Coupling the T7 A1 promoter to the runaway-replication vector as an efficient method for stringent control and high-level expression of lacZ

An expression vector characterized by tight regulation and high expression of cloned genes appears to be indispensable for the engineering need. To achieve this goal, in association with lacI the T7 A1 promoter containing two synthetic lac operators was constructed into a runaway-replication vector. To further examine this vector system, lacZ was subcloned and placed under the control of the T7 A1 promoter on the plasmid. With the application of the thermal induction alone, the Escherichia coli strain harboring the recombinant plasmid was able to produce 15,000 Miller units of beta-galactosidase, while it yielded the recombinant protein with 45,000-50,000 Miller units upon both thermal and chemical induction. In sharp contrast, only 60-90 Miller units of beta-galactosidase was obtained for the cell at an uninduced state. As a result, the production yield of beta-galactosidase over the background level is amplified approximately 170-fold by thermal induction and 500-fold by thermal and chemical induction. To produce the recombinant protein on a large scale, an approach by connecting two fermenters in series was newly developed. By applying the three-stage temperature shift in this dual fermenter system, 55,000 Miller units of beta-galactosidase was obtained. Overall, it shows the potential use of the vector system developed here for its tight control and high production of recombinant proteins.     

Disruption of hexokinase II (HXK2) partly relieves glucose repression to enhance production of human kringle fragment in gratuitous recombinant Saccharomyces cerevisiae

The GAL1 gene encoding galactokinase was disrupted in a recombinant Saccharomyces cerevisiae strain in which production of LK8 protein, a kringle fragment of human apolipoprotein, is under the control of GAL1 promoter. Null mutation of the HXK2 gene was introduced further in the gal1Delta strain to relieve glucose repression. A pattern for LK8 expression was compared for the two recombinant S. cerevisiae systems in continuous and fed-batch cultivations. A critical dilution rate in continuous cultivation that repressed LK8 expression was significantly higher for the gal1Deltahxk2Delta strain than that for the gal1Delta strain to sustain the LK8 production even at high glucose consumption rate. Expressed LK8 for the gal1Delta strain was not detectable when the dilution rate exceeded 0.05 h(-1). Maximum LK8 concentration of 57 mgl(-1) was obtained in glucose-limited fed-batch cultivation of the gal1Deltahxk2Delta strain, corresponding to a 13.8-fold enhancement compared with the gal1Delta strain grown under the same conditions.     

Production of hantavirus Puumala nucleocapsid protein in Saccharomyces cerevisiae for vaccine and diagnostics

The production of hantavirus Puumala nucleocapsid (N) protein for potential applications as a vaccine and for diagnostic purposes was investigated with Saccharomyces cerevisiae as a recombinant host. The N protein gene and the hexahistidine tagged N (h-N) protein gene were expressed intracellular from a 2-microm plasmid vectors under the control of a fused galactose inducible GAL10-PYK promoter. For monitoring the recombinant gene expression, a h-N and a GFP fusion protein was used. Different cultivation strategies and growth media compositions were tested in shake flasks and a 5 l bioreactor. When using defined YNB growth medium, we found the biomass yield to be unsatisfactorily low. Higher concentrated YNB medium, promoted cell growth but showed a pronounced inhibitory effect on heterologous gene expression. This phenomenon could not be attributed to plasmid losses, as we could demonstrate high stability of the vector under the applied cultivation conditions. Supplementation of YNB medium with extracts of plant origin resulted in increased biomass yields with concomitant high expression levels of the recombinant gene. The modified medium was used for fed-batch cultivations where basic metabolic features as well as growth parameters were determined in addition to recombinant gene expression. The maximal volumetric yield of N protein was 316 mg l(-1), the respective yield of h-N protein was 284 mg l(-1). Our study provides a basis for large-scale production of hantavirus vaccines, which satisfies economic efficiency as well as biosafety regulations for human applications.     

Purification and renaturation of Dictyostelium recombinant alkaline phosphatase by continuous elution electrophoresis

A 1583 bp fragment of Dictyostelium alp cDNA (94% of the gene) was cloned in pET32a+. The enzyme was expressed in an inactive form in the inclusion body of the expression host BL21-CodonPlus (DE3)-RIL. The recombinant ALP constituted more than 50% of the total protein in the inclusion body and 25-30% of the total protein in the expression host after 3 h induction with IPTG at 37 degrees C. A continuous elution polyacrylamide gel electrophoresis procedure was used to purify the recombinant enzyme. This technique yielded a homogeneous protein that retained enzymatic activity after dialysis without further treatment. A yield of 5mg per liter of culture broth was obtained with a specific activity of approximately 0.7 nmol/min/mg protein (0.7 mU/mg). Immunoinhibition studies using a polyclonal antibody produced against the recombinant protein showed complete inhibition of enzymatic activity when the enzyme was preincubated with the antibody at a 1:1000 dilution. The enzyme exhibited a pH optimum of approximately 9.0. The substrate specificity indicated that the Dictyostelium enzyme is a typical broad range alkaline phosphatase.     

