CHIMERIC PLASMID VECTOR

Abstract: A chimeric plasmid vector comprising first to fifth DNA fragments, wherein the first DNA fragment has a first LTR re­ gion derived from an MMTV and a 5′-splicing site located downstream of the first LTR region and the second DNA frag­ ment has a 3′-splicing site derived from plasmid pCVSVE and located immediately downstream of the first DNA fragment, which second DNA fragment is followed by a polylinker der­ ived from plasmid pi-AN7. The first LTR region of the first DNA fragment comprises a hormone dependent region (or a hor­ mone receptor complex binding region) and an MMTV prom­ otor region located downstream of the hormone dependent region, The third DNA fragment has a second LTR region der­ ived from an MMN and having an transcription termination site, while the fourth DNA fragment comprises an SV40 prom­ otor region derived from plasmid pSV2gpt and an Eco-gpt gene region derived from the plasmid pSV2gpt and operable as a gene marker. The first and fourth DNA fragments are linked together through the fifth DNA fragment which com­ prises an ampicillin resistance gene region derived from plasmid pBR332 and a replication origin derived from the plasmid pBR332.

Description: FIELD OF THE INVENTION The present invention relates to a chimeric plasmid vector including a DNA sequence of an animal virus and, more particularly, to a chimeric plasmid vector which is useful for introducing a DNA fragment into animal cells for producing a desired protein product. BACKGROUND OF THE INVENTION It is well known in the art that a biogenically active form of protein of a higher animal can be advantageously produced using gene manipulation techniques introducing a corresponding gene into the cell of the higher animal. This is because of the fact that most proteins of higher animals have modified structures having, for example, sugar-chain moieties, disulfide bonds and amidation of C-terminals. For the expression of desired foreign genes in animal cells, it has been an ordinary practice to use animal viruses as vectors. When a vector having a transcription promotor of an animal virus linked with a foreign gene is used for introduction into an animal cell, the genetic information carried on the foreign gene is transcribed into the form of an RNA by the action of the viral promotor on the vector. The transcribed genetic information is translated from the RNA into the form of a protein molecule coded for by the genetic information originating in the foreign gene, as also well known in the art. Examples of animal viruses thus used as vectors include papovaviruses, papillomaviruses and retroviruses. Among these animal viruses, papovaviruse SV40 and a SV40-derived plasmid pSV2 and bovine papillomaviruses in particular have frequently been the subjects of research and investigations and have found a variety of practical applications as vectors. On the other hand, vectors using retroviruses have various unique advantages over vectors using other viruses and are expected to provide useful expedients for the future gene engineering. Such unique advantages of the vectors using retroviruses include: (1) the wide host ranges which the vectors have for animal cells; (2) the abilities of the vectors which can be incorporated into and stably maintained within host chromosomal DNAs; (3) that the transfection with the vectors is not lethal to host animal cells; and (4) the amenability of the vectors to insertion of relatively large foreign DNA fragments. It is further well known in the art that the gene expression of a mouse mammary tumor virus (hereinafter referred to as MMTV), which is one of the retroviruses, is regulated, viz., strengthened by addition of glucocorticoid hormone. Taking advantage of this hormone dependency in the gene expression of the MMTV, attempts have thus far been made to synthesize vectors each carrying both a viral promotor region and a hormone dependent region thereof. Such a hormone dependent region, or hormone receptor complex binding region, of the MMTV is included in a "long terminal repeat" (LTR) region which also includes an MMTV promotor region. It is reported for example that a foreign ras gene can be effectively expressed to yield p²¹ when linked with an LTR region of the MMTV (Cell, 27, 245-255, 1981). Other reports refer to an LTR-linked bovine papillomavirus vector (Mol. Cell. Biol., 3, 2045-2057, 1983) and an LTR-linked vector of SV40-derived pSV2 (Nature, 294, 228-232, 1981). Each of these known LTR-linked vectors has only one LTR region therein and is accordingly devoid of an effectively acting transcription termination site. This means that the genetic information replicated from a foreign gene could not be transcribed into RNA accurately and in a stable condition. In respect of an LTR-linked vector, a problem is further encountered in that the transcription proceeds also for the DNA sequence intervening between the sole LTR region of the vector and the foreign gene region inserted into the vector downstream of the LTR region. For this reason, the transcription proceeds for the DNA sequence spanning from the promotor within the LTR region to