Improvements in or relating to eukaryotic expression

Abstract: There is provided a method for producing a useful substance in a eukaryotic host cell wherein the cell is transformed with a recombinant DNA vector that comprises a eukaryotic promoter, a poly-GT element positioned to stimulate the promoter, and a DNA sequence that encodes the useful substance, the DNA sequence being positioned for expression from the promoter, and wherein the cell containing the vector is cultured under conditions suitable for expression of the useful substance, which comprises: (a) providing the cell with a DNA sequence that codes for the expression of an immediate-early gene product of a large DNA virus, and (b) culturing the cell of step (a) under conditions suitable for expressing the gene product and stimulating the activity of the poly-GT element.

Description: The present invention concerns a method of using a poly-GT element in the presence of an immediate-early gene product of a large DNA virus to increase transcription of a recombinant gene in eukaryotic host cells. The poly-GT element exemplified herein is chemically synthesized (the prototype poly-GT element is depicted in Example 1 below). When inserted into an expression vector, the prototype poly-GT element comprises 21 repeating GT units: However, a wide variety of poly-GT elements, comprising repeating units of varying lengths will be recognized by those skilled in the art as suitable for use in the present invention, including chemically synthesized sequences or human genomic sequences. An enhancer element may be used along with the poly-GT element in the present invention to further increase such transcription. In some constructs, the poly-GT sequence is associated with the BK enhancer. The BK enhancer is a defined segment of DNA that consists of repeated sequences (the prototype BK enhancer and a variant BK enhancer are depicted in Example 18, below). However, a wide variety of BK enhancer variants, not all consisting of varying repeated sequences, are known in the art and suitable for use in the invention along with the poly-GT element. The alternating sequence, poly (dT-dG)· poly (dC - dA), is a highly repeated sequence in the eukaryotic genome and is capable of forming left-handed or Z -DNA. Hamada and Kakunaga. 1982, Nature 298:396-398; Hamada et al., 1982, Proc. Natl. Acad. Sci. USA 79:6465-6469. For example, the human genome contains approximately 100,000 copies of 20 to 60 base pair (bp) tracts of poly (dT-dG) · poly (dC - dA) randomly distributed. Hamada et al., 1984, Mol. Cell. Biol. 4:2610-2621. Hamada et al., 1984, Mol. Cell. Biol. 4:2622-2630, disclose that poly (dT-dG) · poly (dC-dA) itself can act as an enhancer of the expression of chloramphenicol acetyltransferase (CAT) when the poly (dT-dG) · poly (dC-dA) is linked to the SV40 early promoter and to the coding sequence for CAT. According to Hamada et al., supra , the enhancer-like activity of poly (dT-dG) · poly (dC-dA) with the SV40 early promoter was much weaker than that of the SV40 enhancer with the SV40 early promoter, and, unlike many viral enhancers, the poly(dT-dG) · poly (dC-dA) was equally active in monkey (CV-1) or human (HeLa) cells. The BK enhancer sequences exemplified herein are obtained from BK virus, a human papovavirus that was first isolated from the urine of an immunosuppressed patient. BK virus is suspected of causing an unapparent childhood infection and is ubiquitous in the human population. Although BK virus grows optimally in human cells, the virus undergoes an abortive cycle in many non-primate cells, transforms rodent cells in vitro , and induces tumors in hamsters. BK virus is very similar to SV40 virus, but the enhancer sequences of these two papovaviruses, SV40 and BK, differ substantially in nucleotide sequence. The complete nucleotide sequence of BK virus (∼5.2 kb) has been disclosed by Seif et al ., 1979, Cell 18:963, and Yang and Wu, 1979, Science 206:456. Prototype BK virus is available from the American Type Culture Collection (ATCC), 12301 Parklawn Dr., Rockville, MD 20852-1776, under the accession number ATCC VR-837. A restriction site and function map of prototype BK virus is presented in Figure 1 of the accompanying drawings. Enhancer elements are cis -acting elements that increase the level of transcription of an adjacent gene from its promoter in a fashion that is relatively independent of the position and orientation of the enhancer element. In fact, Khoury and Gruss, 1983, Cell 33:313, state that "the remarkable ability of enhancer sequences to function upstream from, within, or downstream from eukaryotic genes distinguishes them from classical promoter elements . . ." and suggest that certain experimental results indicate that "enhancers can act over considerable distances (perhaps >10 kb)." The present invention teaches a novel enhancer system, because the poly-GT element, when placed in combination with certain promoters, such as the adenovirus late promoter, functions as an enhancer only in the presence of a trans -acting early viral gene product, such as the E1A gene product. Other enhancer elements, such as the BK enhancer, may be used along with the poly-GT element in the present invention. The present invention specifically teaches an enhancer system for the adenovirus late promoter that results in unexpected increases in transcription only where there is a combination of a cis -acting poly-GT element with a trans -acting immediate-early gene product of a large DNA virus. Thus, a viral trans -acting protein is required to activate enhancer activity from the poly-GT element. This is in striking contrast to the enhancer system of Hamada et al, supra , where a poly-GT sequence alone was capable of enhancing expression of the CAT gene product using the SV40 early promoter. In the present invention, another enhancer element may be used along with the poly-GT element and the trans -acting viral gene product to further increase transcription and thus expression of a useful gene product. When the BK enhancer is used along with the poly-GT element, still further increases in transcription result upon positioning the BK enhancer immediately upstream of (on the 5' side of) the "CCAAT" region of a eukaryotic promoter that is used in tandem with the BK enhancer to transcribe a DNA sequence encoding a useful substance. The CCAAT region or "immediate upstream region" or "-80 homology sequence" is a cis -acting upstream element that is a conserved region of nucleotides observed in promoters whose sequences for transcriptional activity have been dissected. The CCAAT region is found in many, but not all, promoters. In other promoters, equivalent cis -acting upstream sequences are found, including SP1 binding sites, ATF binding sites, the octa sequence, nuclear factor 1 binding sites (which may be the same binding site as the CCAAT transcription factor), the AP1 and AP2 homologies, glucocorticoid response elements, and heat shock response elements. Recently, a comprehensive compilation of currently known upstream sequences, including those just listed, has been published. Wingender, 1988, Nucl. Acids Res. 16:1879-1912. The CCAAT region equivalent in the adenovirus major late promoter is the upstream transcription factor (UTF) or major late transcription factor (MLTF) binding site (approximate nucleotides -50 to -65 upstream of the CAP site). The CCAAT sequence mediates the efficiency of transcription and, with few exceptions, cannot be deleted without decreasing promoter strength. Enhancer elements have been identified in the DNA of a number of viruses, including murine polyomavirus, papilloma virus, adenovirus, retrovirus, hepatitis virus, cytomegalovirus, herpes virus, papovaviruses, such as SV40 and BK, and in many non-viral genes, such as within immunoglobulin gene introns. Enhancer elements may also be present in the DNA of a wide variety of other organisms. Enhancer elements often function differently in different host cells, and this cellular specificity can be due to differences in host gene products that interact with the enhancer element during gene expression. The activity of enhancer elements can be affected by viral gene products present in the host cell. Velcich and Ziff, 1983, Cell 40:705; Borrelli et al ., 1984, Nature 312:608; and Hen et al ., 1985, Science 230:1391, disclose that the adenovirus-2 early region 1A (E1A) gene products repress activation of transcription induced by the SV40, polyoma virus, mouse immunoglobulin gene and adenovirus-2 E1A enhancers. This effect by E1A can be host cell dependent. Eukaryotic expression vectors that utilized enhancers to increase transcription of recombinant genes consequently were not expected to work better than vectors without enhancers in E1A-containing host cells. In striking contrast to the above-referenced prior art methods of using enhancers, European Patent Publication, EPA 0 245 949 (Appln. No. 87303083.7, published Nov. 19, 1987), teaches a method for using the BK virus enhancer element that involves using the E1A gene product or a similar immediate-early gene product of a large DNA virus to maximize gene expression. Thus, European Pat. Publication, EP A 0 245 949, demonstrates for the first time that the ability of the BK enhancer to promote transcription of DNA is increased in the presence of the E1A gene product of any adenovirus. It has now been unexpectedly found that a poly-GT element may be used along with the BK enhancer to further increase transcription of DNA in the presence of the E1A gene product of any adenovirus. When the adenovirus late promoter is used to drive transcription, the poly-GT element surprisingly does not itself enhance transcription, but is activated and does demonstrate enhancer activity only in the presence of the E1A gene product or a similar immediate-early gene product of a large DNA virus. Because the E1A gene product has been shown to repress activation of transcription by most enhancers tested, it was not expected that E1A would have the ability to activate an enhancer-like activity of a poly-GT element. The E1A gene product (actually, the E1A gene produces two products, which are collectively referred to herein as "the E1A gene product") is one such immediate-early gene product from adenovirus, a large DNA virus. The present invention encompasses the use of any immediate-early gene product of a large DNA virus or any retroviral protein product that functions similarly to the E1A gene product to increase the activity of a poly-GT element with or without another enhancer element, such as the BK enhancer. The herpes simplex virus ICP4 protein, described by DeLuca et al ., 1985, Mol. Cell. Biol. 5: 1997-2008, the pseudorabies virus IE protein, described by Feldman et al ., 1982, Proc. Natl. Acad. Sci. USA 79:4952-4956, the cytomegalovirus IE protein described by Tevethia et al ., 1987, Virology 161:276-285, and the E1B protein of adenovirus are all immediate-early gene products of large DNA viruses that have functions similar to the E1A protein. Therefore, the method of the present invention includes the use of the ICP4, IE, or E1B proteins, or any other similarly acting immediate-early gene product, either in the presence or absence of E1A protein, to produce or activate an enhancer-like activity from a poly-GT element. In addition, other trans -acting viral proteins, such as the TAT gene product of the HIV virus or other retroviral proteins, may be useful in the method to activate a poly-GT element. The present invention concerns a method of using a poly-GT element in the presence of an immediate-early gene product of a large DNA virus, such as the E1A gene product of adenovirus, or a similarly functioning viral gene product, for purposes of increasing transcription and thus improving expression of recombinant genes in eukaryotic host cells. Another significant aspect of the present invention relates to a variety of expression vectors that utilize the poly-GT element with a eukaryotic promoter, such as the adenovirus late promoter (MLP), in the presence of an immediate-early gene product of a large DNA virus, such as the E1A gene product of adenovirus, to drive expression of useful products in eukaryotic host cells. In addition, an enhancer element such as the BK enhancer may be used along with the poly-GT element to further enhance expression of such useful products. Many of these expression vectors comprise a poly-GT - adenovirus late promoter cassette, which can be readily transferred to other vectors for use in the present method. The cassette may further comprise an enhancer element, such as the BK enhancer. The versatility of the present expression vectors is demonstrated by the high-level expression that may be driven by these vectors of such diverse proteins as chloramphenicol acetyltransferase, protein C, tissue plasminogen activator, modified tissue plasminogen activator or an interferon (α, β, or γ interferon). It is a characteristic of enhancer elements that they can enhance transcription from a distance and that their activity is not dependent on their orientation. Consistent with this, the poly-GT element of this invention may be positioned 5' or 3' to the structural gene to be expressed. Thus, in the construction of certain recombinant DNA expression vectors of the present invention, the poly-GT element was placed upstream of the adenovirus late promoter, itself positioned to drive expression of a recombinant gene on a recombinant DNA expression vector. In addition, in certain of such constructions, a BK enhancer element was placed upstream of the poly-GT element. In the construction of the other recombinant DNA expression vectors of the invention, the poly-GT element was placed at the 3' end of a structural gene to be expressed, demonstrating that the poly-GT element could function in various positions and orientations as has been described for other enhancer elements. The practice of the invention results in increases in expression of human protein C in adenovirus-transformed cells. Such cells are especially preferred hosts for the production of fully γ-carboxylated proteins, such as human protein C. Consequently, a further aspect of the invention comprises an improved method for making γ-carboxylated proteins. Yet another important aspect of the present invention concerns a method of further increasing the expression of a useful gene product by (a) positioning the BK enhancer relative to an adjacent eukaryotic promoter, (b) placing a poly-GT element in such an expression vector and (c) activating such a poly-GT element (and the BK enhancer) with an immediate-early gene product of a large DNA virus or any viral gene product that functions similarly to the E1A gene product. In a first step, these derivatives were constructed by enzymatic treatment that positioned the BK enhancer very close to the MLTF binding site. Dramatic increases in expression levels, as compared with constructions that lack this positioning, were observed when these modified BK enhancer-adenovirus late promoter sequences were incorporated into expression vectors and then used to drive expression of useful gene products in eukaryotic host cells. In a second step, these derivatives were further constructed by placing a poly-GT element in either orientation at the 3' end of a gene encoding a useful product. Further increases in expression levels, as compared with constructions that lack the poly-GT element, were observed. Thus, the present invention provides a method for further increasing expression of a useful gene product by increasing the activity of the BK enhancer relative to an adjacent eukaryotic promoter that comprises positioning the enhancer immediately upstream, within 0 to about 300 nucleotides, of the 5' end of the CCAAT region or CCAAT region equivalent of the eukaryotic promoter and by activating a poly-GT element irrespective of its position and orientation with E1A gene product. Yet another aspect of the invention results from attempts to increase expression of recombinant products encoded on the vectors described herein by incorporation of a poly-GT element into certain expression vectors in which have been incorporated portions of the tripartite leader (TPL) sequence of adenovirus. Increases in expression result when the TPL sequence is incorporated along with the poly-GT element in such expression vectors and the poly-GT element is activated in the presence of the E1A gene product of adenovirus. Further increases in expression in such TPL-containing vectors may result in the presence of the VA gene product of adenovirus. For purposes of the present invention, the following terms are as defined below: Antibiotic - a substance produced by a microorganism that, either naturally or with limited chemical modification, will inhibit the growth of or kill another microorganism or eukaryotic cell. Antibiotic Resistance-Conferring Gene - a DNA segment that encodes an activity that confers resistance to an antibiotic. ApR - the ampicillin-resistant phenotype or gene conferring same. Cloning - the process of incorporating a segment of DNA into a recombinant DNA cloning vector. CmR - the chloramphenicol-resistant phenotype or gene conferring same. E1A - an immediate-early gene product of adenovirus which can activate a poly-GT element to express enhancer activity. ep - a DNA segment comprising the SV40 early promoter of the T-antigen gene, the T-antigen binding sites, and the SV40 origin of replication. Eukaryotic promoter - any DNA sequence that functions as a promoter in eukaryotic cells. GT enhancer system - any poly-GT element linked to a promoter, such as MLP, in which the poly-GT element does not itself possess enhancer activity but is activated as an enhancer by an immediate-early gene product of a large DNA virus, such as the E1A gene product or by any similarly activating viral gene product. HmR - the hygromycin-resistant phenotype or gene conferring same. IVS - DNA encoding an intron, also called an intervening sequence. Large DNA virus - a virus that infects eukaryotic cells and has a genome greater than ∼10 kb in size, i.e., any of the pox viruses, adenoviruses, and herpes viruses. MLP - the major late promoter of adenovirus, which is also referred to herein as the adenovirus late promoter, adenovirus-type-2 late promoter, or Ad2 late promoter. MLTF binding site - the site in adenovirus DNA where the major late transcription factor (MLTF) binds; the MLTF is required for MLP activity. NeoR - the neomycin resistance-conferring gene, which can also be used to confer G418 resistance in eukaryotic host cells. ori - a plasmid origin of replication. pA - a DNA sequence encoding a polyadenylation signal. Poly-GT element - a DNA sequence of (GT) n -(CA) n , which is illustrated herein by a sequence where n is 21, but which can also refer to sequences of varying lengths where n is greater or less than 21, and may refer to chemically synthesized (GT) n -(CA) n sequences or human genomic DNA fragments containing a (GT) n -(CA) n tract. Promoter - a DNA sequence that directs transcription of DNA into RNA. Recombinant DNA Cloning Vector - any autonomously replicating or integrating agent that comprises a DNA molecule to which one or more additional DNA segments can be or have been added. Recombinant DNA Expression Vector - any recombinant DNA cloning vector comprising a promoter and associated insertion site, into which a DNA sequence that encodes a useful product can be inserted and expressed. Recombinant DNA Vector - any recombinant DNA cloning or expression vector. Replicon - any DNA sequence that controls the replication of a recombinant DNA vector. Restriction Fragment - any linear DNA generated by the action of one or more restriction enzymes. rRNA - ribosomal ribonucleic acid. Sensitive Host Cell - a host cell that cannot grow in the presence of a given antibiotic or other toxic compound without a DNA segment that confers resistance thereto. Structural Gene - any DNA sequence that encodes a polypeptide, inclusive of that DNA encoding the start and stop codons. TAT - a gene product of the retrovirus HIV which has activity as a trans -acting viral protein. TcR - the tetracycline-resistance phenotype or gene conferring same. Transformant - a recipient host cell that has undergone transformation. Transformation - the introduction of DNA into a recipient host cell. tRNA - transfer ribonucleic acid. Figure 1 is a restriction site and function map of BK virus. Figure 2 is a restriction site and function map of plasmid pBKneol. Figure 3 is a restriction site and function map of plasmid pSV2cat. Figure 4 is a restriction site and function map of plasmid pLP(GT)cat. Figure 5 is a restriction site and function map of plasmid pBL(GT)cat. Figure 6 is a restriction site and function map of plasmid pSBL(GT)cat. Figure 7 depicts the construction and presents a restriction site and function map of plasmid pL133. Figure 8 is a restriction site and function map of plasmid pLPC(GT). Figure 9 is a restriction site and function map of plasmid pLPC4(GT). Figure 10 is a restriction site and function map of plasmid pSV2hyg. Figure 11 is a restriction site and function map of plasmid pLPChyg1(GT). Figure 12 depicts the construction and presents a restriction site and function map of plasmid pBW32. Figure 13 is a restriction site and function map of plasmid pLPChd1(GT). Figure 14 is a restriction site and function map of plasmid phd(GT). Figure 15 is a restriction site and function map of plasmid pLPCE1A(GT). Figure 16 is a restriction site and function map of plasmid pBLT(GT). Figure 17 is a restriction site and function map of plasmid pBLThyg1(GT). Figure 18 is a restriction site and function map of plasmid pBLTdhfr1(GT). Figure 19 is a restriction site and function map of plasmid pTPA603. Figure 20 is a restriction site and function map of plasmid phd(GT)TPA. Figure 21 is a restriction site and function map of plasmid phd(GT)MTPA. Figure 22 is a restriction site and function map of plasmid pBal8cat(GT6). Figure 23 is a restriction site and function map of plasmid pSV2E1A. Figure 24 is a restriction site and function map of plasmid pT6hd(GT). The drawings are meant to aid the reader in understanding the invention. The figures are representations of the noted plasmids and are drawn approximately to scale. Restriction sites other than those expressly drawn may be present on the plasmids. The present invention concerns an improved method for producing a useful substance in a eukaryotic host cell wherein the cell is transformed with a recombinant DNA vector that comprises a eukaryotic promoter, a poly-GT element positioned to stimulate the promoter, and a DNA sequence that encodes the useful substance, the DNA sequence being positioned for expression from the promoter, and wherein the cell containing the vector is cultured under conditions suitable for expression of the useful substance, wherein the improvement comprises: (a) providing the cell with a DNA sequence that codes for the expression of an immediate-early gene product of a large DNA virus; and (b) culturing the cell of step (a) under conditions suitable for expressing the immediate-early gene product and stimulating the activity of the poly-GT element. Those skilled in the art recognize that many established cell lines express an immediate-early gene product of a large DNA virus and that such cell lines are especially useful in the present method. Thus, the present invention also comprises an improved method for producing a useful substance in a eukaryotic host cell wherein the cell is transformed with a recombinant DNA vector that comprises a eukaryotic promoter, a poly-GT element positioned to stimulate the promoter, and a DNA sequence that encodes the useful substance, the sequence being positioned for expression from the promoter, and wherein the cell containing the vector is cultured under conditions suitable for expression of the useful substance, wherein the improvement comprises: (a) inserting the vector into a eukaryotic host cell that expresses an immediate-early gene product of a large DNA virus, and (b) culturing the cell of step (a) under conditions suitable for expressing the immediate-early gene product and stimulating the activity of the poly-GT element. An important aspect of the present invention is the novel group of expression vectors that comprise a poly-GT element with the adenovirus-2 late promoter. The expression vectors of the present invention were constructed so that DNA molecules encoding useful products can be or have been readily inserted in other vectors in the correct position for enhanced expression via the activation of the poly-GT element by the E1A gene product. Furthermore, the poly-GT element and eukaryotic promoter have been constructed to form a "cassette," which can be isolated from the expression vectors on a relatively small restriction fragment. The cassette can be readily shuttled between a variety of expression vectors. The cassette may contain, in addition to the poly-GT element, an enhancer element, such as the BK enhancer. The expression vectors specifically exemplified herein utilize the adenovirus-2 late promoter in the poly-GT - BK enhancer-eukaryotic promoter cassette that drives transcription in the presence of the E1A gene product in the method of the present invention. Single-stranded DNA fragments used in the construction of a poly-GT element were chemically synthesized by using a DNA synthesizer. Many DNA synthesizing instruments are known in the art and can be used to synthesize the single-stranded fragments which are then annealed to form a poly-GT element. A poly-GT element is a DNA sequence of (GT) n - (CA) n , which is illustrated-herein by a sequence wherein n is 21, but also refers to sequences of varying lengths where n is greater or less than 21. Such sequences may be chemically synthesized or may be human genomic DNA fragments. The sequence of a prototype poly-GT element is illustrated in Example 1. The illustrated poly-GT element has Bam HI and Xho I sites and thus may be conveniently inserted into a Bam HI or an Xho I site in a variety of expression vectors. Those skilled in the art will recognize that a synthetic poly-GT element may contain sequences for restriction enzyme recognition sites other than Bam HI or Xho I so as to make the poly-GT element easily insertable into any expression vector. Alternatively, regardless of the restriction enzyme recognition sites available in the poly-GT element, the poly-GT element may be inserted by blunt-end ligation into any suitable expression vector. BK virus, publically available from the American Type Culture Collection (ATCC), 12301 Parklawn Drive, Rockville, MD 20852, under the accession number, ATCC VR-837, can be purchased or readily isolated in large quantities, as described in Example 2, for use as a source of BK viral sequences, in particular, as a source of BK enhancer. It is also convenient to clone the BK viral DNA onto a plasmid cloning vector and use the recombinant vector as a source of BK viral DNA sequences. Consequently, the BK viral genome has been combined with a portion of plasmid pdBPV-MMTneo to construct plasmids pBKneo1 and pBkneo2. Plasmid pdBPV-MMTneo, about 15 kb in size and publically available from the ATCC under the accession number ATCC 37224, comprises the replicon and β-lactamase gene from plasmid pBR322, the mouse metallothionein promoter positioned to drive expression of a structural gene that encodes a neomycin resistance-conferring enzyme, and about 8 kb of bovine papilloma virus (BPV) DNA. Plasmid pdBPV-MMTneo can be digested with restriction enzyme Bam HI to generate two fragments: the ∼8 kb fragment that comprises the BPV DNA and an ∼7 kb fragment that comprises the other sequences described above. BK virus has only one Bam HI restriction site, and plasmids pBKneo1 and pBkneo2 were constructed by ligating the ∼7 kb Bam HI restriction fragment of plasmid pdBPV-MMTneo to Bam HI-linearized BK virus DNA. The construction of plasmids pBKneo1 and pBKneo2, which differ only with respect to the orientation of the BK virus DNA, is described in Example 2, and a restriction site and function map of plasmid pBKneo1 is presented in Figure 2 of the accompanying drawings. Because plasmids pBKneo1 and pBKneo2 comprise the entire genome of the BK virus, including the enhancer sequence, they serve as useful starting materials for the poly-GT containing expression vectors described herein that also contain the BK enhancer. Other expression vectors that do not contain the BK enhancer are also useful in the present invention. One expression vector, plasmid pBLcat, comprises the BK enhancer sequence in tandem with the human adenovirus-type-2 late promoter positioned to drive expression of the chloramphenicol acetyltransferase enzyme (CAT). Plasmid pSV2cat, publically available, serves as a convenient source of the CAT gene and can be obtained from the ATCC under the accession number ATCC 37155. A restriction site and function map of plasmid pSV2cat is presented in Figure 3 of the accompanying drawings. Human adenovirus-type-2 DNA is commercially available and also is publically available from the ATCC under the accession number ATCC VR-2. Another expression vector, plasmid pLPcat, not containing the BK enhancer, comprises the human adenovirus-type-2 late promoter positioned to drive expression of chloramphenicol acetyltransferase. A poly-GT element has been inserted into both plasmids pLPcat and pBLcat to create plasmids pLP(GT)cat and pBL(GT)cat, respectively. Plasmids pLPcat and pBLcat were constructed as follows. The ∼0.32 kb late-promoter-containing Acc I- Pvu II restriction fragment of human adenovirus-type-2 DNA was ligated to blunt-ended Bcl I linkers that attached only to the Pvu II end of the Acc I- Pvu II restriction fragment. The resulting fragment was then ligated to the ∼4.51 kb Acc I- Stu I restriction fragment of plasmid pSV2cat to yield plasmid pLPcat. Plasmid pBLcat was constructed from plasmid pLPcat by ligating the origin of replication and enhancer-containing, ∼1.28 kb Acc I- Pvu II restriction fragment of BK virus DNA to the ∼4.81 kb Acc I- Stu I restriction fragment of plasmid pLPcat. A poly-GT element is inserted into plasmids pLPcat and pBLcat as follows. A poly-GT element with Xho I linkers is ligated into Xho I digested plasmid pLPcat or pBLcat to create plasmids pLP(GT)cat and pBL(GT)cat, respectively. The unique Xho I site in plasmid pLPcat is located upstream of the adenovirus late promoter. The unique Xho I site in plasmid pBLcat is located between the BK enhancer and the adenovirus late promoter. The construction of plasmids pLPcat, pLP(GT)cat, pBLcat, and pBL(GT) cat are further described in Examples 4 and 5. Restriction site and function map of plasmids pLP(GT)cat and pBL(GT)cat are presented in Figures 4 and 5, respectively, of the accompanying drawings. Plasmid pBLcat is a convenient source of the BK enhancer-adenovirus late promoter "cassette". This cassette is an ∼870 bp Hin dIII restriction fragment that can be conveniently inserted into a eukaryotic expression vector to increase expression of a product encoded by that vector. Plasmid pBL(GT)cat is a convenient source of the poly-GT - BK enhancer - adenovirus late promoter cassette of the present invention. This poly-GT containing cassette is an ∼930 bp Hin dIII restriction fragment that can also be conveniently inserted into a eukaryotic expression vector to further increase expression of a useful gene product encoded by that vector in the presence of an immediate-early gene product of a large DNA virus, such as the E1A gene product. Plasmid pSBLcat and pSBL(GT)cat may be prepared using the above described cassettes. For plasmid pSBLcat, this was done by digesting plasmid pSV2cat with restriction enzyme Hin dIII and inserting the BK enhancer-adenovirus late promoter cassette. The resultant plasmid, designated as plasmid pSBLcat, contains the BK enhancer and, in addition, the SV40 origin of replication, SV40 early promoter, and SV40 enhancer and therefore differs from plasmid pBLcat in which those additional sequences have been deleted. The tandem SV40 enhancer-BK enhancer-adenovirus major late promoter (SBL promoter) cassette can be excised from plasmid pSBLcat on a Pvu II restriction enzyme fragment, which can be conveniently inserted into any recombinant DNA expression vector. Plasmid pSBLcat drives expression of CAT to