IMPROVED STRAINS OF YEAST FOR THE EXPRESSION OF HETEROLOGOUS GENES

Abstract: The GAL4 protein is rate-limiting in quantity as a positive regulator for galactose-inducible promoters in strains of yeast. Novel strains are described in which the GAL4 protein can be overproduced in a regulatable fashion. These strains are useful for the regulatable expression in yeast of hetero­ logous genes whose expression is driven by a galactose-­ inducible promoter.

Description: BACKGROUND OF THE INVENTION Saccharomyces cerevisiae has proven versatile as a host species for the expression of foreign polypeptides. Many different proteins from a variety of species have been expressed in S. cerevisiae , some to levels of >10% of total cell protein. Typically, expression has been mediated by a plasmid containing yeast regulatory sequences (transcriptional promoter and terminator) circumscribing the structural gene for the expressed polypeptide as well as other sequences required for the selection and amplification of plasmids in both S. cerevisiae and in Escherichia coli . In addition, it has been possible to integrate the yeast regulatory sequences circumscribing the structural gene for the expressed polypeptide into a yeast chromosome and achieve high-level expression. S. cerevisiae has 4 genes which encode the enzymes responsible for the utilization of galactose as a carbon source. The GAL 1, GAL 2, GAL 7, and GAL 10 genes respectively encode galactokinase, galactose permease, α-D-galactose-1-phosphate uridyltransferase, and uridine diphosphogalactose-4-epimerase. In the absence of galactose, very little expression of these enzymes is detected. If cells initially are grown on medium containing glucose, and galactose is added to the culture, these four enzymes are induced coordinately by at least 1000-fold upon depletion of glucose from the media. This induction has been shown to occur at the level of RNA transcription. The GAL 1, GAL 2, GAL 7 and GAL 10 genes have been molecularly cloned and sequenced. The regulatory and promoter sequences to the 5' sides of some of the respective coding regions have been placed adjacent to the coding regions of the lac Z gene. These experiments have defined those promoter and regulatory sequences which are necessary and sufficient for galactose induction and which are useful for driving the expression of heterologous genes. The GAL 4 and GAL 80 gene products are respectively positive and negative regulators of the expression of the GAL 1, GAL 2, GAL 7, and GAL 10 genes as well as of the MEL 1 gene, which encodes α-galactosidase, an enzyme required for melibiose catabolism. The GAL 4 product acts as a positive regulator at the transcriptional level by binding to specific DNA sequences 5' to the structural information of these genes. In the absence of galactose, the GAL 80 gene product (protein) interacts with the GAL 4 gene product or with a DNA sequence to prevent this transcriptional activation event. In the presence of galactose, the GAL 80 protein apparently cannot antagonize the GAL 4 protein, and the GAL 4 protein can function as a transcriptional activator. The GAL 4 gene is expressed at very low levels, and its product is rate-limiting for the optimal induction of the GAL gene promoters. This is especially true when a cell contains multiple copies of plasmids with galactose-inducible promoters driving the expression of heterologous genes. However, if it is a goal to maintain minimal or no expression of a heterologous gene before the addition of galactose, it is important that the GAL 4 gene be expressed at very low levels in the absence of galactose. In a variety of recombinant microbial expression systems, the synthesis of many different heterologous polypeptides has been shown to be deleterious to the host cell. As a consequence, there is selective pressure against the expression of such heterologous polypeptides, such that the only cells which accumulate in a scale-up of such a recombinant culture are those which do not express the heterologous