has the ability to efficiently channel the products of the PUFA PKS system into both the phospholipids (PL) and triacylglycerols (TAG). In particular, the invention is applicable to any Thraustochytrid microorganism or any other eukaryote that has an endogenous PUFA PKS system, which is described in detail below by way of example. In addition, the invention is applicable to any suitable host organism, into which the modified genetic material for production of various PUFA profiles as described herein can be transformed. For example, in the Examples, the PUFA PKS system from  Schizochytrium  is transformed into an  E. coli . Such a transformed organism could then be further modified to alter the PUFA production profile using the methods described herein. 
     The present invention particularly makes use can make use of genes and nucleic acid sequences which encode proteins or domains from PKS systems other than the PUFA PKS system described herein and in prior applications and includes genes and nucleic acid sequences from bacterial and non-bacterial PKS systems, including PKS systems of Type I (iterative or modular), Type II or Type III, described above. Organisms which express each of these types of PKS systems are known in the art and can serve as sources for nucleic acids useful in the genetic modification process of the present invention. 
     In a preferred embodiment, genes and nucleic acid sequences which encode proteins or domains from PKS systems other than the PUFA PKS system or from other PUFA PKS systems are isolated or derived from organisms which have preferred growth characteristics for production of PUFAs. In particular, it is desirable to be able to culture the genetically modified Thraustochytrid microorganism at temperatures at or greater than about 15° C., at or greater than 20° C., at or greater than 25° C., or at or greater than 30° C., or up to about 35° C., or in one embodiment, at any temperature between about 20° C. and 35° C., in whole degree increments. Therefore, PKS proteins or domains having functional enzymatic activity at these temperatures are preferred. The PUFA PKS genes from  Shewanella olleyana  or  Shewanella japonica  described herein naturally produce EPA and grow at temperatures up to 25° C., 30° C., or 35° C., which makes them particularly useful for this embodiment of the invention (see Examples 1-2). 
     In another preferred embodiment, the genes and nucleic acid sequences that encode proteins or domains from a PUFA PKS system that produces one fatty acid profile are used to modify another PUFA PKS system and thereby alter the fatty acid profile of the host. For example,  Thraustochytrium  23B (ATCC 20892) is significantly different from  Schizochytrium  sp. (ATCC 20888) in its fatty acid profile.  Thraustochytrium  23B can have DHA:DPA(n-6) ratios as high as 40:1 compared to only 2-3:1 in  Schizochytrium  (ATCC 20888).  Thraustochytrium  23B can also have higher levels of C20:5(n-3). However,  Schizochytrium  (ATCC 20888) is an excellent oil producer as compared to  Thraustochytrium  23B.  Schizochytrium  accumulates large quantities of triacylglycerols rich in DHA and docosapentaenoic acid (DPA; 22:5ω6); e.g., 30% DHA+DPA by dry weight. Therefore, the present inventors describe herein the modification of the  Schizochytrium  endogenous PUFA PKS system with  Thraustochytrium  23B PUFA PKS genes to create a genetically modified  Schizochytrium  with a DHA:DPA profile more similar to  Thraustochytrium  23B (i.e., a “super-DHA-producer”  Schizochytrium , wherein the production capabilities of the  Schizochytrium  combine with the DHA:DPA ratio of  Thraustochytrium ). This modification is demonstrated in Example 8. 
     Therefore, the present invention makes use of genes from certain marine bacterial and any Thraustochytrid or other eukaryotic PUFA PKS systems, and further utilizes gene mixing to extend and/or alter the range of PUFA products to include EPA, DHA, DPA, ARA, GLA, SDA and others. The method to obtain these altered PUFA production profiles includes not only the mixing of genes from various organisms into the Thraustochytrid PUFA PKS genes, but also various methods of genetically modifying the endogenous Thraustochytrid PUFA PKS genes disclosed herein. Knowledge of the genetic basis and domain structure of the Thraustochytrid PUFA PKS system and the marine bacterial PUFA PKS system provides a basis for designing novel genetically modified organisms that produce a variety of PUFA profiles. Novel PUFA PKS constructs prepared in microorganisms such as a Thraustochytrid can be isolated and used to transform plants to impart similar PUFA production properties onto the plants. 
     Any one or more of the endogenous Thraustochytrid PUFA PKS domains can be altered or replaced according to the present invention (for example with a domain from a marine bacterium of the present invention), provided that the modification produces the desired result (i.e., alteration of the PUFA production profile of the microorganism). Particularly preferred domains to alter or replace include, but are not limited to, any of the domains corresponding to the domains in  Schizochytrium  OrfB or OrfC (β-keto acyl-ACP synthase (KS), acyltransferase (AT), FabA-like β-hydroxy acyl-ACP dehydrase (DH), chain length factor (CLF), enoyl ACP-reductase (ER), an enzyme that catalyzes the synthesis of trans-2-acyl-ACP, an enzyme that catalyzes the reversible isomerization of trans-2-acyl-ACP to cis-3-acyl-ACP, and an enzyme that catalyzes the elongation of cis-3-acyl-ACP to cis-5-β-keto-acyl-ACP). In one embodiment, preferred domains to alter or replace include, but are not limited to, β-keto acyl-ACP synthase (KS), FabA-like β-hydroxy acyl-ACP dehydrase (DH), and chain length factor (CLF). 
     In one aspect of the invention, Thraustochytrid PUFA-PKS PUFA production is altered by modifying the CLF (chain length factor) domain. This domain is characteristic of Type II (dissociated enzymes) PKS systems. Its amino acid sequence shows homology to KS (keto synthase pairs) domains, but it lacks the active site cysteine. CLF may function to determine the number of elongation cycles, and hence the chain length, of the end product. In this embodiment of the invention, using the current state of knowledge of FAS and PKS synthesis, a rational strategy for production of ARA by directed modification of the non-bacterial PUFA-PKS system is provided. There is controversy in the literature concerning the function of the CLF in PKS systems (Bisang et al.,  Nature  401:502 (1999); Yi et al.,  J. Am. Chem. Soc.  125:12708 (2003)) and it is realized that other domains may be involved in determination of the chain length of the end product. However, it is significant that  Schizochytrium  produces both DHA (C22:6, ω-3) and DPA (C22:5, ω-6). In the PUFA-PKS system the cis double bonds are introduced during synthesis of the growing carbon chain. Since placement of the ω-3 and ω-6 double bonds occurs early in the synthesis of the molecules, one would not expect that they would affect subsequent end-product chain length determination. Thus, without being bound by theory, the present inventors believe that introduction of a factor (e.g. CLF) that directs synthesis of C20 units (instead of C22 units) into the  Schizochytrium  PUFA-PKS system will result in the production of EPA (C20:5, ω-3) and ARA (C20:4, ω-6). For example, in heterologous systems, one could exploit the CLF by directly substituting a CLF from an EPA producing system (such as one from  Photobacterium , or preferably from a microorganism with the preferred growth requirements as described below) into the  Schizochytrium  gene set. The fatty acids of the resulting transformants can then be analyzed for alterations in profiles to identify the transformants producing EPA and/or ARA. 
     By way of example, in this aspect of the invention, one could construct a clone with the CLF of OrfB replaced with a CLF from a C20 PUFA-PKS system, such as the marine bacterial systems described in detail herein. A marker gene could be inserted downstream of the coding region. More specifically, one can use the homologous recombination system for transformation of Thraustochytrids as described herein and in detail in U.S. patent application Ser. No. 10/124,807, supra. One can then transform the wild type Thraustochytrid cells (e.g.,  Schizochytrium  cells), select for the marker phenotype, and then screen for those that had incorporated the new CLF. Again, one would analyze these transformants for any effects on fatty acid profiles to identify transformants producing EPA and/or ARA. Alternatively, and in some cases, preferably, such screening for the effects of swapped domains can be carried out in  E. coli  (as described below) or in other systems such as, but not limited to, yeast. If some factor other than those associated with the CLF is found to influence the chain length of the end product, a similar strategy could be employed to alter those factors. In another embodiment of the invention, an organism is modified by introducing both a chain length factor plus a β-ketoacyl-ACP synthase (KS) domain. 
     In another aspect of the invention, modification or substitution of the β-hydroxy acyl-ACP dehydrase/keto synthase pairs is contemplated. During cis-vaccenic acid (C18:1, Δ11) synthesis in  E. coli , creation of the cis double bond is believed to depend on a specific DH enzyme, β-hydroxy acyl-ACP dehydrase, the product of the fabA gene. This enzyme removes HOH from a β-keto acyl-ACP and initially produces a trans double bond in the carbon chain. A subset of DH&#39;s, FabA-like, possess cis-trans isomerase activity (Heath et al., 1996, supra). A novel aspect of bacterial and non-bacterial PUFA-PKS systems is the presence of two FabA-like DH domains. Without being bound by theory, the present inventors believe that one or both of these DH domains will possess cis-trans isomerase activity (manipulation of the DH domains is discussed in greater detail below). 
     Another aspect of the unsaturated fatty acid synthesis in  E. coli  is the requirement for a particular KS enzyme, β-ketoacyl-ACP synthase, the product of the fabB gene. This is the enzyme that carries out condensation of a fatty acid, linked to a cysteine residue at the active site (by a thio-ester bond), with a malonyl-ACP. In the multi-step reaction, CO 2  is released and the linear chain is extended by two carbons. It is believed that only this KS can extend a carbon chain that contains a double bond. This extension occurs only when the double bond is in the cis configuration; if it is in the trans configuration, the double bond is reduced by enoyl-ACP reductase (ER) prior to elongation (Heath et al., 1996, supra). All of the PUFA-PKS systems characterized so far have two KS domains, one of which shows greater homology to the FabB-like KS of  E. coli  than the other. Again, without being bound by theory, the present inventors believe that in PUFA-PKS systems, the specificities and interactions of the DH (FabA-like) and KS (FabB-like) enzymatic domains determine the number and placement of cis double bonds in the end products. Because the number of 2-carbon elongation reactions is greater than the number of double bonds present in the PUFA-PKS end products, it can be determined that in some extension cycles complete reduction occurs. Thus the DH and KS domains can be used as targets for alteration of the DHA/DPA ratio or ratios of other long chain fatty acids. These can be modified and/or evaluated by introduction of homologous domains from other systems or by mutagenesis of these gene fragments. In one embodiment, the FabA-like DH domain may not require a KS partner domain at all. 
     In another embodiment, the ER (enoyl-ACP reductase—an enzyme which reduces the trans-double bond in the fatty acyl-ACP resulting in fully saturated carbons) domains can be modified or substituted to change the type of product made by the PKS system. For example, the present inventors know that  Schizochytrium  PUFA-PKS system differs from the previously described bacterial systems in that it has two (rather than one) ER domains. Without being bound by theory, the present inventors believe these ER domains can strongly influence the resulting PKS production product. The resulting PKS product could be changed by separately knocking out the individual domains or by modifying their nucleotide sequence or by substitution of ER domains from other organisms, such as the ER domain from the marine bacteria described herein. 
