The construction and characterisation of plasmid vectors suitable for the expression of all DNA phases under the control of the E. coli tryptophan promoter

Characterization of heterologous hemoglobin and flavohemoglobin promoter regulation in Escherichia coli

Bacterial hemoglobins and flavohemoglobins have been used to improve cell growth and productivity in biotechnological applications. The expression of globin genes can be induced by reducing the oxygen supply or applying external stressors, which provide a simple and inexpensive mechanism for induction of heterologous protein production. It is in the interest of the biotechnological industry to seek new promoters, which are non-patented, cheap and simple to induce. Therefore, new globin gene promoters have been isolated from Campylobacter jejuni, Bacillus subtilis, Deinococcus radiodurans, Streptomyces coelicolor, and Salmonella typhi. The goal was to obtain insights about the regulation mechanisms of these promoters in Escherichia coli using in silico and experimental methods. The recognition of these promoters by the E. coli transcriptional machinery was first analyzed by computational methods. Computer analysis revealed that all the promoters, except the promoter of S. coelicolor, should be functional in E. coli and most of them also contain putative binding sites for ArcA, CRP, and FNR global regulators. Furthermore, the expression profiles of the promoters fused to the chloramphenicol acetyl transferase gene were analyzed under various conditions using E. coli mutants devoid of regulatory molecules. In vivo regulation studies of globin promoters mainly verified the in silico predictions.

Tightly regulated vectors for the cloning and expression of toxic genes

A series of low-copy expression vectors that permits the stable maintenance and regulated expression of highly toxic gene products has been developed. These vectors utilize the lactose promoter/operator system, and protect against read-through transcription from other promoters on the plasmid by placement of the rrnB T1T2 terminators upstream of the lactose promoter. For additional regulatory control, the vectors utilize low-copy origins of replication. Either the pMPP6 origin (pSC101-derived) is used for cloning into Escherichia coli or related species, or the broad-host-range RK2 origin of replication is utilized for cloning into the majority of Gram-negative bacteria. The resulting plasmids have no detectable leaky expression. To test these vectors, the genes for the bacteriocidal colicins D, E3, and E7 were cloned and stably maintained in the absence of their immunity genes. Upon induction with isopropyl-beta-D-thiogalactopyranoside (IPTG), cell death was observed, indicating expression of each colicin. These low-copy expression vectors will be useful for the cloning and expression of toxic genes in bacterial systems.

Strategies for achieving high-level expression of genes in Escherichia coli

Progress in our understanding of several biological processes promises to broaden the usefulness of Escherichia coli as a tool for gene expression. There is an expanding choice of tightly regulated prokaryotic promoters suitable for achieving high-level gene expression. New host strains facilitate the formation of disulfide bonds in the reducing environment of the cytoplasm and offer higher protein yields by minimizing proteolytic degradation. Insights into the process of protein translocation across the bacterial membranes may eventually make it possible to achieve robust secretion of specific proteins into the culture medium. Studies involving molecular chaperones have shown that in specific cases, chaperones can be very effective for improved protein folding, solubility, and membrane transport. Negative results derived from such studies are also instructive in formulating different strategies. The remarkable increase in the availability of fusion partners offers a wide range of tools for improved protein folding, solubility, protection from proteases, yield, and secretion into the culture medium, as well as for detection and purification of recombinant proteins. Codon usage is known to present a potential impediment to high-level gene expression in E. coli. Although we still do not understand all the rules governing this phenomenon, it is apparent that "rare" codons, depending on their frequency and context, can have an adverse effect on protein levels. Usually, this problem can be alleviated by modification of the relevant codons or by coexpression of the cognate tRNA genes. Finally, the elucidation of specific determinants of protein degradation, a plethora of protease-deficient host strains, and methods to stabilize proteins afford new strategies to minimize proteolytic susceptibility of recombinant proteins in E. coli.

