EXPRESSION VECTORS FOR USE IN E, COLI

Abstract: The present invention provides a transcriptional and translational activating sequence derived from the lambda pL transcriptional activating sequence and the E. coli 1pp trans­ lational activating sequence. The activating sequence has been cloned into recombinant DNA expression vectors into which DNA sequences encoding functional polypeptides can be readily inserted and expressed. The activating sequence of the present invention has been shown to drive high-level ex­ pression of a bovine growth hormone derivative and a human growth hormone derivative in E. coli. Preferred expression vectors of the present invention also comprise the c1857 tem­ perature-sensitive lambda pL repressor gene, a rop derivat­ ive of the plasmid pBR322 replicon, and a tetracycline resist­ ance-conferring gene.

Description: The present invention provides a novel transcriptional and translational activating sequence and related recombinant DNA expression vectors for use in E . coli . The novel activating sequence is derived from the bacteriophage lambda pL promoter and the E. coli lpp gene translational activating sequence. The preferred expression vectors of the present invention utilize the novel transcriptional and translational activating sequence of the present invention, the temperature-sensitive lambda cI repressor gene cI857, a tetracycline resistance-conferring gene, and a rop ⁻ derivative of the replicon of plasmid pBR322. The expression vectors of the present invention are versatile and allow expression of any functional polypeptide-encoding gene of interest, provided that the gene is introduced into the vector in the correct reading phase. The utility and versatility of the present expression vectors are exemplified by plasmids disclosed herein that, when introduced into E . coli and the resulting transformants cultured at the appropriate temperature, drive expression of a bovine growth hormone derivative or a human growth hormone derivative to levels approaching 30% of the total cellular protein. For purposes of the present invention, as disclosed and claimed herein, the following terms are as defined below. Amino acid residues are abbreviated as follows: ALA is an Alanine ARG is an Arginine ASN is an Asparagine ASP is an Aspartic CYS is a Cysteine GLN is a Glutamine GLU is a Glutamic Acid GLY is a Glycine HIS is a Histidine ILE is an Isoleucine LEU is a Leucine LYS is a Lysine MET is a Methionine PHE is a Phenylalanine PRO is a Proline SER is a Serine THR is a Threonine TRP is a Tryptophan TYR is a Tyrosine VAL is a Valine 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 enzymatic or other activity which confers resistance to an antibiotic. ApR - the ampicillin-resistant phenotype or gene conferring the same. Cloning - the process of incorporating a segment of DNA into a recombinant DNA cloning or expression vector. Deoxyribonucleotide residues are abbreviated as follows: A is deoxyadenyl; G is deoxyguanyl; C is deoxycytidyl; and T is thymidyl. EK-BGH - methionyl-phenylalanyl-leucinyl(aspartyl)₄-lysinyl-bovine growth hormone, which is bovine growth hormone, beginning with the amino-terminal phenylalanyl residue, linked at the amino-terminus to amino acid residues that encode an enterokinase cleavage site. On the plasmid maps herein, the gene that encodes such a bovine growth hormone derivative is represented as EK-BGH. Functional Polypeptide - a recoverable bioactive entirely heterologous or homologous polypeptide or precursor, a recoverable bioactive polypeptide comprising a heterologous polypeptide and a portion or whole of a homologous polypeptide or a recoverable bioinactive fusion polypeptide comprising a heterologous polypeptide and a bioinactivating homologous polypeptide that can be specifically cleaved. KmR - the kanamycin-resistant phenotype or gene conferring the same. lpp T - the transcription terminator sequence of the E . coli lpp gene. MET-ASP-HGH - methionyl-aspartyl human growth hormone. ori - the origin of replication. Permissive Temperature - the temperature range throughout which a temperature-sensitive gene product functions in a manner similar to its wild-type counterpart. pL - the phage lambda leftward transcriptional activating sequence. Promoter - a DNA sequence that directs transcription of DNA into RNA. Recombinant DNA Cloning Vector - any autonomously replicating or integrating agent, including, but not limited to, plasmids, comprising a DNA molecule to which one or more additional DNA molecules can be or have been added. Recombinant DNA Expression Vector - any autonomously replicating or integrating agent, including, but not limited to, plasmids, comprising a DNA molecule that encodes a transcriptional and translational activating sequence and to which one or more additional DNA molecules encoding functional polypeptides can be or have been added so as to be expressed from the activating sequence. Replicon - A DNA sequence that controls and allows for autonomous replication of a plasmid or other vector. Restriction Fragment - any linear DNA molecule generated by the action of one or more restriction enzymes. Selectable Marker - a DNA sequence, such as an antibiotic resistance-conferring gene, that encodes an activity that provides a basis for selection, such that only cells that comprise the DNA sequence can grow and divide. Restrictive Temperature - the temperature range throughout which a temperature-sensitive gene product does not function or functions ineffectively as compared to the wild-type gene product. Sensitive Host Cell - a host cell that cannot grow in the presence of a given antibiotic without a DNA segment that confers resistance thereto. Structural Gene - DNA that encodes the amino acids comprising a functional polypeptide but lacks transcriptional and translational activating sequences. TcR - the tetracycline-resistant phenotype or gene conferring the same. Transcriptional Activating Sequence - a DNA sequence that directs transcription of DNA into RNA. Transformant - a recipient host cell that has undergone transformation. Transformation - the introduction of DNA into a recipient host cell that changes the genotype and results in a change in the recipient cell. Translational Activating Sequence - any DNA sequence, including the Shine-Dalgarno sequence and translational start codon, such as 5'-ATG-3', but not including any sequences downstream from the start codon, that provides for the initiation of translation of a mRNA transcript into a peptide or polypeptide. Description of the Drawings Figure 1 is a restriction site and function map of plasmid pKC283. Figure 2 is a restriction site and function map of plasmid pKC283PX. Figure 3 is a restriction site and function map of plasmid pKC283-L. Figure 4 is a restriction site and function map of plasmid pKC283-LB. Figure 5 is a restriction site and function map of plasmid pKC283PRS. Figure 6 is a restriction site and function map of plasmid pL32. Figure 7 is a restriction site and function map of plasmid pL47. Figure 8 is a restriction site and function map of plasmid pL84. Figure 9 is a restriction site and function map of plasmid pL95. Figure 10 is a restriction site and function map of plasmid pPR12. Figure 11 is a restriction site and function map of plasmid pPR12AR1. Figure 12 is a restriction site and function map of plasmid pL110. Figure 13 is a restriction site and function map of plasmid pL111. Figure 14 is a restriction site and function map of plasmid pCC101. Detailed Description of the Invention The present invention concerns a novel, hybrid pL- lpp transcriptional and translational activating sequence, recombinant DNA expression vectors that comprise the novel activating sequence, and host cells transformed with the expression vectors of the present invention. The novel activating sequence of the present invention has the following structure: The novel activating sequence of the present invention has many features not