polypeptide or express so little of the heterologous polypeptide that the culture becomes an uneconomical source of that polypeptide. An optimal scheme for the scale-up of such a recombinant culture would be to maintain minimal or no expression of the heterologous gene during the expansion of the culture to a large volume and high cell density and then to induce the maximal expression of the heterologous gene only in the final stage of culture growth prior to product isolation. OBJECTS OF THE INVENTION It is an object of the present invention to provide novel strains of yeast in which the GAL 4 protein can be overproduced in a regulatable fashion, such that the expression of heterologous genes driven by galactose-inducible promoters can be minimized during the expansion of the culture to a high cell density and maximized at the final stage of culture growth prior to product isolation. This and other objects of the present invention will be apparent from the following description. SUMMARY OF THE INVENTION The GAL 4 protein is rate-limiting in quantity as a positive regulator for galactose-inducible promoters in strains of yeast. Novel strains are described in which the GAL 4 protein can be overproduced in a regulatable fashion. These strains are useful for the regulatable expression in yeast of heterologous genes whose expression is driven by a galactose-inducible promoter. BRIEF DESCRIPTION OF THE FIGURES Figure 1 depicts the preparation of pKHint-A. Figure 2 depicts the preparation of pKHint-B and pKHint-C. Figure 3 depicts the preparation of Sc252(p GAL 10/ GAL 4) and Sc294(p GAL 10/ GAL 4). Figure 4 depicts the preparation of pRJ178. Figure 5 depicts the preparation of pJC197. Figure 6 depicts the preparation of pKH207-1. Figure 7 depicts the preparation of pKH4. Figure 8 depicts the preparation of pKH4-EBMA. DETAILED DESCRIPTION OF THE INVENTION The present invention is directed to the development of novel strains of yeast in which the GAL 4 gene product can be overexpressed in a regulatable fashion. The present invention also is directed to the use of such novel strains as a host for the expression of heterologous genes driven by a galactose-inducible promoter. Specifically, it is directed to conditions for the scale-up of such recombinant cultures such that heterologous gene expression can be minimized during the expansion of the culture to a large volume and high cell density and then maximized at the final stage of culture growth prior to product isolation. An expression cassette, containing the GAL 10 promoter fused to the GAL 4 structural gene, is constructed in such a way that cells which integrate this cassette at the HIS 3 locus can be selected. The E. coli - S. cerevisiae shuttle vector YEp51 contains the yeast GAL 10 promoter sequences. This plasmid is digested with Sau 3A and Sal I, and the 0.5 kbp GAL 10 promoter fragment is isolated by preparative agarose gel electrophoresis. Two oligonucleotide linkers are synthesized and ligated to the 0.5 kbp fragment, thus converting the 5' Sau 3A and the 3' Sal I ends to Eco RI and Bgl II ends, respectively. After subsequent digestion with Eco RI and Bgl II, the 0.5 kbp Eco RI- Bgl II GAL 10 promoter fragment is isolated by preparative agarose gel electrophoresis. The plasmid vector pJJ42 contains the yeast HIS 3 gene. The yeast GAL 4 structural gene (including the translational start site) is isolated from pSJ3 and subcloned to form pIA. This plasmid is digested with Bam HI and Hind III, and the 2.9 kbp GAL 4 fragment is isolated by preparative agarose gel electrophoresis. The plasmid vector pJJ42 contains the yeast HIS 3 gene. This plasmid is digested with Eco RI and Hind III to remove a segment of the HIS 3 gene extending from the middle of the coding region to just upstream of the HIS 3 promoter. The 12 kbp vector fragment is isolated by preparative agarose gel electrophoresis and ligated simultaneously with the 0.5 kbp and 2.9 kbp fragments described above to yield the plasmid pKHint-A. The p GAL 10/ GAL 4 expression cassette is removed from pKHint-A by digestion with Bam HI, and the 5.6 kbp fragment is isolated by preparative agarose gel electrophoresis. The plasmid vector pJJ98, a modified pBR322 lacking the Hind III site, is digested with Bam HI and ligated to the 5.6 kbp cassette. The resulting plasmid pKHint-B contains a unique Hind III site located beyond the carboxy-terminal end of the GAL 4 coding sequence. The fully functional yeast URA 3 gene is isolated from the E. coli - S. cerevisiae shuttle vector YEp24 by digestion with Hind III and separation of the 1.2 kbp fragment by preparative agarose gel electrophoresis. After digestion with Hind III, plasmid pKHint-B is ligated to the 1.2 kbp Hind III fragment carrying the URA 3 gene to yield pKHint-C, which now contains the complete expression cassette for integration into the yeast chromosome. The salient features of this cassette are: 1) yeast regulatable GAL 10 promoter, 2) yeast GAL 4 translational start site and structural gene, 3) yeast URA 3 promoter and structural gene for positive selection in ura 3⁻ yeast host, and 4) two Bam HI sites which cleave within the 5' and 3' nontranslated sequences of the HIS 3 gene for integrative-replacement transformation into the yeast HIS 3 locus. When digested with Bam HI, the plasmid pKHint-C yields a 6.8 kbp expression cassette flanked on either end by nontranslated HIS 3 DNA sequences. The digested DNA is used to transform S. cerevisiae host strains Sc252 ( MAT a, ura 3-52 leu 2-2-112, ade 1, MEL 1) and Sc294 ( MAT α, ura 3-52, leu 2-2-112, Δ gal 1-10-7, MEL 1). Acting as a targeting device, the Bam HI ends direct the expression cassette to the chromosomal HIS 3 locus where the cassette integrates via homologous recombination. Transformants are selected for URA 3⁺ and screened for his 3⁻. Southern blot analysis of the clonal isolates, referred to as Sc252 (p GAL 10/ Gal 4) and Sc294 (p GAL 10/ GAL 4), confirms that the cassette has integrated at the HIS 3 locus. Transformants and their respective parental strains are grown in glycerol-lactic acid medium either with or without 2% (w/v) galactose. After harvesting at A⁶⁰⁰ = 1.5, RNA extracts are prepared and analyzed by Northern blot techniques. In the presence of galactose, transcription of GAL 4 mRNA is observed to be at least 20-fold enhanced in the Sc252 (p GAL 10/ GAL 4) and Sc294 (p GAL 10/ GAL 4) strains over their respective parental strains. The strains show basal-level expression of GAL 4 mRNA in the absence of galactose. Thus, the novel yeast strains display the desired phenotype of basal-level GAL 4 expression in the absence of galactose and the higher (than wild-type strains) level expression of GAL 4 in the presence of galactose. In order to test whether the novel yeast strains afford an enhancement over wild-type strains of the levels of expression of a heterologous gene, an expression vector containing a galactose inducible promoter is prepared for transforming the strains. The salient features of this plasmid are: 1)\Escherichia coli -derived sequences for the selection and amplification of the plasmid in E. \ coli , 2) S. cerevisiae -derived sequences for the selection and amplification of the plasmid in yeast, 3) yeast GAL 10 promoter, 4) yeast alpha mating factor pre-leader for directing the translational product into the rough endoplasmic reticulum, 5) yeast alpha mating factor pro-leader which encodes N-glycosylation signal sequences and which is cleaved by the KEX 2 and STE 13 proteases (gene products), 6) the coding sequence for the Epstein-Barr virus membrane antigen (EBMA) proteins, which are 350,000 and 220,000 dalton glycoproteins encoded by EB virus (EBV) and resident on the surface of EBV-infected cells and EB virions, 7) a translational termination sequence, and 8) a yeast transcriptional termination sequence. This recombinant plasmid is introduced to S. cerevisiae strains which are wild-type for GAL 4 and to isogenic S. cerevisiae strains which are harboring the p GAL 10/ GAL 4 