An optimization study of a pH-inducible promoter system for high-level recombinant protein production in Escherichia coli

The unique properties exhibited by the pH-inducible promoter system are clearly demonstrated by the plasmid construct, pSM552-545C-. Step changes of pH substantially increase the expression of beta-galactosidase. Very high expression, a level of around 40% of total cellular protein, can be achieved with superbroth. The high level of induction in rich media, typical of those commonly used to achieve high cell density, suggests the system is versatile enough to be adapted to many specific situations. The variable degree of induction by pH within the range of 8.0 and 5.5 makes possible a degree of expression control not easily accomplished with the existing systems. By precise monitoring of induction pH, a "fine tuning" of foreign gene expression and growth rate to optimum levels is possible. The effect of several operating parameters on recombinant protein production are evaluated. Our results show that operating environments play an extremely important role in achieving high recombinant protein expression levels in a dense culture. Under suboptimal conditions, as are shown in this study, only moderately high levels can be obtained. Even for suboptimal cases, an expression level of about 10 to 15% of total cellular protein while achieving an optical density higher than 25 is routinely obtained. Our results also show that a proper balance between cell growth and recombinant protein synthesis processes are critical in maintaining high expression levels in a dense culture. Any imbalance will most likely lead to more cell growth and poorer protein productivity. We have also demonstrated that reactor operating temperature can be a useful parameter to fine-tune this balance, resulting in significantly improved results.

High level heterologous expression in E. coli using the anaerobically-activated nirB promoter

The anaerobically-regulated nirB promoter was used to express heterologous genes in Escherichia coli. Under anaerobic conditions the promoter was able to express tetanus toxin fragment C at approximately 20% total cell protein (tcp) and the Bordetella pertussis antigen pertactin at greater than 30% tcp. These levels are comparable to those obtained for the same products using the tac promoter. The nirB promoter is very well regulated, giving almost two orders of magnitude increase in fragment C on complete removal of oxygen. The use of this anaerobically-induced promoter in the production of recombinant proteins in E. coli is discussed.

Expression of recombinant proteins in Escherichia coli using an oxygen-responsive promoter

The oxygen-dependent promoter of the Vitreoscilla hemoglobin (VHb) gene has been shown to be functional in E. coli. Earlier studies established that the promoter is maximally induced under microaerobic conditions and that its activity is also influenced by the cAMP-CAP complex. We demonstrate here that the promoter can be used for regulated, high-level expression of recombinant proteins in two-stage fed-batch fermentations. The promoter is maximally induced at dissolved oxygen levels lower than 5% air saturation. Despite the influence of catabolite repression, glucose and glycerol-containing media give comparable product levels under carbon-limited conditions such as those encountered in typical fed-batch fermentations. The possibility of a third level of control of promoter activity is also indicated. This mode of induction can be repressed by addition of a complex nitrogen source such as yeast extract to the medium. The observed promoter activity can be modulated at least 30-fold over the course of high-cell density fermentations producing either cloned beta-galactosidase or cloned chloramphenicol acetyltransferase (CAT). Densitometer scanning of SDS-polyacrylamide gels revealed that beta-galactosidase was expressed to a level of approximately 10% of total cellular protein.

High level expression of interleukin-1 beta in a recombinant Escherichia coli strain for use in a controlled bioreactor

A recombinant bacterial strain for the large scale production of human interleukin-1 beta (IL-1 beta) was constructed. The lambda Pr and the tryptophan systems were compared for efficiency of transcription and regulation. The efficiency of IL-1 protein production from these constructs was analyzed. Enhanced protein synthesis was achieved by the fusion of lambda Pr promoter sequences with trp leader sequences which included the trp RBS. A strain (JM101) was selected for use as a host and tested in a one liter bioreactor. A growth and induction regimen was established for use in bioreactors which results in the accumulation of 0.75-0.95 g l-1 of recombinant IL-1.

