Bicistronic expression strategy for high-level expression of recombinant proteins in Corynebacterium glutamicum

Hyper-production of porcine contagious pleuropneumonia subunit vaccine proteins in Escherichia coli by developing a bicistronic T7 expression system

The ApxII toxin and the outer membrane lipoprotein (Oml) of Actinobacillus pleuropneumoniae are important vaccine antigens against porcine contagious pleuropneumonia (PCP), a prevalent infectious disease affecting the swine industry worldwide. Previous studies have reported the recombinant expression of ApxII and Oml in Escherichia coli; however, their yields were not satisfactory. Here, we aimed to enhance the production of ApxII and Oml by constructing a bicistronic expression system based on the widely used T7 promoter. To create efficient T7 bicistronic expression cassettes, 16 different fore-cistron sequences were introduced downstream of the T7 promoter. The expression of three vaccine antigens Oml1, Oml7, and ApxII in the four strongest bicistronic vectors were enhanced compared to the monocistronic control. Further optimization of the fermentation conditions in micro-well plates (MWP) led to improved production. Finally, the production yields reached unprecedented levels of 2.43 g L-1 of Oml1, 2.59 g L-1 of Oml7, and 1.21 g L-1 of ApxII, in a 5 L bioreactor. These three antigens also demonstrated well-protective immunity against A. pleuropneumoniae infection. In conclusion, this study establishes an efficient bicistronic T7 expression system that can be used to express recombinant proteins in E. coli and achieves the hyper-production of PCP vaccine proteins.

Alternative routes for production of the drug candidate d-chiro-inositol with Corynebacterium glutamicum using endogenous or promiscuous plant enzymes

d-chiro-Inositol (DCI) is a promising drug candidate for treating insulin resistance and associated diseases such as type 2 diabetes or polycystic ovary syndrome. In this study, we developed two production processes for DCI using Corynebacterium glutamicum as host. In the first process, myo-inositol (MI) is oxidized to 2-keto-myo-inositol (2KMI) by the inositol dehydrogenase (IDH) IolG and then isomerized to 1-keto-d-chiro-inositol (1KDCI) by the isomerases Cg0212 or Cg2312, both of which were identified in this work. 1KDCI is then reduced to DCI by IolG. Overproduction of IolG and Cg0212 in a chassis strain unable to degrade inositols allowed the production of 1.1 g/L DCI from 10 g/L MI. As both reactions involved are reversible, only a partial conversion of MI to DCI can be achieved. To enable higher conversion ratios, a novel route towards DCI was established by utilizing the promiscuous activity of two plant-derived enzymes, the NAD+-dependent d-ononitol dehydrogenase MtOEPa and the NADPH-dependent d-pinitol dehydrogenase MtOEPb from Medicago truncatula (barrelclover). Heterologous production of these enzymes in the chassis strain led to the production of 1.6 g/L DCI from 10 g/L MI. For replacing the substrate MI by glucose, the two plant genes were co-expressed with the endogenous myo-inositol-1-phosphate synthase gene ino1 either as a synthetic operon or using a novel, bicistronic T7-based expression vector. With the single operon construct, 0.75 g/L DCI was formed from 20 g/L glucose, whereas with the bicistronic construct 1.2 g/L DCI was obtained, disclosing C. glutamicum as an attractive host for of d-chiro-inositol production.

Interrogating the Function of Bicistronic Translational Control Elements to Improve Consistency of Gene Expression

Context independent gene expression is required for genetic circuits to maintain consistent and predicable behavior. Previous efforts to develop context independent translation have leveraged the helicase activity of translating ribosomes via bicistronic design translational control elements (BCDs) located within an efficiently translated leader peptide. We have developed a series of bicistronic translational control elements with strengths that span several orders of magnitude, maintain consistent expression levels across diverse sequence contexts, and are agnostic to common ligation sequences used in modular cloning systems. We have used this series of BCDs to investigate several features of this design, including the spacing of the start and stop codons, the nucleotide identity upstream of the start codon, and factors affecting translation of the leader peptide. To demonstrate the flexibility of this architecture and their value as a generic modular expression control cassette for synthetic biology, we have developed a set of robust BCDs for use in several Rhodococcus species.