     In another aspect of the invention, substitution of one of the DH (FabA-like) domains of the PUFA-PKS system for a DH domain that does not posses isomerization activity is contemplated, potentially creating a molecule with a mix of cis- and trans-double bonds. The current products of the  Schizochytrium  PUFA PKS system are DHA and DPA (C22:5 ω6). If one manipulated the system to produce C20 fatty acids, one would expect the products to be EPA and ARA (C20:4 ω6). This could provide a new source for ARA. One could also substitute domains from related PUFA-PKS systems that produced a different DHA to DPA ratio—for example by using genes from  Thraustochytrium  23B (the PUFA PKS system of which is identified in U.S. patent application Ser. No. 10/124,800, supra). 
     Additionally, in one embodiment, one of the ER domains is altered in the Thraustochytrid PUFA PKS system (e.g. by removing or inactivating) to alter the end product profile. Similar strategies could be attempted in a directed manner for each of the distinct domains of the PUFA-PKS proteins using more or less sophisticated approaches. Of course one would not be limited to the manipulation of single domains. Finally, one could extend the approach by mixing domains from the PUFA-PKS system and other PKS or FAS systems (e.g., type I, type II, type III) to create an entire range of new PUFA end products. 
     As an example of how the bacterial PUFA PKS genes described in detail herein can be used to modify PUFA production in  Schizochytrium , the following discussion is provided. Again, all of the examples described herein may be equally applied to the production of other genetically modified microorganisms or to the production of genetically modified plants. All presently-known examples of PUFA PKS genes from bacteria exist as four closely linked genes that contain the same domains as in the three-gene  Schizochytrium  set. Indeed, the present inventors have demonstrated that the PUFA PKS genes from  Shewanella olleyana  and  Shewanella japonica  are found in this tightly clustered arrangement. The DNA sequences of the bacterial PUFA PKS genes described herein can now be used to design vectors for transformation of  Schizochytrium  strains defective in the endogenous PUFA PKS genes (e.g., see Examples 4, 6 and 7). Whole bacterial genes (coding sequences) may be used to replace whole  Schizochytrium  genes (coding sequences), thus utilizing the  Schizochytrium  gene expression regions, and the fourth bacterial gene may be targeted to a different location within the genome. Alternatively, individual bacterial PUFA PKS functional domains may be “swapped” or exchanged with the analogous  Schizochytrium  domains by similar techniques of homologous recombination. As yet another alternative, bacterial PUFA PKS genes may even be added to PUFA PKS systems from Thraustochytrids to produce organisms having more than one PUFA synthase activity. It is understood that the sequence of the bacterial PUFA PKS genes or domains may have to be modified to accommodate details of  Schizochytrium  codon usage, but this is within the ability of those of skill in the art. 
     It is recognized that many genetic alterations, either random or directed, which one may introduce into a native (endogenous, natural) PKS system, will result in an inactivation of enzymatic functions. Therefore, in order to test for the effects of genetic manipulation of a Thraustochytrid PUFA PKS system in a controlled environment, one could first use a recombinant system in another host, such as  E. coli , to manipulate various aspects of the system and evaluate the results. For example, the FabB strain of  E. coli  is incapable of synthesizing unsaturated fatty acids and requires supplementation of the medium with fatty acids that can substitute for its normal unsaturated fatty acids in order to grow (see Metz et al. (2001), supra). However, this requirement (for supplementation of the medium) can be removed when the strain is transformed with a functional PUFA-PKS system (i.e. one that produces a PUFA product in the  E. coli  host—see (Metz et al. (2001), supra,  FIG. 2A  of that publication). The transformed FabB strain now requires a functional PUFA-PKS system (to produce the unsaturated fatty acids) for growth without supplementation. The key element in this example is that production of a wide range of unsaturated fatty acid will suffice (even unsaturated fatty acid substitutes such as branched chain fatty acids). Therefore, in another preferred embodiment of the invention, one could create a large number of mutations in one or more of the PUFA PKS genes disclosed herein, and then transform the appropriately modified FabB strain (e.g. create mutations in an expression construct containing an ER domain and transform a FabB strain having the other essential domains on a separate plasmid—or integrated into the chromosome) and select only for those transformants that grow without supplementation of the medium (i.e., that still possessed an ability to produce a molecule that could complement the FabB defect). The FabA strain of  E. coli  has a similar phenotype to the FabB strain and could also be used as an alternative strain in the example described above. 
     One test system for genetic modification of a PUFA PKS is exemplified in the Examples section. Briefly, a host microorganism such as  E. coli  is transformed with genes encoding a PUFA PKS system including all or a portion of a Thraustochytrid PUFA PKS system (e.g., Orfs A, B and C of  Schizochytrium ) and a gene encoding a phosphopantetheinyl transferases (PPTase), which is required for the attachment of a phosphopantetheine cofactor to produce the active, holo-ACP in the PKS system. The genes encoding the PKS system can be genetically engineered to introduce one or more modifications to the Thraustochytrid PUFA PKS genes and/or to introduce nucleic acids encoding domains from other PKS systems into the Thraustochytrid genes (including genes from non-Thraustochytrid microorganisms and genes from different Thraustochytrid microorganisms). The PUFA PKS system can be expressed in the  E. coli  and the PUFA production profile measured. In this manner, potential genetic modifications can be evaluated prior to manipulation of the Thraustochytrid PUFA production organism. 
     The present invention includes the manipulation of endogenous nucleic acid molecules in a Thraustochytrid PUFA PKS system and/or the use of isolated nucleic acid molecules comprising a nucleic acid sequence from a  Shewanella japonica  PUFA PKS system, from a  Shewanella olleyana  PUFA PKS system, and can additionally include a nucleic acid sequence from a Thraustochytrid PUFA PKS system, or homologues of any of such nucleic acid sequences. In one aspect, the present invention relates to the modification and/or use of a nucleic acid molecule comprising a nucleic acid sequence encoding a domain from a PUFA PKS system having a biological activity of at least one of the following proteins: malonyl-CoA:ACP acyltransferase (MAT), β-keto acyl-ACP synthase (KS), ketoreductase (KR), acyltransferase (AT), FabA-like β-hydroxy acyl-ACP dehydrase (DH), phosphopantetheine transferase, chain length factor (CLF), acyl carrier protein (ACP), enoyl ACP-reductase (ER), an enzyme that catalyzes the synthesis of trans-2-acyl-ACP, an enzyme that catalyzes the reversible isomerization of trans-2-acyl-ACP to cis-3-acyl-ACP, and/or an enzyme that catalyzes the elongation of cis-3-acyl-ACP to cis-5-β-keto-acyl-ACP. Preferred domains to modify in order to alter the PUFA production profile of a host Thraustochytrid have been discussed previously herein. 
     The genetic modification of an organism according to the present invention preferably affects the type, amounts, and/or activity of the PUFAs produced by the organism, whether the organism has an endogenous PUFA PKS system that is genetically modified, and/or whether recombinant nucleic acid molecules are introduced into the organism. According to the present invention, to affect an activity of a PUFA PKS system, such as to affect the PUFA production profile, includes any genetic modification in the PUFA PKS system or genes that interact with the PUFA PKS system that causes any detectable or measurable change or modification in any biological activity the PUFA PKS system expressed by the organism as compared to in the absence of the genetic modification. According to the present invention, the phrases “PUFA profile”, “PUFA expression profile” and “PUFA production profile” can be used interchangeably and describe the overall profile of PUFAs expressed/produced by a organism. The PUFA expression profile can include the types of PUFAs expressed by the organism, as well as the absolute and relative amounts of the PUFAs produced. Therefore, a PUFA profile can be described in terms of the ratios of PUFAs to one another as produced by the organism, in terms of the types of PUFAs produced by the organism, and/or in terms of the types and absolute or relative amounts of PUFAs produced by the organism. 
     As discussed above, the host organism can include any prokaryotic or eukaryotic organism with or without an endogenous PUFA PKS system and preferably is a eukaryotic microorganism with the ability to efficiently channel the products of the PUFA PKS system into both the phospholipids (PL) and triacylglycerols (TAG). A preferred host microorganism is any member of the order Thraustochytriales, including the families Thraustochytriaceae and Labyrinthulaceae. Particularly preferred host cells of these families have been described above. Preferred host plant cells include plant cells from any crop plant or plant that is commercially useful. 
     In one embodiment of the present invention, it is contemplated that a genetic engineering and/or mutagenesis program could be combined with a selective screening process to obtain a Thraustochytrid microorganism with the PUFA production profile of interest. The mutagenesis methods could include, but are not limited to: chemical mutagenesis, shuffling of genes, switching regions of the genes encoding specific enzymatic domains, or mutagenesis restricted to specific regions of those genes, as well as other methods. 
     For example, high throughput mutagenesis methods could be used to influence or optimize production of the desired PUFA profile. Once an effective model system has been developed, one could modify these genes in a high throughput manner. Utilization of these technologies can be envisioned on two levels. First, if a sufficiently selective screen for production of a product of interest (e.g., EPA) can be devised, it could be used to attempt to alter the system to produce this product (e.g., in lieu of, or in concert with, other strategies such as those discussed above). Additionally, if the strategies outlined above resulted in a set of genes that did produce the PUFA profile of interest, the high throughput technologies could then be used to optimize the system. For example, if the introduced domain only functioned at relatively low temperatures, selection methods could be devised to permit removing that limitation. 
     As described above, in one embodiment of the present invention, a genetically modified microorganism or plant includes a microorganism or plant which has an enhanced ability to synthesize desired bioactive molecules (products) or which has a newly introduced ability to synthesize specific products (e.g., to synthesize a specific antibiotic). According to the present invention, “an enhanced ability to synthesize” a product refers to any enhancement, or up-regulation, in a pathway related to the synthesis of the product such that the microorganism or plant produces an increased amount of the product (including any production of a product where there was none before) as compared to the wild-type microorganism or plant, cultured or grown, under the same conditions. Methods to produce such genetically modified organisms have been described in detail above and indeed, any exemplary modifications described using any of the PUFA PKS systems can be adapted for expression in plants. 
     One embodiment of the present invention is a method to produce desired bioactive molecules (also referred to as products or compounds) by growing or culturing a genetically modified microorganism or plant of the present invention (described in detail above). Such a method includes the step of culturing in a fermentation medium or growing in a suitable environment, such as soil, a microorganism or plant, respectively, that has a genetic modification as described previously herein and in accordance with the present invention. Preferred host cells for genetic modification related to the PUFA PKS system of the invention are described above. 
