PRODUCTION OF POLYUNSATURATED FATTY ACIDS BY EXPRESSION OF POLYKETIDE-LIKE SYNTHESIS GENES IN PLANTS

Abstract: The present invention relates to compositions and methods for preparing polyunsaturated long chain fatty acids in plants, plant parts and plant cells, such as leaves, roots, fruits and seeds. Nucleic acid sequences and constructs encoding PKS-like genes required for the poly-unsaturated long chain fatty acid production, including the genes responsible for eicosapentenoic acid production of Shewanella putrefaciens and novel genes associated with the production of docosahexenoic acid in Vibrio marinus are used to generate transgenic plants, plant parts and cells which contain and express one or more transgenes encoding one or more of the PKS-like genes associated with such long chain polyunsaturated fatty acid production. Expression of the PKS-like genes in the plant system permits the large scale production of polyunsaturated long chain fatty acids such as eicosapentenoic acid and docosahexenoic acid for modification of the fatty acid profile of plants, plant parts and tissues. Manipulation of the fatty acid profiles allows for the production of commercial quantities of novel plant oils and products.

Description: PRODUCTION OF POLYUNSATURATED FATTY ACIDS BY EXPRESSION OF POLYKETIDE-LIKE SYNTHESIS GENES IN PLANTS INTRODUCTION Field ofthe Invention This invention relates to modulating levels of enzymes and/or enzyme components capable of modifying long chain poly-unsaturated fatty acids (PUFAs) in a host cell, and constructs and methods for producing PUFAs in a host cell. The invention is exemplified by production of eicosapentenoic acid (EPA) using genes derived from Shewanella putrefaciens and Vibrio marinus. Background Two main families of poly-unsaturated fatty acids (PUFAs) are the ω3 fatty acids, exemplified by eicosapentenoic acid, and the ω6 fatty acids, exemplified by arachidonic acid. PUFAs are important components ofthe plasma membrane ofthe cell, where they can be found in such forms as phospholipids, and also can be found in triglycerides. PUFAs also serve as precursors to other molecules of importance in human beings and animals, including the prostacyclins, leukotrienes and prostaglandins. Long chain PUFAs of importance include docosahexenoic acid (DHA) and eicosapentenoic acid (EPA), which are found primarily in different types of fish oil, gamma-linolenic acid (GLA), which is found in the seeds of a number of plants, including evening primrose (Oenothera biennis), borage (Borago officinalis) and black currants (Ribes nigrum), stearidonic acid (SDA), which is found in marine oils and plant seeds, and arachidonic acid (ARA), which along with GLA is found in filamentous fungi. ARA can be purified from animal tissues including liver and adrenal gland. Several genera of marine bacteria are known which synthesize either EPA or DHA. DHA is present in human milk along with ARA. PUFAs are necessary for proper development, particularly in the developing infant brain, and for tissue formation and repair. As an example, DHA, is an important constituent of many human cell membranes, in particular nervous cells (gray matter), muscle cells, and spermatozoa and believed to affect the development of brain functions in general and to be essential for the development of eyesight. EPA and DHA have a number of nutritional and pharmacological uses. As an example adults affected by diabetes (especially non insulin-dependent) show deficiencies and imbalances in their levels of DHA which are believed to contribute to later coronary conditions. Therefore a diet balanced in DHA may be beneficial to diabetics. For DHA, a number of sources exist for commercial production including a variety of marine organisms, oils obtained from cold water marine fish, and egg yolk fractions. The purification of DHA from fish sources is relatively expensive due to technical difficulties, making DHA expensive and in short supply. In algae such as Amphidinium and Schyzochytrium and marine fungi such as Thraustochytrium DHA may represent up to 48% ofthe fatty acid content ofthe cell. A few bacteria also are reported to produce DHA. These are generally deep sea bacteria such as Vibrio marinus. For ARA, microorganisms including the genera Mortierella, Entomophthora, Phytium and Porphyridium can be used for commercial production. Commercial sources of SDA include the genera Trichodesma and Echium. Commercial sources of GLA include evening primrose, black currants and borage. However, there are several disadvantages associated with commercial production of PUFAs from natural sources. Natural sources of PUFA, such as animals and plants, tend to have highly heterogeneous oil compositions. The oils obtained from these sources can require extensive purification to separate out one or more desired PUFA or to produce an oil which is enriched in one or more desired PUFA. Natural sources also are subject to uncontrollable fluctuations in availability. Fish stocks may undergo natural variation or may be depleted by overfishing. Animal oils, and particularly fish oils, can accumulate environmental pollutants. Weather and disease can cause fluctuation in yields from both fish and plant sources. Cropland available for production of alternate oil-producing crops is subject to competition from the steady expansion of human populations and the associated increased need for food production on the remaining arable land. Crops which do produce PUFAs, such as borage, have not been adapted to commercial growth and may not perform well in monoculture. Growth of such crops is thus not economically competitive where more profitable and better established crops can be grown. Large -scale fermentation of organisms such as Shewanella also is expensive. Natural animal tissues contain low amounts of ARA and are difficult to process. Microorganisms such as Porphyridium and Shewanella are difficult to cultivate on a commercial scale. Dietary supplements and pharmaceutical formulations containing PUFAs can retain the disadvantages ofthe PUFA source. Supplements such as fish oil capsules can contain low levels ofthe particular desired component and thus require large dosages. High dosages result in ingestion of high levels of undesired components, including contaminants. Care must be taken in providing fatty acid supplements, as overaddition may result in suppression of endogenous biosynthetic pathways and lead to competition with other necessary fatty acids in various lipid fractions in vivo, leading to undesirable results. For example, Eskimos having a diet high in ω3 fatty acids have an increased tendency to bleed (U.S. Pat. No. 4,874,603). Fish oils have unpleasant tastes and odors, which may be impossible to economically separate from the desired product, such as a food supplements. Unpleasant tastes and odors ofthe supplements can make such regimens involving the supplement undesirable and may inhibit compliance by the patient. A number of enzymes have been identified as being involved in PUFA biosynthesis. Linoleic acid (LA, 18:2 Δ 9, 12) is produced from oleic acid (18: 1 Δ9) by a Δl2-desaturase. GLA (18:3 Δ 6, 9, 12) is produced from linoleic acid (LA, 18:2 Δ9, 12) by a Δό-desaturase. ARA (20:4 Δ 5, 8, 11, 14) is produced from DGLA (20:3 Δ 8, 11, 14), catalyzed by a Δ5-desaturase. Eicosapentenoic acid (EPA) is a 20 carbon, omega 3 fatty acid containing 5 double bonds (Δ 5, 8, 11, 14, 17), all in the cis configuration. EPA, and the related DHA (Δ 4, 7, 10, 13, 16, 19, C22:6) are produced from oleic acid by a series of elongation and desaturation reactions. Additionally, an elongase (or elongases) is required to extend the 18 carbon PUFAs out to 20 and 22 carbon chain lengths. However, animals cannot convert oleic acid (18:1 Δ 9) into linoleic acid (18:2 Δ 9, 12). Likewise, μ-linolenic acid (ALA, 18:3 Δ 9, 12, 15) cannot be synthesized by mammals. Other eukaryotes, including fungi and plants, have enzymes which desaturate at positions Δl2 and Δl5. The major poly-unsaturated fatty acids of animals therefore are either derived from diet and/or from desaturation and elongation of linoleic acid (18:2 Δ 9, 12) or μ-linolenic acid (18:3 Δ 9, 12, 15). Poly-unsaturated fatty acids are considered to be useful for nutritional, pharmaceutical, industrial, and other purposes. An expansive supply of poly-unsaturated fatty acids from natural sources and from chemical synthesis are not sufficient for commercial needs. Because a number of separate desaturase and elongase enzymes are required for fatty acid synthesis from linoleic acid (LA, 18:2 Δ 9, 12), common in most plant species, to the more saturated and longer chain PUFAs, engineering plant host cells for the expression of EPA and DHA may require expression of five or six separate enzyme activities to achieve expression, at least for EPA and DHA, and for production of quantities of such PUFAs additional engineering efforts may be required, for instance the down regulation of enzymes competing 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. Relevant Literature Several genera of marine bacteria have been identified which synthesize either EPA or DHA (DeLong and Yayanos, Applied and Environmental Microbiology (1986) 51 : 730-737). Researchers ofthe Sagami Chemical Research Institute have reported EPA production in E. coli which have been transformed with a gene cluster from the marine bacterium, Shewanella putrefaciens. A minimum of 5 open reading frames (ORFs) are required for fatty acid synthesis of EPA in E. coli. To date, extensive characterization of the functions ofthe proteins encoded by these genes has not been reported (Yazawa (1996) Lipids 31, S-297; WO 93/23545; WO 96/21735). The protein sequence of open reading frame (ORF) 3 as published by Yazawa, USPN 5,683,898 is not a functional protein. Yazawa defines the protein as initiating at the methionine codon at nucleotides 9016-9014 ofthe Shewanella PKS-like cluster (Genbank accession U73935) and ending at the stop codon at nucleotides 8185-8183 of the Shewanella PKS-like cluster. However, when this ORF is expressed under control of a heterologous promoter in an E. coli strain containing the entire PKS-like cluster except ORF 3, the recombinant cells do not produce EPA. Polyketides are secondary metabolites the synthesis of which involves a set of enzymatic reactions analogous to those of fatty acid synthesis (see reviews: Hopwood and Sherman, Annu. Rev. Genet. (1990) 24: 37-66, and Katz and Donadio, in Annual Review of Microbiology (1993) 47: 875-912). It has been proposed to use polyketide synthases to produce novel antibiotics (Hutchinson and Fujii, Annual Review of Microbiology (1995) 49:201-238). SUMMARY OF THE INVENTION Novel compositions and methods are provided for preparation of long chain polyunsaturated fatty acids (PUFAs) using polyketide-like synthesis (PKS-like) genes in plants and plant cells. In contrast to the known and proposed methods for production of PUFAs by means of fatty acid synthesis genes, by the invention constructs and methods are provided for producing PUFAs by utilizing genes of a PKS-like system. The methods involve growing a host cell of interest transformed with an expression cassette functional in the host cell, the expression cassette comprising a transcriptional and translational initiation regulatory region, joined in reading frame 5' to a DNA sequence to a gene or component of a PKS-like system capable of modulating the production of PUFAs (PKS- like gene). An alteration in the PUFA profile of host cells is achieved by expression following introduction of a complete PKS-like system responsible for a PUFA biosynthesis into host cells. The invention finds use for example in the large scale production of DHA and EPA and for modification ofthe fatty acid profile of host cells and edible plant tissues and/or plant parts. