A method and composition for increasing the accumulation of squalene and specific sterols in yeast

Abstract: A method of increasing the accumulation of squalene and specific sterols in yeast comprising increasing the expression level of a structural gene encoding a polypeptide having HMG-CoA reductase activity in a mutant yeast having single or double defects in the expression of sterol biosyntheticenzymes is provided. The expression level of a structural gene is preferably increased by transforming yeast with a recombinant DNA molecule comprising a vector operatively linked to an exogenous DNA segment that encodes a polypeptide having HMG-CoA reductase activity and a promoter that is suitable for driving the expression of the encoded polypeptide in the transformed yeast. The polypeptide having HMG-CoA reductase activity is preferably a truncated, active HMG-CoA reductase enzyme. Recombinant DNA molecules useful for transforming yeast and mutant yeast transformed with such recombinant DNA molecules are also disclosed.

Description: Technical Field The present invention relates to a method and composition for increasing the accumulation of squalene and specific sterols in yeast. Squalene and sterol accumulation is increased by increasing the expression level of a gene encoding a polypeptide having HMG-CoA reductase activity. Background of the Invention As used herein, the term "sterol" refers to derivatives of a fused, reduced ring system, cyclopenta-[α]-phenanthrene, comprising three fused cyclohexane rings (A, B and C) in a phenanthrene arrangement, and a terminal cyclopentane ring (D) having the formula and carbon atom position numbering shown below: where R is an 8 to 10 carbon-atom sidechain. Sterols are metabolically derived from acetate. Acetyl coenzyme A (CoA) reacts with acetoacetyl CoA to form 3-hydroxy-3-methylglutaryl CoA (HMG-CoA). HMG-CoA is reduced to mevalonate in an irreversible reaction catalyzed by the enzyme HMG-CoA reductase. Mevalonate is phosphorylated and decarboxylated to isopentenyl-pyrophosphate (IPP). Through the sequential steps of isomerization, condensation and dehydrogenation, IPP is converted to geranyl pyrophosphate (GPP). GPP combines with IPP to form farnesyl pyrophosphate (FPP), two molecules of which are reductively condensed to form squalene, a 30-carbon precursor of sterols. In yeast, squalene is converted to squalene epoxide, which is then cyclized to form lanosterol. Lanosterol has two methyl groups at position 4, a methyl group at position 14, a double bond at position 8(9) and an 8 carbon sidechain of the formula: CH 3 CH(CH 2 ) 2 CH=C(CH 3 ) 2 . Lanosterol is sequentially demethylated at positions 14 and 4 to form zymosterol (cholesta-8,24-dienol), which is converted to ergosterol (ergosta-5,7,22-trienol), the most abundant sterol of naturally occurring, wild-type yeast via a series of five enzymatic reactions schematically diagramed in Figure 1. The five reactions are: a. methylation of the carbon at position 24, catalyzed by a 24-methyltransferase; b. movement of the double bond at position 8(9) to position 7(8), catalyzed by a Δ8→Δ7 isomerase; c. introduction of a double bond at position 5(6), catalyzed by a 5-dehydrogenase (desaturase); d. introduction of a double bond at position 22(23), catalyzed by a 22-dehydrogenase (desaturase); and e. removal of a double bond at position 24(28), catalyzed by a 24(28)-hydrogenase (reductase). In wild-type yeast of the species Saccharomyces cerevisiae ( S . cerevisiae ), the predominant order of these reactions is thought to be a, b, c, d and e. [Parks et al., CRC Critical Reviews in Microbiology , 6 :301-341 (1978)]. According to such a predominant pathway, zymosterol is converted sequentially to fecosterol [ergosta-8,24(28)-dienol], episterol [ergosta-7,24(28)-dienol], ergosta-5,7,24(28)-trienol, ergosta-5,7,22, 24(28)-tetraenol, and finally ergosterol. If the enzymes catalyzing the reactions involved in the predominant pathway are substrate specific, then one would expect to find only the six sterols set forth above in yeast. Such, however, is not the case. Eighteen sterols have been found and described. [See, e.g., Parks et al., CRC Critical Reviews in Microbiology, 6 :301-341 (1978); Woods et al., Microbios , 10(A) :73-80 (1974); Bard et al., Lipids , 12 :645-654 (1977) (See Table 1)]. Thus, at least some of the enzymes are not substrate specific. Sterol Required Enzymes 1. Zymosterol (cholesta-8,24-dienol) none 2. fecosterol (ergosta-8,24(28)-dienol) a 3. episterol (ergosta-7,24(28)-dienol) a,b 4. ergosta-5,7,24(28)-trienol a,b,c 5. ergosta-5,7,22, 24(28)-tetraenol a,b,c,d 6. ergosterol (ergosta-5,7,22-trienol) a,b,c,d,e 7. ergosta-7,22,24 (28)-trienol a,b,d 8. cholesta-7,24-dienol b 9. cholesta-5,7,24-trienol b,c 10. cholesta-5,7,22,24-tetraenol b,c,d 11. ergosta-5,7-dienol a,b,c,e 12. ergosta-7,22-dienol a,b,d,e 13. ergosta-7-enol a,b,e 14. ergosta-5,8-dienol a,c,e 15. ergosta-5,8,22-trienol a,c,d,e 16. ergosta-8,22-dienol a,d,e 17. ergosta-8-enol a,e 18. ergosta-8,14,24(28)-trienol a Despite the lack of substrate specificity, one might expect that specific alterations in the sterol biosynthetic pathway would have predictable consequences. Currently available data show that such predictability is not present. For example, mutant S . cerevisiae with a defect in the expression of zymosterol-24-methyl-transferase (enzyme a), which mutants are designated erg6, might be expected to accumulate sterols 1 and 8-10 of Table 1, which sterols theoretically do not require the action of enzyme a for their synthesis. Parks et al., CRC Critical Reviews in Microbiology , 6 :301-341 (1978), however, report that erg6 mutants accumulate only zymosterol (#1), cholesta-5,7,24-trienol (#9) and cholesta-5,7,22,24-tetraenol (#10). Bard, M. et al., Lipids , 12 :645-654 (1977), on the other hand, report that erg6 mutants accumulate only sterols #1 and #10. Mutant S . cerevisiae with a defect in the expression of ergosta-5,7,24(28)-trienol-22-dehydrogenase (enzyme d), designated erg5, might be expected to accumulate sterols 1-4, 6, 8, 9, 11, 13, 14, 17 and 18. Parks et al., CRC Critical Reviews in Microbiology , 6 :301-341 (1978) report, that erg5 mutants accumulate only ergosta-5,7-dienol (#11), ergosta-5,7,24(28)-trienol (#4), ergosta-8,14,24(28)-trienol (#18) and episterol (#3). In contrast, Bard et al., Lipids , 12 :645-654 (1977) report that erg5 mutants accumulate zymosterol (#1), ergosta-5,7-dienol (#11), ergosta-5,7,24(28)-trienol (#4), ergosta-7,24(28)-dienol (#3) and ergosta-8,14,24(28)-trienol (#18). Still further, mutant S . cerevisiae with a defect in episterol-5-dehydrogenase (enzyme c), designated erg3, might be expected to accumulate sterols 1-3, 7, 8, 12, 13 and 16-18. Parks et al., CRC Critical Reviews in Microbiology , 6 :301-341 (1978) report that erg3 mutants accumulate only ergosta-7,22-dienol (#12), ergosta-8,22-dienol (#16), ergosta-7,22,24(28)-trienol (#7), fecosterol (#2) and episterol (#3). These data, taken together, show that specific defects in the expression of one sterol synthetic enzyme do not lead to predictable changes in sterol accumulation. A similar degree of unpredictability is found when sterol accumulation is examined in mutants having two defects in enzymes of the sterol biosynthetic pathway. Thus, for example, erg5-erg6 double mutants (defects in enzymes d and a) might be expected to accumulate sterols 1, 8 and 9. Parks et al. and Bard et al., above, report that erg5-erg6 double mutants accumulate only zymosterol (#1) and cholesta-5,7,24-trienol (#9). These data relating to sterol accumulation in yeast show that specific alterations in enzyme activity do not result in predictable changes in sterol accumulation. The data further show a lack of agreement between different investigators studying identical alterations. The present invention furnishes a solution to the problem of unpredictability by providing a method and composition for increasing the accumulation of squalene and specific sterols in yeast. Summary of the Invention The present invention generally provides a method of increasing squalene and specific sterol accumulation in mutant yeasts having a single or double defect in the expression of sterol biosynthetic pathway enzymes. This method comprises transforming such mutant yeasts with a recombinant DNA molecule comprising a vector operatively linked to an exogenous DNA segment that encodes a polypeptide having HMG-CoA reductase activity and a promoter suitable for driving the expression of HMG-CoA reductase in the transformed yeast. In particular, the invention provides a method of increasing accumulation of squalene, and at least one further sterol in S. cerevisiae selected from zymosterol, cholesta-7,24-dienol, cholesta-5,7,24-trienol, ergosta-8,22-dienol, ergosta-7,22-dienol, ergosta-8-enol, ergosta-7-enol, ergosta-5,7,24(28)-trienol and ergosta-5,7-dienol, comprising transforming an erg 3, erg 5, erg 3-erg 6 or erg 5-erg 6 mutant of S. cerevisiae with a recombinant DNA molecule comprising a vector operatively linked to an exogenous DNA segment that encodes the catalytic region but is free from the membrane binding region of an HMG-CoA reductase enzyme, and a promoter suitable for driving the expression of said reductase in said yeast. The structural gene encoding a polypeptide having HMG-CoA reductase activity encodes an active, truncated HMG-CoA reductase enzyme, which enzyme comprises the catalytic and at least a portion of the linker region that is free from the membrane binding region of HMG-CoA reductase enzyme. The copy number of the structural gene is increased by transforming the mutant yeast with a recombinant DNA molecule comprising a vector operatively linked to an exogenous DNA segment that encodes a polypeptide having a HMG-CoA reductase activity and a promoter that is suitable for driving the expression of the encoded polypeptide in the transformed yeast. Suitable promoters include promoters that are subject to inducible regulation by factors either extrinsic or intrinsic to yeast. Preferably, both the promoter and the exogenous DNA segment are integrated into the chromosomal DNA of the transformed yeast. The present invention most preferably provides a method of increasing squalene, zymosterol, cholesta-7,24-dienol and cholesta-5,7,24-trienol accumulation in yeast of the species S . cerevisiae comprising increasing the expression level of a structural gene encoding a polypeptide having HMG-CoA reductase activity in a mutant S . cerevisiae having defects in the expression of zymosterol-24-methyltransferase (erg6) and ergosta-5,7,24(28)-trienol-22-dehydrogenase (erg5). In further preferred embodiments, transformation of a mutant yeast having a defect in the expression of the enzyme episterol-5-dehydrogenase (erg 3) results in a transformed, mutant yeast which overaccumulates squalene, ergosta-8,22-dienol, ergosta-7,22-dienol, ergosta-8-enol and ergosta-7-enol. Transformation of a mutant yeast having a double defect in the expression of zymosterol-24-methyltransferase and episterol-5-dehydrogenase enzymes (erg6 and erg3) results in a transformed mutant yeast which overaccumulates squalene, zymosterol and cholesta-7,24-dienol. Transformation of a mutant yeast having a defect in the expression of ergosta-5,7,24(28)-trienol-22-dehydrogenase (erg 5) results in a transformed mutant yeast which overaccumulates zymosterol and a mixture of ergosta-5,7,24(28)-trienol and ergosta-5,7-dienol. Transformation of mutant yeast is preferably accomplished using a recombinant DNA molecule selected from the group of plasmid vectors consisting of plasmids pSOC725ARC, pSOC106ARC, pARC306E, pARC300D, pARC300S, pARC300T and pARC304S. Most preferred is plasmid pARC304S. The present invention further provides for a mutant species of S . cerevisiae , which mutant has a double defect in the expression of zymosterol-24-methyl-transferase and ergosta-5,7,24(28)-trienol-22-dehydrogenase