readily observable from the nucleotide sequence depicted above. Nucleotides 1 - 138 of the sequence are derived from the lambda leftward promoter, whereas nucleotides 143 - 172 are derived from the E . coli lpp translational activating sequence. Nucleotides 139 - 142 are derived neither from lambda nor from the E . coli lpp gene but instead were generated in the fusion of the pL transcriptional activating sequence to the lpp translational activating sequence. Nucleotides 1 - 138 encode not only the pL transcriptional activating sequence but also three pL operator sequences, which serve as repressor binding sites. The operator sequences are encoded at residues 65 - 81, 85 - 101, and 109 - 125. Messenger RNA (mRNA) transcription begins at the deoxyadenyl residue at position 134 in wild-type lambda and proceeds toward the translational start codon; presumably, mRNA transcription begins at the same position in the novel activating sequence of the present invention. The translational start codon of the coding sequence to be expressed is located at residues 170 - 172, which are 5'-ATG-3'. Two restriction enzyme sites in the activating sequence of the present invention are particularly useful. A Bgl II site located at residues 137 - 142 allows for substitution of the lpp translational activating sequence with other activating sequences and also allows for nearly complete control over the sequence of the RNA to be transcribed from the pL promoter. The Xba I site at residues 153 - 158 provides a convenient site for the insertion of functional polypeptide-encoding DNA, although the inserted DNA must also comprise sequences that reconstruct the activating sequence of the present invention. The Xba I site could also be used in conjunction with the Bgl II site for translational activating sequence substitutions. The number of nucleotides between the transcriptional start site, at residue 134, and the translational start site, at residue 170, is also an important feature of the novel activating sequence of the present invention. Most E . coli genes have about 20 - 40 nucleotides between the transcriptional start site and translational start codon. The transcriptional and translational activating sequence of the present invention, with about 36 nucleotides between the transcriptional and translational start sites, is therefore typical of most E . coli activator sequences. In contrast, most of the expression vectors disclosed in the prior art that utilize the lambda pL transcriptional activating sequence comprise well over one hundred nucleotides between the transcriptional start site and the translational start codon. Preferred expression vectors of the present invention comprise not only the novel activating sequence, but also a replicon that allows extrachromosomal maintenance of the vector, a selectable marker, and a DNA sequence that encodes a functional polypeptide. More preferred expression vectors of the present invention further comprise a gene that encodes a temperature-sensitive repressor of the lambda pL transcriptional activating sequence. The most preferred expression vectors of the present invention comprise the novel activating sequence, a tetracycline resistance-conferring gene, the lambda cI857 gene, and a DNA sequence that encodes a functional polypeptide, such as enterokinase-linked bovine growth hormone (EK-BGH) or methionyl-aspartyl-human growth hormone (MET-ASP-HGH). The prototype expression vector of the present invention, into which any gene of interest can be inserted and expressed, was constructed using plasmid pKC283 as starting material. Plasmid pKC283 is ∼9 kilobases (kb) in size and can be conventionally isolated from E . coli K12 BE1201/pKC283, a strain deposited and made part of the permanent stock culture collection of the Northern Regional Research Laboratory, Peoria, Illinois 61604. The strain is available to the public as a preferred source and stock reservoir of plasmid pKC283 under the accession number NRRL B-15830. E . coli K12 BE1201/pKC283 may carry two different plasmids that can be conventionally isolated and separated on the basis of size and the pattern of restriction sites. A restriction site and function map of plasmid pKC283 is provided in Figure 1 of the accompanying drawings. Plasmid pKC283 comprises the lambda pL transcriptional activating sequence and a bacteriophage T₇ late gene translational activating sequence connected in translational reading phase with a gene encoding β-galactosidase activity. Sequences from a λ lac transducing phage are encoded at the 3' end of the β-galactosidase-encoding DNA. The replicon of plasmid pKC283 is derived from plasmid pBR322, but because the ∼2.1 kb Eco RI- Pvu II restriction fragment of plasmid pBR322 has been replaced by the lambda pL, bacteriophage T₇, and β-galactosidase-encoding sequences in plasmid pKC283, plasmid pKC283 is rop ⁻. A plasmid with the rop ⁻ mutation, described in Cesareni et al ., 1982, Proc. Natl. Acad. Sci. U.S.A. 79:6313 and Som et al ., 1983, Proc. Natl. Acad. Sci. U.S.A. 80:3232, has a copy number in E . coli about 5X higher than plasmid pBR322. All of the expression vectors of the present invention exemplified herein utilize the rop ⁻ derivative of the plasmid pBR322 replicon. However, the high level of expression of functional polypeptide achieved by host cells transformed with the plasmids of the present invention is due, primarily, to the novel transcriptional and translational activating sequence disclosed herein. Consequently, the present invention also comprises the use of the novel activating sequence to drive expression of functional polypeptides from expression vectors that utilize other plasmid replicons, such as the runaway replicon described in U.S. Patent No. 4,487,835, or phage (viral) sequences for the purpose of introducing and/or maintaining the expression vector in the host cell. In order to construct the expression vector of the present invention, plasmid pKC283 was digested with Pvu II, ligated to Xho I linkers, and recircularized to form plasmid pKC283PX. These steps deleted ∼3.0 kb of the β-galactosidase-encoding DNA and placed an Xho I restriction enzyme recognition site where the deletion took place. A restriction site and function map of plasmid pKC283PX is presented in Figure 2 of the accompanying drawings. Plasmid pKC283PX was digested with restriction enzymes Bgl II and Xho I, and a synthetic linker encoding an Xba I restriction enzyme recognition sequence and part of the E . coli lpp translational activating sequence site was introduced into the Bgl II- Xho I-digested plasmid pKC283PX. Essentially, this latter step merely replaced ∼0.15 kb of β-galactosidase encoding DNA with a synthetic DNA fragment. The resulting plasmid was designated pKC283-L; a restriction site and function map of plasmid pKC283-L is presented in Figure 3 of the accompanying drawings. Plasmid pKC283-L was digested with Xho I, treated with DNA polymerase I large fragment (Klenow), ligated to Bam HI linkers, digested with Bam HI, and religated to form plasmid pKC283-LB. Plasmid pKC283-LB does not comprise the entire activating sequence of the present invention but can still be used to construct expression vectors. The Xba I and Bam HI sites on plasmid pKC283-LB are situated so that functional polypeptide-encoding DNA can be introduced into plasmid pKC283-LB as an Xba I- Bam HI restriction fragment. High-level expression of the functional polypeptide-encoding DNA will occur when the Xba I- Bam HI restriction fragment comprises sequences that reconstruct the novel activating sequence of the present invention. A restriction site and function map of plasmid pKC283-LB is presented in Figure 4 of the accompanying drawings. Plasmid pKC283-LB comprises ∼2.1 kb of E . coli DNA that is unnecessary for expression vector functions. In order to reduce the size of plasmid pKC283-LB, the following replacement reaction was carried out. Plasmid pKC283PX was digested with Sal