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 provides designations for the ORFs ofthe EPA gene cluster of Shewanella. Figure 1A shows the organization ofthe genes; those ORFs essential for EPA production in E. coli are numbered. Figure IB shows the designations given to subclones. Figure 2 provides the Shewanella PKS-like domain structure, motifs and 'Blast' matches of ORF 6 (Figure 2A), ORF 7 (Figure 2B), ORF 8 (Figure 2C), ORF 9 (Figure 2D) and ORF 3 (Figure 2E). Figure 2F shows the structure ofthe region ofthe Anabeana chromosome that is related to domains present in Shewanella EPA ORFs. Figure 3 shows results for pantethenylation - ORF 3 in E. coli strain SJ16. Figure 4 is the sequence for the PKS-like cluster found in Shewanella, containing ORFs 3, 4, 5, 6, 7, 8 and 9. The start and last codons for each ORF are as follows: ORF3 (published-mactive): 9016, 8186; ORF3 (active in EPA synthesis): 9157, 8186; ORF 6: 13906, 22173; ORF 7: 22203, 24515; ORF 8: 24518, 30529; ORF 9: 30730, 32358. Figure 5 shows the sequence for the PKS-like cluster in an approximately 40 kb DNA fragment of Vibrio marinus, containing ORFs 6, 7, 8 and 9. The start and last condons for each ORF are as follows: ORF 6: 17394, 25352; ORF 7: 25509, 28160; ORF 8: 28209, 34265; ORF 9: 34454, 36118. Figure 6 shows the sequence for an approximately 19 kb portion ofthe PKS-like cluster of Figure 5 which contains the ORFs 6, 7, 8 and 9. The start and last condons for each ORF are as follows: ORF 6: 411, 8369; ORF 7: 8526, 11177; ORF 8: 11226, 17282; ORF 9: 17471, 19135. Figure 7 shows a comparison ofthe PKS-like gene clusters of Shewanella putrefaciens and Vibrio marinus; Figure 7B is the Vibrio marinus operon sequence. Figure 8 is an expanded view ofthe PKS-like gene cluster portion of Vibrio marinus shown in Figure 7B showing that ORFs 6, 7 and 8 are in reading frame 2, while ORF 9 is in reading frame 3. Figure 9 demonstrates sequence homology of ORF 6 of Shewanella putrefaciens and Vibrio marinus. The Shewanella ORF 6 is depicted on the vertical axis, and the Vibrio ORF 6 is depicted on the horizontal axis. Lines indicate regions ofthe proteins that have a 60% identity. The repeated lines in the middle correspond to the multiple ACP domains found in ORF 6. Figure 10 demonstrates sequence homology of ORF 7 of Shewanella putrefaciens and Vibrio marinus. The Shewanella ORF 7 is depicted on the vertical axis, and the Vibrio ORF 7 is depicted on the horizontal axis. Lines indicate regions ofthe proteins that have a 60% identity. Figure 11 demonstrates sequence homology of ORF 8 of Shewanella putrefaciens and Vibrio marinus. The Shewanella ORF 8 is depicted on the vertical axis, and the Vibro. ORF 8 is depicted on the horizontal axis. Lines indicate regions ofthe proteins that have a 60% identity. Figure 12 demonstrates sequence homology of ORF 9 of Shewanella putrefaciens and Vibrio marinus. The Shewanella ORF 9 is depicted on the vertical axis, and the Vibrio ORF 9 is depicted on the horizontal axis. Lines indicate regions ofthe proteins that have a 60% identity. Figure 13 is a depiction of various complementation experiments, and resulting PUFA production. On the right, is shown the longest PUFA made in the E. coli strain containing the Vibrio and Shewanella genes depicted on the left. The hollow boxes indicate ORFs from Shewanella. The solid boxes indicate ORFs from Vibrio. Figure 14 is a chromatogram showing fatty acid production from complementation of pEPAD8 from Shewanella (deletion ORF 8) with ORF 8 from Shewanella, in E. coli Fad E-. The chromatogram presents an EPA (20:5) peak. Figure 15 is a chromatogram showing fatty acid production from complementation of pEPAD8 from Shewanella (deletion ORF 8) with ORF 8 from Vibrio marinus, in E. coli Fad E-. The chromatograph presents EPA (20:5) and DHA (22:6) peaks. Figure 16 is a table of PUFA values from the ORF 8 complementation experiment, the chromatogram of which is shown in Figure 15. Figure 17 is a plasmid map showing the elements of pCGN7770. Figure 18 is a plasmid map showing the elements of pCGN8535. Figure 19 is a plasmid map showing the elements of pCGN8537. Figure 20 is a plasmid map showing the elements of pCGN8525. Figure 21 is a comparison ofthe Shewanella ORFs as defined by Yazawa and those disclosed in Figure 4. When a protein starting at the leucine (TTG) codon at nucleotides 9157-9155 and ending at the stop codon at nucleotides 8185-8183 is expressed under control of a heterologous promoter in an E. coli strain containing the entire PKS-like cluster except ORF 3, the recombinant cells do produce EPA. Thus, the published protein sequence is likely to be wrong, and the coding sequence for the protein may start at the TTG codon at nucleotides 9157-9155 or the TTG codon at nucleotides 9172-9170. This information is critical to the expression of a functional PKS-like cluster heterologous system. Figure 22 is a plasmid map showing the elements of pCGN8560. Figure 23 is plasmid map showing the elements of pCGN8556. Figure 24 shows the translated DNA sequence upstream ofthe published ORF 3. The ATG start codon at position 9016 is the start codon for the protein described by Yazawa et al (1996) supra. The other arrows depict TTG or ATT codons that can also serve as start codons in bacteria. When ORF 3 is started from the published ATG codon at 9016, the protein is not functional in making EPA. When ORF 3 is initiated at the TTG codon at position 9157, the protein is capable of facilitating EPA synthesis. DESCRIPTION OF THE PREFERRED EMBODIMENTS In accordance with the subject invention, novel DNA sequences, DNA constructs and methods are provided, which include some or all ofthe polyketide-like synthesis (PKS-like) pathway genes from Shewanella, Vibrio or other microorganisms, for modifying the poly-unsaturated long chain fatty acid content of host cells, particularly host plant cells. The present invention demonstrates that EPA synthesis genes in Shewanella putrefaciens constitute a polyketide-like synthesis pathway. Functions are ascribed to the Shewanella and Vibrio genes and methods are provided for the production of EPA and DHA in host cells. The method includes the step of transforming cells with an expression cassette comprising a DNA encoding a polypeptide capable of increasing the amount of one or more PUFA in the host cell. Desirably, integration constructs are prepared which provide for integration ofthe expression cassette into the genome of a host cell. Host cells are manipulated to express a sense or antisense DNA encoding a polypeptide(s) that has PKS-like gene activity. By "PKS-like gene" is intended a polypeptide which is responsible for any one or more ofthe functions of a PKS-like activity of interest. By "polypeptide" is meant any chain of amino acids, regardless of length or post-translational modification, for example, glycosylation or phosphorylation. Depending upon the nature ofthe host cell, the substrate(s) for the expressed enzyme may be produced by the host cell or may be exogenously supplied. Of particular interest is the selective control of PUFA production in plant tissues and/or plant parts such as leaves, roots, fruits and seeds. The invention can be used to synthesize EPA, DHA, and other related PUFAs in host cells. There are many advantages to transgenic production of PUFAs. As an example, in transgenic E. coli as in Shewanella, EPA accumulates in the phospholipid fraction, specifically in the sn-2 position. It may be possible to produce a structured lipid in a desired host cell which differs substantially from that produced in either Shewanella or E. coli. Additionally transgenic production of PUFAs in particular host cells offers several advantages over purification from natural sources such as fish or plants. In transgenic plants, by utilizing a PKS-like system, fatty acid synthesis of PUFAs is achieved in the cytoplasm by a system which produces the PUFAs through de novo production ofthe fatty acids utilizing malonyl Co- A and acetyl Co- A as substrates. In this fashion, potential problems, such as those associated with substrate competition and diversion of normal products of fatty acid synthesis in a host to PUFA production, are avoided. Production of fatty acids from recombinant plants provides the ability to alter the naturally occurring plant fatty acid profile by providing new synthetic pathways in the host or by suppressing undesired pathways, thereby increasing levels of desired PUFAs, or conjugated forms thereof, and decreasing levels of undesired PUFAs. Production of fatty acids in transgenic plants also offers the advantage that expression of PKS-like genes in particular tissues and/or plant parts means that greatly increased levels of desired PUFAs in those tissues and/or parts can be achieved, making recovery from those tissues more economical. Expression in a plant tissue and/or plant part presents certain efficiencies, particularly where the tissue or part is one which is easily harvested, such as seed, leaves, fruits, flowers, roots, etc. For example, the desired PUFAs can be expressed in seed; methods of isolating seed oils are well established. In addition to providing a source for purification of desired PUFAs, seed oil components can be manipulated through expression of PKS-like genes, either alone or in combination with other genes such as elongases, to provide seed oils having a particular PUFA profile in concentrated form. The concentrated seed oils then can be added to animal milks and/or synthetic or semisynthetic milks to serve as infant formulas where human nursing is impossible or undesired, or in cases of malnourishment or disease in both adults and infants. Transgenic microbial production of fatty acids offers the advantages that many microbes are known with greatly simplified oil compositions as compared with those of higher organisms, making purification of desired components easier. Microbial production is not subject to fluctuations caused by external variables such as weather and food supply. Microbially produced oil is substantially free of contamination by environmental pollutants. Additionally, microbes can provide PUFAs in particular forms which may have specific uses. For example, Spirulina can provide PUFAs predominantly at the first and third positions of triglycerides; digestion by pancreatic lipases preferentially releases fatty acids from these positions. Following human or animal ingestion of triglycerides derived from Spirulina, thes PUFAs are released by pancreatic lipases as free fatty acids and thus are directly available, for example, for infant brain development. Additionally, microbial oil production can be manipulated by controlling culture conditions, notably by providing particular substrates for microbially expressed enzymes, or by addition of compounds which suppress undesired biochemical pathways. In addition to these advantages, production of fatty acids from recombinant microbes provides the ability to alter the naturally occurring microbial fatty acid profile by providing new synthetic pathways in the host or by suppressing undesired pathways, thereby increasing levels of desired PUFAs, or conjugated forms thereof, and decreasing levels of undesired PUFAs. Production of fatty acids in animals also presents several advantages. Expression of desaturase genes in animals can produce greatly increased levels of desired PUFAs in animal tissues, making recovery from those tissues more economical. For example, where the desired PUFAs are expressed in the breast milk of animals, methods of isolating PUFAs from animal milk are well established. In addition to providing a source for purification of desired PUFAs, animal breast milk can be manipulated through expression of desaturase genes, either alone or in combination with other human genes, to provide animal milks with a PUFA composition substantially similar to human breast milk during the different stages of infant development. Humanized animal milks could serve as infant formulas where human nursing is impossible or undesired, or in the cases of malnourishment or disease. DNAs encoding desired PKS-like genes can be identified in a variety of ways. In one method, a source of a desired PKS-like gene, for example genomic libraries from a Shewanella or Vibrio spp., is screened with detectable enzymatically- or chemically- synthesized probes. Sources of ORFs having PKS-like genes are those organisms which produce a desired PUFA, including DHA-producing or EPA-producing deep sea bacteria growing preferentially under high pressure or at relatively low temperature. Microorgansims such as Shewanella which produce EPA or DHA also can be used as a source of PKS-like genes. The probes can be made from DNA, RNA, or non-naturally occurring nucleotides, or mixtures thereof. Probes can be enzymatically synthesized from DNAs of known PKS-like genes for normal or reduced-stringency hybridization methods. For discussions of nucleic acid probe design and annealing conditions, see, for example, Sambrook et al, Molecular Cloning: A Laboratory Manual (2 n ed.), Vols. 1-3, Cold Spring Harbor Laboratory, (1989) or Current Protocols in Molecular Biology, F. Ausubel et al, ed., Greene Publishing and Wiley-Interscience, New York (1987), each of which is incorporated herein by reference. Techniques for manipulation of nucleic acids encoding PUFA enzymes such as subcloning nucleic acid sequences encoding polypeptides into expression vectors, labelling probes, DNA hybridization, and the like are described generally in