Effects of temperature on Escherichia coli overproducing beta-lactamase or human epidermal growth factor

The effects of temperature on strains of Escherichia coli which overproduce and excrete either beta-lactamase or human epidermal growth factor were investigated. E. coli RB791 cells containing plasmid pKN which has the tac promoter upstream of the gene for beta-lactamase were grown and induced with isopropyl-beta-D-thiogalactopyranoside in batch culture at 37, 30, 25, and 20 degrees C. The lower temperature greatly reduced the formation of periplasmic beta-lactamase inclusion bodies, increased significantly the total amount of beta-lactamase activity, and increased the purity of extracellular beta-lactamase from approximately 45 to 90%. Chemostat operation at 37 and 30 degrees C was difficult due to poor cell reproduction and beta-lactamase production. However, at 20 degrees C, continuous production and excretion of beta-lactamase were obtained for greater than 450 h (29 generations). When the same strain carried plasmid pCU encoding human epidermal growth factor, significant cell lysis was observed after induction at 31 and 37 degrees C, whereas little cell lysis was observed at 21 and 25 degrees C. Both total soluble and total human epidermal growth factor increased with decreasing temperature. These results indicate that some of the problems of instability of strains producing high levels of plasmid-encoded proteins can be mitigated by growth at lower temperatures. Further, lower temperatures can increase for at least some secreted proteins both total plasmid-encoded protein formed and the fraction that is soluble.

Promoter selectivity of Escherichia coli RNA polymerase: omega factor is responsible for the ppGpp sensitivity

Transcription in vitro of stringently controlled Escherichia coli genes by purified RNA polymerase holoenzyme is inhibited by guanosine tetraphosphate (ppGpp). In order to examine possible role of omega factor in this ppGpp sensitivity, RNA polymerases with or without the omega factor were reconstituted and tested for their ppGpp sensitivity using an in vitro mixed transcription system. RNA polymerase lacking the omega factor was found virtually insensitive to ppGpp but the addition of a purified omega factor restored the ppGpp sensitivity of this omega-free RNA polymerase. These results raise a possibility that the omega factor is a regulatory protein of RNA polymerase and is involved in the ppGpp-mediated alteration of the promoter selectivity.

Increased synthesis of human parathyroid hormone in Escherichia coli through alterations of the 5' untranslated region

The expression of human parathyroid hormone (hPTH) in Escherichia coli was optimized by variations of the spacing sequence between the ribosome-binding site (RBS) and the beginning of the gene (ATG) and by increasing the complementarity of the RBS to the 16 S rRNA. The expression level of 3 micrograms/liter increased more than 100-fold to 475 micrograms/liter as a direct consequence of modifications in the region 5' of the gene.

Host control of plasmid replication: requirement for the sigma factor sigma 32 in transcription of mini-F replication initiator gene

Replication of F factor or mini-F plasmid is strongly inhibited in the rpoH (htpR) mutants of Escherichia coli deficient in the sigma factor (sigma 32) known to be required for heat shock gene expression. Transcription of the mini-F repE gene encoding a replication initiator protein (E protein) was examined by operon fusion and by direct determination of repE mRNA. The synthesis rate and the level of repE mRNA were found to increase transiently upon temperature upshift (30 degrees C to 42 degrees C) in wild-type cells but to decrease rapidly in the rpoH mutants. Thus sigma 32 appeared to be directly involved in transcription of repE whose product, E protein, in turn activates DNA replication from the mini-F ori2 region. This scheme of host-controlled plasmid replication is further supported by the analysis of transcription in vitro: RNA synthesis can be initiated from the repE promoter by a minor form of RNA polymerase containing sigma 32 but not by the major polymerase containing the normal sigma factor sigma 70. The sigma 32-mediated transcription from the repE promoter is strongly inhibited by the E protein. We conclude that transcription of the mini-F repE gene is mediated by the host transcription factor sigma 32 and is negatively controlled by its own product.

II-BGlc, a glucose receptor of the bacterial phosphotransferase system: molecular cloning of ptsG and purification of the receptor from an overproducing strain of Escherichia coli