I, treated with Klenow, ligated to Eco RI linkers, digested with Eco RI, and recircularized by ligation to yield plasmid pKC283PRS. A restriction site and function map of plasmid pKC283PRS is presented in Figure 5 of the accompanying drawings. Plasmid pKC283PRS was then digested with restriction enzymes Pst I and Sph I, and the resulting ∼0.85 kb restriction fragment was isolated from the other digestion products and purified. Plasmid pKC283-LB was digested with Pst I and Sph I, and the resulting ∼3.0 kb restriction fragment was isolated and purified. The purified fragments were then ligated together to form plasmid pL32. A restriction site and function map of plasmid pL32 is presented in Figure 6 of the accompanying drawings. Although an intermediate plasmid in the construction of the expression vector of the present invention, plasmid pL32 can drive expression of an inserted functional polypeptide-encoding DNA fragment. A derivative of plasmid pL32, designated pL47, was constructed by insertion of an Xba I- Bam HI restriction fragment encoding Met-Phe-Pro-Leu-Asp-Asp-Asp-Asp-Lys-bovine growth hormone (EK-bGH). The Xba - Bam HI restriction fragment also comprised sequences that, when inserted into plasmid pKC283-LB, reconstructed the novel activating sequence of the present invention. The Xba I- Bam HI restriction fragment is ∼0.6 kb and originates from plasmid pCC101. The construction of plasmid pCC101 is disclosed in Example 3 of European Patent Application 95 361, published 30 November 1983, attorney docket number X-5872, incorporated herein by reference. A restriction site and function map of plasmid pCC101 is presented in Figure 14 of the accompanying drawings. Plasmid pCC101, also known as plasmid pCZ101, is claimed in European Patent Application 85 301 468.6, filed 4 March 1985, attorney docket number X-6125B. A restriction site and function map of plasmid pL47 is presented in Figure 7 of the accompanying drawings. Plasmid pL47 is lethal to E . coli host cells which do not possess a lambda pL repressor gene. Initiation of transcription from the lambda pL promoter is known to be repressed to levels below detection limits by the product of the lambda gene cI. Mutants of the cI gene are available that code for a temperature-sensitive repressor (Lieb, M., 1966, J. Mol. Biol. 16:149), so that transcription from pL is repressed at low temperature and is initiated at high temperature. Because of the availability of repressor genes such as cI, the lambda pL promoter has the distinct advantage of being controllable in E . coli , a characteristic which is desirable where large-scale microbial fermentation production is concerned. The lambda cro gene product also represses transcription from the pL promoter. As stated above, uncontrolled transcription from the lambda pL promoter of the EX-BGH-encoding DNA is lethal to E . coli transformed with plasmid pL47. In the presence of repressor, however, the lambda pL promoter does not function, and no EK-BGH is expressed by E . coli /pL47 transformants. To obtain good production of EK-BGH, the E . coli /pL47 transformants must comprise a gene that encodes a temperature-sensitive repressor of the lambda pL promoter, so that lambda pL can be repressed while the cells are being cultured to the appropriate cell density for expression. The cI857 gene encodes a temperature-sensitive repressor of the lambda pL promoter. E . coli /pL47 transformants that comprise the cI857 gene, either on an extrachromosomally-replicating vector or integrated into the chromosome, can be cultured at the permissive temperature and EK-BGH expressed at levels approaching 30% of the total cellular protein at the restrictive temperature. The presence of a transcription terminator at the 3' end of the expressed gene of expression vectors such as pL47 is known to enhance expression. Because the sequence downstream of the 3' end of the EK-BGH-encoding DNA on plasmid pL47 originated from the λ lac transducing phage and was not known to encode a transcription terminator, a vector was constructed to replace the sequences at the 3' end of the EK-BGH-encoding DNA of plasmid pL47 with sequences known to encode the E . coli lpp terminator. The E . coli lpp terminator can be easily isolated from plasmid pCC101. In order to construct the desired plasmid with the lpp terminator at the 3' end of the EK-BGH-encoding DNA, plasmid pL32 was digested with Bam HI, treated with Klenow, ligated to Sal I linkers, digested with Sal I, and then digested with Xba I. The resulting ∼3.9 kb DNA fragment had Sal I and Xba I cohesive ends and was then ligated to the ∼1.62 kb Xba I- Sal I restriction fragment of plasmid pCC101. This latter ∼1.62 kb restriction fragment encodes EK-BGH and the lpp transcription terminator in the desired orientation. The plasmid resulting from the ligation of the Xba I- Sal I restriction fragments described above was designated plasmid pL84. A restriction site and function map of plasmid pL84 is presented in Figure 8 of the accompanying drawings. To delete extraneous DNA at the 3' end of the lpp terminator, plasmid pL84 was digested with Nde I, treated with Klenow, ligated to Kpn I linkers, digested with Kpn I, and recircularized by ligation to yield plasmid pL95. A restriction site and function map of plasmid pL95 is presented in Figure 9 of the accompanying drawings. When E . coli /pL95 transformants comprising the temperature-sensitive cI857 gene product are cultured for expression of EK-BGH, the desired EK-BGH accumulates to levels approaching 30% of total cellular protein. Therefore, despite the presence of a defined terminator on plasmid pL95, plasmids pL47 and pL95 drive expression of EK-BGH to about the same extent. Because plasmids pL47 and pL95 drive expression of EK-BGH equally well, and because plasmid pL47 comprises λ lac transducing phage sequences at the 3' end of the EK-BGH-encoding sequences, the λ lac transducing phage sequences on plasmid pL47 probably encode a transcription terminator. However, neither plasmid pL47, nor plasmid pL95, are ideal expression vectors. Both plasmids utilize an ampicillin resistance-conferring gene for selection purposes. Because the ampicillin resistance-conferring enzyme destroys the ampicillin in the media, high levels of the antibiotic need to be maintained in the fermentation broth in order that the ampicillin not be depleted. Of course, depletion of the ampicillin would allow cells that do not comprise an expression plasmid to fluorish, resulting in low yields of the desired product. The use of a tetracycline resistance-conferring gene as a selectable marker eliminates the need for high levels of antibiotic and the problem of plasmid loss. The tetracycline resistance-conferring mechanism affects the permeability of the host cell membrane and does not inactivate the tetracycline in the media. Therefore, plasmids pL47 and pL95 could be improved by replacing the ampicillin resistance-conferring gene on the plasmids with the tetracycline resistance-conferring gene. Plasmids pL47 and pL95 could also be improved by placing the cI857 gene on the same plasmid as the pL promoter. As described above, E . coli transformants of plasmid pL47 or plasmid pL95 require the presence of the cI857 gene product, with the cI857 gene encoded either in the chromosome of the host cell or in a separate extrachromosomal vector, to prevent the plasmids from being lethal to the host cell during the period of culture to obtain the requisite number of cells for expression. However, the presence of the cI857 gene in either the host cell chromosomal DNA or a separate vector can result in there being more copies of the pL- lpp hybrid transcriptional and translational activating sequence of the present invention in the host cell than copies of the cI857 gene or gene product. Such a deficiency of cI857 gene product would result in expression of the gene driven by the pL- lpp hybrid transcriptional and translational activating sequence. Because