     One embodiment of the present invention is a method to produce desired PUFAs by culturing a genetically modified microorganism of the present invention (described in detail above). Such a method includes the step of culturing in a fermentation medium and under conditions effective to produce the PUFA(s) a microorganism that has a genetic modification as described previously herein and in accordance with the present invention. An appropriate, or effective, medium refers to any medium in which a genetically modified microorganism of the present invention, including Thraustochytrids and other microorganisms, when cultured, is capable of producing the desired PUFA product(s). Such a medium is typically an aqueous medium comprising assimilable carbon, nitrogen and phosphate sources. Such a medium can also include appropriate salts, minerals, metals and other nutrients. Any microorganisms of the present invention can be cultured in conventional fermentation bioreactors. The microorganisms can be cultured by any fermentation process which includes, but is not limited to, batch, fed-batch, cell recycle, and continuous fermentation. Preferred growth conditions for Thraustochytrid microorganisms according to the present invention are well known in the art and are described in detail, for example, in U.S. Pat. Nos. 5,130,242, 5,340,742, and 5,698,244, each of which is incorporated herein by reference in its entirety. 
     In one embodiment, the genetically modified microorganism is cultured at a temperature of at or greater than about 15° C., and in another embodiment, at or greater than about 20° C., and in another embodiment, at or greater than about 25° C., and in another embodiment, at or greater than about 30° C., and in another embodiment, up to about 35° C. or higher, and in another embodiment, at any temperature between about 20° C. and 35° C., in whole degree increments. 
     The desired PUFA(s) and/or other bioactive molecules produced by the genetically modified microorganism can be recovered from the fermentation medium using conventional separation and purification techniques. For example, the fermentation medium can be filtered or centrifuged to remove microorganisms, cell debris and other particulate matter, and the product can be recovered from the cell-free supernatant by conventional methods, such as, for example, ion exchange, chromatography, extraction, solvent extraction, phase separation, membrane separation, electrodialysis, reverse osmosis, distillation, chemical derivatization and crystallization. Alternatively, microorganisms producing the PUFA(s), or extracts and various fractions thereof, can be used without removal of the microorganism components from the product. 
     Preferably, a genetically modified microorganism of the invention produces one or more polyunsaturated fatty acids including, but not limited to, EPA (C20:5, ω-3), DHA (C22:6, ω-3), DPA (C22:5, ω-6), ARA (C20:4, ω-6), GLA (C18:3, n-6), and SDA (C18:4, n-3)). In one preferred embodiment, a  Schizochytrium  that, in wild-type form, produces high levels of DHA and DPA, is genetically modified according to the invention to produce high levels of EPA. As discussed above, one advantage of using genetically modified Thraustochytrid microorganisms to produce PUFAs is that the PUFAs are directly incorporated into both the phospholipids (PL) and triacylglycerides (TAG). 
     Preferably, PUFAs are produced in an amount that is greater than about 5% of the dry weight of the microorganism, and in one aspect, in an amount that is greater than 6%, and in another aspect, in an amount that is greater than 7%, and in another aspect, in an amount that is greater than 8%, and in another aspect, in an amount that is greater than 9%, and in another aspect, in an amount that is greater than 10%, and so on in whole integer percentages, up to greater than 90% dry weight of the microorganism (e.g., 15%, 20%, 30%, 40%, 50%, and any percentage in between). 
     In the method for production of desired bioactive compounds of the present invention, a genetically modified plant is cultured in a fermentation medium or grown in a suitable medium such as soil. An appropriate, or effective, fermentation medium has been discussed in detail above. A suitable growth medium for higher plants includes any growth medium for plants, including, but not limited to, soil, sand, any other particulate media that support root growth (e.g. vermiculite, perlite, etc.) or hydroponic culture, as well as suitable light, water and nutritional supplements which optimize the growth of the higher plant. The genetically modified plants of the present invention are engineered to produce significant quantities of the desired product through the activity of the PKS system that is genetically modified according to the present invention. The compounds can be recovered through purification processes which extract the compounds from the plant. In a preferred embodiment, the compound is recovered by harvesting the plant. In this embodiment, the plant can be consumed in its natural state or further processed into consumable products. 
     Many genetic modifications useful for producing bioactive molecules will be apparent to those of skill in the art, given the present disclosure, and various other modifications have been discussed previously herein. The present invention contemplates any genetic modification related to a PUFA PKS system as described herein which results in the production of a desired bioactive molecule. 
     Bioactive molecules, according to the present invention, include any molecules (compounds, products, etc.) that have a biological activity, and that can be produced by a PKS system that comprises at least one amino acid sequence having a biological activity of at least one functional domain of a non-bacterial PUFA PKS system as described herein. Such bioactive molecules can include, but are not limited to: a polyunsaturated fatty acid (PUFA), an anti-inflammatory formulation, a chemotherapeutic agent, an active excipient, an osteoporosis drug, an anti-depressant, an anti-convulsant, an anti- Heliobactor pylori  drug, a drug for treatment of neurodegenerative disease, a drug for treatment of degenerative liver disease, an antibiotic, and a cholesterol lowering formulation. One advantage of the PUFA PKS system of the present invention is the ability of such a system to introduce carbon-carbon double bonds in the cis configuration, and molecules including a double bond at every third carbon. This ability can be utilized to produce a variety of compounds. 
     Preferably, bioactive compounds of interest are produced by the genetically modified microorganism in an amount that is greater than about 0.05%, and preferably greater than about 0.1%, and more preferably greater than about 0.25%, and more preferably greater than about 0.5%, and more preferably greater than about 0.75%, and more preferably greater than about 1%, and more preferably greater than about 2.5%, and more preferably greater than about 5%, and more preferably greater than about 10%, and more preferably greater than about 15%, and even more preferably greater than about 20% of the dry weight of the microorganism. For lipid compounds, preferably, such compounds are produced in an amount that is greater than about 5% of the dry weight of the microorganism. For other bioactive compounds, such as antibiotics or compounds that are synthesized in smaller amounts, those strains possessing such compounds at of the dry weight of the microorganism are identified as predictably containing a novel PKS system of the type described above. In some embodiments, particular bioactive molecules (compounds) are secreted by the microorganism, rather than accumulating. Therefore, such bioactive molecules are generally recovered from the culture medium and the concentration of molecule produced will vary depending on the microorganism and the size of the culture. 
     One embodiment of the present invention relates to a method to modify an endproduct so that it contains at least one fatty acid (although the endproduct may already contain at least one fatty acid, whereby at least one additional fatty acid is provided by the present method), comprising adding to the endproduct an oil produced by a recombinant host cell (microbial or plant) that expresses at least one recombinant nucleic acid molecule comprising a nucleic acid sequence encoding at least one biologically active domain of a PUFA PKS system. The PUFA PKS system includes any suitable bacterial or non-bacterial PUFA PKS system described herein, including the bacterial PUFA PKS systems from  Shewanella japonica  or  Shewanella olleyana , or any PUFA PKS system from other bacteria that normally (i.e., under normal or natural conditions) are capable of growing and producing PUFAs at temperatures above 22° C. 
     Preferably, the endproduct is selected from the group consisting of a food, a dietary supplement, a pharmaceutical formulation, a humanized animal milk, and an infant formula. Suitable pharmaceutical formulations include, but are not limited to, an anti-inflammatory formulation, a chemotherapeutic agent, an active excipient, an osteoporosis drug, an anti-depressant, an anti-convulsant, an anti- Heliobactor  pylon drug, a drug for treatment of neurodegenerative disease, a drug for treatment of degenerative liver disease, an antibiotic, and a cholesterol lowering formulation. In one embodiment, the endproduct is used to treat a condition selected from the group consisting of: chronic inflammation, acute inflammation, gastrointestinal disorder, cancer, cachexia, cardiac restenosis, neurodegenerative disorder, degenerative disorder of the liver, blood lipid disorder, osteoporosis, osteoarthritis, autoimmune disease, preeclampsia, preterm birth, age related maculopathy, pulmonary disorder, and peroxisomal disorder. 
     Suitable food products include, but are not limited to, fine bakery wares, bread and rolls, breakfast cereals, processed and unprocessed cheese, condiments (ketchup, mayonnaise, etc.), dairy products (milk, yoghurt), puddings and gelatin desserts, carbonated drinks, teas, powdered beverage mixes, processed fish products, fruit-based drinks, chewing gum, hard confectionery, frozen dairy products, processed meat products, nut and nut-based spreads, pasta, processed poultry products, gravies and sauces, potato chips and other chips or crisps, chocolate and other confectionery, soups and soup mixes, soya based products (milks, drinks, creams, whiteners), vegetable oil-based spreads, and vegetable-based drinks. 
     Yet another embodiment of the present invention relates to a method to produce a humanized animal milk. This method includes the steps of genetically modifying milk-producing cells of a milk-producing animal with at least one recombinant nucleic acid molecule comprising a nucleic acid sequence encoding at least one biologically active domain of a PUFA PKS system as described herein. 
     Methods to genetically modify a host cell and to produce a genetically modified non-human, milk-producing animal, are known in the art. Examples of host animals to modify include cattle, sheep, pigs, goats, yaks, etc., which are amenable to genetic manipulation and cloning for rapid expansion of a transgene expressing population. For animals, PKS-like transgenes can be adapted for expression in target organelles, tissues and body fluids through modification of the gene regulatory regions. Of particular interest is the production of PUFAs in the breast milk of the host animal. 
     The following examples are provided for the purpose of illustration and are not intended to limit the scope of the present invention. 
     EXAMPLES 
     Example 1 
     The following example shows that certain EPA-producing bacteria contain PUFA PKS-like genes that appear to be suitable for modification of  Schizochytrium.    
     Two EPA-producing marine bacterial strains of the genus  Shewanella  have been shown to grow at temperatures typical of  Schizochytrium  fermentations and to possess PUFA PKS-like genes.  Shewanella olleyana  (Australian Collection of Antarctic Microorganisms (ACAM) strain number 644; Skerratt et al.,  Int. J. Syst. Evol. Microbiol.  52, 2101 (2002)) produces EPA and grows up to 25-30° C.  Shewanella japonica  (American Type Culture Collection (ATCC) strain number BAA-316; Ivanova et al.,  Int. J. Syst. Evol. Microbiol.  51, 1027 (2001)) produces EPA and grows up to 30-35° C. 
     To identify and isolate the PUFA-PKS genes from these bacterial strains, degenerate PCR primer pairs for the KS-MAT region of bacterial orf5/pfaA genes and the DH-DH region of bacterial orf7/pfaC genes were designed based on published gene sequences for  Shewanella  SCRC-2738,  Shewanella oneidensis  MR-1;  Shewanella  sp. GA-22;  Photobacter profundum , and  Moritella marina  (see discussion above). Specifically, the primers and PCR conditions were designed as follows: 
     Primers for the KS/AT region; based on the following published sequences:  Shewanella  sp. SCRC-2738;  Shewanella oneidensis  MR-1;  Photobacter profundum; Moritella marina : 
     