Sambrook, supra. Ohgonucleotide probes also can be used to screen sources and can be based on sequences of known PKS-like genes, including sequences conserved among known PKS- like genes, or on peptide sequences obtained from a desired purified protein. Ohgonucleotide probes based on amino acid sequences can be degenerate to encompass the degeneracy ofthe genetic code, or can be biased in favor ofthe preferred codons of the source organism. Alternatively, a desired protein can be entirely sequenced and total synthesis of a DNA encoding that polypeptide performed. Once the desired DNA has been isolated, it can be sequenced by known methods. It is recognized in the art that such methods are subject to errors, such that multiple sequencing ofthe same region is routine and is still expected to lead to measurable rates of mistakes in the resulting deduced sequence, particularly in regions having repeated domains, extensive secondary structure, or unusual base compositions, such as regions with high GC base content. When discrepancies arise, resequencing can be done and can employ special methods. Special methods can include altering sequencing conditions by using: different temperatures; different enzymes; proteins which alter the ability of oligonucleotides to form higher order structures; altered nucleotides such as ITP or methylated dGTP; different gel compositions, for example adding formamide; different primers or primers located at different distances from the problem region; or different templates such as single stranded DNAs. Sequencing of mRNA can also be employed. For the most part, some or all ofthe coding sequences for the polypeptides having PKS-like gene activity are from a natural source. In some situations, however, it is desirable to modify all or a portion ofthe codons, for example, to enhance expression, by employing host preferred codons. Host preferred codons can be determined from the codons of highest frequency in the proteins expressed in the largest amount in a particular host species of interest. Thus, the coding sequence for a polypeptide having PKS-like gene activity can be synthesized in whole or in part. All or portions ofthe DNA also can be synthesized to remove any destabilizing sequences or regions of secondary structure which would be present in the transcribed mRNA. All or portions ofthe DNA also can be synthesized to alter the base composition to one more preferable to the desired host cell. Methods for synthesizing sequences and bringing sequences together are well established in the literature. In vitro mutagenesis and selection, site-directed mutagenesis, or other means can be employed to obtain mutations of naturally occurring PKS-like genes to produce a polypeptide having PKS-like gene activity in vivo with more desirable physical and kinetic parameters for function in the host cell, such as a longer half-life or a higher rate of production of a desired polyunsaturated fatty acid. Of particular interest are the Shewanella putrefaciens ORFs and the corresponding ORFs of Vibrio marinus. The Shewanella putrefaciens PKS-like genes can be expressed in transgenic plants to effect biosynthesis of EPA. Other DNAs which are substantially identical in sequence to the Shewanella putrefaciens PKS-like genes, or which encode polypeptides which are substantially similar to PKS-like genes of Shewanella putrefaciens can be used, such as those identified from Vibrio marinus. By substantially identical in sequence is intended an amino acid sequence or nucleic acid sequence exhibiting in order of increasing preference at least 60%, 80%, 90% or 95%) homology to the DNA sequence ofthe Shewanella putrefaciens PKS-like genes or nucleic acid sequences encoding the amino acid sequences for such genes. For polypeptides, the length of comparison sequences generally is at least 16 amino acids, preferably at least 20 amino acids, and most preferably 35 amino acids. For nucleic acids, the length of comparison sequences generally is at least 50 nucleotides, preferably at least 60 nucleotides, and more preferably at least 75 nucleotides, and most preferably, 110 nucleotides. Homology typically is measured using sequence analysis software, for example, the Sequence Analysis software package ofthe Genetics Computer Group, University of Wisconsin Biotechnology Center, 1710 University Avenue, Madison, Wisconsin 53705, MEGAlign (DNAStar, Inc., 1228 S. Park St., Madison, Wisconsin 53715), and MacVector (Oxford Molecular Group, 2105 S. Bascom Avenue, Suite 200, Campbell, California 95008). BLAST (National Center for Biotechnology Information (WCBI) www.ncbi.nlm.gov; FASTA (Pearson and Lipman, Science (1985) 227:1435-1446). Such software matches similar sequences by assigning degrees of homology to various substitutions, deletions, and other modifications. 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 hydrophihcity (Kyte and Doolittle, J. Mol. Biol. (1982) 157: 105-132), or on the basis ofthe ability to assume similar polypeptide secondary structure (Chou and Fasman, Adv. Enzymol. (1978) 47: 45-148, 1978). A related protein to the probing sequence is identified when p > 0.01, preferably p > 10 " or 10 "8 . Encompassed by the present invention are related PKS-like genes from the same or other organisms. Such related PKS-like genes include variants ofthe disclosed PKS- like ORFs that occur naturally within the same or different species of Shewanella, as well as homologues ofthe disclosed PKS-like genes from other species and evolutionarily related proteins having analogous function and activity. Also included are PKS-like genes which, although not substantially identical to the Shewanella putrefaciens PKS- like genes, operate in a similar fashion to produce PUFAs as part of a PKS-like system. Related PKS-like genes can be identified by their ability to function substantially the same as the disclosed PKS-like genes; that is, they can be substituted for corresponding ORFs of Shewanella or Vibrio and still effectively produce EPA or DHA. Related PKS- like genes also can be identified by screening sequence databases for sequences homologous to the disclosed PKS-like genes, by hybridization of a probe based on the disclosed PKS-like genes to a library constructed from the source organism, or by RT- PCR using mRNA from the source organism and primers based on the disclosed PKS-like gene. Thus, the phrase "PKS-like genes" refers not only to the nucleotide sequences disclosed herein, but also to other nucleic acids that are allelic or species variants of these nucleotide sequences. It is also understood that these terms include nonnatural mutations introduced by deliberate mutation using recombinant technology such as single site mutation or by excising short sections of DNA open reading frames coding for PUFA enzymes or by substituting new codons or adding new codons. Such minor alterations substantially maintain the immunoidentity ofthe original expression product and/or its biological activity. The biological properties ofthe altered PUFA enzymes can be determined by expressing the enzymes in an appropriate cell line and by determining the ability ofthe enzymes to synthesize PUFAs. Particular enzyme modifications considered minor would include substitution of amino acids of similar chemical properties, e.g., glutamic acid for aspartic acid or glutamine for asparagine. When utilizing a PUFA PKS-like system from another organism, the regions of a PKS-like gene polypeptide important for PKS-like gene activity can be determined through routine mutagenesis, expression ofthe resulting mutant polypeptides and determination of their activities. The coding region for the mutants can include deletions, insertions and point mutations, or combinations thereof. A typical functional analysis begins with deletion mutagenesis to determine the N- and C-terminal limits ofthe protein necessary for function, and then internal deletions, insertions or point mutants are made in the open ready frame to further determine regions necessary for function. Other techniques such as cassette mutagenesis or total synthesis also can be used. Deletion mutagenesis is accomplished, for example, by using exonucleases to sequentially remove the 5' or 3' coding regions. Kits are available for such techniques. After deletion, the coding region is completed by ligating oligonucleotides containing start or stop codons to the deleted coding region after 5' or 3' deletion, respectively. Alternatively, oligonucleotides encoding start or stop codons are inserted into the coding region by a variety of methods including site-directed mutagenesis, mutagenic PCR or by ligation onto DNA digested at existing restriction sites. Internal deletions can similarly be made through a variety of methods including the use of existing restriction sites in the DNA, by use of mutagenic primers via site directed mutagenesis or mutagenic PCR. Insertions are made through methods such as linker-scanning mutagenesis, site-directed mutagenesis or mutagenic PCR. Point mutations are made through techniques such as site-directed mutagenesis or mutagenic PCR. Chemical mutagenesis also can be used for identifying regions of a PKS-like gene polypeptide important for activity. A mutated construct is expressed, and the ability of the resulting altered protein to function as a PKS-like gene is assayed. Such structure- function analysis can determine which regions may be deleted, which regions tolerate insertions, and which point mutations allow the mutant protein to function in substantially the same way as the native PKS-like gene. All such mutant proteins and nucleotide sequences encoding them are within the scope ofthe present invention. EPA is produced in Shewanella as the product of a PKS-like system, such that the EPA genes encode components of this system. In Vibrio, DHA is produced by a similar system. The enzymes which synthesize these fatty acids are encoded by a cluster of genes which are distinct from the fatty acid synthesis genes encoding the enzymes involved in synthesis of the C16 and C18 fatty acids typically found in bacteria and in plants. As the Shewanella EPA genes represent a PKS-like gene cluster, EPA production is, at least to some extent, independent ofthe typical bacterial type II FAS system. Thus, production of EPA in the cytoplasm of plant cells can be achieved by expression ofthe PKS-like pathway genes in plant cells under the control of appropriate plant regulatory signals. EPA production in E. coli transformed with the Shewanella EPA genes proceeds during anaerobic growth, indicating that O2-dependent desaturase reactions are not involved. Analyses ofthe proteins encoded by the ORFs essential for EPA production reveals the presence of domain structures characteristic of PKS-like systems. Fig. 2A shows a summary ofthe domains, motifs, and also key homologies detected by "BLAST" data bank searches. Because EPA is different from many ofthe other substances produced by PKS-like pathways, i.e., it contains 5, cis double bonds, spaced at 3 carbon intervals along the molecule, a PKS-like system for synthesis of EPA is not expected. Further, BLAST searches using the domains present in the Shewanella EPA ORFs reveal that several are related to proteins encoded by a PKS-like gene cluster found in Anabeana. The structure of that region ofthe Anabeana chromosome is shown in Fig. 2F. The Anabeana PKS-like genes have been linked to the synthesis of a long-chain (C26), hydroxy-fatty acid found in a glycolipid layer of heterocysts. The EPA protein domains with homology to the Anabeana proteins are indicated in Fig. 2F. ORF 6 of Shewanella contains a KAS domain which includes an active site motif (DXAC*) as well as a "GFGG" motif which is present at the end of many Type II KAS proteins (see Fig. 2A). Extended motifs are present but not shown here. Next is a malonyl-CoA:ACP acyl transferase (AT) domain. Sequences near the active site motif (GHS*XG) suggest it transfers malonate rather than methylmalonate, i.e., it resembles the acetate-like ATs. Following a linker region, there is a cluster of 6 repeating domains, each -100 amino acids in length, which are homologous to PKS-like ACP sequences. Each contains a pantetheine binding site motif (LGXDS*(L/I)). The presence of 6 such ACP domains has not been observed previously in fatty acid synthases (FAS) or PKS-like systems. Near the end ofthe protein is a region which shows homology to β-keto-ACP reductases (KR). It contains a pyridine nucleotide binding site motif "GXGXX(G/A/P)". The Shewanella ORF 8 begins with a KAS domain, including active site and ending motifs (Fig. 2C). The best match in the data banks is with the Anabeana HglD. There is also a domain