many recombinant gene products exert deleterious effects on the host cell, regulated expression such that a gene product is expressed only when the culture is at a certain density and condition is highly desirable. Such regulated expression can be assured by placing the cI857 gene on the same expression vector as the pL- lpp hybrid transcriptional and translational activating sequence of the present invention. The improvements of pL47 and pL95 discussed above were achieved in the following manner. The desired cI857 and tetracycline resistance-conferring genes were isolated from plasmid pPR12, which is disclosed and claimed in U.S. Patent No. 4,436,815, issued 13 March 1984. A restriction site and function map of plasmid pPR12 is presented in Figure 10 of the accompanying drawings. To isolate both the cI857 and tetracycline resistance-conferring genes on a Pst I- Eco RI restriction fragment, plasmid pPR12 was digested with Eco RI, treated with Klenow, and recircularized by ligation to yield plasmid pPR12ΔR1. Plasmid pPR12ΔR1 was digested with Ava I, treated with Klenow, ligated to Eco RI linkers, digested with Eco RI, and recircularized by ligation to yield plasmid pPR12AR1. A restriction site and function map of plasmid pPR12AR1 is presented in Figure 11 of the accompanying drawings. The ∼2.9 kb Pst I- Eco RI restriction fragment of plasmid pPR12AR1 was ligated to the ∼1.03 kb Eco RI- Bam HI and ∼2.7 kb Bam HI- Pst I restriction fragments of plasmid pL47 to yield plasmid pL110. A restriction site and function map of plasmid pL110 is presented in Figure 12 of the accompanying drawings. In an analogous construction, the ∼2.9 kb Pst I- Eco RI restriction fragment of plasmid pPR12AR1 was ligated to the ∼1.03 kb Eco RI- Bam HI restriction fragment of plasmid pL47 and to the ∼2.03 kb Bam HI- Pst I restriction fragment of plasmid pL95 to yield plasmid pL111. A restriction site and function map of plasmid pL111 is presented in Figure 13 of the accompanying drawings. Plasmids pL110 and pL111 are useful, controllable expression vectors. A variety of functional polypeptide-encoding DNA fragments can be introduced into and expressed from plasmids pL110 and pL111. The functional polypeptide-encoding DNA is best introduced into the plasmid on an Xba I- Bam HI restriction fragment, in a reaction that replaces the EK-BGH-encoding DNA of plasmids pL110 and pL111 with the functional polypeptide-encoding DNA desired to be expressed. The modifications necessary to convert any functional polypeptide-encoding DNA into the required Xba I- Bam HI restriction fragment employ reactions within the capacity of one skilled in the art. Plasmid pL110 serves as starting material in the construction of plasmid pNM1093, a plasmid that utilizes the activating sequence of the present invention to drive expression of MET-ASP-HGH in E . coli host cells. Plasmid pNM1093 is constructed by ligating the ∼6.0 kb Xba I- Bam HI restriction fragment of plasmid pL110 to an ∼0.6 kb Xba I- Bam HI restriction fragment that encodes a small portion of the transcriptional and translational activating sequence of the present invention and the complete coding sequence MET-ASP-HGH. Plasmid pNM1093 is ∼6.6 kb in size; E . coli K12 RV308/pNM1093 transformants express MET-ASP-HGH at levels approaching 30% of the total cellular protein when cultured at the restrictive temperature for the cI857 gene product. The following Examples describe the construction of the expression vectors of the present invention. Example 1 Isolation of Plasmid pKC283 Lyophils of E . coli K12 BE1201/pKC283 are obtained from the Northern Regional Research Laboratory, Peoria, Illinois 61604, under the accession number NRRL B-15830. The lyophils are decanted into tubes containing 10 ml LB medium (10 g Bacto-tryptone, 5 g Bacto-yeast extract, and 10 g NaCl per liter; pH is adjusted to 7.5) 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 the 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 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 LB-agar (LB medium with 15 g/l Bacto-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 LB medium containing 50 µg/ml ampicillin and incubated overnight at 32°C with vigorous shaking. The 10 ml overnight culture was inoculated into 500 ml LB medium and incubated at 32°C with vigorous shaking until the culture reached stationary phase. The following procedure is adapted from Maniatis et al ., 1982, Molecula r Cloning (Cold Spring Harbor Laboratory). The cells were harvested by centrifugation at 4000 g for 10 minutes at 4°C, and the supernatant was discarded. The cell pellet was washed in 100 ml of ice-cold STE buffer (0.1 M NaCl; 10 mM Tris-HCl, pH 7.8; and 1 mM EDTA). After washing, the cell pellet was resuspended in 10 ml of Solution 1 (50 mM glucose; 25 mM Tris-HCl, pH 8.0; and 10 mM EDTA) containing 5 mg/ml lysozyme and left at room temperature for 10 minutes. Twenty ml of Solution 2 (0.2 N NaOH and 1% SDS) were then added to the lysozyme-treated cells, and the solution was gently mixed by inversion. The mixture was incubated on ice for 10 minutes. Fifteen ml of ice-cold 5 M potassium acetate, pH 4.8, were added to the lysed-cell mixture and the solution mixed by inversion. The solution was incubated on ice for 10 minutes. The 5 M potassium acetate solution was prepared by adding 11.5 ml of glacial acetic acid to 28.5 ml of water and 60 ml of 5 M potassium acetate; the resulting solution is 3 M with respect to potassium and 5 M with respect to acetate. The lysed cell mixture was centrifuged in a Beckman SW27 (or its equivalent) at 20,000 rpm for 20 minutes at 4°C. The cell DNA and debris formed a pellet on the bottom of the tube. About 36 ml of supernatant were recovered, and 0.6 volumes of isopropanol were added, mixed, and the resulting solution left at room temperature for 15 minutes. The plasmid DNA was collected by centrifugation at 12,000 g for 30 minutes at room temperature. The supernatant was discarded, and the DNA pellet was washed with 70% ethanol at room temperature. The ethanol wash was decanted, and the pellet was dried in a vacuum desiccator. The pellet was then resuspended in 8 ml of TE buffer (10 mM Tris-HCl, pH 8.0, and 1 mM EDTA). Eight grams of CsCl were added to the DNA solution. About 0.8 ml of a 10 mg/ml solution of ethidium bromide in water were added for each 10 ml of CsCl-DNA solution. The final density of the solution was about 1.55 g/ml, and the ethidium bromide concentraton was about 600 µg/ml. The solution was transferred to a Beckman Type 50 centrifuge tube, filled to the top with paraffin oil, sealed, and centrifuged at 45,000 rpm for 24 hours at 20°C. After centrifugation, two bands of DNA were visible in ordinary light. After removing the cap from the tube, the lower DNA band was removed by using a syringe with a No. 21 hypodermic needle inserted through the side of the centrifuge tube. The ethidium bromide was removed by several extractions with water-saturated 1-butanol. The CsCl was removed by dialysis against TE buffer. After extractions with buffered phenol and then chloroform, the DNA was precipitated, washed with 70% ethanol, and dried. About 1 mg of plasmid pKC283 was obtained and stored at 4°C in TE buffer at a concentration of about 1 µg/ul. A restriction site and function map of plasmid pKC283 is presented in Figure 1 of the accompanying drawings. Example 2 Construction of Plasmid pKC283PX About 10 µl of the plasmid pKC283 DNA prepared in Example 1 were mixed with 20 µl 10 X medium-salt restriction buffer (500 mM NaCl; 100 mM Tris-HCl, pH 7.5; 100 mM MgCl₂; and 10 mM DTT), 20 µl 1 mg/ml BSA, 5 µl restriction enzyme Pvu II (∼50 Units, as defined by Bethesda Research Laboratories (BRL), from which all restriction enzymes used herein were obtained), and 145 µl of water, 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 5 X Kinase Buffer (300 mM Tris-HCl, pH 7.8; 50 mM MgCl₂; and 25 mM DTT), 5 µl 5 mM ATP, 24 µl H₂O, 0.5 µl of T4 polynucleotide