       
         
           
               
               
            
               
                 prRZ23 
                   
               
               
                 GGYATGMTGRTTGGTGAAGG 
                 (forward; SEQ ID NO: 25) 
               
               
                   
               
               
                 prRZ24 
                   
               
               
                 TRTTSASRTAYTGYGAACCTTG 
                 (reverse; SEQ ID NO: 26) 
               
            
           
         
       
     
     Primers for the DH region; based on the following published sequences:  Shewanella  sp. GA-22;  Shewanella  sp. SCRC-2738;  Photobacter profundum; Moritella marina : 
                        prRZ28           ATGKCNGAAGGTTGTGGCCA   (forward; SEQ ID NO: 27)               prRZ29           CCWGARATRAAGCCRTTDGGTTG   (reverse; SEQ TD NO: 28)            
The PCR conditions (with bacterial chromosomal DNA as templates) were as follows:
 
     Reaction Mixture: 
     0.2 μM dNTPs 
     0.1 μM each primer 
     8% DMSO 
     250 ng chromosomal DNA 
     2.5 U Herculase® DNA polymerase (Stratagene) 
     1× Herculase® buffer 
     50 μL total volume 
     PCR Protocol: (1) 98° C. for 3 min.; (2) 98° C. for 40 sec.; (3) 56° C. for 30 sec.; (4) 72° C. for 90 sec.; (5) Repeat steps 2-4 for 29 cycles; (6) 72° C. for 10 min.; (7) Hold at 6° C. 
     For both primer pairs, PCR gave distinct products with expected sizes using chromosomal DNA templates from either  Shewanella olleyana  or  Shewanella japonica . The four respective PCR products were cloned into pCR-BLUNT II-TOPO (Invitrogen) and insert sequences were determined using the M13 forward and reverse primers. In all cases, the DNA sequences thus obtained were highly homologous to known bacterial PUFA PKS gene regions. 
     The DNA sequences obtained from the bacterial PCR products were compared with known sequences and with PUFA PKS genes from  Schizochytrium  ATCC 20888 in a standard Blastx search (BLAST parameters: Low Complexity filter: On; Matrix: BLOSUM62; Word Size: 3; Gap Costs: Existance11, Extension 1 (BLAST described in Altschul, S. F., Madden, T. L., Schääffer, A. A., Zhang, J., Zhang, Z., Miller, W. &amp; Lipman, D. J. (1997) “Gapped BLAST and PSI-BLAST: a new generation of protein database search programs.” Nucleic Acids Res. 25:3389-3402, incorporated herein by reference in its entirety)). 
     At the amino acid level, the sequences with the greatest degree of homology to the  Shewanella olleyana  ACAM644 ketoacyl synthase/acyl transferase (KS-AT) deduced amino acid sequence were:  Photobacter profundum  pfaA (identity=70%; positives=81%);  Shewanella oneidensis  MR-1 “multi-domain β-ketoacyl synthase” (identity=66%; positives=77%); and  Moritella marina  ORF8 (identity=56%; Positives=71%). The  Schizochytrium  sp. ATCC20888 orfA was 41% identical and 56% positive to the deduced amino acid sequence for  Shewanella olleyana  KS-AT. 
     At the amino acid level, the sequences with the greatest degree of homology to the  Shewanella japonica  ATCC BAA-316 ketoacyl synthase/acyl transferase (KS-AT) deduced amino acid sequence were:  Shewanella oneidensis  MR-1 “multi-domain β-ketoacyl synthase” (identity=67%; positives=79%);  Shewanella  sp. SCRC-2738 orf5 (identity=69%; positives=77%); and  Moritella marina  ORF8 (identity=56%; positives=70%). The  Schizochytrium  sp. ATCC20888 orfA was 41% identical and 55% positive to the deduced amino acid sequence for  Shewanella japonica  KS-AT. 
     At the amino acid level, the sequences with the greatest degree of homology to the  Shewanella olleyana  ACAM644 dehydrogenase (DH) deduced amino acid sequence were:  Shewanella  sp. SCRC-2738 orf7 (identity=77%; positives=86%);  Photobacter profundum  pfaC (identity=72%; positives=81%); and  Shewanella oneidensis  MR-1 “multi-domain β-ketoacyl synthase” (identity=75%; positives=83%). The  Schizochytrium  sp. ATCC20888 orfC was 26% identical and 42% positive to the deduced amino acid sequence for  Shewanella olleyana  DH. 
     At the amino acid level, the sequences with the greatest degree of homology to the  Shewanella japonica  ATCC BAA-316 dehydrogenase (DH) deduced amino acid sequence were:  Shewanella  sp. SCRC-2738 orf7 (identity=77%; positives=86%);  Photobacter profundum  pfaC (identity=73%; positives=83%) and  Shewanella oneidensis  MR-1 “multi-domain β-ketoacyl synthase” (identity=74%; positives=81%). The  Schizochytrium  sp. ATCC20888 orfC was 27% identical and 42% positive to the deduced amino acid sequence for  Shewanella japonica  DH. 
     Example 2 
     The following example demonstrates the generation, identification, sequencing and analysis of DNA clones encoding the complete PUFA PKS systems from  Shewanella japonica  and  Shewanella olleyana.    
       Shewanella japonica  and  Shewanella olleyana  recombinant libraries, consisting of large genomic DNA fragments (approximately 40 kB), were generated by standard methods in the cosmid vector Supercos-1 (Stratagene). The cosmid libraries were screened by standard colony hybridization procedures. The  Sh. olleyana  cosmid library was screened using two separate digoxigenin-labeled probes. Each probe contained a fragment of DNA homologous to a segment of EPA biosynthetic gene clusters described in Example 1 above and respectively represent both ends of the clusters. These probes were generated by PCR using  Sh. olleyana  DNA as a template and primers prRZ23 (SEQ ID NO:25) and prRZ24 (SEQ ID NO:26) for one probe and prRZ28 (SEQ ID NO:27) and prRZ29 (SEQ ID NO:28) for a second probe. Example 1 above describes these degenerate primers and the derived PCR products containing DNA fragments homologous to segments of EPA biosynthetic genes.  Sh. japonica  specific probes were generated in a similar manner and the cosmid library was screened. In all cases, strong hybridization of the individual probes to certain cosmids indicated clones containing DNA homologous to EPA biosynthetic gene clusters. 
     Clones with strong hybridization to both probes were then assayed for heterologous production of EPA in  E. coli . Cells of individual isolates of  E. coli  cosmid clones were grown in 2 mL of LB broth overnight at 30° C. with 200 rpm shaking. 0.5 mL of this subculture was used to inoculate 25 mL of LB broth and the cells were grown at 20° C. for 20 hours. The cells were then harvested via centrifugation and dried by lyophilization. The dried cells were analyzed for fat content and fatty acid profile and content using standard gas chromatography procedures. No EPA was detected in fatty acids prepared from control cells of  E. coli  containing the empty Supercos-1 vector.  E. coli  strains containing certain cosmids from  S. japonica  and  S. olleyana  typically produced between 3-8% EPA of total fatty acids. 
     Cosmid 9A10 from  Sh. olleyana  and cosmid 3F3 from  Sh. japonica  were selected for total random sequencing. The cosmid clones were randomly fragmented and subcloned, and the resulting random clones were sequenced. The chromatograms were analyzed and assembled into contigs with the Phred, Phrap and Consed programs (Ewing, et al., Genome Res. 8(3):175-185 (1998); Ewing, et al., Genome Res. 8(3): 186-194 (1998); Gordon et al., Genome Res. 8(3):195-202 (1998)). Each nucleotide base pair of the final contig was covered with at least a minimum aggregated Phred score of 40 (confidence level 99.995%). 
     The nucleotide sequence of the 39669 bp contig from cosmid 3F3 is shown as SEQ ID NO:1. The nucleotide sequence of the 38794 bp contig from cosmid 9A10 is shown as SEQ ID NO:7. The sequences of the various domains and proteins for the PUFA PKS gene clusters from  Shewanella japonica  (cosmid 3F3) and  Shewanella olleyana  (cosmid 9A10) are described in detail previously herein, and are represented in SEQ ID NOs:2-6 and 8-12, respectively. 
     Protein comparisons described herein were performed using standard BLAST analysis (BLAST parameters: Blastp, low complexity filter On, program—BLOSUM62, Gap cost—Existence: 11, Extension 1; (BLAST described in Altschul, S. F., Madden, T. L., Schääffer, A. A., Zhang, J., Zhang, Z., Miller, W. &amp; Lipman, D. J. (1997) “Gapped BLAST and PSI-BLAST: a new generation of protein database search programs.” Nucleic Acids Res. 25:3389-3402)). Domain identification was performed using the Conserved Domain Database and Search Service (CD-Search), v2.01. The CD-Search is a public access program available through the public database for the National Center for Biotechnology Information, sponsored by the National Library of Medicine and the National Institutes of Health. The CD-Search contains protein domains from various databases. The CD-Search uses a BLAST algorithm to identify domains in a queried protein sequence (Marchler-Bauer A, Bryant SH. “CD-Search: protein domain annotations on the fly.” Nucleic Acids Res. 32:W327-331 (2004)). Finally, Open Reading Frame (ORF) identification was aided by the use of the EasyGene 1.0 Server (Larsen T S, Krogh A. “EasyGene—a prokaryotic gene finder that ranks ORFs by statistical significance”, BMC Bioinformatics 2003, 4:21) and GeneMark.hmm 2.1 (Lukashin A. and Borodovsky M., “GeneMark.hmm: new solutions for gene finding” Nucleic Acids Res., Vol. 26, No. 4, pp. 1107-1115. 1998). The default settings were used in the EasyGene analysis and  Vibrio cholerae  was used as the reference organism. The default settings were used with the GeneMark.hmm program and the Pseudonative.model as the setting for the model organism. These programs use a Hidden Markov Models algorithms to predict bacterial genes. 
     Table 1 shows an overview/analysis of ORFs from cosmid 3F3 from  Shewanella japonica , including start and stop codon coordinates based on SEQ ID NO:1, total nucleotide length of each ORF, total amino acids for each predicted protein, calculated molecular weight of each predicted protein, highest homolog in a BLASTp query against the public GenBank database, GI accession number (“GenInfo Identifier” sequence identification number) of the most homologous entry in the GenBank database, and proposed function (if related to EPA production). 
     Table 2 shows an overview/analysis of ORFs from cosmid 9A10 from  Shewanella olleyana , including start and stop codon coordinates based on SEQ ID NO:7, and the same additional information that was presented in Table 1 for  Shewanella japonica.    
     Table 3 shows the percent identity of deduced proteins from EPA clusters of  Shewanella japonica  (cosmid 3F3) compared to  Shewanella olleyana  (cosmid 9A10) and also compared to proteins from EPA-producing organisms having the highest levels of identity in the public sequence database. Table 4 shows the same analysis as Table 3 with regard to nucleotide identity. 
     Table 5 shows the 23 nucleotides upstream from all of the annotated pfa ORFs with possible ribosome binding sites being underlined, as well as the alternative start codon and upstream nucleotides for ORFs that are annotated to start with the TTG start codon. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 ORF analysis of cosmid 3F3 from  Shewanella japonica   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                   
                 Start 
                 Stop 
                 total nt 
                 total 
                   