the foreign gene region. Such an extended coverage of the transcription would result in instability of the transcription agent. With a view to providing a solution to these problems and to exploiting the unique advantages of vectors derived from retroviruses, efforts have been made by the present inventors to develop new and useful chimeric plasmid vectors capable of producing proteins in animal cells accurately and in stable condition through the expression of the genetic information originally carried by foreign genes. The present invention is credited to such efforts and is based on the discovery that proteins can be produced stably in an animal cell by a chimeric plasmid vector which has at least two, first and second specific LTR regions. The first LTR region includes a hormone dependent region (or hormone receptor complex binding region) and an MMTV promotor region and the second LTR region is located downstream of the first LTR region with a polylinker carrying the insertion sites located between the two LTR regions. Between the polylinker and the first LTR region are provided a 5'-splicing site and a 3'-splicing site with the former located closer to the first LTR region and the latter located closer to the polylinker. Thus, it is a prime object of the present invention to provide a novel chimeric plasmid vector which is specifically useful for producing a desired protein preferably having biogenic activities through accurate and stabilized expression of a foreign gene within animal cells. SUMMARY OF THE INVENTION In accordance with the present invention, there is provided a chimeric plasmid vector which comprises first to fifth DNA fragments. The first DNA fragment has a first LTR region derived from an MMTV and a 5'-splicing site located downstream of the first LTR region, wherein the first LTR region comprises a hormone dependent region (or a hormone receptor complex binding region) and an MMTV promotor region located downstream of the hormone dependent region. The third DNA fragment has a second LTR region derived from the MMTV and having a transcription termination site. The first and third DNA fragments are linked with the fifth DNA fragment which comprises an ampicillin resistance gene region derived from plasmid pBR322 and a replication origin derived from the plasmid pBR322. A polylinker derived from plasmid pi-AN7 is inserted downstream of the first DNA fragment, whereupon the second DNA fragment which has a 3'-splicing site derived from plasmid pCVSVE is inserted immediately upstream of the polylinker. The fourth DNA fragment, which comprises an SV40 promotor region derived from plasmid pSV2gpt and an Eco-gpt gene region derived from the plasmid pSV2gpt, is then inserted as a gene marker between the fourth DNA fragment and the third DNA fragment. BRIEF DESCRIPTION OF THE DRAWINGS The features and advantages of a chimeric plasmid vector according to the present invention will be more clearly appreciated from the following description taken in conjunction with the accompanying drawings in which: Fig. 1 is a view showing first to fifth intermediate plasmids (pMM1 - pMM5) produced by some earlier steps of a process of synthesizing a chimeric plasmid vector (pM102) according to the present invention; Fig. 2 is a view similar to Fig. 1 but shows fifth to ninth intermediate plasmids (pMM5 - pMM8) produced by steps subsequent to the steps shown in Fig. 1 as well as the chimeric plasmid vector (pMM102) resulting from the process shown in Figs. 1 and 2; Fig. 3 is a schematic representation of a restriction enzyme cleavage map of the chimeric plasmid vector (pMM102) according to the present invention; Fig. 3A is a schematic representation of the general structure composed of a total of five DNA fragments forming the chimeric plasmid vector (pMM102) shown in Fig. 3; Fig. 4 is a view showing the nucleotide sequence of a polylinker which may be used in the chimeric plasmid vector according to the present invention; and Fig. 5 is a view showing the palindromic nucleotide sequence of the double strand Xho I linker DNA used in the chimeric plasmid vector according to the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS In a process of synthesizing a novel chimeric plasmid vector according to the present invention, an LTR-containing Bam HI- Xho I digested fragment of clone H (Cell, 23, 335-345, 1981) is cloned at the Bam HI- Sal I cleavage site of E. coli plasmid pBR322 to create plasmid pMM1 as a first intermediate plasmid, as shown in Fig. 1. The clone H herein used has resulted from the cloning of MMTV at the Eco RI cleavage site of a lambda-phage vector lambda-gtWES .lambda-B (Cell, 23, 335-345, 1981). In Fig. 1, the alphabetic denotations used for the clone H refer to enzymes as follows: Ba: Bam HI; Bg: Bgl II; E: Eco RI; H: Hin dIII; K: Kpn I; P: Pst I; S: Sst I; Xb: Xba I; Xh: Xho I. In Fig. 1 (and also in Fig. 2), furthermore, the restriction enzyme sites shown enclosed within parentheses are those cleavage sites which have been made extinct. The first intermediate plasmid pMM1 thus created is cleaved with a restriction enzyme Pvu II at the two Pvu II cleavage sites of the