Influence of dilution rate and induction of cloned gene expression in continuous fermentations of recombinant yeast

The effects of growth rate on cloned gene product synthesis in recombinant Saccharomyces cerevisiae have been studied in continuous culture. The plasmid employed contains a yeast GAL10-CYC1 hybrid promoter directing expression of the E. coli lacZ gene. beta-Galactosidase production was therefore controlled by the yeast galactose regulatory circuit, and the induction process and its effects were studied at the various dilution rates. At all dilution rates plasmid stability decreased with induction of lacZ gene expression. In some instances, two induced "steady states" were observed, the first 10-15 residence times after induction and the second after 40-50 residence times. The second induced steady state was characterized by greater biomass concentration and lower beta-galactosidase specific activity relative to the first induced "steady-state." beta-Galactosidase specific activity and biomass concentration increased as dilution rate was reduced, and despite lower flow rate and plasmid stability, overall productivity (activity/L/hr) was substantially higher at low dilution rate. Important factors influencing all of the trends were the glucose and galactose (inducer) concentrations in the vessel and inducer metabolism.     

Improved coexpression and multiassembly properties of recombinant human ferritins subunit in Escherichia coli

Human heavy chain (H-) and light chain (L-) ferritins were amplified from a human cDNA library. Each ferritin gene was inserted downstream of the T7 promoter of bacterial expression vectors, and two types of coexpression vectors were constructed. The expression levels of recombinant ferritins ranged about 26-36% of whole-cell protein. Hferritin exhibited a lower expression ratio compared with L-ferritin, by a coexpression system. However, the coexpression of HL-ferritins was significantly increased above the expression ratio of H-ferritin by cultivation without IPTG induction overnight. Purified recombinant H-, L-, HL-, and LHferritins were shown to be homo- and heteropolymeric high molecular complexes and it was indicated that their assembled subunits would be able to work functionally in the cell. Thus, these results indicate an improvement in the expression strategy of H-ferritin for heteropolymeric production and studies of ferritin assembly in Escherichia coli.     

Uricase production by a recombinant <Emphasis Type="Italic">Hansenula polymorpha</Emphasis> strain harboring <Emphasis Type="Italic">Candida utilis</Emphasis> uricase gene

Uricase is an important medical enzyme which can be used to determine urate in clinical analysis, to therapy gout, hyperuricemia, and tumor lysis syndrome. Uricase of Candida utilis was successfully expressed in Hansenula polymorpha under the control of methanol oxidase promoter using Saccharomyces cerevisiae alpha-factor signal peptide as the secretory sequence. Recombinant H. polymorpha MU200 with the highest extracellular uricase production was characterized with three copies of expression cassette and selected for process optimization for the production of recombinant enzyme. Among the parameters investigated in shaking flask cultures, the pH value of medium and inoculum size had great influence on the recombinant uricase production. The maximum extracellular uricase yield of 2.6 U/ml was obtained in shaking flask culture. The yield of recombinant uricase was significantly improved by the combined use of a high cell-density cultivation technique and a pH control strategy of switching culture pH from 5.5 to 6.5 in the induction phase. After induction for 58 h, the production of recombinant uricase reached 52.3 U/ml (about 2.1 g/l of protein) extracellularly and 60.3 U/ml (about 2.4 g/l) intracellularly in fed-batch fermentation, which are much higher than those expressed in other expression systems. To our knowledge, this is the first report about the heterologous expression of uricase in H. polymorpha.     