                   
                 Accession 
                 Proposed function 
               
               
                 ORF 
                 Codon 
                 Codon 
                 length 
                 AA 
                 MW 
                 Homology of deduced protein 
                 Number 
                 of deduced protein 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 orf1* 
                 1195 
                 548 
                 648 
                 215 
                 24561.35 
                 syd protein 
                 GI: 24373178 
                   
               
               
                   
                   
                   
                   
                   
                   
                   Shewanella oneidensis  MR-1 
               
               
                 orf2 
                 1255 
                 2109 
                 855 
                 284 
                 32825.47 
                 conserved hypothetical protein 
                 GI: 24373177 
               
               
                   
                   
                   
                   
                   
                   
                   Shewanella oneidensis  MR-1 
               
               
                 orf3 
                 2196 
                 2834 
                 639 
                 212 
                 23779.30 
                 pseudouridylate synthase 
                 GI: 23123676 
               
               
                   
                   
                   
                   
                   
                   
                 
                   Nostoc punctiforme 
                 
               
               
                 orf4* 
                 3832 
                 2873 
                 960 
                 319 
                 36135.31 
                 LysR transcriptional regulator 
                 GI: 24373176 
               
               
                   
                   
                   
                   
                   
                   
                   Shewanella oneidensis  MR-1 
               
               
                 orf5 
                 3962 
                 5956 
                 1995 
                 664 
                 73468.40 
                 metallo-beta-lactamase 
                 GI: 24373175 
               
               
                   
                   
                   
                   
                   
                   
                 superfamily protein 
               
               
                   
                   
                   
                   
                   
                   
                   Shewanella oneidensis  MR-1 
               
               
                 pfaE* 
                 7061 
                 6150 
                 912 
                 303 
                 34678.40 
                 orf2 
                 GI: 2529415 
                 phosphopantetheinyl 
               
               
                   
                   
                   
                   
                   
                   
                   Shewanella  sp. SCRC-2738 
                   
                 transferase 
               
               
                 orf6* 
                 9249 
                 7222 
                 2028 
                 675 
                 73367.16 
                 Translation elongation factor 
                 GI: 27358908 
               
               
                   
                   
                   
                   
                   
                   
                   Vibrio vulnificus  CMCP6 
               
               
                 orf7 
                 9622 
                 10494 
                 873 
                 290 
                 32540.64 
                 putative transcriptional regulator 
                 GI: 24373172 
               
               
                   
                   
                   
                   
                   
                   
                   Shewanella oneidensis  MR-1 
               
               
                 pfaA 
                 10491 
                 18854 
                 8364 
                 2787 
                 294907.67 
                 PfaA polyunsaturated fatty acid 
                 GI: 46913082 
                 EPA synthase 
               
               
                   
                   
                   
                   
                   
                   
                 synthase 
               
               
                   
                   
                   
                   
                   
                   
                 
                   Photobacterium profundum 
                 
               
               
                 pfaB 
                 18851 
                 21130 
                 2280 
                 759 
                 82727.25 
                 PfaB polyunsaturated fatty acid 
                 GI: 46913081 
                 EPA synthase 
               
               
                   
                   
                   
                   
                   
                   
                 synthase 
               
               
                   
                   
                   
                   
                   
                   
                 
                   Photobacterium profundum 
                 
               
               
                 pfaC 
                 21127 
                 27186 
                 6060 
                 2019 
                 219255.74 
                 PfaC polyunsaturated fatty acid 
                 GI: 15488033 
                 EPA synthase 
               
               
                   
                   
                   
                   
                   
                   
                 synthase 
               
               
                   
                   
                   
                   
                   
                   
                 
                   Photobacterium profundum 
                 
               
               
                 pfaD 
                 27197 
                 28825 
                 1692 
                 542 
                 59116.36 
                 orf8 
                 GI: 2529421 
                 EPA synthase 
               
               
                   
                   
                   
                   
                   
                   
                   Shewanella  sp. SCRC-2738 
               
               
                 orf8 
                 29445 
                 30926 
                 1482 
                 493 
                 56478.03 
                 putative cellulosomal protein 
                 GI: 7208813 
               
               
                   
                   
                   
                   
                   
                   
                 
                   Clostridium thermocellum 
                 
               
               
                 orf9 
                 31105 
                 32712 
                 1608 
                 535 
                 59618.32 
                 methyl-accepting chemotaxis 
                 GI: 24374914 
               
               
                   
                   
                   
                   
                   
                   
                 protein 
               
               
                   
                   
                   
                   
                   
                   
                   Shewanella oneidensis  MR-1 
               
               
                 orf10 
                 32988 
                 33845 
                 858 
                 285 
                 32119.88 
                 Glutathione S-transferase 
                 GI: 27359215 
               
               
                   
                   
                   
                   
                   
                   
                   Vibrio vulnificus  CMCP6 
               
               
                   
               
               
                 *on the reverse complementary strand 
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 ORF analysis of cosmid 9A10 from  Shewanella olleyana   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                   
                 Start 
                 Stop 
                 total nt 
                 total 
                   
                   
                 Accession 
                 Proposed function 
               
               
                 ORF 
                 Codon 
                 Codon 
                 length 
                 AA 
                 MW 
                 Homology of deduced protein 
                 Number 
                 of deduced protein 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 orf1* 
                 4160 
                 3531 
                 630 
                 209 
                 23724.40 
                 acetyltransferase, GNAT family 
                 GI: 24373183 
                   
               
               
                   
                   
                   
                   
                   
                   
                   Shewanella oneidensis  MR-1 
               
               
                 orf2* 
                 4992 
                 4606 
                 387 
                 128 
                 14034.86 
                 hypothetical protein 
                 GI: 24373181 
               
               
                   
                   
                   
                   
                   
                   
                   Shewanella oneidensis  MR-1 
               
               
                 orf3 
                 5187 
                 5522 
                 336 
                 111 
                 12178.79 
                 hypothetical protein 
                 GI: 24373180 
               
               
                   
                   
                   
                   
                   
                   
                   Shewanella oneidensis  MR-1 
               
               
                 orf4 
                 5644 
                 6417 
                 774 
                 257 
                 29674.73 
                 hypothetical protein 
                 GI: 24373179 
               
               
                   
                   
                   
                   
                   
                   
                   Shewanella oneidensis  MR-1 
               
               
                 orf5* 
                 7148 
                 6495 
                 654 
                 217 
                 24733.33 
                 syd protein 
                 GI: 24373178 
               
               
                   
                   
                   
                   
                   
                   
                   Shewanella oneidensis  MR-1 
               
               
                 orf6 
                 7208 
                 8062 
                 855 
                 284 
                 32749.29 
                 hypothetical protein 
                 GI: 24373177 
               
               
                   
                   
                   
                   
                   
                   
                   Shewanella oneidensis  MR-1 
               
               
                 orf7 
                 8841 
                 8131 
                 711 
                 236 
                 26178.32 
                 putative phosphatase 
                 GI: 28899965 
               
               
                   
                   
                   
                   
                   
                   
                 
                   Vibrio parahaemolyticus 
                 
               
               
                 orf8 
                 9167 
                 9808 
                 642 
                 213 
                 23849.14 
                 pseudouridylate synthase 
                 GI: 23123676 
               
               
                   
                   
                   
                   
                   
                   
                 
                   Nostoc punctiforme 
                 
               
               
                 orf9* 
                 10797 
                 9805 
                 993 
                 330 
                 37337.29 
                 LysR transcriptional regulator 
                 GI: 24373176 
               
               
                   
                   
                   
                   
                   
                   
                   Shewanella oneidensis  MR-1 
               
               
                 orf10 
                 10968 
                 12962 
                 1995 
                 664 
                 72982.72 
                 metallo-beta-lactamase 
                 GI: 24373175 
               
               
                   
                   
                   
                   
                   
                   
                 superfamily protein 
               
               
                   
                   
                   
                   
                   
                   
                   Shewanella oneidensis  MR-1 
               
               
                 pfaE* 
                 13899 
                 13027 
                 873 
                 290 
                 32864.30 
                 orf2 
                 GI: 2529415 
                 phosphopantetheinyl 
               
               
                   
                   
                   
                   
                   
                   
                   Shewanella  sp. SCRC-2738 
                   
                 transferase 
               
               
                 orf11* 
                 16195 
                 14156 
                 2040 
                 679 
                 74070.34 
                 Translation elongation factor 
                 GI: 27358908 
               
               
                   
                   
                   
                   
                   
                   
                   Vibrio vulnificus  CMCP6 
               
               
                 orf12 
                 16568 
                 17440 
                 873 
                 290 
                 32741.82 
                 putative transcriptional regulator 
                 GI: 24373172 
               
               
                   
                   
                   
                   
                   
                   
                   Shewanella oneidensis  MR-1 
               
               
                 pfaA 
                 17437 
                 25743 
                 8307 
                 2768 
                 293577.27 
                 PfaA polyunsaturated fatty acid 
                 GI: 46913082 
                 EPA synthase 
               
               
                   
                   
                   
                   
                   
                   
                 synthase 
               
               
                   
                   
                   
                   
                   
                   
                 
                   Photobacterium profundum 
                 
               
               
                 pfaB 
                 25740 
                 27971 
                 2232 
                 743 
                 80446.82 
                 PfaB polyunsaturated fatty acid 
                 GI: 46913081 
                 EPA synthase 
               
               
                   
                   
                   
                   
                   
                   
                 synthase 
               
               
                   
                   
                   
                   
                   
                   
                 
                   Photobacterium profundum 
                 
               
               
                 pfaC 
                 27968 
                 34030 
                 6063 
                 2020 
                 218810.57 
                 PfaC polyunsaturated fatty acid 
                 GI: 15488033 
                 EPA synthase 
               
               
                   
                   
                   
                   
                   
                   
                 synthase 
               
               
                   
                   
                   
                   
                   
                   
                 
                   Photobacterium profundum 
                 
               
               
                 pfaD 
                 34041 
                 35669 
                 1629 
                 542 
                 59261.59 
                 orf8 
                 GI: 2529421 
                 EPA synthase 
               
               
                   
                   
                   
                   
                   
                   
                   Shewanella  sp. SCRC-2738 
               
               
                   
               
               
                 *on the reverse complementary strand 
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 Amino Acid Percent Identity 
               
            
           
           
               
               
               
            
               
                   
                 
                   Shewanella 
                 
                 
                   Shewanella 
                 
               
               
                   
                 
                   japonica 
                 
                 
                   olleyana 
                 
               
               
                   
                 (3F3) 
                 (9A10) 
               
               
                   
                   
               
            
           
           
               
            
               
                 PfaA 
               
            
           
           
               
               
               
            
               
                   Shewanella japonica  (3F3) 
                   