plasmid, whereupon the larger one of the resultant two DNA fragments, viz., the fragment including the Bam HI cleavage site is isolated from the other and is ligated with T4 DNA ligase to produce plasmid pMM2 as a second intermediate plasmid, as also shown in Fig. 1. Subsequently, a Bgl II-digested DNA fragment containing the LTR region of the clone H is inserted at the Bam HI cleavage site of the second intermediate plasmid pMM2 to yield plasmid pMM3 as a third intermediate plasmid. The third intermediate plasmid pMM3 is cleaved with restriction enzymes Eco RI and Hin dIII and the resultant DNA fragment is treated with Klenow enzyme to make its ends blunt and further with T4 DNA ligase so as to be circularized for thereby forming plasmid pMM4 as a fourth intermediate plasmid as shown in Fig. 1. It may be noted that the fourth intermediate plasmid pMM4 thus obtained has no Eco RI and Hin dIII cleavage sites for the ends of the cleaved fragment. Such a fourth intermediate plasmid pMM4 is cleaved with a restriction enzyme Xba I and the resultant fragment is also treated with Klenow enzyme to make its ends blunt. The fragment is then circularized with its blunt ends linked together with a polylinker derived from a Hin dIII- Eco RI digested pi-AN7 fragment treated with Klenow enzyme, thereby producing plasmid pMM5 as a fifth intermediate plasmid as shown in Fig. 1 (Gene, 30, 257-260, 1984 for pi AN7). This polylinker consists of a DNA fragment having cleavage sites for restriction enzymes Hin dIII (AAGCTT), Xba I (CTAG), Bgl II (AGATCT), Pst I (CTGCAG), Sal I (GTCGAC), Bam HI (GGATCC), Sma I (CCGG), Eco RI (GAATTC), as shown in Fig. 4. The fifth intermediate plasmid pMM5 is digested with restriction enzymes Apa I and Kpn I and the resultant cleaved fragment is treated with T4 DNA polymerase to make its ends blunt and then with T4 DNA ligase so as to be circularized in shrunk form into plasmid pMM6 as a sixth intermediate plasmid as shown in Fig. 2. The sixth intermediate plasmid pMM6 thus produced is partially digested with a restriction enzyme Nde I for a short period of time. Of a number of DNA fragments produced by the cleavage of a plenty of sixth intermediate plasmid pMM6, the fragments each resulting from a plasmid cleaved at one of its three Nde I cleavage sites are isolated by electrophoresis against an agarose gel plate from the DNA fragments each resulting from a plasmid cleaved at two or three of its Nde I cleavage sites. The DNA fragments isolated in this manner are digested with a restriction enzyme Hin dIII and the resultant DNA fragments are treated with Klenow enzyme and further with T4 DNA ligase for being circularized in shrunk form. It may be noted that a number of DNA fragments thus isolated are broken down to two categories, a DNA fragment of one category having two Nde I cleavage sites with one Nde I cleavage site made extinct and a DNA fragment of the other category having three Nde I cleavage sites. Of these two kinds of plasmids, those each having two Nde I cleavage sites, one of which has been derived from the plasmid pBR322, are herein used each as a seventh intermediate plasmid pMM7 as shown in Fig. 2. The seventh intermediate plasmid pMM7 thus having two Nde I cleavage sites is partially digested with a restriction enzyme Nde I and the resultant DNA fragment is treated with Klenow enzyme and further with T4 DNA ligase so as to be circularized in shrunk form with an Xho I linker inserted between the blunt ends of the fragment. Of the number of plasmids thus obtained, those each of which has the Xho I linker located at the Nde I cleavage site derived from the plasmid pBR322 is selectively extracted as an eighth intermediate plasmid pMM7X1 as shown in Fig. 2. Fig. 5 shows the palindromic nucleotide sequence of the double strand Xho I linker DNA herein used. A Pvu II- Bam HI digested fragment of SV40-derived plasmid pSV2gpt (Science, 209, 1422-1427, 1980) containing an SV40 promotor region, an Eco-gpt gene (which inherently has myconophenolic acid resistance expressed) and an SV40 splicing site therein is treated with Klenow enzyme. An Xho I linker is linked to the resultant fragment, which is then inserted into the Xho I cleavage site of the eighth intermediate plasmid pMM7X1 to produce plasmid pMM8 as a ninth intermediate plasmid as shown in Fig. 2. The polylinker sites ranging between the Hin dIII and Pst I cleavage sites of this ninth intermediate plasmid pMM8 is substituted by a Hin dIII- Pst I digested DNA fragment containing a 3'-splicing site of plasmid pCVSVE (Mol. Cell. Biol., 2, 1304-1319, 1982), the 3'-splicing site of which has been derived from an immunoglobulin variable region. A chimeric plasmid vector according to the present invention is now finally synthesized, which is herein named pMM102. Fig. 3 represents a restriction enzyme cleavage map of the chimeric plasmid vector pMM102 thus provided in accordance with the present invention. Furthermore, Fig. 3A schematically represents the general structure composed of the first to fifth DNA fragments forming the chimeric plasmid vector (pMM102) according to the present invention. As noted previously, the