Improvement of the thermoregulated T7 expression system by using the heat-sensitive lacI

The thermoregulated T7 expression system was previously reported to be an effective way to produce massive amounts of recombinant proteins (Chao, Y. P.; Law, W. S.; Chen, P. T.; Hung, W. B. High production of heterologous proteins in Escherichia coli using the thermo-regulated T7 expression system. Appl. Microbiol. Biotechnol. 2002b, 58, 446-453). To ensure its practical applicability, the system was improved for stringency with the construction of the T7lac-promoter-containing plasmid associated with the thermolabile lacI gene (lacIts). Owing to the recessive feature of lacIts, the wild-type lacI was removed from the genome of the cell. Moreover, the cell was engineered to carry the chromosomal copy of the T7 gene 1 subject to the regulation of lambdaPL and lambdaPR promoters. To characterize the system, the lacZ gene was fused to the T7lac promoter, and subsequent experiments showed that various amounts of LacZ could be synthesized in the plasmid-bearing cell in response to heat. Among the producers, the cell with the plasmid containing lacIts (substitution of Gly265 with Asp in lacI) was able to produce the maximal LacZ, the production accounting for an amplification of more than 200-fold over the uninduced level. A further demonstration was carried out to illustrate the practical usefulness of the developed system by producing carbamoylase on a 4000 L scale. Cultured to reach high cell density, the carbamolyase-producing cell was shown to retain plasmids with 95% stability and to be capable of producing soluble protein equal to 13% of the total cell proteins. Overall, it illustrates the remarkable features of the developed system with tightness, high expression level, thermal inducibility, and high stability.     

Construction and characterization of thermo-inducible vectors derived from heat-sensitive lacI genes in combination with the T7 A1 promoter

The lack of stringency and the cost of induction are two major disadvantages of using lac-derived vectors for recombinant protein productions. To compensate for these drawbacks, a series of thermo-inducible vectors was developed by coupling heat-sensitive lacI (lacIts) with the T7 A1 promoter on a multiple-copynumber plasmid. The lacIts genes were created by the introduction of Gly187-->Ser substitution along with three alternative mutation sites, Leu233-->Lys, Ala241-->Thr, and Gly265-->Asp, generated by site-directed mutagenesis into the wild-type lacI gene. With the LacZ production as a model, the induction profiles for various vectors containing distinct lacIts exhibited a positive trend as the temperature increased. The fully induced level was achieved by applying the temperature shift from 30 degrees C to 42, 40, or 37 degrees C to the cells harboring the plasmid with the Gly187-->Ser, Ala241-->Thr, or Gly265-->Asp substitution in lacI, respectively. As a result, it produced the maximal LacZ production ranging between 46,000 and 54,000 Miller units, corresponding to a 100- to 400-fold amplification over the uninduced level. As a whole, these novel expression vectors are characterized as having tight regulation and facile inducibility, and their practical usefulness in industrial production of recombinant proteins appears promising.     

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.     

抗菌肽Adenoregulin基因工程菌培养条件的优化及分批发酵研究

denoregulin（ADR）是来源于南美树蛙Phyllomedusa bicolor皮肤的含有33个氨基酸的抗菌肽,在非极性环境中形成α_螺旋型结构,具有抗菌活性强、抗菌谱广的特点。将ADR基因克隆于pET32a载体上,转化大肠杆菌BL21(DE3),对这一工程菌株的培养条件进行了优化。通过正交试验,考察诱导时机、诱导剂量和诱导时间三个因素的不同水平对蛋白表达的影响,结果发现诱导时机的影响尤为显著,考察了9种不同培养基对表达量的影响,发现培养基中加入葡萄糖对目标蛋白的稳定表达起了重要的作用,确定最佳培养条件为：培养基为2×YT+0.5%葡萄糖,诱导时机为OD600=0.9左右,诱导剂IPTG加入的终浓度为0.1mmol/L,诱导时间为4h。采用前期恒pH、后期指数流加的策略进行工程菌BL21(DE3)/pET32a-adr的高密度培养,在整个流加过程中,通过控制葡萄糖的加入量,将菌株的比生长速率控制在015h^-1,乙酸浓度也被控制在较低的水平(<2g/L),但是质粒丢失严重,在发酵结束时,约有40%的大肠杆菌中不带质粒,这导致了目标蛋白的表达量下降严重,但是表达的目标蛋白90%以上为可溶性形式。表达的融合蛋白无抑菌活性,而裂解后得到的ADR单体具有明显的抑菌活性。     

Development of efficient suicide mechanisms for biological containment of bacteria.