which has sequence homology to the N- terminal one half of the Anabeana HglC. This region also shows weak homology to KAS proteins although it lacks the active site and ending motifs. It has the characteristics ofthe so-called chain length factors (CLF) of Type LI PKS-like systems. ORF 8 appears to direct the production of EPA versus DHA by the PKS-like system. ORF 8 also has two domains with homology to β-hydroxyacyl-ACP dehydrases (DH). The best match for both domains is with E. coli FabA, a bi-functional enzyme which carries out both the dehydrase reaction and an isomerization (trans to cis) ofthe resulting double bond. The first DH domain contains both the active site histidine (H) and an adjacent cysteine (C) implicated in FabA catalysis. The second DH domain has the active site H but lacks the adjacent C (Fig. 2C). Blast searches with the second DH domain also show matches to FabZ, a second E. coli DH, which does not possess isomerase activity. The N-terminal half of ORF 7 (Fig. 2B) has no significant matches in the data banks. The best match ofthe C-terminal half is with a C-terminal portion ofthe Anabeana HglC. This domain contains an acyl-transferase (AT) motif (GXSXG). Comparison ofthe extended active site sequences, based on the crystal structure ofthe E. coli malonyl-CoA:ACP AT, reveals that ORF 7 lacks two residues essential for exclusion of water from the active site (E. coli nomenclature; QI 1 and Rl 17). These data suggest that ORF 7 may function as a thioesterase. ORF 9 (Fig. 2D) is homologous to an ORF of unknown function in the Anabeana Hgl cluster. It also exhibits a very weak homology to NIFA, a regulatory protein in nitrogen fixing bacteria. A regulatory role for the ORF 9 protein has not been excluded. ORF 3 (Fig. 2Ε) is homologous to the Anabeana Hetl as well as EntD from E. coli and Sfp of Bacillus. Recently, a new enzyme family of phosphopantetheinyl transferases has been identified that includes Hetl, EntD and Sfp (Lamblot RH, et al. (1996) A new enzyme superfamily - the phophopantetheinyl transferases. Chemistry & Biology, Vol 3, #11, 923-936 ). The data of Fig. 3 demonstrates that the presence of ORF 3 is required for addition of β-alanine (i.e. pantetheine) to the ORF 6 protein. Thus, ORF 3 encodes the phosphopantetheinyl transferase specific for the ORF 6 ACP domains. (See, Haydock SF et al. (1995) Divergent sequence motifs correlated with the substrate specificity of (methyl)malonyl-CoA:acyl carrier protein transacylase domains in modular polyketide synthases, EEBS Lett., 374, 246-248). Malonate is the source ofthe carbons utilized in the extension reactions of ΕPA synthesis. Additionally, malonyl-CoA rather than malonyl-ACP is the AT substrate, i.e., the AT region of ORF 6 uses malonyl Co-A. Once the DNA sequences encoding the PKS-like genes of an organism responsible for PUFA production have been obtained, they are placed in a vector capable of replication in a host cell, or propagated in vitro by means of techniques such as PCR or long PCR. Replicating vectors can include plasmids, phage, viruses, cosmids and the like. Desirable vectors include those useful for mutagenesis ofthe gene of interest or for expression ofthe gene of interest in host cells. A PUFA synthesis enzyme or a homologous protein can be expressed in a variety of recombinantly engineered cells. Numerous expression systems are available for expression of DNA encoding a PUFA enzyme. The expression of natural or synthetic nucleic acids encoding PUFA enzyme is typically achieved by operably linking the DNA to a promoter (which is either constitutive or inducible) within an expression vector. By expression vector is meant a DNA molecule, linear or circular, that comprises a segment encoding a PUFA enzyme, operably linked to additional segments that provide for its transcription. Such additional segments include promoter and terminator sequences. An expression vector also may include one or more origins of replication, one or more selectable markers, an enhancer, a polyadenylation signal, etc. Expression vectors generally are derived from plasmid or viral DNA, and can contain elements of both. The term "operably linked" indicates that the segments are arranged so that they function in concert for their intended purposes, for example, transcription initiates in the promoter and proceeds through the coding segment to the terminator. See Sambrook et al, supra. The technique of long PCR has made in vitro propagation of large constructs possible, so that modifications to the gene of interest, such as mutagenesis or addition of expression signals, and propagation ofthe resulting constructs can occur entirely in vitro without the use of a replicating vector or a host cell. In vitro expression can be accomplished, for example, by placing the coding region for the desaturase polypeptide in an expression vector designed for in vitro use and adding rabbit reticulocyte lysate and cofactors; labeled amino acids can be incorporated if desired. Such in vitro expression vectors may provide some or all ofthe expression signals necessary in the system used. These methods are well known in the art and the components ofthe system are commercially available. The reaction mixture can then be assayed directly for PKS-like enzymes for example by determining their activity, or the synthesized enzyme can be purified and then assayed. Expression in a host cell can be accomplished in a transient or stable fashion. Transient expression can occur from introduced constructs which contain expression signals functional in the host cell, but which constructs do not replicate and rarely integrate in the host cell, or where the host cell is not proliferating. Transient expression also can be accomplished by inducing the activity of a regulatable promoter operably linked to the gene of interest, although such inducible systems frequently exhibit a low basal level of expression. Stable expression can be achieved by introduction of a nucleic acid construct that can integrate into the host genome or that autonomously replicates in the host cell. Stable expression ofthe gene of interest can be selected for through the use of a selectable marker located on or transfected with the expression construct, followed by selection for cells expressing the marker. When stable expression results from integration, integration of constructs can occur randomly within the host genome or can be targeted through the use of constructs containing regions of homology with the host genome sufficient to target recombination with the host locus. Where constructs are targeted to an endogenous locus, all or some ofthe transcriptional and translational regulatory regions can be provided by the endogenous locus. To achieve expression in a host cell, the transformed DNA is operably associated with transcriptional and translational initiation and termination regulatory regions that are functional in the host cell. Transcriptional and translational initiation and termination regions are derived from a variety of nonexclusive sources, including the DNA to be expressed, genes known or suspected to be capable of expression in the desired system, expression vectors, chemical synthesis The termination region can be derived from the 3' region ofthe gene from which the initiation region was obtained or from a different gene. A large number of termination regions are known to and have been found to be satisfactory in a variety of hosts from the same and different genera and species. The termination region usually is selected more as a matter of convenience rather than because of any particular property. When expressing more than one PKS-like ORF in the same cell, appropriate regulatory regions and expression methods should be used. Introduced genes can be propagated in the host cell through use of replicating vectors or by integration into the host genome. Where two or more genes are expressed from separate replicating vectors, it is desirable that each vector has a different means of replication. Each introduced construct, whether integrated or not, should have a different means of selection and should lack homology to the other constructs to maintain stable expression and prevent reassortment of elements among constructs. Judicious choices of regulatory regions, selection means and method of propagation ofthe introduced construct can be experimentally determined so that all introduced genes are expressed at the necessary levels to provide for synthesis ofthe desired products. A variety of procaryotic expression systems can be used to express PUFA enzyme. Expression vectors can be constructed which contain a promoter to direct transcription, a ribosome binding site, and a transcriptional terminator. Examples of regulatory regions suitable for this purpose in E. coli are the promoter and operator region ofthe E. coli tryptophan biosynthetic pathway as described by Yanofsky (1984) J. Bacteriol., 158:1018-1024 and the leftward promoter of phage lambda (Pλ) as described by Herskowitz and Hagen, (1980) Ann. Rev. Genet., 14:399-445. The inclusion of selection markers in DNA vectors transformed in E.coli is also useful. Examples of such markers include genes specifying resistance to ampicillin, tetracycline, or chloramphenicol. Vectors used for expressing foreign genes in bacterial hosts generally will contain a selectable marker, such as a gene for antibiotic resistance, and a promoter which functions in the host cell. Plasmids useful for transforming bacteria include pBR322 (Bolivar, et al, (1977) Gene 2:95-113), the pUC plasmids (Messing, (1983) Meth. Enzymol. 101 :20-77, Vieira and Messing, (1982) Gene 19:259-268), pCQV2 (Queen, ibid.), and derivatives thereof. Plasmids may contain both viral and bacterial elements. Methods for the recovery ofthe proteins in biologically active form are discussed in U.S. Patent Nos. 4,966,963 and 4,999,422, which are incorporated herein by reference. See Sambrook, et al for a description of other prokaryotic expression systems. For expression in eukaryotes, host cells for use in practicing the present invention include mammalian, avian, plant, insect, and fungal cells. As an example, for plants, the choice of a promoter will depend in part upon whether constitutive or inducible expression is desired and whether it is desirable to produce the PUFAs at a particular stage of plant development and/or in a particular tissue. Considerations for choosing a specific tissue and/or developmental stage for expression ofthe ORFs may depend on competing substrates or the ability ofthe host cell to tolerate expression of a particular PUFA. Expression can be targeted to a particular location within a host plant such as seed, leaves, fruits, flowers, and roots, by using specific regulatory sequences, such as those described in USPN 5,463,174, USPN 4,943,674, USPN 5,106,739, USPN 5,175,095, USPN 5,420,034, USPN 5,188,958, and USPN 5,589,379. Where the host cell is a yeast, transcription and translational regions functional in yeast cells are provided, particularly from the host species. The transcriptional initiation regulatory regions can be obtained, for example from genes in the glycolytic pathway, such as alcohol dehydrogenase, glyceraldehyde-3 -phosphate dehydrogenase (GPD), phosphoglucoisomerase, phosphoglycerate kinase, etc. or regulatable genes such as acid phosphatase, lactase, metallothionein, glucoamylase, etc. Any one of a number of regulatory sequences can be used in a particular situation, depending upon whether constitutive or induced transcription is desired, the particular efficiency ofthe promoter in conjunction with the open-reading frame of interest, the ability to join a strong promoter with a control region from a different promoter which allows for inducible transcription, ease of construction, and the like. Of particular interest are promoters which are activated in the presence of galactose. Galactose-inducible promoters (GAL1, GAL7, and GAL10) have been extensively utilized for high level and regulated expression of protein in yeast (Lue et al, (1987) Mol. Cell. Biol. 7:3446; Johnston, (1987) Microbiol. Rev. 51 :458). Transcription from the GAL promoters is activated by the GAL4 protein, which binds to the promoter region and activates transcription when galactose is present. In the absence of galactose, the antagonist GAL80 binds to GAL4 and prevents GAL4 from activating transcription. Addition of galactose prevents GAL80 from inhibiting activation by GAL4. Preferably, the termination region is derived from a yeast gene, particularly Saccharomyces, Schizosaccharomyces, Candida or Kluyveromyces. The 3' regions of two mammalian genes, γ interferon and α2 interferon, are also known