Preparation of recombinant alpha-thrombin: high-level expression of recombinant human prethrombin-2 and its activation by recombinant ecarin

We have established a large-scale manufacturing system to produce recombinant human alpha-thrombin. In this system, a high yield of alpha-thrombin is prepared from prethrombin-2 activated by recombinant ecarin. We produced human prethrombin-2 using mouse myeloma cells and an expression plasmid carrying the chicken beta-actin promoter and mutant dihydrofolate reductase gene for gene amplification. To increase prethrombin-2 expression further, we performed fed-batch cultivation with the addition of vegetable peptone in 50 liters of suspension culture. After five feedings of vegetable peptone, the expression level of the recombinant prethrombin-2 reached 200 micro g/ml. Subsequently, the recombinant prethrombin-2 could be activated to alpha-thrombin by recombinant ecarin expressed in a similar manner. Finally, recombinant alpha-thrombin was purified to homogeneity by affinity chromatography using a benzamidine-Sepharose gel. The yield from prethrombin-2 in culture medium was approximately 70%. The activity of the purified recombinant alpha-thrombin, including hydrolysis of a chromogenic substrate, release of fibrinopeptide A, and activation of protein C, was indistinguishable from that of plasma-derived alpha-thrombin. Our system is suitable for the large-scale production of recombinant alpha-thrombin, which can be used in place of clinically available alpha-thrombin derived from human or bovine plasma.     

Genetic manipulation of stationary-phase genes to enhance recombinant protein production in Escherichia coli

Genetic manipulation of the host strain, by which cell physiology could be modulated, was exploited to enhance recombinant protein production in Escherichia coli. The effects of an inactivated stationary-phase gene (rmf or katF) on recombinant protein production in strains with two different expression systems (the pH-inducible and the lac promoters) were investigated. An improvement of recombinant protein production in the katF mutant at low growth rates was observed for both expression systems. A fourfold and a 30% increase in the volumetric recombinant protein activity were observed for the pH-inducible and the lac promoter system, respectively. The effect of the rmf mutation, on the other hand, depends on the expression system. A twofold increase in the volumetric recombinant protein activity was found for the pH-inducible promoter system, but there was no improvement for the lac promoter system. Improvement in culture performance for slow-growing cultures may have an impact on the design strategy of the host/vector system used in fed-batch cultures, where the specific growth rate is usually slow. The information may also be useful for developing optimal host/vector gene expression systems for recombinant protein production. (c) 1996 John Wiley & Sons, Inc.     

特异腐质霉内切葡聚糖酶Ⅱ基因在毕赤酵母中的表达及酶学性质

【目的】在毕赤酵母中表达特异腐质霉Humicola insolens的中性内切葡聚糖酶Ⅱ,并对其性质加以研究。【方法】利用RT-PCR的方法,以特异腐质霉(Humicola insolens)NC3总RNA为模板,克隆到中性内切葡聚糖酶Ⅱ基因(egⅡ)的cDNA。将其插入表达载体pPIC9K,重组质粒经线性化后电击转化毕赤酵母(Pichia pastoris)菌株GS115。【结果】SDS-PAGE和酶活的检测结果均表明:egⅡ基因在毕赤酵母中成功表达。重组酶的部分酶学性质研究表明,该酶的最适反应温度为70°C,且在65°C以下具有较好的热稳定性。最适反应pH为6.5,在pH 6.0?7.0之间有较好的稳定性。【结论】用重组毕赤酵母可高效表达外源中性内切葡聚糖酶,为其今后在工业应用奠定了基础。     

Two-dimensional gel electrophoresis for controlling and comparing culture supernatants of mammalian cell culture productions systems

A recombinant Chinese hamster ovary cell line, producing human erythropoietin, was cultivated in a continuous mode in a stirred tank reactor applying different dilution rates. In order to monitor the stability of this expression system, product and non-product proteins of the cell culture supernatant were analyzed by two-dimensional electrophoresis. The consistency of the isoforms of the recombinant product was determined by western blot combined with specific staining.The same cell line was propagated in a high cell density cultivation system based on macro-cell-aggregates. The patterns of secreted proteins of the cell line cultivated in the different systems were compared in order to detect modifications in protein expression of the product and of non product proteins relevant for cell culture supernatant.Hardly any alterations in two-dimensional pattern were detectable. The isoforms of erythropoietin, as well as the overall pattern of secreted proteins, detectable with the two-dimensional electrophoresis method were remarkably stable under different cultivation conditions.Abbreviations 2-DE two-dimensional electrophoresis - rCHO recombinant Chinese hamster ovary cell - EPO erythropoietin - FBR fluidized bed reactor - IEF isoelectric focusing - IPG immobilized pH gradient - pI isoelectric point - SDS sodium dodecylsulfate - STR stirred tank reactor     

The use of elements of the E. coli Ntr-system for the design of an optimized recombinant expression system for high cell density cultivations.