integrant, and transformed clones are selected. Cells are grown in synthetic selective glycerol-lactic acid medium; galactose is added to the cultures to induce expression of the EBMA gene. RNA is prepared from both strains and subjected to Northern blot analysis. The mRNA encoding EBMA is amplified at least 4-5 fold in the p GAL 10/ GAL 4 integrant over the wild-type strain. Lysates are prepared, electrophoresed in polyacrylamide gels, and Western blotted to nitrocellulose. EBMA proteins reactive specifically with human convalescent EBV sera are amplified approximately 10-fold in the p GAL 10/ GAL 4 integrant over the wild-type strain. No EBMA mRNA or EBMA protein is detectable in the p GAL 10/ GAL 4 integrant strain prior to the addition of galactose to the culture. The GAL 4 protein functions as a transcriptional activator of galactose-inducible promoters and does not a priori influence the way in which heterologous gene products whose expression is driven by galactose-inducible promoters would be translated or processed post-translationally. Therefore, it is obvious to those skilled in the art that the concept of utilizing a yeast strain in which the GAL 4 protein can be overproduced in a regulatable fashion as a host for heterologous gene expression extends to any heterologous gene whose expression is driven from a galactose-inducible promoter. Furthermore, it is obvious to those skilled in the art that any one of the galactose-inducible promoters, GAL 1, GAL 2, GAL 7, GAL 10, or MEL 1, all being subject to GAL 4-mediated transcriptional activation, can be utilized to drive the expression of heterologous genes in a yeast host in which the GAL 4 protein can be overproduced in a regulatable fashion. Another key concept is that the GAL 4 protein overproduction should be regulatable, meaning that GAL 4 gene expression should be driven by a promoter whose activity is regulatable by any physiological means, including but not limited to the addition of galactose to a culture. Therefore, it is obvious to those skilled in the art that a novel strain in which the GAL 4 protein can be overproduced in a regulatable fashion would contain a GAL 4 gene whose expression is driven by a promoter other than one which is galactose-inducible, including but not limited to the promoters of the alcohol dehydrogenase 2, acid phosphatase, phosphoglycerate kinase, invertase, yeast heat shock and copperthionein genes. Furthermore, it is obvious to those skilled in the art that those sequences containing a regulatable promoter and the GAL 4 coding sequence can reside in the yeast host either in an integrated form or on a plasmid and still allow for the regulatable overproduction of the GAL 4 protein. The genus Saccharomyces is composed of a variety of species. Most commonly used is Saccharomyces cerevisiae , or baker's yeast, as a host for the recombinant-DNA mediated expression of a variety of foreign polypeptides. However, the distinctions among the other species of the genus Saccharomyces are not always well-defined. Many of these species are capable of interbreeding with S. cerevisiae and have been demonstrated to possess galactose-inducible genes and positive regulatory proteins for the transcriptional activation of such genes. The positive regulator, which in S. cerevisiae is known as GAL 4, may be of different structure or name from GAL 4 and still function as a transcriptional activator of galactose-inducible promoters. Therefore, it will be obvious to those skilled in the art that the concept of utilizing a strain in which the GAL 4 protein, or other positive regulator for transcriptional activation of galactose-inducible promoters, can be overproduced in a regulatable fashion and can be extended to other species of the genus Saccharomyces , including but not limited to carlsbergensis , norbensis , diastaticus , oviformis , uvarum , rouxii , montanus , kluyveri , and elongisporus . Several yeast genera, such as Hansenula , Candida , Torulonsis , and Pichia , have been shown to contain