to function in yeast. Nucleotide sequences surrounding the translational initiation codon ATG have been found to affect expression in yeast cells. If the desired polypeptide is poorly expressed in yeast, the nucleotide sequences of exogenous genes can be modified to include an efficient yeast translation initiation sequence to obtain optimal gene expression. For expression in Saccharomyces, this can be done by site-directed mutagenesis of an inefficiently expressed gene by fusing it in-frame to an endogenous Saccharomyces gene, preferably a highly expressed gene, such as the lactase gene. As an alternative to expressing the PKS-like genes in the plant cell cytoplasm, is to target the enzymes to the chloroplast. One method to target proteins to the chloroplast entails use of leader peptides attached to the N-termini ofthe proteins. Commonly used leader peptides are derived from the small subunit of plant ribulose bis phosphate carboxylase. Leader sequences from other chloroplast proteins may also be used. Another method for targeting proteins to the chloroplast is to transform the chloroplast genome (Stable transformation of chloroplasts of Chlamydomonas reinhardtii (1 green alga) using bombardment of recipient cells with high- velocity tungsten microprojectiles coated with foreign DNA has been described. See, for example, Blowers et al Plant Cell (1989) 7:123-132 and Debuchy et al EMBO J (1989) 5:2803-2809. The transformation technique, using tungsten microprojectiles, is described by Kline et al, Nature (London) (1987) 327:70-73). The most common method of transforming chloroplasts involves using biolistic techniques, but other techniques developed for the purpose may also be used. (Methods for targeting foreign gene products into chloroplasts (Shrier et al EMBO J. (1985) 4:25-3>2) or mitochnodria (Boutry et al, supra) have been described. See also Tomai et al Gen. Biol. Chem. (1988) 253:15104-15109 and US Patent No. 4,940,835 for the use of transit peptides for translocating nuclear gene products into the chloroplast. Methods for directing the transport of proteins to the chloroplast are reviewed in Kenauf TIBTECH( 1987) 5:40-47. For producing PUFAs in avian species and cells, gene transfer can be performed by introducing a nucleic acid sequence encoding a PUFA enzyme into the cells following procedures known in the art. If a transgenic animal is desired, pluripotent stem cells of embryos can be provided with a vector carrying a PUFA enzyme encoding transgene and developed into adult animal (USPN 5,162,215 ; Ono et al. ( 1996) Comparative Biochemistry and Physiology A 773(3):287-292; WO 9612793; WO 9606160). In most cases, the transgene is modified to express high levels ofthe PKS-like enzymes in order to increase production of PUFAs. The transgenes can be modified, for example, by providing transcriptional and/or translational regulatory regions that function in avian cells, such as promoters which direct expression in particular tissues and egg parts such as yolk. The gene regulatory regions can be obtained from a variety of sources, including chicken anemia or avian leukosis viruses or avian genes such as a chicken ovalbumin gene. Production of PUFAs in insect cells can be conducted using baculovirus expression vectors harboring PKS-like transgenes. Baculovirus expression vectors are available from several commercial sources such as Clonetech. Methods for producing hybrid and transgenic strains of algae, such as marine algae, which contain and express a desaturase transgene also are provided. For example, transgenic marine algae can be prepared as described in USPN 5,426,040. As with the other expression systems described above, the timing, extent of expression and activity ofthe desaturase transgene can be regulated by fitting the polypeptide coding sequence with the appropriate transcriptional and translational regulatory regions selected for a particular use. Of particular interest are promoter regions which can be induced under preselected growth conditions. For example, introduction of temperature sensitive and/or metabolite responsive mutations into the desaturase transgene coding sequences, its regulatory regions, and/or the genome of cells into which the transgene is introduced can be used for this purpose. The transformed host cell is grown under appropriate conditions adapted for a desired end result. For host cells grown in culture, the conditions are typically optimized to produce the greatest or most economical yield of PUFAs, which relates to the selected desaturase activity. Media conditions which may be optimized include: carbon source, nitrogen source, addition of substrate, final concentration of added substrate, form of substrate added, aerobic or anaerobic growth, growth temperature, inducing agent, induction temperature, growth phase at induction, growth phase at harvest, pH, density, and maintenance of selection. Microorganisms such as yeast, for example, are preferably grown using selected media of interest, which include yeast peptone broth (YPD) and minimal media (contains amino acids, yeast nitrogen base, and ammonium sulfate, and lacks a component for selection, for example uracil). Desirably, substrates to be added are first dissolved in ethanol. Where necessary, expression ofthe polypeptide of interest may be induced, for example by including or adding galactose to induce expression from a GAL promoter. When increased expression ofthe PKS-like gene polypeptide in a host cell which expresses PUFA from a PKS-like system is desired, several methods can be employed. Additional genes encoding the PKS-like gene polypeptide can be introduced into the host organism. Expression from the native PKS-like gene locus also can be increased through homologous recombination, for example by inserting a stronger promoter into the host genome to cause increased expression, by removing destabilizing sequences from either the mRNA or the encoded protein by deleting that information from the host genome, or by adding stabilizing sequences to the mRNA (see USPN 4,910,141 and USPN 5,500,365). Thus, the subject host will have at least have one copy ofthe expression construct and may have two or more, depending upon whether the gene is integrated into the genome, amplified, or is present on an extrachromosomal element having multiple copy numbers. Where the subject host is a yeast, four principal types of yeast plasmid vectors can be used: Yeast Integrating plasmids (Yips), Yeast Replicating plasmids (YRps), Yeast Centromere plasmids (YCps), and Yeast Episomal plasmids (YEps). Yips lack a yeast replication origin and must be propagated as integrated elements in the yeast genome. YRps have a chromosomally derived autonomously replicating sequence and are propagated as medium copy number (20 to 40), autonomously replicating, unstably segregating plasmids. YCps have both a replication origin and a centromere sequence and propagate as low copy number (10-20), autonomously replicating, stably segregating plasmids. YEps have an origin of replication from the yeast 2μm plasmid and are propagated as high copy number, autonomously replicating, irregularly segregating plasmids. The presence ofthe plasmids in yeast can be ensured by maintaining selection for a marker on the plasmid. Of particular interest are the yeast vectors pYES2 (a YEp plasmid available from Invitrogen, confers uracil prototrophy and a GAL1 galactose- inducible promoter for expression), and pYX424 (a YEp plasmid having a constitutive TP1 promoter and conferring leucine prototrophy; (Alber and Kawasaki (1982). J. Mol. & Appl. Genetics 1 : 419). The choice of a host cell is influenced in part by the desired PUFA profile ofthe transgenic cell, and the native profile ofthe host cell. Even where the host cell expresses PKS-like gene activity for one PUFA, expression of PKS-like genes of another PKS-like system can provide for production of a novel PUFA not produced by the host cell. In particular instances where expression of PKS-like gene activity is coupled with expression of an ORF 8 PKS-like gene of an organism which produces a different PUFA, it can be desirable that the host cell naturally have, or be mutated to have, low PKS-like gene activity for ORF 8. As an example, for production of EPA, the DNA sequence used encodes the polypeptide having PKS-like gene activity of an organism which produces EPA, while for production of DHA, the DNA sequences used are those from an organism which produces DHA. For use in a host cell which already expresses PKS-like gene activity it can be necessary to utilize an expression cassette which provides for overexpression ofthe desired PKS-like genes alone or with a construct to downregulate the activity of an existing ORF ofthe existing PKS-like system, such as by antisense or co-suppression. Similarly, a combination of ORFs derived from separate organisms which produce the same or different PUFAs using PKS-like systems may be used. For instance, the ORF 8 of Vibrio directs the expression of DHA in a host cell, even when ORFs 3, 6, 7 and 9 are from Shewanella, which produce EPA when coupled to ORF 8 of Shewanella. Therefore, for production of eicosapentanoic acid (EPA), the expression cassettes used generally include one or more cassettes which include ORFs 3, 6, 7, 8 and 9 from a PUFA-producing organism such as the marine bacterium Shewanella putrefaciens (for EPA production) or Vibrio marinus (for DHA production). ORF 8 can be used for induction of DHA production, and ORF 8 of Vibrio can be used in conjunction with ORFs 3, 6, 7 and 9 of Shewanella to produce DHA. The organization and numbering scheme ofthe ORFs identified in the Shewanella gene cluster are shown in Fig 1 A. Maps of several subclones referred to in this study are shown in Fig IB. For expression of a PKS-like gene polypeptide, transcriptional and translational initiation and termination regions functional in the host cell are operably linked to the DNA encoding the PKS-like gene polypeptide. Constructs comprising the PKS-like ORFs of interest can be introduced into a host cell by any of a variety of standard techniques, depending in part upon the type of host cell. These techniques include transfection, infection, holistic impact, electroporation, microinjection, scraping, or any other method which introduces the gene of interest into the host cell (see USPN 4,743,548, USPN 4,795,855, USPN 5,068,193, USPN 5,188,958, USPN 5,463,174, USPN 5,565,346 and USPN 5,565,347). Methods of transformation which are used include lithium acetate transformation (Methods in Enzymology, (1991) 194:186-187). For convenience, a host cell which has been manipulated by any method to take up a DNA sequence or construct will be referred to as "transformed" or "recombinant" herein. The subject host will have at least have one copy ofthe expression construct and may have two or more, depending upon whether the gene is integrated into the genome, amplified, or is present on an extrachromosomal element having multiple copy numbers. For production of PUFAs, depending upon the host cell, the several polypeptides produced by pEPA, ORFs 3, 6, 7, 8 and 9, are introduced as individual expression constructs or can be combined into two or more cassettes which are introduced individually or co-transformed into a host cell. A standard transformation protocol is used. For plants, where less than all PKS-like genes required for PUFA synthesis have been inserted into a single plant, plants containing a complementing gene or genes can be crossed to obtain plants containing a full complement of PKS-like genes to synthesize a desired PUFA. The PKS-like-mediated production of PUFAs can be performed in either prokaryotic or eukaryotic host cells. The cells can be cultured or formed as part or all of a host organism including an animal. Viruses and bacteriophage also can be used with appropriate cells in the production of PUFAs, particularly for gene transfer, cellular targeting and selection. Any type of plant cell can be used for host cells, including dicotyledonous plants, monocotyledonous plants, and cereals. Of particular interest are crop plants such as Brassica, Arabidopsis, soybean, corn, and the like. Prokaryotic cells of interest include Eschericia, Baccillus, Lactobaccillus, cyanobacteria and the like. Eukaryotic cells include plant cells, mammalian cells such as those of lactating animals, avian cells such as of chickens, and other cells amenable to genetic manipulation including insect, fungal, and algae cells. Examples of host animals include mice, rats, rabbits, chickens, quail, turkeys, 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 ofthe gene regulatory regions. Of particular interest is