The inducible glnA promoter 2 of the E. coli glutamine synthetase gene is suitable as an expression unit for the production of recombinant proteins at low and high cell densities. It is active when the concentration of ammonium as the sole nitrogen source in the culture medium is below 1 mM. This nitrogen regulatory system was optimized by introduction of expression cassettes consisting of additional elements of the ntr-system. These artificial constructions result in enhanced recombinant gene expression in the production phase. Furthermore, the basic recombinant protein level during the growth phase is reduced due to a tighter promoter control. A three- to four-fold higher accumulation of chloramphenicol-acetyltransferase (as reporter protein) and of anti-EGF-receptor miniantibodies was achieved by increasing the amount of the final regulator molecule NtrC approximately P via plasmidal co-expression of the ntrC gene. The introduction of a modified glnA promoter 1 inverse to glnAp2 lowered the basic activity of glnAp2 to about one half. It is assumed that under nitrogen excess conditions sigma 70-RNA polymerase binds at glnAp1 and thereby prevents most of the binding of sigma 54-RNA polymerase at glnAp2. The optimized expression systems were successfully applied in low and high cell density cultivations. In the fed-batch phase of high cell density cultivations recombinant protein formation was induced through external nitrogen limitation under FIA-controlled concentration of glucose as carbon source.     

Overexpression and purification of recombinant human interferon alpha2b in Escherichia coli

Overexpression of rhIFN-alpha2b was obtained by synthesizing a codon optimized gene for IFN-alpha2b and expressing it in the form of inclusion bodies (IBs) in Escherichia coli. The recombinant plasmid pRSET-IFNalpha, which had the IFN-alpha2b gene under the T7 promoter, was coexpressed with plasmid pGP1-2, which carried the gene for T7 RNA polymerase under the heat inducible lambdaP(L) promoter. This two plasmid expression system was optimized with respect to heat shock time, media, and time of induction in shake flask cultures. This was then scaled up into a bioreactor to get a maximum volumetric product yield of 5.2g/L at a final OD(600) of 67. At this point, the IBs represented approximately 40% of the total cellular protein. This high specific product yields eased the further downstream processing steps and improved product recoveries. The IBs were isolated and purified through ion exchange followed by step refolding to give a final product yield of approximately 3g/L, which is maximum reported in the literature. The bioassay of the refolded protein gave a specific activity of approximately 3 x 10(9)IU/mg protein.     

Relationship of c-myc gene copy number and gene expression: cellular effects of elevated c-myc protein

The relationship between gene copy number and expression and cellular consequences of elevated levels of c-myc protein has been investigated using recombinant Chinese hamster ovary cell lines transfected with DNA coding for the murine c-myc gene. HC-8 and LC-5 recombinant cells carry approximately 800 and 50 copies of c-myc sequences, respectively, under control of an inducible heat shock promoter. Multivariate flow cytometric analysis and clonogenic assays were used to measure the relationship among c-myc expression, rate of DNA synthesis, and cell survival. Following heat exposure, maximally induced HC-8 cells produced approximately tenfold more c-myc protein than heated LC-5 cells, suggesting a close relationship between gene copy number and level of expression. However, considerable heterogeneity in the level and time of c-myc expression was observed following heat induction, even though the amounts of genomic c-myc were relatively constant. Heterogeneity in gene expression was not attributable to variation in heat induction methodologies and/or cell cycle phase distributions. The presence of high levels of recombinant c-myc protein was associated with a decreased rate of bromodeoxyuridine incorporation into DNA. High levels of c-myc protein in HC-8 cells were inversely correlated with cell survival postheating, suggesting that high levels of c-myc protein are incompatible with cell survival.     

High-density Escherichia coli cultivation process for hyperexpression of recombinant porcine growth hormone.

Fermentation studies were performed on an Escherichia coli culture that carries a recombinant plasmid composed of an ampicillin-resistant gene, a temperature-regulated pL promoter, and a porcine pituitary cDNA sequence coding for growth hormone. The objective was to achieve high cell density while maintaining the specific expression level of recombinant porcine growth hormone (r-pGH) observed in shake flasks. At a specific expression level of 20% of total cell protein, the cell density of a glucose-limited fed-batch process reached 38 units of OD600 in 14 h, compared to flask cultivation, which resulted in only 1.4 units of OD600 in the same period. The observed critical fermentation conditions for maximal expression included (1) limiting glucose concentration below 1 g l-1 throughout the fed-batch growth and induction phases, (2) keeping postinduction temperature at 42 degrees C for 5-7 h, and (3) maintaining a postinduction growth rate around 0.17-0.21 h-1.     