To optimize plasmid containment, we have systematically investigated the factors that limit the killing efficiency of a suicide system based on the relF gene from Escherichia coli controlled by inducible lac promoters and placed on plasmids. In induction experiments with this suicide system, killing efficiency was unaffected by temperature and growth medium; there was no requirement for great promoter strength or high plasmid copy number. We could demonstrate that the factors limiting killing were the mutation rate of the suicide function and the reduced growth rate caused by a basal level of expression of the suicide gene during normal growth, which can give a selective growth advantage to cells with mutated suicide functions. The capacity of the plasmid-carried killing system to contain the plasmid was tested in transformation, transduction, and conjugational mobilization. The rate of plasmid transfer detected in these experiments seemed too high to provide adequate biological containment. As expected from the induction experiments, plasmids that escaped containment in these transfer experiments turned out to be mutated in the suicide function. With lac-induced suicide as a test, the efficiency of the system was improved by tightening the repression of the suicide gene, thereby preventing selection of cells mutated in the killing function. Reduction of the mutational inactivation rate of the suicide system by duplication of the suicide function augmented the efficiency of the suicide dramatically. These results permit the construction of extremely efficient biological containment systems.     

Development of efficient suicide mechanisms for biological containment of bacteria

To optimize plasmid containment, we have systematically investigated the factors that limit the killing efficiency of a suicide system based on the relF gene from Escherichia coli controlled by inducible lac promoters and placed on plasmids. In induction experiments with this suicide system, killing efficiency was unaffected by temperature and growth medium; there was no requirement for great promoter strength or high plasmid copy number. We could demonstrate that the factors limiting killing were the mutation rate of the suicide function and the reduced growth rate caused by a basal level of expression of the suicide gene during normal growth, which can give a selective growth advantage to cells with mutated suicide functions. The capacity of the plasmid-carried killing system to contain the plasmid was tested in transformation, transduction, and conjugational mobilization. The rate of plasmid transfer detected in these experiments seemed too high to provide adequate biological containment. As expected from the induction experiments, plasmids that escaped containment in these transfer experiments turned out to be mutated in the suicide function. With lac-induced suicide as a test, the efficiency of the system was improved by tightening the repression of the suicide gene, thereby preventing selection of cells mutated in the killing function. Reduction of the mutational inactivation rate of the suicide system by duplication of the suicide function augmented the efficiency of the suicide dramatically. These results permit the construction of extremely efficient biological containment systems.     

Cloning and characterization of several dominant-negative and tight-binding mutants of lac repressor

Using both general recombination and molecular cloning techniques, 13I-, I-d and Itb missense mutations in the lacI gene were transferred from F'lacIq episomes to ColE1 derivative plasmids. Two deletion derivatives of the lacI genes encoding the wild-type (wt) and the tight-binding (Itb) B3 and B5 repressors were also constructed. The mutant repressors were examined for polypeptide size and stability, and for binding to the inducer isopropyl-beta-D-thiogalactoside (IPTG). Several of the I-d repressors were shown to be partially degraded in vivo, in confirmation of earlier results based on [14C]IPTG binding [Miwa and Sadler, J. Mol. Biol. 117 (1977) 843-868]. The sizes of polypeptides produced by lacI deletion derivatives were consistent with expectations based on the extent of deletion and the location of termination sites within the plasmid sequence. The first 400 bp of several mutant lacI genes were sequenced. Our wt lacI gene differs from another wt lacI sequence (Farabaugh, 1978), containing a single bp change that results in an Ala to Thr substitution at amino acid (aa) 109. We identified bp substitutions and the resultant aa changes for two Itb and two I-d genes; the positions correlated with prior genetic mapping data. Three of these new changes were in the N-terminal domain (headpiece) of repressor, with one change in the core domain at aa 99.     