similar metabolic pathways for the utilization of methanol as a sole carbon source for growth. The gene for alcohol oxidase, an enzyme which participates in this metabolic pathway, has been isolated from Pichia pastoris . The P. pastoris alcohol oxidase promoter has been isolated and shown to be sensitive to methanol induction of expression. The promoter has been placed on an expression vector, where it has been shown to promote the expression of foreign genes cloned adjacent to it. Thus, the principle of utilizing an inducible promoter to direct expression of foreign genes in species of other yeast genera is well-established. Moreover, species of other yeast genera can utilize a variety of carbon sources, including galactose, for growth. Therefore, it will be obvious to those skilled in the art that the concept of increased and regulated expression of a positive regulatory element as a transcriptional activator of galactose inducible promoters to increase expression of a heterologous gene whose expression is driven by a galactose-inducible promoter extends the range of suitable yeast hosts to species of yeast from the families Saccharomycetaceae and Cryptococcaceae , including but not limited to species from the genera Pichia , Candida , Hansenula , Torulopsis , Kluyveromyces , and Saccharomycopsis . The following examples illustrate the present invention without, however, limiting the same thereto. The disclosure of each reference mentioned in the following examples is hereby incorporated by reference. EXAMPLE 1 The E. coli - S. cerevisiae shuttle vector YEp51 (Broach et al ., Experimental Manipulation of Gene Expression , p. 83, Academic Press, 1983) contains the yeast GAL 10 promoter sequences. This plasmid was digested with Sau 3A and Sal I, and the 0.5 kbp GAL 10 promoter fragment was isolated by preparative agarose gel electrophoresis (Figure 1, top). Two oligonucleotide linkers were synthesized and ligated to the 0.5 kbp fragment, thus converting the 5' Sau 3A and the 3' Sal I ends to Eco RI and Bgl II ends, respectively. The respective structures of these oligonucleotides are After subsequent digestion with Eco RI and Bgl II, the 0.5 kbp Eco RI- Bgl II GAL 10 promoter fragment was isolated by preparative agarose gel electrophoresis (Figure 1, top). In order to isolate the GAL 4 structural gene free of its own transcriptional control sequences, pIA (Figure 1, middle) was constructed according to the following procedure. pYe(CEN3)41 [Clark and Carbon, Nature 287 : 504 (1980)] was digested with Pst I and the large fragment self-ligated to remove the LEU 2 gene and reconstitute the bla gene. The resulting plasmid pYC was digested with Bam HI and Sal I and the large Bam HI- Sal I fragment was ligated with a 2.4 kbp Bgl II- Sal I fragment bearing the yeast LEU 2 gene, which had been isolated by digestion of YEp13 [Broach et al. , Gene 8: 121 (1979)] with Bgl II and Sal I. As a result of this ligation, the Bam HI site is destroyed in the resulting plasmid pYCL-1. Plasmid pAAR6 (Ammerer, Methods in Enzymology 101 : 192 (1983)] was digested with Bam HI and Hind III, and the 1.5 kbp Bam HI- Hind III fragment containing the yeast alcohol dehydrogenase I (ADHI) gene promoter was isolated. The plasmid pYCL-1 was then digested with Bgl II and Hind III and the resulting large vector fragment was ligated with the 1.5 kbp Bam HI- Hind III ADHI promoter fragment to yield pYCLF, which does not contain a Bam HI site. The GAL 4 gene then was isolated from pSJ3 [Johnston and Hopper, Proc. Natl. Acad. Sci., U.S.A. 79 : 6971 (1982)] by digestion with Hind III. The resulting 3.3 kbp Hind III fragment containing the GAL 4 gene then was inserted into the unique Hind III site of pYCLF in the appropriate orientation, such that the ADH I promoter was upstream of the GAL 4 structural gene. The resulting plasmid YCpLF+4 contains a unique Bam HI site which is equidistant (approximately 0.45 kbp) from the 3'-end of the ADH I promoter and the translational start of the GAL 4 gene. YCpLF+4 was