                 87.7 
               
               
                   Shewanella olleyana  (9A10) 
                 87.7 
               
               
                   Shewanella  sp. SCRC-2738 Orf5 
                 63 
                 63.4 
               
               
                   Photobacterium profundum  S9 PfaA 
                 60.9 
                 62.2 
               
               
                   Moritella marina  Orf8 
                 41.6 
                 42.9 
               
            
           
           
               
            
               
                 PfaB 
               
            
           
           
               
               
               
            
               
                   Shewanella japonica  (3F3) 
                   
                 70.3 
               
               
                   Shewanella olleyana  (9A10) 
                 70.3 
               
               
                   Shewanella  sp. SCRC-2738 Orf6 
                 39.8 
                 38.4 
               
               
                   Photobacterium profundum  S9 PfaB 
                 39 
                 39.6 
               
               
                   Moritella marina  Orf9 
                 19 
                 18.4 
               
            
           
           
               
            
               
                 PfaC 
               
            
           
           
               
               
               
            
               
                   Shewanella japonica  (3F3) 
                   
                 85.7 
               
               
                   Shewanella olleyana  (9A10) 
                 85.7 
               
               
                   Shewanella  sp. SCRC-2738 Orf7 
                 65.1 
                 64.8 
               
               
                   Photobacterium profundum  S9 PfaC 
                 64.6 
                 64.6 
               
               
                   Moritella marina  Orf10 
                 47.3 
                 47.1 
               
            
           
           
               
            
               
                 PfaD 
               
            
           
           
               
               
               
            
               
                   Shewanella japonica  (3F3) 
                   
                 98.2 
               
               
                   Shewanella olleyana  (9A10) 
                 98.2 
               
               
                   Shewanella  sp. SCRC-2738 Orf8 
                 84.2 
                 84 
               
               
                   Photobacterium profundum  S9 PfaD 
                 93.8 
                 64.6 
               
               
                   Moritella marina  Orf11 
                 63 
                 62.6 
               
            
           
           
               
            
               
                 PfaE 
               
            
           
           
               
               
               
            
               
                   Shewanella japonica  (3F3) 
                   
                 61.2 
               
               
                   Shewanella olleyana  (9A10) 
                 61.2 
               
               
                   Shewanella  sp. SCRC-2738 Orf2 
                 36.7 
                 38 
               
               
                   Anabaena  sp. PCC 7120 HetI 
                 22.6 
                 24.8 
               
               
                   Bacillus subtilis  Sfp 
                 20.1 
                 20.7 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 4 
               
             
            
               
                   
               
               
                 Nucleic Acid Percent Identity 
               
            
           
           
               
               
               
            
               
                   
                 
                   Shewanella 
                 
                 
                   Shewanella 
                 
               
               
                   
                 
                   japonica 
                 
                 
                   olleyana 
                 
               
               
                   
                 (3F3) 
                 (9A10) 
               
               
                   
                   
               
            
           
           
               
            
               
                 pfaA 
               
            
           
           
               
               
               
            
               
                   Shewanella japonica  (3F3) 
                   
                 83.1 
               
               
                   Shewanella olleyana  (9A10) 
                 83.1 
               
               
                   Shewanella  sp. SCRC-2738 orf5 
                 65.5 
                 65.5 
               
               
                   Photobacterium profundum  S9 pfaA 
                 63.5 
                 64.4 
               
               
                   Moritella marina  orf8 
                 56 
                 56.2 
               
            
           
           
               
            
               
                 pfaB 
               
            
           
           
               
               
               
            
               
                   Shewanella japonica  (3F3) 
                   
                 70.4 
               
               
                   Shewanella olleyana  (9A10) 
                 70.4 
               
               
                   Shewanella  sp. SCRC-2738 orf6 
                 54.7 
                 54.5 
               
               
                   Photobacterium profundum  S9 pfaB 
                 53.4 
                 52.6 
               
               
                   Moritella marina  orf9 
                 42.2 
                 40.6 
               
            
           
           
               
            
               
                 pfaC 
               
            
           
           
               
               
               
            
               
                   Shewanella japonica  (3F3) 
                   
                 79.6 
               
               
                   Shewanella olleyana  (9A10) 
                 79.6 
               
               
                   Shewanella  sp. SCRC-2738 orf7 
                 66.2 
                 67.2 
               
               
                   Photobacterium profundum  S9 pfaC 
                 66 
                 66.7 
               
               
                   Moritella marina  orf10 
                 58.3 
                 58.8 
               
            
           
           
               
            
               
                 pfaD 
               
            
           
           
               
               
               
            
               
                   Shewanella japonica  (3F3) 
                   
                 89.5 
               
               
                   Shewanella olleyana  (9A10) 
                 89.5 
               
               
                   Shewanella  sp. SCRC-2738 orf8 
                 77.4 
                 77.8 
               
               
                   Photobacterium profundum  S9 pfaD 
                 75.9 
                 76.0 
               
               
                   Moritella marina  orf11 
                 63.5 
                 62.9 
               
            
           
           
               
            
               
                 pfaE 
               
            
           
           
               
               
               
            
               
                   Shewanella japonica  (3F3) 
                   
                 65 
               
               
                   Shewanella olleyana  (9A10) 
                 65 
               
               
                   Shewanella  sp. SCRC-2738 orf2 
                 43 
                 44.4 
               
               
                   Anabaena  sp. PCC 7120 hetI 
                 43.1 
                 38.6 
               
               
                   Bacillus subtilis  sfp 
                 34.6 
                 32.9 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 5 
               
               
                   
               
               
                 Predicted start sites of ORFs from EPA biosynthesis 
               
               
                 clusters (start codons shown in bold) Possible ribo- 
               
               
                 some binding sites are underlined 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
            
               
                 ALL pfa ORFs 
               
            
           
           
               
               
               
            
               
                 3F3 
                   
                   
               
               
                 CTGAACACT GGAGA CTCAAA  ATG   
                 pfaA 
                 SEQ ID NO: 33 
               
               
                 GCTGACTTGC AGGAG TCTGT  GTG   
                 pfaB 
                 SEQ ID NO: 34 
               
               
                 CAATTAGA AGGAGAA CAATC  TTG   
                 pfaC 
                 SEQ ID NO: 35 
               
               
                 AGAGGCAT AAAGGAA TAATA  ATG   
                 pfaD 
                 SEQ ID NO: 36 
               
               
                 GCGACCTA GAACAA GCGACA  ATG   
                 pfaE 
                 SEQ ID NO: 37 
               
               
                   
               
               
                 9A10 
                   
                   
               
               
                 CTGAACACT GGAGA CTCAAA  ATG   
                 pfaA 
                 SEQ ID NO: 38 
               
               
                 GCTGATTT GCAGGAG TCTGT  GTG   
                 pfaB 
                 SEQ ID NO: 39 
               
               
                 CAATTAG AAGGAGAA CAATC  TTG   
                 pfaC 
                 SEQ ID NO: 40 
               
               
                 AGAGGCAT AAAGGAA TAATA  ATG   
                 pfaD 
                 SEQ ID NO: 41 
               
               
                 CAATTTA GCCTGAG CCTAGT  TTG   
                 pfaE 
                 SEQ ID NO: 42 
               
               
                   
               
            
           
           
               
            
               
                 pfaC Alternate Start Comparisons 
               
            
           
           
               
               
               
            
               
                 3F3 
                   
                   
               
               
                 CAATTAG AAGGAGAA CAATC  TTG   
                 pfaC 
                   
               
               
                 TAAATCGCACT GGTA TTGTC  ATG   
                 pfaC alternate #1 
                 SEQ ID NO: 43 
               
               
                 AAGCACTC AATGAT GCTGGT  GTG   
                 pfaC alternate #2 
                 SEQ ID NO: 44 
               
               
                   
               
            
           
           
               
            
               
                 pfaC alternate #1 starts at nucleotide 21514 of SEQ ID NO: 1 
               
               
                 This is 387 nucleotides downstream of annotated pfaC start 
               
               
                   
               
               
                 pfaC alternate #2 starts at nucleotide 21460 of SEQ ID NO: 1 
               
               
                 This is 333 nucleotides downstream of annotated pfaC start 
               
               
                   
               
            
           
           
               
               
               
            
               
                 9A10 
                   
                   
               
               
                 CAATTAG AAGGAGAA CAATC  TTG   
                 pfaC 
                   
               
               
                 TAAACC GCACCGGTA TTGTC  ATG   
                 pfaC alternate #1 
                 SEQ ID NO: 45 
               
               
                 ACCCAGCT GACTA TCAAGGT  GTG   
                 pfaC alternate #2 
                 SEQ ID NO: 46 
               
               
                   
               
            
           
           
               
            
               
                 pfaC alternate #1 starts at nucleotide 28370 of SEQ ID NO: 7 
               
               
                 This is 402 nucleotides downstream of annotated pfaC start 
               
               
                   
               
               
                 pfaC alternate #2 starts at nucleotide 28151 of SEQ ID NO: 7 
               
               
                 This is 183 nucleotides downstream of annotated pfaC start 
               
               
                   
               
               
                 pfaE Alternate Start Comparisons 
               
            
           
           
               
               
               
            
               
                 9 A 10 
                   
                   
               
               
                 CAATTTA GCCTGAG CCTAGT  TTG   
                 pfaE 
                   
               
               
                 ATGAATC GACTGCG TCTATT  GTG   
                 pfaE alternate #1 
                 SEQ ID NO: 47 
               
               
                 CATCTA GAGAA CAAGGTTTA  ATG   
                 pfaE alternate #2 
                 SEQ ID NO: 48 
               
               
                   
               
            
           
           
               
            
               
                 pfaE alternate #1 starts at nucleotide 13821 of SEQ ID NO: 7 
               
               
                 This is 78 nucleotides upstream of the annotated pfaE start 
               
               
                   
               
               
                 pfaE alternate #2 starts at nucleotide 13743 of SEQ ID NO: 7 
               
               
                 This is 156 nucleotides upstream of the annotated pfaE start 
               
               
                   
               
            
           
         
       
     