enzymes (erg 5 and erg6). That mutant S . cerevisiae is designated ATC0402mu. The present invention still further provides for a mutant species of S . cerevisiae having a single or double defect in the expression of enzymes that catalyze the conversion of squalene to ergosterol that is transformed with a recombinant DNA molecule comprising as described before. In particular, the invention provides a mutant of S. cerevisiae having a single or double defect in the expression of enzymes that catalyze the conversion of squalene to ergosterol selected from erg 3, erg 5, erg 3-erg 6 and erg 5-erg 6 mutants, which is transformed with a recombinant DNA molecule comprising a vector operatively linked to an exogenous DNA segment that encodes the catalytic region and at least a portion of the linker region but is free from the membrane binding region of an HMG-CoA reductase enzyme, and a promoter suitable for driving the expression of said reductase in said yeast. The present invention still further provides for recombinant DNA molecules used to transform mutant yeasts such that the transformed mutant yeast overaccumulates squalene and specific sterols. The recombinant DNA molecules are plasmids pARC304S, pARC300S, pARC300T, pARC300D, pARC306E, pSOC106ARC and pSOC725ARC. The present invention provides several benefits and advantages. One advantage of the present invention is the provision of methods known to result in the predictable accumulation of specific sterols. Another advantage of the present invention is the ability to accumulate specific sterols to levels markedly greater than levels found in non-transformed yeast. Still further benefits and advantages will be apparent to the skilled worker from the description that follows. Brief Descriptions of the Drawings Figure 1 is a schematic diagram illustrating the various transformation steps involved in the metabolic conversion of zymosterol to ergosterol as shown and discussed in Bard et al., Lipids , 12(8) :645 (1977). The letters (a-e) indicate the five enzymes responsible for catalyzing the individual transformation steps. Numerals alone or with the letter "C" and an enzymic name indicate the position of the enzymes' activities and the activity of each enzyme. Figure 2, shown as twelve panels designated Figure 2-1 through 2-12, is the nucleotide base sequence (SEQ ID NO:1) and derived amino acid residue sequence (SEQ ID NO:2) for S . cerevisiae HMG-CoA reductase 1 published by Basson et al., Mol. Cell Biol. , 8(9) :3797-3808 (1988). Nucleotides are numbered (left-hand side) in the 5' to 3' direction. Position 1 corresponds to the first nucleotide of the ATG triplet coding for the initiator methionine. The predicted amino acid sequence is shown below the nucleotide sequence. The amino acid residues are numbered (right-hand side) beginning with the initiator methionine. Figure 3 is a schematic diagram showing the physical structure and genetic organization of plasmid pSOC725ARC. Plasmid pSOC725ARC was constructed to place a coding sequence for a truncated HMG-CoA reductase gene under control of a GAL 1-10 promoter. This plasmid also contains the TRP-1 gene and the yeast 2 micron origin of replication. Certain restriction sites indicated by lines linked to the arcs and abbreviation for their respective restriction endonuclease enzymes are indicated. Figure 4 is a schematic diagram showing the physical structure and genetic organization of plasmid pSOC106ARC. Plasmid pSOC106ARC was constructed to place a coding sequence for an intact HMG-CoA reductase gene under the control of a GAL 1-10 promoter. Plasmid pSOC106ARC also contains the TRP-1 gene and the yeast 2 micron origin of replication. Certain restriction sites are indicated as in Figure 3. Figure 5 is a schematic diagram showing the physical structure and genetic organization of plasmid pARC306E. Plasmid pARC306E was constructed to place a coding sequence for a truncated HMG-CoA reductase gene under control of a GAL-1 promoter. Plasmid pARC306E also contains the TRP-1 gene. Certain restriction sites are indicated as in Figure 3. Figure 6 is schematic diagram showing the physical structure and genetic organization of plasmid pARC300D. Plasmid pARC300D was constructed to place a coding sequence for a truncated HMG-CoA reductase gene under the control of a PGK promoter. Plasmid pARC300D also contains the TRP-1 gene. Certain restriction sites are indicated as in Figure 3. Figure 7 is a schematic diagram showing the physical structure and genetic organization of plasmid pARC300S. Plasmid pARC300S was constructed to place a coding sequence for a truncated HMG-CoA reductase gene under control of a PGK promoter. Plasmid pARC300S also contains a URA 3 selectable marker. Certain restriction sites are indicated as in Figure 3. Figure 8 is a schematic diagram showing the physical structure and genetic organization of plasmid pARC300T. Plasmid pARC300T was constructed to place a coding sequence for a truncated HMG-coA reductase gene under control of a PGK promoter. Plasmid pARC300T also contains a URA 3 selectable marker. Certain restriction sites are indicated as in Figure 3. Figure 9 is a schematic diagram showing the physical structure and genetic organization of plasmid pARC304S. Plasmid pARC304S was constructed to place a coding sequence of a truncated HMG-CoA reductase gene under the control of an ADH promoter. Plasmid pARC304S also contains a URA 3 selectable marker. Certain restriction sites are indicated as in Figure 3. Detailed Description of the Invention I. Definitions The following words and phrases have the meanings set forth below. Expression: The combination of intracellular processes, including transcription and translation, undergone by a structural gene to produce a polypeptide. Expression vector: A DNA sequence that forms control elements that regulate expression of structural genes when operatively linked to those genes. Operatively linked: A structural gene is covalently bonded in correct reading frame to another DNA (or RNA as appropriate) segment, such as to an expression vector so that the structural gene is under the control of the expression vector. Promoter: A recognition site on a DNA sequence or group of DNA sequences that provide an expression control element for a structural gene and to which RNA polymerase specifically binds and initiates RNA synthesis (transcription) of that gene. Recombinant DNA molecule: A hybrid DNA sequence comprising at least two nucleotide sequences not normally found together in nature. Structural gene: A DNA sequence that is expressed as a polypeptide, i.e., an amino acid residue sequence. Vector: A DNA molecule capable of replication in a cell and/or to which another DNA segment can be operatively linked so as to bring about replication of the attached segment. Alternatively, a vector can be a non-replicating vector that is integrated into the chromosome of the transformed cell. A plasmid is an exemplary vector. II. The Invention The present invention relates to compositions and methods for increasing the accumulation of squalene and specific sterols in yeast cultures as well as to the yeast that exhibit increased squalene and sterol accumulation relative to a non-transformed yeast. The yeast is Saccharomyces cerevisiae ( S. cerevisiae). A yeast contemplated by this invention is transformed with an added structural gene that encodes a polypeptide having HMG-CoA reductase activity, that encoded polypeptide being expressed in the transformed yeast. Non-transformed yeasts are mutant species having a single or double defect in the expression of enzymes involved in converting zymosterol to ergosterol (sterol biosynthetic pathway enzymes). The non-transformed and transformed yeasts compared are of the same species, S. cerevisiae. Sterol production in a yeast culture of the present invention is increased by increasing the cellular activity of the enzyme HMG-CoA reductase, which enzyme catalyzes the conversion of 3-hydroxy-3-methylglutaryl Coenzyme A (HMG-CoA) to mevalonate. As used herein, "cellular activity" means the total catalytic activity of HMG-CoA reductase in a yeast cell. Cellular HMG-CoA reductase activity is increased by increasing the expression level of a structural gene encoding a polypeptide having HMG-CoA reductase catalytic activity. Expression of that encoded structural gene enhances the cellular activity of that enzyme. The expression level is increased by methods well known in the art. For example, expression of a structural gene is increased by deregulating the promoter, which controls expression of such a structural gene. The promoter that regulates expression of the HMG-CoA reductase gene in a normal, wild-type yeast can be identified and excised from the genome. A new promoter, which allows for overexpression of the HMG-CoA reductase gene, is then inserted according to standard transformation techniques. Increasing the expression level of a structural gene encoding a polypeptide having HMG-CoA reductase catalytic activity involves increasing the copy number of a structural gene encoding such a polypeptide. The copy number is increased by transforming a yeast cell with a recombinant DNA molecule comprising a vector operatively linked to an exogenous DNA segment that encodes a polypeptide having HMG-CoA reductase activity, and a promoter suitable for driving the expression of said polypeptide in said yeast. Such a polypeptide is catalytically active, and a truncated HMG-CoA reductase protein. Thus, a transformed yeast cell has one or more added genes that encode a polypeptide having HMG-CoA reductase activity relative to a non-transformed yeast of the same species. As such, a transformed yeast can be distinguished from a non-transformed yeast by standard technology such as agarose separation of DNA fragments or mRNAs followed by transfer and appropriate blotting with DNA or RNA or by use of polymerase chain reaction technology, as are well known. Relative HMG-CoA reductase activity of the transformed and non-transformed yeasts can also be compared, with a relative increase in HMG-CoA reductase activity in transformed yeasts being indicative of transformation. The accumulation of squalene and specific sterols can also be used to distinguish between non-transformed and transformed yeasts. A. Structural Genes The present invention contemplates transforming a yeast with a structural gene that encodes a polypeptide having HMG-CoA reductase activity. The HMG-CoA reductase enzymes of both animal and yeast cells comprise three distinct amino acid residue sequence regions, which regions are designated the catalytic region, the membrane binding region and the linker region. The catalytic region contains the active site of the HMG-CoA reductase enzyme and comprises about forty percent of the total, localized on the COOH-terminal portion of intact HMG-CoA reductase enzyme. The membrane binding region contains hydrophobic amino acid residues and comprises about fifty percent of the total, localized on the NH 2 -terminal portion of intact HMG-CoA reductase enzyme. The linker region connects the catalytic and membrane binding regions, and constitutes the remaining about ten percent of the intact enzyme. As discussed in greater detail below, only the catalytic region of HMG-CoA reductase is needed herein. Thus, a structural gene that encodes a polypeptide corresponding to that catalytic region is the minimal gene required for transforming yeasts. However, larger polypeptide enzymes and their structural genes are preferred. Thus, the present invention contemplates use of truncated structural genes that encode the active catalytic region, or the catalytic region plus at least a portion of the linker region that is free from the membrane binding region of HMG-CoA reductase. A structural gene encoding a polypeptide having HMG-CoA reductase activity can be obtained or constructed from a variety of sources and by a variety of methodologies, [See, e.g., Carlson