Production of phoA promoter-controlled human epidermal growth factor in fed-batch cultures of Escherichia coli YK537 (pAET-8)

Factors involved in heterologous expression of proteins in E. coli host

The production of recombinant proteins is one of the most significant achievements of biotechnology in the last century. These proteins are produced in the eukaryotic or prokaryotic heterologous hosts. By increasing the omics data especially related to different heterologous hosts as well as the presence of new amenable genetic engineering tools, we can artificially engineer heterologous hosts to produce recombinant proteins in sufficient quantities. Numerous recombinant proteins have been produced and applied in various industries, and the global recombinant proteins market size is expected to be cast to reach USD 2.4 billion by 2027. Therefore, identifying the weakness and strengths of heterologous hosts is critical to optimize the large-scale biosynthesis of recombinant proteins. E. coli is one of the popular hosts to produce recombinant proteins. Scientists reported some bottlenecks in this host, and due to the increasing demand for the production of recombinant proteins, there is an urgent need to improve this host. In this review, we first provide general information about the E. coli host and compare it with other hosts. In the next step, we describe the factors involved in the expression of the recombinant proteins in E. coli. Successful expression of recombinant proteins in E. coli requires a complete elucidation of these factors. Here, the characteristics of each factor will be fully described, and this information can help to improve the heterologous expression of recombinant proteins in E. coli.

Recombinant Protein Production in E. coli Using the phoA Expression System

Auto-inducible promoter systems have been reported to increase soluble product formation in the periplasm of E. coli compared to inducer-dependent systems. In this study, we investigated the phosphate (PO4)-sensitive phoA expression system (pAT) for the production of a recombinant model antigen-binding fragment (Fab) in the periplasm of E. coli in detail. We explored the impact of non-limiting and limiting PO4 conditions on strain physiology as well as Fab productivity. We compared different methods for extracellular PO4 detection, identifying automated colorimetric measurement to be most suitable for at-line PO4 monitoring. We showed that PO4 limitation boosts phoA-based gene expression, however, the product was already formed at non-limiting PO4 conditions, indicating leaky expression. Furthermore, cultivation under PO4 limitation caused physiological changes ultimately resulting in a metabolic breakdown at PO4 starvation. Finally, we give recommendations for process optimization with the phoA expression system. In summary, our study provides very detailed information on the E. coli phoA expression system, thus extending the existing knowledge of this system, and underlines its high potential for the successful production of periplasmic products in E. coli.

β-Galactosidase: From its source and applications to its recombinant form

Carbohydrate-active enzymes are a group of important enzymes playing a critical role in the degradation and synthesis of carbohydrates. Glycosidases can hydrolyze glycosides into oligosaccharides, polysaccharides, and glycoconjugates via a cost-effective approach. Lactase is an important member of β-glycosidases found in higher plants, animals, and microorganisms. β-Galactosidases can be used to degrade the milk lactose for making lactose-free milk, which is sweeter than regular milk and is suitable for lactose-intolerant people. β-Galactosidase is employed by many food industries to degrade lactose and improve the digestibility, sweetness, solubility, and flavor of dairy products. β-Galactosidase enzymes have various families and are applied in the food-processing industries such as hydrolyzed-milk products, whey, and galactooligosaccharides. Thus, this enzyme is a valuable protein which is now produced by recombinant technology. In this review, origins, structure, recombinant production, and critical modifications of β-galactosidase for improving the production process are discussed. Since β-galactosidase is a valuable enzyme in industry and health care, a study of its various aspects is important in industrial biotechnology and applied biochemistry.

Study on the isoprene-producing co-culture system of Synechococcus elongates-Escherichia coli through omics analysis

BACKGROUND

The majority of microbial fermentations are currently performed in the batch or fed-batch manner with the high process complexity and huge water consumption. The continuous microbial production can contribute to the green sustainable development of the fermentation industry. The co-culture systems of photo-autotrophic and heterotrophic species can play important roles in establishing the continuous fermentation mode for the bio-based chemicals production.

RESULTS

In the present paper, the co-culture system of Synechococcus elongates-Escherichia coli was established and put into operation stably for isoprene production. Compared with the axenic culture, the fermentation period of time was extended from 100 to 400 h in the co-culture and the isoprene production was increased to eightfold. For in depth understanding this novel system, the differential omics profiles were analyzed. The responses of BL21(DE3) to S. elongatus PCC 7942 were triggered by the oxidative pressure through the Fenton reaction and all these changes were linked with one another at different spatial and temporal scales. The oxidative stress mitigation pathways might contribute to the long-lasting fermentation process. The performance of this co-culture system can be further improved according to the fundamental rules discovered by the omics analysis.

CONCLUSIONS

The isoprene-producing co-culture system of S. elongates-E. coli was established and then analyzed by the omics methods. This study on the co-culture system of the model S. elongates-E. coli is of significance to reveal the common interactions between photo-autotrophic and heterotrophic species without natural symbiotic relation, which could provide the scientific basis for rational design of microbial community.

Upstream development of Escherichia coli fermentation process with PhoA promoter using design of experiments (DoE)

In this work, a fed-batch fermentation development was performed with recombinant E. coli carrying the PhoA promoter system. The phosphate concentrations tested for this PhoA strain, 2.79 mM to 86.4 mM, were beyond the concentrations previously evaluated for cell growth and product titer. The results from the scouting work was used for design of experiments (DoE) where a range of phosphate levels from 27.1 mM to 86.4 mM was simultaneously evaluated with temperature, pH and DO set points. Definitive screening was used to evaluate these parameters simultaneously and the results indicate that fermentation temperature and phosphate content are the major contributors of product titer. The other factors tested such as pH had a minimal effect and DO had no impact on product titer.