higher levels than does plasmid pBLcat, so long as no E1A gene product is present. This increased expression in the absence of EIA gene product indicates that the two enhancers, one from SV40 and the other from BK, have an additive, enhancing effect on transcription from nearby promoters in some cell lines. Thus, tandem enhancer sequences upstream of a eukaryotic promoter may be used for the efficient expression of genes in a wide variety of mammalian cells. In the presence of E1A gene product, plasmid pBLcat drives expression of CAT to higher levels than does plasmid pSBLcat, presumably because the SV40 enhancer is inhibited by the E1A gene product. Higher levels of expressions of CAT therefore would be expected with plasmid pBL(GT)cat as compared with pSBL(GT)cat in the presence of E1A gene product. Conversely, in HeLa cells, the SV40 enhancer markedly stimulated transcription from the adenovirus 2 late promoter, but the BK enhancer only minimally stimulated transcription from the adenovirus 2 late promoter in HeLa cells. Because the basal level of BK activity in HeLa cells is so low, stimulation of that activity with the immediate-early gene product of a large DNA virus, such as E1A gene product, still does not result in optimal expression levels. Inserting a poly-GT element along with the BK enhancer appears to have no effect on increasing expression in HeLa cells. This low level activity of the BK enhancer in HeLa cells is thought to be due to a repressor activity present in HeLa cells that interacts with the BK enhancer. This repressor activity in HeLa cells can be titrated out by introducing more copies of the BK enhancer into the HeLa cell. In fact, in the HeLa cell line, E1A may increase the level of the repressor. However, optimal expression levels can be obtained in HeLa cells using the tandem SV40 enhancer - BK-enhancer. This tandem enhancer thus has the advantage of avoiding cell-specific negative interactions that may be encountered, as in HeLa cells, in some host cells. A restriction site and function map of plasmid pSBL(GT)cat is presented in Figure 6 of the accompanying drawings, and the construction of plasmids pSBLcat and pSBL(GT)cat are described in Example 6. The BK enhancer-adenovirus late promoter cassette has been used for expression of human protein C. The poly-GT - BK enhancer - adenovirus late promoter cassette has been used in the present invention to improve expression of human protein C. Increased expression may be achieved by ligating either of the above-described cassettes into plasmid pL133, a plasmid disclosed in U.S. Patent No. 4,775,624, issued October 4, 1988, incorporated herein by reference. A restriction site and function map of plasmid pL133 is presented in Figure 7 of the accompanying drawings. Plasmid pL133, the construction of which is given in Example 7, was digested with restriction enzyme Hin dIII and then ligated to the ∼0.87 kb Hin dIII restriction fragment of plasmid pBLcat to yield plasmid pLPC. A poly-GT element is inserted into the unique Xho I site of plasmid pLPC to yield plasmid pLPC(GT). A restriction site and function map of plasmid pLPC(GT) is presented in Figure 8 of the accompanying drawings, and the construction of plasmids pLPC, pLPC(GT) are further described in Example 8. Alternatively, plasmid pLPC(GT) is prepared by digesting pL133 with Hin dIII and then ligating it with the ∼0.93 kb Hin dIII fragment of pBL(GT). Plasmids pLPC(GT) and pLPC, like plasmid pL133, comprise the enhancer, early and late promoters, T-antigen-binding sites, and origin of replication of SV40. Thus, use of plasmid pLPC or pLPC(GT) and derivatives thereof in any recombinant host cells is illustrative of the tandem enhancer expression method described herein. Plasmid pLPC or pLPC(GT) may serve as a useful starting material for many vectors described herein, including plasmids pSBL and pSBL(GT). Plasmids pSBL and pSBL(GT) may be constructed by deleting the protein C-encoding DNA on plasmids pLPC and pLPC(GT), respectively. This deletion merely requires excision of plasmid pLPC's or pLPC(GT)'s single Bcl I restriction fragment by digestion with Bcl I and self-ligation. The resulting plasmid pSBL or pSBL(GT) serves as a convenient expression vector for use in the tandem enhancer method described herein, because coding sequences of interest can be readily inserted at the sole remaining Bcl I site. The SV40 regulatory elements present on plasmid pLPC or pLPC(GT) are situated closely together and difficult to delineate. The binding of T antigen to the T-antigen-binding sites, which is necessary for SV40 replication, is known to enhance transcription from the SV40 late promoter. Because the high level of T-antigen-driven replication of a plasmid that comprises the SV40 origin of replication is generally lethal to the host cell, neither plasmid pLPC nor plasmid pL133, for example, are stably maintained in the presence of SV40 T antigen. The overall structure of the BK regulatory region is quite similar to that of SV40, for the BK enhancer, origin of replication, early and late promoters, and the BK analogue of the T-antigen-binding sites are all closely situated and each functional domain is not easily separated from the other. However, when grown in the present of BK T antigen, a plasmid that comprises the BK origin of replication and T-antigen-binding sites does not necessarily replicate to an extent that proves lethal and can bestably maintained as an episomal element in the host cell. In addition, the T-antigen-driven replication can be used to increase the copy number of a vector comprising the BK origin of replication so that when selective pressure is applied more copies of the plasmid integrate into the host cell's chromosomal DNA. Apparently due to the similar structure-function relationships between the BK and SV40 T antigens and their respective binding sites, BK replication is also stimulated by SV40 T antigen. To construct a derivative of plasmid pLPC that can exist as a stably-maintained element in a transformed eukaryotic cell, the entire BK genome, as an Eco RI-linearized restriction fragment, was inserted into the single Eco RI restriction site of plasmid pLPC. This insertion produced two plasmids, designated pLPC4 and pLPC5, which differ only with respect to the orientation of the BK Eco RI fragment. The construction of plasmids pLPC4 and pLPC5 is further described in Example 9A. A poly-GT element is inserted into the unique Xho I site of plasmids pLPC4 or pLPC5 to create plasmids pLPC4(GT) and pLPC5(GT), respectively, as described in Example 9B. A restriction site and function map of plasmid pLPC4(GT) is presented in Figure 9 of the accompanying drawings. Episomal maintenance of a recombinant DNA expression vector is not always preferred over integration into the host cell chromosome. However, due to the absence of a selectable marker that functions in eukaryotic cells, the identification of stable, eukaryotic transformants of plasmid pLPC or pLPC(GT) is difficult, unless the plasmid is cotransformed with another plasmid that does comprise a selectable marker. Consequently, plasmid pLPC or plasmid pLPC(GT) may be modified to produce derivative plasmids that are selectable in eukaryotic host cells. In the case of plasmid pLPC, this was done by ligating the plasmid to a portion of plasmid pSV2hyg; plasmid pSV2hyg is a plasmid that comprises a hygromycin resistance-conferring gene. A restriction site and function map of plasmid pSV2hyg, which can be obtained from the Northern Regional Research Laboratory (NRRL), Peoria, IL 61640, under the accession number NRRL B-18039 (deposited Feb. 11, 1986), is presented in Figure 10 of the accompanying drawings. Plasmid pSV2hyg was digested with restriction enzyme Bam HI, and the ∼2.5 kb Bam HI restriction fragment, which comprises the entire hygromycin resistance-conferring gene, was isolated, treated with Klenow enzyme (the large fragment produced upon subtilisin cleavage of E . coli DNA polymerase I), and then ligated to the Klenow-treated, ∼5.82 kb Nde I- Stu I restriction fragment of plasmid pLPC to yield plasmids pLPChyg1 and pLPChyg2. Plasmids pLPChyg1 and pLPChyg2 differ only with respect to the orientation of the hygromycin resistance-conferring fragment. The construction protocol for plasmids pLPChyg1 and pLPChyg2 is described in Example 10A. A poly-GT element is inserted into the unique Xho I site of plasmids pLPChyg1 and pLPChyg2, to create plasmids pLPChyg1(GT) and pLPChyg2(GT), respectively, as described in Example 10B. A restriction site and function map of plasmid pLPChyg1(GT) is presented in Figure 11 of the accompanying drawings. Human protein C expression plasmids similar to plasmids pLPChyg1 and pLPChyg2 containing the dihydrofolate reductase (dhfr) gene were constructed by inserting the dhfr gene-containing, Klenow-treated ∼1.9 kb Bam HI restriction fragment of plasmid pBW32 into the ∼5.82 kb Nde I- Stu I restriction fragment of plasmid pLPC. The resulting plasmids, designated as pLPCdhfr1 and pLPCdhfr2, differ only with respect to the orientation of the dhfr gene. A poly-GT element is inserted in the Xho I site of plasmids pLPCdhfr1 and pLPCdhfr2, to create plasmids pLPCdhfr1(GT) and pLPCdhfr2(GT), respectively. The construction of these plasmids is described in Example 12. Plasmid pLPChyg1 was further modified to introduce a dihydrofolate reductase (dhfr) gene. The dhfr gene is a selectable marker in dhfr-negative cells and can be used to increase the copy number of a DNA segment by exposing the host cell to increasing levels of methotrexate. The dhfr gene can be obtained, for example, from plasmid pBW32, a plasmid disclosed in Japanese Patent Application, Serial No. 201225/86, published as Publication No. 48378/87 on March 3, 1987, incorporated herein by reference. Other sources of this gene will be apparent to one skilled in the art (e.g. ATCC 37146, plasmid pSV2-dhfr). A restriction site and function map of plasmid pBW32 is presented in Figure 12 of the accompanying drawings. The construction protocol for plasmid pBW32 is described in Example 11. The dhfr gene-containing, ∼1.9 kb Bam HI restriction fragment of plasmid pBW32 was isolated, treated with Klenow enzyme, and inserted into partially- Eco RI-digested plasmid pLPChyg1 to yield plasmids pLPChd1 and pLPChd2. Plasmid pLPChyg1 contains two Eco RI restriction enzyme recognition sites, one in the hygromycin resistance-conferring gene and one in the plasmid pBR322-derived sequences. The fragment comprising the dhfr gene was inserted into the Eco RI site located in the pBR322-derived sequences of plasmid pLPChyg1 to yield plasmids pLPChd1 and pLPChd2. The construction of plasmids pLPChd1 and pLPChd2, which differ only with respect to the orientation of the dhfr gene-containing DNA segment, is described in Example 12A. A poly-GT element is inserted in the unique Xho I site of plasmids pLPChd1 and pLPChd2 to create plasmids pLPChd1(GT) and pLPChd2(GT), respectively, as described in Example 12B. A restriction site and function map of plasmid pLPChd1(GT) is presented in Figure 13 of the accompanying drawings. Plasmid pLPChd1 was modified to form plasmid phd, a plasmid that contains both the BK enhancer - adenovirus late promoter cassette and also the hygromycin resistance-conferring and dhfr genes. To construct plasmid phd, plasmid pLPChd1 was prepared from dam ⁻ E . coli host cells, digested with restriction enzyme Bcl I, and recircularized, thus deleting the human protein C-encoding DNA. Plasmid phd contains a single Bcl I restriction enzyme recognition site, which is conveniently positioned for the insertion of any sequence desired to be expressed from the BK enhancer-adenovirus late promoter as described herein. The construction protocol for plasmid phd is described in Example 13A. Plasmid phd has been transformed into E. coli GM48 ( dam ⁻, dcm ⁻) and has been deposited (July 26, 1989) in the Northern Regional Research Laboratory (NRRL) depository, 1815 N. University Street, Peoria, IL 61604. This transformed host cell is available under the accession number NRRL B-18525. The plasmid can be isolated from this host be conventional means known to those skilled in the art. A poly-GT element is inserted into the Xho I site of plasmid phd to create plasmid phd(GT), as further described in Example 13B. The single Bcl I restriction enzyme recognition site of plasmid phd(GT) is conveniently positioned for the insertion of any sequence desired to be expressed from the BK enhancer-poly-GT-adenovirus late promoter of the present invention. A restriction site and function map of plasmid phd(GT) is presented in Figure 14 of the accompanying drawings. Other expression vectors that drive expression of human protein C are plasmids pLPCE1A and pLPCE1A(GT). Plasmids pLPCE1A and pLPCE1A(GT) contain the E1A gene of human adenovirus type 2, the gene product of which, as described above, increases the activity of the BK enhancer, and, as described in the present invention, activates the enhancer activity of the poly-GT element. Thus, transcription from a promoter in tandem with the BK enhancer and the poly-GT element increases in the presence of the E1A gene product. Plasmid pLPCE1A was constructed by ligating the E1A gene-containing, ∼1.8 kb Ba1 I restriction fragment of human adenovirus-type-2 DNA with the ∼5.82 kb Nde I- Stu I restriction fragment of plasmid pLPC. The construction protocol for plasmid pLPCE1A is described in Example 14A. A poly-GT element is inserted into the unique Xho I site of plasmid pLPCE1A to created plasmid pLPCE1A(GT), as further described in Example 14B. A restriction site and function map of plasmid pLPCE1A(GT) is presented in Figure 15 of the accompanying drawings. A variety of expression vectors of the present invention utilize the poly-GT - BK enhancer - adenovirus late promoter cassette or the BK enhancer - adenovirus late promoter cassette to drive expression of tissue plasminogen activator (TPA) or modified TPA (MTPA). To construct vectors with TPA or MTPA, plasmid pBW32 (Figure 12) was digested with restriction enzyme Bam HI, and the resultant ∼5.6 kb fragment was recircularized to yield plasmid pBW32del. Plasmid pBW32del, which encodes modified TPA and contains only one Hin dIII restriction site, was digested with Hin dIII and then ligated with the ∼0.65 kb Hin dIII restriction fragment of plasmid pBal8cat to yield plasmid pBLT. Plasmid pBal8cat comprises an improved BK enhancer-adenovirus late promoter cassette and is described in Example 18. A poly-GT element is inserted into the unique Bam HI site of plasmid pBLT to create plasmids pBLT(GT). A restriction site and function map of plasmid pBLT(GT) is presented in Figure 16 of the accompanying drawings, and the construction protocol for plasmids pBLT and pBLT(GT) is described in Example 15. Selectable markers may be introduced into Bam HI-digested plasmid pBLT or pBLT(GT). For example, with plasmid pBLT, in one construction, the hygromycin resistance gene-containing, ∼2.5 kb Bam HI restriction fragment of plasmid pSV2hyg was inserted to yield plasmids pBLThyg1 and pBLThyg2, and in another construction, the dhfr gene-containing ∼1.9 kb Bam HI restriction fragment of plasmid pBW32 was inserted to yield plasmids pBLTdhfr1 and pBLTdhfr2. The four plasmids, pBLThyg1, pBLThyg2, pBLTdhfr1, and pBLTdhfr2, differ only with respect to the type and/or orientation of the selectable marker. The construction protocol of plasmids pBLThyg1, pBLThyg2, pBLTdhfr1, and pBLTdhfr2 and their poly-GT containing derivatives, pBLThyg1(GT), pBLThyg2(GT), pBLTdhfr1(GT), and pBLTdhfr2(GT), is described in Example 16. A restriction site and function map of each of plasmids pBLThyg1(GT) and pBLTdhfr1(GT) is presented in Figures 17 and 18, respectively, of the accompanying drawings. Other expression vectors described herein that drive expression of TPA or modified TPA were derived from plasmid pTPA103, an intermediate used in the construction of plasmid pBW32. The construction protocol for plasmid pTPA103 is described in Example 11, and a restriction site and function map of plasmid pTPA103 is presented in Figure 19 of the accompanying drawings. To construct these derivatives, a Bam HI restriction site was introduced immediately before the 5' end of the TPA coding region of plasmid pTPA103. Plasmid pTPA103 was digested with restriction enzyme Hga I to isolate the ∼0.52 kb Hga I restriction fragment that comprises the 5' end of the TPA coding region. After Klenow treatment, the Hga I was ligated to Bam HI linkers, digested with restriction enzyme Bam HI, and inserted into Bam HI-digested plasmid pBR322 to form plasmids pTPA601 and pTPA602. Next, plasmid pTPA602 was digested with restriction enzymes Bgl II and Sal I, and the resultant ∼4.2 kb Bgl II- Sal I restriction fragment was ligated to the ∼2.05 kb Sal I- Bgl II restriction fragment of plasmid pTPA103 to form plasmid pTPA603. Plasmid pTPA603 thus contains the complete coding sequence for TPA bounded by a Bam HI restriction site on both ends. A restriction site and function map of plasmid pTPA603 is presented in Figure 19 of the accompanying drawings. To construct a plasmid that is analogous to plasmid pTPA603 but that encodes a modified form of TPA, plasmid pTPA603 was digested with restriction enzymes Bgl II and Sst I, and the resultant ∼5.02 kb Bgl II- Sst I fragment was ligated to the ∼0.69 kb Bgl II- Sst I restriction fragment of plasmid pBLT. The resultant plasmid, designated as pMTPA603, was then digested with restriction enzyme Bam HI, and the resultant ∼1.35 kb fragment was isolated. This fragment and the ∼1.90 kb Bam HI restriction fragment of plasmid pTPA603 were individually ligated in separate ligations to Bcl I-digested plasmid phd (Figure 14) to form the respective plasmids phdMTPA and phdTPA. The construction of plasmids phdTPA and phdMTPA, and their poly-GT containing derivatives, phdTPA(GT) and phdMTPA(GT), beginning with the construction protocol for plasmid pTPA602, is described in Example 17. Restriction site and function maps of plasmids phd(GT)TPA and phd(GT)MTPA are presented in Figures 20 and 21, respectively, of the accompanying drawings. The present invention comprises a method for using a poly-GT element with a BK enhancer in tandem with a eukaryotic promoter to drive transcription and expression of DNA sequences in eukaryotic host cells that express an immediate-early gene of a large DNA virus. Skilled artisans will recognize that virtually any eukaryotic promoter can be used in tandem with the poly-GT element and the BK enhancer in the present method. For example, the SV40 early and late promoters, BK early and late promoters, early and late promoters of any of the polyoma viruses or papovaviruses, promoters of the herpes virus family, interferon α1 promoter, mouse metallothionein promoter, promoters of the retroviruses, β-globin promoter, promoters of the adenoviruses, promoters of poxviruses, sea urchin H2A promoter, conalbumin promoter, ovalbumin promoter, mouse β-globin promoter, and human β-globin promoter, can all serve as the eukaryotic promoter in the method of the present invention. Moreover, any sequence that can be used to initiate transcription, for example, a sequence containing a transcription start site, composed of a "TATA"-like sequence with or without an upstream "CCAAT" sequence, can serve as the promoter in the present invention. Such promoters can be utilized in the present method by conventionally inserting the promoters into expression vectors comprising a poly-GT element with the BK enhancer as exemplified herein using the adenovirus-2 late promoter, which is the preferred eukaryotic promoter for use in the present method. The BK enhancer used in the vectors described herein was isolated from the prototype strain of BK virus (ATCC VR-837). However, a number of BK virus variants have been isolated and described. Gardner et al ., 1971, Lancet 1:1253, (see also Gardner, 1973, Brit. Med. J. 1:77-78) described the first isolation of a BK virus, and the Gardner strain is thus referred to as the prototype or wild-type BK virus. The Gardner strain of BK virus (Figure 1) is available from the ATCC under the accession number ATCC VR-837. In fact, when ATCC VR-837 was obtained for use in constructing the vectors described herein, it was observed that BK variants were present in the population of viruses. Others have observed this phenomenon, i.e., Chuke et al ., 1986, J. Virology 60(3):960. Neither the method of using a poly-GT element along with the BK enhancer in tandem with a eukaryotic promoter to drive expression of useful substances, such as nucleic acid and protein, in the presence of an immediate-early gene product of a large DNA virus nor any other method of the present invention is limited to the Gardner strain or particular BK variant as a source of the BK enhancer, although a variant enhancer isolated from the prototype strain (Example 18) is preferred. The following Table lists a representative number of BK variants that can be used in the methods described herein. In addition, a BK-like virus (simian agent 12) contains enhancer elements homologous to the BK enhancer and can be used in the methods as described herein. The enhancer elements of simian agent 12 are described in Cunningham et al ., 1985, J. Virol. 54:483-492 and, for purposes of the present invention, are BK enhancer variants. Skilled artisans will understand that a variety of eukaryotic host cells can be used in present method, so long as the host cell expresses an immediate-early gene product of a large DNA virus. Because the immediate-early gene product can be introduced into host cells by many means, such a transformation with a plasmid or other vector, virtually any eukaryotic cell can be used in the present method. In particular, those skilled in the art will recognize that the E1A gene product which activates the GT enhancer sytem of the present invention may be provided in a variety of ways including: (1) introducing a single plasmid into the eukaryotic host cell which encodes the E1A gene along with the poly-GT element, the eukaryotic promoter and a structural gene to be expressed, for example, plasmid pLPCE1A(GT); (2) introducing two plasmids by co-transformation of the eukaryotic host cell, where one plasmid encodes E1A, for example, plasmid pSV2E1A, and one plasmid encodes the poly-GT element, the eukaryotic promoter and a structural gene to be expressed, for example, plasmid pBL(GT)cat; or (3) using an E1A expressing cell line such as 293 or AV12, and introducting a single plasmid encoding the poly-GT element, the eukaryotic promoter and the structural gene to be expressed. Human cells are preferred host cells in the method of the present invention. While human kidney cells are especially preferred as host cells, the adenovirus 5-transformed human embryonic kidney cell line 293, which expresses the E1A gene product, is most preferred and is available from the ATCC under the accession number ATCC CRL 15753. The 293 cell line is preferred not only because 293 cells express the E1A gene product but also because of the ability of the 293 cells to γ-carboxylate and otherwise properly process complex gene products such as protein C. "γ-carboxylation" refers to a reaction in which a carboxyl group is added to a glutamic acid residue at the γ-carbon, and a γ-carboxylated protein is a protein in which some amino acid residues have undergone γ-carboxylation. Kidney cells normally γ-carboxylate and otherwise process certain proteins, but 293 cells are transformed with adenovirus, which generally results in a loss of specialized functions. Consequently, the present invention also comprises an improvement in the method for producing a protein that is naturally gamma carboxylated, properly folded, and processed wherein said protein is encoded in a recombinant DNA vector containing a poly-GT element and BK enhancer with the adenovirus late promoter. The 293N3S derivative of the 293 cell line is also suitable for use in the present invention and is able to grow in suspension culture as described in Graham, 1987, J. Gen. Virol. 68:937. This method of producing a γ-carboxylated protein is not limited to adenovirus-transformed human embryonic kidney cells. Instead, the method of producing a γ-carboxylated protein is broadly applicable to all adenovirus-transformed host cells. Those skilled in the art also will recognize that the method can be practiced by first transforming a eukaryotic cell with an expression vector for a γ-carboxylated protein and then transforming the resulting transformant with adenovirus. Harold Ginsberg, in The Adenoviruses (1984, Plenum Press, New York), describes a number of adenoviruses and methods of obtaining adenovirus-transformed host cells. One especially preferred adenovirus-transformed host cell for purposes of expressing a γ-carboxylated protein encoded on a recombinant DNA expression vector is the Syrian hamster cell line AV12-664 (hereinafter AV12). The AV12 cell line was constructed by injecting adenovirus type 12 into the scruff of the neck of the Syrian hamster and isolating cells from the resulting tumor. The AV12 cell line is a preferred host for purposes of producing a γ-carboxylated protein. Examples of γ-carboxylated proteins include, but are not limited to, Factor VII, Factor IX, Factor V, protein C, protein S, protein Z, and prothrombin. The novel poly-GT - BK enhancer - eukaryotic promoter constructions described in Examples 18 and 19 were constructed using a method for improving expression using a poly-GT element and the BK enhancer with a eukaryotic promoter. Such method comprises placing a poly-GT element 5' or 3' relative to the eukaryotic promoter in the presence of the E1A gene product, and placing the BK enhancer within 0 to 300 nucleotides upstream of the 5' end of the CCAAT region or CCAAT region equivalent of the eukaryotic promoter used in tandem with the BK enhancer. Other viral gene products, such as the VA gene product of adenovirus, increase the translation efficiency of mRNA molecules that contain the tripartite leader of adenovirus (Kaufman, 1985, Proc. Natl. Acad. Sci. U.S.A., 82:689-693; Svensson and Akusjarul, 1985, EMBO, 4:957-964). The vectors of the present invention can be readily modified to encode the entire tripartite leader of adenovirus; the VA gene product may be used to increase translation of a given mRNA that only contains the first part of the adenovirus tripartite leader. The sequence of the tripartite leader of adenovirus is depicted below: wherein A is riboadenyl, G is riboguanyl, C is ribocytidyl, and U is uridyl. As encoded in adenovirus DNA, the tripartite leader is interrupted by large introns. The presence of these introns or portions of the introns does not adversely affect expression levels. Plasmids p4-14 and p2-5 described herein contain the tripartite leader of adenovirus and are described more fully in Example 24, below. Plasmids p4-14(GT) and p2-5(GT) further contain a poly-GT element. Many of the illustrative vectors of the invention, such as plasmids pBL(GT)cat and pLPC(GT), contain only the first part of the tripartite leader of adenovirus. As used herein, the "first part" of the tripartite leader of adenovirus, when transcribed into mRNA, comprises at least the sequence: 5'-ACUCUCUUCCGCAUCGCUGUCUGCGAGGGCCAG-3' Thus, the present invention comprises an improvement in the method for producing a useful substance in a eukaryotic host cell that is transformed with a recombinant DNA vector that contains both a eukaryotic promoter and a DNA sequence that encodes the useful substance, the DNA sequence being positioned for expression from the promoter, and wherein the cell containing the recombinant DNA vector is cultured under conditions suitable for expression of the useful substance, wherein the improvement comprises: (a) incorporating a poly-GT element into the vector; (b) incorporating DNA that encodes the first part of the tripartite leader of an adenovirus into the vector of step (a) such that, upon transcription, the mRNA produced encodes the useful product and, at the 5' end, contains said first part of the tripartite leader; (c) providing the cell containing the vector of step (b) with a first DNA sequence that codes for the expression of a VA gene product and a second DNA sequence that codes for the expression of an E1A gene product of the adenovirus; and (d) culturing the cell of step c) under conditions suitable for (i) expressing the VA gene product and for stimulating translation of the mRNA and (ii) expressing the E1A gene product add for stimulating the enhancer activity of the poly-GT element,subject to the limitation that the mRNA does not contain the entire tripartite leader of the adenovirus. Plasmids coding for VA have been constructed from adenovirus DNA. A restriction fragment of ∼1723 bp, defined by a Sal I site (at nucleotide 9833) and a Hin dIII site (at nucleotide 11556), was isolated from adenovirus-2 DNA and cloned into Hin dIII- Sal I-digested plasmid pBR322, thus replacing the ∼622 bp Sal I- Hin dIII fragment of pBR322, to construct plasmid pVA. A plasmid coding for neomycin resistance and VA has been constructed by isolating a ∼1826 bp Nru I fragment from plasmid pVA and inserting that fragment into Klenow-treated, Bam HI-digested plasmid pSVNneo (available from BRL). The resultant plasmid, designated pVA-Neo, can be used to insert the VA gene into any cell line by selection of neomycin (G418) resistance after transformation. The VA gene product of adenovirus, however, may exert its greatest positive effect on expression of recombinant genes containing either the first part of the tripartite leader of adenovirus, or the entire tripartite leader, in the first few days following transformation of the host cell with a VA-encoding vector. Subsequent expression of the VA gene product in the host cell after the first few days may not give optimal expression levels. The T antigen of SV40, BK virus, or any other polyomavirus can also be used with the vectors described herein to increase promoter activity and/or increase copy number of the plasmid by stimulating replication. SV40 T antigen stimulates transcription from both the adenovirus and BK late promoters. SV40 T antigen does not appear to trans-activate the poly-GT element. However, by including T-antigen-coding sequences on the expression vectors of the present invention or by cotransfection of the vectors with a plasmid(s) carrying T-antigen-coding sequences, amplification of copy number can be obtained prior to the application of selective pressure as outlined in Example 22. This will allow for high copy number integration of the expression vector. Thus, in the preferred embodiment of the present invention, the recombinant DNA expression vector comprises a poly-GT element, the BK enhancer isolated from the prototype strain positioned less than 300 nucleotides upstream of the adenovirus late promoter, which itself is positioned to drive expression of a gene that encodes at least the first part of the tripartite leader and a useful substance. This preferred vector is used to transform human embryonic kidney 293 cells which constitutively express the E1A gene product and that have been modified, either before or after transformation with the expression vector, to express the VA gene product of an adenovirus. For stable transformants, however, presence of the VA gene product may not be desired. The following Examples more fully describe the methods, compounds, and recombinant organisms of the present invention. Those skilled in the art will recognize that the particular reagents, equipment, and procedures described in the Examples are merely illustrative and do not limit the present invention. Example 1 Preparation of Poly-GT Element The single-stranded DNA fragments used in the construction of a poly-GT element were chemically synthesized either by using a Systec 1450A DNA Synthesizer (Systec, Inc., 3816 Chandler Drive, Minneapolis, MN 55401) or an ABS 380A DNA Synthesizer (Applied Biosystems, Inc., 850 Lincoln Centre Drive, Foster City, CA 94404). Many DNA synthesizing instruments are known in the art and can be used to make the fragments. In addition, the fragments can also be conventionally prepared in substantial accordance with the procedures of Itakura et al ., 1977, Science, 198:1056 and Crea et al , 1978, Proc. Nat. Acad. Sci. USA, 75 :5765. Five hundred nanograms of each single strand of