et al., Cell , 28 :145 (1982); Rine et al., Proc. Nat. Acad. Sci. U.S.A. , 80 :6750 (1983)]. Exemplary of such structural genes are the mammalian and yeast genes encoding HMG-CoA reductase. The mammalian genome contains a single gene encoding HMG-CoA reductase. The nucleotide base sequence of the hamster and human gene for HMG-CoA reductase have been described. A composite nucleotide sequence of cDNA corresponding to the mRNA, as well as the derived amino acid residue sequence, for hamster HMG-CoA reductase is found in Chin et al., Nature , 308 :613 (1984) and SEQ ID NO:3. The composite nucleotide sequence in that paper, comprising about 4606 base pairs, includes the nucleotide sequence encoding the intact hamster HMG-CoA reductase enzyme. Intact hamster HMG-CoA reductase comprises about 887 amino acid residues, shown in SEQ ID NO:4. A preferred structural gene is one that encodes a polypeptide corresponding to only the catalytic region of the enzyme. Two catalytically active segments of hamster HMG-CoA reductase have been defined, [Liscum et al., J. Biol. Chem. , 260(1) :522 (1985)]. One catalytic region has an apparent size of about 63 kDa and comprises amino acid residues from about position 373 to about position 887 of SEQ ID NO:4. A second catalytic region has an apparent size of about 53 kDa and comprises amino acid residues from about position 460 to about position 887 of SEQ ID NO:4. The about 63 kDa catalytically active segment is encoded by base pairs from about nucleotide position 1282 to about nucleotide position 2824 of the sequence in SEQ ID NO:3. The about 53 kDa catalytically active segment is encoded by base pairs from about nucleotide position 1543 to about nucleotide position 2824 of the sequence in SEQ ID NO:3. In a preferred embodiment, the utilized structural gene encodes the catalytic region and at least a portion of the linker region of HMG-CoA reductase. The linker region of hamster HMG-CoA reductase comprises amino acid residues from about position 340 to about position 373 or from about position 340 to about position 460, depending upon how the catalytic region is defined. These linker regions are encoded by base pairs from about nucleotide position 1183 to about nucleotide position 1282 or from about position 1183 to about position 1543 respectively of the sequence in SEQ ID NO:3. The structural gene encoding the linker region is operatively linked to the structural gene encoding the catalytic region. In one particularly preferred embodiment, a structural gene encoding a catalytically active, truncated HMG-CoA reductase enzyme can optionally contain base pairs encoding a small portion of the membrane region of the enzyme. A truncated hamster HMG-CoA reductase gene, designated HMGR-Δ227, comprising nucleotides 164-190 and 1187-2824 of the sequence in SEQ ID NO:3, which encodes amino acid residues 1-9 (from the membrane binding region) and 342-887 has been used to transform cells lacking HMG-CoA reductase, [Gil et al., Cell , 41 :249 (1985)]. A structural gene encoding a polypeptide comprising a catalytically active, truncated or intact HMG-CoA reductase enzyme from other organisms such as yeast can also be used in accordance with the present invention. Yeast cells contain two genes encoding HMG-CoA reductase. The two yeast genes, designated HMG1 and HMG2, encode two distinct forms of HMG-CoA reductase, designated HMG-CoA reductase 1 and HMG-CoA reductase 2. The nucleotide base sequence of HMG1 (SEQ ID NO:1) as well as the amino acid residue sequence of HMG-CoA reductase 1 (SEQ ID NO:2) are presented in Figure 2, reprinted from Basson et al., Mol. Cell Biol. , 8(9) :3797 (1988). The entire HMG1 gene comprises about 3360 base pairs. Intact HMG-CoA reductase 1 comprises an amino acid sequence of about 1054 amino acid residues. The entire HMG2 gene comprises about 3348 base pairs shown in SEQ ID NO:5. Intact HMG-CoA reductase 2 comprises about 1045 amino acid residues shown in SEQ ID NO:6 (Basson et al., above). By analogy to the truncated hamster structural gene, structural genes encoding polypeptides comprising catalytically active, truncated HMG-CoA reductase enzymes from yeast can also be used in accordance with the present invention. The catalytic region of HMG-CoA reductase 1 comprises amino acid residues from about residue 618 to about residue 1054: i.e., the COOH-terminus. A structural gene that encodes the catalytic region comprises base pairs from about nucleotide position 1974 to about position 3282 of Figure 2 and SEQ ID NO:1. The linker region of HMG-CoA reductase 1 comprises an amino acid sequence from about residue 525 to about residue 617. A structural gene that encodes the linker region comprises nucleotides from about position 1695 to about position 1974 of Figure 2. A structural gene encoding a polypeptide comprising the catalytic region and at least a portion of the linker region of yeast HMG-CoA reductase 1 preferably comprises the structural gene encoding the linker region of the enzyme operatively linked to the structural gene encoding the catalytic region of the enzyme. Also by analogy to the truncated hamster gene, a truncated HMG1 gene can optionally contain nucleotide base pair sequences encoding a small portion of the membrane binding region of the enzyme. Such a structural gene preferably comprises base pairs from about nucleotide position 121 to about position 146 and from about position 1695 to about position 3282 of Figure 2 and SEQ ID NO:1. A construct similar to those above from an analogous portion of yeast HMG-CoA reductase 2 can also be utilized. It will be apparent to those of skill in the art that the nucleic acid sequences set forth herein, either explicitly, as in the case of the sequences set forth above, or implicitly with respect to nucleic acid sequences generally known and not presented herein, can be modified due to the built-in redundancy of the genetic code and non-critical areas of the polypeptide that are subject to modification and alteration. In this regard, the present invention contemplates allelic variants of structural genes encoding a polypeptide having HMG-CoA reductase activity. The previously described DNA segments are noted as having a minimal length, as well as total overall lengths. That minimal length defines the length of a DNA segment having a sequence that encodes a particular polypeptide having HMG-CoA reductase activity. As is well known in the art, so long as the required DNA sequence is present and in proper reading frame, (including start and stop signals), additional base pairs can be present at either end of the segment and that segment can still be utilized to express the protein. This, of course, presumes the absence in the segment of an operatively linked DNA sequence that represses expression, expresses a further product that consumes the enzyme desired to be expressed, expresses a product other than the desired enzyme or otherwise interferes with the structural gene of the DNA segment. Thus, so long as the DNA segment is free of such interfering DNA sequences, the maximum size of a recombinant DNA molecule, particularly an expression vector, is governed mostly by convenience and the vector size that can be accommodated by a host cell, once all of the minimal DNA sequences required for replication and expression, when desired, are present. Typically, a DNA segment of the invention can be up to 15,000 base pairs in length. Minimal vector sizes are well known. B. Recombinant DNA Molecules A recombinant DNA molecule of the present invention can be produced by operatively linking a vector to a useful DNA segment to form a plasmid such as discussed herein. Particularly preferred recombinant DNA molecules are discussed in detail in Examples 2 to 7, hereafter. A vector capable of directing the expression of a polypeptide having HMG-CoA reductase activity is referred to herein as an "expression vector". Such expression vectors contain expression control elements including the promoter. The polypeptide coding genes are operatively linked to the expression vector to permit the promoter sequence to direct RNA polymerase binding and expression of the desired polypeptide coding gene. Useful in expressing the polypeptide coding gene are promoters that are inducible, viral, synthetic, constitutive as described by Poszkowski et al., EMBO J. , 3 :2719 (1989) and Odell et al., Nature , 313 :810 (1985), and temporally regulated, spatially regulated, and spatiotemporally regulated as disclosed in Chau et al., Science , 244 :174-181 (1989). The promoter preferably comprises a promoter sequence whose function in regulating expression of the structural gene is substantially unaffected by the amount of sterol in the cell. As used herein, the term "substantially unaffected" means that the promoter is not responsive to direct feedback control by the sterols accumulated in transformed cells. A promoter is also selected for its ability to direct the transformed yeast's transcriptional activity to the structural gene encoding a polypeptide having HMG-CoA reductase activity. Structural genes can be driven by a variety of promoters in yeast. Promoters utilized with the present invention are those preferably regulated by factors, which can be monitored and controlled in the internal or external environment of the transformed cell. Examples of promoters inducibly regulated by factors in the cell's external environment (extrinsic factors) are the GAL 1 promoter, the GAL 10 promoter, the GAL 1-10 promoter, the GAL 7 promoter, the metallothionine promoter, the a-factor promoter, the invertase promoter and the enolase promoter. Preferred are the well known GAL 1 , the GAL 10 and the GAL 1-10 promoters. Examples of promoters subject to inducible regulation by factors in the cell's internal environment (intrinsic factors) are the phosphoglycerate kinase ( PGK ) promoter, the triose-phosphate isomerase ( TPI ) promoter, the alcohol dehydrogenase ( ADH ) promoter and the repressible acid phosphatase promoter. Preferred are the well known PGK and the ADH promoters. The choice of which expression vector and ultimately to which promoter a polypeptide coding gene is operatively linked depends directly on the functional properties desired, e.g. the location and timing of protein expression, and the host cell to be transformed. These are well known limitations inherent in the art of constructing recombinant DNA molecules. However, a vector useful in practicing the present invention is capable of directing the expression of the polypeptide coding gene included in the DNA segment to which it is operatively linked. The present method contemplates a plasmid vector. The plasmid vectors of the present invention can be incorporated either within (integrated) or without (episomal) the chromosomes of the transformed cell. An episomal plasmid includes an origin of replication for yeast, the nucleic acid sequence that encodes a polypeptide having HMG-CoA reductase activity, a promoter, and a selective marker. The selective marker can include genes conveying antibiotic resistance, or permitting an auxotrophic host to metabolize a substrate that it would not otherwise be able, but for the presence of the plasmid vector. However, the use of antibiotic resistance as a selective marker requires growing organisms in an antibiotic culture media. Due to the expense of the antibiotic, organisms dependent on antibiotics are difficult to develop commercially. Generally, auxotrophic organisms are used for yeast. Auxotrophic organisms can be produced by mutation and culture techniques which are well known in the art. Selective markers which can complement an auxotrophic host organism include the well known TRP 1 gene encoding phosphoribosyl anthraniline isomerase, the URA 3 gene encoding orotine-5' phosphate decarboxylate, the LEU 2 gene encoding isopropylmalate isomerase, and the HIS 3 gene encoding histidinol dehydrogenase. A preferred