chimeric plasmid vector pMM102 according to the present invention can be replicated in E. coli . Such a novel chimeric plasmid vector is capable of transforming E.coli K-12 khi-1776 in a manner like that described in Example 1, and is carried on, for example, E.coli K-12 khi-1776 (pMM102) which has been deposited at Fermentation Research Institute, Agency of Industrial Science and Technology, Japan under the deposition number FERM P-8429. The chimeric plasmid vector pMM102 according to the present invention is characterized inter alia in that the plasmid vector has two, first and second LTR regions as will be readily seen from Fig. 3A. The first LTR region, contained in the first DNA fragment, is derived from an MMTV and includes a hormone dependent region (or a hormone receptor complex binding region) and an MMTV promotor region. The second LTR region is derived from an MMTV and has a transcription termination site. The second LTR region is contained in the third DNA fragment and is located downstream of the first LTR region across a polylinker into which a foreign gene is to be inserted. With the foreign gene inserted into the polylinker for being expressed within host animal cells, the RNA transcription initiated at the site immediately subsequent to the MMTV promotor region is assuredly brought to a stop when the transcription reaches the transcription termination site on the second LTR region. Protein molecules of a particular type which is specific to the foreign gene used can thus be produced accurately and in stabilized condition within the host animal cells. The chimeric plasmid vector pMM102 according to the present invention is further characterized in that the first DNA fragment containing the first LTR region also contains a 5'-splicing site located downstream of the first LTR region and that the second DNA fragment is provided downstream of this 5'-splicing site and has a 3'-splicing site located immediately upstream of the polylinker into which a foreign gene is to be inserted, as will also be readily seen from Fig. 3A. By addition of these 5'- and 3'-splicing sites between the first LTR region and the polylinker, the DNA sequence which intervenes between the two splicing sites acts as if it were an intron and is disqualified as a coding sequence. The RNA transcription from the foreign gene as performed within host animal cells may therefore skip over the lengthy DNA sequence spanning from the 5'-splicing site to the 3'splicing site upstream of the polylinker. This means that the proteins which are coded for by the lengthy DNA region thus skipped over are not produced so that the translation of the genetic information from the foreign gene into the form of desired proteins through the RNA transcription will proceed smoothly. From the foregoing description it will have been appreciated that the present invention provides solution to the various problems which have been inherent in vectors containing a sole MMTV-derived LTR region as hereinbefore pointed out. The chimeric plasmid vector pMM102 according to the present invention is also characterized in that the vector further includes the fourth DNA fragment having an SV40 promotor region derived from plasmid pSV2gpt and a mycophenolic acid resistant Eco-gpt gene region derived from the plasmid pSV2gpt, as will also be readily seen from Fig. 3A. When a foreign gene is to be expressed within host animal cells, the Eco-gpt gene region of the fourth DNA fragment serves as a gene marker to aid in the selection of the host cell transformed by the chimeric plasmid vector pMM102 according to the present invention. It therefore follows that the transformed host cell acquires a mycophenolic acid resistance stemming from the Eco-gpt gene region and can be isolated with utmost ease. It may also be noted in this instance that the function of the Eco-gpt gene region as the gene marker can not affect the hormonal regulation over the MMTV promotor region since the SV40 promotor region per se is free from the influence of hormones. The chimeric plasmid vector pMM102 according to the present invention is still further characterized in that the vector further includes the fifth DNA fragment located downstream of the fourth DNA fragment. As will be seen from Fig. 3A, the fifth DNA fragment comprises an ampicillin resistance gene region and a replication origin both of which have been derived from the E. coli plasmid pBR332. Where the chimeric plasmid vector pMM102 having a foreign gene inserted at the polylinker thereof is to be amplified in a cell of E. coli prior to the transformation of host animal cells, the ampicillin resistance gene region of the fifth DNA fragment of the plasmid vector acts as a gene marker in the cell of E. coli so that the cell comes to be blessed with an ampicillin resistance when transformed. This provide ease of isolating the transformed cells. Still another important aspect of the chimeric plasmid vector pMM102 according to the present invention is that the plasmid vector comprises the first DNA fragment having a first LTR region and a virus packing region located downstream of the first LTR region and capable of