the poly-GT element were annealed after synthesis of the single strands as follows. Sixteen µl of strand 1 and 11 µl of strand 2 were mixed together in an eppendorf tube, and 3 µl of 10X TM buffer (0.5 M Tris-HCl pH = 7.6, 0.1 M MgCl₂) were added. The tube was placed in a boiling water bath for 2 minutes and then allowed to cool slowly by incubation at room temperature and then at 4°C. This process annealed the single strands to form the double-stranded poly-GT element. After this incubation, the 30 µl sample was lyophilized and then suspended in 7 µl of glass-distilled H₂O (dH₂O). The poly-GT element constructed had the following structure: As shown, this poly-GT element was constructed to include a (GT) n -(CA) n segment where n = 21, where this segment is flanked by DNA sequences that are recognition sites for the following restriction enzymes: Bam HI (G↓GATCC), Xho I (C↓TCGAG) and Eco RI (G↓AATTC). This poly-GT element can therefore be easily inserted into a Bam HI site or an Xho I site in a variety of expression vectors to create a poly-GT containing derivative of the expression vector as described in a number of following Examples. Alternatively, the poly-GT element can be inserted by blunt end ligation into any restriction site of an expression vector. Example 2 Preparation of BK Virus DNA BK virus is obtained from the American Type Culture Collection under the accession number ATCC VR-837. The virus is delivered in freeze-dried form and resuspended in Hank's balanced salts (Gibco, 3175 Staley Road, Grand Island, NY 14072) to a titer of about 10⁵ plaque-forming units ("pfu")/ml. The host of choice for the preparation of BK virus DNA is primary human embryonic kidney (PHEK) cells, which can be obtained from Flow Laboratories, Inc., 7655 Old Springhouse Road, McLean, VA 22101, under catalogue number 0-100 or from M.A. Bioproducts under catalogue number 70-151. About five 75 mm² polystyrene flasks comprising confluent monolayers of about 10⁶ PHEK cells are used to prepare the virus. About 1 ml of BK virus at a titer of 10⁵ pfu/ml is added to each flask, which is then incubated at 37°C for one hour, and then, fresh culture medium (Dulbecco's Modified Eagle's Medium, Gibco, supplemented with 10% fetal bovine serum) is added, and the infected cells are incubated at 37°C for 10-14 days or until the full cytopathogenic effect of the virus is noted. This cytopathogenic effect varies from cell line to cell line and from virus to virus but usually consists of cells rounding up, clumping, and sloughing off the culture disk. The virus is released from the cells by three freeze-thaw cycles, and the cellular debris is removed by centrifugation at 5000Xg. The virus in 1 liter of supernatant fluid is precipitated and collected by the addition of 100 g of PEG-6000, incubation of the solution for 24 hours at 4°C, and centrifugation at 5000Xg for 20 minutes. The pellet is dissolved in 0.1X SSC buffer (1XSSC = 0.15 M NaCl and 0.015 M sodium citrate, pH = 7) at 1/100th of the original volume. The virus suspension is layered onto a 15 ml solution of saturated KBr in a tube, which is centrifuged at 75,000Xg for 3 hours. Two bands are evident in the KBr solution after centrifugation. The lower band, which contains the complete virion, is collected and desalted on a Sephadex® G-50 column (Sigma Chemical Co., P.O. Box 14508, St. Louis, MO 63178) using TE (10mM Tris-HCl, pH = 7.8, and 1 mM EDTA) as an elution buffer. Sodium dodecyl sulfate (SDS) is added to the solution of purified virions obtained from the column to a concentration of 1%; pronase is added to a concentration of 100 µg/ml, and the solution is incubated at 37°C for 2 hours. Cesium chloride is then added to the solution to a density of 1.56 g/ml, and ethidium bromide is added to the solution to a final concentration of 100 µg/ml. The solution is centrifuged in a Sorvall (DuPont Inst. Products, Biomedical Division, Newton, CT 06470) 865 rotor or similar vertical rotor at 260,000Xg for 24 hours. After centrifugation, the band of virus DNA is isolated and extracted five times with isoamyl alcohol saturated with 100 mM Tris-HCl, pH = 7.8. The solution of BK virus DNA is then dialyzed against TE buffer until the 260 nm/280 nm absorbance ratio of the DNA is between 1.75 and 1.90. The DNA is precipitated by adjusting the NaCl concentration to 0.15 M, adding two volumes of ethanol, incubating the solution at -70°C for at least 2 hours, and centrifuging the solution at 12,000Xg for 10 minutes. The resulting pellet of BK virus DNA is suspended in TE buffer at a concentration of 1 mg/ml. Example 3 Construction of Plasmids pBKneo1 and pBKneo2 E . coli K12 HB101/pdBPV-MMTneo cells are obtained in lyophilized form from the American Type Culture Collection under the accession number ATCC 37224. The lyophilized cells are plated on L-agar (L broth with 15 g agar per liter) plates containing 100 µg/ml ampicillin and incubated at 37°C to obtain single colony isolates. One liter of L broth (10 g tryptone, 10 g NaCl, and 5 g yeast extract per liter) containing 50 µg/ml ampicillin was inoculated with a colony of E . coli K12 HB101/pdBPV-MMTneo and incubated in an airshaker at 37°C until the O.D.₅₉₀ was ∼1 absorbance unit, at which time 150 mg of chloramphenicol were added to the culture. The incubation was continued for about 16 hours; the chloramphenicol addition inhibits protein synthesis, and thus inhibits further cell division, but allows plasmid replication to continue. The culture was centrifuged in a GSA rotor (Sorvall) at 6000 rpm for 5 minutes at 4°C. The resulting supernatant was discarded, and the cell pellet was washed in 40 ml of TES buffer (10 mM Tris-HCl, pH=7.5; 10 mM NaCl; and 1 mM EDTA) and then repelleted. The supernatant was discarded, and the cell pellet was frozen in a dry ice-ethanol bath and then thawed. The thawed cell pellet was resuspended in 10 ml of a solution of 25% sucrose and 50 mM EDTA. About 1 ml of a 5 mg/ml lysozyme solution, 3 ml of 0.25 M EDTA, pH=8.0, and 100 µl of 10 mg/ml RNAse A were added to the solution, which was then incubated on ice for 15 minutes. Three ml of lysing solution (prepared by mixing 3 ml of 10% Triton-X 100, 75 ml of 0.25 M EDTA, pH=8.0, 15 ml of 1 M Tris-HCl, pH=8.0, and 7 ml of dH₂O) were added to the lysozyme-treated cells, mixed, and the resulting solution incubated on ice for another 15 minutes. The lysed cells were frozen in a dry ice-ethanol bath and then thawed. The cellular debris was removed from the solution by centrifugation at 25,000 rpm for 40 minutes in SW28.1 rotor (Beckman, Scientific Instrument Division, Campus Drive at Jamboree Blvd., Irvine, CA 92713) and by extraction with buffered phenol. About 30.44 g of CsCl and ∼1 ml of a 5 mg/ml ethidium bromide solution were added to the cell extract, and then, the volume of the solution was adjusted to 40 ml with TES buffer. The solution was decanted into a VTi50 ultracentrifuge tube (Beckman), which was then sealed and centrifuged in a VTi50 rotor at 42,000 rpm for ∼16 hours. The resulting plasmid band, visualized with ultraviolet light, was isolated and then placed in a Ti75 tube and rotor (Beckman) and centrifuged at 50,000 rpm for 16 hours. Any necessary volume adjustments were made using TES containing 0.761 g/ml CsCl. The plasmid band was again isolated, extracted with salt-saturated isopropanol to remove the ethidium bromide, and diluted 1:3 with TES buffer. Two volumes of ethanol and 1 volume of 3 M sodium acetate were then added to the solution, which was then incubated overnight at -20°C. The plasmid DNA was pelleted by centrifuging the solution in an SS34 rotor (Sorvall) for 15 minutes at 10,000 rpm. The ∼1 mg of plasmid pdBPV-MMTneo DNA obtained by this procedure was suspended in 1 ml of TE buffer and stored at -20°C. The foregoing plasmid isolation procedure is generally used when large amounts of very pure plasmid DNA are desired. The procedure can be modified to rapidly obtain a smaller, less pure amount of DNA, such as is needed when screening transformants for the presence of a given plasmid, by using only about 5 ml of cultured cells, lysing the cells in an appropriately scaled-down amount of lysis buffer, and replacing the centrifugation steps with phenol and chloroform extractions. About 5 µg (5 µl) of the plasmid pdBPV-MMTneo DNA prepared above and 5 µg (5 µl) of the BK virus DNA prepared in Example 2 were each digested at 37°C for 2 hours in a solution containing 2 µl of 10X Bam HI buffer (1.5 M NaCl; 60 M Tris-HCl, pH=7.9; 60 mM MgCl₂; and 1 mg/ml BSA), 1 µl (about 20 units; all enzyme units referred to herein, unless otherwise indicated, refer to the unit definitions of New England Biolabs, 32 Tozer Road, Beverly, MA 01915, although the actual source of enzymes may have been different) of restriction enzyme Bam HI, and 7 µl of dH₂O. The reaction was stopped by an extraction with an equal volume of phenol, followed by two extractions with chloroform. Each Bam HI-digested DNA was then precipitated, collected by centrifugation, and resuspended in 5 µl of dH₂O. About 1 µl of 10X ligase buffer was added to a mixture of Bam HI-digested plasmid pdBPV-MMTneo (1 µl) and Bam HI-digested BK virus DNA (1 µl). After 1 µl (∼1000 units) of T4 DNA ligase and 6 µl of dH₂O were added to the mixture of DNA, the resulting reaction was incubated at 16°C overnight. The ligated DNA constituted the desired plasmids pBKneo1 and pBKneo2, which differ only with respect to the orientation of the BK virus DNA. A restriction site and function map of plasmid pBKneo1 is presented in Figure 2 of the accompanying drawings. E . coli K12 HB101 cells are available in lyophilized form from the Northern Regional Research Laboratory under the accession number NRRL B-15626 (deposited Sept. 28, 1983). E . coli K12 HB101 cells were cultured, made competent for transformation, and transformed with the ligated DNA prepared above as follows. A 50 ml culture of E . coli K12 HB101 in L broth was grown to an optical density at 650 nanometers (O.D.₆₅₀) of approximately 0.4 absorbance units. The culture was chilled on ice for ten minutes, and the cells were collected by centrifugation. The cell pellet was resuspended in 25 ml of cold 100 mM MgCl₂ and incubated on ice for 25 minutes. The cells were once again pelleted by centrifugation, and the pellet was resuspended in 2.5 ml of cold 100 mM CaCl₂ and incubated for 30 minutes on ice. After the incubation, the cells are competent for the uptake of transforming DNA. Two hundred µl of this cell suspension were mixed with the ligated DNA prepared above and incubated on ice for 30 minutes. At the end of this period, the cells were placed in a water bath at 42°C for 2 minutes and then returned to the ice for an additional 10 minutes. The cells were collected by centrifugation and resuspended in one ml of L broth and incubated at 37°C for 1 hour. Aliquots of the cell mixture were plated on L-agar plates containing 100 µg/ml ampicillin. The plates were incubated at 37°C overnight. E . coli K12 HB101/pBKneo1 and E . coli K12/pBKneo2 transformants were identified by their ampicillin-resistant phenotype and by restriction enzyme analysis of their plasmid DNA. Plasmid DNA was obtained from the E . coli K12 HB101/pBKneo1 and E . coli K12/pBKneo2 cells as described above. Example 4 Construction of Plasmids pLPcat and pLP(GT)cat A. Construction of Plasmid pLPcat The virion DNA of adenovirus 2 (Ad2) is a double-stranded linear molecule about 35.94 kb in size. The Ad2 late promoter can be isolated on an ∼0.316 kb Acc I- Pvu II restriction fragment of the Ad2 genome; this ∼0.32 kb restriction fragment corresponds to the sequence between nucleotide positions 5755 and 6071 of the Ad2 genome. To isolate the desired ∼0.32 kb Acc I- Pvu II restriction fragment, Ad2 DNA is first digested with restriction enzyme Bal I, and the ∼2.4 kb Bal I restriction fragment that comprises the entire sequence of the ∼0.32 kb Acc I- Pvu II restriction fragment is isolated. Then, the ∼2.4 kb Bal I restriction fragment is digested with Acc I and Pvu II to obtain the desired fragment. About 50 µg of Ad2 DNA (available from BRL) are dissolved in 80 µl of dH₂O and 10 µl of 10X Bal I buffer (100 mM Tris-HCl, pH = 7.6; 120 mM MgCl₂; 100 mM DTT; and 1 mg/ml BSA). Ten µl (about 20 units) of restriction enzyme Bal I are added to the solution of Ad2 DNA, and the resulting reaction is incubated at 37°C for 4 hours. The Bal I-digested DNA is loaded onto a 1% agarose gel and electrophoresed until the restriction fragments are well separated. Visualization of the electrophoresed DNA is accomplished by staining the gel in a dilute solution (0.5 µg/ml) of ethidium bromide and exposing the stained gel to long-wave ultraviolet (UV) light. The DNA is isolated from the agarose gel as follows. A narrow section of the gel containing the desired fragment (visualized with UV light) is cut out from the gel and placed in an eppendorf tube. The tube containing the gel section is first placed at -70°C or on dry ice to freeze the gel, then placed at room temperature to thaw, then frozen and thawed again. After these two freeze-thaw cycles, liquid in the tube is collected and the DNA fragment is removed from the liquid by 3 successive extractions with isoamyl alcohol, followed by ethanol precipitation. The purified fragment obtained is dissolved in 10 µl of TE buffer. About 6 µl of dH₂O and 2 µl of 10X Acc I buffer (60 mM NaCl; 60 mM Tris-HCl, pH = 7.5; 60 mM MgCl₂; 60 mM DTT; and 1 mg/ml BSA) are added to the solution of the ∼2.4 kb Bal I restriction fragment of Ad2. After the addition of 2 µl (about 10 units) of restriction enzyme Acc I to the solution of DNA, the reaction is incubated at 37°C for 2 hours. After the Acc I digestion, the DNA is collected by ethanol precipitation and resuspended in 16 µl of dH₂O and 2 µl of 10X Pvu II buffer (600 mM NaCl; 60 mM Tris-HCl, pH = 7.5; 60 mM MgCl₂; 60 mM DTT; and 1 mg/ml BSA). After the addition of about 2 µl (about 10 units) of restriction enzyme Pvu II to the solution of DNA, the reaction is incubated at 37°C for 2 hours. The Acc I- Pvu II-digested, ∼2.4 kb Bal I restriction fragment of Ad2 is loaded onto an ∼6% polyacrylamide gel and electrophoresed until the ∼0.32 kb Acc - Pvu II restriction fragment that comprises the Ad2 late promoter is separated from the other digestion products. The gel is stained with ethidium bromide and viewed using UV light, and the segment of gel containing the ∼0.32 kb Acc I- Pvu II restriction fragment is cut from the gel, crushed, and soaked overnight at room temperature in 250 µl of extraction buffer (500 mM ammonium acetate; 10 mM magnesium acetate; 1 mM EDTA; and 0.1% SDS). The following morning, the mixture is centrifuged, and the pellet is discarded. The DNA in the supernatant is precipitated with ethanol; about 2 µl of tRNA are added to ensure complete precipitation of the desired fragment. About 0.2 µl of the ∼0.32 kb Acc I- Pvu II restriction fragment are obtained and suspended in 7 µl of dH₂O. About 0.25 µg (in 0.5 µl) of Bcl I linkers (5'-CTGATCAG-3', available from New England Biolabs), which had been kinased in substantial accordance with the procedure described in Example 11A, below, was added to the solution of the ∼0.32 kb Acc I- Pvu II restriction fragment, and then, 1 µl (about 1000 units) of T4 DNA ligase and 1 µl of 10X ligase buffer (0.5M Tris-HCl, pH = 7.8; 100 mM MgCl₂; 200 mM DTT; 10 mM ATP; and 0.5 mg/ml BSA) were added to the solution of DNA, and the resulting reaction was incubated at 16°C overnight. The Bcl I linkers could ligate only to the Pvu II end of the Acc I- Pvu II restriction fragment. DNA sequencing later revealed that four Bcl I linkers attached to the Pvu II end of the Acc I- Pvu II restriction fragment. These extra Bcl I linkers can be removed by Bcl I digestion and religation; however, the extra Bcl I linkers were not removed as the linkers do not interfere with the proper functioning of the vectors that comprise the extra linkers. E . coli K12 HB101/pSV2cat cells are obtained in lyophilized form from the ATCC under the accession number ATCC 37155, and plasmid pSV2cat DNA was isolated from the cells in substantial accordance with the procedure of Example 3. A restriction site and function map of plasmid pSV2cat is presented in Figure 3 of the accompanying drawings. About 1 mg of plasmid pSV2cat DNA is obtained and dissolved in 1 ml of TE buffer. About 3 µg (3 µl) of the plasmid pSV2cat DNA were added to 2 µl of 10X Acc I buffer and 16 µl of dH₂O, and then, 3 µl (about 9 units) of restriction enzyme Acc I were added to the solution of pSV2cat DNA, and the resulting reaction was incubated at 37°C for 2 hours. The Acc I-digested plasmid pSV2cat DNA was then digested with restriction enzyme Stu I by adding 3 µl of 10X Stu I buffer (1.0M NaCl; 100 mM Tris-HCl, pH = 8.0; 100 mM MgCl₂; 60 mM DTT; and 1 mg/ml BSA), 5 µl of dH₂O, and about 2 µl (about 10 units) of restriction enzyme Stu I. The resulting reaction was incubated at 37°C for 2 hours. The reaction was terminated by extracting the reaction mixture once with phenol, then twice with chloroform. About 0.5 µl of the desired fragment was obtained and dissolved in 20 µl of TE buffer. About 4 µl of the Acc I- Stu I-digested plasmid pSV2cat DNA were mixed with about 7 µl of the ∼0.32 kb Acc I- Pvu II (with Bcl I linkers attached) restriction fragment of Ad2, and after the addition of 3 µl of 10X ligase buffer, 15 µl of dH₂O, and 2 µl (about 1000 units) of T4 DNA ligase, the ligation reaction was incubated at 16°C overnight. The ligated DNA constituted the desired plasmid pLPcat, a plasmid that comprises the Ad2 late promoter positioned so as to drive transcription, and thus expression, of the chloramphenicol acetyltransferase gene. The ligated DNA was used to transform E . coli K12 HB101 cells in substantial accordance with the procedure of Example 3. The transformed cells were plated on L-agar containing 50 µg/ml ampicillin; restriction enzyme analysis of plasmid DNA was used to identify the E . coli K12 HB101/pLPcat transformants. Plasmid pLPcat DNA was isolated from the transformants for use in subsequent constructions in substantial accordance with the plasmid isolation procedure described in Example 3. B. Construction of pLP(GT)cat 1. Construct with Plasmid pLPcat About 1 µg of the poly-GT element as prepared in Example 1 in 30 µl dH₂O is added to 15 µl 10X Core buffer (50mM Tris-HCl, pH = 8.0; 10 mM MgCl₂; and 50 mM NaCl₂) and 95 µl dH₂O. Ten µl (about 100 units) of restriction enzyme Xho I are added to the solution of poly-GT element, and the resulting reaction is incubated at 37°C for 3 hours. After the Xho I digest, the DNA is collected by ethanol precipitation and resuspended in 7 µl of dH₂O. About 5 µg of plasmid pLPcat DNA in 10 µl of TE buffer is dissolved in 10 µl of 10X Core buffer and 75 µl dH₂O. Five µl (about 60 units) of Xho I are added to the solution of DNA, and the resulting reaction is incubated at 37°C for 2 hours. The reaction is stopped by extracting the reaction mixture first with phenol: chloroform, then with chloroform:isoamyl alcohol. The Xho I-digested plasmid pLPcat DNA is precipitated with ethanol and resuspended in 100 µl of TE buffer. The Xho I-digested plasmid pLPcat DNA is treated with calf-intestinal alkaline phosphatase as follows. The Xho I-digested plasmid pLPcat DNA was diluted to 100 µl in TE buffer; about 0.06 units of calf-intestinal alkaline phosphatase (Collaborative Research, Inc., 128 Spring Street, Lexington, MA 02173) were added to the solution, and the resulting reaction was incubated at 37°C for 30 minutes. The solution was adjusted to contain 1X SET (5 mM Tris-HCl, pH = 7.8; 5 mM EDTA; and 15 mM NaCl), 0.3M sodium acetate, and 0.5% SDS and then incubated at 65°C for 45 minutes. The phosphatase treatment prevents the pLPcat DNA from self ligating. The DNA was collected by ethanol precipitation and then resuspended in 10 µl of 10 mM Tris-HCl, pH 8.0 buffer. About 1 µl of 10X ligase buffer (0.5 M Tris-HCl, pH = 7.8; 100 mM MgCl₂; 200 mM DTT; 10 mM ATP; and 0.5 mg/ml BSA) is added to a mixture of 1 µl (about 50 ng) of the Xho I-digested, phosphatased plasmid pLPcat DNA and 7 µl (about 100 ng) of the Xho I-digested poly-GT element. One µl (about 1000 units) of T4 DNA ligase is added to the solution of DNA, and the resulting reaction is incubated at 16°C overnight. The ligated DNA constitutes the desired plasmid pLP(GT)cat. A restriction site and function map of plasmid pLP(GT)cat is presented in Figure 4 of the accompanying drawings. The ligated DNA is used to transform E . coli K12 HB101 cells in substantial accordance with the procedure of Example 3. The transformed cells are plated on an L-agar containing 50 µg/ml ampicillin; restriction enzyme analysis of plasmid DNA is used to identify the E . coli K12 HB101/pLP(GT)cat transformants. Plasmid pLP(GT)cat DNA is isolated from the transformants for use in subsequent constructions in substantial accordance with the plasmid isolation procedure described in Example 3. 2. Construct with Plasmid pBL(GT)cat Alternatively, plasmid pLP(GT)cat has been prepared starting with plasmid pBL(GT)cat DNA (described in Example 5 below) and deleting a fragment containing the BK enhancer according to the following procedure. About 9.5 µg of plasmid pBL(GT)cat DNA prepared as in Example 5B in 10 µl of Tris buffer (10 mM Tris-HCl, pH = 8.0) were added to 10 µl of 10X high salt buffer and 69 µl dH₂O. About 9 µl (about 18 units) of restriction enzyme Mst II and 2 µl (about 20 units) of restriction enzyme Bgl II were added to the DNA solution, and the resulting reaction was incubated at 37°C for 2 hours. The reaction was then stopped and the DNA extracted and precipitated as above. The DNA precipitate was dissolved in 45 µl of dH₂O. To the 45 µl of Mst II/ Bgl II-digested plasmid pBL(GT)cat DNA were added 5 µl of 10X Klenow buffer and 1 µl (about 5 units as marketed by BRL) Klenow enzyme and incubated in substantial accordance with the procedure of Example 10. The Klenow-treated, Mst II/ Bgl II-digested plasmid pBL(GT)cat DNA was extracted once with phenol:chloroform, once with chloroform:isoamyl alcohol, then precipitated with ethanol, and finally resuspended in 10 µl of Tris buffer. To about 1 µl (about 0.5 µg) of the Klenow-treated, Mst II/ Bgl II-digested plasmid pBL(GT)cat DNA was added 1 µl 10X ligase buffer, 7 µl dH₂O and 1 µl (about 100 units) of T4 DNA ligase. The resulting reaction was incubated at 16°C overnight and the ligated DNA constituted an alternative construct of the desired plasmid pLP(GT)cat, which construct may also be designated as plasmid pBL(GT)catΔ Bgl II/ Mst II. The deletion of the ∼480 bp Bgl II/ Mst II fragment from pBL(GT)cat removes the BK enhancer that was ligated into plasmid pLPcat in Example 5A to create plasmid pBLcat. Example 5 Construction of Plasmids pBLcat and pBL(GT)cat A. Construction of Plasmid pBLcat About 88 µg of plasmid pBKneo1 DNA in 50 µl of TE buffer were added to 7.5 µl of 10X Acc I buffer, 30 µl of dH₂O and 15 µl (about 75 units) of restriction enzyme Acc I, and the resulting reaction was incubated at 37°C for 2 hours. The Acc I-digested BK virus DNA was loaded on an agarose gel, and the ∼1.4 kb fragment that contains the BK enhancer was separated from the other digestion products. The ∼1.4 kb Acc I restriction fragment was then isolated in substantial accordance with the procedure described in Example 4A. About 5 µg of the fragment were resuspended in 5 µl of 10X Pvu II buffer, 45 µl of dH₂O, and 5 µl (about 25 units) of restriction enzyme Pvu II, and the resulting reaction was incubated at 37°C for 2 hours. The Pvu II-digested DNA was then isolated and prepared for ligation in substantial accordance with the procedure of Example 4A. About 2 µg of the desired ∼1.28 kb Acc I- Pvu II fragment were obtained and dissolved in 5 µl of TE buffer. About 1 µg of plasmid pLPcat DNA was dissolved in 5 µl of 10X Acc I buffer and 40 µl of dH₂O. Five µl (about 25 units) of restriction enzyme Acc I were added to the solution of plasmid pLPcat DNA, and the resulting reaction was incubated at 37°C. The Acc I-digested plasmid pLPcat DNA was precipitated with ethanol and resuspended in 5 µl of 10X Stu I buffer, 40 µl of dH₂O, and 5 µl (about 25 units) of restriction enzyme Stu I, and the resulting reaction was incubated at 37°C for 2 hours. The Acc I- Stu I-digested plasmid pLpcat DNA was precipitated with ethanol several times to purify the ∼4.81 kb Acc I- Stu I restriction fragment that comprises the E . coli origin of replication and Ad2 late promoter away from the other digestion product, a restriction fragment about 16 bp in size. About 1 µg of the desired ∼4.81 kb restriction fragment was obtained and dissolved in 20 µl of TE buffer. The 5 µl of ∼4.81 kb Acc I- Stu I restriction fragment of plasmid pLPcat were added to 5 µl of ∼1.28 kb Acc I- Pvu II restriction fragment of BK virus. After the addition of 3 µl of 10X ligase buffer, 15 µl of dH₂O, and 2 µl (about 1000 units) of T4 DNA ligase to the mixture of DNA, the resulting ligation reaction was incubated at 16°C overnight. The ligated DNA constituted the desired plasmid pBLcat. The ligated DNA was used to transform E . coli K12 HB101 cells in substantial accordance with the procedure described in Example 3. E . coli K12 HB101/pBLcat transformants were identified by restriction enzyme analysis of their plasmid DNA. Plasmid pBLcat DNA was prepared for use in subsequent constructions in substantial accordance with the procedure of Example 3. B. Construction of Plasmid pBL(GT)cat About 1 µg of the poly-GT element prepared as in Example 1 in 30 µl dH₂O was digested with Xho I in substantial accordance with the procedure of Example 4B.1. About 23 µg of plasmid pBLcat DNA in 10 µl of TE buffer were digested with Xho I and phosphatased in substantial accordance with the procedure of Example 4B.1. The Xho I-digested, phosphatased plasmid pBLcat DNA and the Xho I-digested poly-GT element were ligated in substantial accordance with the procedure of Example 4B.1. The ligated DNA constituted the desired plasmid pBL(GT)cat. A restriction site and function map of plasmid pBL(GT)cat is presented in Figure 5 of the accompanying drawings. The ligated DNA was used to transform E . coli K12 HB101 cells in substantial accordance with the procedure of Example 3. The transformed cells were plated on L-agar containing 50 µg/ml ampicillin; restriction enzyme analysis of plasmid DNA was used to identify E . coli K12 HB101/pBL(GT)cat transformants. Plasmid pBL(GT)cat DNA was isolated from the transformants in substantial accordance with the plasmid isolation procedure described in Example 3. Example 6 Construction of Plasmids pSBLcat and pSBL(GT)cat A. Construction of Plasmid pSBLcat About 100 µg of plasmid pBLcat DNA were dissolved in 10 µl of 10X Hin dIII buffer (0.5 M NaCl; 0.1 M Tris-HCl, pH = 8.0; 0.1 M MgCl₂; and 1 mg/ml BSA) and 80 µl of dH₂O. About 10 µl (about 100 units) of restriction enzyme Hin dIII were added to the solution of plasmid pBLcat DNA, and the resulting reaction was incubated at 37°C for 2 hours. The Hin dIII-digested plasmid pBLcat DNA was loaded onto a 1% agarose gel and electrophoresed until the ∼0.87 kb Hin dIII restriction fragment that comprises the BK enhancer and Ad2 late promoter was well separated from the other digestion products; then, the ∼0.87 kb fragment was isolated and prepared for ligation in substantial accordance with the procedure of Example 4A. About 10 µg of the desired fragment were obtained and dissolved in 50 µl of TE buffer. About 1 µg of plasmid pSV2cat DNA in 1 µl of TE buffer was dissolved in 2 µl of 10X Hin dIII buffer and 16 µl of dH₂O. One µl (about 10 units) of restriction enzyme Hin dIII was added to the solution of DNA, and the resulting reaction was incubated at 37° for 2 hours. The reaction was stopped by extracting the reaction mixture first with phenol, then twice with chloroform. The Hin dIII-digested plasmid pSV2cat DNA was precipitated with ethanol and resuspended in 100 µl of TE buffer. The Hin dIII-digested plasmid pSV2cat DNA was treated with calf-intestinal alkaline phosphatase in substantial accordance with the procedure of Example 4B and the resuspended in 10 µl of TE buffer. About 5 µl of the ∼0.87 kb Hin dIII restriction fragment of plasmid pBLcat were added to the 10 µl of Hin dIII-digested plasmid pSV2cat, and then, 3 µl of 10X ligase buffer, 2 µl (about 1000 units) of T4 DNA ligase, and 13 µl of dH₂O were added to the solution of DNA, and the resulting reaction was incubated at 16°C for 2 hours. The ligated DNA constituted the desired plasmid pSBLcat. The ligated DNA was used to transform E . coli K12 HB101 in substantial accordance with the procedure of Example 3. The transformed cells were plated on L-agar containing ampicillin, and the plasmid DNA of the ampicillin-resistant transformants was examined by restriction enzyme analysis to identify the E . coli K12 HB101/pSBLcat transformants. The ∼0.87 kb Hin dIII restriction fragment that encodes the BK enhancer and Ad2 late promoter could insert into Hin dIII-digested plasmid pSBLcat in one of two orientations, only one of which yields plasmid pSBLcat. B. Construction of Plasmids pSBL(GT)cat A poly-GT element is inserted into the unique Xho I site in plasmid pSBLcat, between the BK enhancer and the Ad2 late promoter, in substantial accordance with the procedure of Example 4B.1. The Xho I-digested poly-GT element and Xho I-digested plasmid pSBLcat DNA are ligated and the ligated DNA constitutes the desired plasmid pSBL(GT)cat. A restriction site and function map of plasmid pSBL(GT)cat is presented in Figure 6 of the accompanying drawings. Example 7 Construction of Plasmid pL133 A. Construction of Intermediate Plasmid pSV2-HPC8 Plasmid pHC7 comprises a DNA sequence that encodes human protein C. One liter of L broth containing 15 µg/ml tetracycline was inoculated with a culture of E . coli K12 RR1/pHC7 (NRRL B-15926, deposited Jan. 29, 1985), and plasmid pHC7 DNA was isolated and purified in substantial accordance with the procedure of Example 3. About 1 mg of plasmid pHC7 DNA was obtained by this procedure, suspended in 1 ml of TE buffer, and stored at -20°C. A restriction site and function map of plasmid pHC7 is presented in Figure 7 of the accompanying drawings. Fifty µl of the plasmid pHC7 DNA were mixed with 5 µl (about 50 units) of restriction enzyme Ban I, 10 µl of 10X Ban I reaction buffer (1.5 M NaCl; 60 mM Tris-HCl, pH = 7.9; 60 mM MgCl₂; and 1 mg/ml BSA), 35 µl of dH₂O, and incubated until the digestion was complete. The Ban I-digested plasmid pHC7 DNA was then electrophoresed on a 3.5% polyacrylamide gel (29:1, acrylamide: bisacrylamide), until the ∼1.25 kb Ban I restriction fragment was separated from the other digestion products. The region of the gel containing the ∼1.25 kb Ban I restriction fragment was cut from the gel, placed in a test tube, and broken into small fragments. One ml of extraction buffer (500 mM ammonium acetate, 10 mM magnesium acetate, 1 mM EDTA, 1% SDS, and 10 mg/ml tRNA) was added to the tube containing the fragments, and the tube was placed at 37°C overnight. Centrifugation was used to pellet the debris, and the supernatant was transferred to a new tube. The debris was washed once with 200 µl of extraction buffer; the wash supernatant was combined with the first supernatant from the overnight extraction. After passing the supernatant through a plug of glass wool, two volumes of ethanol were added to and mixed with the supernatant. The resulting solution was placed in a dry ice-ethanol bath for ∼10 minutes, and then, the DNA was pelleted by centrifugation. Approximately 8 µg of the ∼1.25 kb Ban I restriction fragment were obtained by this procedure. The purified fragment was suspended in 10 µl of TE buffer and stored at -20°C. The Ban I restriction fragment had to be modified by the addition of a linker to construct plasmid pSV2-HPC8. The DNA fragments used in the construction of the linker were synthesized in substantial accordance with the procedures described in Example 1. Five hundred picomoles of each single strand of the synthesized linker were kinased in 20 µl of reaction buffer, which contained 15 units (about 0.5 µl) T4 polynucleotide kinase, 2 µl 10X ligase buffer, 10 µl of 500 µM ATP, and 7.5 µl of dH₂O. The kinase reaction was incubated at 37°C for 30 minutes, and the reaction was terminated by incubation at 100°C for 10 minutes. In order to ensure