Enhanced production of recombinant proteins in Corynebacterium glutamicum using a molecular chaperone

Protein synthesis in Corynebacterium glutamicum is critical for applications in biotechnology and medicine. However, the use of C. glutamicum for protein production is limited by its low expression and aggregation. To overcome these limitations, a molecular chaperone plasmid system was developed in this study to improve the efficiency of recombinant protein synthesis in C. glutamicum. The effect of molecular chaperones on target protein synthesis (Single-chain variable fragment, Scfv) under three different promoter strengths was tested. In addition, the plasmid containing the molecular chaperone and target protein was verified for growth stability and plasmid stability. This expression model was further validated using two recombinant proteins, human interferon-beta (Hifn) and hirudin variant III (Rhv3). Finally, the Rhv3 protein was purified, and analysis of Rhv3 activity confirmed that the use of a molecular chaperone led to an improvement in test protein synthesis. Thus, the use of molecular chaperones is believed to will improve recombinant proteins synthesis in C. glutamicum.

Production of Trehalose from Maltose by Whole Cells of Permeabilized Recombinant Corynebacterium glutamicum

Trehalose (α-D-glucopyranosyl-1,1-α-D-glucopyranoside) is a stable and nonreducing disaccharide; can be used as sweetener, stabilizer, and humectant; and has many applications in the food, pharmaceutical, and cosmetic industries. Trehalose production from maltose catalyzed by trehalose synthase (TreS) is simple and economically feasible for industrial-scale application. Reducing the cost and enhancing the efficiency of TreS synthesis and the conversion of maltose to trehalose is critical for trehalose production. In this study, the homologous TreS was constitutively overexpressed in Corynebacterium glutamicum ATCC13032 by removing the repressor gene lacIq fragment in the plasmid, and TreS expression could be exempt from the inducer addition and induction process. For cell permeabilization, Triton X-100 was used as a permeabilization agent, and the treatment time was 3 h. In the conversion system, the permeabilized cells of recombinant C. glutamicum were used as biocatalysts, 300 g/L maltose was used as a substrate, and 173.7 g/L trehalose was produced within 12 h under 30 °C and pH 7.0 conditions. In addition, the whole-cell biocatalysts showed promising reusability. This study provides a safe, convenient, practical, and low-cost pathway for the production of trehalose.

Development of a novel platform for recombinant protein production in Corynebacterium glutamicum on ethanol

Corynebacterium glutamicum represents an emerging recombinant protein expression factory due to its ideal features for protein secretion, but its applicability is harmed by the lack of an autoinduction system with tight regulation and high yield. Here, we propose a new recombinant protein manufacturing platform that leverages ethanol as both a delayed carbon source and an inducer. First, we reanalysed the native inducible promoter P_ICL from the acetate uptake operon and found that its limited capacity is the result of the inadequate translation initial architecture. The two strategies of bicistronic design and ribozyme-based insulator can ensure the high activity of this promoter. Next, through transcriptional engineering that alters transcription factor binding sites (TFBSs) and the first transcribed sequence, the truncated promoter P_A256 with a dramatically higher transcription level was generated. When producing the superfolder green fluorescent protein (sfGFP) under 1% ethanol conditions, P_A256 exhibited substantially lower protein accumulation in prophase but an approximately 2.5-fold greater final yield than the strong promoter P_H36. This superior expression mode was further validated using two secreted proteins, camelid antibody fragment (VHH) and endoxylanase (XynA). Furthermore, utilizing CRISPRi technology, ethanol utilization blocking strains were created, and P_A256 was shown to be impaired in the phosphotransacetylase (PTA) knockdown strains, indicating that ethanol metabolism into the tricarboxylic acid cycle is required for P_A256 upregulation. Finally, this platform was applied to produce the “de novo design” protein NEO-2/15, and by introducing the N-propeptide of CspB, NEO-2/15 was effectively secreted with the accumulation 281 mg/L obtained after 24 h of shake-flask fermentation. To the best of our knowledge, this is the first report of NEO-2/15 secretory overexpression.