packing a virus into globular form and the third DNA fragment located downstream of the first DNA fragment and including a second LTR region as will also be seen from Fig. 3A. A foreign gene is to be inserted into such a plasmid vector at the site of a polylinker intervening between the first and third DNA fragments. The products of replication (which include the foreign gene per se ) produced on the DNA sequence ranging from the first LTR region to the second LTR region can therefore be easily recovered in the form of globular viruses. If the globular viruses thus recovered are transfected into a vast number of adequate host animal cells, desired proteins can be produced on a large scale by the expression of foreign genes in the host cells. In the meantime, the retrovirus-derived chimeric plasmid vector pMM102 according to the present invention stays in stable condition within the chromosome of host animal cells when the plasmid vector is incorporated therein. In addition, the chimeric plasmid vector pMM102 is suitable for the production of a vast amount of gene products by reason of the positive activity of the MMTV promotor region located in one of the LTR regions of the plasmid vector. The MMTV promotor region is sensitive to glucocorticoid hormone and has its activity significantly enhanced by the addition of such a hormone thereto. This characteristic of the MMTV promotor region is useful for regulating the transformative expression of a foreign gene. For example, the transformative expression of a foreign gene within a host animal cell can be regulated with the gene inserted into the chimeric plasmid vector pMM102 for the purpose of assaying the physiologic effects which the particular gene will exhibit within the host animal cell. Where a given foreign gene happens to be potentially detrimental to the host cell, the MMTV promotor region in the chimeric plasmid vector pMM102 will allow such a gene into the host cell in a condition inhibited from being expressed within the cell. It may be further noted that the polylinker sequence in the chimeric plasmid vector pMM102 according to the present invention has a unique cleavage site for each of the restriction enzymes Sal I, Sma I, Eco RI and Xba I. Furthermore, the chimeric plasmid vector pMM102 has two cleavage sites for each of the restriction enzymes Bam HI and Acc I, one of the two cleavage site being located within the polylinker sequence. This will provide ease of cloning of a DNA fragment digested with any of the restriction enzymes Sal i, Bam HI, Bgl II, Bcl I, Sma I, Eco RI, Xba I, Xho I, Acc I, Tag I, Sau 3A. Synthesis of Chimeric Plasmid Vector pMM102 The features of the chimeric plasmid vector pMM102 provided in accordance with the present invention will be better understood from the following description regarding a typical example of the process of synthesizing the plasmid vector. (1) Synthesis of First Intermediate Plasmid pMM1 To 2 µl of plasmid pBR322 (0.5 µg/µl) were added 2 µl of a 10-fold concentrated buffer solution containing 100 mM Tris-HCl (pH 7.6), 70 mM MgCl₂, 70 mM β-mercaptoethanol, and 500 mM NaCl, followed by addition of water to give a total volume of 18 µl. To the aqueous solution thus prepared were added 1 µl of 10 unit/µl Bam HI and 1 µl of 8 unit/µl Sal I. The resultant mixture was allowed to react at 37°C for 1 hour, was then heated at 65°C for 5 minutes to terminate the reaction and was thereafter subjected to electrophoresis against a 1% agarose gel plate. Thereupon, a band for the 4.1 Kbp DNA was cut from the gel plate and was taken into a dialysis tube containing a Tris-acetate buffer solution (which consisted of 0.04 M Tris-acetate (pH 7.5) and 0.001 M EDTA (ethylenediaminetetraacetate) to extract the DNA therewith through electrophoresis. The DNA preparation accordingly produced in the dialysis tube was then taken out of the tube and was mixed with an equal volume of phenol. The resultant mixture was, upon vigorous shaking, subjected to centrifugation at 15,000 rpm for 5 minutes to collect an aqueous layer containing the DNA. The DNA-containing aqueous layer was mixed with an equal volume of phenol, followed by further centrifugation under the same conditions. Such a procedure was repeated on the resultant DNA-containing aqueous layer after an equal volume of chloroform had been added thereto. The DNA solution obtained as a result of such a repeated procedure was mixed with one-tenth volume of 3 M sodium acetate (pH 7.0) and three volumes of ethanol and was chilled at -70°C for 10 minutes. After cooling, the DNA solution was centrifuged at 15,000 rpm at 0°C for 10 minutes to precipitate DNA and, upon removal of the residual ethanol by evaporation, the resultant DNA preparation was dissolved in 10 µl of water. On the other hand, 20 µl of 1 µg/µl clone H (Cell, 23, 335-345, 1981) was digested with restriction enzymes Bam HI and Xho I (each being of 6 unit/µl activity) and the resultant solution was electrophoresed against a 1% agarose gel plate. A band for the 3.6 Kbp DNA was cut from the gel plate to extract the DNA following the steps used for the separation of DNA using the plasmid pBR322 as