complete kination, the reaction was chilled on ice, 2 µl of 0.2 M DDT, 2.5 µl of 5 mM ATP, and 15 units of T4 polynucleotide kinase were added to the reaction mixture and mixed, and the reaction mixture was incubated another 30 minutes at 37°C. The reaction was stopped by another 10 minute incubation at 100°C and then chilled on ice. Although kinased separately, the two single strands of the DNA linker were mixed together after the kinase reaction. To anneal the strands, the kinase reaction mixture was incubated at 100°C for 10 minutes in a water bath containing about 150 ml of water. After this incubation, the water bath was shut off and allowed to cool to room temperature, a process taking about 3 hours. The water bath, still containing the tube of kinased DNA, was then incubated at 4°C overnight. This process annealed the single strands. The annealed linker had the following structure: The linker was stored at -20°C until use. About 8 µg of ∼1.25 kb Ban I fragment were added to and mixed with the ∼50 µl of linker (about 500 picomoles), 1 µl of T4 DNA ligase (about 500 units), 10 µl of 10X ligase buffer, and 29 µl of dH₂O, and the resulting ligation reaction was incubated at 4°C overnight. The ligation reaction was stopped by a 10 minute incubation at 65°C. The DNA was pelleted by adding sodium acetate to a final concentration of 0.3 M, adding 2 volumes of ethanol, chilling in a dry ice-ethanol bath, and then centrifuging the solution. The DNA pellet was dissolved in 10 µl of 10X Apa I reaction buffer (60mM NaCl; 60 mM Tris-HCl, pH = 7.4; 60 mM MgCl₂; and 60 mM 2-mercaptoethanol), 5 µl (about 50 units) of restriction enzyme Apa I, and 85 µl of dH₂O, and the reaction was placed at 37°C for two hours. The reaction was then stopped and the DNA pelleted as above. The DNA pellet was dissolved in 10 µl of 10X Hin dIII reaction buffer, 5 µl (about 50 units) of restriction enzyme Hin dIII, and 85 µl of dH₂O, and the reaction was placed at 37°C for two hours. After the Hin dIII digestion, the reaction mixture was loaded onto a 3.5% polyacrylamide gel, and the desired ∼1.23 kb Hin dIII- Apa I restriction fragment was isolated in substantial accordance with the procedure described in Example 4A. Approximately 5 µg of the desired fragment were obtained, suspended in 10 µl of TE buffer, and stored at -20°C. Fifty µl of plasmid pHC7 DNA were mixed with 5 µl (about 50 units) of restriction enzyme Pst I, 10 µl of 10X Pst I reaction buffer (1.0 M NaCl; 100 mM Tris-HCl, pH = 7.5; 100 mM MgCl₂; and 1 mg/ml BSA), and 35 µl of dH₂O and incubated at 37°C for two hours. The Pst I-digested plasmid pHC7 DNA was then electrophoresed on a 3.5% polyacrylamide gel, and the desired ∼0.88 kb fragment was purified in substantial accordance with the procedure described above. Approximately 5 µg of the desired fragment were obtained, suspended in 10 µl of TE buffer, and stored at -20°C. About 5 µg of the ∼0.88 kb Pst I fragment were added to and mixed with about 50 µl of the following linker, which was constructed on an automated DNA synthesizer as described in Example 1: About 1 µl of T4 DNA ligase (about 10 units), 10 µl 10X ligase buffer, and 29 µl dH₂O were added to the mixture of DNA, and the resulting ligation reaction was incubated at 4°C overnight. The ligation reaction was stopped by a 10 minute incubation at 65°C. After precipitation of the ligated DNA, the DNA pellet was dissolved in 10 µl of 10X Apa I reaction buffer, 5 µl (about 50 units) of restriction enzyme Apa I and 85 µl of dH₂O, and the reaction was placed at 37°C for two hours. The reaction was then stopped and the DNA pelleted once again. The DNA pellet was dissolved in 10 µl 10X Bgl II reaction buffer (1 M NaCl; 100 mM Tris-HCl, pH = 7.4; 100 mM MgCl₂; 100 mM 2-mercaptoethanol; and 1 mg/ml BSA), 5 µl (about 50 units) of restriction enzyme Bgl II, and 85 µl glass distilled dH₂O, and the reaction was placed at 37°C for two hours. After the Bgl II digestion, the reaction mixture was loaded onto a 3.5% polyacrylamide gel, and the desired ∼0.19 kb Apa I- Bgl II restriction fragment was isolated in substantial accordance with the procedure described above. Approximately 1 µg of the desired fragment was obtained, suspended in 10 µl of TE buffer, and stored at -20°C. Approximately 10 µg of publically available plasmid pSV2gpt DNA (ATCC 37145) were dissolved in 10 µl of 10X Hin dIII reaction buffer, 5 µl (about 50 units) of restriction enzyme Hin dIII, and 85 µl of dH₂O, and the reaction was placed at 37°C for 2 hours. The reaction mixture was then made 0.25 M in sodium acetate, and after the addition of two volumes of ethanol and incubation in a dry ice-ethanol bath, the DNA was pelleted by centrifugation. The DNA pellet was dissolved in 10 µl of 10X Bgl II buffer, 5 µl (about 50 units) of restriction enzyme Bgl II, and 85 µl of dH₂O, and the reaction was placed at 37°C for two hours. After the Bgl II digestion, the reaction mixture was loaded onto a 1% agarose gel, and the fragments were separated by electrophoresis. The gel was stained with ethidium bromide and viewed under UV light, and the band containing the desired ∼5.1 kb Hin dIII- Bgl II fragment was cut from the gel and placed in dialysis tubing, and electrophoresis was continued until the DNA was electrophoresed out of the agarose. The buffer containing the DNA from the dialysis tubing was extracted with phenol and chloroform and then, the DNA was ethanol precipitated. The pellet was resuspended in 10 µl of TE buffer and constituted about 5 µg of the desired ∼5.1 kb Hin dIII- Bgl II restriction fragment of plasmid pSV2gpt. Two µl of the ∼1.23 kb Hin dIII- Apa I restriction fragment, 3 µl of the ∼0.19 kb Apa I- Bgl II fragment, and 2 µl of the ∼5.1 kb Hin dIII- Bgl II fragment were mixed together and then incubated with 10 µl of 10X ligase buffer, 1 µl of T4 DNA ligase (about 500 units), and 82 µl of dH₂O at 16°C overnight. The ligated DNA constituted the desired plasmid pSV2-HPC8; a restriction site and function map of the plasmid is presented in Figure 7 of the accompanying drawings. E . coli K12 RR1 (NRRL B-15210, deposited Oct. 19, 1982) cells were made competent for transformation in substantial accordance with the procedure described in Example 3. The ligated DNA prepared above was used to transform the cells, and aliquots of the transformation mix were plated on L-agar plates containing 100 µg/ml ampicillin. The plates were then incubated at 37°C. E . coli K12 RR1/pSV2-HPC8 transformants were verified by restriction enzyme analysis of their plasmid DNA. B. Final Construction of Plasmid nL133 Fifty µg of plasmid pSV2-HPC8 were dissolved in 10 µl of 10X Hin dIII reaction buffer, 5 µl (about 50 units) of restriction enzyme Hin dIII, and 85 µl of dH₂O, and the reaction was incubated at 37°C for two hours. After the Hin dIII digestion, the DNA was precipitated, and the DNA pellet was dissolved in 10 µl 10X Sal I reaction buffer (1.5 M NaCl; 60 mM Tris-HCl, pH = 7.9; 60 mM MgCl₂; 60 mM 2-mercaptoethanol; and 1 mg/ml BSA), 5 µl (about 50 units) of restriction enzyme Sal I, and 85 µl of dH₂O. The resulting Sal I reaction mixture was incubated for 2 hours at 37°C. The Hin dIII- Sal I-digested plasmid pSV2-HPC8 was loaded onto a 3.5% polyacrylamide gel and electrophoresed until the desired ∼0.29 kb Hin dIII- Sal I restriction fragment was separated from the other reaction products. The desired fragment was isolated from the gel in substantial accordance with the procedure described above in part A; about 2 µg of the fragment were obtained and suspended in 10 µl of TE buffer. Fifty µg of plasmid pSV2-HPC8 were dissolved in 10 µl of 10X Bgl II reaction buffer, 5 µl (about 50 units) of restriction enzyme Bgl II, and 85 µl of dH₂O, and the reaction was incubated at 37°C for two hours. After the Bgl II digestion, the DNA was precipitated, and the DNA pellet was dissolved in 10 µl of 10X Sal I reaction buffer, 5 µl (about 50 units) of restriction enzyme Sal I, and 85 µl of dH₂O. The resulting Sal I reaction mixture was incubated for 2 hours at 37°C. The Sal I- Bgl I-digested plasmid pSV2-HPC8 was loaded onto a 3.5% polyacrylamide gel and electrophoresed until the desired ∼1.15 kb Sal I- Bgl II restriction fragment was separated from the other reaction products. The ∼1.15 kb Sal I- Bgl II restriction fragment was isolated from the gel in substantial accordance with the procedure described above in part A; about 8 µg of fragment were obtained and suspended in 10 µl of TE buffer. Approximately 10 µg of plasmid pSV2-β-globin DNA (NRRL B-15928, deposited January 29, 1985) were dissolved in 10 µl of 10X Hin dIII reaction buffer, 5 µl (about 50 units) of restriction enzyme Hin dIII, and 35 µl of dH₂O, and the reaction was placed at 37°C for 2 hours. The reaction mixture was then made 0.25 M in sodium acetate, and after the addition of two volumes of ethanol and incubation in a dry ice-ethanol bath, the DNA was pelleted by centrifugation. The Hin dIII-digested plasmid pSV2-β-globin DNA was dissolved in 10 µl of 10X Bgl II buffer, 5 µl (about 50 units) of restriction enzyme Bgl II, and 85 µl of dH₂O, and the reaction was placed at 37°C for two hours. After the Bgl II digestion, the reaction mixture was loaded onto a 1% agarose gel, and the fragments were separated by electrophoresis. The desired ∼4.2 kb Hin dIII- Bgl II restriction fragment was isolated from the gel in substantial accordance with the procedure described in Example 4A; about 5 µg of the desired fragment were obtained and suspended in 10 µl of TE buffer. Two µl of the ∼0.29 kb Hin dIII- Sal I fragment of plasmid pSV2-HPC8, 2 µl of the ∼1.15 kb Sal I- Bgl II fragment of plasmid pSV2-HPC8, and 2 µl of the ∼4.2 kb Hin dIII- Bgl II fragment of plasmid pSV2-β-globin were mixed together and ligated in substantial accordance with the procedure of part A above: The ligated DNA constituted the desired plasmid pL133; a restriction site and function map of plasmid pL133 is presented in Figure 7 of the accompanying drawings. The desired E . coli K12 RR1/pL133 transformants were constructed in substantial accordance with the teaching of part A above, with the exception that plasmid pL133, rather than plasmid pSV2-HPC8, was used as the transforming DNA. Example 8 Construction of Plasmids pLPC and pLPC(GT) A. Construction of Plasmid pLPC About 20 µg of plasmid pBLcat DNA as prepared in Example 5A were dissolved in 10 µl of 10X Hin dIII buffer and 80 µl of dH₂O. About 10 µl (about 100 units) of restriction enzyme Hin dIII were added to the solution of plasmid pBLcat DNA, and the resulting reaction was incubated at 37°C for 2 hours. The Hin dIII-digested plasmid pBLcat DNA was loaded onto a 1% agarose gel and electrophoresed until the ∼0.87 kb Hin dIII restriction fragment that comprises the BK enhancer and Ad2 late promoter was separated from the other digestion products; then, the ∼0.87 kb fragment was isolated and prepared for ligation in substantial accordance with the procedure of Example 4A. About 2 µg of the desired fragment were obtained and dissolved in 5 µl of TE buffer. About 1.5 µg of plasmid pL133 DNA as prepared in Example 7 was dissolved in 2 µl of 10X Hin dIII buffer and 16 µl of dH₂O. About 1 µl (about 10 units) of restriction enzyme Hin dIII was added to the solution of DNA, and the resulting reaction was incubated at 37°C for 2 hours. The DNA was then diluted to 100 µl with TE buffer and treated with calf-intestinal alkaline phosphatase in substantial accordance with the procedure in Example 3. The Hin dIII-digested plasmid pL133 DNA was extracted twice with phenol and once with chloroform, precipitated with ethanol, and resuspended in 10 µl of TE buffer. About 5 µl of the ∼0.87 kb Hin dIII restriction fragment of plasmid pBLcat were added to the 1.5 µl of Hin dIII-digested plasmid pL133, and then, 1 µl of 10X ligase buffer, 1 µl (about 1000 units) of T4 DNA ligase, and 1.5 µl of dH₂O were added to the solution of DNA, and the resulting reaction was incubated at 16°C overnight. The ligated DNA constituted the desired plasmid pLPC. The ligated DNA was used to transform E . coli K12 HB101 in substantial accordance with the procedure of Example 3. The transformed cells were plated on L-agar containing ampicillin, and the plasmid DNA of the ampicillin-resistant transformants was examined by restriction enzyme analysis to identify the E . coli K12 HB101/pLPC transformants. The ∼0.87 kb Hin dIII restriction fragment that encodes the BK enhancer and Ad2 late promoter could insert into Hin dIII-digested plasmid pL133 in one of two orientations, only one of which yields plasmid pLPC. B. Construction of Plasmid pLPC(GT) Plasmid pLPC(GT) is prepared by the same procedures as described in part A except that the ∼0.93 kb Hin dIII fragment of plasmid pBL(GT)cat is used in the construction instead of the ∼0.87 kb Hin dIII fragment of plasmid pBLcat to yield the desired plasmid pLPC(GT). The ∼0.93 kb Hin dIII restriction fragment that encodes the BK enhancer, the poly-GT element and the Ad2 late promoter can be inserted into Hin dIII-digested plasmid pL133 in one of two orientations, only one of which yields plasmid pLPC(GT). A restriction site and function map of plasmid pLPC(GT) is presented in Figure 8 of the accompanying drawings. The only difference between plasmids pLPC and pLPC(GT) is the presence of a poly-GT element in plasmid pLPC(GT). Alternatively, plasmid pLPC(GT) is prepared by inserting a poly-GT element into the unique Xho I site in plasmid pLPC, between the BK enhancer and the Ad2 late promoter, in substantial accordance with the procedure of Example 4B.1. Example 9 Construction of Plasmids pLPC4, pLPC5, pLPC4(GT) and pLPC5(GT) A. Construction of Plasmids pLPC4 and pLPC5 About 1 µg (1 µl) of the BK virus DNA prepared as in Example 2 and 1 µg (1 µl) of plasmid pLPC prepared as in Example 8A were dissolved in 2 µl of 10X Eco RI buffer (1.0 M Tris-HCl, pH 7.5; 0.5 M NaCl; 50 mM MgCl₂; and 1 mg/ml BSA) and 14 µl of dH₂O. Two µl (about 10 units) of restriction enzyme Eco RI were added to the solution of DNA, and the resulting reaction was incubated at 37°C for 2 hours. The Eco RI-digested mixture of BK virus and plasmid pLPC DNA was extracted once with buffered phenol and once with chloroform. Then, the DNA was collected by adjusting the NaCl concentration to 0.25 M, adding two volumes of ethanol, incubating the solution in a dry ice-ethanol bath for 2 minutes, and centrifuging the solution to pellet the DNA. The supernatant was discarded, and the DNA pellet was rinsed with 70% ethanol, dried, and resuspended in 12 µl of TE buffer. About 13 µl of dH₂O and 3 µl of 10X ligase buffer were added to the Eco RI-digested mixture of BK virus and plasmid pLPC DNA. Two µl (about 100 units) of T4 DNA ligase were added to the solution of DNA, and the resulting reaction was incubated at 16°C for 2 hours. The ligated DNA constituted the desired plasmids pLPC4 and pLPC5, which differ only with respect to the orientation of the inserted BK virus DNA. The ligated DNA constituted the desired plasmids pLPC4 and pLPC5 and was used to transform E . coli K12 HB101 competent cells in substantial accordance with the procedure of Example 3. The transformed cells were plated on L-agar containing 100 µg/ml ampicillin. The E . coli K12 HB101/pLPC4 and E . coli K12 HB101/pLPC5 transformants were identified by their ampicillin-resistant phenotype and by restriction enzyme analysis of their plasmid DNA. B. Construction of Plasmids pLPC4(GT) and pLPC5(GT) Plasmids pLPC4(GT) and pLPC5(GT) are prepared by the same procedures as described in part A above for plasmids pLPC4 and pLPC5, except that plasmid pLPC(GT) prepared as in Example 8B is used in the constructions instead of plasmid pLPC. Alternatively, plasmids pLPC4(GT) and pLPC5(GT) are prepared by inserting a poly-GT element into the unique Xho I site of plasmids pLPC4 and pLPC5, respectively, in substantial accordance with the procedure of Example 4B.1. A restriction site and function map of plasmid pLPC4(GT) is presented in Figure 9 of the accompanying drawings. Example 10 Construction of Plasmids pLPChyg1, pLPChyg2, pLPChyg1(GT) and pLPChyg2(GT) A. Construction of Plasmids pLPChyg1 and pLPChyg2 E . coli K12 RR1/pSV2hyg cells are obtained from the Northern Regional Research Laboratory under the accession number NRRL B-18039. Plasmid pSV2hyg DNA is obtained from the cells in substantial accordance with the procedure of Example 3. A restriction site and function map of plasmid pSV2hyg is presented in Figure 10 of the accompanying drawings. About 10 µg (in 10 µl of TE buffer) of plasmid pSV2hyg were added to 2 µl of 10X Bam HI buffer and 6 µl of dH₂O. Two µl (about 20 units) of restriction enzyme Bam HI were added to the solution of DNA, and the resulting reaction was incubated at 37°C for 2 hours. The reaction was extracted first with phenol and then extracted twice with chloroform. The Bam HI-digested plasmid pSV2hyg DNA was loaded onto a 1% agarose gel, and the ∼2.5 kb hygromycin resistance gene-containing restriction fragment was isolated in substantial accordance with the procedure described in Example 4A. About 5 µl of 10X Klenow buffer (0.2 mM in each of the four dNTPs; 0.5 M Tris-HCl, pH = 7.8; 50 mM MgCl₂; 0.1 M 2-mercaptoethanol; and 100 µg/ml BSA) and 35 µl of dH₂O were added to the solution of Bam HI-digested plasmid pSV2hyg DNA, and then, about 25 units of Klenow enzyme (about 5 µl, as marketed by BRL) were added to the mixture of DNA, and the resulting reaction was incubated at 16°C for 30 minutes. The Klenow-treated Bam HI-digested plasmid pSV2hyg DNA was extracted once with phenol and once with chloroform and then precipitated with ethanol. About 2 µg of the desired fragment were obtained and suspended in 5 µl of TE buffer. About 10 µg (10 µl) of plasmid pLPC DNA were added to 2 µl of 10X Stu I buffer and 6 µl of dH₂O. Two µl (about 10 units) of restriction enzyme Stu I were added to the solution of DNA, and the resulting reaction was incubated at 37°C for 2 hours. The Stu I-digested plasmid pLPC DNA was precipitated with ethanol, collected by centrifugation, and resuspended in 2 µl of 10X Nde I buffer (1.5 M NaCl; 0.1 M Tris-HCl, pH = 7.8; 70 mM MgCl₂; 60 mM 2-mercaptoethanol; and 1 mg/ml BSA) and 16 µl of dH₂O. Two µl (about 10 units) of restriction enzyme Nde I were added to the solution of Stu I-digested DNA, and the resulting reaction was incubated at 37°C for 2 hours. The Nde I- Stu I-digested plasmid pLPC DNA was precipitated with ethanol, collected by centrifugation, and resuspended in 5 µl of 10X Klenow buffer and 40 µl of dH₂O. Five µl (about 25 units) of Klenow enzyme were added to the solution of DNA, and the resulting reaction was incubated at 16°C for 30 minutes. After the Klenow reaction, the reaction mixture was loaded onto a 1% agarose gel, and the ∼5.82 kb Nde I- Stu I restriction fragment was isolated from the gel. About 5 µg of the desired fragment were obtained and suspended in 5 µl of TE buffer. About 2 µl of the ∼2.5 kb Klenow-treated Bam HI restriction fragment of plasmid pSV2hyg were mixed with about 1 µl of the ∼5.82 kb Klenow-treated Nde I- Stu I restriction fragment of plasmid pLPC, and about 3 µl of 10X ligase buffer, 2 µl of T4 DNA ligase (about 1000 units), 1 µl of T4 RNA ligase (about 1 unit), and 14 µl of dH₂O were added to the solution of DNA. The resulting reaction was incubated at 16°C overnight. The ligated DNA constituted the desired plasmids pLPChyg1 and pLPChyg2, which differ only with respect to the orientation of the ∼2.5 kb Klenow-treated, Bam HI restriction fragment of plasmid pSV2hyg. The ligated DNA was used to transform E . coli K12 HB101 in substantial accordance with the procedure of Example 3. The desired E . coli K12 HB101/pLPChyg1 and E . coli K12 HB101/pLPChyg2 transformants were plated on L-agar containing ampicillin and identified by restriction enzyme analysis of their plasmid DNA. B. Construction of Plasmids pLPChyg1(GT) and pLPChyg2(GT) A poly-GT element is inserted into the unique Xho I site in plasmid pLPChyg1 or pLPChyg2, between the BK enhancer and the Ad2 late promoter, in substantial accordance with the procedure of Example 4B.1. The Xho I-digested poly-GT element and Xho I-digested plasmid pLPChyg1 or pLPChyg2 DNA are ligated and the ligated DNA constitutes the desired plasmids pLPChyg1(GT) and pLPChyg2(GT), respectively. A restriction site and function map of plasmid pLPChyg1(GT) is presented in Figure 11 of the accompanying drawings. Example 11 Construction of Plasmid pBW32 A. Construction of Intermediate Plasmid pTPA103 Plasmid pTPA102 comprises the coding sequence of human tissue plasminogen activator (TPA). Plasmid pTPA102 can be isolated from E . coli K12 MM294/pTPA102, a strain available from the Northern Regional Research Laboratory under the accession number NRRL B-15834 (deposited Aug. 10, 1984). A restriction site and function map of plasmid pTPA102 is presented in Figure 12 of the accompanying drawings. Plasmid pTPA102 DNA is isolated from E . coli K12 MM294/pTPA102 in substantial accordance with the procedure of Example 3. About 50 µg of plasmid pTPA102 (in about 50 µl of TE buffer) were added to 10 µl of 10X Tth 111I buffer (0.5 M NaCl; 80 mM Tris-HCl, pH = 7.4; 80 mM MgCl₂; 80 mM 2-mercaptoethanol; and 1 mg/ml BSA) and 80 µl of dH₂O. Ten µl (about 50 units) of restriction enzyme Tth 111I were added to the solution of DNA, and the resulting reaction was incubated at 65°C for 2 hours. The reaction mixture was loaded onto a 1% agarose gel, and the ∼4.4 kb Tth 111I restriction fragment that comprises the TPA coding sequence was isolated from the gel. The other digestion products, 3.1 kb and 0.5 kb restriction fragments, were discarded. About 10 µg of the desired ∼4.4 kb Tth 111I restriction fragment were obtained and suspended in 10 µl of TE buffer. About 5 µl of 10X Klenow buffer and 30 µl of dH₂O were added to the solution comprising the ∼4.4 kb Tth 111I restriction fragment, and after the further addition of 5 µl of Klenow enzyme (about 5 units), the reaction mixture was incubated at 16°C for 30 minutes. After the Klenow reaction, the DNA was precipitated with ethanol and resuspended in 3 µl of 10X ligase buffer and 14 µl of dH₂O. Bam HI linkers (New England Biolabs), which had the following sequence: were kinased and prepared for ligation by the following procedure. Four µl of linkers (about 2 µg) were dissolved in 20.15 µl of dH₂O and 5 µl of 10X kinase buffer (500 mM Tris-HCl, pH = 7.6 and 100 mM MgCl₂), incubated at 90°C for two minutes, and then cooled to room temperature. Five µl of γ-³²P-ATP (about 20 mCi), 2.5 ml of 1 M DTT, and 5 µl of polynucleotide kinase (about 10 units) were added to the mixture, which was then incubated at 37°C for 30 minutes. Then, approximately 3.35 µl of 10 mM ATP and 5 µl of kinase were added, and the reaction was continued for another 30 minutes at 37°C. The radioactive ATP aids in determining whether the linkers have ligated to the target DNA. About 10 µl of the kinased Bam HI linkers were added to the solution of ∼4.4 kb Tth 111I restriction fragment, and after the addition of 2 µl of T4 RNA ligase (about 2 units), the ligation reaction was incubated overnight at 4°C. The ligated DNA was precipitated with ethanol and resuspended in 5 µl of 10X Hin dIII buffer and 40 µl of dH₂O. Five µl (about 50 units) of restriction enzyme Hin dIII were added to the solution of DNA, and the resulting reaction was incubated at 37°C for 2 hours. The Hin dIII-digested DNA was precipitated with ethanol and resuspended in 10 µl of 10X Bam HI buffer and 90 µl of dH₂O. Ten µl (about 100 units) of restriction enzyme Bam HI were added to the solution of DNA, and the resulting reaction was incubated at 37°C for 2 hours. After the Bam HI digestion, the reaction mixture was loaded onto a 1% agarose gel, and the ∼2.0 kb Bam HI- Hin dIII restriction fragment was isolated from the gel in substantial accordance with the procedure of Example 4A. About 4 µg of the desired fragment were obtained and suspended in about 5 µl of TE buffer. To construct plasmid pTPA103, the ∼2.0 kb Bam HI- Hin dIII restriction fragment derived from plasmid pTPA102 was inserted into Bam HI- Hin dIII-digested plasmid pRC. Plasmid pRC was constructed by inserting an ∼288 bp Eco RI- Cla I restriction fragment that comprises the promoter and operator ( trp PO) sequences of the E . coli trp operon into Eco RI- Cla I-digested plasmid pKC7. Publically available plasmid pKC7 can be obtained from the American Type Culture Collection in E . coli K12 N100/pKC7 under the accession number ATCC 37084. The ∼288 bp Eco RI- Cla I restriction fragment that comprises the trp PO can be isolated from plasmid pTPA102, which can be isolated from E . coli K12 MM294/pTPA102 (NRRL B-15834). Plasmid pKC7 and plasmid pTPA102 DNA can be obtained from the aforementioned cell lines in substantial accordance with the procedure of Example 3. This ∼0.29 kb Eco RI- Cla I restriction fragment of plasmid pTPA102 comprises the transcription activating sequence and most of the translation activating sequence of the E . coli trp gene and has the sequence depicted below: Thus, to construct plasmid pRC, about 2 µg of plasmid pKC7 in 10 µl of TE buffer were added to 2 µl of 10X Cla I buffer (0.5 M NaCl; 60 mM Tris-HCl, pH = 7.9, 60 mM MgCl₂; and 1 mg/ml BSA) and 6 µl of dH₂O. Two µl (about 10 units) of restriction enzyme Cla I were added to the solution of plasmid pKC7 DNA, and the resulting reaction was incubated at 37°C for 2 hours. The Cla I-digested plasmid pKC7 DNA was precipitated with ethanol and resuspended in 2 µl of 10X Eco RI buffer and 16 µl of dH₂O. Two µl (about 10 units) of restriction enzyme Eco RI were added to the pKC7 DNA, and the resulting reaction was incubated at 37°C for 2 hours. The Eco RI- Cla I-digested plasmid pKC7 DNA was extracted once with phenol and then twice with chloroform. The DNA was then precipitated with ethanol and resuspended in 3 µl of 10X ligase buffer and 20 µl of dH₂O. A restriction site and function map of plasmid solution of Cla I-digested plasmid pKC7 can be obtained from Maniatis et al ., Molecular Cloning (Cold Spring Harbor Laboratory, 1982), page 8. About 20 µg of plasmid pTPA102 in about 20 µl of TE buffer were added to 10 µl of 10X Cla I buffer and 60 µl of dH₂O. Ten µl (about 50 units) of restriction enzyme Cla I were added to the solution of plasmid pTPA102 DNA, and the resulting reaction was incubated at 37°C for 2 hours. The Cla I-digested plasmid pTPA102 DNA was precipitated with ethanol and resuspended in 10 µl of 10X Eco RI buffer and 80 µl of dH₂O. Ten µl (about 50 units) of restriction enzyme Eco RI were added to the solution of Cla I-digested plasmid pTPA102 DNA, and the resulting reaction was incubated at 37°C for 2 hours. The Eco RI- Cla I-digested plasmid pTPA102 DNA was extracted once with phenol, loaded onto a 7% polyacrylamide gel, and electrophoresed until the ∼288 bp Eco RI- Cla I restriction fragment that comprises the trp PO was separated from the other digestion products. The ∼288 bp Eco RI- Cla I restriction fragment was isolated from the gel; about 1 µg of the desired fragment was obtained, suspended in 5 µl of TE buffer, and added to the solution of Eco RI- Cla I-digested plasmid pKC7 DNA prepared as described above. Two µl (about 1000 units) of T4 DNA ligase were then added to the mixture of DNA, and the resulting ligation reaction was incubated at 16°C for 2 hours. The ligated DNA constituted the desired plasmid pRC DNA. The ligated DNA was used to transform E . coli K12 HB101 competent cells in substantial accordance with the procedure of Example 3. The transformed cells were plated on L-agar containing 100 µg/ml ampicillin, and the ampicillin-resistant transformants were screened by restriction enzyme analysis of their plasmid DNA to identify the desired E . coli K12 HB101/pRC colonies. Plasmid pRC DNA was obtained from the E . coli K12 HB101/pRC transformants in substantial accordance with the procedure of Example 3. About 2 µg of plasmid pRC DNA in 2 µl of TE buffer were added to 2 µl of 10X Hin dIII buffer and 16 µl of dH₂O. Two µl (about 10 units) of restriction enzyme Hin dIII were added to the solution of plasmid pRC DNA, and the resulting reaction was incubated at 37°C for two hours. The Hin dIII-digested plasmid pRC DNA was precipitated with ethanol and resuspended in 2 µl of 10X Bam HI buffer and 16 µl of dH₂O. Two µl (about 10 units) of restriction enzyme Bam HI were added to the solution of Hin dIII-digested plasmid pRC DNA, and the resulting reaction was incubated at 37°C for 2 hours. The Bam HI- Hin dIII-digested plasmid pRC DNA was extracted once with phenol and then twice with chloroform. The DNA was precipitated with ethanol and resuspended in 3 µl of 10X ligase buffer and 20 µl of dH₂O. Approximtely 4 µg (in about 5 µl of TE buffer) of ∼2.0 kb Hin dIII- Bam HI restriction fragment of plasmid pTPA102 were then added to the solution of Bam HI- Hin dIII-digested plasmid pRC DNA. Two µl (about 1000 units) of T4 DNA ligase were added to the mixture of DNA, and the resulting reaction was incubated at 16°C for 2 hours. The ligated DNA constituted the desired plasmid pTPA103 DNA. To reduce undesired transformants, the ligated DNA was digested with restriction enzyme Nco I, which cuts plasmid pRC but not plasmid pTPA103. Thus, digestion of the ligated DNA with Nco I reduces undesired transformants, because linearized DNA transforms E . coli at a lower frequency than closed circular DNA. To digest the ligated DNA, the DNA was first precipitated with ethanol and then resuspended in 2 µl of 10X Nco I buffer (1.5 M NaCl; 60 mM Tris-HCl, pH = 7.8; 60 mM MgCl₂; and 1 mg/ml BSA) and 16 µl of dH₂O. Two µl (about 10 units) of restriction enzyme Nco I were added to the solution of DNA, and the resulting reaction was incubated at 37°C for 2 hours. The ligated and then Nco I-digested DNA was used to transform E . coli K12 RV308 (NRRL B-15624, deposited Sept. 28, 1983). E . coli K12 RV308 cells were made competent and transformed in substantial accordance with the procedure of Example 3. The transformation mixture was plated on L-agar containing 100 µg/ml ampicillin. The ampicillin-resistant transformants were tested for sensitivity to kanamycin, because plasmid pRC confers kanamycin resistance, but plasmid pTPA103 does not. The ampicillin-resistant, kanamycin-sensitive transformants were then used to prepare plasmid DNA, and the plasmid DNA was examined by restriction enzyme analysis to identify the E . coli K12 RV308/pTPA103 transformants. A restriction site and function map of plasmid pTPA103 is presented in Figure 12 of the accompanying drawings. Plasmid pTPA103 DNA was isolated from the E . coli K12 RV308/pTPA103 cells in substantial accordance with the procedure of Example 3. B. Construction of Intermediate Plasmid pBW25 About 1 µg of plasmid pTPA103 DNA in 1 µl of TE buffer was added to 2 µl of 10X Bgl II buffer and 16 µl of dH₂O. One µl (about 5 units) of restriction enzyme Bgl II was added to the solution of plasmid pTPA103 DNA, and the resulting reaction was incubated at 37°C for 2 hours. The Bgl II-digested plasmid pTPA103 DNA was precipitated with ethanol and resuspended in 5 µl of 10X Klenow buffer and 44 µl of dH₂O. One µl of Klenow enzyme (about 1 unit) was added to the solution of Bgl II-digested plasmid pTPA103 DNA, and the resulting reaction was incubated at 16°C for 2 hours. The Klenow-treated, Bgl II-digested plasmid pTPA103 DNA was precipitated with ethanol and resuspended in 3 µl of 10X ligase buffer and 22 µl of dH₂O. About 2 µl (0.2 µg) of unkinased Nde I linkers (New England Biolabs) of sequence: were added to the solution of Klenow-treated, Bgl II-digested plasmid pTPA103 DNA, together with 2 µl (about 1000 units) of T4 DNA ligase and 1 µl (about 2 units) of T4 RNA ligase, and the resulting ligation reaction was incubated at 4°C overnight. The ligated DNA constituted plasmid pTPA103der Nde I, which is substantially similar to pTPA103, except plasmid pTPA103der Nde I has an Nde I recognition sequence where plasmid pTPA103 has a Bgl II recognition sequence. The ligated DNA was used to transform E . coli K12 RV308 competent cells in substantial accordance with the procedure described in Example 3. The transformed cells were plated on L-agar containing ampicillin, and the E . coli K12 RV308/pTPA103der Nde I transformants were identified by restriction enzyme analysis of their plasmid DNA. Plasmid pTPA103der Nde I DNA was isolated from