kinase (about 2.5 units as defined by P-L Biochemicals, Inc., 1037 W. McKinley Ave., Milwaukee, WI, 53205), 5 µl 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 10 X ligase buffer (300 mM Tris-HCl, pH 7.6; 100 mM MgCl₂; and 50 mM DTT), 2.5 µl of 1 mg/ml BSA, 7 µl of 5 mM ATP, 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 3 µl of water 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; 10.0 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 2 of the accompanying drawings. Example 3 Construction of E . coli K12 MO(λ⁺)/pKC283PX E . coli K12 MO(λ⁺)can be obtained from the Northern Regional Research Laboratories in lyophylized form under the accession number NRRL B-15993. E . coli K12 MO(λ⁺) comprises the wild-type lambda pL cI repressor gene, so that transcription from the hybrid pL- lpp promoter of the present invention does not occur in E . coli K12 MO(λ⁺) cells. The lyophils are reconstituted, single colonies of MO(λ⁺) are isolated, and a 10 ml overnight culture of the MO(λ⁺)cells is prepared in substantial accordance with the procedure of Example 1, except that the temperature of incubation is 37°C and no ampicillin is used in the growth media. Fifty µl of the overnight culture were used to inoculate 5 ml of LB media 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 LB media containing 10 mM MgSO₄ and 10 mM MgCl₂. The diluted culture was incubated at 37°C with vigorous shaking until the absorbance at 550 nm (A₅₅₀) 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 4000 g for 10 minutes at 4°C. The cell pellet was resuspended in 100 ml of cold 10 mM NaCl and then immediately re-pelleted 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 one-half ml aliquot of the cells was added to the ligated DNA prepared in Example 2; 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 LB media in 125 ml flasks and incubated at 37°C for one hour. One hundred µl aliquots were plated on LB-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 accordance with the procedure of Example 1, 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 2 of the accompanying drawings. Example 4 Construction of E . coli K12 MO(λ⁺)/pKC283-L Ten µg of plasmid pKC283PX DNA prepared in accordance with the procedure of Example 1 were dissolved in 20 µl of 10X high-salt buffer, 20 µl 1 mg/ml BSA, 5 µl (∼50 units) restriction enzyme Bgl II, 5µl (∼50 units) restriction enzyme Xho I, and 150 µl of water, and the resulting reaction was incubated at 37°C for two hours. The reaction was stopped, and after precipitating the Bgl II- Xho I digested DNA, 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 and kinased. The linker was kinased in substantial accordance with the procedure of Example 2. The DNA linker had the following structure: The linker depicted above was synthesized from single-stranded deoxyoligonucleotides by procedures well known in the art. The single-stranded deoxyoligonucleotides can be synthesized with commercially available instruments, such as the 380A DNA Synthesizer marketed by Applied Biosystems (850 Lincoln Centre Drive, Foster City, CA 94404), which utilizes phosphoramidite chemistry. Other procedures for synthesizing DNA are also known in the art. The conventional modified phosphotriester method of synthesizing single stranded DNA is described in Itakura et al ., 1977, Science 198:1056 and in Crea et al ., 1978, Proc. Nat. Acad. Sci. USA 75:5765. In addition, an especially preferred method of synthesizing DNA is disclosed in Hsiung et al ., 1983, Nucleic Acid Research 11:3227 and Narang et al ., 1980, Methods in Enzymology 68:90. The linker and Bgl II- Xho I-digested plasmid pKC283PX were ligated in substantial accordance with the procedure of Example 2. The ligated DNA constituted the desired plasmid pKC283-L. A restriction site and function map of plasmid pKC283-L is presented in Figure 3 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 in substantial accordance with the procedure of Example 3. Example 5 Construction of E . coli K12 MO(λ⁺)/pKC283-LB About 10 µg of plasmid pKC283-L DNA, prepared in substantial accordance with the procedures of Example 1, were dissolved in 20 µl 10X high-salt buffer, 20 µl 1 mg/ml BSA, 5 µl (∼50 units) restriction enzyme Xho I, and 155 µl of H₂O, and the resulting reaction was incubated at 37°C for two hours. The Xho I-digested plasmid pKC283-L DNA was then precipitated from the reaction mixture by the addition of three volumes of 95% ethanol and one-tenth volume of 3 M sodium acetate, incubation in a dry ice-ethanol bath for five minutes, and centrifugation. The resulting DNA pellet was washed with 70% ethanol, dried, and resuspended in 2 µl 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 H₂O, 1 µl (∼6 units as defined by P-L Biochemicals) of Klenow, which is the large fragment of E . coli DNA polymerase I, 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 procedure of Example 2. 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 in substantial accordance with the procedure of Example 2. 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 of Examples 2 and 3. The E . coli K12 MO(λ⁺)/pKC283-LB transformants were identified, and then plasmid pKC283-LB DNA was prepared in substantial accordance with the procedure of Example 1. A restriction site and function map of plasmid pKC283-LB is presented in Figure 4 of the accompanying drawings. Example 6 Construction of E . coli K12 MO(λ⁺)/pL32 About 10 µg of plasmid pKC283PX were digested with restriction enzyme Sal I in high-salt buffer, treated with Klenow, and ligated to Eco RI linkers (5'-GAGGAATTCCTC-3') in substantial accordance with the procedure of Example 5, with the exception of the starting plasmid, restriction enzymes, and linkers used. 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⁺) in substantial accordance with the procedure of Example 3. After the E . coli K12 MO(λ⁺)/pKC283PRS transformants were identified, plasmid pKC283PRS DNA was prepared in substantial accordance with the procedure of Example 1. A restriction site and function map of plasmid pKC283PRS is presented in Figure 5 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-gelling-temperature agarose (FMC Corporation, Marine Colloids Division, Rockland, Maine 04841) gel for 2-3 hours at ∼130 V and ∼75 mA in Tris-Acetate buffer. 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 segment was determined by weight and density of the segment, and an equal volume of 10 mM Tris-HCl, pH 7.6, was added to the tube containing the segment. The segment was then melted by incubation at 72°C. About 1 ug of the ∼0.85 kb Pst I- Sph I restriction fragment of plasmid pKC283PRS was obtained in a volume of about 100 µl. 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 in substantial accordance with the procedure of Example 2. The ligated DNA constituted the desired plasmid pL32. A restriction site and function map of plasmid pL32 is presented in Figure 6 of the accompanying drawings. Plasmid pL32 was transformed into E . coli K12 MO(λ⁺) cells in substantial accordance with the procedure of Example 3. Plasmid pL32 DNA was prepared from the E. coli K12 MO(λ⁺)/pL32 transformants in substantial accordance with the procedure of Example 1. 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. Example 7 Construction of E . coli K12 MO(λ⁺)/pL47 Plasmid pCC101 is disclosed in Example 3 of European Patent Application 95 361, published 30 November 1983, incorporated herein by reference. A restriction site and function map of plasmid pCC101 is presented in Figure 14 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 and prepared for ligation in substantial accordance with the procedure of Example 6. 