The bacterial phosphoenolpyruvate:glycose phosphotransferase system (PTS) consists of interacting cytoplasmic and membrane proteins that catalyze the phosphorylation and translocation of sugar substrates across the cell membrane. One PTS protein, II-BGlc, is the membrane receptor specific for glucose and methyl D-glucopyranosides; the protein has been purified to homogeneity from Salmonella typhimurium [Erni, B., Trachsel, H., Postma, P. & Rosenbusch, J. (1982) J. Biol. Chem. 257, 13726-13730]. In the present experiments, the Escherichia coli ptsG locus, which encodes II-BGlc, was isolated from a transducing phage library and subcloned into plasmid vectors. The resulting plasmids complement the following phenotypic defects of ptsG mutants: growth on glucose, uptake and phosphorylation of methyl alpha-D-glucoside, and repression of the utilization of non-PTS sugars by methyl alpha-glucoside. The transformed cells overproduce II-BGlc 4- to 10-fold, and a Mr 43,000 polypeptide was synthesized from the plasmids in an in vitro transcription/translation system. The E. coli and S. typhimurium II-BGlc proteins differ in their physical properties, and a modified, three-step purification procedure was developed for isolating the E. coli protein.

The trfB region of broad host range plasmid RK2: the nucleotide sequence reveals incC and key regulatory gene trfB/korA/korD as overlapping genes

We report the nucleotide sequence of the trfB region of broad host range plasmid RK2. This region encodes the following loci: trfB, identical to korA and korD, which encodes a key transcriptional repressor of certain RK2 operons; incC, which appears to be involved in plasmid maintenance, possibley through post-transcriptional regulation of trfA product levels; the start of korB, which encodes a second transcriptional repressor of operons involved in stable inheritance of RK2. These loci are expressed as part of the trfB operon. In combination with deletion analysis, transcriptional and translation fusions and 'maxicell' analysis of polypeptides, the DNA sequence allows a number of conclusions to be drawn. First, the korB ORF start codon overlaps the incC ORF stop codon, suggesting the possibility of translational coupling between these two genes. Second, the trfB ORF lies entirely within the first third of the incC ORF using a different phase. Third, the incC ORF appears to contain a second transcriptional start whose function appears to be coupled to translation of the trfB ORF. Analysis of codon usage in the region of overlap between incC and trfB suggests that the incC gene may have evolved before the trfB gene. Determination of the DNA sequence of a mutant in which the product of trfB is rendered defective for transcriptional repression reveals an amino acid alteration within a region of this polypeptide which exhibits homology to the alpha helix-turn-alpha helix motif characteristic of many DNA binding proteins, and which is probably responsible for recognition of the trfB operator by this protein.

Use of a portable ribosome-binding site for maximizing expression of a eukaryotic gene in Escherichia coli

To maximize expression of a eukaryotic gene in Escherichia coli, a series of plasmids were constructed containing various synthetic ribosome-binding sites (RBS). These sites consist of a Shine-Dalgarno (SD) region (with translation stop codons in all three reading frames) positioned at distances 5-9 nucleotides (nt) from the AUG initiator codon of the gene coding for human T-cell growth factor (TCGF or IL-2). The region encompassing the RBS through the TCGF structural gene from each of these plasmids was inserted as a 'cassette' into seven different E. coli expression vectors, and TCGF production was measured. Our results demonstrate a greater than 2000-fold range of TCGF synthesis dependent upon the promoter and the synthetic RBS used. The translational efficiency of the TCGF gene was found to be influenced by the quality of the RBS, which is in part determined by the external sequence context of this site. The synthetic RBS, containing the necessary information for the translation initiation process, readily accessible by restriction sites, should be of general usefulness in obtaining maximum expression of eukaryotic genes in E. coli.

Plasmid vector pBR322 and its special-purpose derivatives--a review

The plasmid pBR322 was one of the first EK2 multipurpose cloning vectors to be designed and constructed (ten years ago) for the efficient cloning and selection of recombinant DNA molecules in Escherichia coli. This 4363-bp DNA molecule has been extensively used as a cloning vehicle because of its simplicity and the availability of its nucleotide sequence. The widespread use of pBR322 has prompted numerous studies into its molecular structure and function. These studies revealed two features that detract from the plasmid's effectiveness as a cloning vector: plasmid instability in the absence of selection and, the lack of a direct selection scheme for recombinant DNA molecules. Several vectors based on pBR322 have been constructed to overcome these limitations and to extend the vector's versatility to accommodate special cloning purposes. The objective of this review is to provide a survey of these derivative vectors and to summarize information currently available on pBR322.