cut at the Bam HI site and then treated with exonuclease Bal-31 to generate various length deletions. The resulting ends were made flush-ended with the Klenow fragment of DNA polymerase I. Bam HI linkers then were added and the ends religated, producing a family of plasmids with various fusions of the ADH I promoter to the GAL 4 gene. Subsequent characterization of the resulting plasmids led to the isolation of a particular plasmid pIA in which the Bam HI site at the 3'-end of the ADH I promoter was located five base pairs upstream of the ATG translational initiation codon for the GAL 4 structural gene. The yeast GAL 4 structural gene (including the translational start site) was isolated from the plasmid pIA. This plasmid was digested with Bam HI and Hind III and a 2.9 kbp GAL 4 fragment was isolated by preparative agarose gel electrophoresis (Figure 1, middle). The plasmid vector pJJ42 [Struhl et al ., Cold spring Harbor Symp. Quant. Biol., 47 : 901 (1982)] contains the yeast HIS 3 gene. This plasmid was digested with Eco RI and Hind III which cleave within the HIS 3 promoter and within the HIS 3 gene, respectively. A 12 kbp vector fragment was isolated by preparative agarose gel electrophoresis and ligated simultaneously to the 0.5 kbp and 2.9 kbp fragments described above to yield the plasmid pKHint-A (Figure 1, bottom). The p GAL 10/ GAL 4 expression cassette was removed from pKHint-A by digestion with Bam HI and isolation of the 5.6 kbp fragment by preparative agarose gel electrophoresis (Figure 2, top). The plasmid vector pJJ98, a modified pBR322 lacking the Hind III site, was digested with Bam HI and ligated to the 5.6 kbp fragment (Figure 2, middle). The resulting plasmid pKHint-B contains a unique Hind III site located at the carboxy-terminal end of the GAL 4 coding sequence. The fully functional yeast URA 3 gene was isolated from the E. coli - S. cerevisiae shuttle vector YEp24 [Botstein et al ., Cell 8: 17 (1979)] by digestion with Hind III and separation of the 1.2 kbp fragment by preparative agarose gel electrophoresis (Figure 2, middle). After digestion with Hind III, plasmid pKHint-B was ligated to the 1.2 kbp Hind III fragment carrying the URA 3 gene to yield pKHint-C, which now contains the complete expression cassette for integration into the yeast chromosome. The salient features of this cassette are: 1) yeast regulatable GAL 10 promoter, 2) yeast GAL 4 translational start site and structural gene, 3) yeast URA 3 promoter and structural gene for positive selection in ura 3⁻ yeast host, 4) two Bam HI sites which cleave within the 5' and 3' nontranslated sequence of the HIS 3 gene for integrative transformation into the yeast HIS 3 chromosomal locus. EXAMPLE 2 When digested with Bam HI, the plasmid pKHint-C yielded a 6.8 kbp expression cassette flanked on either end by nontranslated HIS 3 DNA sequences (Figure 3, top). The digested DNA was used to transform S. cerevisiae host strains Sc252 (MATa, ura 3-52, leu 2-2-112, ade 1, MEL 1) [Johnston et al ., Proc. Natl. Acad. Sci. U.S.A., 79 : 697 (1982)] and Sc294 ( Mat α, ura 3-52, leu 2-2-112, Δgal1-10-7, MEL 1). Acting as a targeting device, the Bam HI ends directed the expression cassette to the chromosomal HIS 3 locus where the cassette integrated via homologous recombination (Figure 3, middle). Transformants were selected for URA 3⁺ and screened for his 3⁻. Southern blot analysis of the clonal isolates confirmed that the cassette had integrated at the HIS 3 locus. Transformants and their respective parental strains were grown in synthetic selective (leu⁻) glycerol-lactic acid medium [Sherman et al ., Methods in Yeast Genetics . A Laboratory Manual , Cold Spring Harbor Press (1979)], either with or without 2% (w/v) galactose. After harvesting at A⁶⁰⁰ = 1.5, RNA extracts were prepared [Zitomer et al ., Proc. Natl. Acad. Sci. U.S.A. 76: 627 (1979)] and analyzed by Northern blot techniques. [Thomas, Proc. Natl. Acad. Sci. U.S.A. 77 : 5201 (1980)]. In the presence of galactose, transcription of GAL 4 mRNA