A facile and efficient method to achieve LacZ overproduction by the expression vector carrying the thermoregulated promoter and plasmid copy number

On the basis of the runaway-replication vector, an expression plasmid was developed to achieve tight regulation as well as high-level expression of cloned genes by thermal control of the promoter together with the plasmid copy number. To demonstrate the feasibility of this approach, the lacZ gene was fused with the heat-inducible promoter on the vector, and the result showed that protein production levels in the Escherichia coli strain harboring the recombinant plasmid could be varied in response to various degrees of heat shock. The maximal soluble LacZ ranging between 45 000 and 50 000 Miller units was obtained as the recombinant strain received a 30 --> 40 degrees C stepwise upshift, and it accounted for a 450-fold amplification over an uninduced level. Further analyses by SDS-PAGE indicated the maximal protein production (including soluble and insoluble forms) in the bacteria reaching approximately 30% total cell protein. In addition, two approaches were demonstrated to be very useful in enhancing the total soluble LacZ production on a fermenter scale. One was to shuttle the culture between two fermenters connected in series and set at different temperatures. The other resorted to the use of two-step temperature alteration in a batch fermenter, namely, raising the temperature to 40 degrees C for a certain period of time followed by reducing the temperature to 37 degrees C. Overall, it illustrates the remarkable features of the expression system with stringent regulation, high-level production capacity, facile induction, and high stability, and the usefulness of this system for recombinant protein productions is promising.     

Improved production of recombinant ovine interferon-tau by mut(+) strain of Pichia pastoris using an optimized methanol feed profile

Recombinant ovine interferon-tau (r-oIFN-tau) production by Pichia pastoris was studied using methanol as the sole carbon source during induction. The cells were grown on glycerol up to a certain cell density before induction of the AOX1 promoter by methanol for expression of the recombinant protein. Cell growth on methanol has been modeled using a substrate-feed equation, which served as the basis for an effective computer control of the process. The r-oIFN-tau concentration in the culture began to decline despite continued cell growth after 50 (+/- 6) h of induction, which was associated with an increase in proteolytic activity of the fermentation broth. A specific growth rate of 0.025 h(-1) was found to be optimal for r-oIFN-tau production. No significant improvement in r-oIFN-tau production was observed when the specific growth rate was stepped up before the critical point when r-oIFN-tau concentration started decreasing during fermentation. However, best results were obtained when the specific growth rate was stepped down from 0.025 to 0.02 h(-1) at 38 h of induction, whereby the active production period was prolonged until 70 h of induction and the broth protease activity was correspondingly reduced. The corresponding maximum protein yield was 391.7 mg x L(-1) after 70 h of fermentation. The proteolytic activity could be reduced by performing fermentations at specific growth rates of 0.025 h(-1) or below. The recombinant protein production can be performed at an optimal yield by directly controlling the methanol feed rate by a computer-controlled model. The production profile of r-oIFN-tau was found to be significantly different from other secreted and intracellular recombinant protein processes, which is an indication that recombinant protein production in Pichia pastoris needs to be optimized as individual processes following established principles.     

Sustained and constitutive high levels of protein production in continuous cultures of bacillus subtilis

The feasibility of continuous production of proteins in chemostat cultures of Bacillus subtilis was investigated. An expression system consisting of the bacterium B. subtilis BR151 carrying plasmid p602/19 was used. The plasmid contains the cat (chioramphenicol acetyltrans-ferase) gene downstream of a strong vegetative T5 promoter. It was found that, at a dilution rate of 0.2 h(-1) production of relatively high levels of CAT protein (about 4% ofcellular protein) can be sustained. But, experiments at a higher dilution rate of 0.4 h(-1) were unproductive because of high acidformation and washout. Combination of low cell yield, which results from excessive acid formation, and low dilution rate led to a low volumetric CAT productivity. Our recent work with the nonrecombinant cells, has demonstrated that uptake of small amounts of citrate significantly reduces or entirelyeliminates the acid formation. This superior performance in the presence ofcitrate was hypothesized, based on strong experimental evidence, to be the result of a reduction in glycolysis flux through a sequence of events leading to a reduction in pyruvate kinase and phosphof- ructokinase activities, the regulatory enzymes of glycol-ysis. In this study, it is demonstrated that cofeeding of glucose and citrate substantially reduces theorganic acid formation and significantly increases the recombinant culture productivity. The combination of high specific CAT activity and cell density resulted in a total of six- to tenfold higher culture productivitywhen citrate and glucose were cometabolized than when glucose was the only carbon source. (c) 1995 John Wiley & Sons Inc.