     Example 3 
     The following example demonstrates that  Schizochytrium  Orfs A, B and C encode a functional DHA/DPA synthesis enzyme via functional expression in  E. coli.    
     General Preparation of  E. coli  Transformants 
     The three genes encoding the  Schizochytrium  PUFA PKS system that produce DHA and DPA (Orfs A, B &amp; C; SEQ ID NO:13, SEQ ID NO:15 and SEQ ID NO:17, respectively) were cloned into a single  E. coli  expression vector (derived from pET21c (Novagen)). The genes are transcribed as a single message (by the T7 RNA-polymerase), and a ribosome-binding site cloned in front of each of the genes initiates translation. Modification of the Orf B coding sequence was needed to obtain production of a full-length Orf B protein in  E. coli  (see below). An accessory gene, encoding a PPTase (see below) was cloned into a second plasmid (derived from pACYC184, New England Biolabs). 
     The Orf B gene is predicted to encode a protein with a mass of ˜224 kDa. Initial attempts at expression of the gene in  E. coli  resulted in accumulation of a protein with an apparent molecular mass of ˜165 kDa (as judged by comparison to proteins of known mass during SDS-PAGE). Examination of the Orf B nucleotide sequence revealed a region containing 15 sequential serine codons—all of them being the TCT codon. The genetic code contains 6 different serine codons, and three of these are used frequently in  E. coli . The present inventors used four overlapping oligonucleotides in combination with a polymerase chain reaction protocol to resynthesize a small portion of the Orf B gene (a ˜195 base pair, BspHI to SacII restriction enzyme fragment) that contained the serine codon repeat region. In the synthetic Orf B fragment, a random mixture of the 3 serine codons commonly used by  E. coli  was used, and some other potentially problematic codons were changed as well (i.e., other codons rarely used by  E. coli ). The BspHI to SacII fragment present in the original Orf B was replaced by the resynthesized fragment (to yield Orf B*) and the modified gene was cloned into the relevant expression vectors. The modified OrfB* still encodes the amino acid sequence of SEQ ID NO:16. Expression of the modified Orf B* clone in  E. coli  resulted in the appearance of a ˜224 kDa protein, indicating that the full-length product of OrfB was produced. The sequence of the resynthesized Orf B* BspHI to SacII fragment is represented herein as SEQ ID NO:29. Referring to SEQ ID NO:29, the nucleotide sequence of the resynthesized BspHI to SacII region of Orf B is shown. The BspHI restriction site and the SacII restriction site are identified. The BspHI site starts at nucleotide 4415 of the Orf B CDS (SEQ ID NO:15) (note: there are a total of three BspHI sites in the Orf B CDS, while the SacII site is unique). 
     The ACP domains of the Orf A protein (SEQ ID NO:14 in  Schizochytrium ) must be activated by addition of phosphopantetheine group in order to function. The enzymes that catalyze this general type of reaction are called phosphopantetheine transferases (PPTases).  E. coli  contains two endogenous PPTases, but it was anticipated that they would not recognize the Orf A ACP domains from  Schizochytrium . This was confirmed by expressing Orfs A, B* (see above) and C in  E. coli  without an additional PPTase. In this transformant, no DHA production was detected. The inventors tested two heterologous PPTases in the  E. coli  PUFA PKS expression system: (1) sfp (derived from  Bacillus subtilis ) and (2) Het I (from the cyanobacterium Nostoc strain 7120). 
     The sfp PPTase has been well characterized and is widely used due to its ability to recognize a broad range of substrates. Based on published sequence information (Nakana, et al., 1992 , Molecular and General Genetics  232: 313-321), an expression vector for sfp was built by cloning the coding region, along with defined up- and downstream flanking DNA sequences, into a pACYC-184 cloning vector. The oligonucleotides: 
                            CGGGGTACCCGGGAGCCGCCTTGGCTTTGT           (forward; SEQ ID NO: 30);           and                   AAACTGCAGCCCGGGTCCAGCTGGCAGGCACCCTG           (reverse; SEQ ID NO: 31),            
were used to amplify the region of interest from genomic  B. subtilus  DNA. Convenient restriction enzyme sites were included in the oligonucleotides to facilitate cloning in an intermediate, high copy number vector and finally into the EcoRV site of pACYC184 to create the plasmid: pBR301. Examination of extracts of  E. coli  transformed with this plasmid revealed the presence of a novel protein with the mobility expected for sfp. Co-expression of the sfp construct in cells expressing the Orf A, B*, C proteins, under certain conditions, resulted in DHA production. This experiment demonstrated that sfp was able to activate the  Schizochytrium  Orf A ACP domains. In addition, the regulatory elements associated with the sfp gene were used to create an expression cassette into which other genes could be inserted. Specifically, the sfp coding region (along with three nucleotides immediately upstream of the ATG) in pBR301 was replaced with a 53 base pair section of DNA designed so that it contains several unique (for this construct) restriction enzyme sites. The initial restriction enzyme site in this region is NdeI. The ATG sequence embedded in this site is utilized as the initiation methionine codon for introduced genes. The additional restriction sites (BglLL, NotI, SmaI, PmelI, HindIII, SpeI and XhoI) were included to facilitate the cloning process. The functionality of this expression vector cassette was tested by using PCR to generate a version of sfp with a NdeI site at the 5′ end and an XhoI site ate the 3′ end. This fragment was cloned into the expression cassette and transferred into  E. coli  along with the Orf A, B* and C expression vector. Under appropriate conditions, these cells accumulated DHA, demonstrating that a functional sfp had been produced.
 