Use of a modified Escherichia coli trpR gene to obtain tight regulation of high-copy-number expression vectors

It has been found that with high-copy-number vectors utilising the Escherichia coli trp promoter the amount of repressor protein produced from the single chromosomally located trpR gene is inadequate for tight repression to be obtained. An attempt has been made to overcome this problem by inserting the trpR gene in cis into the expression vector. This proved unsuccessful because transcription from the trp promoter of such a plasmid could not be induced with 3,beta-indole acrylic acid, probably because the trpR gene is autogenously regulated. However, it was found that when the natural trpR promoter was replaced with a relatively weak constitutive promoter a useful self-repressible vector could be formed. A modified trpR gene of this type has been used to obtain tightly controlled expression of human interferon-beta (IFN-beta) from a vector having a copy number of 400. Tight regulation is particularly important in this case as IFN-beta is highly toxic to the E. coli cell.

Properties of a human immunoglobulin epsilon-chain fragment synthesized in Escherichia coli

A fragment of the cloned gene for the human myeloma ND epsilon chain, coding for the second, third, and fourth domains of the immunoglobulin, has been coupled to the tryptophan control region of an expression plasmid and subcloned in Escherichia coli. Induction of gene expression results in the synthesis of the expected, antigenically active polypeptide of Mr 40,000, which constitutes 18% of total bacterial protein and yields 55 mg/liter of culture. The immunoglobulin, which is aggregated and packed into large inclusion bodies within the bacterial cell, can be dissolved by denaturing solvents and purified by affinity chromatography using anti-IgE Sepharose. Reduced monomeric chains assemble spontaneously into dimers. On assay to measure the inhibition of binding of 125I-labeled human E myeloma protein to Fc epsilon receptors on cultured human basophils, the cloned gene product exhibited 20% of the activity of the native protein.

Construction and application of a novel plasmid "ATG vector" for direct expression of foreign genes in Escherichia coli

A new type of plasmid expression vector was developed for direct expression of foreign genes in Escherichia coli. The plasmid vector, designated pTrS3, carries the E. coli tryptophan (trp) promoter and the Shine-Dalgarno (SD) sequence for the trp leader peptide as well as an ATG sequence located 13 bp downstream from the SD sequence. The dG residue of this ATG overlaps with the first dG residue of the single Sph I recognition sequence (GCATGC) of the vector DNA. After cleaving pTrS3 DNA by Sph I, the 3' protruding Sph I ends were converted into blunt ends using the Klenow fragment of E. coli DNA polymerase I. Subsequently, the DNA fragments coding for mature human interferon-beta or for the interferon lacking several aminoterminal amino acids, were ligated to this vector DNA and cloned in E. coli. Interferon activity was detected in the extracts of bacterial strains harboring the recombinant plasmids and the results indicated that the interferon-beta polypeptides without the five aminoterminal amino acids might be less active than the mature form.

Cloning and expression of a porcine prorelaxin gene in E. coli

A porcine prorelaxin gene has been constructed partly by synthetic means and partly from its natural messenger RNA. A gene coding for the 32 N-terminal amino acids including a chain initiator methionine codon (B gene) was synthesised and inserted in a plasmid at a site downstream from a tryptophan promoter in such a way that its expression is under the control of the trp promoter. DNA corresponding to the rest of the prorelaxin was prepared using reverse transcriptase extension of a primer complementary to relaxin mRNA and joined at a suitable restriction site to the B gene. Transformation of E. coli with this plasmid followed by suitable induction resulted in the synthesis of a new protein identified as prorelaxin by its size and its antigenic similarity to relaxin.

Expression plasmid vectors containing Escherichia coli tryptophan promoter transcriptional units lacking the attenuator

Two DNA fragments which contain the Escherichia coli tryptophan promoter operator region but lack the attenuator have been used in the construction of a series of pAT153 based plasmids suitable for the regulated expression of foreign genes in E. coli. The first, a 139-bp HhaI fragment includes 59 bp of the trp leader sequence, ending within the "attenuator peptide" coding sequence, eleven codons from the N-terminus. A fusion-type expression plasmid incorporating this fragment has been constructed. The second, a 99-bp HaeIII-TaqI fragment contains no coding sequence but includes the "attenuator peptide" SD site situated 4 bp upstream of the TaqI site. This fragment has been incorporated in expression vectors which result in the direct expression of cloned gene sequences. To further maximise expression, plasmids with directly repeating trp promoter HaeIII-TaqI units have been constructed.