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 in substantial accordance with the procedure of Example 2 to yield plasmid pL47. A restriction site and function map of plasmid pL47 is presented in Figure 7 of the accompanying drawings. Plasmid pL47 was transformed into E . coli K12 MO(λ⁺) in substantial accordance with the procedure of Example 3, 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 1. Example 8 Construction of E . coli K12 MO(λ⁺)/pL84 About 10 µg of plasmid pL32 were digested with about 50 units of restriction enzyme Bam HI in 200 µl of high-salt buffer at 37°C for about two hours. The Bam HI-digested plasmid pL32 DNA was precipitated, treated with Klenow, and ligated to Sal I linkers (5'-CGTCGACG-3') in substantial accordance with the procedure of Example 5. After the Sal I linkers were ligated, about 100 units of restriction enzyme Sal I and about 50 units of restriction enzyme Xba I were added to the ligation mixture, which was adjusted to have the composition of high-salt buffer, and the resulting reaction was incubated at 37°C for two hours. The reaction products were separated by agarose gel electrophoresis, and the ∼3.9 kb Sal I- Xba I restriction fragment was isolated and prepared for ligation in substantial accordance with the procedure of Example 6. About 10 µg of plasmid pCC101 were digested with about 50 units each of restriction enzymes Sal I and Xba I in 200 µl of high-salt buffer at 37°C for 2 hours. The reaction products were separated by agarose gel electrophoresis, and the ∼1.6 kb Sal I- Xba I restriction fragment which encodes EK-BGH and the transcription terminator of the E . coli lpp gene was isolated and prepared for ligation in substantial accordance with the procedure of Example 6. The ∼3.9 kb Sal I- Xba I restriction fragment derived from plasmid pL32 was ligated to the ∼1.6 kb Sal I- Xba I restriction fragment of plasmid pCC101 in substantial accordance with the procedure of Example 2. The ligated DNA constituted the desired plasmid pL84. A restriction site and function map of plasmid pL84 is presented in Figure 8 of the accompanying drawings. The ligated DNA was used to transform E . coli K12 MO(λ⁺) cells in substantial accordance with the procedure of Example 3. Plasmid pL84 DNA was prepared from the E . coli K12 MO(λ⁺)/pL84 transformants in substantial accordance with the procedure of Example 1. Example 9 Construction of E . coli K12 MO(λ⁺)/pL95 About 10 µg of plasmid pL84 were digested with about 50 units restriction enzyme Nde I in about 200 µl of high-salt buffer for two hours at 37°C. The Nde I-digested DNA was then precipitated and treated with Klenow in substantial accordance with the procedure of Example 5. The Nde I-digested, Klenow-treated plasmid pL84 DNA was then ligated to Kpn I linkers (5'-GGGTACCC-3') in substantial accordance with the procedure of Example 2. After the linker ligation, the DNA was precipitated and the resulting DNA pellet was resuspended in 20 µl of 10X low-salt buffer (0.1 M Tris-HCl, pH 7.6; 0.1 M MgCl₂; and 10 mM DTT), 20 µl of 1 mg/ml BSA, 5 µl (about 50 units) restriction enzyme Kpn I, and 155 µl of H₂O. The resulting reaction was incubated at 37°C for two hours. After the Kpn I digestion, the DNA was loaded onto a low-melting-temperature agarose gel, and the ∼3.8 kb restriction fragment was isolated and recircularized by ligation in substantial accordance with the procedure of Example 6. The ligated DNA constituted plasmid pL95, which was used to transform E . coli K12 MO(λ⁺) in substantial accordance with the procedure of Example 3. A restriction site and function map of plasmid pL95 is presented in Figure 9 of the accompanying drawings. Example 10 Construction of E . coli K12 RV308/pPR12AR1 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 #4,436,815, issued 13 March 1984. A restriction site and function map of plasmid pPR12 is presented in Figure 10 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 treated with Klenow in substantial accordance with the procedure of Example 5. After the Klenow reaction, the Eco RI-digested, Klenow-treated plasmid pPR12 DNA was recircularized by ligation in substantial accordance with the procedure of Example 2. 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, except that selection was based on tetracycline (5 ug/ml) resistance, not ampicillin resistance. E . coli K12 RV308 is available from the NRRL under the accession number NRRL B-15624. 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 1. 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 treated with Klenow in substantial accordance with the procedure of Example 5. After the Klenow reaction, the Ava I-digested, Klenow-treated plasmid pPR12ΔR1 DNA was ligated to Eco R1 linkers (5'-GAGGAATTCCTC-3') in substantial accordance with the procedure of Example 2. After the linker ligation, the DNA was precipitated and then resuspended in about 200 µl of high-salt buffer containing about 50 units of restriction enzyme Eco R1. The resulting reaction was incubated at 37°C for about 2 hours. After the Eco R1 digestion, the reaction mixture was loaded onto an agarose gel, and the ∼5.1 kb Eco R1 restriction fragment was purified in substantial accordance with the procedure of Example 6. The ∼5.1 kb Eco R1 restriction fragment was recircularized by ligation in substantial accordance with the procedure of Example 2. The ligated DNA constituted the desired plasmid pPR12AR1. The plasmid pPR12AR1 DNA was transformed into E . coli K12 RV308 in substantial accordance with the procedure of Example 3, except that selection was based on tetracycline resistance, not ampicillin resistance. After identifying the E . coli K12 RV308/pPR12AR1 transformants, plasmid pPR12AR1 DNA was prepared in substantial accordance with the procedure of Example 1. A restriction site and function map of plasmid pPR12AR1 is presented in Figure 11 of the accompanying drawings. Example 11 Construction of E . coli K12 RV308/pL110 About 10 µg of plasmid pPR12AR1 DNA were suspended in about 200 ml 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 an agarose gel, and the ∼2.9 kb Pst I- Eco R1 restriction fragment of plasmid pPR12AR1 was isolated and prepared for ligation in substantial accordance with the procedure of Example 6. About 10 ug of plasmid pL47 were digested with restriction enzymes Pst I and Bam HI in 200 ul of high-salt buffer at 37°C for two hours. The Pst I- Bam HI-digested DNA was loaded onto an 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 6. In a separate reaction, about 10 ug of plasmid pL47 DNA were digested with restriction enzymes Eco RI and Bam HI in 200 ul of high-salt buffer at 37°C for two hours, and the ∼1.03 kb Eco RI- Bam HI restriction fragment that comprised the novel transcriptional and translational activating sequence and the EK-BGH-encoding DNA was isolated and prepared for ligation in substantial accordance with the procedure of Example 6. The ∼2 ug of the ∼1.03 kb Eco RI- Bam HI restriction fragment obtained were used in the construction of both plasmid pL110 and plasmid pL111. The construction of plasmid pL111 is described in Example 12. 