the transformants for use in subsequent constructions in substantial accordance with the procedure of Example 3. About 10 µg of plasmid pTPA103der Nde I DNA in 10 µl of TE buffer were added to 2 µl of 10X Ava II buffer (0.6 M NaCl; 60 mM Tris-HCl, pH = 8.0; 0.1 M MgCl₂; 60 mM 2-mercaptoethanol; and 1 mg/ml BSA) and 6 µl of dH₂O. Two µl (about 10 units) of restriction enzyme Ava II were added to the DNA, and the resulting reaction was incubated at 37°C for 2 hours. The Ava II-digested DNA was loaded onto a 1% agarose gel and electrophoresed until the ∼1.4 kb restriction fragment was separated from the other digestion products. The ∼1.4 kb Ava II restriction fragment of plasmid pTPA103der Nde I was isolated from the gel; about 2 µg of the desired fragment were obtained and suspended in 5 µl of TE buffer. Five µl of 10X Klenow buffer, 35 µl of dH₂O, and 5 µl (about 5 units) of Klenow enzyme were added to the solution of ∼1.4 kb Ava II restriction fragment, and the resulting reaction was incubated at 16°C for thirty minutes. The Klenow-treated DNA was precipitated with ethanol and resuspended in 3 µl of 10X ligase buffer and 14 µl of dH₂O. About 2 µl of Hpa I linkers of sequence: were kinased in substantial accordance with the procedure of part A above. About 10 µl of the kinased linkers were added to the solution of Klenow-treated, ∼1.4 kb Ava II restriction fragment of plasmid pTPA103der Nde I together with 2 µl (about 1000 units) of T4 DNA ligase and 1 µl (about 1 unit) of T4 RNA ligase, and the resulting reaction was incubated at 16°C overnight. The ligated DNA was extracted once with phenol, extracted twice with chloroform, precipitated with ethanol, and resuspended in 2 µl of 10X Eco RI buffer and 16 µl of dH₂O. Two µl (about 10 units) of restriction enzyme Eco RI were added to the solution of DNA, and the resulting reaction was incubated at 37°C for 2 hours. The Eco RI-digested DNA was extracted once with phenol, extracted twice with chloroform, precipitated with ethanol, and resuspended in 3 µl of 10X ligase buffer and 20 µl of dH₂O. The fragment, which is ∼770 bp in size and encodes the trp PO and the amino-terminus of TPA, thus prepared had one Eco RI-compatible end and one blunt end and was ligated into Eco RI- Sma I-digested plasmid pUC19 to form plasmid pUC19TPAFE as follows. About 2 µg of plasmid pUC19 (available from BRL) were dissolved in 2 µl of 10X Sma I buffer (0.2 M KCl; 60 mM Tris-HCl, pH = 8.0; 60 mM MgCl₂; 60 mM 2-mercaptoethanol; and 1 mg/ml BSA) and 16 µl of dH₂O. Two µl (about 10 units) of restriction enzyme Sma I were added to the solution of DNA, and the resulting reaction was incubated at 25°C for 2 hours. The Sma I-digested plasmid pUC19 DNA was precipitated with ethanol, collected by centrifugation, and resuspended in 2 µl of 10X Eco RI buffer and 16 µl of dH₂O. Two µl (about 10 units) of restriction enzyme Eco RI were added to the solution of Sma I-digested plasmid pUC19 DNA, and the resulting reaction was incubated at 37°C for 2 hours. The Eco RI- Sma I-digested plasmid pUC19 DNA was extracted once with phenol, extracted twice with chloroform, and resuspended in 5 µl of TE buffer. The Eco RI- Sma I-digested plasmid pUC19 DNA was added to the solution containing the ∼770 bp Eco RI-blunt end restriction fragment derived from plasmid pTPA103der Nde I. Two µl (about 1000 units) of T4 DNA ligase were added to the mixture of DNA, and the resulting reaction was incubated at 16°C overnight. The ligated DNA constituted the desired plasmid pUC19TPAFE. A restriction site and function map of plasmid pUC19TPAFE is presented in Figure 12 of the accompanying drawings. The multiple-cloning site of plasmid pUC19, which comprises the Eco RI and Sma I recognition sequences utilized in the construction of plasmid pUC19TPAFE, is located within the coding sequence for the lac Z α fragment. Expression of the lac Z α fragment in cells that contain the lac Z ΔM15 mutation, a mutation in the lac Z gene that encodes β-galactosidase, allows those cells to express a functional β-galactosidase molecule and thus allows those cells to hydrolyze X-Gal (5-bromo-4-chloro-3-indolyl-β-D-galactopyranoside), a colorless compound, to its indigo-colored hydrolysis product in the presence of the inducer IPTG (isopropylthiogalactoside). Insertion of DNA into the multiple-cloning site of plasmid pUC19 interrupts the coding sequence for the lac Z α fragment, and cells with the lac Z ΔM15 mutation that host such a plasmid are unable to hydrolyze X-Gal (such that the colonies are white, not indigo). The ligated DNA that constituted plasmid pUC19TPAFE was used to transform E . coli K12 RR1ΔM15 (NRRL B-15440, deposited May 27, 1983) cells made competent for transformation in substantial accordance with the procedure of Example 3. The transformed cells were plated on L-agar containing 100 µg/ml ampicillin; 40 µg/ml X-Gal; and 1 mM LRTG. Colonies that failed to exhibit the indigo color were subcultured and used to prepare plasmid DNA; the E . coli K12 RR1ΔM15/pUC19TPAFE transformants were identified by restriction enzyme analysis of their plasmid DNA. Plasmid pUC19TPAFE DNA was isolated from the E . coli K12 RRΔM15/pUC19TPAFE cells for use in subsequent constructions in substantial accordance with the procedure of Example 3. About 7 µg of plasmid pUC19TPAFE in 20 µl of TE buffer were added to 10 µl of 10X Hpa I buffer (0.2 M KCl; 0.1 M Tris-HCl, pH = 7.4; and 0.1 M MgCl₂) and 70 µl of dH₂O. Three µl (about 6 units) of restriction enzyme Hpa I were added to the solution of plasmid pUC19TPAFE DNA, and the resulting reaction was incubated at 37°C for 20 minutes; the short reaction period was designed to yield a partial Hpa I digest. The reaction was adjusted to 150 µl of 1X Bam HI buffer (150 mM NaCl; 10 mM Tris-HCl, pH = 8.0; and 10 mM MgCl₂; raising the salt concentration inactivates Hpa I). One µl (about 16 units) of restriction enzyme Bam HI were added to the solution of Hpa I (partial) digest of DNA, and the resulting reaction was incubated at 37°C for 90 minutes. The Bam HI- Hpa I (partial) digest of plasmid pUC19TPAFE DNA was concentrated by ethanol precipitation, loaded onto a 1.5% agarose gel, and the ∼3.42 kb Hpa I- Bam HI restriction fragment that comprises the replicon, β-lactamase gene, and all of the TPA-encoding DNA of plasmid pUCATPAFE was isolated from the gel in substantial accordance with the procedure of Example 4A. About 1 µg of the desired fragment was obtained and suspended in 20 µl of TE buffer. About 10 µg of plasmid pTPA103 in 10 µl of TE buffer were dissolved in 10 µl of 10X Sca I buffer (1.0 M NaCl; 60 mM Tris-HCl, pH = 7.4; 60 mM MgCl₂; 10 mM DTT; and 1 mg/ml BSA) and 80 µl of dH₂O. Three µl (about 18 units) of restriction enzyme Sca I were added to the solution of plasmid pTPA103 DNA, and the resulting reaction was incubated at 37°C for 90 minutes. The reaction volume was adjusted to 150 µl of 1X Bam HI buffer, and 1 µl (about 16 units) of restriction enzyme Bam HI was added to the mixture, which was then incubated at 37°C for 90 minutes. The DNA was precipitated with ethanol, collected by centrifugation, and resuspended in preparation for electrophoresis. The Sca I- Bam HI-digested plasmid pTPA103 DNA was loaded onto a 1.5% agarose gel and electrophoresed until the ∼1.015 kb Sca I- Bam HI restriction fragment was separated from the other digestion products. The ∼1.015 Sca I- Bam HI restriction fragment that comprises the TPA carboxy-terminus-encoding DNA of plasmid pTPA103 was isolated from the gel as described above; about 0.5 µl of the desired fragment were obtained and dissolved in 20 µl of dH₂O. About 2 µl of the ∼3.42 kb Bam HI- Hpa I restriction fragment of plasmid pUC19TPAFE were added to 2 µl of the ∼1.015 kb Sca I- Bam HI restriction fragment of plasmid pTPA103 together with 2 µl of 10X T4 DNA ligase buffer and 1 µl (about 1 Weiss unit; the ligase was obtained from Promega Biotec, 2800 S. Fish Hatchery Road, Madison, WI 53711) of T4 DNA ligase, and the resulting reaction was incubated at 16°C overnight. The ligated DNA constituted the desired plasmid pBW25. A restriction site and function map of plasmid pBW25 is presented in Figure 14 of the accompanying drawings. The ligated DNA was used to transform E . coli K12 JM105 (available from BRL) that were made competent for transformation in substantial accordance with the procedure of Example 3, except that 50 mM CaCl₂ was used in the procedure. The transformed cells were plated on Brain Heart Infusion agar (BHI) (Difco Laboratories, Detroit, MI) containing 100 µg/ml ampicillin, and the E . coli K12 JM105/pBW25 transformants were identified by restriction enzyme analysis of their plasmid DNA. Digestion of plasmid pBW25 with restriction enzyme Eco RI yields ∼3.38 kb and ∼1.08 kb restriction fragments. Plasmid pBW25 is prepared for use in subsequent constructions in substantial accordance with the procedure of Example 3. C. Site-Specific Mutagenesis of the TPA Coding Region and Construction of Plasmid pBW28 About 5 µg of plasmid pBW25 in 10 µl of dH₂O were added to 10 µl of 10X Hin dIII reaction buffer and 80 µl of dH₂O. One µl (about 20 units) of restriction enzyme Hin dIII was added to the solution of plasmid pBW25 DNA, and the resulting reaction was incubated at 37°C for 90 minutes. Three µl (about 24 units) of restriction enzyme Eco RI and 10 µl of 1M Tris-HCl, pH = 7.6, were added to the solution of Hin dIII-digested plasmid pBW25 DNA, and the resulting reaction was incubated at 37°C for 90 minutes. The Eco RI- Hin dIII-digested plasmid pBW25 DNA was concentrated by ethanol precipitation, loaded onto a 1.5% agarose gel, and electrophoresed until the ∼810 bp Eco RI- Hin dIII restriction fragment was separated from the other digestion products. About 0.5 µg of the ∼810 bp Eco RI- Hin dIII restriction fragment was isolated from the gel in substantial accordance with the procedure of Example 4A, prepared for ligation, and resuspended in 20 µl of dH₂O. About 4.5 µg of the replicative form (RF) of M13mp8 DNA (available from New England Biolabs) in 35 µl of dH₂O were added to 10 µl of 10X Hin dIII buffer and 55 µl of dH₂O. One µl (about 20 units) of restriction enzyme Hin dIII was added to the solution of M13mp8 DNA, and the resulting reaction was incubated at 37°C for 1 hour. Three µl (about 24 units) of restriction enzyme Eco RI and 10 µl of 1M Tris-HCl, pH = 7.6, were added to the solution of Hin dIII-digested M13mp8 DNA, and the resulting reaction was incubated at 37°C for 1 hour. The Hin dIII- Eco RI-digested M13mp8 DNA was collected by ethanol precipitation, resuspended in preparation for agarose gel electrophoresis, and the large restriction fragment isolated by gel electrophoresis as described above. About 1 µg of the large Eco RI- Hin dIII restriction fragment of M13mp8 was obtained and suspended in 20 µl of dH₂O. About 2 µl of the large Eco RI- Hin dIII restriction fragment of M13mp8, 2 µl of 10X T4 DNA ligase buffer, 12 µl of dH₂O and 1 µl (about 1 Weiss unit) of T4 DNA ligase were added to 3 µl of the ∼810 bp Eco RI- Hin dIII restriction fragment of plasmid pBW25, and the resulting ligation reaction was incubated at 16°C overnight. E . coli JM103 cells (available from BRL) were made competent and transfected with the ligation mix in substantial accordance with the procedure described in the BRL M13 Cloning/'Dideoxy' Sequencing Instruction Manual, except that the amount of DNA used per transfection was varied. Recombinant plaques were identified by insertional inactivation of the β-galactosidase α-fragment-encoding gene, which results in the loss of the ability to cleave X-Gal to its indigo-colored cleavage product. For screening purposes, each of six white plaques were picked into 2.5 ml of L broth, to which was added 0.4 ml of E . coli K12 JM103 in logarithmic growth phase, which had been cultured in minimal media stock to insure retention of the F episome that carries pro AB. The 2.5 ml plaque-containing cell suspensions were incubated in an air-shaker at 37°C for 8 hours. Cells from 1.5 ml aliquots were pelleted and RF DNA isolated in substantial accordance with the alkaline miniscreen procedure of Birnboim and Doly, 1979, Nuc. Acids Res. 7:1513. The remainder of each culture was stored at 4°C for stock. The desired phage, designated MP8BW26, contained the ∼810 bp Eco RI- Hin dIII restriction fragment of plasmid pBW25 ligated to the ∼7.2 kb Eco RI- Hin dIII restriction fragment of M13mp8. About fifty ml of log phase E . coli JM103 were infected with MP8BW26 and incubated in an air-shaker at 37°C for 18 hours. The infected cells were pelleted by low speed centrifugation, and single-stranded MP8BW26 DNA was prepared from the culture supernatant by scaling up the procedure given in the Instruction Manual. Single-stranded MP8BW26 was mutagenized in substantial accordance with the teaching of Adelman et al ., 1983, DNA 2(3): 183-193, except that the Klenow reaction was done at room temperature for 30 minutes, then at 37°C for 60 minutes, then at 10°C for 18 hours. In addition, the S1 treatment was done at 20°C, the salt concentration of the buffer was one-half that recommended by the manufacturer, and the M13 sequencing primer (BRL) was used. The synthetic oligodeoxyribonucleotide primer used to delete the coding sequence for amino acid residues 87 through 261 of native TPA was 5'-GGGAAGTGCTGTGAAATATCCACCTGCGGCCTGAGA-3'. The resulting mutagenesis mix was used to transfect E . coli K12 JM 103 in substantial accordance with the infection procedure described above. Desired mutants were identified by restriction enzyme analysis of RF DNA and by Maxam and Gilbert DNA sequencing. The desired mutant, which had the coding sequence for amino acid residues 87 through 261 of native TPA deleted, was designated MP8BW27. To construct plasmid pBW28, a variety of DNA fragments are needed. The first of these fragments was obtained by adding about 20 µg of RF MP8BW27 DNA in 20 µl of dH₂O to 10 µl of 10X Nde I buffer and 60 µl of dH₂O. Ten µl (about 50 units) of restriction enzyme Nde I were added to the mixture of MP8BW27 DNA, and the resulting reaction was incubated at 37°C for 2 hours. The Nde I-digested MP8BW27 DNA was precipitated with ethanol, collected by centrifugation, and resuspended in 10 µl of 10X Eco RI buffer and 90 µl of dH₂O. Ten µl (∼50 units) of restriction enzyme Eco RI were added to the solution of Nde I-digested plasmid MP8BW27 DNA, and the resulting reaction was incubated at 37°C for 2 hours. The Eco RI- Nde I-digested MP8BW27 DNA was electrophoresed on an agarose gel until the ∼560 bp Nde I-EcoRI restriction fragment, which contains the portion of TPA coding sequence that spans the site of deletion, was separated from the other digestion products. The ∼560 bp Nde I- Eco RI restriction fragment was isolated from the gel as described above; about 0.5 µg of the desired fragment was obtained and suspended in 20 µl of dH₂O. The second fragment needed to construct plasmid pBW28 is synthesized one strand at a time on an automated DNA synthesizer in substantial accordance with the procedure of Example 1. The two complementary strands, which will hybridize to form a double-stranded DNA segment with Xba I and Nde I overlaps, are kinased and annealed in substantial accordance with the procedure of Example 7A. The linker has the following structure: The third fragment needed to construct plasmid pBW28 was prepared by adding about 20 µg of plasmid pTPA103 in 20 µl of TE buffer to 10 µl of 10X Bam HI buffer and 60 µl of dH₂O. Ten µl (about 50 units) of restriction enzyme Bam HI were added to the solution of plasmid pTPA103 DNA, and the resulting reaction was incubated at 37°C for 2 hours. The Bam HI-digested plasmid pTPA103 DNA was precipitated with ethanol, collected by centrifugation, and resuspended in 10 µl of 10X Eco RI buffer and 80 µl of dH₂O. Ten µl (about 50 units) of restriction enzyme Eco RI were added to the solution of Bam HI-digested plasmid pTPA103 DNA, and the resulting reaction was incubated at 37°C for 2 hours. The Bam HI- Eco RI-digested plasmid pTPA103 DNA was loaded onto a 1.5% agarose gel and electrophoresed until the ∼689 bp Eco RI- Bam HI restriction fragment, which comprises the coding sequence for the carboxy-terminus of TPA, was separated from the other digestion products. About 0.5 µg of the ∼689 bp fragment was isolated from the gel as described above, and then resuspended in 10 µl of dH₂O. The final fragment necessary to construct plasmid pBW28 was isolated from plasmid pL110, a plasmid disclosed in European Patent Appln. No. 86306452.3 (Published on April 8, 1987 as EP A 0 217 529) . A restriction site and function map of plasmid pL110 is presented in Figure 12 of the accompanying drawings, and the construction of plasmid pL110 is disclosed below in part D of this Example. About 25 µg of plasmid pL110 in 25 µl of TE buffer were added to 10 µl of 10X Xba I buffer (0.5 M NaCl; 60 mM Tris-HCl, pH = 7.9; 60 mM MgCl₂; and 1 mg/ml BSA) and 55 µl of dH₂O. Ten µl (about 50 units) of restriction enzyme Xba I were added to the solution of plasmid pL110 DNA, and the resulting reaction was incubated at 37°C for 2 hours. The Xba I-digested plasmid pL110 DNA was precipitated with ethanol, collected by centrifugation, and resuspended in 10 µl of 10X Bam HI buffer and 89 µl of dH₂O. One µl (about 5 units) of restriction enzyme Bam HI was added to the solution of Xba I-digested plasmid pL110 DNA, and the resulting reaction was incubated at 37°C for 30 minutes to obtain a partial Bam HI digest. The Xba I - Bam HI (partial) digest of plasmid pL110 DNA was loaded onto a 1% agarose gel and electrophoresed until the ∼6.0 kb Xba I- Bam HI fragment was clearly separated from the other digestion products. The ∼6.0 kb restriction fragment was isolated from the gel as described above; about 0.5 µg of the ∼6.0 kb Xba I- Bam HI restriction fragment was obtained and suspended in about 40 µl of dH₂O. This ∼6.0 kb Xba I- Bam HI restriction fragment comprises all of plasmid pL110 except the EK-BGH-encoding DNA. To construct plasmid pBW28, the following fragments are mixed together: (1) about 0.1 µg (8 µl) of the ∼6.0 kb Bam HI- Xba I restriction fragment of plasmid pL110; (2) about 0.05 µg (2 µl) of the ∼560 bp Nde I- Eco RI restriction fragment of MP8BW27; (3) about 0.1 µg (2 µl) of the ∼689 bp Eco RI- Bam HI restriction fragment of plasmid pTPA103; and (4) about 0.02 µg (1 µl) of the ∼45 bp Xba I- Nde I synthetic linker. Two µl of 10X T4 DNA ligase buffer and 1 µl (about 1 Weiss unit) of T4 DNA ligase are added to the mixture of DNA, and the resulting ligation reaction is incubated at 4°C overnight. The ligated DNA constituted the desired plasmid pBW28. A restriction site and function map of plasmid pBW28 is presented in Figure 12 of the accompanying drawings. The ligated DNA was used to transform E . coli K12 MM294 (NRRL B-15625, deposited Sept. 28, 1983) made competent in substantial accordance with the procedure of Example 3, except that 50 mM CaCl₂ was used in the procedure. Due to the presence of the lambda pL promoter and the gene encoding the temperature-sensitive lambda pL repressor on plasmid pBW28, the transformation procedure and culturing of transformants were varied somewhat. The cells were not exposed to temperatures greater than 32°C during transformation and subsequent culturing. Part D of this Example relates more fully the procedures for handling plasmids that encode the lambda pL promoter and its temperature-sensitive repressor. The desired E . coli K12 MM294/pBW28 transformants were identified by their tetracycline-resistant, ampicillin-sensitive phenotype and by restriction enzyme analysis of their plasmid DNA. D. Construction of Plasmid pL110 Plasmid pL110 was constructed using plasmid pKC283 as starting material. E . coli K12 BE1201/pKC283 in lyophilized form is obtained from the NRRL under the accession number NRRL B-15830 (deposited Aug. 3, 1984). The lyophilized samples are decanted into tubes containing 10 ml of L broth and incubated two hours at 32°C at which time the cultures are made 50 µg/ml in ampicillin and then incubated at 32°C overnight. The E . coli K12 BE1201/pKC283 cells were cultured at 32°C, because plasmid pKC283 comprises the pL promoter and because E . coli K12 BE1201 cells comprise a temperature-sensitive cI repressor gene integrated into the cellular DNA. When cells that comprise a wild-type lambda pL repressor gene or when cells that do not comprise a lambda pL promoter are utilized in this plasmid isolation procedure, as described in subsequent Examples herein, the temperature of incubation is 37°C. A small portion of the overnight culture is placed on L-agar plates containing 50 µg/ml ampicillin in a manner so as to obtain a single colony isolate of E . coli K12 BE1201/pKC283. The single colony obtained was inoculated into 10 ml of L broth containing 50 µg/ml ampicillin and incubated overnight at 32°C with vigorous shaking. The 10 ml overnight culture was inoculated into 500 ml of L broth and incubated at 32°C with vigorous shaking until the culture reached stationary phase. Plasmid pKC283 DNA was then prepared from the cells in substantial accordance with the procedure of Example 3. About 1 mg of plasmid pKC283 was obtained and stored at 4°C in TE buffer at a concentration of about 1 µg/ml. A restriction site and function map of plasmid pKC283 is presented in Figure 12 of the accompanying drawings. Ten µl (about 10 µg) of the plasmid pKC283 DNA were mixed with 20 µl of 10X medium-salt restriction buffer (500 mM NaCl; 100 mM Tris-HCl, pH = 7.5; 100 mM MgCl₂; and 10 mM DTT), 20 µl of 1 mg/ml BSA, 5 µl of restriction enzyme Pvu II (about 25 units), and 145 µl of dH₂O, and the resulting reaction was incubated at 37°C for 2 hours. Restriction enzyme reactions described herein were routinely terminated by phenol and then chloroform extractions, which were followed by precipitation of the DNA, an ethanol wash, and resuspension of the DNA in TE buffer. After terminating the Pvu II digestion as described above, the Pvu II-digested plasmid pKC283 DNA was precipitated and then resuspended in 5 µl of TE buffer. About 600 picomoles (pM) of Xho I linkers (5'-CCTCGAGG-3') were kinased in a mixture containing 10 µl of 5X Kinase Buffer (300 mM Tris-HCl, pH = 7.8; 50 mM MgCl₂; and 25 mM DTT), 5 µl of 5 mM ATP, 24 µl of dH₂O, 0.5 µl of T4 polynucleotide kinase (about 2.5 units as defined by P-L Biochemicals), 5 µl of 1 mg/ml BSA, and 5 µl of 10 mM spermidine by incubating the mixture at 37°C for 30 minutes. About 12.5 µl of the kinased Xho I linkers were added to the 5 µl of Pvu II-digested plasmid pKC283 DNA, and then, 2.5 µl of 10X ligase buffer, 2.5 µl (about 2.5 units as defined by P-L Biochemicals) of T4 DNA ligase, 2.5 µl of 10 mM spermidine, and 12.5 µl of dH₂O were added to the DNA. The resulting ligation reaction was incubated at 4°C overnight. After the ligation reaction, the reaction mixture was adjusted to have the composition of high-salt buffer (0.1 M NaCl; 0.05 M Tris-HCl, pH 7.5; 10 mM MgCl₂; and 1 mM DTT). About 10 µl (100 units) of restriction enzyme Xho I were added to the mixture, and the resulting reaction was incubated at 37°C for 2 hours. The reaction was terminated, and the Xho I-digested DNA was precipitated, resuspended, and ligated as described above, except that no Xho I linkers were added to the ligation mixture. The ligated DNA constituted the desired plasmid pKC283PX. A restriction site and function map of plasmid pKC283PX is presented in Figure 12 of the accompanying drawings. E . coli K12 MO (λ⁺), available from the NRRL under the accession number NRRL B-15993, deposited August 14, 1985, comprises the wild-type lambda pL cI repressor gene, so that transcription from the lambda pL promoter does not occur in E . coli K12 MO (λ⁺) cells. Single colonies of E . coli K12 MO(λ⁺) are isolated, and a 10 ml overnight culture of the cells is prepared; no ampicillin is used in the growth media. Fifty µl of the overnight culture were used to inoculate 5 ml of L broth, which also contained 10 mM MgSO₄ and 10 mM MgCl₂. The culture was incubated at 37°C overnight with vigorous shaking. The following morning, the culture was diluted to 200 ml with L broth containing 10 mM MgSO₄ and 10 mM MgCl₂. The diluted culture was incubated at 37°C with vigorous shaking until the O.D.₅₉₀ was about 0.5, which indicated a cell density of about 1 x 10⁸ cells/ml. The culture was cooled for ten minutes in an ice-water bath, and the cells were then collected by centrifugation at 4000Xg for 10 minutes at 4°C. The cell pellet was resuspended in 100 ml of cold 10 mM NaCl and then immediately repelleted by centrifugation. The cell pellet was resuspended in 100 ml of 30 mM CaCl₂ and incubated on ice for 20 minutes. The cells were again collected by centrifugation and resuspended in 10 ml of 30 mM CaCl₂. A 0.5 ml aliquot of the cells was added to the ligated DNA prepared above; the DNA had been made 30 mM in CaCl₂. The cell-DNA mixture was incubated on ice for one hour, heat-shocked at 42°C for 90 seconds, and then chilled on ice for about two minutes. The cell-DNA mixture was diluted into 10 ml of L broth in 125 ml flasks and incubated at 37°C for one hour. Aliquots of 100 µl were plated on L-agar plates containing ampicillin and incubated at 37°C until colonies appeared. The colonies were individually cultured, and the plasmid DNA of the individual colonies was examined by restriction enzyme analysis and gel electrophoresis. Plasmid DNA isolation was performed on a smaller scale in substantial accordance with the procedure of Example 3, but the CsCl gradient step was omitted until the desired E . coli K12 MO(λ⁺)/pKC283PX transformants were identified. A restriction site and function map of plasmid pKC283PX is presented in Figure 12 of the accompanying drawings. Ten µg of plasmid pKC283PX DNA were dissolved in 20 µl of 10X high-salt buffer, 20 µl of 1 mg/ml BSA, 5 µl (about 50 units) of restriction enzyme Bgl II, 5 µl (about 50 units) of restriction enzyme Xho I, and 150 µl of dH₂O, and the resulting reaction was incubated at 37°C for two hours. The reaction was stopped; the Bgl II- Xho I digested DNA was precipitated, and the DNA was resuspended in 5 µl of TE buffer. A DNA linker with single-stranded DNA ends characteristic of Bgl II and Xho I restriction enzyme cleavage was synthesized using an automated DNA synthesizer as described in Example 1 and kinased as described in Example 7A. The DNA linker had the following structure: The linker and Bgl II- Xho I-digested plasmid pKC283PX were ligated in substantial accordance with the ligation procedure described above. The ligated DNA constituted the desired plasmid pKC283-L. A restriction site and function map of plasmid pKC283-L is presented in Figure 12 of the accompanying drawings. The plasmid pKC283-L DNA was used to transform E . coli K12 MO(λ⁺), and the resulting E . coli K12 MO(λ⁺)/pKC283-L transformants were identified by their ampicillin-resistant phenotype and by restriction enzyme analysis of their plasmid DNA. About 10 µg of plasmid pKC283-L DNA were dissolved in 20 µl 10X high-salt buffer, 20 µl of 1 mg/ml BSA, 5 µl (about 50 units) of restriction enzyme Xho I, and 155 µl of dH₂O, and the resulting reaction was incubated at 37°C for two hours. The Xho I-digested plasmid pKC283-L DNA was then precipitated and resuspended in 2 µl of 10X nick-translation buffer (0.5 M Tris-HCl, pH = 7.2; 0.1 M MgSO₄; and 1 mM DTT), 1 µl of a solution 2 mM in each of the deoxynucleotide triphosphates, 15 µl of dH₂O, 1 µl (about 6 units as defined by P-L Biochemicals) of Klenow enzyme, and 1 µl of 1 mg/ml BSA. The resulting reaction was incubated at 25°C for 30 minutes; the reaction was stopped by incubating the solution at 70°C for five minutes. Bam HI linkers (5'-CGGGATCCCG-3') were kinased and ligated to the Xho I-digested, Klenow-treated plasmid pKC283-L DNA in substantial accordance with the linker ligation procedures described above. After the ligation reaction, the DNA was digested with about 100 units of Bam HI for about 2 hours at 37°C in high-salt buffer. After the Bam HI digestion, the DNA was prepared for ligation, and the ∼5.9 kb Bam HI restriction fragment was circularized by ligation and transformed into E . coli K12 MO(λ⁺) in substantial accordance with the procedures described above. The E . coli K12 MO(λ⁺)/pKC283-LB transformants were identified, and then, plasmid pKC283-LB DNA was prepared from the transformants in substantial accordance with the procedure of Example 3. A restriction site and function map of plasmid pKC283-LB is presented in Figure 12 of the accompanying drawings. About 10 µg of plasmid pKC283PX were digested with restriction enzyme Sal I in high-salt buffer, treated with Klenow enzyme, and ligated to Eco RI linkers (5'-GAGGAATTCCTC-3') in substantial accordance with the procedures described above. After digestion with restriction enzyme Eco RI, which results in the excision of ∼2.1 kb of DNA, the ∼4.0 kb Eco RI restriction fragment was circularized by ligation to yield plasmid pKC283PRS. The ligated DNA was used to transform E. coli K12 MO(λ⁺), and after the E . coli K12 MO(λ⁺)/pKC283PRS transformants were identified, plasmid pKC283PRS DNA was prepared from the transformants in substantial accordance with the procedure of Example 3. A restriction site and function map of plasmid pKC283PRS is presented in Figure 12 of the accompanying drawings. About 10 µg of plasmid pKC283PRS were digested in 200 µl of high-salt buffer with about 50 units each of restriction enzymes Pst I and Sph I. After incubating the reaction at 37°C for about 2 hours, the reaction mixture was electrophoresed on a 0.6% low-melting-temperature agarose (FMC Corporation, Marine Colloids Division, Rockland, Maine 04841) gel. The gel was stained in a dilute solution of ethidium bromide, and the band of DNA constituting the ∼0.85 kb Pst I- Sph I restriction fragment, which was visualized with long-wave UV light, was cut from the gel in a small segment. The volume of the gel segment was determined by its weight and density, and an equal volume of 10 mM Tris-HCl, pH 7.6, was added to the tube containing the gel segment. The gel segment was then melted by incubation at 72°C. About 1 µg of the ∼0.85 kb Pst I- Sph I restriction fragment of plasmid pKC283PRS was obtained in a volume of about 100 µl after 2 phenol extractions and ethanol precipitation. In an analogous manner, plasmid pKC283-LB was digested with restriction enzymes Pst I and Sph I, and the resulting ∼3.0 kb restriction fragment was isolated by agarose gel electrophoresis and prepared for ligation. The ∼0.85 kb Pst I- Sph I restriction fragment of plasmid pKC283PRS was ligated to the ∼3.0 kb Pst I- Sph I restriction fragment of plasmid pKC283-LB. The ligated DNA constituted the desired plasmid pL32. A restriction site and function map of plasmid pL32 is presented in Figure 12 of the accompanying drawings. Plasmid pL32 was transformed into E . coli K12 MO(λ⁺) cells; plasmid pL32 DNA was prepared from the E . coli K12 MO(λ⁺)/pL32 transformants in substantial accordance with the procedure of Example 3. Analysis of the plasmid pL32 DNA demonstrated that more than one Eco RI linker attached to the Klenow-treated, Sal I ends of plasmid pKC283PX. The presence of more than one Eco RI linker does not affect the utility of plasmid pL32 or derivatives of plasmid pL32 and can be detected by the presence of an Xho I restriction site, which is generated whenever two of the Eco RI linkers are ligated together. Plasmid pCC101 is disclosed in Example 3 of U.S. Patent No. 4,745,069, issued May 17, 1988, incorporated herein by reference. A restriction site and function map of plasmid pCC101 is presented in Figure 12 of the accompanying drawings. To isolate the EK-BGH-encoding DNA, about 10 µg of plasmid pCC101 were digested in 200 µl of high-salt buffer containing about 50 units each of restriction enzymes Xba I and Bam HI. The digestion products were separated by agarose gel electrophoresis, and the ∼0.6 kb Xba I- Bam HI restriction fragment which encodes EK-BGH was isolated from the gel and prepared for ligation. Plasmid pL32 was also digested with restriction enzymes Xba I and Bam HI, and the ∼3.9 kb restriction fragment was isolated and prepared for ligation. The ∼3.9 kb Xba I- Bam HI restriction fragment of plasmid pL32 was ligated to the ∼0.6 kb Xba I- Bam HI restriction fragment of plasmid pCC101 to yield plasmid pL47. A restriction site and function map