selective marker for an auxotrophic host is TRP 1. Preferred episomal plasmid vectors are pSOC725ARC and pSOC106ARC. Episomally replicating vectors are sometimes difficult to maintain in host organisms for long periods of time in liquid culture, especially when the selective pressure used to maintain the vector is complementation of a nutritional auxotrophy. A preferred embodiment of the present invention includes an integrating vector which requires little or no selective pressure to maintain base sequences for the polypeptide having HMG-CoA reductase activity and the promoter. Integrating vectors, in accordance with the present invention, include base sequences that encode a polypeptide having HMG-CoA reductase activity, a promoter, a selective marker and sequences homologous to host chromosomal DNA that permit the base sequences to be incorporated within the chromosome via homologous recombination. The homologous region includes restriction sites that permit the plasmid to become linear. In linear form, the plasmid can recombine at homologous regions of the chromosome. Integrating vectors do not include origins of replication for the host organism. Preferred integrating vectors are pARC300S, pARC300T, pARC300D, pARC306E and pARC304S. Plasmid vector pARC304S is most preferred as evidenced by its ability to generate the greatest enhancement in sterol accumulation (see Example 15). The basic genetic characteristics of preferred plasmid vectors are summarized in Table 2, below. Plasmid Vector Genetic Characteristics pSOC106 TRP1-2µori-GAL 1-HMG1 pSOC725 TRP1-2µori-GAL 10-tHMG1 pARC306E TRP1-GAL 1-tHMG1 pARC300D TRP1-PGK-tHMG1 pARC300S,T URA3-PGK-tHMG1-ura3 term pARC304S URA3-ADH-tHMG1-ura3 term Individuals skilled in the art will readily recognize that episomal and integrating vectors are often amplified in organisms other than the intended host and require means of replication and selection in the non-host organism. Generally, the non-host organism is Escherichia coli due to its well-known features and characteristics. In preferred embodiments, the vector used to express the polypeptide coding gene includes a selection marker that is effective in a yeast cell, such as the URA 3 or TRP I markers. Other suitable selection means for use in amplifying the vectors in bacteria include antibiotic markers, such as genes encoding for beta lactamase (penicillin resistance), chloramphenicol transacetylase (chloramphenicol resistance), and neomycin phosphotransferase (kanamycin and neomycin resistance). A variety of methods has been developed to operatively link DNA to vectors via complementary cohesive termini or blunt ends. For instance, complementary homopolymer tracts can be added to the DNA segment to be inserted and to the vector DNA. The vector and DNA segment are then joined by hydrogen bonding between the complementary homopolymeric tails to form recombinant DNA molecules. Alternatively, synthetic linkers containing one or more restriction endonuclease sites can be used to join the DNA segment to the expression vector. The synthetic linkers are attached to blunt-ended DNA segments by incubating the blunt-ended DNA segments with a large excess of synthetic linker molecules in the presence of an enzyme that is able to catalyze the ligation of blunt-ended DNA molecules, such as bacteriophage T4 DNA ligase. Thus, the products of the reaction are DNA segments carrying synthetic linker sequences at their ends. These DNA segments are then cleaved with the appropriate restriction endonuclease and ligated into an expression vector that has been cleaved with an enzyme that produces termini compatible with those of the synthetic linker. Synthetic linkers containing a variety of restriction endonuclease sites are commercially available from a number of sources including New England BioLabs, Beverly, MA. Also contemplated by the present invention are RNA equivalents of the above described recombinant DNA molecules. C. Transformed Yeasts and Methods of Transformation The copy number of a gene coding for a polypeptide having HMG-CoA reductase activity is increased by transforming a desired yeast with a suitable vector that contains that structural gene. Expression of that gene in the transformed yeast enhances the activity of HMG-CoA reductase. Yeast cells are transformed in accordance with the present invention by methods known and readily apparent to those of skill in the yeast transformation art, [See, e.g., Hinnen et al., Proc. Natl. Acad. Sci. USA , 75 :1929-(1978); Ito et al., Bact. , 5 :163-168 (1983)]. A preferred general method of transformation is the lithium acetate procedure of Ito et al., above. Yeast cells are grown to a concentration about 2 X 10 7 cells/ml in a medium containing yeast extract, bactopeptone and dextrose. Cells are collected by low speed centrification and resuspended in a transformation medium containing lithium acetate in a Tris-EDTA buffer. Cells are maintained in the transformation medium for about one hour at about 30°C. Recombinant DNA molecules of desired composition are added to the transformation medium cell suspension and the mixture is maintained at about 30°C for about one-half hour. Polyethylene glycol (M.W. 4000) is then added to the cell suspension such that the final concentration of polyethylene glycol is about 35 percent weight/volume (w/v). Cells are maintained in the polyethylene glycol-containing solution at about 30°C for about two hours and then at about 42°C for an additional five minutes. Sterile distilled water is added to the cell suspension, and the cells collected by low speed centrification. Further specifics are provided hereinafter. Successfully transformed cells are identified by growing the transformed cells on selection medium, identifying cell characteristics indicative of transformation (i.e., increased accumulation of squalene or specific sterols), analyzing nucleic acids isolated from such transformed cells with standard techniques such as Southern blot analysis, [Holm et al., Gene, 42 :169 (1986)]. D. Mutated Yeasts The yeasts utilized in accordance with the present invention are mutated yeasts having single or double defects in the expression of enzymes that catalyze the conversion of zymosterol to ergosterol. Such enzymes are referred to herein as "erg" gene products. Table 3 below lists the particular erg designations for specific enzyme expression defects. Enzyme Expression Defect Mutant Designation zymosterol-24-methyltransferase erg6 ergosta-5,7,24(28)-trienol-22-dehydrogenase erg5 episterol-5-dehydrogenase erg3 Mutants used in accordance with the present invention can be purchased or generated from commercially available sources such as the Yeast Genetic Stock Center (Berkeley, CA.). For example, erg5 and erg5-erg6 double mutants are produced from commercially available sources. Mutant yeast ATC0402mu, an erg5-erg6 double mutant, is constructed by crossing a commercially available erg6 mutant yeast, M610-12B, with a commercially available erg5 mutant, po15αΔ22, and then crossing the resultant double mutant, ATC0403mu, with a wild-type yeast. Mutant yeast ATC0402mu and its derviative mutant yeast ATC0315rc are the most preferred mutants for transformation with the plasmid vectors of the present invention. Alternatively, ATC0403 is crossed with a different wild-type, and mutants having desired genotypes are back-crossed twice with wild-type yeast to yield species ATC4124, an ergs mutant. Mutants are also obtained by well known methods of inducing mutations. See , e.g. , Boeke et al., Mol. Gen. Genet. , 197 :345-346 (1984); Sherman et al., Methods and Yeast Genetics, Cold Spring Harbor Laboratory, N.Y. (1986). In a preferred embodiment, wild-type yeasts are transformed with an inducible "TY1-neo" transposon as a mutagenic agent. Plasmid pJEF1105, containing a GAL:TY1-neo expression cassette, is used as the transforming agent. Boeke et al., Science, 239 :280-282 (1989). Competent transformants demonstrating both neomycin and nystatin resistance are then evaluated for sterol content. Transformation of wild-type yeast with pJEF1105 yields mutant ATC6118, an erg3 mutant, and mutant ATC0501, an erg6 mutant. Mutants having single expression defects are then crossed to generate mutants having double defects in enzyme expression. For example, the crossing of mutant ATC6118 with mutant ATC0501 yields mutant ATC6119, an erg3-erg6 double mutant. The genotype of exemplary mutants contemplated for use in the present invention are presented in Table 4 below. Genotype symbols are used in accordance with convention cited in Mortimer et al. Yeast , 5 :321-403 (1989) and Broach, The Molecular Biology of the Yeast Saccharomyces, Life Cycle and Inheritance , Strathern, Jones and Broach, eds., Cold Spring Harbor Laboratory, pp. 653-727 (1981). Species genotype po15αΔ22 a, erg5 M610-12β α, ile3, erg6-5, trp1, gal2 DBY745 α, adel, ura3-52, leu2-100, leu2-122, MEL, gal 1 gal 10 YNN281 α, trp1-Δ, his3Δ-200, ura 3-52,lys 2 ATC0403mu a, trp1, gal, erg5, erg6 ATC0402mu a, trp1, GAL, ergs, erg6 ATC6118 a, his3Δ-200, erg3, ura3-52, GAL ATC4124 α, erg5,trp1, GAL ATC4154 a, ura3-52, erg7, gal ATC6119 α, erg3, erg6, ura3-52, GAL ATC1500cp a, erg5, erg6 ATC0315rc a, ura3, erg5, erg6 ATC1551 a, erg5, erg6 E. Squalene and Sterol Accumulation in Transformed Yeast The transformed mutant yeast species of the present invention overaccumulate squalene and specific sterols relative to non-transformed mutants of the same species. Relative to a non-transformed erg3 mutant, an erg3 mutant transformed with a plasmid vector used herein overaccumulates squalene, ergosta-8,22-dienol, ergosta-7,22-dienol, ergosta-8-enol and ergosta-7-enol. Relative to a non-transformed erg5 mutant, an erg5 mutant transformed with a plasmid vector used herein overaccumulates squalene, zymosterol, and a mixture of ergosta-5,7,24(28)-trienol and ergosta-5,7 dienol. Similar results are seen when mutants having double defects in enzymes of the sterol synthetic pathway are transformed. Relative to a non-transformed erg3-erg6 mutant, an erg3-erg6 mutant transformed with a useful plasmid vector overaccumulates squalene, zymosterol and cholesta-7,24-dienol. Relative to a non-transformed erg5-erg6 mutant, an erg5-erg6 double mutant transformed with the plasmid vector useful herein overaccumulates squalene, zymosterol, cholesta-5,7,24-trienol and cholesta-7,24-dienol. F. HMG-CoA Reductase Activity In Transformed Yeasts The expression of a structural gene encoding a polypeptide having HMG-CoA reductase activity in the transformed yeast of the present invention enhances the cellular activity of said HMG-CoA reductase. As a result of transformation, the copy number of an added gene encoding a polypeptide having HMG-CoA reductase activity is increased from 1 to about 2 to about 10. Cellular activity of HMG-CoA reductase in such transformed cells is almost linearly proportional to the increase in copy number through a copy number of about 6 and then falls slightly when a copy number of 9 is reached. Thus, when the copy number is increased to about 2, HMG-CoA reductase activity is elevated to a level about 1.4 times the activity observed in non-transformed yeast. A further increase in the copy number to a level of about 6 is accompanied by a further increase in HMG-CoA reductase activity to a level about 2.6 times that found in non-transformed yeast. Increases in the copy number beyond about 6 to about 9 are not accompanied by further increases in HMG-CoA reductase activity. A transformed yeast having a copy number of about 9 has a level of HMG-CoA reductase activity about equal to about twice that seen in non-transformed yeast. G. Harvesting of Sterols If desired, transformed yeasts are harvested to recover the sterol product. Most of the sterol in our genetically transformed yeast of this invention occurs in the form of fatty acid esters. To obtain free sterols, it is therefore necessary to saponify the "yeast pulp" in