is observed to be at least 20-fold enhanced in the Sc252 (p GAL 10/ GAL 4) and Sc294 (p GAL 10/ GAL 4) strains over their respective parental strains. The strains show basal-level expression of GAL 4 mRNA in the absence of galactose. Thus, the novel yeast strains display the desired phenotype of basal-level GAL 4 expression in the absence of galactose and much higher (than wild-type strains) level expression of GAL 4 in the presence of galactose. EXAMPLE 3 The alpha mating factor (α factor) gene MFαl was cloned onto a plasmid vector from S. cerevisiae genomic DNA [Kurjan et al ., Cell 30 : 933 (1982); Singh et al ., Nucleic Acids Res. 11 : 4049 (1983)]. The resulting plasmid pKH2 was digested with Eco RI and the 1.7 kbp fragment bearing the alpha mating factor gene was purified by preparative agarose gel electrophoresis (Figure 4, top). Plasmid pRJ148 (a modified pBR322 lacking the Hind III site) was digested with Eco RI and ligated to the 1.7 kbp fragment to yield the plasmid pRJ159 (Figure 4, middle). This DNA was digested with HindIII and self-ligated to form plasmid pRJ167, which now has a unique Hind III site (Figure 4, middle). Plasmid pRJ167 was digested with Hind III and modified by the insertion of a synthetic oligonucleotide adaptor to yield a new plasmid (pRJ178) containing a unique Hind III site which is to the 3' side of the promoter and pre-pro-leader and to the 5' side of the translational termination signals in all three reading frames (Figure 4, bottom). The Hind III site was converted to a Bam HI site by digestion with Hind III, flush-ending with the Klenow fragment of DNA polymerase I, addition of Bam HI linkers and self-ligation to form plasmid pJC193 (Figure 5, top). This plasmid was digested with Eco RI, flush-ended with the Klenow fragment of DNA polymerase I, modified by the addition of Bcl I linkers, digested with Bcl I, and the 1.5 kbp fragment bearing the alpha mating factor gene isolated by preparative agarose gel electrophoresis (Figure 5, middle). This resulting Bcl I fragment then was treated with calf intestine alkaline phosphatase and inserted into the unique Bam HI site of pCl/l, destroying the original Bam HI site in the process (plasmid pJC194; Figure 5, bottom). This DNA was digested with Bam HI and self-ligated to remove excess Bam HI linkers (plasmid pJC197, Figure 5, bottom). pJC197 was digested with Eco RI and Pst I. A 0.7 kbp fragment containing a portion of the alpha mating factor pre-pro-leader, a three-frame translational terminator and the alpha mating factor transcriptional terminator was isolated by preparative agarose gel electrophoresis (Figure 6, middle left). The E. coli - S. cerevisiae shuttle vector YEp51 contains the yeast GAL 10 gene promoter sequences. This DNA was digested with Sau 3A, made flush-ended with the Klenow fragment of DNA polymerase I, and ligated with synthetic Bam HI linkers of the structure: The linkered DNA was digested with Sal I and a 0.5 kbp GAL 10 promoter fragment was isolated by preparative agarose gel electrophoresis (Figure 6, top left). A 35 bp Sal I - Pst I synthetic oligonucleotide adapter was synthesized, having the structure: This adapter contains 11 base pairs of the alpha mating factor non-translated leader and the ATG and first 8 amino acids of the alpha mating factor pre-pro-leader. The 0.5 kbp GAL 10 promoter and the 35 bp adapter fragments were ligated together and digested with Bam HI to generate a 5' Bam HI end. The resulting 0.5 kbp Bam HI- Pst I GAL 10 promoter-alpha mating factor fragment was isolated by preparative agarose gel electrophoresis (Figure 6, middle). The plasmid vector pBR322 was digested with Eco RI and Bam HI, and a 4.0 kbp vector fragment was isolated by preparative agarose gel electrophoresis. The 4.0 kbp, 0.7 kbp and 0.5 kbp fragments were ligated together to create the plasmid pKH207-1 (Figure 6, bottom left). pKH207-1 was digested to completion with Eco RI and partially digested with Bam HI. The 1.2 kbp Bam HI - Eco RI GAL10 promoter-alpha mating factor expression cassette was isolated by preparative agarose gel electrophoresis and made flush-ended with the Klenow fragment of DNA polymerase I (Figure 7, middle). The E. coli - S. cerevisiae shuttle vector pCl/l [Beggs, Nature 275 : 104 (1978); Rosenberg et al ., Nature 312 : 77 (1984)] was digested with Bam HI and Sal I, made flush-ended with the Klenow fragment of DNA polymerase I, and ligated with the 1.2 kbp cassette to create a new expression plasmid, pKH4 (Figure 7, bottom right). The B68' plasmid [Hummel et al ., J. Virol. 