Expression of a cloned K88ac adhesion antigen determinant: identification of a new adhesion cistron and role of a vector-encoded promoter

The determinant for the K88ac adherence antigen of porcine enterotoxigenic Escherichia coli has been cloned previously onto the vector plasmid pBR322 to form the K88ac-pBR322 hybrid plasmid pMK005 (M. Kehoe et al., Nature [London] 291:122-126). Further studies on the expression of the K88ac antigen from pMK005 are presented in this paper. Expression was found to be dependent mainly on the P1 promoter of the pBR322 vector. The natural K88ac promoter was apparently not cloned from the original parental K88ac plasmid. The P1 promoter was deleted and replaced by a DNA sequence encoding the promoter-operator region of the E. coli tryptophan (Trp) operon. Cells harboring the Trp-pMK005 hybrid plasmid expressed high levels of K88ac antigen when the Trp promoter was repressed. If the promoter was derepressed either by growing the cells in low concentrations of tryptophan or in the presence of indole acrylic acid, growth of the cells harboring the Trp-pMK005 hybrid plasmid was inhibited. A quantitative assay was used to measure the levels of K88ac antigen expressed by cells harboring different pMK005::Tn5 plasmids. All cells were found to express a reduced level of K88ac antigen, providing evidence that a single transcription unit, initiating at promoter P1 of pBR322, may be involved in the expression of the K88ac antigen. By constructing specific deletion and insertion mutants of pMK005, a fifth adhesion cistron, tentatively named adhE, was identified and mapped at the proximal end of the K88ac determinant. Although the cistron is required for high-level expression of K88ac surface-associated fimbriae, as yet no gene product has been assigned to adhE.

Inducible high level synthesis of mature human fibroblast interferon in Escherichia coli

We have obtained high level synthesis in Escherichia coli of mature human fibroblast interferon using a plasmid vector that was designed to allow easy coupling of a DNA coding region to the initiator AUG of the replicase gene of the RNA phage MS2 cloned downstream of phage lambda's leftward promoter. The activity of the promoter can be regulated by temperature. Induced cells accumulated the interferon up to 4% of the total cellular protein. The biological activity of the product amounted to 4 X 10(9) international units per litre of culture. The synthesis of human fibroblast interferon was shown to drastically inhibit the growth rate of the bacterial host.

Determination of the promoter strength in the mixed transcription system. II. Promoters of ribosomal RNA, ribosomal protein S1 and recA protein operons from Escherichia coli

Using the in vitro mixed transcription system (Kajitani, M. and Ishihama, A. (1983) Nucleic Acids Res. 11, 671-686), we determined the two parameters of the promoter strength, i.e., the rate of open complex formation between RNA polymerase and promoter, and the saturation level of the open complex formation at equilibrium, for the promoters of ribosomal RNA (rrnE), ribosomal protein S1 (rpsA) and recA protein (recA) operons from Escherichia coli. Taken together with the previous determinations for lactose (lac(UV5)), tryptophan (trp) and ribosomal protein L10 (rp1J) operons, these studies revealed that the relative promoter strengths with respect to the kinetic parameter are 200, 70, 50, 40, 30, 20 and 2% of the reference promoter lacP(UV5) for recAp, rp1Jp, rpsAp3, trpP, rpsAp1, rrnEp1 and rrnEp2, respectively, under our standard reaction conditions (50 mM NaCl and 37 degrees C); and those with respect to the thermodynamic parameter are 70, 35, 20, 10, 10, 10 and 5% the level of lacP(UV5) for rrnEp2, trpP, rpsAp3, rp1Jp, rpsAp1, rrnEp1 and recAp, respectively. The order of the promoter strength, however, changes with variation of the salt concentration or reaction temperature.