base, e.g., as described in the Examples below (2:1 EtOH/H 2 O containing 20 percent w/v KOH). In a preferred embodiment, harvesting comprises: (i) homogenizing sterol-containing transformed yeasts to produce a pulp; and (ii) extracting the sterol(s) from the pulp with an appropriate basic solvent such as an organic solvent or by supercritical extraction followed by base saponification in an appropriate solvent [Favati et al., J. Food Sci. , 53 :1532 (1988) and the citations therein] to produce a sterol-containing liquid solution or suspension; and (iii) isolating the sterol(s) from the solution or suspension. Transformed yeasts are homogenized to produce a pulp using methods well known to one skilled in the art. This homogenization can be done manually, by a machine, or by a chemical means. The pulp consists of a mixture of the sterol of interest, residual amounts of precursors, cellular particles and cytosol contents, which is subjected to extraction procedures. Sterol(s) can be extracted from the pulp produced above to form a sterol-containing solution or suspension. Such extraction processes are common and well known to one skilled in this art. For example, the extracting step can consist of soaking or immersing the pulp in a suitable solvent. This suitable solvent is capable of dissolving or suspending the sterol present in the pulp to produce a sterol-containing solution or suspension. Solvents useful for such an extraction process are well known to those skilled in the art and include several organic solvents and combinations thereof such as methanol, ethanol, isopropanol, acetone, acetonitrile, tetrahydrofuran (THF), hexane, and chloroform as well as water-organic solvent mixtures. A vegetable oil such as peanut, corn, soybean and similar oils can also be used for this extraction. Yeasts transformed with a structural gene for an active, truncated HMG-CoA reductase enzyme are grown under suitable culture conditions for a period of time sufficient for sterols to be synthesized. The sterol-containing yeast cells are then lysed chemically or mechanically, and the sterol is extracted from the lysed cells using a liquid organic solvent, as described before, to form a sterol-containing liquid solution or suspension. The sterol is thereafter isolated from the liquid solution or suspension by usual means such as chromatography. The sterol is isolated from the solution or suspension produced above using methods that are well known to those skilled in the art of sterol isolation. These methods include, but are not limited to, purification procedures based on solubility in various liquid media, chromatographic techniques such as column chromatography and the like. Best Mode For Carrying Out The Invention The following examples illustrate the best mode of carrying out the invention and are not to be construed as limiting of the specification and claims in any way. EXAMPLE 1: Transformation of S. Cerevisiae Yeast of the species S. cerevisiae were transformed in accordance with a lithium acetate procedure, [Ito et al., J. Bacteriol. , 153 :163-168 (1983)]. Yeast cells were grown in about 50 ml of YEPD medium (yeast extract 1 percent w/v, bactopeptone, 2 percent w/v; and dextrose, 2 percent w/v) overnight at about 30°C. When the concentration of cells was about 2 x 10 7 cells/ml, the cells were collected by low speed centrifugation. Cells appearing in the pellet of the centrifugation were suspended in about 50 mls of TE buffer (10 mM Tris Cl, 1 mM EDTA) and repelleted by centrifugation. The pellet from this second centrifugation was resuspended in about 1.0 ml of TE buffer. To 0.5 ml of this cell suspension were added 0.5 ml of 0.2 M lithium acetate (LiOAc), and the suspension was maintained at about 30°C for one hour with constant shaking. Recombinant DNA (about 10 µg in up to 15 µl of TE buffer) was added to 100 µl of the TE-LiOAc cell suspension and the admixture maintained at about 30°C for one-half hour without shaking. The DNA-containing cell suspension was then well mixed with polyethylene glycol (44 percent w/v) such that the final concentration of polyethylene glycol (PEG) was about 35 percent (w/v). The cells were maintained in this PEG solution at about 30°C for about two hours and then at about 42°C for about five minutes. About 10 ml of sterile, distilled water was added to each suspension and the cells were collected by low speed centrifugation. This procedure was repeated, and the collected cells were dispersed in about 1.0 ml of distilled water. Approximately 100 to 200 µl of this suspension were then spread-plated on selective medium. Transformation of cells was confirmed by growth on selection medium, identification of cell characteristics indicative of transformation (i.e., increased levels of selected sterols or squalene), and Southern blot analysis of nucleic acid isolated from such transformed cells [Holm et al., Gene , 42 :169-173 (1986)]. EXAMPLE 2: Construction of Episomal Plasmid pSOC725ARC Plasmid pSOC725ARC (See Figure 3) was constructed to place a coding sequence for a truncated HMG1 gene under control of the GAL 1 portion of a GAL 1-10 promoter. Plasmid pSOC725ARC also contains the TRP 1 gene and the yeast 2 micron origin of replication (IR1). This plasmid was prepared from intermediate plasmids as follows. The TRP 1-ARS gene of S . cerevisiae was removed from plasmid YRP12 [Stinchcomb et al. Nature, 282 :39 (1979)] by digestion with Eco RI. The 1445 base pair DNA fragment containing the TRP 1-ARS gene was purified on an agarose gel and ligated into plasmid pUC8 (Viera et al., Gene, (1982)), which had been digested with Eco RI to form plasmid pSOC742. A yeast episomal replication origin, obtained from purified S . cerevisiae two-micron plasmid DNA, was digested with Eco RI and then treated with the Klenow fragment of E . coli DNA polymerase 1 to yield an about 2240 base pair fragment containing the two-micron origin of DNA replication. The about 2240 base pair fragment was purified by agarose gel electrophoresis and ligated into plasmid pUC8, which had been digested with Sma I to form plasmid pSOC743. Plasmid pSOC742 was cleaved with Bam HI and Bgl II to yield an 857 base pair, TRP 1 -containing gene fragment, which was inserted into pSOC743 that had been cut with Bam HI to form plasmid pSOC744. The MEL1 gene was removed from plasmid pMP550 [Sumner-Smith et al., Gene , 36 :333-340 (1985)] with restriction endonucleases Eco RI and Bam HI, and the about 2858 base pair restriction fragment containing MEL1 was purified on an agarose gel. The purified fragment was then ligated into plasmid pUC8 which had been digested with Eco RI and Bam HI to form plasmid pSOC741. The final stage of assembly of pSOC740 was achieved by purifying an about 3101 base pair, Eco RI restriction fragment of pSOC744 that contained the TRP 1 and two-micron origin, and ligating it into Eco RI-cleaved plasmid pSOC741 to form plasmid pSOC740. The GAL 1-10 promoter was excised from pBM258, [Johnston et al., Proc. Natl. Acad. Sci. USA , 79:6971-6975 (1982)] as a 685 base pair Bam HI-Eco RI restriction fragment, and ligated into pUC18, which had been digested with Bam HI and Eco RI to form plasmid pSOC711. Plasmid pSOC740 was digested with Eco RI and the resulting 3101 base pair fragment, containing the two-micron origin of replication and the TRP 1 gene, was isolated and ligated into the Eco RI digested plasmid pSOC711 to produce plasmid pSOC712, in which the TRP 1 gene is proximal to the GAL 1-10 promoter. A Pst I restriction site spanning the coding sequence for amino acid residues 529-530 of HMG-CoA reductase 1 was chosen as the point at which to introduce both a new Bam HI restriction site and a new initiator methionine codon. A 1706 base pair Pst I-Eco RI restriction fragment, containing the coding sequence for the COOH-terminal half of HMG-CoA reductase 1, was purified from a digest of pJR59, [Basson et al., Proc. Natl. Acad. Sci. USA , 83 :5563-5567 (1986)]. This purified pJR59 fragment and a synthetic oligonucleotide: were ligated with pUC18 [Yanisch-Perron et al., Gene , 33:103-119 (1985)], which had been cleaved with Bam HI and Eco RI. The resulting plasmid, pSOC937, contained a Bam HI restriction site 12 base pairs upstream of the truncated HMG-CoA reductase coding sequence initiator methionine. The polypeptide formed from initiation at that point had initial methionine and proline residues followed by amino acid residues 530 through 1054 of the natural HMG-CoA reductase 1. The Eco RI restriction site, which is at the 3' end of the gene, is located 135 base pairs past the end of the coding sequence for the truncated HMG-CoA reductase protein. The truncated gene for HMG-CoA reductase was placed into plasmid pSOC712 by converting the Eco RI site at the 3' end of the truncated reductase gene to a Bam HI site (Klenow polymerase filled, ligated to an oligonucleotide, d5-CGGATCCG, specifying the Bam HI restriction site) and cleaving the preparation with endonuclease Bam HI. A purified, resulting 1728 base pair Bam HI ended restriction fragment from pSOC937 was ligated into the Bam HI-digested pSOC712 to produce plasmid pSOC725ARC, whose schematic restriction map is shown in Figure 3. EXAMPLE 3: Construction of Episomal Plasmid pSOC106ARC Plasmid pSOC106ARC (See Figure 4) was constructed to place a coding sequence for intact HMG1 under the control of the GAL 1 portion of a GAL 1-10 promoter. A 610 base pair Bgl II fragment from pJR59 (about positions 9026-9636), containing the DNA surrounding the beginning of the HMG-CoA reductase coding sequence, was isolated and further restricted with Dde I to provide a DNA fragment (about positions 9151-9636) starting 68 base pairs upstream of the first codon of the HMG-CoA reductase coding sequence. The Dde I and Bgl II fragments were treated with the Klenow fragment of DNA polymerase to render the ends "blunt." The fragments were then ligated to oligonucleotide linkers, d5'-CCGGATCCGG-3 (SEQ ID NO:9), specifying a Bam HI cleavage site (BRL linkers). The ligated fragments were digested with Bam HI to produce ligateable Bam HI restriction ends, and the resulting 499 base pair fragment containing the start of the HMG-CoA reductase coding sequence was ligated into Bam HI digested pBR322 to form plasmid pSOC104. The remainder of the HMG-CoA reductase coding sequence was reconstructed downstream of the new 5' Bam HI site by ligating a 1477 base pair Xba I-Sac I DNA fragment of pJR59, which specifies the 5' half of the HMG-CoA reductase coding sequence, and a 2101 base pair Sac I-Sal I fragment of pJR59, which specifies the 3' half of the HMG-CoA reductase coding sequence, into pSOC104 digested with Xba I and Sal I to form plasmid pSOC105 containing a 3903 base pair Bam HI-Sal I restriction fragment having the entire coding sequence for HMG-CoA reductase. This 3903 base pair fragment was ligated into Bam HI-Sal I-restricted pSOC712 (See Example 2) to form plasmid pSOC106ARC. EXAMPLE 4: Construction of Integrating Plasmid pARC306E Plasmid pARC306E (See Figure 5) was constructed to place a coding sequence for truncated HMGl under control of the GAL 1 portion of a GAL 1-10 promoter. Plasmid pARC306E contains the S . cerevisiae TRP 1 gene and a GAL 1 promoter-driven, truncated HMG-CoA reductase gene housed on an E . coli replicon, which specifies ampicillin resistance. There are no S . cerevisiae replicators on plasmid pARC306E. Unique restriction sites within both the TRP 1 gene (Eco RV, position 865) and the truncated HMG-CoA reductase gene (Cla I, position 4280) serve as sites for the generation of linear plasmids with DNA homologous to S . cerevisiae chromosomal DNA on both sides of the restriction site. Thus, plasmid pARC306E can be incorporated into the chromosome at either site via homologous recombination. The multiple restriction recognition site of plasmid pUC8, located between the Eco RI and Hind III sites, was replaced by the oligonucleotide: to create