Hypersecretion of OmlA antigen in Corynebacterium glutamicum through high-throughput based development process

Outer membrane lipoprotein A (OmlA) is a vaccine antigen against porcine contagious pleuropneumonia (PCP), a disease severely affecting the swine industry. Here, we aimed to systematically potentiate the secretory production of OmlA in Corynebacterium glutamicum (C. glutamicum), a widely used microorganism in the food industry, by establishing a holistic development process based on our high-throughput culture platform. The expression patterns, expression element combinations, medium composition, and induction conditions were comprehensively screened or optimized in microwell plates (MWPs), followed by fermentation parameter optimization in a 4 × 1 L parallel fermentation system (CUBER4). An unprecedented yield of 1.01 g/L OmlA was ultimately achieved in a 5-L bioreactor following the scaling-up strategy of fixed oxygen mass transfer coefficient (k_La), and the produced OmlA antigen showed well-protective immunity against Actinobacillus pleuropneumoniae challenge. This result provides a rapid and reliable pipeline to achieve the hyper-production of OmlA, and possibly other recombinant vaccines, in C. glutamicum. KEY POINTS: • Established a holistic development process and applied it to potentiate the secretion of OmlA. • The secretion of OmlA reached an unprecedented yield of 1.01 g/L. • The recombinant OmlA antigen induced efficient protective immunity.

Bicistronic design as recombinant expression enhancer: characteristics, applications, and structural optimization

The bicistronic design (BCD) is characterized by a short fore-cistron sequence and a second Shine-Dalgarno (SD2) sequence upstream of the target gene. The outstanding performance of this expression cassette in promoting recombinant protein production has attracted attention. Recently, the application of the BCD has been further extended to gene expression control, protein translation monitoring, and membrane protein production. In this review, we summarize the characteristics, molecular mechanisms, applications, and structural optimization of the BCD expression cassette. We also specifically discuss the challenges that the BCD system still faces. This is the first review of the BCD expression strategy, and it is believed that an in-depth understanding of the BCD will help researchers to better utilize and develop it. KEY POINTS: • Summary of the characteristics and molecular mechanisms of the BCD system. • Review of the actual applications of the BCD expression cassette. • Summary of the structural optimization of the BCD system.

Effect of Clp protease from Corynebacterium glutamicum on heterologous protein expression

The protease present in a host may reduce the yield and biological activity of heterologous proteins. In this study, we used protease overexpression and deletion strategies to examine the effect of the Clp protease system in Corynebacterium glutamicum on the recombinant protein and to produce a highly efficient heterologous protein expression host. In this study, we identified seven genes in the Clp protease family in Corynebacterium glutamicum ATCC 13032 through bioinformatics analysis, and studied their effects on the enhanced green fluorescent protein (EGFP) reporter protein. The fluorescence intensity of the knockout strain was significantly higher, and the effect of the clpS deletion strain was the most obvious. To verify the universal effect of the lack of clpS, the excellent industrial strain C. glutamicum 1.15647 was transformed to form recombinant 15647-ΔclpS. Based on the results, 15647-ΔclpS had a more significant effect on improving protein expression. Furthermore, recombinant human teriparatide (rhPTH) and variable domain of heavy chain of heavy-chain antibody (VHH) were selected to verify the universal applicability of the knockout strain for expressing heterologous proteins. Accordingly, we found that protease deficiency could increase the production of heterologous proteins. Finally, through a large-scale fermentation, the 15647-ΔclpS strain was used to produce VHH. Its yield was approximately 530 mg/L, which was 65% higher than that of WT-15647. In this study, a host that could effectively increase heterologous protein expression was successfully obtained.