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 in substantial accordance with the procedure of Examples 2 and 3, except that tetracycline resistance, not ampicillin 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. Example 12 Construction of E . coli K12 RV308/pL111 Plasmid pL111 was constructed in substantial accordance with the procedure of Example 11. The ∼1.03 kb Eco RI- Bam HI restriction fragment of plasmid pL47 was ligated to the ∼2.03 kb Bam HI- Pst I restriction fragment of plasmid pL95 and to the ∼2.9 kb Pst I- Eco RI restriction fragment of plasmid pPR12AR1 to construct plasmid pL111, so plasmid pL111 differs from plasmid pL110 only in the sequences located downstream of the 3' end of the EK-BGH coding region. The ligation of the restriction fragments and the transformation of the resulting plasmid pL111 DNA into E . coli K12 RV308 was done in substantial accordance with the procedures of Examples 2 and 3, although tetracycline resistance, not ampicillin resistance, was used as the basis for selecting transformants. A restriction site and function map of plasmid pL111 is presented in Figure 13 of the accompanying drawings. Example 13 Protocol for Induction of Functional Polypeptide Expression From the Lambda pL- lpp Hybrid Transcriptional and Translational Activating Sequence Although the following procedure specifically exemplifies induction of EK-BGH expression from the activating sequence of the present invention, the procedure is equally applicable to the induction of expression of other functional polypeptides using the expression vectors of the present invention. The "cells" and "culture" referred to in the following procedure represent individual cultures of the following transformants of the present invention: E . coli K12 RV308/pRK248cIts, pL47; E . coli K12 RV308/pRK248cIts, pL84; E . coli K12 RV308/pRK248cIts, pL95; E . coli K12 RV308/pL110; and E . coli K12 RV308/pL111. Because E . coli K12 MO(λ⁺) comprises a wild-type lambda cI repressor gene, plasmids pL47, pL84, and pL95 had to be introduced into a host that comprised a temperature-sensitive lambda pL repressor gene to successfully conduct the induction experiments. Consequently, E . coli K12 RV308/pRK284cIts host cells were prepared and transformed with plasmids pL47, pL84, and pL95. Plasmid pRK248cIts can be isolated from E . coli K12 JMB9/pRK248cIts, also known as E . coli K12 MCB3604, a strain deposited and made part of the permanent culture collection of the Northern Regional Research Laboratories. Plasmid pRK248cIts can be obtained from the NRRL under the accession number NRRL B-15631. Plasmid pRK248cIts comprises a temperature-sensitive mutant of the lambda pL repressor gene cI; the construction of plasmid pRK248cIts is described in Bernard and Helinski, 1979, Methods of Enzymology, 68:482, and Bernard et al ., 1979, Gene 5:59. Plasmid pRK248cIts was isolated and purified in substantial accordance with the procedure of Example 1 and then transformed into E . coli K12 RV308 in substantial accordance with the procedure of Example 3. Because plasmid pRK248cIts comprises the tetracycline resistance-conferring gene, and not the ampicillin resistance-conferring gene, tetracycline resistance was used as the basis for selection of the E . coli K12 RV308/pRK248cIts transformants. However, a variety of plasmids could be used in place of plasmid pRK248cIts for the purposes of this Example. Such plasmids would necessarily comprise a temperature-sensitive lambda pL repressor gene, a selectable marker independent of the ampicillin resistance-conferring gene, and a replicon, such as the replicon from plasmid pACYC184 or an R factor, that is compatible with the replicon of plasmid pBR322. Furthermore, temperature-sensitive control of the novel activating sequence of the present invention could also be obtained by using a host cell that comprised a chromosomally-integrated, temperature-sensitive cI repressor gene. The E . coli K12 RV308/pRK248cIts transformants were made competent for transformation in substantial accordance with the procedure of Example 3, except that the E . coli K12 RV308/pRK248cIts culture was grown at 30°C rather than 37°C, and the CaCl₂-treated cells were heat-shocked at 34°C rather than 42°C. In separate transformations, plasmids pL47, pL84 and pL95 were transformed into the E . coli K12 RV308/pRK248cIts competent cells. Because the desired transformants comprise both the tetracycline resistance-conferring gene from plasmid pRK248cIts and also the ampicillin resistance-conferring gene from plasmid pL47, plasmid pL84, or plasmid pL95, both ampicillin and tetracycline selection were maintained to retain the plasmids in the transformants. About 10 µl aliquots of cells at stationary phase were used to inoculate about 10 ml of LB media containing the appropriate antibiotic(s) for selection of cells containing the desired plasmid(s). The culture was incubated with shaking at 32°C for about 16 hours. The culture was diluted with fresh media containing antibiotic(s) for plasmid selection so that the A₅₅₀ of the diluted culture was less than 0.1. The culture was incubated with shaking at 32°C until the A₅₅₀ was ∼0.3. Then, the culture was diluted 1:1 into fresh media, which had been warmed to 42°C and contained antibiotics for plasmid selection, and incubation with shaking was continued at 42°C for 2-4 hours. One ml aliquots of the culture were removed at various points between two hours and four hours after the shift to 42°C. The cells were collected by centrifugation and resuspended in about 0.1 ml of sample buffer. Five ml of sample buffer are prepared by mixing into solution 1.8 g of urea, 2 mg bromophenol blue, 0.25 ml β-mercaptoethanol, 1.5 ml 10% SDS, 0.625 ml 4 X upper-Tris buffer (0.5 M Tris-HCl, pH 6.8, and 0.4% SDS), and 0.75 ml glycerol. The samples were then boiled and electrophoresed on a 12% SDS-polyacrylamide gel. After Coomassie-Blue staining, the gel demonstrated that EK-BGH represented about 30% of the total cellular protein. Example 14 Construction of E . coli K12 RV308/pNM1093 About 10 µg of plasmid pL110 DNA is digested with about 50 units of restriction enzyme Xba I and about 2 units of restriction enzyme Bam HI in 200 µl of high-salt buffer. The reaction is carried out so that the Xba I digest goes to near completion but the Bam HI reaction is incomplete; the partial Bam HI digest is necessary to isolate fragments that comprise the complete tetracycline resistance-conferring gene. The Xba I- Bam HI-digested plasmid pL110 DNA is then loaded onto an agarose gel, and the ∼6.0 kb Xba I- Bam HI restriction fragment that comprises all of plasmid pL110 except the EK-BGH-encoding sequences is isolated and prepared for ligation in substantial accordance with the procedure of Example 6. An ∼0.6 kb Xba I- Bam HI restriction fragment encoding MET-ASP-HGH is prepared and ligated to the ∼6.0 kb Xba I- Bam HI restriction fragment of plasmid pL110 to construct plasmid pNM1093. The sequence, numbered to facilitate discussion, of the ∼0.6 kb Xba I- Bam HI restriction fragment is depicted below: The coding region for MET-ASP-HGH is located from nucleotide residue 17 to residue 595 in the sequence depicted above. The amino acid sequence of MET-ASP-HGH is depicted below, beginning with the residue at the amino terminus: The ∼0.6 kb Xba - Bam HI restriction fragment encoding MET-ASP-HGH can be prepared by synthetic means using procedures well-known in the art. Martial et al ., 1979, Science 205:602, have disclosed the cloning of much of the human growth hormone coding sequence by isolating human pituitary mRNA that comprised human growth hormone mRNA, preparing complementary DNA (cDNA) with reverse transcriptase, and inserting the cDNA into a plasmid vehicle. Goodman et al ., 1979, Methods in Enzymology 68:75, have disclosed the procedure for isolating human pituitary mRNA. Furthermore, U. S. Patent No. 4,363,877, issued 14 December 1982 to Goodman et al ., discloses that human growth hormone cDNA can be isolated from plasmid pHGH-1, which was deposited in the American Type Culture Collection, 12301 Parklawn Drive, Rockville, Maryland 20852, under the accession number ATCC 40,000. Human growth hormone cDNA can be used to prepare the ∼0.6 kb Xba I- Bam HI restriction fragment used in the construction of plasmid pNM1093. Once the cDNA has been cloned into a suitable plasmid vector, the ∼0.6 kb Xba I- Bam HI restriction fragment can be constructed by using restriction enzymes to isolate much of the human growth hormone coding sequence and then attaching synthetic DNA linkers designed to recreate the sequence of the ∼0.6 kb Xba I- Bam HI restriction fragment depicted above. Plasmid pNM1093 was used to transform E . coli K12 RV308 in substantial accordance with the procedure of Example 3, except that tetracycline resistance, not ampicillin resistance, was used as the basis for selecting transformants. The E . coli K12 RV308/pNM1093 transformants express MET-ASP-HGH at levels approaching 30% of the total cellular protein when cultured in accordance with the procedure of Example 13.