plasmid pUC8NL, which has a single restriction site for the nuclease enzyme Cla I. Plasmid pSOC712 (See Example 2) was digested with Eco RI and the fragments treated with nuclease S1 and bacteriophage T4 DNA polymerase plus deoxynucleotides to remove the overhanging 5' Eco RI restriction ends. These ends were ligated to the oligonucleotide: and the fragments treated with Cla I nuclease to produce Cla I restriction ends. The resulting 3108 base pair Cla I-Cla I fragment, containing the yeast TRP 1 gene and the two-micron replicator, was purified by gel electrophoresis and ligated into pUC8NL, which had been cleaved with Cla I, to create plasmid pARC300A. A 2031 base pair fragment containing the two-micron replication origin was removed from pARC300A by treatment with nuclease Pst I. The resulting modified plasmid pARC300A was treated with nuclease S1 and bacteriophage T4 DNA polymerase plus deoxynucleotides to remove the Pst I restriction overhangs and with calf intestinal alkaline phosphatase to disallow reclosure of the plasmid. The modified pARC300A plasmid was coligated with the oligonucleotide: to introduce a Cla I site just downstream (to the 3' end) of the TRP 1 gene to form a plasmid, and then closed to form pARC306B. The TRP 1 gene was separate from yeast replicators, and bounded by Cla I restriction sites. Plasmid pARC306B was digested with Cla I, purified by polyacrylamide gel electrophoresis and the Cla I-Cla I restriction fragment was introduced into plasmid pUC8, which had been cleaved with nuclease Acc I, to form plasmid pARC306C. As the integration of exogenous DNA into yeast chromosomes is best carried out using homologous recombination, a dispensable fragment of yeast DNA was desired. This DNA would be used to drive homologous recombination if for some reason, recombination at the TRP 1 or HMG-CoA reductase gene were not utilizable. The DNA chosen for this purpose was the HIS3 gene. An 1800 pair Bam HI-Bam HI restriction fragment was removed from plasmid YEP6 [Struhl et al., Proc. Natl. Acad. Sci. USA , 76 :1035 (1979)] and introduced into plasmid pARC306C, which had been cleaved with Bam HI, to create plasmid pARC306D. Plasmid pSOC725 (See Example 2) was digested with Eco RI to yield a GAL 1-10 promoter linked to a truncated HMG-CoA reductase gene, which was then inserted into Eco RI-digested plasmid pARC306D, to form plasmid pARC306E. EXAMPLE 5: Construction of Integrating Plasmid pARC300D Plasmid pARC300D (See Figure 6) was constructed to place a coding sequence for a truncated HMG1 gene under the control of a PGK promoter. This plasmid was prepared from intermediate plasmids as follows. Plasmid pSOC611 was constructed to determine the efficacy of the mouse metallothionine promoter as a transcriptional driver for the truncated HMG-CoA reductase gene in yeast. Construction of pSOC611 began with restriction of plasmid pSOC744 (See Example 2) with Eco RI endonuclease, followed by treatment with Klenow Polymerase I and deoxynucleotide triphosphates to fill in the Eco RI restriction ends. The resulting about 3101 base pair 2-micron- and TRP 1 -containing fragment of pSOC744 was ligated to pUC18 which had been cleaved with Hinc II, to form plasmid pSOC517. Plasmid pSOC517 was then cleaved with Kpn I and Eco RI and the mouse metallothionine promoter was inserted as a Kpn I-Eco RI restriction fragment to form plasmid pSOC518. This promoter region is composed of the Kpn I to Bgl II fragment originally in pJYMMT(e) [Hammer et al., Journal of Applied Molecular Genetics, Vol. 1:273 (1982)] as well as a short Bgl II, Eco RI DNA fragment of unknown sequence. The truncated HMG-CoA reductase gene was added to pSOC518 in two steps. First, the truncated HMG-CoA reductase gene was removed from pSOC725 as a Bam HI restriction fragment. This fragment was then ligated into M13mp7 which had been cleaved with Bam HI. The new M13 derivative formed was designated pSOC610. The truncated HMG-CoA reductase gene was removed from pSOC610 as an Eco RI fragment and inserted into Eco RI-digested plasmid pSOC518. The resulting plasmid was designated pSOC611. Plasmid pUC8 was partially digested with restriction endonuclease Hae II and religated. Transformants arising from this procedure were screened to find a plasmid missing the Hae II restriction fragment containing the portion of the lac operon which was originally present in plasmid pUC8. This new plasmid was designated pSOC505ARC. Restriction sites for the endonucleases Eco RI, Hind III and Kpn I were introduced into the Nde I site of plasmid pSOC505ARC by ligation of the oligonucleotide: into Nde I-digested pSOC505ARC to form plasmid pARC303A. To form the new multi-cloning site, the normal multi-cloning site present in M13mp18 was altered by ligating the oligonucleotide: into Bam HI-Kpn I digested M13mp18. This resulted in an altered M13 virus, designated pARC303B. This construct lacked both the Kpn I and Sma I sites normally found in the M13mp18 multi-cloning site. The new multi-cloning site was removed as an Eco RI, Hind III restriction fragment from pARC303B, and was ligated into Eco RI, Hind III restricted plasmid pARC303A to form plasmid pARC303C. In addition to a variation in the normal array of sites included in the multi-cloning site, another smaller multi-cloning site was introduced into the vector, at a point some distance away from the first multi-cloning site to allow for independent manipulation of yeast auxotrophic complementation markers and other features which did not have to be proximal to the promoters and coding sequences which would be inserted in the large multi-cloning site. The new array of restriction sites was introduced by ligation of the oligonucleotide: into pARC303C cleaved with endonuclease Aat II to form plasmid pARC300E, which contained the series of cloning sites, Aat II, Sma I, and Cla I at the former Aat II site. The yeast TRP 1 gene was isolated as an 820 base pair fragment from pARC306B (See Example 4) with the restriction endonuclease Cla I. The 820 base pair Cla I-Cla I fragment was purified by agarose gel electrophoresis and ligated into plasmid pARC300E, which had been digested with Cla I, to create plasmid pARC300B. Plasmid pSOC611 was digested with Bam HI and Ssp I to yield a 1667 base pair coding sequence for the truncated HMG-CoA reductase gene which was purified by agarose gel purification. The 1667 base pair fragment was ligated to Bam HI, Hinc II restricted plasmid pARC300B to generate plasmid pARC300C. A source of an alternate promoter to the GAL 1-10 promoter which has been used to drive transcription of the truncated HMG-CoA reductase gene was desired. Use of the GAL 1-10 promoter requires that the yeast be cultured on galactose, an expensive substrate. In order to achieve high levels of transcription through the truncated HMG-CoA reductase gene during culture, growth in the presence of the much less expensive substrate, glucose, the promoter from the S . cerevisiae phosphoglycerate kinase ( PGK ) gene was isolated. The sequence of the gene is available from the literature, [Hitzeman, et al., Nucl. Acid Res. , 10 :7791-7808 (1982)]. From the known sequence, an oligonucleotide probe sufficiently complementary to the gene to be used as a hybridization probe was synthesized: This probe was labelled by T 4 polynucleotide kinase treatment in the presence of 32 P-ATP, and used to screen a library of bacteriophage λ subclones of the yeast genome, supplied by Maynard Olson (Washington University School of Medicine, Department of Genetics, St. Louis, Mo.). The gene was removed from this clone as an Eco RI-Hind III fragment, and subcloned into M13mp18, forming a new phage mARC127. To make the PGK promoter useful, the restriction site at the 5' end of the promoter was changed to an Eco RI restriction site, and a Bgl II restriction site was introduced into the DNA fragment to the 3' side of the transcriptional start site. The Bgl II restriction site was introduced by using the oligonucleotide: to mutagenize mARC127 according to the procedure of Kunkel et al., Proc. Natl. Acad. Sci. USA, 82 :4778 (1985). This resulted in the M13 phage designated mARC128. The Hind III site beyond the 5' end of the promoter region was converted to an Eco RI site by cutting mARC128 with nuclease Hind III, treating with the Klenow fragment of DNA polymerase and the four deoxynucleotide triphosphates, followed by ligation in the presence of the oligonucleotide: which specifies an Eco RI site. The resulting M13 derivative was designated pARC306L. Plasmid pARC306L was digested with Eco RI and Bgl II and a 1500 base pair fragment containing the PGK promoter, was purified by agarose gel electrophoresis and ligated into pARC300C, which had been restricted with Eco RI and Bam HI, to produce plasmid pARC300D. EXAMPLE 6: Construction of Integrating Plasmids pARC300S and pARC300T Plasmids pARC300S (See Figure 7) and pARC300T (See Figure 8) were constructed to incorporate a URA 3 selectable marker into an integrating vector, in which a coding sequence for a truncated HMG1 gene was under the control of a PGK promoter. The only difference between plasmid pARC300S and pARC300T is the length of the PGK promoter driving transcription of the truncated reductase coding sequence. A unique Eco RV restriction site found within the URA 3 gene allows the plasmids to be linearized and integrated via homologous recombination into the chromosomal URA 3 gene. The URA 3 gene from plasmid YEP24 (Botstein, et al., Gene , 8 :17-24 (1979)) was removed as an 1127 base pair Eco RI-Sma I ended restriction fragment and ligated into plasmid pUC19, cut with Eco RI and Sma I to form a new plasmid LpARCLH550. An 1141 base pair Hind III ended restriction fragment was removed from LpARCLH550 and ligated into Hind III-cleaved pUC18 to form plasmid LpARCLH553a. An 1108 base pair Sma I-Hind III restriction fragment was removed from LpARCLH553a and inserted into Sma I-Hind III cleaved M13mp19 nucleic acid to create a new phage nucleic acid pARC306K. The unique PstI site within the URA 3 gene was eliminated by mutagenesis with the oligonucleotide: using the method of Kunkel, L.M.; et al., Proc. Nat'l. Acad. Sci. USA, 82:4778 (1985), to form plasmid pARC300Z. Plasmid pARC300Z was cut with Hind III, the ends filled in with the Klenow fragment of DNA polymerase and deoxynucleotide triphosphates, and the modified pARC300Z ligated with oligonucleotide d5'-CCCCGGGG-3', which specified a Sma I restriction site. This new M13 derivative, which contains the URA 3 gene on a Sma I restriction fragment, was named plasmid pARC300Y. Plasmid pARC304A was constructed to provide a source of a modified URA 3 transcription terminator fragment which could then be introduced at the 3' end of the coding sequence region in a yeast integrating transformation vector. The transcription terminator would function to improve mRNA stability in species transformed with integrating vectors containing coding sequences either lacking the terminator or having only weak terminator sequences. Improved mRNA stability could mean increased activity of the protein encoded by the coding sequence region. The terminator chosen was a region of the S . cerevisiae URA 3 , which functions as a terminator, [Yarger et al., Molecular and Cellular Biology , 6 :1095 (1986)]. The terminator sequence was constructed using 4 synthetic oligomers: These oligomers were designed to provide Hind III and Kpn I restriction ends, respectively. The modified URA 3 transcription terminator was assembled by ligating all four oligomers to each other and digesting the ligation product with Hind III and Kpn I to produce ligatable Hind III-Kpn I restriction ends. The 67 base pair fragment was isolated on a polyacrylamide gel, purified by electroeluting the DNA from the gel fragment, and then ligated into Hind III-Kpn I restricted pUC118, (ATCC 37462). This construction created a new plasmid designated pARC304A. A 67 base pair Hind III-Kpn I fragment