of plasmid pL47 is presented in Figure 12 of the accompanying drawings. Plasmid pL47 was transformed into E . coli K12 MO(λ⁺), and the E . coli K12 MO(λ⁺)/pL47 transformants were identified. Plasmid pL47 DNA was prepared from the transformants in substantial accordance with the procedures of Example 3. Plasmid pPR12 comprises the temperature-sensitive pL repressor gene cI857 and the plasmid pBR322 tetracycline resistance-conferring gene. Plasmid pPR12 is disclosed and claimed in U.S. Patent No. 4,436,815, issued March 13, 1984. A restriction site and function map of plasmid pPR12 is presented in Figure 12 of the accompanying drawings. About 10 µg of plasmid pPR12 were digested with about 50 units of restriction enzyme Eco RI in 200 µl of high-salt buffer at 37°C for two hours. The Eco RI-digested plasmid pPR12 DNA was precipitated and then treated with Klenow enzyme in substantial accordance with the procedure described above. After the Klenow reaction, the Eco RI-digested, Klenow-treated plasmid pPR12 DNA was recircularized by ligation, and the ligated DNA, which constituted the desired plasmid pPR12ΔR1, was used to transform E . coli K12 RV308 in substantial accordance with the procedure of Example 3; transformants were selected based on tetracycline (10 µg/ml) resistance. After the E . coli K12 RV308/pPR12ΔR1 transformants were identified, plasmid pPR12ΔR1 DNA was prepared from the transformants in substantial accordance with the procedure of Example 3. About 10 µg of plasmid pPR12ΔR1 were digested with about 50 units of restriction enzyme Ava I in 200 µl of medium-salt buffer at 37°C for 2 hours. The Ava I-digested plasmid pPR12ΔR1 DNA was precipitated and then treated with Klenow enzyme. After the Klenow reaction, the Ava I-digested, Klenow-treated plasmid pPR12ΔR1 DNA was ligated to Eco RI linkers (5'-GAGGAATTCCTC-3'), precipitated, resuspended in about 200 µl of high-salt buffer containing about 50 units of restriction enzyme Eco RI, and incubated at 37°C for about 2 hours. After the Eco RI digestion, the reaction mixture was loaded onto a 0.6% low-melting agarose gel, and the ∼5.1 kb Eco RI restriction fragment was purified from the gel and recircularized by ligation to yield the desired plasmid pPR12AR1, all in substantial accordance with the procedures described above. The plasmid pPR12AR1 DNA was transformed into E . coli K12 RV308 in substantial accordance with the procedure of Example 3; selection of transformants was based on tetracycline resistance. Plasmid pPR12AR1 DNA was prepared from the transformants in substantial accordance with the procedure of Example 3. A restriction site and function map of plasmid pPR12AR1 is presented in Figure 12 of the accompanying drawings. About 10 µg of plasmid pPR12AR1 DNA were suspended in about 200 µl of high-salt buffer containing about 50 units each of restriction enzymes Pst I and Eco RI, and the digestion reaction was incubated at 37°C for about 2 hours. The reaction mixture was then loaded onto a 1% agarose gel, and the ∼2.9 kb Pst I- Eco RI restriction fragment of plasmid pPR12AR1 was isolated and prepared for ligation in substantial accordance with the procedure of Example 3. About 10 µg of plasmid pL47 were digested with restriction enzymes Pst I and Bam HI in 200 µl of high-salt buffer at 37°C for two hours. The Pst I- Bam HI-digested DNA was loaded onto a 1% agarose gel, and the ∼2.7 kb Pst I- Bam HI restriction fragment that comprised the origin of replication and a portion of the ampicillin resistance-conferring gene was isolated and prepared for ligation in substantial accordance with the procedure of Example 3. In a separate reaction, about 10 µg of plasmid pL47 DNA were digested with restriction enzymes Eco RI and Bam HI in 200 µl of high-salt buffer at 37°C for two hours, and the ∼1.03 kb Eco RI- Bam HI restriction fragment that comprised the lambda pL transcription activating sequence, the E . coli lpp translation activating sequence, and the EK-BGH-encoding DNA was isolated and prepared for ligation. The ∼2.7 kb Pst I- Bam HI and ∼1.03 kb Eco RI- Bam HI restriction fragments of plasmid pL47 were ligated to the ∼2.9 kb Pst I- Eco RI restriction fragment of plasmid pPR12AR1 to construct plasmid pL110, and the ligated DNA was used to transform E . coli K12 RV308. Tetracycline resistance was used as the basis for selecting transformants. Two Pst I restriction enzyme recognition sites are present in the EK-BGH coding region that are not depicted in the restriction site and function maps presented in the accompanying drawings. A restriction site and function map of plasmid pL110 is presented in Figure 12 of the accompanying drawings. E. Final Construction of Plasmid pBW32 Approximately 10 µg of plasmid pSV2-β-globin DNA (NRRL B-15928) were dissolved in 10 µl of 10X Hin dIII reaction buffer, 5 µl (about 50 units) of restriction enzyme Hin dIII, and 85 µl of dH₂O, and the reaction was placed at 37°C for 2 hours. The reaction mixture was then made 0.15 M in LiCl, and after the addition of 2.5 volumes of ethanol and incubation in a dry-ice-ethanol bath, the DNA was pelleted by centrifugation. The DNA pellet was dissolved in 10 µl of 10X Bgl II buffer, 5 µl (∼50 units) of restriction enzyme Bgl II, and 85 µl of dH₂O, and the reaction was placed at 37°C for two hours. After the Bgl II digestion, the reaction mixture was loaded onto a 0.85% agarose gel, and the fragments were separated by electrophoresis. The gel was visualized using ethidium bromide and ultraviolet light, and the band containing the desired ∼4.2 kb Hin dIII- Bgl II fragment was excised from the gel as previously described and purified in substantial accordance with the procedure of Example 4A. The pellet was resuspended in 10 µl of dH₂O and constituted ∼5 µg of the desired ∼4.2 kb Hin dIII- Bgl II restriction fragment of plasmid pSV2-β-globin. The ∼2.0 kb Hin dIII- Bam HI restriction fragment of plasmid pTPA103 that encodes TPA was isolated from plasmid pTPA103 in substantial accordance with the foregoing teaching. About 5 µl of the ∼2.0 kb Hin dIII- Bam HI restriction fragment of plasmid pTPA103 were obtained, suspended in 10 µl of dH₂O, and stored at -20°C. Two µl of the ∼4.2 kb Bgl II- Hin dIII restriction fragment of plasmid pSV2-β-globin and 4 µl of the ∼2.0 kb Hin dIII- Bam HI fragment of plasmid pTPA103 were mixed together and then incubated with 2 µl of 10X ligase buffer, 11 µl of dH₂O, and 1 µl of T4 DNA ligase (about 500 units) at 4°C overnight. The ligated DNA constituted the desired plasmid pTPA301; a restriction site and function map of the plasmid is presented in Figure 12 of the accompanying drawings. The ligated DNA was used to transform E . coli K12 RR1 cells (NRRL B-15210) made competent for transformation in substantial accordance with the procedure of Example 3. Plasmid DNA was obtained from the E . coli K12 RR1/pTPA301 transformants in substantial accordance with the procedure of Example 3. Plasmid pSV2-dhfr comprises a dihydrofolate reductase (dhfr) gene useful for selection of transformed eukaryotic cells and amplification of DNA covalently linked to the dhfr gene. Ten µg of plasmid pSV2-dhfr (isolated from publically available E . coli K12 HB101/pSV2-dhfr, ATCC 37146) were mixed with 10 µl of 10X Pvu II buffer, 2 µl (about 20 units) of Pvu II restriction enzyme, and 88 µl of dH₂O, and the resulting reaction was incubated at 37°C for two hours. The reaction was terminated by phenol and chloroform extractions, and then, the Pvu II-digested plasmid pSV2-dhfr DNA was precipitated and collected by centrifugation. Bam HI linkers (5'-CGGATCCCG-3') were kinased and prepared for ligation by the following procedure. To 1 µg of linker in 5 µl of dH₂O was added: 10 µl of 5X kinase buffer, 5 µl of 5 mM ATP, 5 µl of BSA (1 mg/ml), 5 µl of 10 mM spermidine, 19 µl of dH₂O, and 1 µl of T4 polynucleotide kinase (about 10 units). This reaction was then incubated at 37°C for 60 minutes and stored at -20°C. Five µl (about 5 µg) of the Pvu II-digested plasmid pSV2-dhfr and 12 µl (about 0.25 µg) of the kinased Bam HI linkers were mixed and incubated with 11 µl of dH₂O, 2 µl 10X ligase buffer, and 1 µl (about 1000 units) of T4 DNA ligase at 16°C overnight. Ten µl of 10X Bam HI reaction buffer, 10 µl (about 50 units) of Bam HI restriction enzyme, and 48 µl of dH₂O were added to the ligation reaction mixture, which was then incubated at 37°C for 3 hours. The reaction was loaded onto a 1% agarose gel, and the desired ∼1.9 kb fragment, which comprises the dhfr gene, was isolated from the gel in substantial accordance with the procedure of Example 4A. All linker additions performed in these examples were routinely purified on an agarose gel to reduce the likelihood of multiple linker sequences in the final vector. Approximately 3 µg of fragment were obtained and suspended in 10 µl of TE buffer. Next, approximately 15 µl (about 1 µg) of plasmid pTPA301 were digested with Bam HI restriction enzyme as taught above. Because there is a unique Bam HI site in plasmid pTPA301, this Bam HI digestion generates linear plasmid pTPA301 DNA. The Bam HI-digested plasmid pTPA301 was precipitated with ethanol and resuspended in 94 µl of dH₂O and phosphatased using 1 µl of calf-intestinal alkaline phosphatase, and 5 µl of 1 M Tris-HCl, pH = 9.0, at 65°C for 45 min. The DNA was extracted with phenol:chloroform, then extracted with chloroform:isoamyl alcohol, ethanol precipitated, and resuspended in 20 µl dH₂O. Ten µl (about 0.25 µg) of phosphatased plasmid pTPA301 were added to 5 µl of the ∼1.9 kb Bam HI dhfr-gene-containing restriction fragment (about 1.5 µg), 3 µl of 10X ligase buffer, 3 µl (about 1500 units) of T4 DNA ligase, and 9 µl dH₂O. This ligation reaction was incubated at 15°C overnight; the ligated DNA constituted the desired plasmid pTPA303 DNA. Plasmid pTPA303 was used to transform E . coli K12 RR1 (NRRL B-15210), and the resulting E . coli K12 RR1/pTPA303 transformants were identified by their ampicillin-resistant phenotype and by restriction enzyme analysis of their plasmid DNA. Plasmid pTPA303 was isolated from the transformants in substantial accordance with the procedure of Example 3. To isolate the ∼2.7 kb Eco RI- Bgl II restriction fragment that encodes the pBR322 replicon and β-lactamase gene from plasmid pTPA301, about 10 µg of plasmid pTPA301 are digested to completion in 400 µl total reaction volume with 20 units Bgl II restriction enzyme in 1X Bgl II buffer at 37°C. After the Bgl II digestion, the Tris-HCl concentration is adjusted to 110 mM, and 20 units of Eco RI restriction enzyme are added to the Bgl II-digested DNA at 37°C. After the Eco RI digestion, the Eco RI- Bgl II-digested DNA is loaded onto a 1% agarose gel and electrophoresed until the ∼2.7 kb Eco RI- Bgl II restriction fragment is separated from the other digestion products, and then, the ∼2.7 kb fragment is isolated and prepared for ligation. To isolate a restriction fragment that comprises the dhfr gene, plasmid pTPA303 was double-digested with Hin dIII and Eco RI restriction enzymes, and the ∼2340 bp Eco RI- Hin dIII restriction fragment that comprises the dhfr gene was isolated and recovered. To isolate the ∼2.0 kb Hin dIII- Sst I restriction fragment of plasmid pTPA303 that comprises the coding region for the carboxy-terminus of TPA and the SV40 promoter, plasmid pTPA303 was double digested with Hin dIII and Sst I restriction enzymes in 1X Hin dIII buffer. The ∼1.7 kb fragment was isolated from the gel and prepared for ligation. To isolate the ∼680 bp Xho II (compatible for ligation with the Bgl II overlap)- Sst I restriction fragment of plasmid pBW28 that comprises the coding region for the amino terminus of modified TPA, about 10 µg of plasmid pBW28 were digested with Xho II enzyme to completion in 1X Xho II buffer (0.1 M Tris-HCl, pH = 8.0; 0.1 M MgCl₂; 0.1% Triton X-100; and 1 mg/ml BSA). The Xho II-digested DNA was recovered by ethanol precipitation and subsequently digested to completion with Sst I enzyme. The Xho II- Sst I-digested DNA was loaded onto an acrylamide gel, and the desired fragment was isolated from the gel and prepared for ligation in substantial accordance with the procedure of Example 4A. About 0.1 µg of each of the above fragments: (1) the ∼2.7 kb Eco RI- Bgl II restriction fragment of plasmid pTPA301; (2) the ∼2.34 kb Eco RI- Hin dIII restriction fragment of plasmid pTPA303; (3) the ∼1.7 kb Sst I- Hin dIII restriction fragment of plasmid pTPA303; and (4) the ∼0.68 kb Sst I- Xho II restriction fragment of plasmid pBW28 were ligated together to form plasmid pBW32. The ligation mix was used to transform E . coli K12 MM294 as taught in Example 3, except that 50 mM CaCl₂ was used in the procedure. Transformants were identified by their ampicillin-resistant phenotype and by restriction analysis of their plasmid DNA. Plasmid pBW32 DNA was obtained from the E . coli K12 MM294/pBW32 transformants in substantial accordance with the procedure of Example 3. A restriction site and function map of plasmid pBW32 is presented in Figure 12 of the accompanying drawings. Example 12 Construction of Plasmids pLPChd1, pLPChd2, pLPChd1(GT), pLPChd2(GT), pLPCdhfr1, pLPCdhfr2, pLPCdhfr1(GT) and pLPCdhfr2(GT) A. Construction of Plasmids pLPChd1 and pLPChd2 About 20 µg of plasmid pBW32 (Example 11) in 20 µl of TE buffer were added to 10 µl of 10X Bam HI buffer and 60 µl of dH₂O. Ten µl (about 50 units) of restriction enzyme Bam HI were added to the solution of plasmid pBW32 DNA, and the resulting reaction was incubated at 37°C for two hours. The Bam HI-digested plasmid pBW32 DNA was precipitated with ethanol, collected by centrifugation, and resuspended in 5 µl of 10X Klenow buffer, 45 µl of dH₂O, and 2 µl (about 100 units) of Klenow enzyme. The reaction was incubated at 16°C for 30 minutes; then, the reaction mixture was loaded onto a 1% agarose gel and electrophoresed until the digestion products were clearly separated. The ∼1.9 kb Klenow-treated, Bam HI restriction fragment of plasmid pBW32 that comprises the dhfr gene was isolated from the gel and prepared for ligation in substantial accordance with the procedure of Example 4A. About 4 µg of the desired fragment were obtained and suspended in 5 µl of TE buffer. About 200 µg of plasmid pLPChyg1 (Example 10) in 100 µl of TE buffer were added to 15 µl of 10X Eco RI buffer and 30 µl of dH₂O. About 5 µl (about 50 units) of restriction enzyme Eco RI were added to the solution of plasmid pLPChyg1 DNA, and the resulting reaction was incubated at 37°C for about 10 minutes. The short reaction time was calculated to produce a partial Eco RI digestion. Plasmid pLPChyg1 has two Eco RI restriction sites, one of which is within the coding sequence of the hygromycin resistance-conferring (HmR) gene, and it was desired to insert the ∼1.9 kb dhfr-gene-containing restriction fragment from plasmid pBW32 into the Eco RI site of plasmid pLPChyg1 that is not in the HmR gene. The Eco RI (partial) digest of plasmid pLPChyg1 DNA was loaded onto an agarose gel and electrophoresed until the singly-cut plasmid pLPChyg1 DNA was separated from uncut plasmid DNA and the other digestion products. The singly-cut DNA was isolated from the gel and prepared for ligation in substantial accordance with the procedure of Example 4A. About 2 µl of the singly- Eco RI-cut plasmid pLPChyg1 were obtained and suspended in 25 µl of TE buffer. To this sample, 5 µl (about 25 units) of Klenow enzyme, 5 µl of 10X Klenow buffer, and 40 µl of dH₂O were added, and the resulting reaction was incubated at 16°C for 60 minutes. The Klenow-treated, Eco RI (partial) digest of plasmid pLPChyg1 DNA was then extracted twice with phenol and then once with chloroform, precipitated with ethanol, and resuspended in 25 µl of TE buffer. About 5 µl of the ∼1.9 kb Klenow-treated Bam HI restriction fragment of plasmid pBW32 and about 5 µl of the singly- Eco RI-cut plasmid pLPChyg1 DNA were mixed together, and 1 µl of 10X ligase buffer, 5 µl of dH₂O, 1 µl (about 500 units) of T4 DNA ligase, and 1 µl (about 2 units) of T4 RNA ligase were added to the mixture of DNA, and the resulting reaction was incubated at 16°C overnight. The ligated DNA constituted the desired plasmids pLPChd1 and pLPChd2, which differ only with respect to the orientation of the ∼1.9 kb fragment that comprises the dhfr gene. The ligated DNA was used to transform E . coli K12 HB101 cells made competent for transformation in substantial accordance with the procedure of Example 3. The transformed cells were plated onto L-agar containing 100 µg/ml ampicillin, and the ampicillin-resistant transformants were analyzed by restriction enzyme analysis of their plasmid DNA to identify the E . coli K12 HB101/pLPChd1 and E . coli K12 HB101/pLPChd2 transformants. Plasmid pLPChd1 and plasmid pLPChd2 DNA were isolated from the appropriate transformants in substantial accordance with the procedure of Example 3. Plasmids pLPChd3 and pLPChd4 are similar in structure to plasmids pLPChd1 and pLPChd2. Plasmids pLPChd3 and pLPChd4 are constructed in substantial accordance with the procedure used to construct plasmids pLPChd1 and pLPChd2, except plasmid pLPChyg2 is used as starting material in the procedure rather than plasmid pLPChyg1. B. Construction of Plasmids pLPChd1(GT), pLPChd2(GT), pLPChd3(GT), and pLPChd4(GT) A poly-GT element is inserted into the unique Xho I site, between the BK enhancer and the Ad2 late promoter, in plasmid pLPChd1, pLPChd2, pLPChd3 or pLPChd4 in substantial accordance with the procedure of Example 4B.1. The Xho I-digested poly-GT element and Xho I-digested plasmid pLPChd1, pLPChd2, pLPChd3 or pLPChd4 DNA are ligated and the ligated DNA constitutes the desired plasmids pLPChd1(GT), pLPChd2(GT), pLPChd3(GT) and pLPChd4(GT), respectively. A restriction site and function map of plasmid pLPChd1(GT) is presented in Figure 13 of the accompanying drawings. C. Construction of Plasmids pLPCdhfr1 and pLPCdhfr2 About 100 µg of plasmid pBW32 in 100 µl of TE buffer were added to 15 µl of 10X Bam HI buffer and 25 µl of dH₂O. Ten µl (about 25 units) of restriction enzyme Bam HI were added to the solution of plasmid pBW32 DNA, and the resulting reaction was incubated at 37°C for 2 hours. The Bam HI-digested plasmid pBW32 DNA was treated with Klenow enzyme in substantial accordance with the procedure in Example 10A. The blunt-ended fragment was precipitated with ethanol, resuspended in 10 µl of TE buffer, loaded onto a 1% agarose gel, and electrophoresed until the ∼1.9 kb Bam HI restriction fragment that comprises the dhfr gene was separated from the other digestion products. The ∼1.9 kb restriction fragment was then isolated from the gel and prepared for ligation in substantial accordance with the procedure of Example 4A; about 10 µg of the desired fragment were obtained and suspended in 50 µl of TE buffer. About 5 µl of Nde - Stu I-digested plasmid pLPC DNA, as prepared in Example 10, were added to 5 µl of the Klenow-treated, ∼1.9 kb Bam HI restriction fragment of plasmid pBW32, 1.5 µl of 10X ligase buffer, 1 µl (about 1000 units) of T4 DNA ligase, 1 µl (about 2 units) of T4 RNA ligase, and 1.5 µl of dH₂O. The resulting ligation reaction was incubated at 16°C overnight; the ligated DNA constituted the desired plasmids pLPCdhfr1 and pLPCdhfr2, which differ only with respect to the orientation of the ∼1.9 kb fragment that contains the dhfr gene. The ligated DNA was used to transform E . coli K12 HB101 in substantial accordance with the procedure of Example 3. The transformed cells were plated onto L-agar containing ampicillin, and the ampicillin-resistant E . coli K12 HB101/pLPCdhfr1 and E . coli K12 HB101/pLPCdhfr2 transformants were identified by restriction enzyme analysis of their plasmid DNA. D. Construction of Plasmids pLPCdhfr1(GT) and pLPCdhfr2(GT) A poly-GT element is inserted into the unique Xho I site in pLPCdhfr1 or pLPCdhfr2, between the BK enhancer and the Ad2 late promoter, in substantial accordance with the procedure of Example 4B.1. The Xho I-digested poly-GT element and Xho I-digested plasmid pLPCdhfr1 or pLPCdhfr2 DNA are ligated and the ligated DNA constitutes the desired plasmids pLPCdhfr1(GT) and pLPCdhfr2(GT), respectively. Example 13 Construction of Plasmids phd and phd(GT) A. Construction of Plasmid phd To construct plasmid phd, it was necessary to prepare the plasmid pLPChd1 DNA (Example 12), used as starting material in the construction of plasmid phd, from E . coli host cells that lack an adenine methylase, such as that encoded by the dam gene, the product of which methylates the adenine residue in the sequence 5'-GATC-3'. E . coli K12 GM48 (NRRL B-15725, deposited November 23, 1983) lacks a functional dam methylase and so is a suitable host to use for the purpose of preparing plasmid pLPChd1 DNA for use as starting material in the construction of plasmid phd. E . coli K12 GM48 cells were cultured and made competent for transformation, and plasmid pLPChd1 was used to transform the E . coli K12 GM48 cells in substantial accordance with the procedure of Example 3. The transformed cells were plated on L-agar containing ampicillin, and once the ampicillin-resistant, E . coli K12 GM48/pLPChd1 transformants had formed colonies, one such colony was used to prepare plasmid pLPChd1 DNA in substantial accordance with the procedure of Example 3. About 1 mg of plasmid pLPChd1 DNA was obtained and suspended in about 1 ml of TE buffer. About 2 µg of plasmid pLPChd1 DNA in 2 µl of TE buffer were added to 2 µl of 10X Bcl I buffer (750 mM KCl; 60 mM Tris-HCl, pH = 7.4; 100 mM MgCl₂; 10 mM DTT and 1 mg/ml BSA) and 14 µl of dH₂O. Two µl (about 10 units) of restriction enzyme Bcl I were added to the solution of plasmid pLPChd1 DNA, and the resulting reaction was incubated at 50°C for two hours. The reaction was stopped by extracting the mixture once with phenol and twice with chloroform. About 1 µl of the Bcl I-digested plasmid pLPChd1 DNA was added to 1 µl of 10X T4 DNA ligase buffer, 8 µl of dH₂O and 1 µl (about 500 units) of T4 DNA ligase. The ligation reaction was incubated at 16°C overnight, and the ligated DNA constituted the desired plasmid phd. Plasmid phd results from the deletion of the extra Bcl I linkers that attached during the construction of plasmid pLPcat and the two adjacent Bcl I restriction fragments of a total size of about 1.45 kb from plasmid pLPChd1. Plasmid phd facilitates the expression of any DNA sequence from the BK virus enhancer-adenovirus late promoter as described herein, because the DNA to be expressed can be readily inserted in the correct position for expression at the single Bcl I site on plasmid phd. The ligated DNA was used to transform E . coli K12 GM48 in substantial accordance with the procedure of Example 3. The transformed cells were plated on L-agar containing ampicillin, and the ampicillin-resistant E . coli K12 GM48/phd transformants were identified by restriction enzyme analysis of their plasmid DNA. Plasmids analogous to plasmid phd can be constructed in substantial accordance with the foregoing procedure for constructing plasmid phd using any of plasmids pLPChd2, pLPChd3, or pLPChd4 as starting material rather than plasmid pLPChd1. These analogous plasmids differ from plasmid phd only with respect to the orientation of the hygromycin resistance-conferring and/or dhfr genes. B. Construction of Plasmid phd(GT) 1. Construction using Plasmid phd A poly-GT element is inserted into the unique Xho I site in phd, between the BK enhancer and the Ad2 late promoter, in substantial accordance with the procedure of Example 4B.1. The Xho I-digested poly-GT element and Xho I-digested plasmid phd DNA are ligated and the ligated DNA constitutes the desired plasmid phd(GT). A restriction site and function map of plasmid phd(GT) is presented in Figure 14 of the accompanying drawings. Plasmid phd(GT) facilitates the expression of any DNA sequence from the poly-GT - BK virus enhancer - adenovirus late promoter of the present invention, because the DNA to be expressed can be readily inserted in the correct position for expression at the single Bcl I site on plasmid phd(GT). In the presence of an immediate-early gene product of a large DNA virus, such as the E1A gene product, the plasmid phd(GT) provides an improved expression vector for any inserted DNA sequence. The ligated DNA is used to transform E . coli K12 GM48 and E . coli K12 GM48/phd(GT) transformants are selected, all in substantial accordance with the procedures of part A above. 2. Construction Using Plasmid pLPChd1(GT), pLPChd2(GT), pLPChd3(GT) or pLPChd4(GT) Alternatively, plasmid phd(GT) is prepared in substantial accordance with the procedure of part A of this Example, except that plasmid pLPChd1(GT), pLPChd2(GT), pLPChd3(GT) or pLPChd4(GT) (as described in Example 12B) is used as starting material, instead of plasmid pLPChd1, pLPChd2, pLPChd3 or pLPChd4. Example 14 Construction of Plasmids pLPCE1A and pLPCE1A(GT) A. Construction of Plasmid pLPCE1A To isolate the E1A gene of adenovirus 2 DNA, about 20 µg of adenovirus 2 DNA (from BRL) were dissolved in 10 µl of 10X Bal I buffer (100 mM Tris-HCl, pH = 7.6; 120 mM MgCl₂; 100 mM 2-mercaptoethanol; and 1 mg/ml BSA) and 80 µl of dH₂O. Ten µl (about 20 units) of restriction enzyme Bal I were added to the solution of adenovirus 2 DNA, and the resulting reaction was incubated at 37°C for two hours. The Bal I-digested DNA was loaded onto a 1% agarose gel and electrophoresed until the ∼1.8 kb restriction fragment that comprises the E1A gene was separated from the other digestion products. The ∼1.8 kb fragment was isolated from the gel and prepared for ligation in substantial accordance with the procedure of Example 4A. About 3 µg of the desired fragment was obtained and suspended in 20 µl of TE buffer. About 5 µg of plasmid pLPC DNA in 5 µl of TE buffer were added to 2 µl of 10X Stu I buffer and 11 µl of dH₂O. Two µl (about 10 units) of restriction enzyme Stu I were added to the solution of plasmid pLPC DNA, and the resulting reaction was incubated at 37°C for 2 hours. The Stu I-digested plasmid pLPC DNA was precipitated with ethanol and resuspended in 2 µl of 10X Nde I buffer and 16 µl of dH₂O. Two µl (about 10 units) of restriction enzyme Nde I were added to the solution of Stu I-digested plasmid pLPC DNA, and the resulting reaction was incubated at 37°C for 2 hours. The Nde I- Stu I-digested plasmid pLPC DNA was precipitated with ethanol and resuspended in 5 µl of 10X Klenow buffer and 42 µl of H₂O. Three µl (about 6 units) of Klenow enzyme were added to the solution of DNA, and the resulting reaction was incubated at 37°C for 30 minutes. The reaction mixture was then loaded onto a 1% agarose gel and electrophoresed until the ∼5.82 kb, Klenow-treated, Nde I- Stu I restriction fragment was clearly separated from the other reaction products. The fragment was isolated from the gel and prepared for ligation in substantial accordance with the procedure of Example 4A. About 2 µg of the ∼5.82 kb, Klenow-treated, Nde I- Stu I restriction fragment of plasmid pLPC were obtained and suspended in 25 µl of TE buffer. About 9 µl of the ∼1.8 kb Bal I restriction fragment of adenovirus 2 that encodes the E1A gene and 3 µl of the ∼5.82 kb, Klenow-treated, Nde I- Stu I restriction fragment of plasmid pLPC were added to 2 µl of 10X ligase buffer and 4 µl of dH₂O. One µl (about 500 units) of T4 DNA ligase and 1 µl (about 2 units) of T4 RNA ligase were added to the solution of DNA, and the resulting reaction was incubated at 16°C overnight. The ligated DNA constituted the desired plasmids pLPCE1A and pLPCE1A1, which differ with respect to the orientation of the E1A gene and possibly differ with respect to the expression-enhancing effect the BK enhancer has on the E1A gene on the plasmid. Because the E1A promoter is located closer to the BK enhancer on plasmid pLPCE1A than plasmid pLPCE1A1, E1A expression may be higher when plasmid pLPCE1A is used as opposed to plasmid pLPCE1A1. The ligated DNA was used to transform E . coli K12 HB101 in substantial accordance with the procedure of Example 3. The transformed cells were plated on L-agar containing ampicillin, and the ampicillin-resistant transformants were screened by restriction enzyme analysis of their plasmid DNA to identify the E . coli K12 HB101/pLPCE1A and E . coli K12 HB101/pLPCE1A1 transformants. Plasmid DNA was obtained from the transformants for use in later experiments in substantial accordance with the procedure of Example 3. B. Construction of Plasmids pLPCE1A(GT) and pLPCE1A1(GT) A poly-GT element is inserted into the unique Xho I site in plasmid pLPCE1A or pLPCE1A1, between the BK enhancer and the Ad2 late promoter, in substantial accordance with the procedure of Example 4B.1. The Xho I-digested poly-GT element and Xho I-digested plasmid pLPCE1A or pLPCE1A1 DNA are ligated, and the ligated DNA constitutes the desired plasmids pLPCE1A(GT) and pLPCE1A1(GT), respectively. A restriction site and function map of plasmid pLPCE1A(GT) is presented in Figure 15 of the accompanying drawings. Example 15 Construction of Plasmids pBLT and pBLT(GT) A. Construction of Plasmid pBLT About 1 µg of plasmid pBW32 DNA (Figure 12, Example ii) in 1 µl of TE buffer was added to 2 µl of 10X Bam HI buffer and 15 µl of dH₂O. About 2 µl (about 10 units) of restriction enzyme Bam HI were added to the solution of plasmid pBW32 DNA, and the resulting reaction was incubated to 37°C for 2 hours. The reaction was stopped by first extracting the reaction mixture with phenol and then extracting the reaction mixture twice with chloroform. About 1 µl of the Bam HI-digested plasmid pBW32 DNA was added to 1 µl of 10X ligase buffer and 8 µl of dH₂O, and after 1 µl (about 500 units) of T4 DNA ligase was added to the solution of DNA, the resulting reaction was incubated at 16°C overnight. The ligated DNA constituted the desired plasmid pBW32del, which is about 5.6 kb in size and comprises a single Hin dIII restriction site. The ligated DNA was used to transform E . coli K12 HB101 in substantial accordance with the procedure of Example 3. The desired E . coli K12 HB101/pBW32del transformants were identified by their ampicillin-resistant phenotype and by restriction enzyme analysis of their plasmid DNA. Plasmid pBW32del DNA was obtained from the transformants for use in subsequent constructions in substantial accordance with the procedure of Example 3. About 1 µg of plasmid pBW32del in 1 µl of TE buffer was added to 2 µl of 10X Hin dIII buffer and 15 µl of dH₂O. Two µl (about 10 units) of restriction enzyme Hin dIII were added to the solution of plasmid pBW32del DNA, and the resulting reaction was incubated at 37°C for 2 hours. The sample was diluted to 100 µl with TE buffer and treated with calf-intestinal alkaline phosphatase in substantial accordance with the procedure described in Example 4B. The reaction was extracted twice with phenol then once with chloroform. The Hin dIII-digested plasmid pBW32del DNA was then precipitated with ethanol and resuspended in 10 µl of dH₂O. Plasmid pBal8cat (Example 18) was digested with restriction enzyme Hin dIII, and the ∼0.65kb Hin dIII restriction fragment that comprises the modified BK enhancer - adenovirus 2 late promoter cassette was isolated and prepared for ligation in substantial accordance with the procedure of Example 6A. About 0.1 µg of the ∼0.65 kb Hin dIII restriction fragment of plasmid pBal8cat in 5 µl of TE buffer was added to 3 µl of the solution of Hin dIII-digested plasmid pBW32del. One µl (about 500 units) of T4 DNA ligase and 1 µl of 10X ligase buffer was added to the mixture of DNA, and the resulting reaction was incubated at 16°C overnight. The ligated DNA constituted the desired plasmid pBLT. The ligated DNA was used to transform E . coli K12 HB101 in substantial accordance with the procedure of Example 3. The transformed cells were plated on L-agar containing ampicillin, and the ampicillin-resistant E . coli K12 HB101/pBLT transformants were identified by restriction enzyme analysis of their plasmid DNA. Because the ∼0.65 kb Hin dIII restriction fragment could insert into Hin dIII-digested plasmid pBW32del in either one of two orientations, only one of which yields plasmid pBLT, the orientation of the ∼0.65 kb Hin dIII restriction fragment had to be determined to identify the E . coli K12 HB101/pBLT transformants. Plasmid pBLT DNA was prepared from the transformants for use in subsequent constructions in substantial accordance with the procedure of Example 3. B. Construction of Plasmid