the production of PUFAs in the breast milk ofthe host animal. Examples of host microorganisms include Saccharomyces cerevisiae, Saccharomyces carlsbergensis, or other yeast such as Candida, Kluyveromyces or other fungi, for example, filamentous fungi such as Aspergillus, Neurospora, Penicillium, etc. Desirable characteristics of a host microorganism are, for example, that it is genetically well characterized, can be used for high level expression ofthe product using ultra-high density fermentation, and is on the GRAS (generally recognized as safe) list since the proposed end product is intended for ingestion by humans. Of particular interest is use of a yeast, more particularly baker's yeast (S. cerevisiae), as a cell host in the subject invention. Strains of particular interest are SC334 (Mat α pep4-3 prbl-1122 ura3-52 leu2- 3, 112 regl-501 gall; (Hovland et al (1989) Gene 83:57-64); BJ1995 (Yeast Genetic Stock Centre, 1021 Donner Laboratory, Berkeley, CA 94720), INVSC1 (Mat hiw3Δl leu2 trpl-289 ura3-52 (Invitrogen, 1600 Faraday Ave., Carlsbad, CA 92008) and INVSC2 (Mat α his3Δ200 ura3-167; (Invitrogen). Bacterial cells also may be used as hosts. This includes E. coli, which can be useful in fermentation processes. Alternatively, a host such as a Lactobacillus species can be used as a host for introducing the products ofthe PKS- like pathway into a product such as yogurt. The transformed host cell can be identified by selection for a marker contained on the introduced construct. Alternatively, a separate marker construct can be introduced with the desired construct, as many transformation techniques introduce multiple DNA molecules into host cells. Typically, transformed hosts are selected for their ability to grow on selective media. Selective media can incorporate an antibiotic or lack a factor necessary for growth ofthe untransformed host, such as a nutrient or growth factor. An introduced marker gene therefor may confer antibiotic resistance, or encode an essential growth factor or enzyme, and permit growth on selective media when expressed in the transformed host cell. Desirably, resistance to kanamycin and the amino glycoside G418 are of particular interest (see USPN 5,034,322). For yeast transformants, any marker that functions in yeast can be used, such as the ability to grow on media lacking uracil, lencine, lysine or tryptophan. Selection of a transformed host also can occur when the expressed marker protein can be detected, either directly or indirectly. The marker protein can be expressed alone or as a fusion to another protein. The marker protein can be one which is detected by its enzymatic activity; for example β-galactosidase can convert the substrate X-gal to a colored product, and luciferase can convert luciferin to a light-emitting product. The marker protein can be one which is detected by its light-producing or modifying characteristics; for example, the green fluorescent protein of Aequorea victoria fluoresces when illuminated with blue light. Antibodies can be used to detect the marker protein or a molecular tag on, for example, a protein of interest. Cells expressing the marker protein or tag can be selected, for example, visually, or by techniques such as FACS or panning using antibodies. The PUFAs produced using the subject methods and compositions are found in the host plant tissue and/or plant part as free fatty acids and/or in conjugated forms such as acylglycerols, phospholipids, sulfolipids or glycolipids, and can be extracted from the host cell through a variety of means well-known in the art. Such means include extraction with organic solvents, sonication, supercritical fluid extraction using for example carbon dioxide, and physical means such as presses, or combinations thereof. Of particular interest is extraction with methanol and chloroform. Where appropriate, the aqueous layer can be acidified to protonate negatively charged moieties and thereby increase partitioning of desired products into the organic layer. After extraction, the organic solvents can be removed by evaporation under a stream of nitrogen. When isolated in conjugated forms, the products are enzymatically or chemically cleaved to release the free fatty acid or a less complex conjugate of interest, and are then subjected to further manipulations to produce a desired end product. Desirably, conjugated forms of fatty acids are cleaved with potassium hydroxide. If further purification is necessary, standard methods can be employed. Such methods include extraction, treatment with urea, fractional crystallization, HPLC, fractional distillation, silica gel chromatography, high speed centrifugation or distillation, or combinations of these techniques. Protection of reactive groups, such as the acid or alkenyl groups, can be done at any step through known techniques, for example alkylation or iodination. Methods used include methylation ofthe fatty acids to produce methyl esters. Similarly, protecting groups can be removed at any step. Desirably, purification of fractions containing DHA and EPA is accomplished by treatment with urea and/or fractional distillation. The uses ofthe subject invention are several. Probes based on the DNAs ofthe present invention find use in methods for isolating related molecules or in methods to detect organisms expressing PKS-like genes. When used as probes, the DNAs or oligonucleotides need to be detectable. This is usually accomplished by attaching a label either at an internal site, for example via incorporation of a modified residue, or at the 5 ' or 3' terminus. Such labels can be directly detectable, can bind to a secondary molecule that is detectably labeled, or can bind to an unlabelled secondary molecule and a detectably labeled tertiary molecule; this process can be extended as long as is practicable to achieve a satisfactorily detectable signal without unacceptable levels of background signal. Secondary, tertiary, or bridging systems can include use of antibodies directed against any other molecule, including labels or other antibodies, or can involve any molecules which bind to each other, for example a biotin-streptavidin/avidin system. Detectable labels typically include radioactive isotopes, molecules which chemically or enzymatically produce or alter light, enzymes which produce detectable reaction products, magnetic molecules, fluorescent molecules or molecules whose fluorescence or light- emitting characteristics change upon binding. Examples of labelling methods can be found in USPN 5,011,770. Alternatively, the binding of target molecules can be directly detected by measuring the change in heat of solution on binding of a probe to a target via isothermal titration calorimetry, or by coating the probe or target on a surface and detecting the change in scattering of light from the surface produced by binding of a target or a probe, respectively, is done with the BIAcore system. PUFAs produced by recombinant means find applications in a wide variety of areas. Supplementation of humans or animals with PUFAs in various forms can result in increased levels not only ofthe added PUFAs, but of their metabolic progeny as well. Complex regulatory mechanisms can make it desirable to combine various PUFAs, or to add different conjugates of PUFAs, in order to prevent, control or overcome such mechanisms to achieve the desired levels of specific PUFAs in an individual. In the present case, expression of PKS-like gene genes, or antisense PKS-like gene transcripts, can alter the levels of specific PUFAs, or derivatives thereof, found in plant parts and/or plant tissues. The PKS-like gene polypeptide coding region is expressed either by itself or with other genes, in order to produce tissues and/or plant parts containing higher proportions of desired PUFAs or containing a PUFA composition which more closely resembles that of human breast milk (Prieto et al, PCT publication WO 95/24494) than does the unmodified tissues and/or plant parts. PUFAs, or derivatives thereof, made by the disclosed method can be used as dietary supplements for patients undergoing intravenous feeding or for preventing or treating malnutrition. For dietary supplementation, the purified PUFAs, or derivatives thereof, can be incorporated into cooking oils, fats or margarines formulated so that in normal use the recipient receives a desired amount of PUFA. The PUFAs also can be incorporated into infant formulas, nutritional supplements or other food products, and find use as anti-inflammatory or cholesterol lowering agents. Particular fatty acids such as EPA can be used to alter the composition of infant formulas to better replicate the PUFA composition of human breast milk. The predominant triglyceride in human milk is reported to be 1 ,3-di-oleoyl-2-palmitoyl, with 2-palmitoyl glycerides reported as better absorbed than 2-oleoyl or 2-lineoyl glycerides (see USPN 4,876,107). Typically, human breast milk has a fatty acid profile comprising from about 0.15 % to about 0.36 % as DHA, from about 0.03 % to about 0.13 % as EPA, from about 0.30 % to about 0.88 % as ARA, from about 0.22 % to about 0.67 % as DGLA, and from about 0.27 % to about 1.04 % as GLA. A preferred ratio of GLA:DGLA:ARA in infant formulas is from about 1 : 1 :4 to about 1 :1 :1, respectively. Amounts of oils providing these ratios of PUFA can be determined without undue experimentation by one of skill in the art. PUFAs, or host cells containing them, also can be used as animal food supplements to alter an animal's tissue or milk fatty acid composition to one more desirable for human or animal consumption. For pharmaceutical use (human or veterinary), the compositions generally are administered orally but can be administered by any route by which they may be successfully absorbed, e.g., parenterally (i.e. subcutaneously, intramuscularly or intravenously), rectally or vaginally or topically, for example, as a skin ointment or lotion. Where available, gelatin capsules are the preferred form of oral administration. Dietary supplementation as set forth above also can provide an oral route of administration. The unsaturated acids of the present invention can be administered in conjugated forms, or as salts, esters, amides or prodrugs ofthe fatty acids. Any pharmaceutically acceptable salt is encompassed by the present invention; especially preferred are the sodium, potassium or lithium salts. Also encompassed are the N-alkylpolyhydroxamine salts, such as N- methyl glucamine, described in PCT publication WO 96/33155. Preferred esters are the ethyl esters. The PUFAs ofthe present invention can be administered alone or in combination with a pharmaceutically acceptable carrier or excipient. As solid salts, the PUFAs can also be administered in tablet form. For intravenous administration, the PUFAs or derivatives thereof can be incorporated into commercial formulations such as Intralipids. Where desired, the individual components of formulations can be individually provided in kit form, for single or multiple use. A typical dosage of a particular fatty acid is from 0J mg to 20 g, or even 100 g daily, and is preferably from 10 mg to 1, 2, 5 or 10 g daily as required, or molar equivalent amounts of derivative forms thereof. Parenteral nutrition compositions comprising from about 2 to about 30 weight percent fatty acids calculated as triglycerides are encompassed by the present invention. Other vitamins, and particularly fat-soluble vitamins such as vitamin A, D, E and L-carnitine optionally can be included. Where desired, a preservative such as a tocopherol can be added, typically at about 0.1% by weight. The following examples are presented by way of illustration, not of limitation. EXAMPLES Example 1 The Identity of ORFs Derived from Vibrio marinus Using polymerase chain reaction (PCR) with primers based on ORF 6 of Shewanella (Sp ORF 6) sequences (FW 5' primers CUACUACUACUACCAAGCT AAAGCACTTAACCGTG, and CUACUACUACUAACAGCGAAATGCTTATCAAG for Vibrio and SS9 respectively and 3' BW primers: CAUCAUCAUCAUGCGACC AAAACCAAATGAGCTAATAC for both Vibrio and SS9) and genomic DNAs templates from Vibrio and a borophyllic photobacter producing EPA (provided by Dr. Bartlett, UC San Diego), resulted in PCR products of cα.400 bases for Vibrio marinus (Vibrio) and cα.900 bases for SS9 presenting more than 75% homology with corresponding fragments of Sp ORF 6 (see Figure 25) as determined by direct counting of homologous amino acids. A Vibrio cosmid library was then prepared and using the Vibrio ORF 6 PCR product as a probe (see Figure 26); clones containing at least ORF 6 were selected by colony hybridization. Through additional sequences ofthe selected cosmids such as cosmid #9 and cosmid #21, a Vibrio cluster (Figure 5) with ORFs homologous to, and organized in the same sequential order (ORFs 6-9) as ORFs 6-9 of Shewanella, was obtained (Figure 7). The Vibrio ORFs from this sequence are found at 17394 to 361 15 and comprehend ORFs 6-9. Table Vibrio operon figures 17394 to 25349 length = 7956 nt 25509 to 28157 length = 2649 nt 28209 to 34262 length = 6054 nt 34454 to 36115 length = 1662 nt The ORF designations for the Shewanella genes are based on those disclosed in Figure 4, and differ from those published for the Shewanella cluster (Yazawa et al, USPN 5,683,898). For instance, ORF 3 of Figure 4 is read in the opposite direction from the other ORFs and is not disclosed in Yazawa et al USPN 5,683,898 (See Fig. 24) for comparison with Yazawa et al USPN 5,683,898). Sequences homologous to ORF 3, were not found in the proximity of ORF 6 (17000 bases upstream of ORF 6) or of ORF 9 (CΩ.4000 bases downstream of ORF 9). Motifs characteristic of phosphopantethenyl transferases (Lambalot et al (1996) Current Biology 3:923-936) were absent from the Vibrio sequences screened for these motifs. In addition, there was no match to Sp ORF 3 derived probes in genomic digests of Vibrio and of SC2A Shewanella (another bacterium provided by the University of San Diego and also capable of producing EPA). Although ORF 3 may exist in Vibrio, its DNA may not be homologous to that of Sp ORF 3 and/or could be located in portions ofthe genome that were not sequenced. Figure 6 provides the sequence of an approximately 19 kb Vibrio clone comprising ORFs 6-9. Figures 7 and 8 compare the gene cluster organizations ofthe PKS-like systems of Vibrio marinus and Shewanella putrefacians. Figures 9 through 12 show the levels of sequence homology between the corresponding ORFs 6, 7, 8 and 9, respectively. Example 2 ORF 8 Directs DHA Production As described in example 1 , DNA homologous to Sp ORF 6 was found in an unrelated species, SS9 Photobacter, which also is capable of producing EPA. Additionally, ORFs homologous to Sp ORF 6-9 were found in the DHA producing Vbrio marinus (Vibrio). From these ORFs a series of experiments was designed in which deletions in each of Sp ORFs 6-9 that suppressed EPA synthesis in E. coli (Yazawa (1996) supra) were complemented by the corresponding homologous genes from Vibrio. The Sp EPA cluster was used to determine if any ofthe Vibrio ORFs 6-9 was responsible for the production of DHA. Deletion mutants provided for each ofthe Sp ORFs are EPA and DHA null. Each deletion was then complemented by the corresponding Vibrio ORF expressed behind a lac promoter (Figure 13). The complementation of a Sp ORF 6 deletion by a Vibrio ORF 6 reestablished the production of EPA. Similar results were obtained by complementing the Sp ORF 7 and ORF 9 deletions. By contrast, the complementation of a Sp ORF 8 deletion resulted in the production of C22:6. Vibrio ORF 8 therefore appears to be a key element in the synthesis of DHA. Figures 14 and 15 show chromatograms of fatty acid profiles from the respective complementations of Sp del ORF 6 with Vibrio ORF 6 (EPA and no DHA) and Sp del ORF 8 with Vibrio ORF 8 (DHA). Figure 16 shows the fatty acid percentages for the ORF 8 complementation, again demonstrating that ORF 8 is responsible for DHA production. These data show that polyketide-like synthesis genes with related or similar ORFs can be combined and expressed in a heterologous system and used to produce a distinct PUFA species in the host system, and that ORF 8 has a role in determining the ultimate chain length. The Vibrio ORFs 6, 7, 8, and 9 reestablish EPA synthesis. In the case of Vibrio ORF 8, DHA is also present (ca. 0.7%) along with EPA (ca. 0.6%) indicating that this gene plays a significant role in directing synthesis of DHA vs EPA for these systems. Example 3 Requirements for Production of DHA To determine how Vibrio ORFs ofthe cluster ORF 6-9 are used in combination with Vibrio ORF 8, some combinations of Vibrio ORF 8 with some or all ofthe other Vibrio ORFS 6-9 cluster were created to explain the synthesis of DHA. Vibrio ORFs 6-9 were complemented with Sp ORF 3. The results of this complementation are presented in Figures 16b and 16c. The significant amounts of DHA measured (greater than about 9%) and the absence of EPA suggest that no ORFs other than those of Vibrio ORFs 6-9 are required for DHA synthesis when combined with Sp ORF 3. This suggests that Sp ORF 3 plays a general function in the synthesis of bacterial PUFAs. With respect to the DHA vs EPA production, it may be necessary to combine Vibrio ORF 8 with other Vibrio ORFs ofthe 6-9 cluster in order to specifically produce DHA. The roles of Vibrio ORF 9 and each ofthe combinations of Vibrio ORFs (6,8), (7, 8), (8, 9), etc in the synthesis of DHA are being studied. Example 4 Plant Expression Constructs A cloning vector with very few restriction sites was designed to facilitate the cloning of large fragments and their subsequent manipulation. An adapter was assembled by annealing oligonucleotides with the sequences AAGCCCGGGCTT and GTACAAGCCCGGGCTTAGCT. This adapter was ligated to the vector pBluescript LI SK+ (Stratagene) after digestion ofthe vector with the restriction endonucleases Asp7\% and Sstl. The resulting vector, pCGN7769 had a single Srfl (and embedded Smal) cloning site for the cloning of blunt ended DNA fragments. A plasmid containing the napin cassette from pCGN3223, (USPN 5,639,790) was modified to make it more useful for cloning large DNA fragments containing multiple restriction sites, and to allow the cloning of multiple napin fusion genes into plant binary transformation vectors. An adapter comprised ofthe self annealed ohgonucleotide of sequence CGCGATTTAAATGGCGCGCCCTGCAGGCGGCCGCCTGCAGGGCGC GCCATTTAAAT was ligated into the vector pBC SK+ (Stratagene) after digestion ofthe vector with the restriction endonuclease TfosHLI to construct vector pCGN7765. Plamids pCGN3223 and pCGN7765 were digested with Nøtl and ligated together. The resultant vector, pCGΝ7770 (Figure 17), contains the pCGN7765 backbone and the napin seed specific expression cassette from pCGN3223. Shewanella constructs Genes encoding the Shewanella proteins were mutagenized to introduce suitable cloning sites 5' and 3' ORFs using PCR. The template for the PCR reactions was DNA of the cosmid pEPA (Yazawa et al, supra). PCR reactions were performed using Pfu DNA polymerase according to the manufacturers' protocols. The PCR products were cloned into Srfi digested pCGN7769. The primers CTGCAGCTCGAGACAATGTTGATT TCCTTATACTTCTGTCC and GGATCCAGATCTCTAGCTAGTCTTAGCTGAAGC TCGA were used to amplify ORF 3, and to generate plasmid pCGN8520. The primers TCTAGACTCGAGACAATGAGCCAGACCTCTAAACCTACA and CCCGGGCTC GAGCTAATTCGCCTCACTGTCGTTTGCT were used to amplify ORF 6, and generate plasmid pCGN7776. The primers GAATTCCTCGAGACAATGCCGCTGCGCATCG CACTTATC and GGTACCAGATCTTTAGACTTCCCCTTGAAGTAAATGG were used to amplify ORF 7, and generate plasmid pCGN7771. The primers GAATTCGTCG ACACAATGTCATTACCAGACAATGCTTCT and TCTAGAGTCGACTTATAC AGATTCTTCGATGCTGATAG were used to amplify ORF 8, and generate plasmid pCGN7775. The primers GAATTCGTCGACACAATGAATCCTACAGCAA CTAACGAA and TCTAGAGGATCCTTAGGCCATTCTTTGGTTTGGCTTC were used to amplify ORF 9, and generate plasmid pCGN7773. The integrity ofthe PCR products was verified by DNA sequencing ofthe inserts of pCGN7771, PCGN8520, and pCGN7773. ORF 6 and ORF 8 were quite large in size. In order to avoid sequencing the entire clones, the center portions ofthe ORFs were replaced with restriction fragments of pEPA. The 6.6 kilobase PacVBamHl fragment of pEPA containing the central portion of ORF 6 was ligated into PacVBamHl digested pCGN7776 to yield pCGN7776B4. The 4.4 kilobase BamHUBglΩ. fragment of pEPA containing the central portion of ORF 8 was ligated into BamHUBglH digested pCGN7775 to yield pCGN7775A. The regions flanking the pEPA fragment and the cloning junctions were verified by DNA sequencing. Plasmid pCGN7771 was cut withΛTzoI and BglU and ligated to pCGN7770 after digestion with Sail and BglLI. The resultant napin/ORF 7 gene fusion plasmid was designated pCGN7783. Plasmid pCGN8520 was cut with Xhol and BglU and ligated to pCGN7770 after digestion with Sail and BglU. The resultant napin ORF 3 gene fusion plasmid was designated pCGN8528. Plasmid pCGN7773 was cut with Sail and BamHl and ligated to pCGN7770 after digestion with Sail and BglU. The resultant napin/ORF 9 gene fusion plasmid was designated pCGN7785. Plasmid pCGN7775A was cut with Sail and ligated to pCGN7770 after digestion with Sail. The resultant napin ORF 8 gene fusion plasmid was designated pCGN7782. Plasmid pCGN7776B4 was cut with Xhol and ligated to pCGN7770 after digestion with Sail. The resultant napin/ORF 6 gene fusion plasmid was designated pCGN7786B4. A binary vector for plant transformation, pCGN5139, was constructed from pCGN1558 (McBride and Summerfelt (1990) Plant Molecular Biology, 14:269-276). The polylinker of pCGN1558 was replaced as a HinάHUAsp7\ i\ fragment with a polylinker containing unique restriction endonuclease sites, Ascl, Pad, Xbal, Swal, BamHl, dNotl. The Asp7\S and HindUl restriction endonuclease sites are retained in pCGN5139. PCGN5139 was digested with NotI and ligated with NotI digested pCGΝ7786B4. The resultant binary vector containing the napin/ORF 6 gene fusion was designated ρCGN8533. Plasmid pCGN8533 was digested with Sse8387I and ligated with Sse8387I digested pCGN7782. The resultant binary vector containing the napin/ORF 6 gene fusion and the napin ORF 8 gene fusion was designated pCGN8535 (Figure 18). The plant binary transformation vector, pCGN5139, was digested with Asp718 and ligated with Asp7\ 8 digested pCGN8528. The resultant binary vector containing the napin/ORF 3 gene fusion was designated pCGN8532. Plasmid pCGN8532 was digested with NotI and ligated with NotI digested pCGΝ7783. The resultant binary vector containing the napin/ORF 3 gene fusion and the napin/ORF 7 gene fusion was designated pCGN8534. Plasmid pCGN8534 was digested with Sse8387I and ligated with S?e8387I digested pCGN7785. The resultant binary vector containing the napin/ORF 3 gene fusion, the napin/ORF 7 gene fusion and the napin/ORF 9 gene fusion was designated pCGN8537 (Figure 19). Vibrio constructs The Vibrio ORFs for plant expression were all obtained using Vibrio cosmid #9 as a starting molecule. Vibrio cosmid #9 was one ofthe cosmids isolated from the Vibrio cosmid library using the Vibrio ORF 6 PCR product described in Example 1. A gene encoding Vibrio ORF 7 (Figure 6) was mutagenized to introduce a Sail site upstream ofthe open reading frame and BamHl site downstream ofthe open reading frame using the PCR primers: TCTAGAGTCGACACAATGGCGGAATTAGCTG TTATTGGT and GTCGACGGATCCCTATTTGTTCGTGTTTGCTATATG. A gene encoding Vibrio ORF 9 (Figure 6) was mutagenized to introduce a BamHl site upstream ofthe open reading frame and XhoHl site downstream ofthe open reading frame using the PCR primers: GTCGACGGATCCACAATGAATATAGTAAGTAATCATTCGGCA and GTCGACCTCGAGTTAATCACTCGTACGATAACTTGCC. The restriction sites were introduced using PCR, and the integrity ofthe mutagenized plasmids was verified by DNA sequence. The Vibrio ORF 7 gene was cloned as a SaR-BamHl fragment into the napin cassette of Sal-BgR digested pCGN7770 (Figure 17) to yield pCGN8539. The Vibrio ORF 9 gene was cloned as a Sall-BamHl fragment into the napin cassette of Sal- Ball digested pCGN7770 (Figure 17) to yield pCGN8543. Genes encoding the Vibrio ORF 6 and ORF 8 were mutagenized to introduce Sail sites flanking the open reading frames. The Sail sites flanking ORF 6 were introduced using PCR. The primers used were: CCCGGGTCGACACAATGGCTAAAAAGAACA CCACATCGA and CCCGGGTCGACTCATGACATATCGTTCAAAATGTCACTGA. The central 7.3 kb BamHl-Xhol fragment ofthe PCR product was replaced with the corresponding fragment from Vibrio cosmid #9. The mutagenized ORF 6 were cloned into the Sail site ofthe napin cassette of pCGN7770 to yield plasmid pCGN8554. The mutagenesis of ORF 8 used a different strategy. A BamHl fragment containing ORF 8 was subcloned into plasmid pHC79 to yield cosmid #9". A Sail site upstream ofthe coding region was introduced on and adapter comprised ofthe oligonucleotides TCGACATGGAAAATATTGCAGTAGTAGGTATTGCTAATTT GTTC and CCGGGAACAAATTAGCAATACCTACTACTGCAATATTTTCCATG. The adapter was ligated to cosmid #9" after digestion with Sail and Xmal. A Sail site was introduced downstream ofthe stop codon by using PCR for mutagenesis. A DNA fragment containing the stop codon was generated using cosmid #9" as a template with the primers TCAGATGAACTTTATCGATAC and TCATGAGACGTCGTCGACTTA CGCTTCAACAATACT. The PCR product was digested with the restriction endonucleases Clal andJαtU and was