containing a URA 3 transcription terminator was isolated from plasmid pARC304A and ligated into Hind III-Kpn I restricted pARC300E to form plasmid pARC300M. A truncated HMG-CoA reductase coding sequence was isolated as a 1667 base pair Bam HI-Ssp I fragment from pSOC611, (See Example 5) purified by agarose gel electrophoresis, and ligated to pARC300M, which had been restricted with Bam HI and Hinc II, to form plasmid pARC300R. A URA 3 complementing gene was removed from plasmid pARC300Y as an Xma I restriction fragment, and ligated into the Xma I site of pARC300R to create plasmid pARC300U. One other change in the restriction sites available on the DNA specifying the PGK promoter was made. The minimum DNA required to specify full PGK promoter activity has been determined, [Stanway, Nucleic Acids Research , 15 :6855-6873 (1987)]. A new Eco RI site was added to the DNA specifying the PGK promoter at a region just past the minimal 5' required DNA. The site was added by utilizing the oligonucleotide: to mutagenize mARC1228 by the method of Kunkel et al., Proc. Natl. Acad. Sci. USA , 82 :4778 (1985). This new M13 derivative was designated pARC306M. A 1500 base pair phosphoglycerate kinase promoter ( PGK ) was removed from plasmid pARC306L (See Example 5) using Eco RI and Bgl II restriction enzymes. The PGK promoter fragment was purified by agarose gel electrophoresis and ligated to Eco RI and Bam HI restricted pARC300U, to form plasmid pARC300S. A shortened PGK promoter (555 base pair fragment) was isolated from Eco RI and Bgl II restricted plasmid pARC306M and inserted into Eco RI-Bam HI digested plasmid pARC300U to form plasmid pARC300T. The only difference between plasmid pARC300S and plasmid pARC300T is the length of the PGK promoter driving transcription of the truncated reductase coding sequence. A unique Eco RV restriction site found within the URA 3 gene allows the plasmids to be linearized and integrated via homologous recombination into the chromosomal URA 3 gene. EXAMPLE 7: Construction of Plasmid pARC304S Plasmid pARC304S (see Figure 9) was constructed to place the coding sequence of a truncated HMG1 gene under the control of an ADH promoter. Plasmid pBR322 was digested with Eco RI and Bam HI to yield a fragment containing the ADH1 promoter. The ADH1 -containing fragment was ligated into plasmid pARC300U (See Example 6), which had been cut with Eco RI and Bam HI, to form pARC304S. Plasmid pARC304S was deposited pursuant to the Budapest Treaty requirements with the American Type Culture Collection (ATCC) at 12301 Parklawn Drive, Rockville, MD 20852 U.S.A. on November 9, 1990 and was assigned Accession No. ATCC40916. EXAMPLE 8: Generation of Mutant S. cerevisiae ATC0402mu Mutant ATC0402mu was generated to have the GAL, a, and trp1 phenotype as well as having defects in the expression of zymosterol-24-methyltransferase and ergosta-5,7,24(28)-trienol-22-dehydrogenase enzymes. These enzymes are respectively the erg6 and erg5 gene products of S . cerevisiae . An erg6 deficient mutant S . cerevisiae , M610-12B, obtained from the Yeast Genetic Stock Center (Univ. of California, Berkeley, CA), was crossed with an erg5 deficient mutant S . cerevisiae (obtained as a gift from Dr. Leo Parks, North Carolina State Univ., Raleigh, NC) to produce an erg6-erg5 double mutant, ATC0403mu. ATC0403mu was then crossed with wild-type S . cerevisiae, DBY745 (Yeast Genetic Stock Center) to produce mutant ATC0402mu. Mutant ATC0402mu was deposited pursuant to the Budapest Treaty Requirements with the American Type Culture Collection (ATCC) at 12301 Parklawn Drive, Rockville MD 20852 U.S.A. on November 9, 1990, and was assigned Accession No. ATCC 74027. EXAMPLE 9: Generation of Transformed Mutants ATC1500cp, ATC1502, ATC1503, ATC1551 and ATC2401 Several mutants were generated from the transformation of ATC0402mu using the method of Example 1, with various expression systems (plasmids) containing HMG-CoA reductase coding sequences under the transcriptional control of various promoters. The introduction into ATC0402mu of plasmid pSOC106ARC, constructed according to the method of Example 3, created ATC1503. The introduction into ATC0402mu of plasmid pSOC725ARC, constructed according to the method of Example 2, created ATC2401mu. The introduction into ATCO402mu of plasmid pARC306E, constructed according to the method of Example 4, created ATC1502. The introduction into ATC0402mu of plasmid pARC300D, constructed according to the method of Example 5, created ATC1500cp. The creation of strain ATC1551 required the generation of a ura3 derivative of strain ATC1500cp, which has no auxotrophic markers. The ura3 derivative was created by transforming ATC1500cp with a mutagenic oligonucleotide using the method of Moerschell et al. [ Proc. Natl. Acad. Sci. USA , 85 :524-528 (1988)]. The sequence of the mutagenic oligonucleotide used is: Mutated yeast cells were selected by their resistance to 5'-fluoro-orotic acid (5-FOA), as described in Ausubel et al., (eds.), Current Protocols in Molecular Biology , John Wiley and Sons, New York, (1989), and screened for their inability to grow in the absence of uracil. The resulting ura3 strain was designated ATC0135rc. Strain ATC0315rc was then transformed with plasmid pARC304S, constructed according to the method of Example 7, to create strain ATC1551. Transformation of strain ATC0315rc with plasmid pARC304S of the present invention resulted in the greatest degree of sterol accumulation. Further, the growth of a transformed ATC0315rc mutant under conditions of restricted aeration as compared to usual culture conditions, resulted in an increased accumulation of squalene relative to other sterols as well as an increase in the total accumulation of squalene and total sterols. Mutant ATC0315rc was deposited pursuant to the Budapest Treaty Requirements with the American Type Culture Collection (ATCC) at 12301 Parklawn Drive, Rockville, MD 20852 U.S.A. on September 16, 1991, and was assigned Accession No. ATCC 74090. EXAMPLE 10: Generation of Mutant S. cerevisiae ATC6118, ATC0501 and ATC6119 Mutants were obtained using an inducible "TY1-neo" transposon as the mutagenic agent, [Boeke, et al., Science, 239:280-282 (1989)]. Wild type S . cerevisiae JB516 was transformed with plasmid pJEF1105 [Boeke et al., Science, 239:280-282 (1989)3, containing an inducible GAL : TY1neo expression cassette, and plasmid pCGS286, containing a GAL : lacZ control. The transformed yeast were then spread onto petri dishes containing two kinds of Xgal chromogenic indicator dye: synthetic dextrose (SD) agar media minus uracil and synthetic galactose (SG) agar media minus uracil. Yeast transformed with plasmid pJEF1105 appeared normal on dextrose but smaller than untransformed control yeast on galactose media. The stability of plasmid pJEF1105 was confirmed by shuttling into E . coli for propagation and restriction analysis. Once plasmid pJEF1105-transformed yeasts were shown to be competent, the pJEF1105 transformants were placed on SG-minus uracil agar at a density of no more than 1000 transformants per petri plate. The plates were incubated at 22°C for five days, during which the mutagenic transposition of the plasmid borne TY1 -neo occurred. The transformants were then replica plated onto another SG-minus uracil plate and incubated another five days. Those colonies that survived were replica plated onto YEPD agar plates containing 100 units/ml of nystatin to select for sterol production and 100 units/ml of G418 (a neomycin analog) to select for the "neo" phenotype. Transformants that were both nystatin and G418 resistant were evaluated for sterol content and distribution using gas chromatographic and mass spectrographic analysis and then classified as to the specific sterol biosynthetic step affected by the mutation. A yeast deficient in the enzyme episterol-5-dehydrogenase (the erg3 gene product) was isolated and designated ATC6118. A yeast deficient in the enzyme zymosterol-24-methyltransferase (erg6) was isolated from plasmid pJEF1105 mutated yeast DBY745 (Yeast Genetic Stock Center) and designated ATC0501. ATC0501 was crossed with ATC6118 to produce an erg3-erg6 double mutant designated ATC6119. EXAMPLE 11: Generation of Transformed Mutant S. cerevisiae ATC2100, ATC2104 and ATC2109 Following the method of Example 1, the introduction into ATC6119 of plasmids pARC300S and pARC300T, constructed according to the method of Example 6, created ATC2100 and ATC2104 respectively, whereas the introduction into ATC6118 of plasmid pARC300S created ATC2109. EXAMPLE 12: Generation of Mutant S. cerevisiae ATC4124 ATC4124 (Yeast Genetic Stock Centers) was generated by crossing ATC0403mu with YNN281 (Yeast Genetic Stock Centers) and selecting for the desired mutation. The resulting segregants were then backcrossed twice with YNN281. Resulting ATC4124 had a defect in the expression of cholesta-5,7,24(28)-trienol-22-dehydrogenase (the ergs gene product). EXAMPLE 13: Generation of Transformed Mutant S. cerevisiae ATC2107 and ATC2108 Following the method of Example 1, introduction into ATC4124 of plasmid pARC306E, constructed according to the method of Example 4, created ATC2107 and ATC2108. EXAMPLE 14: HMG-CoA Reductase Activity in Mutant and Transformed Yeast HMG-CoA reductase activity was measured in non-transformed and transformed erg5-erg6 mutant yeasts. About 0.2 ml of 50 mM potassium phosphate buffer, pH 6.8, containing 125 mM sucrose, 20 mM EDTA and 100 mM KCl was combined with 10 mM DTT (freshly made), 1 mM NADPH, enzyme preparation and water to make an enzyme solution of about 0.475 ml final volume. The enzyme solution was preincubated at 37°C for 20 minutes and the incubation reaction initiated with the addition of 100 µM 14 C-HMG-CoA (60,000 dpm in 0.025 ml). After five minutes, the reaction was stopped by the addition of 50 µl of HCl (1:1) and further incubation at 37°C for 30 minutes to lactonize the product. The product, mevalonolactone, was separated from HMG on an anion exchanger AGI-X8 (Bio-Rad) and the radioactivity associated with the product was counted in a scintillation counter. The results are shown in Table 5, below. The copy number of an added structural gene encoding a polypeptide having HMG-CoA reductase activity was estimated using standard procedures well known to those of skill in the transformation art. Mutant Estimated Copy # of Added Structural Gene Specific Activity HMG-CoA Reductase (mmols/min/mg dry wt) Non-transformed ATC0402mu 0 0.52 Transformed ATC1503 1,2 0.69 ATC1500cp 5,6 1.33 ATC1512 8,9 1.01 EXAMPLE 15. Squalene and Sterol Accumulation in Yeast The accumulation of squalene and specific sterols was determined in non-transformed and transformed mutant yeast cultures. Fifty to one hundred mg of lyophilized yeast cells were extracted/saponified in 10 ml of an ethanol/water (2:1) solution containing 20 percent (w/v) KOH for two hours at 80°C. Extracts were partially neutralized with 10 ml 1N HCl and extracted twice with 15 ml n-heptane. The sterol-containing heptane fractions were evaporated to dryness under a stream of N 2 and resuspended to an appropriate volume with n-heptane containing an internal standard (5-alpha-cholestane). The resuspended samples were analyzed for sterol accumulation by capillary GC with flame ionization detection. Table 6 contains summary data for non-transformed (control) and transformed mutants having a single defect (erg3, erg5) in the expression of sterol biosynthetic pathway enzymes. Table 7 contains summary data for non-transformed (control) and transformed mutants having double defects (erg3-erg6, erg5-erg6) in the expression of sterol biosynthetic pathway enzymes. In both Table 6 and Table 7, the transformants were all made by transforming the control mutant having the same erg mutation. Sterol levels are expressed as a percent of the dry biomass. ERG3 Mutants Percent of Biomass Sterol Non-transformed Transformed ATC6118 ATC2109 a. Squalene N.D. 0.26 