the starting material. The DNA thus obtained was dissolved in 10 µl of water, to which solution was added 0.1 µl of Bam HI- Sal I digested DNA fragment of the plasmid pBR322, 2 µl of a 10-fold concentrated buffer solution containing 660 mM Tris-HCl (pH 7.5), 66 mM MgCl₂, 100 mM DTT (dithiothreitol) and 1 mM ATP, followed by dilution with water to give a total volume of 18 µl. To the aqueous solution thus prepared was added 2 µl of T4 ligase (1.4 unit/µl) and the resultant mixture was allowed to react overnight at 4°C and was thereafter diluted to five folds. One µl of the DNA preparation obtained in this manner was mixed with 20 µl of competent cell suspension of E. coli HB101 and the mixture was allowed to stand at 0°C for 30 minutes, followed by incubation at 42°C for 2 minutes. After addition of 80 µl of L-broth, the mixture was incubated at 37°C for 30 minutes and was placed onto the surface of an agar plate contain,ng 40 µg/ml of ampicillin. The agar plate was then incubated overnight at 37°C. There were thus obtained colonies, which were assayed for isolation of plasmids pMM1 consisting of the Bam HI- Sal I digested DNA fragments of plasmid pBR322 linked with the Bam HI- Xho I digested DNA fragments of the clone H. (2) Synthesis of Second Intermediate Plasmid pMM2 The first intermediate plasmid pMM1 thus produced was cleaved with a restriction enzyme Pvu II at the two Pvu II cleavage sites of the plasmid. The resultant reaction product was electrophoresed against an agarose gel plate, whereupon a band for the 4.5 Kbp DNA was cut from the gel plate as in step (1). The DNA contained in the cut was recovered therefrom and was ligated with T4 ligase also as in step (1). The resultant circular DNA was cloned by the transformation of E. coli to produce plasmid pMM2 as the second intermediate plasmid. (3) Synthesis of Third Intermediate Plasmid pMM3 The clone H was cleaved with a restriction enzyme Bgl II, whereupon a 4.5 Kbp DNA fragment was recovered by agarose gel electrophoresis and was ligated with the Bam HI digested DNA fragment of the plasmid pMM2 as in step (1), thereby yielding plasmid pMM3 as the third intermediate plasmid. (4) Synthesis of Fourth Intermediate Plasmid pMM4 The third intermediate plasmid pMM3 was cleaved with restriction enzymes Eco RI and Hin dIII and the resultant DNA fragment was precipitated in ethanol and was thereafter dissolved in 21 µl of water. To the resultant DNA solution were added 2.5 µl of buffer solution containing 0.5 M Tris-HCl (pH7.2), 0.1 M MgCl₂, 1mM DTT and 500 µg/ml of BSA and 1µl of deoxyribonucleoside triphosphate mixture (2 mM for each of dATP, dGTP, dCTP and dTTP), followed by addition of 0.5 µl of 2.6 unit/µl Klenow enzyme. The resultant mixture was allowed to stand at room temperature for 30 minutes, was then heated at 65°C for 5 minutes, and was treated with 2 µl of T4 ligase (1.4 unit/µl) to produce plasmid pMM4 as the fourth intermediate plasmid devoid of Eco RI and Hin dIII cleavage sites. (5) Synthesis of Fifth Intermediate Plasmid pMM5 The fourth intermediate plasmid pMM4 was cleaved with a restriction enzyme Xba I and was treated with Klenow enzyme. The resultant fragment was linked to the polylinker sites (multiple restriction enzyme cleavage sites) consisting of a Hin dIII- Eco RI digested fragment of pi-AN7 which had been treated with Klenow enzyme. The DNA fragment thus obtained was treated also as in step (1) to produce plasmid pMM5 as the fifth intermediate plasmid. (6) Synthesis of Sixth intermediate Plasmid pMM6 The fifth intermediate plasmid pMM5 was cleaved with restriction enzymes Apa I and Kpn I and the resultant fragment was recovered by precipitation in ethanol and was dissolved in 16 µl of water. To the resultant solution were added 2 µl of buffer solution containing 0.33 M Tris-acetic acid (pH 7.9), 0.66 M sodium acetate, 0.10 M magnesium acetate, 5.0 mM DTT and 1mg/ml of BSA, followed by addition of 1 µl of deoxyribonucleoside triphosphate mixture (2 mM for each component) and 1 µl of T4 DNA polymerase. The resultant mixture was incubated at 37°C for 5 minutes. The reaction was terminated with addition of 1 µl of 0.5 M EDTA, whereupon a DNA fragment was extracted by treatment with phenol and was precipitated with ethanol. The resultant DNA fragment was treated as in step (1) to produce plasmid pMM6 as the sixth intermediate plasmid (Fig. 2). (7) Synthesis of Seventh Intermediate Plasmid pMM7 The sixth intermediate plasmid pMM6 thus obtained was partially digested with a restriction enzyme Nde I and the resultant DNA fragments were extracted by treatment with phenol arid were electrophoresed against an agarose gel plate. A band for the 7.5Kbp DNA was cut from the gel plate and the DNA contained in the cut was extracted by treatment with phenol and was precipitated in ethanol to recover the DNA fragment. The DNA fragment thus recovered was digested with a restriction enzyme Hin dIII and the resultant DNA fragment was treated with Klenow enzyme and was subjected to treatments similar to those used in step (1) to produce plasmid pMM7 as the seventh intermediate plasmid pMM7. (8) Synthesis of Eighth Intermediate Plasmid