cloned into the cosmid 9" derivative digested with the same enzymes to yield plasmid 8P3. The Sail fragment from 8P3 was cloned into Sail digested pCGN7770 to yield pCGN8515. PCGN8532, a binary plant transformation vector that contains a Shewannella ORF 3 under control ofthe napin promoter was digested with NotI, and a NotI fragment of pCGΝ8539 containing a napin Vibrio ORF 7 gene fusion was inserted to yield pCGN8552. Plasmid pCGN8556 (Figure 23), which contains Shewannella ORF 3, and Vibrio ORFs 7 and 9 under control ofthe napin promoter was constructed by cloning the Sse8357 fragment from pCGN8543 into Sse8387 digested pCGN8552. The NotI digested napin/ORF 8 gene from plasmid pCGΝ8515 was cloned into a NotI digested plant binary transformation vector pCGΝ5139 to yield pCGN8548. The &e8387 digested napin/ORF 6 gene from pCGN8554 was subsequently cloned into the S?e8387 site of pCGN8566. The resultant binary vector containing the napin/ORF 6 gene fusion and napin/ORF 8 gene fusion was designated pCGN8560 (Figure 22). Example 5 Plant Transformation and PUFA Production EPA production The Shewanella constructs pCGN8535 and pCGN8537 can be transformed into the same or separate plants. If separate plants are used, the transgenic plants can be crossed resulting in heterozygous seed which contains both constructs. pCGN8535 and pCGN8537 are separately transformed into Brassica napus. Plants are selected on media containing kanamycin and transformation by full length inserts ofthe constructs is verified by Southern analysis. Immature seeds also can be tested for protein expression ofthe enzyme encoded by ORFs 3, 6, 7, 8, or 9 using western analysis, in which case, the best expressing pCGNE8535 and pCGN8537 Tl transformed plants are chosen and are grown out for further experimentation and crossing. Alternatively, the Tl transformed plants showing insertion by Southern are crossed to one another producing T2 seed which has both insertions. In this seed, half seeds may be analyzed directly from expression of EPA in the fatty acid fraction. Remaining half-seed of events with the best EPA production are grown out and developed through conventional breeding techniques to provide Brassica lines for production of EPA. Plasmids pCGN7792 and pCGN7795 also are simultaneously introduced into Brassica napus host cells. A standard transformation protocol is used (see for example USPN 5,463,174 and USPN 5,750,871, however Agrobacteria containing both plasmids are mixed together and incubated with Brassica cotyledons during the cocultivation step. Many ofthe resultant plants are transformed with both plasmids. DHA production A plant is transformed for production of DHA by introducing pCGN8556 and pCGN8560, either into separate plants or simultaneously into the same plants as described for EPA production. Alternatively, the Shewanella ORFs can be used in a concerted fashion with ORFs 6 and 8 of Vibrio, such as by transforming with a plant the constructs pCGN8560 and pCGN7795, allowing expression ofthe corresponding ORFs in a plant cell. This combination provides a PKS-like gene arrangement comprising ORFs 3, 7 and 9 of Shewanella, with an ORF 6 derived from Vibrio and also an OFR 8 derived from Vibrio. As described above, ORF 8 is the PKS-like gene which controls the identity ofthe final PUFA product. Thus, the resulting transformed plants produce DHA in plant oil. Example 6 Transgenic plants containing the Shewanella PUFA genes Brassica plants Fifty-two plants cotransformed with plasmids pCGN8535 andpCGN8537 were analyzed using PCR to determine if the Shewanella ORFs were present in the transgenic plants. Forty-one plants contained plasmid pCGN8537, and thirty-five plants contained pCGN8535. 11 ofthe plants contained all five ORFs required for the synthesis of EPA. Several plants contained genes from both ofthe binary plasmids but appeared to be missing at least one ofthe ORFs. Analysis is currently being performed on approximately twenty additional plants. Twenty-three plants transformed with pCGN8535 alone were analyzed using PCR to determine if the Shewanella ORFs were present in the transgenic plants. Thirteen of these plants contained both Shewanella ORF 6 and Shewanella ORF 8. Six ofthe plants contained only one ORF. Nineteen plants transformed with pCGN8537 were alone analyzed using PCR to determine if the Shewanella ORFs were present in the transgenic plants. Eighteen ofthe plants contained Shewanella ORF 3, Shewanella ORF 7, and Shewanella ORF 9. One plant contained Shewanella ORFs 3 and 7. Arabidopsis More than 40 transgenic Arabidopsis plants cotransformed with plasmids pCGN8535 and pCGN8537 are growing in our growth chambers. PCR analysis to determine which ofthe ORFs are present in the plants is currently underway. By the present invention PKS-like genes from various organisms can now be used to transform plant cells and modify the fatty acid compositions of plant cell membranes or plant seed oils through the biosynthesis of PUFAs in the transformed plant cells. Due to the nature ofthe PKS-like systems, fatty acid end-products produced in the plant cells can be selected or designed to contain a number of specific chemical structures. For example, the fatty acids can comprise the following variants: Variations in the numbers of keto or hydroxyl groups at various positions along the carbon chain; variations in the numbers and types (cis or trans) of double bonds; variations in the numbers and types of branches off of the linear carbon chain (methyl, ethyl, or longer branched moieties); and variations in saturated carbons. In addition, the particular length ofthe end-product fatty acid can be controlled by the particular PKS-like genes utilized. All publications and patent applications mentioned in this specification are indicative ofthe level of skill of those skilled in the art to which this mvention pertains. All publications and patent applications are herein incorporated by reference to the same extent as if each individual publication or patent application was specifically and individually indicated to be incorporated by reference. The invention now being fully described, it will be apparent to one of ordinary skill in the art that many changes and modifications can be made thereto without departing from the spirit or scope ofthe appended claims.

Claims: What is claimed is: 1. An isolated nucleic acid comprising: a Vibrio marinus nucleotide sequence selected from the group consisting ofthe ORF 6, ORF 7, ORF 8 and ORF 9 as shown in Figure 6. 2. An isolated nucleic acid comprising: a nucleotide sequence which encodes a polypeptide of a polyketide-like synthesis system, wherein said system produces a docosahexenoic acid when expressed in a host cell. 3. The isolated nucleic acid according to Claim 2, wherein said nucleotide sequence is derived from a marine bacterium. 4. The isolated nucleic acid according to Claim 2, wherein said nucleotide sequence is a Vibrio marinus ORF 8 as shown in Figure 6. 5. An isolated nucleic acid comprising: a nucleotide sequence which is substantially identical to a sequence of at least 50 nucleotides of a Vibrio marinus nucleotide sequence selected from the group consisting of ORF 6, ORF 7, ORF 8 and ORF 9 as shown in Figure 6. 6. A recombinant microbial cell comprising at least one copy of an isolated nucleic acid according to Claim 1 or Claim 2. 7. The recombinant microbial cell according to Claim 6, wherein said cell comprises each element of a polyketide-like synthesis system required to produce a long chain polyunsaturated fatty acid. 8. The recombinant microbial cell according to Claim 7, wherein said cell is a eukaryotic cell. 9. The recombinant microbial cell according to Claim 8, wherein said eukaryotic cell is a fungal cell, an algae cell or an animal cell. 10. The recombinant microbial cell according to Claim 9, wherein said fungal cell is a yeast cell and said algae cell is a marine algae cell. 11. The recombinant microbial cell according to Claim 6, wherein said cell is a prokaryotic cell. 12. The recombinant microbial cell according to Claim 11, wherein said cell is a bacterial cell or a cyanobacterial cell. 13. The microbial cell according to Claim 6, wherein said recombinant microbial cell is enriched for 22:6 fatty acids as compared to a non-recombinant microbial cell which is devoid of said isolated nucleic acid. 14. A method for production of docosahexenoic acid in a microbial cell culture, said method comprising: growing a microbial cell culture having a plurality of microbial cells, wherein said microbial cells or ancestors of said microbial cells were transformed with a vector comprising one or more nucleic acids having a nucleotide sequence which encodes a polypeptide of a polyketide synthesizing system, wherein said one or more nucleic acids are operably linked to a promoter, under conditions whereby said one or more nucleic acids are expressed and docosahexenoic acid is produced in said microbial cell culture. 15. A method for production of a long chain polyunsaturated fatty acid in a plant cell, said method comprising: growing a plant having a plurality of plant cells, wherein said plant cells or ancestors of said plant cells were transformed with a vector comprising one or more nucleic acids having a nucleotide sequence which encodes one or more polypeptides of a polyketide synthesizing system which produces a long chain polyunsaturated fatty acid, wherein each of said nucleic acids are operably linked to a promoter functional in a plant cell, under conditions whereby said polypeptides are expressed and a long chain polyunsaturated fatty acid is produced in said plant cells. 16. The method according to Claim 15, wherein said long chain polyunsaturated fatty acid produced in said plant cells is a 20:5 and 22:6 fatty acid. 17. The method according to Claim 15, wherein said nucleic acids comprise nucleotide sequences encoding any one ofthe polypeptides selected from the group consisting of Vibrio marinus ORF 6, ORF 7, ORF 8 and ORF 9 as shown in Figure 6 and Shewanella putrefaciens ORF 3, ORF 6, ORF 7, ORF 8 and ORF 9 as shown in Figure 4. 18. The method according to Claim 15, wherein said nucleic acid constructs are derived from two or more polyketide synthesizing systems. 19. A recombinant plant cell which produces an long chain polyunsaturated fatty acid exogenous to said plant cell, wherein said recombinant plant cell is produced according to a method comprising: transforming a plant cell or an ancestor or said plant cell with a vector comprising one or more nucleic acids having a nucleotide sequence which encodes one or more polypeptides of a polyketide synthesizing system which produces a long chain polyunsaturated fatty acid, wherein each of said nucleic acids are operably linked to a promoter functional in said plant cell whereby a recombinant plant cell is obtained; and growing said recombinant plant cell under conditions whereby said polypeptides are expressed and a long chain polyunsaturated fatty acid is produced in said plant cell. 20. The recombinant plant cell according to Claim 19, wherein said recombinant plant cell is a recombinant seed cell. 21. The recombinant plant cell according to Claim 20, wherein said recombinant seed cell is a recombinant embryo cell. 22. The method according to Claim 15, wherein said long chain polyunsaturated fatty acid produced in said plant cells is eicosapentenoic acid. 23. The method according to Claim 15, wherein said long chain polyunsaturated fatty acid produced in said plant cells is docosahexenoic acid. 24. The recombinant plant cell according to Claim 19, wherein said recombinant plant cell is from a plant selected from the group consisting of Brassica, soybean, safflower, and sunflower. 25. A plant oil produced by a recombinant plant cell according to Claim 19, wherein said plant oil comprises eicosapentenoic acid. 26. A plant oil produced by a recombinant plant cell according to Claim 19, wherein said plant oil comprises docosahexenoic acid. 27. The plant oil according to Claim 25 or Claim 26, wherein said plant oil is encapsulated. 28. A dietary supplement comprising a plant oil according to Claim 27. 29. A recombinant E. coli cell which produces docosahexenoic acid. 30. A plant oil comprising eicosapentenoic acid. 31. A plant oil comprising docosahexenoic acid. 32. The recombinant microbial cell according to Claim 12, wherein said bacterial cell is a lactobacillus cell.

Inventors: FACCIOTTI DANIEL, LASSNER MICHAEL, METZ JAMES GEORGE, FACCIOTTI, DANIEL, LASSNER, MICHAEL, METZ, JAMES, GEORGE
Applicants: CALGENE LLC, CALGENE, LLC
Application year: 1998
Main classification: C12N15
IPC codes: A01H5, C07K14, C12N1, C12N5, C12N15, C12P7, C12R1