b. ergosta-8,22-dienol 0.31 1.08 c. ergosta-7,22-dienol 0.66 1.64 d. ergosta-8-enol 0.27 0.42 e. ergosta-7-enol 0.63 0.72 ERG5 Mutants Percent of Biomass Sterol Non-transformed Transformed ATC4124 ATC2107 ATC2108 a. Squalene N.D. 1.10 0.49 b. Zymosterol 0.05 0.25 0.25 c. ergosta-5,7,24(28)-trienol and ergosta-5,7-dienol 0.17 1.75 1.19 The above data illustrate that transformation of mutants having a single defect in the expression of sterol biosynthetic pathway enzymes resulted in an increased accumulation of squalene and specific sterols (See Table 6). Relative to a non-transformed erg3 mutant, erg3 mutants transformed with a plasmid vector useful in the present invention overaccumulated squalene, ergosta-8,22-dienol, ergosta-7,22-dienol, ergosta-8-enol and ergosta-7-enol. Relative to a non-transformed erg5 mutant, erg5 mutants transformed with a plasmid vector useful in the present invention overaccumulated squalene, zymosterol, and a mixture of ergosta-5,7,24(28)-trienol and ergosta-5,7-dienol. Similarly, transformation of mutants having double defects in the sterol biosynthetic pathway enzymes led to the overaccumulation of squalene and specific sterols. Relative to a non-transformed erg3-erg6 mutant, erg3-erg6 mutants transformed with a plasmid vector useful in the present invention overaccumulated squalene, zymosterol and cholesta-7,24-dienol. Relative to a non-transformed erg5-erg6 mutant, erg5-erg6 double mutants transformed with a plasmid vector useful in the present invention overaccumulated squalene, zymosterol, cholesta-5,7,24-trienol and cholesta-7,24-dienol. The greatest increases in squalene and specific sterol accumulation are seen when erg5-erg6 mutant ATC0315rc is transformed with plasmid vector pARC304S (mutant ATC1551), as described in Example 9. Further, the data show that species ATC0402mu, the grandparent strain of mutant ATC0315rc, has elevated levels of sterols relative to either an erg5 or an erg6 single mutant (see Table 6). EXAMPLE: 16 Induction of Squalene Accumulation in Yeast Transformant ATC1551 It is generally known that restricted aeration induces squalene accumulation at the expense of sterols in yeast cultures. This occurs because oxygen is required for the enzymatic conversion of squalene to squalene monoepoxide, which in turn is converted into lanosterol and other yeast sterols. To determine if high levels of squalene accumulation could be induced in transformants, cultures of ATC1551 were grown under varying degrees of aeration by varying the volume (and hence the surface-to-volume ratio) of growth medium in shake-flask cultures and assaying squalene and total sterol at one day intervals over a period of four days. Triplicate 250 ml baffled shake-flasks respectively containing 50, 100, 150 and 200 ml of YEP/2 percent glucose growth medium were inoculated with two ml of a 24 hour liquid culture of ATC1551 grown on a rotary shaker (200 rpm) at 30°C. Fifty ml culture aliquots were harvested by centrifugation after one, two, three and four days growth under the aforementioned conditions and lyophilized overnight. To insure efficient squalene extraction, approximately 100 mg of each lyophilized sample was agitated for 10 minutes in 15 ml conical tubes containing a suitable quantity of glass beads and a small amount of water. The disrupted cell material was then extracted three successive times with 10 ml of 100 percent ethanol with vigorous agitation for one hour at 80°C. The combined ethanol extracts were reduced to dryness under a stream of nitrogen and redissolved in two ml of heptane containing 5α-cholestane as the internal standard. GC analyses of squalene were conducted as previously described. For total sterol analyses, the same samples were reduced under a stream of nitrogen and saponified in 5 ml of 95 percent ethanol/water solution containing 0.3 M KOH for one hour at 80°C. An equivalent volume of water was added and the samples were twice extracted with 10 ml aliquots of heptane. The heptane extracts were combined, reduced to a suitable volume and analyzed by GC. The results are shown in Table 8 (data averaged from triplicate cultures and expressed as percent of dry biomass). Growth Medium Volume 50 ml 100 ml 150 ml 200 ml Time to Harvest Percent of Dry Biomass Day 1 squalene 4.25 5.40 3.61 2.63 total sterol 9.40 9.52 6.81 5.46 Day 2 squalene 4.78 6.43 11.89 8.32 total sterol 8.29 6.44 3.72 2.98 Day 3 squalene 4.75 8.82 13.54 13.38 total sterol 7.96 7.65 4.36 4.19 Day 4 squalene 4.03 7.08 15.99 14.72 total sterol 7.09 8.62 5.10 3.39 The data show that in transformed, erg5-erg6 mutants, squalene is preferentially accumulated as compared to total sterol by restricting the level of aeration as compared to usual culture conditions (50 mls of growth medium), particularly after more than about one day of culture. The data also show that restricting the level of aeration (lowering the surface-to-volume ratio) also increases the sum total of squalene and total sterol accumulation, after more than about two days of culure. Although the present invention has now been described in terms of certain preferred embodiments, and exemplified with respect thereto, one skilled in the art will readily appreciate that various modifications, changes, omissions and substitutions may be made without departing from the spirit thereof. SEQUENCE LISTING (1) GENERAL INFORMATION: (i) APPLICANT: Saunders, Court A. Wolf, Fred R. Mukharji, Indrani (ii) TITLE OF INVENTION: A Method and Composition for Increasing the Accumulation of Squalene and Specific Sterols in Yeast (iii) NUMBER OF SEQUENCES: 24 (iv) CORRESPONDENCE ADDRESS: (A) ADDRESSEE: Amoco Corp., Patents and Licensing Dept. (B) STREET: 200 East Randolph St. (C) CITY: Chicago (D) STATE: Illinois (E) COUNTRY: USA (F) ZIP: 60680-0703 (v) COMPUTER READABLE FORM: (A) MEDIUM TYPE; Floppy disk (B) COMPUTER: IBM PC compatible (C) OPERATING SYSTEM: PC-DOS/MS-DOS (D) SOFTWARE: PatentIn Release #1.0, Version #1.25 (vi) CURRENT APPLICATION DATA: (A) APPLICATION NUMBER: 07/613,380 (B) FILING DATE: November 15, 1990 (C) CLASSIFICATION: (viii) ATTORNEY/AGENT INFORMATION: (A) NAME: Galloway, Norvall B. (ix) TELECOMMUNICATION INFORMATION: (A) TELEPHONE: 312 856-7180 (B) TELEFAX: 312 856-4972 (2) INFORMATION FOR SEQ ID NO:1: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 3360 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: DNA (genomic) (ix) FEATURE: (A) NAME/KEY: CDS (B) LOCATION: 121..3282 (xi) SEQUENCE DESCRIPTION: SEQ ID NO:1: (2) INFORMATION FOR SEQ ID NO:2: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 1054 amino acids (B) TYPE: amino acid (D) TOPOLOGY: linear (ii) MOLECULE TYPE: protein (xi) SEQUENCE DESCRIPTION: SEQ ID NO:2: (2) INFORMATION FOR SEQ ID NO:3: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 4768 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: cDNA (ix) FEATURE: (A) NAME/KEY: CDS (B) LOCATION: 164..2827 (xi) SEQUENCE DESCRIPTION: SEQ ID NO:3: (2) INFORMATION FOR SEQ ID NO:4: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 887 amino acids (B) TYPE: amino acid (D) TOPOLOGY: linear (ii) MOLECULE TYPE: protein (xi) SEQUENCE DESCRIPTION: SEQ ID NO:4: (2) INFORMATION FOR SEQ ID NO:5: (i) SEQUENCE CHARACTEROSTOCS: (A) LENGTH: 3348 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: DNA (genomic) (ix) FEATURE: (A) NAME/KEY: CDS (B) LOCATION: 121..3255 (xi) SEQUENCE DESCRIPTION: SEQ ID NO:5: (2) INFORMATION FOR SEQ ID NO:6: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 1045 amino acids (B) TYPE: amino acid (D) TOPOLOGY: linear (ii) MOLECULE TYPE: protein (xi) SEQUENCE DESCRIPTION: SEQ ID NO:6: (2) INFORMATION FOR SEQ ID NO:7: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 22 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: DNA (genomic) (xi) SEQUENCE DESCRIPTION: SEQ ID NO:7: (2) INFORMATION FOR SEQ ID NO:8: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 14 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: DNA (genomic) (xi) SEQUENCE DESCRIPTION: SEQ ID NO:8: (2) INFORMATION FOR SEQ ID NO:9: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 10 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: DNA (genomic) (xi) SEQUENCE DESCRIPTION: SEQ ID NO:9: (2) INFORMATION FOR SEQ ID NO:10: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 33 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: DNA (genomic) (xi) SEQUENCE DESCRIPTION: SEQ ID NO:10: (2) INFORMATION FOR SEQ ID NO:11; 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Claims: A method of increasing accumulation of squalene, and at least one further sterol in S. cerevisiae selected from zymosterol, cholesta-7,24-dienol, cholesta-5,7,24-trienol, ergosta-8,22-dienol, ergosta-7,22-dienol, ergosta-8-enol, ergosta-7-enol, ergosta-5,7,24 (28)-trienol and ergosta-5,7-dienol, comprising transforming an erg 3, erg 5, erg 3-erg 6 or erg 5-erg 6 mutant of S. cerevisiae with a recombinant DNA molecule comprising a vector operatively linked to an exogenous DNA segment that encodes the catalytic region but is free from the membrane binding region of an HMG-CoA reductase enzyme, and a promoter suitable for driving the expression of said reductase in said yeast. The method according to claim 1 wherein said DNA segment further encodes at least a portion of the linker region of the HMG-CoA reductase enzyme. The method according to claim 1 wherein squalene is accumulated relative to the additional sterol by culturing said yeast under conditions of restricted aeration. The method according to claim 1 wherein the promoter is selected from GAL 1, GAL 10, GAL 1-10, PGK and ADH promoters. The method according to claim 1 wherein the promoter and the exogenous DNA segment are integrated into the chromosomal DNA of said yeast. The method according to claim 1 for increasing squalene, ergosta-8,22-dienol, ergosta-7,22-dienol, ergosta-8-enol and ergosta-7-enol where an erg 3 mutant of S. cerevisiae is tranformed. The method according to claim 1 for increasing squalene, zymosterol and cholesta-7,24-dienol wherein an erg 3-erg 6 mutant S. cerevisiae is transformed. The method according to claim 1 for increasing squalene, zymosterol, ergosta-5,7,24(28)-trienol and ergosta-5,7-dienol wherein an erg 5 mutant S. cerevisiae is transformed. A method according to claim 6, 7 or 8 wherein the recombinant DNA molecule is selected from the group of plasmid vectors consisting of plasmids pSOC725ARC (Figure 3), pSOC106ARC (Figure 4), pARC300D (Figure 6), pARC306E (Figure 5), pARC300S (Figure 7), pARC300T (Figure 8) and pARC304S (Figure 9). A mutant of S. cerevisiae having a single or double defect in the expression of enzymes that catalyze the conversion of squalene to ergosterol selected from erg 3, erg 5, erg 3-erg 6 and erg 5-erg 6 mutants, which is transformed with a recombinant DNA molecule comprising a vector operatively linked to an exogenous DNA segment that encodes the catalytic region and at least a portion of the linker region but is free from the membrane binding region of an HMG-CoA reductase enzyme, and a promoter suitable for driving the expression of said reductase in said yeast. The mutant according to claim 10 wherein the mutant is selected from a mutant deposited as ATCC 74090, or a mutant obtained by transforming a cell deposited as ATCC 74027 with a plasmid of Figure 3, Figure 4, Figure 5 or Figure 6, or a mutant obtained by transforming a cell of ATCC 74090 with a plasmid of Figure 9. A recombinant DNA molecule selected from the group of plasmids designated plasmids pARC304S (Figure 9), pARC300S (Figure 7), pARC300T (Figure 8), pARC300D (Figure 6), pARC306E (Figure 5), pSOC106ARC (Figure 4) and pSOC725ARC (Figure 3).

Inventors: MUKHARJI INDRANI, SAUNDERS COURT A, WOLF FRED RICHARD, MUKHARJI, INDRANI, SAUNDERS, COURT A., WOLF, FRED RICHARD
Applicants: BP CORP NORTH AMERICA INC, BP CORPORATION NORTH AMERICA INC.
Application year: 1991
Main classification: C12N15
IPC codes: C12N15, C12N1, C12N9, C12P33, C12P5, C12R1