Claims: The lambda pL- lpp hybrid transcriptional and translational activating sequence of the following structure: wherein A is deoxyadenyl; G is deoxyguanyl; C is deoxycytidyl; and T is thymidyl. A recombinant DNA expression vector that comprises the sequence of Claim 1. The vector of Claim 2 which is a plasmid. The vector of Claim 3 that further comprises the following elements: a. a rop ⁻ replicon derived from plasmid pBR322; b. a selectable marker; and c. a DNA sequence that encodes a functional polypeptide. The vector of Claim 4 that is: plasmid pL47 shown in Figure 7 and obtained by inserting the ∼0.6kb Xba I- Bam HI restriction fragment from plasmid pCC101 into pL32 obtained by digesting pKC283PRS with restriction enzymes Pst I and Sph I, isolating the ∼0.85kb restriction fragment, digesting pKC283-LB with Pst I and Sph I, isolating the ∼3.0kb restriction fragment and ligating it with the ∼0.85kb restriction fragment; plasmid pL84 shown in Figure 8 and obtained by digesting pL32 with Bam HI, treating with Klenow, ligating to Sal I linkers, digesting with Sal I and Xba I to give a ∼3.9kb DNA fragment and ligating it with the ∼1.62kb Xba I- Sal I restriction fragment of plasmid pCC101; or plasmid pL95 shown in Figure 9 and obtained by digesting pL84 with Nde I, treating with Klenow, ligating to Kpm I linkers, digesting with KpnI and recircularizing. The vector of Claim 4 that further comprises a gene that encodes a temperature-sensitive cI repressor of lambda pL. The vector of Claim 6 wherein the selectable marker is the tetracycline resistance-conferring gene, the gene that encodes a temperature-sensitive repressor of lambda pL is CI857, and the DNA sequence that encodes a functional polypeptide is a DNA sequence that encodes EK-BGH. The vector of Claim 6 wherein the selectable marker is the tetracycline resistance-conferring gene, the gene that encodes a temperature-sensitive repressor of lambda pL is cI857, and the DNA sequence that encodes a functional polypeptide is a DNA sequence that encodes MET-ASP-HGH. The vector of Claim 7 that is: plasmid pL110 shown in Figure 12 and obtained by ligating the ∼2.9kb Pst I- Eco RI restriction fragment of plasmid pPR12AR1 obtained by digesting pPR12ΔRI with Ava I, treating with Klenow, ligating to Eco RI linkers, digesting with Eco RI and recircularizing, to the ∼1.03kb Eco RI- Bam HI and ∼2.7kb Bam HI- Pst I restriction fragments of plasmid pL47; or plasmid pL111 shown in Figure 13 and obtained by ligating the ∼2.9kb Pst I- Eco RI restriction fragment of plasmid pPR12AR1 to the ∼1.03kb Eco RI- Bam HI restriction fragment of plasmid pL47 and to the ∼2.03kb Bam HI- Pst I restriction fragment of pL95. The vector of Claim 8 that is plasmid pNM1093 obtained by ligating the ∼6.0kb Xba I- Bam HI restriction fragment of plasmid pL110 to an ∼0.6kb Xba I- Bam HI restriction fragment encoding a small portion of the transcriptional and translational activating sequence according to Claim 1 and the complete coding sequence MET-ASP-HGH. A host cell transformed with an expression vector of any of Claims 2 to 5. The host cell of Claim 11 that is E . coli K12 RV308/pRK248cIts obtained by transforming E . coli K12 RV308 (NRRL B-15624) with plasmid pRK248cIts obtained from NRRL B-15631. A host cell transformed with a plasmid of any of Claims 6 to 10. The host cell of Claim 13 that is E . coli K12 RV308. Plasmid pKC283PX shown in Figure 2 and obtained by digesting pKC283 (from E . coli K12 BE1201/pKC283, NRRL B-15830) with Pvu II, ligating to Xho I linkers, and recircularizing; plasmid pKC283-L shown in Figure 3 and obtained by digesting pKC283PX with restriction enzymes Bgl II and Xho I, and introducing a synthetic linker encoding an Xba I restriction enzyme recognition sequence and part of the E . coli lpp translational activating sequence site; plasmid pKC283-LB shown in Figure 4 and obtained by digesting pKC283-L with Xho I, treating with DNA polymerase I large fragment (Klenow), ligated to Bam HI linkers, digested with Bam HI and religated; plasmid pL32 shown in Figure 6 and obtained by digesting pKC283PRS with restriction enzymes Pst I and Sph I, isolating the ∼0.85kb restriction fragment, digesting pKC283-LB with Pst I and Sph I, isolating the ∼3.0kb restriction fragment and ligating it with the ∼0.85kb restriction fragment; plasmid pKC283PRS shown in Figure 5 and obtained by digesting pKC283PX with Sal I, treating with Klenow, ligating to Eco RI linkers, digesting with Eco RI and recircularizing by ligation, pPR12ΔR1 obtained by digesting plasmid pPR12 with Eco RI, treating with Klenow and recircularizing; or plasmid pPR12AR1 shown in Figure 11 and obtained by digesting pPR12ΔR1 with Ava I, treating with Klenow, ligating to Eco RI linkers digesting with Eco RI and recircularizing.

Inventors: JASKUNAS, STANLEY RICHARD, JR.
Applicants: ELI LILLY AND COMPANY
Application year: 1986
Main classification: C12N15
IPC codes: C12N15, C12N1, C07K14, C12R1