pMM7X1 The seventh intermediate plasmid pMM7 was partially digested with a restriction enzyme Nde I to obtain a 6.5 Kbp DNA fragment as a result of electrophoresis on an agar plate. To this DNA fragment was added 1 µl of 0.02OD unit/µl of an Xho linker (commercially available from Takara Shuzo Co., Ltd.) so that the fragment was circularized in a total volume of 20 µl of the reaction mixture having T4 DNA ligase contained therein. Further using a procedure similar to that taken in step (1), plasmid pMM7X1 was obtained as the eighth intermediate plasmid having the Xho I linker located at the Nde I cleavage site remoter from the polylinker site. (9) Synthesis of Ninth Intermediate Plasmid pMM8 Ten µg of plasmid pSV2gt (Science, 209, 1422-1427, 1980) was digested with restriction enzymes Pvu II and Bam HI to obtain a 2.2 Kbp DNA fragment by agarose gel electrophoresis. The DNA fragment was treated with Klenow enzyme and was linked to an Xho I linker. The resultant DNA fragment was cleaved with a restriction enzyme Xho I whereupon a 2.2 Kbp DNA fragment was recovered by agarose gel electrophoresis and was linked to the Xho I cleavage site of the intermediate plasmid pMM7X1, thus producing plasmid pMM8 as the ninth intermediate plasmid. (10) Synthesis of Chimeric Plasmid Vector pMM102 Ten µg of plasmid pCVSVE (Mol. Cell. Biol., 2, 1304-1319, (1982) was digested with restriction enzymes Hin dIII and Pst I whereupon a DNA fragment of about 90 bp was recovered by the electrophoresis conducted against an acrylic amide gel. The intermediate plasmid pMM8 was then partially digested with a restriction enzyme Pst I and a DNA fragment of about 8.7 Kbp was recovered by agarose gel electrophoresis. This 8.7 Kbp DNA fragment was further cleaved with a restriction enzyme Hin dIII and the resultant fragment was linked with the aforesaid DNA fragment of 90 bp, thus producing the chimeric plasmid vector pMM102 according to the present invention. The various natures of the chimeric plasmid vector pMM102 thus obtained in accordance with the present invention will be further understood from the following Examples. Example 1 Tests were conducted to introduce the chimeric plasmid vector pMM102 into an animal cell and to observe the expression of a gene carried by the plasmid within the animal cell. For this purpose, 100 µg of plasmid pMM102 was dissolved in sterile distilled water. To this solution was added 0.5 ml of 2.5 M CaCl₂. The DNA-CaCl₂ solution was dropwise added to 2.5 ml of HNP solution in a test tube with bubbling by blowing air into the solution, the HNP solution containing 50 mM HEPES (N-2-hydroxyethyl-piperazine-N'-2-ethansulphonic acid), 280 mM NaCl and 1.5 mM Na₂HPO₄ (pH 7.1). The resultant solution was allowed to stand at room temperature for 30 minutes to precipitate the DNA. On the other hand, mink lung cells (ATCC CCL64) were cultured on a Dulbecco's modified Eagle medium (In vitro, 9, 468-469, 1974) supplemented with 10% fetal calf serum to reach a concentration of 3x10⁵ cells per plate of 60cm diameter (which cell culture corresponds to 10⁴ cells/cm²). To this cell culture was added 0.5 ml per plate of the DNA precipitate produced in the test tube, followed by incubation for 16 hours. To the resultant cell culture was added 0.6 ml of DMSO (glycerol-ethyleneglycol-dimethylsulfoxide) to reach a final concentration of 10%. The culture was then allowed to stand for 30 minutes and thereafter the culture medium was exchanged with new one to continue the incubation. The cells thus yielded were grown on a selective culture medium about fortnight to obtain mycophenolic acid resistant colonies, the medium used consisting of a Dulbecco's modified Eagle medium supplemented with 10% fetal calf serum, 15 µg/ml of hypoxanthine, 10 µg/ml of thymidine, 5 µg/ml of glycine, 2 µg/ml of aminopterin, 25 µg/ml of mycophenolic acid, and 250 µg/ml of xanthine. The growth of such resistive colonies shows that the plasmid pMM102 was incorporated into the mink lung cells and an Eco-gpt gene was expressed in each of the cells. Example 2 Tests were then conducted to examine the glucocorticoid hormone denendency of the agent of the promotor region of the MMTV. For this purpose, fifteen strains of the mycophenolic acid resistant cells obtained by Example 1 were cultured individually of one another. The resultant cell cultures were grouped into two different categories whereby the cell cultures of one category were incubated with addition of 10⁻⁶ M dexamethasone thereto and the cell cultures of the other category were incubated without addition of dexamethasone. In 28 hours, a total quantity of RNA was extracted from the resultant cell cultures using the guanidium-CsCl method. A DNA fragment was obtained by cleaving the plasmid pMM102 downstream of the MMTV promotor region with a restriction enzyme Bgl II and was labeled with isotope ³²P. Analysis was made using a hybridization method into the quantity of the RNA complementary to the DNA fragment resulting from the Bgl II-digested plasmid pMM102, revealing that the transcription rate had increased about ten times for two of the strains to which dexamethasone had been