Stabilization of proteins by a bacteriophage T4 gene cloned in Escherichia coli

The cloned bacteriophage T4 pin gene functions to stabilize several different kinds of proteins in Escherichia coli bacteria. Incomplete proteins such as puromycyl polypeptides, abnormal but complete proteins such as the lambda phage tsO protein, and labile eukaryotic proteins encoded by genes cloned in E. coli such as mature human fibroblast interferon are stabilized in cells in which the T4 pin gene is expressed. The cloned T4 pin gene does not seem to affect the turnover of normal E. coli proteins.

Determination of the promoter strength in the mixed transcription system: promoters of lactose, tryptophan and ribosomal protein L10 operons from Escherichia coli

In vitro transcription was performed in a single reaction mixture, which contained three species of truncated E. coli DNA template, each carrying one specific promoter, lacP (UV5), trpP or rplJp, and the transcripts of distinct sizes were analyzed by electrophoresis on the same gel. Using this "mixed transcription" system, the order of the promoter strength, i.e., the capacity to form stable open complex, was determined in the single-round transcription under the standard conditions (50 mM NaCl and 37 degrees C) to be lacP greater than trpP greater than rplJp, the latter two promoters being 30--40% and 5--10% the strength of lacP, respectively. After the multiple-round transcription, however, the level of trp transcription was the lowest due to low cyclic-reaction rate but became the highest when another trp fragment containing the natural terminator was used as the template. The order of the transcription level also varied depending on the ionic strength and the reaction temperature and, as a result, lacP was no more the strongest under high salt concentration and at high temperature.

Expression vehicles used in recombinant DNA technology

We survey cloning vehicles whose function is to carry and express a gene in host cells including Escherichia coli, Saccharomyces cerevisiae and mammalian cells. In E. coli these include vehicles based on the lac operon, the trp operon, the rho leftward operon, and the recA gone; open reading frame cloning vehicles are also discussed, as are steps that can be taken to extrude a gene product from the cell and the use of plasmids with runaway replication. In S. cerevisiae we discuss vehicles based on the PGK gene, the ADH1 gene, the acid phosphatase gene and the GAL1-GAL10 gene cluster. In mammalian cells we discuss vehicles based on SV40 promoters, the metallothionein gene, retroviral LTR promoters, bovine papilloma virus and vaccinia virus.

Chemical synthesis and cloning of a gene for human beta-urogastrone

A DNA duplex coding for the 53 amino acids of human beta-urogastrone has been synthesised. Computer assisted design of the gene included restriction endonuclease sites for plasmid insertion, a termination codon and two triplets coding for lysine at the 5'-end of the structural gene. The synthesis involved preparation of 23 oligodeoxyribonucleotides by phosphotriester procedures coupled to rapid HPLC techniques. The gene was constructed in two halves by enzymatic ligation of the oligonucleotides and cloned into a specially constructed chimeric plasmid vector. Escherichia coli K12 MRC8 was transformed by the plasmid and clones containing the full gene sequence were isolated and characterised.

Cloning vectors derived from bacterial plasmids

A wide variety of plasmid cloning vectors, most of which utilize the basic Co1E1I replicon have been constructed. Utilizing these vectors, in conjunction with the newly developed techniques of gene isolation and oligonucleotide synthesis, essentially any gene which can be identified can be cloned. We anticipate that future work in this area will be directed at improving techniques for the regulated expression of cloned genes and the further development of plasmid replicons in which the copy number can be readily controlled.

The expression in E. coli of synthetic repeating polymeric genes coding for poly(L-aspartyl-L-phenylalanine)

Two dodecadeoxynucleotides of defined sequence have been synthesised by phosphotriester methodology. They can be polymerised to give a double stranded DNA which codes, when read in the correct phase, for the repeating dipeptide poly(aspartyl-phenylalanine). This polymeric DNA has been cloned in E. coli K12 using as vector a plasmid having a controllable bacterial promoter upstream of the insertion site. Clones containing genes coding for up to 150 repeats of (aspartyl-phenylalanine) have been isolated and characterised. The polymeric inserts appear to be stable over many generations and are expressed in E. coli under the control of the bacterial promoter, to give a polymer of phenylalanine and aspartic acid which may be broken down enzymically to yield aspartyl-phenylalanine.