49 : 413 (1984)] carrying the Bam HI-L DNA fragment containing the EBMA gene was digested with Xho II and Sca I, and the 2.5 kbp fragment containing the EBMA structural gene was made flush-ended with the Klenow fragment of DNA polymerase I (Figure 8, top). The expression vector pKH4 was digested with Bam HI and made flush-ended with the Klenow fragment of DNA polymerase I. The 2.5 kbp EBMA fragment was ligated to the vector. This recombinant plasmid pKH4-EBMA (Figure 8, bottom) has the following salient features: (1) E. coli -derived sequences for the selection and amplification of the plasmid in E. \ coli , (2) S. cerevisiae -derived sequences for the selection and amplification of the plasmid in yeast, (3) yeast GAL 10 promoter, (4) yeast alpha mating factor pre-leader for directing the translational product into the rough endoplasmic reticulum, (5) yeast alpha mating factor pro-leader which encodes N-glycosylation signal sequences and which is cleaved by the KEX 2 and STE 13 proteases (gene products) [Julius et al ., Cell 32 : 839 (1983); Julius et al ., Cell 37 : 1075 (1984)], (6) the coding sequence for EBMA, (7) a translational termination sequence, and (8) a transcriptional termination sequence. EXAMPLE 4 The expression vector pKH4-EBMA (Figure 8, bottom) was introduced to S. cerevisiae strains Sc252, Sc252 (p GAL 10/ GAL 4) and Sc294 (p GAL 10/ GAL 4). The transformants (with pKH4-EBMA) were grown in synthetic selective glycerol-lactic acid medium to A⁶⁰⁰ = 0.5-1.0, at which time galactose was added to a final concentration of 2% (w/v). Lysates were prepared, electrophoresed in polyacrylamide gels and Western blotted to nitrocellulose. The 375,000 and 225,000 and 175,000 dalton proteins were found to be specific to EBMA by virtue of their presence only in transformants, their reactivity with human convalescent EBV sera and an EBMA-specific monoclonal antibody [Qualtiere et al ., Proc. Natl. Acad. Sci. 79 : 616 (1982)], and their lack of reactivity with pooled negative human sera. No EBMA-specific proteins were detected in cultures grown in the absence of galactose. At 24 hours after the addition of galactose, the 2 strains Sc252 (p GAL 10/ GAL 4) and Sc294 (p GAL 10/ GAL 4) each contain 10-fold more EBMA-specific proteins than does strain Sc252.

Claims: A strain of species of the families Saccharomycetaceae or Cryptococcaceae containing sequences comprised of a physiologically regulatable promoter linked to the gene for a positive regulatory element which serves as the transcriptional activator of galactose-inducible promoters, said strain being the host for a heterologous gene whose expression is driven by a galactose-inducible promoter. A strain according to Claim 1, wherein the host is a species of the genus Saccharomyces . A strain according to Claim 2, wherein the host is Saccharomyces cerevisiae . A strain according to Claim 1, wherein the promoter linked to the gene for the positive-regulatory element is galactose-inducible. A strain according to Claim 4, wherein the promoter is GAL 10. A strain according to Claim 1, wherein the gene for the positive regulatory element is GAL 4. A strain according to Claim 1, wherein the galactose-inducible promoter is GAL 1, GAL 2, GAL 7, GAL 10, or MEL 1.

Inventors: ELLIS, RONALD W., HOFMANN, KATHRYN J., HOPPER, JAMES E., SCHULTZ, LOREN D.
Applicants: MERCK & CO. INC., RESEARCH CORPORATION TECHNOLOGIES, INC.
Application year: 1987
Main classification: C12N15
IPC codes: C12N1, C12N15, C12R1