added. This means that the transcription conducted by the MMTV promotor region is dependent on the hormone used. Example 3 Tests were further conducted to observe the expression of a CAT gene (choramphenicol resistance gene) within an animal cell. Plasmid pBR325 (Gene, 14, 289, 1981) was cleaved with a restriction enzyme Tag I to obtain a DNA fragment containing CAT gene. The DNA fragment was treated with Klenow enzyme and a Sal I linker was linked to the resultant fragment, which was then cloned at the Sal I cleavage site on a polylinker of the plasmid vector pMM102. A DNA fragment was then selected which had a CAT gene inserted into the polylinker in a positive direction with respect to the MMTV promotor region and was introduced into a mink lung cell as in a manner used in Example 1. After incubation for 48 hours, the cell was suspended in 0.25 M Tris buffer solution and the suspension thus prepared was subjected to ultra-sonication for 1 minute to cause the cell to disrupt, followed by centrifugation at 15,000 rpm for 5 minutes to collect the supernatant. To this supernatant were added ¹⁴C-labeled chloramphenicol and acetyl-CoA. After incubation at 30°C for 60 minutes and subsequent extraction of the liquid substance of the cell into ethyl acetate, an organic layer of ethyl acetate was dried and the resultant substance was dissolved in 10 µl of ethyl acetate. The preparation thus obtained was subjected to silica-gel thin-layer chromatography to develop using a 95:5 (v/v) chloroform-methanol mixture. After the development and drying procedures, analysis was made into the chromatogram generated by an x-ray film held in contact with the resultant preparation by means of a radioautography method. The analysis revealed that chloramphenicol was acetylated in the extracted liquid substance of the cell into which the CAT gene had been introduced. This means that the CAT gene was expressed within the cell.

Claims: A chimeric plasmid vector which comprises (a) a first DNA (deoxyribonucleic acid) fragment having a first LTR (long terminal repeat) region derived from a mouse mammary tumor virus (MMTV) and a 5'-splicing site located downstream of the first LTR region, the first LTR region comprising (a-1) a hormone dependent region, and (a-2) a MMTV promotor region located downstream of the hormone dependent region; (b) a second DNA fragment located downstream of said first DNA fragment and having a 3'-splicing site derived from plasmid pCVSVE; (c) a polylinker derived from plasmid pi-AN7 and located downstream of said second DNA fragment; (d) a third DNA fragment located downstream of said polylinker and having a second LTR region derived from said MMTV, the second LTR comprising (d-1) a transcription termination site; (e) a fourth DNA fragment located downstream of said third DNA fragment, the fourth DNA fragment comprising (e-1) an SV40 promotor region derived from plasmid pSV2gpt, and (f) a fifth DNA fragment located downstream of said fourth DNA fragment and between said first DNA fragment and said fourth DNA fragment, the fifth DNA fragment comprising (f-1) an ampicillin resistance gene region derived from plasmid pBR322, and (f-2) a replication origin derived from said plasmid pBR322. A chimeric plasmid vector according to claim 1 in which said first DNA fragment consists of a DNA fragment of clone H cleaved with restriction enzymes Bam HI and Xho I, said clone H being derived by cloning an MMTV at a Eco RI cleavage site of a lambda-phage vector lambda-gtWES lambda-B. A chimeric plasmid vector according to claim 1 or 2 in which said third DNA fragment consists of a DNA fragment of clone H cleaved with restriction enzyme Bg1 II. A chimeric plasmid vector according to any one of claims 1-3 in which said second DNA fragment having said 3'-splicing site contained therein consists of a DNA fragment of plasmid pCVSVE cleaved with restriction enzymes Pst I and Hin dIII. A chimeric plasmid vector according to any of the preceding claims in which said fourth DNA fragment having said Eco-gpt gene region contained therein consists of a DNA fragment of plasmid SV2gpt cleaved with restriction enzymes Pvu II and Bam HI. A chimeric plasmid vector, denoted pMM102 herein, and deposited in E. Coli K-12 under FERM BP-1157, the restriction enzyme map of which is as shown in Figures 3 and 3A of the accompanying drawings. A chimeric plasmid vector claimed in any one of the preceding claims which has a foreign gene inserted in its polylinker (c). A method employing a vector to convey a foreign gene into an animal host cell, to retain and express the foreign gene therein, in which method there is used as vector a chimeric plasmid vector claimed in claim 7. An animal host cell transformed by containing a chimeric plasmid vector claimed in claim 7. A method of producing a protein by animal cells in which the animal cells are animal cells transformed by containing a chimeric plasmid claimed in claim 7 wherein the foreign gene codes for the protein.

Inventors: SHIRASAWA, YUKIO, TSUCHIDA, NOBUO
Applicants: KABUSHIKI KAISHA YAKULT HONSHA
Application year: 1986
Main classification: C12N15
IPC codes: C12N15, C12R1, C12P21