     To the present inventors&#39; knowledge, Het I had not been tested previously in a heterologous situation. Het I is present in a cluster of genes in Nostoc known to be responsible for the synthesis of long chain hydroxy-fatty acids that are a component of a glyco-lipid layer present in heterocysts of that organism. The present inventors, without being bound by theory, believe that Het I activates the ACP domains of a protein, Hgl E, present in that cluster. The two ACP domains of Hgl E have a high degree of sequence homology to the ACP domains found in  Schizochytrium  Orf A. SEQ ID NO:32 represents the amino acid sequence of the Nostoc Het I protein. The endogenous start codon of Het I has not been identified (there is no methionine present in the putative protein). There are several potential alternative start codons (e.g., TTG and ATT) near the 5′ end of the open reading frame. No methionine codons (ATG) are present in the sequence. A Het I expression construct was made by using PCR to replace the furthest 5′ potential alternative start codon (TTG) with a methionine codon (ATG, as part of the above described NdeI restriction enzyme recognition site), and introducing an XhoI site at the 3′ end of the coding sequence. The modified HetI coding sequence was then inserted into the NdeI and XhoI sites of the pACYC184 vector construct containing the sfp regulatory elements. Expression of this Het I construct in  E. coli  resulted in the appearance of a new protein of the size expected from the sequence data. Co-expression of Het I with  Schizochytrium  Orfs A, B*, C in  E. coli  under several conditions resulted in the accumulation of DHA and DPA in those cells. In all of the experiments in which sfp and Het I were compared, more DHA and DPA accumulated in the cells containing the Het I construct than in cells containing the sfp construct. 
     Production of DHA and DPA in  E. coli  Transformants 
     The two plasmids encoding: (1) the  Schizochytrium  PUFA PKS genes (Orfs A, B* and C) and (2) the PPTase (from sfp or from Het I) were transformed into  E. coli  strain BL21 which contains an inducible T7 RNA polymerase gene. Synthesis of the  Schizochytrium  proteins was induced by addition of IPTG to the medium, while PPTase expression was controlled by a separate regulatory element (see above). Cells were grown under various defined conditions and using either of the two heterologous PPTase genes. The cells were harvested and the fatty acids were converted to methyl-esters (FAME) and analyzed using gas-liquid chromatography. 
     Under several conditions, DHA and DPA were detected in  E. coli  cells expressing the  Schizochytrium  PUFA PKS genes, plus either of the two heterologous PPTases (data not shown). No DHA or DPA was detected in FAMEs prepared from control cells (i.e., cells transformed with a plasmid lacking one of the Orfs). The ratio of DHA to DPA observed in  E. coli  approximates that of the endogenous DHA and DPA production observed in  Schizochytrium . The highest level of PUFA (DHA plus DPA), representing ˜17% of the total FAME, was found in cells grown at 32° C. in 765 medium (recipe available from the American Type Culture Collection) supplemented with 10% (by weight) glycerol. PUFA accumulation was also observed when cells were grown in Luria Broth supplemented with 5 or 10% glycerol, and when grown at 20° C. Selection for the presence of the respective plasmids was maintained by inclusion of the appropriate antibiotics during the growth, and IPTG (to a final concentration of 0.5 mM) was used to induce expression of Orfs A, B* and C. 
     Example 4 
     The following example demonstrates that genes encoding the  Schizochytrium  PUFA PKS enzyme complex can be selectively inactivated (knocked out), and that it is a lethal phenotype unless the medium is supplemented with polyunsaturated fatty acids. 
     Homologous recombination has been demonstrated in  Schizochytrium  (see copending U.S. patent application Ser. No. 10/124,807, incorporated herein by reference in its entirety). A plasmid designed to inactivate  Schizochytrium  Orf A (SEQ ID NO:13) was made by inserting a Zeocin™ resistance marker into the Sma I site of a clone containing the Orf A coding sequence. The Zeocin™ resistance marker was obtained from the plasmid pMON50000-expression of the Zeocin™ resistance gene is driven by a  Schizochytrium  derived tubulin promoter element (see U.S. patent application Ser. No. 10/124,807, ibid.). The knock-out construct thus consists of: 5′  Schizochytrium  Orf A coding sequence, the tub-Zeocin™ resistance element and 3′  Schizochytrium  Orf A coding sequence, all cloned into pBluescript II SK (+) vector (Stratagene). 
     The plasmid was introduced into  Schizochytrium  cells by particle bombardment and transformants were selected on plates containing Zeocin™ and supplemented with polyunsaturated fatty acids (PUFA) (see Example 5). Colonies that grew on the Zeocin™ plus PUFA plates were tested for ability to grow on plates without the PUFA supplementation and several were found that required the PUFA. These PUFA auxotrophs are putative Orf A knockouts. Northern blot analysis of RNA extracted from several of these mutants confirmed that a full-length Orf A message was not produced in these mutants. 
     These experiments demonstrate that a  Schizochytrium  gene (e.g., Orf A) can be inactivated via homologous recombination, that inactivation of Orf A results in a lethal phenotype, and that those mutants can be rescued by supplementation of the media with PUFA. 
     Similar sets of experiments directed to the inactivation of  Schizochytrium  Orf B (SEQ ID NO:15) and Orf C (SEQ ID NO:17) have yielded similar results. That is, Orf B and Orf C can be individually inactivated by homologous recombination and those cells require PUFA supplementation for growth. 
     Example 5 
     The following example shows that PUFA auxotrophs can be maintained on medium supplemented with EPA, demonstrating that EPA can substitute for DHA in  Schizochytrium.    
     As indicated in Example 4,  Schizochytrium  cells in which the PUFA PKS complex has been inactivated required supplementation with PUFA to survive. Aside from demonstrating that  Schizochytrium  is dependent on the products of this system for growth, this experimental system permits the testing of various fatty acids for their ability to rescue the mutants. It was discovered that the mutant cells (in which any of the three genes have been inactivated) grew as well on media supplemented with EPA as they did on media supplemented with DHA. This result indicates that, if the endogenous PUFA PKS complex which produces DHA were replaced with one whose product was EPA, the cells would be viable. Additionally, these mutant cells could be rescued by supplementation with either ARA or GLA, demonstrating the feasibility of producing genetically modified  Schizochytrium  that produce these products. It is noted that a preferred method for supplementation with PUFAs involves combining the free fatty acids with partially methylated beta-cyclodextrin prior to addition of the PUFAs to the medium. 
     Example 6 
     The following example shows that inactivated PUFA genes can be replaced at the same site with active forms of the genes in order to restore PUFA synthesis. 
     Double homologous recombination at the acetolactate synthase gene site has been demonstrated in  Schizochytrium  (see U.S. patent application Ser. No. 10/124,807, supra). The present inventors tested this concept for replacement of the  Schizochytrium  PUFA PKS genes by transformation of a  Schizochytrium  Orf A knockout strain (described in Example 3) with a full-length  Schizochytrium  Orf A genomic clone. The transformants were selected by their ability to grow on media without supplemental PUFAs. These PUFA prototrophs were then tested for resistance to Zeocin™ and several were found that were sensitive to the antibiotic. These results indicate that the introduced  Schizochytrium  Orf A has replaced the Zeocin™ resistance gene in the knockout strain via double homologous recombination. This experiment demonstrates the proof of concept for gene replacement within the PUFA PKS genes. Similar experiments for  Schizochytrium  Orf B and Orf C knock-outs have given identical results. 
     Example 7 
     This example shows that all or some portions of the  Thraustochytrium  23B PUFA PKS genes can function in  Schizochytrium.    
     As described in U.S. patent application Ser. No. 10/124,800 (supra), the DHA-producing protist  Thraustochytrium  23B (Th. 23B) has been shown to contain orfA, orfB, and orfC homologs. Complete genomic clones of the three Th. 23B genes were used to transform the Zeocin™-resistant  Schizochytrium  strains containing the cognate orf “knock-out” (see Example 4). Direct selection for complemented transformants was carried out in the absence of PUFA supplementation. By this method, it was shown that the Th. 23B orfA and orfC genes could complement the  Schizochytrium  orfA and orfC knock-out strains, respectively, to PUFA prototrophy. Complemented transformants were found that either retained or lost Zeocin™ resistance (the marker inserted into the  Schizochytrium  genes thereby defining the knock-outs). The Zeocin™-resistant complemented transformants are likely to have arisen by a single cross-over integration of the entire  Thraustochytrium  gene into the  Schizochytrium  genome outside of the respective orf region. This result suggests that the entire  Thraustochytrium  gene is functioning in  Schizochytrium . The Zeocin™-sensitive complemented transformants are likely to have arisen by double cross-over events in which portions (or conceivably all) of the  Thraustochytrium  genes functionally replaced the cognate regions of the  Schizochytrium  genes that had contained the disruptive Zeocin™ resistance marker. This result suggests that a fraction of the  Thraustochytrium  gene is functioning in  Schizochytrium.    
     Example 8 
     In this example, the entire  Schizochytrium  orfC coding sequence is completely and exactly replaced by the  Thraustochytrium  23B orfC coding sequence resulting in a PUFA profile shifted toward that of  Thraustochytrium.    
     To delete the  Schizochytrium  orfC coding sequence, approximately 2 kb of DNA immediately upstream (up to but not including the ATG start codon) and immediately downstream (beginning just after the TAA stop codon) were cloned around the Zeocin™ resistance marker. The upstream and downstream regions provide homology for double crossover recombination effectively replacing the orfC coding sequence with the marker. Transformants are selected for Zeocin™ resistance in the presence of supplemental PUFA, screened for PUFA auxotrophy, and characterized by PCR and Southern blot analysis. Similarly, a plasmid was constructed in which the same upstream and downstream sequences of the  Schizochytrium  orfC gene region were cloned around the Th. 23B orf C coding sequence (SEQ ID NO:23). Transformation of this plasmid into the Zeocin™ resistant PUFA auxotroph described above was carried out with selection for PUFA prototrophy, thus relying on the Th. 23B orfC gene to function correctly in  Schizochytrium  and complement the PUFA auxotrophy. Subsequent screening for Zeocin™ sensitive transformants identified those likely to have arisen from a replacement of the Zeocin™ resistance marker with the Th. 23B orfC gene. The DHA:DPA ratio in these orfC replacement strains was on average 8.3 versus a normal (“wild type”) value of 2.3. This higher ratio approximates the value of 10 for  Thraustochytrium  23B under these growth conditions. Therefore, it is shown that the PUFA profile of  Schizochytrium  can be manipulated by substituting components of the PUFA synthase enzyme complex. 
     More specifically, the first pair of plasmids captures the regions immediately “upstream” and “downstream” of the  Schizochytrium  orfC gene and was used to construct both the orfC deletion vector as well as the Th. 23B replacement vector. 
     Primers prRZ15 (SEQ ID NO:49) and prRZ16 (SEQ ID NO:50) were used to amplify a 2000 bp fragment upstream of the orfC coding region from a clone of the  Schizochytrium  orfC region. Primer prRZ15 incorporates a KpnI site at the 5-prime end of the fragment and prRZ16 contains homology to  Schizochytrium  sequence up to but not including the ATG start codon and incorporates a BamHI site at the 3-prime end of the fragment. The PCR product was cloned into pCR-Blunt II (Invitrogen) resulting in plasmid pREZ21. In a similar manner, primers prRZ17 (SEQ ID NO:51) and prRZ18 (SEQ ID NO:52) were used to amplify a 1991 bp fragment immediately downstream of the orfC coding region (not containing the TAA stop codon) but incorporating a BamHI site at the 5-prime end and a XbaI site at the 3-prime end. This PCR fragment was cloned into pCR-Blunt II (Invitrogen) to create pREZ18. In a three-component ligation, the upstream region from pREZ21 (as a KpnI-BamHI fragment) and the downstream region from pREZ18 (as a BamHI-XbaI fragment) were cloned into the KpnI-XbaI site of pBlueScriptII SK(+) to yield pREZ22. The Zeocin™ resistance marker from pTUBZEO11-2 (a.k.a. pMON50000; see U.S. patent application Ser. No. 10/124,807, supra) as an 1122 bp BamHI fragment was inserted into the BamHI site of pREZ22 to produce pREZ23A and pREZ23B (containing the Zeocin™ resistance marker in either orientation). The pREZ23 plasmids were then used to create the precise deletion of the orfC coding region by particle bombardment transformation as described above. A strain with the desired structure is named B32-Z1. 
     To develop the plasmid for insertion of the Th. 23B orfC gene, intermediate constructs containing the precise junctions between 1) the  Schizochytrium  upstream region and the 5-prime end of the Th. 23B orfC coding region and 2) the 3-prime end of the Th. 23B orfC coding region and the  Schizochytrium  downstream region are first produced. Then, the internal section of the Th. 23B orfC coding region is introduced. 
     Primers prRZ29a (SEQ ID NO:53) and prRZ30 (SEQ ID NO:54) are used to amplify approximately 100 bp immediately upstream of the  Schizochytrium  orfC coding sequence. Primer prRZ29a includes the SpeI restriction site approximately 95 bp upstream of the  Schizochytrium  orfC ATG start codon, and prRZ30 contains homology to 19 bp immediately upstream of the  Schizochytrium  orfC ATG start codon and 15 bp homologous to the start of the Th. 23B orfC coding region (including the start ATG). Separately, an approximately 450 bp PCR product is generated from the 5-prime end of the Th. 23B orfC coding region using the cloned Th. 23B gene as a template. Primer prRZ31 contains 15 bp of the  Schizochytrium  orfC coding sequence immediately upstream of the start ATG and homology to 17 bp at the start of the Th. 23B orfC coding region, and primer prRZ32 incorporates the NruI site located at approximately 450 bp downstream of the Th. 23B orfC ATG start codon and further includes an artificial SwaI restriction site just downstream of the NruI site. These two PCR products therefore have about 30 bp of overlapping homology with each other at the start ATG site essentially comprising the sequences of prRZ30 (SEQ ID NO:54) and prRZ31 (SEQ ID NO:55). A second round of PCR using a mix of the two first-round PCR products (prRZ29a (SEQ ID NO:53) X prRZ30 (SEQ ID NO:54); ca. 100 bp; prRZ31 (SEQ ID NO:55) X prRZ32 (SEQ ID NO:56); ca. 450 bp) as template and the outside primers prRZ29a (SEQ ID NO:53) and prRZ32 (SEQ ID NO:56) resulted in an approximately 520 bp product containing the “perfect stitch” between the upstream  Schizochytrium  orfC region and the start of the Th. 23B orf C coding region. This PCR product was cloned into plasmid pCR-Blunt II to create pREZ28, and the sequence of the insert was confirmed. 
     Primers prRZ33 (SEQ ID NO:57) and prRZ34 (SEQ ID NO:58) were used for PCR to generate a fragment of approximately 65 bp at the 3-prime end of the Th. 23B orf C coding region using the cloned Th. 23B gene as a template. The upstream end of this fragment (from prRZ33) contains an artificial SwaI restriction site and encompasses the SphI restriction site at approximately 60 bp upstream of the Th. 23B orfC TAA termination codon. The downstream end of this fragment (from prRZ34) contains 16 bp at the 3-prime end of the Th. 23B orf C coding region and 18 bp with homology to  Schizochytrium  sequences immediately downstream from the orfC coding region (including the termination codon). Primers prRZ35 (SEQ ID NO:59) and prRZ36 (SEQ ID NO:60) were used to generate a fragment of approximately 250 bp homologous to  Schizochytrium  DNA immediately downstream of the orfC coding region. The upstream end of this PCR fragment (from prRZ35) contained 15 bp homologous to the end of the Th. 23B orf C coding region (counting the TAA stop codon), and the downstream end contained the SalI restriction site about 240 bp downstream of the  Schizochytrium  stop codon. A second round of PCR using a mix of the two first-round PCR products (prRZ33 (SEQ ID NO:57) X prRZ34 (SEQ ID NO:58); ca. 65 bp; prRZ35 (SEQ ID NO:59) X prRZ36 (SEQ ID NO:60); ca. 250 bp) as template and the outside primers prRZ33 (SEQ ID NO:57) and prRZ36 (SEQ ID NO:60) resulted in an approximately 310 bp product containing the “perfect stitch” between the end of the  Thraustochytrium  23B orfC coding region and the region of  Schizochytrium  DNA immediately downstream of the orfC coding region. This PCR product was cloned into plasmid pCR-Blunt II to create pREZ29, and the sequence of the insert was confirmed. 
     Next, the upstream and downstream “perfect stitch” regions were combined into pREZ22 (see above). In a three component ligation, the SpeI/SwaI fragment from pREZ28 and the SwaI/SalI fragment of pREZ29 were cloned into the SpeI/SalI sites of pREZ22 to create pREZ32. Lastly, the internal bulk of the  Thraustochytrium  23B orfC coding region was cloned into pREZ32 as a NruI/SphI fragment to create pREZ33. This plasmid was then used to transform the orfC knock-out strain B32-Z1 with selection for PUFA prototrophy. 
     Each publication cited or discussed herein is incorporated herein by reference in its entirety. 
     While various embodiments of the present invention have been described in detail, it is apparent that modifications and adaptations of those embodiments will occur to those skilled in the art. It is to be expressly understood, however, that such modifications and adaptations are within the scope of the present invention, as set forth in the following claims.