Overexpression of cloned genes using recombinant Escherichia coli regulated by a T7 promoter: I. Batch cultures and kinetic modeling

A novel Eschericha coli expression system directed by bacteriophage T7 RNA Polymerase utilized for overexpression of the cloned gene. The recombinant cell contains the plasmid with a bacteriophage promoter, the T7 promoter, to regulate the expression of the target gene. This promoter is recongnized only by T7 RNA polymerase, whose gene has been fused into the host chromosome and is under control of the lacUV5 promoter. Therefore, the target gene on the plasmid can be expressed only in the presence of T7 RNA polymerase, which is induced by isopropyl-beta-D-thiogalactopyranoside (IPTG). The batch cultures were performed to investigate the effect of induction on kinetics of cell growth and foreign protein formation and to determine the optimal induction strategy. It was observed that the specific growth rates of the recombinant cells dramatically decrease after induction, and that there is an optimal induction time for maximizing the accumulated intracellular foreign protein. This optimal induction time varies singificantly with inducer concentration. To better understand the optimal behavior, a lumped mechanistic model was constructed to analyze the induced cell growth and foreign protein formation rates. (c) 1992 John Wiley & Sons, Inc.     

Maintenance of pBR322-derived plasmids without functional RNAI

pBR322-derived plasmids that lack the bla gene and 40% of the gene for the replication inhibitor, RNAI, have been constructed. Since the RNAI gene totally overlaps with the gene for the replication primer, RNAII, this primer is similarly defective and also lacks its normal promoter. The primer is presumed to by synthesized either from the counter-tet promoter (plasmid pCL59) or from an inserted lacUV5 promoter (plasmid pCL59-65). Based mainly on the observation that the plasmid Rom protein, which normally assists in the RNAI/RNAII interaction, has no effect on the replication of the RNAI/RNAII-defective plasmids, we suggest that the defective RNAI is not functional while the defective RNAII primer, although less efficient, still allows plasmid replication. The defective plasmids are fully compatible with the intact parent plasmid, indicating that they do not share a common control of replication. In the absence of antibiotics, the bacteria lose the defective plasmid, beginning after 80 generations; under the same conditions, the parent plasmid is retained even after 140 generations. During exponential growth of their host, the number of defective plasmids in a culture increases exponentially with a doubling time either smaller or greater than that of the host cell growth, depending on the growth medium and, in the case of pCL59-65, on the presence or absence of lac inducer IPTG. As a result of these differences in host cell growth and plasmid replication, the plasmids are either gradually diluted out or their copy number continually increases. This shows that, without RNAI, plasmid replication is uncoupled from the host cell growth and not, as usual, adjusted to it. It also implies that the RNAI mechanism is the only means of replication control for ColE1-type plasmids that senses and adjusts the copy number; limiting host factors cannot provide a back-up control to stabilize copy numbers.     

AN OPTIMIZED PROTOCOL FOR OVERPRODUCTION OF RECOMBINANT PROTEIN EXPRESSION IN Escherichia coli

The gram-negative bacterium Escherichia coli (E. coli) offers a means for rapid, high-yield, and economical production of recombinant proteins. Here, a protocol for optimization of parameters involved in bacterial expression conditions is described. L-Asparaginase (ASNase II) was chosen as a model protein for our experiments. ASNase II gene (ansB) was cloned into the pAED4 plasmid and transformed into E. coli BL21pLysS (DE3)-competent cells. It was assumed that high cell density and high copy number of recombinant plasmid in the bacteria host could result in very high production of the recombinant protein. Circumstances for the overproduction of recombinant ASNase II including cell growth conditions, isopropyl β-D-1-thiogalactopyranoside (IPTG) level, ampicillin (Amp) concentration before and during IPTG induction, and cell density were optimized. Regarding the final optimization, overexpression of ASNase II was assessed on a large scale in LB medium. Periplasmic ASNase II was extracted using an alkaline lysis method. The extracted protein was purified by one-step DEAE-Sepharose fast-flow chromatography. ASNase II activity was considered an index for the protein expression. Applying the optimized practical protocol, protein production was significantly enhanced in comparison to the traditional IPTG induction method in the absence of a fermentor and can be applied for overexpression of other recombinant proteins.     