A PhoA-STII Based Method for Efficient Extracellular Secretion and Purification of Fab from Escherichia coli

In comparison with full-length IgGs, antigen binding fragments (Fabs) are smaller in size and do not require the complexed post-translational modification. Therefore, Fab can be cost-effectively produced using an Escherichia coli (E. coli) expression system. However, the disulfide-bonds containing exogenous protein, including Fab, tend to form insoluble inclusion bodies in E. coli, which has been the bottleneck for exogenous protein expressions using this system. The secretory expression of proteins in periplasm or extracellular medium are promising strategies to prevent the formation of inclusion bodies to improve the efficiency to produce Fabs from E. coli. The extracellular expression is of particularly interest since it releases the product into the medium, while periplasmic expression yield is limited to the periplasm space. In addition, the extracellular expression allows for the direct harvesting of proteins from the culture supernatant, sparing the procedures of cell lysis and reducing contamination of host cell protein or DNA. Using anti-VEGF Fab as an example, here we provide a protocol based on the alkaline phosphatase (phoA) promoter and the heat-stable enterotoxin II (STII) leader sequence. Using phosphate starvation to induce the secretory expression, the protocol could be generally used for the efficient production of Fabs.

High purity recombinant human growth hormone (rhGH) expression in Escherichia coli under phoA promoter

ABSTACT Recombinant human Growth Hormone (rhGH) is an important protein for human growth and is in high demand in clinics. Hence, it is necessary to develop an efficient fermentation process to produce highly pure rhGH. In this study, rhGH was expressed in Escherichia coli under alkaline phosphatase (phoA) promoter. The cultivation conditions for high expression level and purity of rhGH were investigated. The best initial phosphate concentration for rhGH expression, out of the 4 levels of initial phosphate concentration tests performed, was 12.6 mmol/L. Subsequently, 2 fed-batch cultivations under low dissolved oxygen (DO) (0% - 10%) and high DO (20% - 30%) conditions were carried out. High purity rhGH (92%) was obtained from 20% - 30% DO-stat cultivation, although the biomass did not show any significant difference. In summary, this research provided an efficient fermentation process for high purity rhGH production from E. coli under phoA promoter, which can lower the production and purification costs for large-scale production of rhGH.

Improvement of the crystallizability and expression of an RNA crystallization chaperone

Crystallizing RNA has been an imperative and challenging task in the world of RNA research. Assistive methods such as chaperone-assisted RNA crystallography (CARC), employing monoclonal antibody fragments (Fabs) as crystallization chaperones have enabled us to obtain RNA crystal structures by forming crystal contacts and providing initial phasing information. Despite the early successes, the crystallization of large RNA-Fab complex remains a challenge in practice. The possible reason for this difficulty is that the Fab scaffold has not been optimized for crystallization in complex with RNA. Here, we have used the surface entropy reduction (SER) technique for the optimization of ΔC209 P4-P6/Fab2 model system. Protruding lysine and glutamate residues were mutated to a set of alanines or serines to construct Fab2SMA or Fab2SMS. Expression with the shake flask approach was optimized to allow large scale production for crystallization. Crystal screening shows that significantly higher crystal-forming ratio was observed for the mutant complexes. As the chosen SER residues are far away from the CDR regions of the Fab, the same set of mutations can now be directly applied to other Fabs binding to a variety of ribozymes and riboswitches to improve the crystallizability of Fab-RNA complex.

Utilizing high-throughput experimentation to enhance specific productivity of an E.coli T7 expression system by phosphate limitation

BACKGROUND

The specific productivity of cultivation processes can be optimized, amongst others, by using genetic engineering of strains, choice of suitable host/vector systems or process optimization (e.g. choosing the right induction time). A further possibility is to reduce biomass buildup in favor of an enhanced product formation, e.g. by limiting secondary substrates in the medium, such as phosphate. However, with conventional techniques (e.g. small scale cultivations in shake flasks), it is very tedious to establish optimal conditions for cell growth and protein expression, as the start of protein expression (induction time) and the degree of phosphate limitation have to be determined in numerous concerted, manually conducted experiments.

RESULTS

We investigated the effect of different induction times and a concurrent phosphate limitation on the specific productivity of the T7 expression system E.coli BL21(DE3) pRhotHi-2-EcFbFP, which produces the model fluorescence protein EcFbFP upon induction. Therefore, specific online-monitoring tools for small scale cultivations (RAMOS, BioLector) as well as a novel cultivation platform (Robo-Lector) were used for rapid process optimization. The RAMOS system monitored the oxygen transfer rate in shake flasks, whereas the BioLector device allowed to monitor microbial growth and the production of EcFbFP in microtiter plates. The Robo-Lector is a combination of a BioLector and a pipetting robot and can conduct high-throughput experiments fully automated. By using these tools, it was possible to determine the optimal induction time and to increase the specific productivity for EcFbFP from 22% (for unlimited conditions) to 31% of total protein content of the E.coli cells via a phosphate limitation.

CONCLUSIONS

The results revealed that a phosphate limitation at the right induction time was suitable to redirect the available cellular resources during cultivation to protein expression rather than in biomass production. To our knowledge, such an effect was shown for the first time for an IPTG-inducible expression system. Finally, this finding and the utilization of the introduced high-throughput experimentation approach could help to find new targets to further enhance the production capacity of recombinant E.coli-strains.