pBLT(GT) A poly-GT element is inserted into the unique Bam HI site in plasmid pBLT, between the BK enhancer and the Ad2 late promoter, in substantial accordance with the procedure of Example 19. The poly-GT element and Bam HI-digested plasmid pBLT DNA are ligated and the ligated DNA constitutes the desired plasmid pBLT(GT). A restriction site and function map of plasmid pBLT(GT) is presented in Figure 16 of the accompanying drawings. Example 16 Construction of Plasmids pBLThyg1, pBLThyg2, pBLThyg1(GT), pBLThyg2(GT), pBLTdhfr1, pBLTdhfr2, pBLTdhfr1(GT), and pBLTdhfr2(GT) A. Construction of Plasmids pBLThyg1 and pBLThyg2 About 4 µg of plasmid pBLT DNA (Example 15A) in 4 µl of TE buffer were added to 2 µl of 10X Bam HI buffer and 12 µl of dH₂O. Two µl (about 10 units) of restriction enzyme Bam HI were added to the solution of plasmid pBLT DNA, and the resulting reaction was incubated at 37°C for 2 hours. The reaction was stopped by extracting the reaction mixture first with phenol and then with chloroform. The Bam HI-digested plasmid pBLT DNA was then precipitated with ethanol and resuspended in 2 µl of TE buffer. About 10 µg of plasmid pSV2hyg in 10 µl of TE buffer were added to 10 µl of 10X Bam HI buffer and 75 µl of dH₂O. Five µl (about 25 units) of restriction enzyme Bam HI were added to the solution of plasmid pSV2hyg DNA, and the resulting reaction was incubated at 37°C for 2 hours. The Bam HI-digested plasmid pSV2hyg DNA was precipiated with ethanol, resuspended in 10 µl of TE buffer, loaded onto a 1% agarose gel, and electrophoresed until the ∼2.5 kb Bam HI restriction fragment that comprises the hygromycin resistance conferring gene was separated from the other digestion products. The ∼2.5 kb restriction fragment was then isolated from the gel and prepared for ligation in substantial accordance with the procedure of Example 4A; about 2 µg of the desired fragment were obtained and suspended in 10 µl of TE buffer. About 2 µl of the Bam HI-digested plasmid pBLT DNA and 1 µl of the ∼2.5 kb Bam HI restriction fragment of plasmid pSV2hyg were added to 1 µl of 10X ligase buffer, 5 µl of dH₂O, and 1 µl (about 500 units) of T4 DNA ligase, and the resulting reaction was incubated at 16°C overnight. The ligated DNA constituted the desired plasmids pBLThyg1 and pBLThyg2. Plasmids pBLThyg1 and pBLThyg2 differ only with respect to the orientation of the ∼2.5 kb Bam HI restriction fragment that encodes the hygromycin resistance-conferring gene. The ligated DNA was used to transform E . coli K12 HB101 in substantial accordance with the procedure of Example 3. The transformed cells were plated onto L-agar containing ampicillin, and the ampicillin-resistant E . coli K12 HB101/pBLThyg1 and E . coli K12 HB101/pBLThyg2 transformants were identified by restriction enzyme analysis of their plasmid DNA. B. Construction of Plasmids pBLThyg1(GT) and pBThyg2(GT) A poly-GT element is inserted into the Bam HI site in plasmid pBLThyg1 or pBLThyg2, between the tPA and hygromycin cistrons following partial digestion of the plasmids with Bam HI. The partial digestion was carried out in substantial accordance with the general methodology for Bam HI partial digestion as described in Example 11C. The poly-GT element and partially Bam HI-digested linear plasmid pBLThyg1 or pBLThyg2 DNA are ligated and the ligated DNA constitutes the desired plasmids pBLThyg1(GT) and pBLThyg2(GT), respectively. A restriction site and function map of plasmid pBLThyg1(GT) is presented in Figure 17 of the accompanying drawings. C. Construction of Plasmids pBLTdhfr1 and pBLTdhfr2 About 100 µg of plasmid pBW32 (Example 11) in 100 µl of TE buffer were added to 15 µl of 10X Bam HI buffer and 25 µl of dH₂O. Ten µl (about 50 units) of restriction enzyme Bam HI were added to the solution of plasmid pBW32 DNA, and the resulting reaction was incubated at 37°C for 2 hours. The Bam HI-digested plasmid pBW32 DNA was precipitated with ethanol, resuspended in 10 µl of TE buffer, loaded onto a 1% agarose gel, and electrophoresed until the ∼1.9 kb Bam HI restriction fragment that comprises the dhfr gene was separated from the other digestion products. The ∼1.9 kb restriction fragment was then isolated from the gel and prepared for ligation in substantial accordance with the procedure of Example 4A; about 10 µg of the desired fragment were obtained and suspended in 50 µl of-TE buffer. About 2 µl of the Bam HI-digested plasmid pBLT DNA, obtained as in part A of this Example, and 1 µl of the ∼1.9 kb Bam HI restriction fragment of plasmid pBW32 were added to 1 µl of 10X ligase buffer, 5 µl of dH₂O, and 1 µl (about 500 units) of T4 DNA ligase, and the resulting reaction was incubated at 16°C overnight. The ligated DNA constituted the desired plasmids pBLTdhfr1 and pBLTdhfr2. Plasmids pBLTdhfr1 and pBLTdhfr2 differ only with respect to the orientation of the ∼1.9 kb Bam HI restriction fragment that encodes the dhfr gene. The ligated DNA was used to transform E . coli K12 HB101 in substantial accordance with the procedure of Example 3. The transformed cells were plated onto L-agar containing ampicillin, and the ampicillin-resistant E . coli K12 HB101/pBLTdhfr1 and E . coli K12 HB101/pBLTdhfr2 transformants were identified by restriction enzyme analysis of their plasmid DNA. D. Construction of Plasmids pBLTdhfr1(GT) and pBLTdhfr2(GT) A poly-GT element is inserted into the Bam HI site in plasmid pBLTdhfr1 or pBLTdhfr2, in substantial accordance with the procedure of Example 16B. The poly-GT element and partially Bam HI-digested linear plasmid pBLTdhfr1 or pBLTdhfr2 DNA are ligated and the ligated DNA constitutes the desired plasmids pBLTdhfr1(GT) and pBLTdhfr2(GT), respectively. A restriction site and function map of plasmid pBLTdhfr1(GT) is presented in Figure 18 of the accompanying drawings. Example 17 Construction of Plasmids phdTPA, phd(GT)TPA, phdMTPA, and phd(GT)MTPA A. Construction of Intermediate Plasmid pTPA602 About 50 µg of plasmid pTPA103 (Example 11, Figure 12) in 45 µl of dH₂O were added to 30 µl of 10X Eco RI buffer and 225 µl of dH₂O. Ten µl (about 80 units) of restriction enzyme Eco RI were added to the solution of plasmid pTPA103 DNA, and the resulting reaction was incubated at 37°C for 90 minutes. The Eco RI-digested plasmid pTPA103 DNA was precipitated with ethanol, resuspended in 50 µl of 1X loading buffer (10% glycerol and 0.02% bromophenol blue), loaded onto a 1% agarose gel, and electrophoresed until the ∼1.1 kb Eco RI restriction fragment was separated from the other reaction products. The ∼1.1 kb Eco RI restriction fragment that comprises the TPA amino-terminal-encoding DNA and was isolated from the gel by electrophoresing the fragment into a dialysis bag. The fragment was then precipitated with ethanol and resuspended in 160 µl of dH₂O. About 40 µl of 10X Hga I buffer (0.5 M NaCl; 60 mM Tris-HCl, pH = 7.4; and 0.1 M MgCl₂), 200 µl of dH₂O, and 20 µl (about 10 units) of restriction enzyme Hga I were added to the solution of ∼1.1 kb Eco RI restriction fragment, and the resulting reaction was incubated at 37°C for 4 hours. The Hga I-digested DNA was precipitated with ethanol and then electrophoresed on a 5% acrylamide gel, and the ∼520 bp restriction fragment that encodes the amino terminus of TPA was isolated onto DE81 paper and recovered. About 5 µg of the ∼520 bp Hga I fragment were obtained and suspended in 50 µl of dH₂O. About 12.5 µl of 10X Klenow buffer (0.5 M Tris-HCl, pH = 7.4, and 0.1 M MgCl₂), 2 µl of a solution that was 6.25 mM in each of the four deoxynucleotide triphosphates, 2 µl of 0.2 M DTT, 1 µl of 7 µg/ml BSA, 57.5 µl of dH₂O, and 2 µl (about 10 units) of Klenow enzyme (Boehringer-Mannheim Biochemicals, 7941 Castleway Dr., P.O. Box 50816, Indianapolis, IN 46250) were added to the solution of the ∼520 bp Hga I restriction fragment, and the resulting reaction was incubated at 20°C for 30 minutes. The Klenow-treated DNA was incubated at 70°C for 15 minutes and precipated with ethanol. About 500 picomoles of Bam HI linker (5'-CGGGATCCCG-3', double-stranded and obtained from New England Biolabs) were phosphorylated using polynucleotide kinase in a total reaction volume of 25 µl. The reaction was carried out in substantial accordance with the procedure described in Example 7A. The kinased Bam HI linkers were added to the solution of Klenow-treated, ∼520 bp Hga I restriction fragment together with 15 µl of 10X ligase buffer, 7 µl (about 7 Weiss units) of T4 DNA ligase, and enough dH₂O to bring the total reaction volume to 150 µl. The resulting reaction was incubated at 16°C overnight. The ligation reaction was heat-inactivated, and the DNA was precipitated with ethanol and resuspended in 5 µl of 10X Bam HI buffer and 45 µl of dH₂O. One µl (about 16 units) of restriction enzyme Bam HI was added to the solution of DNA, and the resulting reaction was incubated at 37°C for 90 minutes. Then, another 16 units of Bam HI enzyme were added to the reaction mixture, and the reaction was incubated at 37°C for another 90 minutes. The reaction mixture was then electrophoresed on a 5% polyacrylamide gel, and the ∼530 bp Hga I restriction fragment, now with Bam HI ends, was purified from the gel in substantial accordance with the procedure of Example 7A. About 2 µg of the desired fragment were obtained and suspended in 20 µl of dH₂O. Bam HI-digested, dephosphorylated plasmid pBR322 DNA can be obtained from New England Biolabs. About 0.1 µg of Bam HI-digested, dephosphorylated plasmid pBR322 in 2 µl of dH₂O was added to 1 µl of the ∼530 bp Hga I restriction fragment, with Bam HI ends, of plasmid pTPA103, 14 µl of dH₂O, and 1 µl (about 1 Weiss unit) of T4 DNA ligase, and the resulting reaction was incubated at 16°C overnight. The ligated DNA constituted the desired plamsid pTPA602 and an equivalent plasmid designated pTPA601, which differs from plasmid pTPA602 only with respect to the orientation of the inserted ∼530 bp restriction fragment. The ligated DNA was used to transform E . coli K12 MM294 in substantial accordance with the procedure of Example 3, except that 50 mM CaCl₂ was used in the procedure. The transformed cells were plated on L-agar containing ampicillin, and the ampicillin-resistant E . coli K12 MM294/pTPA602 and E . coli K12 MM294/pTPA601 cells were identified by restriction enzyme analysis of their plasmid DNA. Presence of an ∼530 bp Bam HI restriction fragment indicated that the plasmid was either plasmid pTPA602 or pTPA601. B. Construction of Intermediate Plasmid pTPA603 About 5 µg of plasmid pTPA602 was dissolved in 20 µl of 10X Bgl II buffer and 180 µl of dH₂O. Three µl (about 24 units) of restriction enzyme Bgl II were added to the solution of plasmid pTPA602 DNA, and the resulting reaction was incubated at 37°C for 90 minutes. Then, about 13 µl of 10X Bam HI buffer were added to the reaction mixture to bring the salt concentration of the reaction mixture up to that recommended for Sal I digestion, and 2 µl (about 20 units) of restriction enzyme Sal I were added to the reaction. The reaction was incubated at 37°C for another 2 hours; then, the DNA was precipitated with ethanol, resuspended in 75 µl of loading buffer, loaded onto a 1% agarose gel, and electrophoresed until the ∼4.2 kb Bgl II- Sal I restriction fragment was separated from the other digestion products. The region of the gel containing the ∼4.2 kb Bgl II- Sal I restriction fragment was excised from the gel, frozen, and the frozen segment was wrapped in plastic and squeezed to remove the ∼4.2 kb fragment. The DNA was precipitated and resuspended in 20 µl of H₂O; about 200 ng of the desired fragment were obtained. About 12 µl of plasmid pTPA103 were dissolved in 15 µl of 10X Bgl II buffer and 135 µl of dH₂O. Two µl (about 16 units) of restriction enzyme Bgl II were added to the solution of plasmid pTPA103 DNA, and the resulting reaction was incubated at 37°C for 90 minutes. About 10 µl of 10X Bam HI buffer were added to the solution of Bgl II-digested plasmid pTPA103 DNA to bring the salt concentration of the reaction mixture up to that required for Sal I digestion. Then, about 2 µl (about 20 units) of restriction enzyme Sal I were added to the solution of Bgl II-digested plasmid pTPA103 DNA, and the reaction was incubated at 37°C for another 90 minutes. The Bgl II- Sal I digested plasmid pTPA103 DNA was concentrated by ethanol precipitation and then loaded onto a 1% agarose gel, and the ∼2.05 kb Bgl II- Sal I restriction fragment that encodes all but the amino-terminus of TPA was isolated from the gel, precipitated with ethanol and resuspended in 20 µl of dH₂O. About 2 µg of the desired fragment were obtained. About 5 µl of the ∼4.2 kb Bgl II- Sal I restriction fragment of plasmid pTPA602 and 2 µl of the ∼2.05 kb Bgl II- Sal I restriction fragment of plasmid pTPA103 were added to 2 µl of 10X ligase buffer, 10 µl of dH₂O, and 1 µl (about 1 Weiss unit) of T4 DNA ligase, and the resulting ligation reaction was incubated at 16°C overnight. The ligated DNA constituted the desired plasmid pTPA603. A restriction site and function map of plasmid pTPA603 is presented in Figure 19 of the accompanying drawings. The ligated DNA was used to transform E . coli K12 MM294 in substantial accordance with the procedure of Example 3, except that 50 mM CaCl₂ was used in the procedure. The transformed cells were plated on L-agar containing ampicillin, and the ampicillin-resistant E . coli K12 MM294/pTPA603 transformants were identified by restriction enzyme analysis of their plasmid DNA. C. Construction of Plasmid pMTPA603 About 100 µg of plasmid pBLT (Example 15) in 100 µl of TE buffer were added to 10 µl of 10X Sst I ( Sst I is equivalent to restriction enzyme Sac I) buffer (60 mM Tris-HCl, pH = 7.4; 60 mM MgCl₂; 60 mM 2-mercaptoethanol; and 1 mg/ml BSA) and 25 µl of dH₂O. Ten µl (about 50 units) of restriction enzyme Sst I were added to the solution of plasmid pBLT DNA, and the resulting reaction was incubated at 37°C for 2 hours. The Sst I-digested plasmid pBLT DNA was precipitated with ethanol and resuspended in 10 µl of 10X Bgl II buffer and 85 µl of dH₂O. Five µl (about 50 units) of restriction enzyme Bgl II were added to the solution of Sst I-digested plasmid pBLT DNA, and the resulting reaction was incubated at 37°C for 2 hours. The Bgl II- Sst I-digested plasmid pBLT DNA was precipitated with ethanol, resuspended in 10 µl of dH₂O, loaded onto a 1.5% agarose gel, electrophoresed, and the ∼690 bp Bgl II- Sst I restriction fragment of plasmid pBLT (which contains that portion of the modified TPA coding sequence wherein the deletion to get the modified TPA coding sequence has occurred) was isolated from the gel in substantial accordance with the procedure of Example 4A. About 5 µg of the desired ∼690 bp Bgl II- Sst I restriction fragment of plasmid pBLT was obtained and suspended in 100 µl of dH₂O. About 5 µg of plasmid pTPA603 (part B of this Example, Figure 19) in 5 µl of TE buffer were added to 10 µl of 10X Sst I buffer and 95 µl of dH₂O. Five µl (about 50 units) of restriction enzyme Sst I were added to the solution of plasmid pTPA603 DNA, and the resulting reaction was incubated at 37°C for 2 hours. The Sst I-digested plasmid pTPA603 DNA was precipitated with ethanol and resuspended in 10 µl of 10X Bgl II buffer and 85 µl of dH₂O. Five µl (about 50 units) of restriction enzyme Bgl II were added to the solution of Sst I-digested plasmid pTPA603 DNA, and the resulting reaction was incubated at 37°C for 2 hours. The Bgl II- Sst I-digested plasmid pTPA603 DNA was diluted to 100 µl in TE buffer and treated with calf-intestinal alkaline phosphatase in substantial accordance with the procedure of Example 4B. The DNA was then precipitated with ethanol and resuspended in 10 µl of dH₂O. About 5 µl of the Bgl II- Sst I-digested plasmid pTPA603 and 2 µl of the ∼690 bp Bgl II- Sst I restriction fragment of plasmid pBLT were added to 2 µl of 10X ligase buffer, 10 µl of dH₂O, and 1 µl (about 1000 units) of T4 DNA ligase, and the resulting ligation reaction was incubated at 16°C overnight. The ligated DNA constituted the desired plasmid pMTPA603. Plasmid pMTPA603 is thus analogous in structure to plasmid pTPA603 (Figure 19), except that plasmid pMTPA603 encodes modified TPA, and plasmid pTPA603 encodes TPA. The ligated DNA was used to transform E . coli K12 HB101 in substantial accordance with the procedure of Example 3. The transformed cells were plated on L-agar containing ampicillin, and the ampicillin-resistant E . coli K12 HB101/pMTPA603 transformants were identified by restriction enzyme analysis of their plasmid DNA. D. Construction of Plasmid phdTPA About 10 µg of plasmid pTPA603 (part B of this Example, Figure 19) in 10 µl of TE buffer were added to 10 µl of 10X Bam HI buffer and 85 µl of dH₂O. Five µl (about 50 units) of restriction enzyme Bam HI were added to the solution of plasmid pTPA603 DNA, and the resulting reaction was incubated at 37°C for 2 hours. The Bam HI-digested plasmid pTPA603 DNA was precipitated with ethanol, resuspended in 10 µl of dH₂O, loaded onto a 1% agarose gel, and electrophoresed until the ∼1.90 kb Bam HI restriction fragment that encodes TPA was separated from the other digestion products. The ∼1.90 kb Bam HI restriction fragment was isolated from the gel and resuspended in 50 µl of TE buffer; about 4 µg of the desired fragment were obtained. About 2 µg of plasmid phd (Example 13) in 2 µl of TE buffer were added to 2 µl of 10X Bcl I buffer and 14 µl of dH₂O. Two µl (about 10 units) of restriction enzyme Bcl I were added to the solution of plasmid phd DNA, and the resulting reaction was incubated at 50°C for 2 hours. The reaction was stopped by extracting the reaction mixture first with phenol and then twice with chloroform. The Bcl I-digested plasmid phd DNA was then precipitated with ethanol and resuspended in 20 µl of TE buffer. About 1 µl of the Bcl I-digested plasmid phd and 2 µl of the ∼1.90 kb Bam HI restriction fragment of plasmid pTPA603 were added to 1 µl of 10X ligase buffer, 5 µl of dH₂O, and 1 µl (about 500 units) of T4 DNA ligase. The resulting ligation reaction was incubated at 16°C overnight. The ligated DNA constituted the desired plasmid phdTPA. The ligated DNA was used to transform E . coli K12 HB101 (NRRL B-15626) in substantial accordance with the procedure of Example 3. The transformation mixture was plated on L-agar containing ampicillin, and the ampicillin-resistant E . coli K12 HB101/phdTPA cells were identified by restriction enzyme analysis. The ∼1.90 kb Bam HI restriction fragment could insert into Bcl I-digested plasmid phd in either one of two orientations, only one of which places the TPA coding sequence in the proper position to be expressed under the control of the BK enhancer - adenovirus late promoter cassette and thus results in the desired plasmid phdTPA. E. Construction of Plasmid phd(GT)TPA A poly-GT element is inserted into the unique Xho I site in plasmid phdTPA (part D of this Example), between the BK enhancer and the Ad2 late promoter, in substantial accordance with the procedure of Example 4B.1. The Xho I-digested poly-GT element and Xho I-digested plasmid phdTPA DNA are ligated and the ligated DNA constitutes the desired plasmid phd(GT)TPA. A restriction site and function map of plasmid phd(GT)TPA is presented in Figure 20 of the accompanying drawings. F. Construction of Plasmid phdMTPA About 10 µg of plasmid pMTPA603 (part C of this Example) in 10 µl of TE buffer were added to 10 µl of 10X Bam HI buffer and 85 µl of dH₂O. Five µl (about 50 units) of restriction enzyme Bam HI were added to the solution of plasmid pMTPA603 DNA, and the resulting reaction was incubated at 37°C for 2 hours. The Bam HI-digested plasmid pMTPA603 DNA was precipitated with ethanol, resuspended in 10 µl of dH₂O, loaded onto a 1% agarose gel, and electrophoresed until the ∼1.35 kb Bam HI restriction fragment that encodes modified TPA was separated from the other digestion products. The ∼1.35 kb Bam HI restriction fragment was isolated from the gel and resuspended in 20 µl of TE buffer; about 4 µg of the desired fragment were obtained. About 1 µl of the Bcl I-digested plasmid phd (part D of this Example) and 2 µl of the ∼1.35 kb Bam HI restriction fragment of plasmid pMTPA603 were added to 1 µl of 10X ligase buffer, 5 µl of dH₂O, and 1 µl (about 500 units) of T4 DNA ligase. The resulting ligation reaction was incubated at 16°C overnight. The ligated DNA constituted the desired plasmid phdMTPA. The ligated DNA was used to transform E . coli K12 HB101 in substantial accordance with the procedure of Example 3. The transformation mixture was plated on L-agar containing ampicillin, and the ampicillin-resistant E . coli K12 HB101/phdMTPA cells were identified by restriction enzyme analysis of their plasmid DNA. The ∼1.35 kb Bam HI restriction fragment could insert into Bcl I-digested plasmid phd in either one of two orientations, only one of which places the TPA coding sequence in the proper position to be expressed under the control of the BK enhancer-adenovirus late promoter and thus results in the desired plasmid phdMTPA. G. Construction of Plasmid phd(GT)MTPA A poly-GT element is inserted into the unique Xho I site in plasmid phdMPTA (part F of this Example), between the BK enhancer and the Ad2 late promoter, in substantial accordance with the procedure of Example 4B.1. The Xho I-digested poly-GT element and Xho I-digested plasmid phdMTPA DNA are ligated and the ligated DNA constitutes the desired plasmid phd(GT)MTPA. A restriction site and function map of plasmid phd(GT)MTPA is presented in Figure 21 of the accompanying drawings. Example 18 Construction of an Improved Enhancer-Promoter Cassette The transcription-enhancing effect of the BK enhancer can be significantly increased by placing the enhancer from 0 to 300 nucleotides upstream of the 5' end of the CCAAT region or CCAAT region equivalent of an adjacent eukaryotic promoter. Enhancement of transcription may be further increased by placing a poly-GT element upstream or downstream from the BK enhancer - Ad2 late promoter cassette in the presence of an immediate-early gene product of a large DNA virus, such as the E1A gene product, as described in Example 19. The sequence and functional elements of the present BK enhancer-Adenovirus 2 late promoter cassette, before modification to achieve greater enhancing activity, is depicted as follows: wherein A is deoxyadenyl; G is deoxyguanyl; C is deoxycytidyl; and T is thymidyl. This depiction assumes that the BK enhancer is from the prototype strain of BK virus, available from the ATCC under the VR-837. However, ATCC VR-837 consists of a mixture of BK variants. Plasmid pBal8cat and the other BK enhancer-containing plasmids described herein comprise a BK enhancer variant isolated from the prototype strain, but not the BK prototype enhancer depicted. The sequence of the variant enhancer corresponding to positions 243 to 467 of the above depicted sequence of the prototype enhancer found in plasmid pBal8cat and other plasmids herein is depicted as follows: As stated above, however, any BK enhancer variant can be used in the methods and compounds of the present invention. The prototype BK enhancer is defined by the three repeated sequences indicated in the sequence above and functions similarly, with respect to an adjacent sequence, in either orientation. To bring the enhancer, more specifically, the 3' end of the third repeat (which depends on the orientation) of the BK enhancer, closer to the 5' end of the CCAAT region of the adenovirus-2 late promoter, about 82 µg of Sst II-digested plasmid pBLcat DNA (Example 5) in 170 µl of TE buffer were added to 20 µl of 5X Bal 31 nuclease buffer (0.1 M Tris-HCl, pH = 8.1; 0.5 M NaCl; 0.06 M CaCl₂; and 5 mM Na₂EDTA) and 9 µl of Bal 31 nuclease, which was composed of 6 µl (about 6 units) of "fast" and 3 µl (about 3 units) of "slow" Bal 31 enzyme (marketed by International Biotechnologies, Inc., P.O. Box 9558, 275 Winchester Ave., New Haven, CT 06506). The reaction was incubated at 30°C for about 3 minutes; then, after about 10 µl of 1.0 M EDTA were added to stop the reaction, the Bal 31-digested DNA was collected by ethanol precipitation and centrifugation. The DNA pellet was resuspended in 1X Klenow buffer and treated with Klenow enzyme in substantial accordance with procedures previously described herein. The Klenow-treated DNA was resuspended in 10 µl of TE buffer; about 1 µl of the DNA was then self-ligated in 10 µl of 1X ligase buffer using T4 DNA and RNA ligase as previously described. The ligated DNA was used to transform E . coli K12 HB101, and then the transformants were plated onto L-agar containing ampicillin. Restriction enzyme analysis was used to determine which transformants contained plasmids with an appropriately-sized BK enhancer-adenovirus 2 late promoter cassette. The foregoing procedure generates a number of plasmids in which the BK enhancer is placed within 0 to 300 nucleotides upstream of the CCAAT region of the adenovirus late promoter. One plasmid resulting from the above procedure was designated plasmid pBal8cat. Plasmid pBal8cat is available from the NRRL under the accession number NRRL B-18267 (deposited Dec. 1, 1987). Plasmid pBal8cat contains a variant of the BK enhancer that contains two repeat sequences of about 100 bp each as shown above. This variant enhancer can be used in the method of the present invention by placing the 3' end of the second repeat within 0 to 300 nucleotides of the CCAAT region of the adenovirus late promoter. Those skilled in the art will recognize that the foregoing procedure produced a number of distinct plasmids, of which plasmid pBal8cat is illustrative. These plasmids, as a group, represent placing the BK enhancer at a variety of distance less than 300 nucleotides from the CCAAT region of the Ad2 late promoter. This method for improving the activity of a BK enhancer, which can be achieved using the foregoing procedure or others known to those skilled in the art, can be used with any BK enhancer and any eukaryotic promoter. Example 19 Construction of an Improved Poly-GT-Enhancer-Promoter Cassette A poly-GT element may be inserted into the variety of distinct plasmids described in Example 18 above, of which plasmid pBal8cat is illustrative. These poly-GT containing plasmids, as a group, represent placing a poly-GT element in either orientation, and either 5' or 3' to the CAT gene, in expression vectors in which the BK enhancer has been placed at a variety of distances less than 300 nucleotides from the CCAAT region of the Ad2 late promoter and thus comprise an important aspect of the present invention. A poly-GT element was inserted into plasmid pBal8cat as follows. About 1 µg of pBal8cat DNA (Example 18) in 1 µl of TE buffer was dissolved in 2 µl of 10X Core buffer and 16 µl of dH₂O. One µl (about 10 units) of restriction enzyme Bam HI was added to the solution of DNA, and the resulting reaction was incubated at 37°C for 2 hours. Following this incubation, the mixture was heat-activated at 70°C for 10 minutes. One µl of 10X ligase buffer was added to a mixture of 1 µl (about 50 ng) of the Bam HI-digested pBal8cat DNA prepared above and 7 µl (about 500 ng) of the poly-GT synthetic fragment prepared as in Example 1. One µl (about 400 units) of T4 DNA ligase was added to the solution of DNA, and the resulting reaction was incubated at 16°C overnight. The ligated DNA constituted the desired plasmids pBal8cat(GT6) (sense) and pBal8cat(GT9) (antisense) which differ only with respect to the orientation of the inserted poly-GT element relative to the CAT coding sequence. The enhancer activity of the poly-GT element in the presence of an immediate-early gene product of a large DNA virus, such as the E1A gene product, does not depend on the orientation of the poly-GT element in the plasmid expression vector into which it has been inserted. Thus, for example, plasmid pBal8cat(GT6) and plasmid pBal8cat(GT9) may be equally effective expression vectors for the CAT gene product in the presence of E1A. A restriction site and function map of plasmid pBal8cat(GT6) is presented in Figure 22 of the accompanying drawings. E . coli K12 HB101 cells (NRRL B-15626) were cultured, made competent for transformation, and transformed with the ligated DNA prepared above in substantial accordance with the procedure of Example 3. The transformed cells were plated on L-agar plates containing 100 µg/ml ampicillin. E . coli K12 HB101/pBal8cat(GT6) and E . coli K12 HB101/pBal8cat(GT9) transformants were identified by their ampicillin-resistant phenotype and by restriction enzyme analysis of their plasmid DNA. Plasmid pBal8cat(GT6) or pBal8cat(GT9) DNA was prepared in substantial accordance with the procedure of Example 3. Example 20 Construction of Plasmid pSV2E1A E . coli K12 HB101/pSV2cat cells are obtained in lyophilized form from the ATCC under the accession number ATCC 37155, and plasmid pSV2cat DNA was isolated from the cells in substantial accordance with the procedure of Example 3. A restriction site and function map of plasmid pSV2cat is presented in Figure 3 of the accompanying drawings. About 1 µg (1 µl) of the plasmid pSV2cat DNA were added to 2 µl of 10X core buffer and 14 µl dH₂O, and then 1.5 µl (about 15 units) of restriction enzyme Stu I and 1.5 µl (about 15 units) of restriction enzyme Eco RI were added to the solution of pSV2cat DNA, and the resulting reaction was incubated at 37°C for 2 hours. The reaction was terminated by heat inactivation at 70°C for 10 minutes. After heat inactivation, 25 µl of dH₂O were added, then about 5 µl of 10X Klenow buffer and 2 µl (about 10 units) of Klenow enzyme were added to the mixture of DNA, and the resulting reaction was incubated at 16°C for 1 hour. The Klenow-treated, Stu I/ Eco RI-digested plasmid pSV2cat DNA was then extracted twice with phenol:chloroform and then once with chloroform:isoamyl alcohol, precipitated with ethanol and resuspended in 20 µl of Tris buffer. A fragment containing the E1A gene of adenovirus-2 DNA was isolated in substantial accordance with the procedure of Example 12A. The ∼1.8 kb Bal I fragment containing the E1A gene was isolated and about 9 µl (about 500 ng) of this fragment were added to 10 µl (about 100 ng) of the Klenow-treated, Stu I/ Eco RI-digested plasmid pSV2cat DNA described above, along with 1 µl dH₂O and 2.5 µl of 10X ligase buffer. Two and one-half µl (about 1250 units) of T4 DNA ligase were added to the solution of DNA, and the resulting reaction was incubated at 16°C overnight. The ligated DNA constituted the desired plasmid pSV2E1A. A restriction site and function map of plasmid pSV2E1A is presented in Figure 23 of the accompanying drawings. Plasmid pSV2E1A is useful in the present invention as a source of the E1A gene in cotransformation experiments along with any other plasmid containing a poly-GT element, a eukaryotic promoter, and a recombinant gene to be expressed in the transformed host cells. Such plasmids include: pLP(GT)cat, pBL(GT)cat, pSBL(GT)cat, pLPC(GT), pLPC4(GT), pLPC5(GT), pLPChyg1(GT), pLPChyg2(GT), pLPCdhfr1(GT), pLPCdhfr2(GT), pLPChd1(GT), pLPChd2(GT), pLPChd3(GT), pLPChd4(GT), pBLT(GT), pBLThyg1(GT), pBLThyg2(GT), pBLTdhfr1(GT), pBLTdhfr2(GT), phdTPA(GT), phdMTPA(GT), pBal8cat(GT6), pBal8cat(GT9) p4-14(GT), p2-5(GT) and pT6hd(GT). Alternatively, the E1A gene may be provided on a single plasmid along with the poly-GT element, eukaryotic promoter, and a recombinant gene to be expressed in transformed host cells, for example, plasmid pLPCE1A(GT) described in Example 14B. Example 21 Construction of Plasmids pT6hd and pT6hd(GT) A. Site-Specific Mutagenesis of the TPA Coding Region and Construction of Plasmid pT6hd Site specific mutagenesis of the TPA coding region and the construction of plasmid pT6hd is accomplished as follows. About 5 µg of plasmid pTPA103 (Example 17) in 10 µl of dH₂O are added to about 10 µl of 10X Hin dIII buffer and 80 µl of dH₂O. One µl (about 20 units) of restriction enzyme Hin dIII is added to the solution of plasmid pTPA103 DNA, and the resulting reaction is incubated at 37°C for 90 minutes. One µl (about 20 units) of restriction enzyme Sst I and 10 µl of 1M Tris-HCl, pH = 7.6, is added to the solution of Hin dIII-digested plasmid pTPA103 DNA, and the resulting reaction is incubated at 37°C for 90 minutes. The Hin dIII- Sst I-digested plasmid pTPA103 DNA is concentrated by ethanol precipitation, loaded onto a 1.5% agarose gel, and electrophoresed until the ∼1.4 kb Hin