Generation of an AraC-araBAD promoter-regulated T7 expression system

An alternative and facile delivery system for T7 RNA polymerase has been devised and constructed. T7 gene 1 has been placed under control of the araBAD promoter element regulated by the AraC protein. Cotransformation of the resultant plasmid, pTara, with one containing a target gene under T7 promoter-regulated expression potentially allows repression by glucose and induction by arabinose in the range of 0.5 to 20 mM sugar concentration. To demonstrate the efficacy of this expression system, the p53 gene under T7 promoter control in two different plasmids was expressed in Escherichia coli using pTara as the source of T7 RNA polymerase. Repression and induction of p53 were achieved in both a lower and higher copy number plasmid, although the levels of induction were higher with the lower copy number expression vector. Cotransformation of an expression plasmid with pTara provides a low-cost method of T7 RNA polymerase-regulated expression that can be fine-tuned using glucose and arabinose concentrations to balance protein expression with potential solubility or toxicity problems.     

Modifications of the E.coli Lac repressor for expression in eukaryotic cells: effects of nuclear signal sequences on protein activity and nuclear accumulation.

Eukaryotic expression vectors designed to produce E. coli Lac repressor protein targeted to the nucleus of mammalian cells were constructed. These constructions carry the lac repressor gene (lacI) fused at different positions to a nuclear localization sequence (NLS) from either the SV40 large T antigen or the adenovirus E1a. When the NLS's were fused to the lacI gene at the 5' end, the protein produced exhibited tighter repression of beta-galactosidase expression than the unmodified LacI protein. Localization sequences at the extreme 3' end of the gene generally diminished induction by IPTG, while introduction of the SV40 NLS nine base pairs upstream of the 3' end eliminated repressor activity. When either NLS was placed at the 3' end behind a random nine base pair linker, the activity of the LacI protein depended on the sequence of the linker, and in 9 of 10 linkers tested, activity of the protein was adversely affected. The one exception was the fusion protein from p3'ss, which had the NLS at the 3' end of lacI behind the nine base pair linker, AGC AGC CTG (ser-ser-leu). This protein exhibited efficient nuclear accumulation, strong repressor activity and greater sensitivity to IPTG induction. The functional linker from the p3'ss fusion protein extends the leucine zipper heptad repeat located at the C-terminus of the protein. These data support the role of the leucine zipper in tetramer formation and predict that extension of this zipper will further stabilize the protein. This modified lacI gene should be valuable for improved adaptation of the prokaryotic regulatory system to eukaryotic cells.     

猪抵抗素(resistin)的原核表达及表达产物的纯化鉴定

用PCR方法扩增到抵抗素基因(Resistin, RSTN)并将其亚克隆至pET-32a(+)表达载体,获得重组质粒pET-RSTN。将重组质粒转化大肠杆菌BL-21（DE3）感受态细胞,用IPTG诱导表达。SDS-PAGE检测结果表明,重组resistin蛋白分子量大小约30kDa。对表达条件如温度、IPTG浓度及诱导时间进行优化并用SDS-PAGE检测。结果表明,30℃、4h、IPTG浓度为1mmol/L时,可溶性重组resistin的含量最高。表达产物经Western blot检测证实是Resistin蛋白,并用镍离子亲和层析的方法获得纯化的Resistin蛋白。     

Flow cytometric analysis of metabolic stress effects due to recombinant plasmids and proteins in Escherichia coli production strains

Overexpression of recombinant proteins in Escherichia coli often leads to a severe growth retardation of the host cells. Using flow cytometry, we analyzed the temporal development of the cellular content of DNA, total protein, and the recombinant product (human superoxide dismutase) in different strains. In cells carrying plasmids utilizing the phage T7 promoter 10 (pET vectors), induction with IPTG leads to an increase in protein content and size, an increase and a wide spreading of DNA content distribution, and a termination of cell division. These effects occurred with pET plasmids with or without an insert, but not with another plasmid which utilizes the tac promoter.     

Direct detection and quantification of horizontal gene transfer by using flow cytometry and gfp as a reporter gene

A new cultivation-independent method for studying conjugal gene transfer between bacteria was evaluated. The method was based on direct detection and enumeration of donor and transconjugant bacterial cells by flow cytometry. Specific detection of transconjugants was obtained by using a conjugative plasmid tagged with a reporter gene (gfp) encoding green fluorescent protein. A chromosomal encoded repressor (lacI(ql)) repressed expression of GFP in the donor bacteria. Enumeration of the donor cells was performed after induction of GFP expression by the addition of inducer isopropyl-thio-beta-D-galactoside (IPTG). The method presented here provided simple and precise quantification of horizontal gene transfer between both Escherichia coli and Pseudomonas putida strains.