dIII- Sst I restriction fragment is separated from the other digestion products. About 0.5 µl of the ∼1.4 bp Hin dIII- Sst I restriction fragment is isolated from the gel in substantial accordance with the procedure of Example 4A, prepared for ligation, and resuspended in 20 µl of dH₂O. About 4.5 µg of the replicative form (RF) of M13mp18 DNA (available from New England Biolabs) in 35 µl of dH₂O is added to 10 µl of 10X Hin dIII buffer and 55 µl of dH₂O. One µl (about 20 units) of restriction enzyme Hin dIII is added to the solution of M13mp18 DNA, and the resulting reaction was incubated at 37°C for 1 hour. One µl (about 20 units) of restriction enzyme Sst I and 10 µl of 1M Tris-HCl, pH = 7.6, is added to the solution of Hin dIII-digested M13mp18 DNA, and the resulting reaction is incubated at 37°C for 1 hour. The Hin dIII- Sst I-digested M13mp18 DNA is collected by ethanol precipitation, resuspended in preparation for agarose gel electrophoresis, and the large restriction fragment isolated by gel electrophoresis as described above. About 1 µl of the large Hin dIII- Sst I restriction fragment of M13mp18 is obtained and suspended in 20 µl of dH₂O. About 2 µl of the large Hin dIII- Sst I restriction fragment of M13mp18, 2 µl of 10X T4 DNA ligase buffer, 12 µl of dH₂O and ∼1 µl (about 1 Weiss unit) of T4 DNA ligase is added to 3 µl of the ∼1.4 kb Hin dIII- Sst I restriction fragment of plasmid pTPA103, and the resulting ligation reaction is incubated at 16°C overnight. E . coli JM103 cells (available from BRL) are made competent and transfected with the ligation mix in substantial accordance with the procedure described in the BRL M13 Cloning/'Dideoxy' Sequencing Instruction Manual, except that the amount of DNA used per transfection is varied. Recombinant plaques are identified by insertional inactivation of the β-galactosidase α-fragment-encoding gene, which results in the loss of the ability to cleave X-Gal to its indigo-colored cleavage product. For screening purposes, several white plaques are picked into 2.5 ml of L broth, to which is added 0.4 ml of E . coli K12 JM103 in logarithmic growth phase, that are cultured in minimal media stock to insure retention of the F episome that carries pro AB. The 2.5 ml plaque-containing cell suspensions are incubated in an air-shaker at 37°C for 8 hours. Cells from 1.5 ml aliquots are pelleted and RF DNA isolated in substantial accordance with the alkaline miniscreen procedure of Birnboim and Doly, 1979, Nuc. Acids Res. 7:1513. The remainder of each culture is stored at 4°C for stock. The desired phage, designated MP18BW47, contains the ∼1.4 kb Hin dIII- Sst I restriction fragment of plasmid pBW25 ligated to the ∼7.2 kb Eco RI- Hin dIII restriction fragment of M13mp18. About fifty ml of log phase E . coli JM103 are infected with MP18BW47 and incubated in an air-shaker at 37°C for 18 hours. The infected cells are pelleted by low speed centrifugation, and single-stranded MP18BW47 DNA is prepared from the culture supernatant by scaling up the procedure given in the Instruction Manual. Single-stranded MP18BW47 is mutagenized in substantial accordance with the teaching of Adelman et al ., 1983, DNA 2(3): 183-193, except that the Klenow reaction is done at room temperature for 30 minutes, then at 37°C for 60 minutes, then a 10°C for 18 hours. In addition, the S1 treatment is done at 20°C, the salt concentration of the buffer is one-half that recommended by the manufacturer, and the M13 sequencing primer (BRL) is used. The synthetic oligodeoxyribonucleotide primer used to delete the coding sequence for amino acid residues 4 through 175 of native TPA is 5'-GGAGCCAGATCTTACCAAGGAAACAGTGACTGCTAC-3'. The resulting mutagenesis mix is used to transfect E . coli K12 JM103 in substantial accordance with the infection procedure described above. Desired mutants are identified by restriction enzyme analysis of RF DNA and by Maxam and Gilbert DNA sequencing. The desired mutant, which has the coding sequence for amino acid residues 4 through 175 of native TPA deleted, is designated MP18BW52. The restriction enzyme digestions, fragment isolations and ligations were all performed in substantial accordance with the procedures of Example 4. Plasmid pT6hd was constructed as follows. First, an ∼1.9 kb Bam HI fragment was obtained from plasmid pTPA603 (Example 17, Figure 19). Next, plasmid pAc373 (Miyamoto et al ., 1985, Mol. Cell. Biol. 5:2860-2865) was digested with Bam HI and ligated with the ∼1.9 kb Bam HI fragment from plasmid pTPA603 to create intermediate plasmid pL100. Plasmid pL100 DNA was then digested with restriction enzymes Bgl II and Sst I and an ∼1.2 kb Bgl II- Sst I fragment was isolated. MP18BW52 DNA, obtained as above, was digested with restriction enzymes Bgl II and Sst I and an ∼718 bp fragment was isolated. The ∼718 bp Bgl II- Sst I fragment of MP18BW52 was ligated with the ∼1.2 kb Bgl II- Sst I fragment of plasmid pL100 to create intermediate plasmid pL229. Plasmid pL229 DNA was then digested with restriction enzyme Bam HI and an ∼1.4 kb Bam HI fragment was isolated. Plasmid phd (Example 13) was digested with Bcl I and ligated with the ∼1.4 kb Bam HI fragment of plasmid pL229 to create the desired plasmid pT6hd. B. Construction of pT6hd(GT) A poly-GT element is inserted into the unique Xho I site in plasmid pT6hd, between the BK enhancer and the Ad2 late promoter, in substantial accordance with the procedure of Example 4B.1. The Xho I-digested poly-GT element and Xho I digested plasmid pT6hd are ligated and the ligated DNA constitutes the desired plasmid pT6hd(GT). A restriction site and function map of plasmid pT6hd(GT) is presented in Figure 24 of the accompanying drawings. Alternatively, plasmid pT6hd(GT) is constructed by ligating the ∼1.4 kb Bam HI fragment from plasmid pL229 (described in part A above) with Bcl I-digested plasmid phd(GT) (Example 13, Figure 14) to create plasmid pT6hd(GT). Example 22 Construction of Eukaryotic Host Cell Transformants of Certain Expression Vectors An important aspect of the expression vectors described herein concerns the use of a poly-GT element and a BK enhancer in such expression vectors to stimulate gene expression in the presence of the E1A gene product. Because 293 cells constitutively express the E1A gene product, 293 cells are the preferred host for the eukaryotic expression vectors of the present invention. The 293 cells are human embryonic kidney cells transformed with adenovirus type 5 (note that any particular type of adenovirus can be used to supply the E1A gene product in the method of the present invention) and are publically available from the ATCC under the accession number CRL 1573. Many of the expression vectors described herein function in a wide variety of host cells, even if the E1A gene product is not present. However, the E1A gene product (or similarly-acting immediate-early gene product of a large DNA virus) is required to activate the poly-GT element to enhance gene expression from the poly-GT-containing expression vectors of the present invention. The E1A gene product can be introduced into a non-E1A-producing cell line either by transformation with a vector that comprises the E1A gene, such as plasmids pSV2E1A, pLPCE1A and pLPCE1A1, or with sheered adenovirus DNA, or by infection with adenovirus. The transformation procedure described below refers to 293 cells as the host cell line; however, the procedure is generally applicable to most eukaryotic cell lines. A variety of cell lines may be transformed with the vectors described herein. Because of the great number of expression vectors described herein, the transformation procedure is described generically. The 293 cells are obtained from the ATCC under the accession number CRL 1573 in a 25 mm² flask containing a confluent monolayer of about 5.5 x 10⁶ cells in Eagle's Minimum Essential Medium with 10% heat-inactivated horse serum. The flask is incubated at 37°C; medium is changed twice weekly. The cells are subcultured by removing the medium, rinsing with Hank's Balanced Salts solution (HBS) (Gibco), adding 0.25% trypsin for 1-2 minutes, rinsing with fresh medium, aspirating, and dispensing into new flasks at a subcultivation ratio of 1:5 or 1:10. One day prior to transfection, cells are seeded at 0.7 x 10⁶ cells per dish. The medium is changed 4 hours prior to transfection. Sterile, ethanol-precipitated plasmid DNA dissolved in TE buffer is used to prepare a 2X DNA-CaCl₂ solution containing 40 µg/ml DNA and 250 mM CaCl₂. 2X HBS is prepared containing 280 mM NaCl, 50 mM HEPES, and 1.5 mM sodium phosphate, with the pH adjusted to 7.05-7.15. The 2X DNA-CaCl₂ solution is added dropwise to an equal volume of sterile 2X HBS. A one ml sterile plastic pipette with a cotton plug is inserted into the mixing tube that contains the 2X HBS, and bubbles are introduced by blowing while the DNA is being added. The calcium-phosphate-DNA precipitate is allowed to form without agitation for 30-45 minutes at room temperature. The precipitate is then mixed by gentle pipetting with a plastic pipette, and one ml (per plate) of precipitate is added directly to the 10 ml of growth medium that covers the recipient cells. After 4 hours of incubation at 37°C, the medium is replaced with DMEM with 10% fetal bovine serum and the cells allowed to incubated for an additional 72 hours before providing selective pressure. For transformants expressing recombinant human protein C, the growth medium contained 1 to 10 µg/ml vitamin K, a cofactor required for γ-carboxylation of the protein. For plasmids that do not comprise a selectable marker, that functions in eukaryotic cells, the transformation procedure utilizes a co-transformation technique with a mixture of plasmids: the expression vector described herein that lacks a selectable marker, and, an expression vector that comprises a selectable marker that functions in eukaryotic cells. This co-transformation technique allows for the identification of cells that comprise both of the transforming plasmids. For cells transfected with plasmids containing the hygromycin resistance-conferring gene, hygromycin is added to the growth medium to a final concentration of about 200 to 400 µg/ml. The cells are then incubated at 37°C for 2-4 weeks with medium changes at 3 to 4 day intervals. The resulting hygromycin-resistant colonies are transferred to individual culture flasks for characterization. The selection of neomycin-resistant colonies (G418 is also used in place of neomycin) is performed in substantial accordance with the selection procedure for hygromycin-resistant cells, except that G418 is added to a final concentration of 400 µg/ml rather than hygromycin. 293 cells are dhfr positive, so 293 transformants that contain plasmids comprising the dhfr gene are not selected solely on the basis of the dhfr-positive phenotype, which is the ability to grow in media that lacks hypoxanthine and thymine. Cell lines that do lack a functional dhfr gene and are transformed with dhfr-containing plasmids can be selected for on the basis of the dhfr - positive phenotype. The use of the dhfr gene as a selectable marker for introducing a gene or plasmid into a dhfr-deficient cell line and the subsequent use of methotrexate to amplify the copy number of the plasmid has been well established in the literature. Although the use of dhfr as a selectable and amplifiable marker in dhfr-producing cells has not been well studied, evidence in the literature would suggest that dhfr can be used as a selectable marker in dhfr-producing cells and for gene amplification. The use of the present invention is not limited by the selectable marker used. Moreover, amplifiable markers such as metallothionein genes, adenosine deaminase genes, or members of the multigene resistance family, exemplified by P-glycoprotein, can be utilized. In 293 cells, it is advantageous to transform with a vector that contains a selectable marker such as hygromycin B resistance-conferring gene and then amplify using methotrexate, which cannot be used for direct selection of murine dhfr-containing plasmids in 293 cells. Example 23 AV12 Eukaryotic Host Cell Transformants Cell line AV12 (ATCC CRL 9595) can be transformed in substantial accordance with the procedure described for 293 cells in Example 22. However, unlike 293 cells, AV12 cells can be directly selected with methotrexate (200-500 nM) when transformed with a vector containing the murine dhfr gene. The advantages of producing a γ-carboxylated protein, for example, activated human protein C, in an adenovirus-transformed host cell are that such cells can produce protein C which is fully γ-carboxylated and fully active. The transformants are selected using hygromycin B or methotrexate; such transformants can produce about 2 to 4 µg/ml of human protein C. Protein C levels can be increased to about 50 µg/ml by amplification with methotrexate. The protein C is activated and its activity determined as described in Grinnell et al., 1987, Bio/Technology 5:1189. The recombinant protein C activity produced in an adenovirus-transformed host cell such as AV12 is at least as active as that found in human blood. In non-adenovirus-transformed host cells, the anticoagulant activity of the recombinant protein C produced has not exceeded 60% of the activity of human blood-derived protein C. Example 24 Construction of Plasmids p4-14 p2-5, p4-14(GT) and p2-5(GT), Plasmids that Encode the Tripartite Leader of Adenovirus Plasmids p4-14(GT) and p2-5(GT) both utilize an improved poly-GT - BK-enhancer - adenovirus late promoter system and the tripartite leader (TPL) of adenovirus to drive high level expression of human protein C in eukaryotic host cells. Plasmids p4-14 and p2-5 are identical to plasmids p4-14(GT) and p2-5(GT), respectively, but lack a poly-GT element. The DNA encoding the adenovirus tripartite leader (TPL) was isolated from adenovirus; numbers in parentheses after restriction enzyme cut sites refer to map units of adenovirus. Plasmid pUC13 (commercially available from BRL) was digested with restriction enzymes Sph I and Bam HI and then ligated with the TPL-encoding ∼7.2 kb Sph I (5135)- Bcl I (12,301) restriction fragment of adenovirus type 2 to yield plasmid pTPL4. Part of an intron was deleted from the TPL-encoding DNA by digesting plasmid pTPL4 with restriction enzymes Sau I (7616) and Bgl II (8904), treating with Klenow enzyme, and religating to yield plasmid pDTPL. Plasmid pvTPL was then digested with restriction enzyme Xho I, and the ∼2.62 kb Xho I fragment encoding the TPL ( Xho I sites at 5799 and 9689 of adenovirus) was isolated and prepared for ligation. Plasmid pBLcat was digested with restriction enzyme Bcl I. A linker having the following sequence was synthesized and annealed in substantial accordance with the procedure of Example 1: and this linker was ligated to Bcl I-digested plasmid pBLcat, followed by digestion with Xho I. The Xho I-digested plasmid pBLcat DNA with the ligated linker was then ligated to yield plasmid pBΔLcat. This construction replaces the adenovirus late promoter on plasmid pBLcat with the linker sequence. Plasmid pBΔLcat was digested with restriction enzyme Xho I and ligated with the ∼2.62 kb Xho I restriction fragment of plasmid pΔTPL to yield plasmid pBΔL-TPL, in which the TPL-encoding fragment is correctly positioned to place the BK enhancer, adenovirus major late promoter, and TPL in alignment for expression of the CAT gene. Plasmid p2-5 was then constructed by ligating these fragments: (1) the Aat II- Bcl I restriction fragment of plasmid pLPChd1, which encodes the dhfr gene; (2) the protein C-encoding, Bcl I restriction fragment of plasmid pLPChd1; (3) the TPL-encoding Pvu II- Bcl I restriction fragment of plasmid pBΔL-TPL; and (4) the BK-enhancer-Ad2MLP-encoding Pvu II- Aat II restriction fragment of plasmid pBal8cat. Plasmid p2-5 thus contains the dhfr gene as a selectable, amplifiable marker and the BK enhancer, Ad2 MLP, and Ad2 TPL correctly positioned to drive expression of human protein C. Plasmid p4-14 is analogous to plasmid p2-5 but was constructed via an intermediate plasmid designated pBal8TPL. Plasmid pBal8TPL was constructed by ligating fragments 1, 3, and 4, used in the construction of plasmid p2-5, as described in the preceding paragraph. Plasmid pBal8TPL was then digested with restriction enzyme Xho I, treated with Klenow enzyme to make the Xho I ends blunt-ended, and then ligated with the human protein C-encoding, Klenow-treated Bcl I restriction fragment of plasmid pLPChd1 to yield plasmid p4-14. Thus, plasmid p4-14 only differs from plasmid p2-5 in that the protein C-encoding DNA was inserted at the Xho I site in the fragment derived from plasmid pBΔL-TPL, whereas in plasmid p2-5, this DNA was inserted at the Bcl I site in the DNA derived from plasmid pBΔL-TPL. Plasmid p4-14 and p2-5 drive high-level expression of human protein C. In AV12 cells, plasmids p4-14 and p2-5 can be directly selected using 200-500 nM methotrexate. AV12/p4-14 transformants, before amplification, express 5-6 times more human protein C than AV12/pLPCdhfr transformants. Amplification with methotrexate further increases the amount of human protein C produced by the cells. Plasmids p4-14 and p2-5 are thus illustrative of the higher expression levels achieved using the Bal8 promoter and the TPL of adenovirus. Still higher levels of expression in the presence of E1A gene product may be achieved by inserting a poly-GT element into plasmid p4-14 or p2-5 as follows. A poly-GT element is inserted into the unique Bcl I site in plasmid p4-14 at the 3' end of the protein C coding sequence ( Bcl I is compatible with Bam HI) to yield plasmid p4-14(GT). A poly-GT element is inserted into the unique Xho I site in plasmid p2-5 at the junction between the TPL and the protein C coding sequence, to yield plasmid p2-5(GT). Example 25 Determination of Recombinant Gene Expression Levels in Eukaryotic Host Cell Transfectants of Certain Poly-GT Expression Vectors A variety of eukaryotic host cells have been transfected with the poly-GT-containing expression vectors of the present invention. In certain of these transfectants, expression levels of the structural gene encoded in these recombinant poly-GT-containing expression vectors have been determined and compared with expression levels from corresponding expression vectors that lack a poly-GT element. In several experiments, three different cell lines were transfected with plasmids pLPcat, pLP(GT)cat, pBLcat, pBL(GT)cat, pBal8cat and pBal8cat(GT6) in substantial accordance with the procedure of Example 21. The above listed plasmids each contain the gene-coding sequence for chloramphenicol acetyltransferase (CAT). Relative levels of CAT activity were determined by the method of Gorman et . al ., 1982, Mol. Cell. Biol. 2:1044-1051. The level of CAT activity by each plasmid is presented in Table 2 below as the average of 1 to 5 separate experiments. Relative CAT activities varied by only 10 to 15% between experiments. Relative Levels of Chloramphenicol Acetyltransferase (CAT) Produced by Recombinant Plasmids in Various Human and Monkey Kidney Cell Lines Plasmid Relative Level of CAT in Cell Line: 293 (ATCC CRL 1573) MK2 (ATCC CCL7) COS-1 (ATCC CRL 1650) pLPcat 1 1 1 pLP(GT)cat 4 1 NT pBLcat 66 28 8 pBL(GT)cat 208 21 8 pBal8cat 479 20 NT pBal8cat(GT6) 958 NT 11 Plasmid pLP(GT)cat is illustrative of an expression vector that contains a poly-GT element, no enhancer, and the Ad2 late promoter, whereas plasmid pBL(GT)cat is illustrative of an expression vector that contains a poly-GT element, the BK enhancer and the Ad2 late promoter. Plasmid pBal8cat(GT6) is similar to plasmid pBL(GT)cat except that pBal8cat(GT6) contains a modified enhancer-promoter region with the poly-GT element 3' to the CAT gene. To determine the effect of the poly-GT element, the plasmid expression vectors listed in Table 2 were transfected into various cell lines and CAT activity determined, as described above. Table 2 shows that in the MK2 cell line, the poly-GT element had no stimulatory effect or enhancer-like activity with the Ad2 late promoter; the relative levels of CAT produced from plasmids pLPcat and pLP(GT) were approximately the same. However, in the presence of E1A, as shown by the results with plasmids pLPcat and pLP(GT)cat in the 293 (E1A⁺) cell line, a 4 fold increase in CAT activity was observed with plasmid pLP(GT)cat which contains a poly-GT element, as compared with plasmid pLPcat. This illustrates the trans -activation of the poly-GT element by E1A. When plasmids pBL(GT)cat and pBLcat were used to transfect 293 cells, the poly-GT element was again trans -activated by E1A. An approximately 4 fold increase in CAT activity was observed with plasmid pBL(GT)cat as compared with plasmid pBLcat. Plasmid pBL(GT)cat shows greater than 200 times the activity of plasmid pLPcat, due to the combined enhancer activity of the poly-GT element and the BK enhancer in the presence of E1A. Using the MK2 cell line and the T antigen-producing COS-1 cell line, levels of expression from plasmids pBL(GT)cat and pBLcat were approximately the same, again demonstrating the poly-GT element requires E1A for trans -activation and further demonstrating that the poly-GT element is not trans -activated by T antigen. Highest levels of expression were achieved with plasmid pBal8cat(GT6), which showed nearly a 1000 fold increase in relative levels of CAT over those with plasmid pLPcat. Plasmid pBal8cat(GT6) is illustrative of the improved expression vectors of this invention, which contain an E1A activated poly-GT element and a modifed BK enhancer-promoter region.

Claims: Claims for the following Contracting States : AT, BE, CH, DE, FR, GB, GR, IT, NL, SE A method for producing a useful substance in a eukaryotic host cell wherein the cell is transformed with a recombinant DNA vector that comprises an adenovirus late promoter, a poly-GT element positioned to stimulate the promoter, and a DNA sequence that encodes the useful substance, the DNA sequence being positioned for expression from the promoter, and wherein the cell containing the vector is cultured under conditions suitable for expression of the useful substance, which comprises: (a) providing the cell with a DNA sequence that codes for the expression of an immediate-early gene product of an adenovirus, and (b) culturing the cell of step (a) under conditions suitable for expressing the gene product and stimulating the activity of the poly-GT element. A method as claimed in Claim 1 in which the immediate-early gene product of an adenovirus is adenovirus E1A protein or adenovirus E1B protein. A method as claimed in Claim 1 or Claim 2 in which the immediate-early gene product of a large DNA virus is adenovirus E1A protein. A method as claimed in any one of Claims 1 to 3 in which the useful substance is human protein C. A method as claimed in any one of Claims 1 to 4 in which the eukaryotic host cell cultured in step (b) is an adenovirus-transformed host cell. A method as claimed in Claim 5 in which the eukaryotic host cell is a 293 (ATCC CRL 1573) or AV12 cell (ATCC CRL 9595). Plasmids: pLP(GT)cat (Figure 4) pBL(GT)cat (Figure 5) pSBL(GT)cat (Figure 6) pLPC(GT) (Figure 8) pLPC4(GT)(Figure 9) pLPC5(GT), differing from pLPC4(GT) with respect to the orientation of the BK virus DNA pLPChyg1(GT) (Figure 11) pLPChyg2(GT), differing from pLPChyg1(GT) with respect to the orientation of the ∼2.5 kb Bam HI fragment of pSV2hyg pLPCdhfr1(GT) produced by inserting a poly-GT element into the Xho I site in pLPCdhfr1, said pLPCdhfr1 being produced by ligating the Nde - Stu I-digested pLPC DNA (Example 8) to ∼1.9 kb Bam HI restriction fragment of pBW32 (Figure 12) pLPCdhfr2(GT) produced by inserting a poly-GT element into the Xho I site in pLPCdhfr2, said pLPCdhfr2 being produced by ligating the Nde - Stu I-digested pLPC DNA (Example 8) to ∼1.9 kb Bam HI restriction fragment of pBW32 (Figure 12) pLPChd1(GT) (Figure 13) pLPChd2(GT), differing from pLPChd1(GT) with respect to the orientation of the ∼1.9 kb Bam HI fragment comprising the dhfr gene pLPChd3(GT) produced by ligating the ∼2.5 Klenow-treated Bam HI fragment of pSV2hyg (NRRL B-18039) with the ∼5.82 kb ∼Klenow-treated Nde - Stu I fragment of pLPC (Example 8) to give plasmid pLPChyg2, ligating the ∼1.9 kb Klenow-treated Bam HI fragment of pBW32 (Figure 12) with Eco RIcut pLPChyg2 to give plasmid pLPChd3, and inserting a poly-GT element into the Xho I site in pLPChd3 pLPChd4(GT) produced by ligating the ∼2.5 Klenow-treated Bam HI fragment of pSV2hyg (NRRL B-18039) with the ∼5.62 kb ∼Klenow-treated Nde - Stu I fragment of pLPC (Example 8) to give plasmid pLPChyg2, ligating the ∼1.9 kb Klenow-treated Bam HI fragment of pBW32 (Figure 12) with Eco RIcut pLPChyg2 to give plasmid pLPChd4, and inserting a poly-GT element into the Xho I site in pLPChd4 pBLT(GT) (Figure 16) pBLThyg1(GT) (Figure 17) pBLThyg2(GT), differing from pBLThyg1(GT) with respect to the orientation of the ∼2.5 kb Bam HI fragment encoding the hygromycin resistance-conferring gene pBLTdhfr1(GT) (Figure 18) pBLTdhfr2(GT), differing from pBLTdhfr1(GT) with respect to the orientation of the ∼1.9 kb Bam HI fragment encoding the dhfr gene phd(GT)TPA (Figure 20) phd(GT)MTPA (Figure 21) pBal 8 cat(GT6) (Figure 22) pBal 8 cat(GT9), differing from pBal 8 cat(GT6) with respect to the orientation of the poly-GT element p4-14(GT) produced by inserting a poly-GT element into the Bcl I site of p4-14 (Example 24) p2-5(GT) produced by inserting a poly-GT element into the Xho I site of p2-5 (Example 24) pT6hd(GT) (Figure 24) pLPCEIA(GT) (Figure 15) pLPCEIAI(GT), differing from pLPCEIA(GT) with respect to the orientation of the EIA gene. A plasmid that comprises a poly-GT element positioned to stimulate transcription from the adenovirus-2 late promoter in the presence of an immediate-early gene product of a large DNA virus. A plasmid of Claim 8 that further comprises the BK enhancer positioned to stimulate transcription from the adenovirus-2 late promoter. Plasmid phd(GT) (Figure 14). A plasmid of Claim 8 that comprises a poly-GT element positioned to stimulate transcription, from the adenovirus-2 late promoter in the presence of an immediate-early gene product of a large DNA virus, of a DNA sequence that encodes a protein. A plasmid of Claim 11 in which the protein is chloramphenicol acetyltransferase. A plasmid of Claim 12 that further comprises the BK enhancer positioned to stimulate transcription from the adenovirus-2 late promoter. A plasmid of Claim 11 in which the protein is human protein C. A plasmid as claimed in Claim 11 in which the protein is tissue plasminogen activator. A plasmid as claimed in Claim 11 in which the protein is a modified tissue plasminogen activator. A plasmid as claimed in Claim 11 in which the protein is α-interferon, β-interferon or γ-interferon. A plasmid as claimed in Claim 11 that further comprises DNA that encodes the tripartite leader of adenovirus between the adenovirus-2 late promoter and the DNA sequence that encodes a protein. A eukaryotic host cell transformed with a plasmid as claimed in any one of Claims 7 to 18. A eukaroytic host cell of Claim 19 which is a 293 (ATCC CRL 1573) or AV12 cell (ATCC CRL 9595). A eukaryotic host cell of Claim 20 that is 293/pLPC(GT), 293/pLPC4(GT), 293/pLPChyg1(GT), 293/pLPCdhfr1(GT), 293/pLPChd1(GT), 293/p4-14(GT), 293/pT6hd(GT), AV12/pLPC(GT), AV12/pLPC4(GT), AV12/pLPChyg1(GT), AV12/pLPCdhfr1(GT), AV12/pLPChd1(GT), AV12/p4-14(GT), AV12/pT6hd(GT), said plasmids being defined in Claim 7. An improved method for producing a protein that is naturally γ-carboxylated, properly folded, end processed and wherein the protein is encoded in a recombinant DNA vector such that the protein is expressed when a eukaryotic host cell containing the vector is cultured under suitable expression conditions, the improvement comprising: (a) inserting a poly-GT element into the recombinant DNA vector; (b)inserting the vector of step (a) into an adenovirus-transformed host cell expressing the E1A gene product; (c)culturing the host cell of step (b) under growth conditions and in media containing sufficient vitamin K for carboxylation. A method as claimed in Claim 22 in which the adenovirus-transformed host cell is a human embryonic kidney 293 or AV12 cell. The method of Claim 23 in which the γ-carboxylated protein is human protein C. A method of producing a protein in a eukaryotic host cell that comprises: (a)transforming the host cell with a recombinant DNA vector that comprises: (1) a poly-GT element; (2) a first enhancer; (3) a second enhancer that is different from the first enhancer; (4) a eukaryotic promoter; and (5) a DNA sequence that encodes the protein and that is positioned for expression from the promoter; and (b)culturing the host cell transformed in step (a) under conditions that allow for gene expression. The method of Claim 25 in which the first enhancer is an SV40 enhancer and the second enhancer is a BK enhancer. The method of Claim 25 in which the protein is human protein C. Claims for the following Contracting State : ES A method for producing a useful substance in a eukaryotic host cell wherein the cell is transformed with a recombinant DNA vector that comprises an adenovirus late promoter, a poly-GT element positioned to stimulate the promoter, and a DNA sequence that encodes the useful substance, the DNA sequence being positioned for expression from the promoter, and wherein the cell containing the vector is cultured under conditions suitable for expression of the useful substance, which comprises: (a) providing the cell with a DNA sequence that codes for the expression of an immediate-early gene product of an adenovirus, and (b) culturing the cell of step (a) under conditions suitable for expressing the gene product and stimulating the activity of the poly-GT element. A method as claimed in Claim 1 in which the immediate-early gene product of an adenovirus is adenovirus E1A protein or adenovirus E1B protein. A method as claimed in Claim 1 or Claim 2 in which the immediate-early gene product of a large DNA virus is adenovirus E1A protein. A method as claimed in any one of Claims 1 to 3 in which the useful substance is human protein C. A method as claimed in any one of Claims 1 to 4 in which the eukaryotic host cell cultured in step (b) is an adenovirus-transformed host cell. A method as claimed in Claim 5 in which the eukaryotic host cell is a 293 (ATCC CRL 1573) or AV12 cell (ATCC CRL 9595). An improved method for producing a protein that is naturally γ-carboxylated, properly folded, and processed and wherein the protein is encoded in a recombinant DNA vector such that the protein is expressed when a eukaryotic host cell containing the vector is cultured under suitable expression conditions, the improvement comprising: (a) inserting a poly-GT element into the recombinant DNA vector; (b)inserting the vector of step (a) into on adenovirus-transformed host cell expressing the E1A gene product; (c)culturing the host cell of step (b) under growth conditions and in media containing sufficient vitamin K for carboxylation. A method as claimed in Claim 7 in which the adenovirus-transformed host cell is a human embryonic kidney 293 (ATCC CRL 1573) or AV12 cell (ATCC CRL 9595). The method of Claim 8 in which the γ-carboxylated protein is human protein C. A method of producing a protein in a eukaryotic host cell that comprises: (a)transforming the host cell with a recombinant DNA vector that comprises: (1) a poly-GT element; (2) a first enhancer; (3) a second enhancer that is different from the first enhancer; (4) a eukaryotic promoter; and (5) a DNA sequence that encodes the protein and that is positioned for expression from the promoter; and (b)culturing the host cell transformed in step (a) under conditions that allow for gene expression. The method of Claim 10 in which the first enhancer is an SV40 enhancer and the second enhancer is a BK enhancer. The method of Claim 10 in which the protein is human protein C. A method as claimed in any one of Claims 1 to 12 in which the host cell is cultured at a temperature of about 25° C to about 40°C. A method as claimed in Claim 13 in which the culturing temperature is about 37°C.

Inventors: BERG DAVID THOMPSON, GRINNELL BRIAN WILLIAM, BERG, DAVID THOMPSON, GRINNELL, BRIAN WILLIAM
Applicants: LILLY CO ELI, ELI LILLY AND COMPANY
Application year: 1989
Main classification: C12N15
IPC codes: C12N9, C12R1, C12N15, C12P21