Construction of a 3A system from BioBrick parts for expression of recombinant hirudin variants III in Corynebacterium glutamicum

Standardized parts can be efficiently assembled into novel biological systems using the three antibiotic (3A) system, ensuring the reusability of components and repeatability of experiments. In this study, we created the 3A expression system for easy construction of gene expression cassettes in Corynebacterium glutamicum (C. glutamicum), which was applied to screen combinations of promoters and signal peptides for improved secreted rhv3 production. We first obtained three strong promoters P₂₂₅₂, P_odhi, and P_yweA from all of promoters, which drive the highest expression of green fluorescent protein (egfp). The three promoters were then assembled with different signal peptides to generate a series of constructs using the 3A expression system developed in this study, from which the highest activity of rhv3 reached 3187.5 ATU/L of P_yweA-CspA-rhv3. Further increased production of rhv3 achieved large-scale fermentation using 5-L jar bioreactor, with the highest rhv3 accumulation 1.21 g/L obtained after 40 h of cultivation, which is higher than 0.95 g/L reported in E. coli. To the best of our knowledge, this is the first report of rhv3 secretory expression in C. glutamicum, which could be applied for the production of other recombinant proteins with significant applications.Key points• We have exploited a 3A system for the genetic manipulation in C. glutamicum.• We constructed element libraries for assembling standard sequence in C. glutamicum.• The secreted expression of rhv3 was realized by 3A system in C. glutamicum.

Enhanced production of recombinant proteins in Corynebacterium glutamicum by constructing a bicistronic gene expression system

BACKGROUND

Corynebacterium glutamicum is a traditional food-grade industrial microorganism, in which an efficient endotoxin-free recombinant protein expression factory is under developing in recent years. However, the intrinsic disadvantage of low recombinant protein expression level is still difficult to be solved. Here, according to the bacteria-specific polycistronic feature that multiple proteins can be translated in one mRNA, efforts have been made to insert a leading peptide gene upstream of target genes as an expression enhancer, and it is found that this can remarkably improve the expression level of proteins under the control of inducible tac promoter in C. glutamicum.

RESULTS

In this research, the Escherichia coli (E. coli) tac promoter combined with 24 different fore-cistron sequences were constructed in a bicistronic manner in C. glutamicum. Three strong bicistronic expression vectors were isolated and exhibited high efficiency under different culture conditions. The compatibility of these bicistronic vectors was further validated using six model proteins- aldehyde dehydrogenase (ALDH), alcohol dehydrogenase (ADH), RamA (regulator of acetate metabolism), Bovine interferon-α (BoIFN-α), glycoprotein D protein (gD) of infectious bovine rhinotracheitis virus (IBRV) and procollagen type Ι N-terminal peptide (PΙNP). All examined proteins were highly expressed compared with the original vector with tac promoter. Large-scale production of PΙNP was also performed in fed-batch cultivation, and the highest PΙNP production level was 1.2 g/L.

CONCLUSION

In this study, the strength of the inducible tac promoter for C. glutamicum was improved by screening and inserting fore-cistron sequences in front of the target genes. Those vectors with bicistronic expression patterns have strong compatibility for expressing various heterogeneous proteins in high yield. This new strategy could be used to further improve the performance of inducible promoters, achieving double competence of inducible control and high yield.

Development of a secretory expression system with high compatibility between expression elements and an optimized host for endoxylanase production in Corynebacterium glutamicum

BACKGROUND

In terms of protein production, the internal environment of the host influences the activity of expression elements, thus affecting the expression level of the target protein. Native expression elements from a specific strain always function well in the original host. In the present study, to enhance the endoxylanase (XynA) production level in Corynebacterium glutamicum CGMCC1.15647 with its native expression elements, approaches to reduce host expression obstacles and to promote expression were evaluated.

RESULTS

We identified the signal peptide of CspB2 in C. glutamicum CGMCC1.15647 by MALDI-TOF and applied it along with its promoter for the production of endoxylanase (XynA) in this strain. The native cspB2 promoter and cspB2 signal peptide are superior to the well-used cspB1 promoter and cspA signal peptide for XynA expression in C. glutamicum CGMCC1.15647, and expression in this strain is superior to the expression in C. glutamicum ATCC13032. The highest XynA secretion efficiency level in deep 24-well plates level (2492.88 U/mL) was achieved by disruption of the cell wall protein CspB2 and the protease ClpS, chromosomal integration of xynA and coexisting plasmid expression, which increased expression 11.43- and 1.35-fold compared to that of chromosomal expression and pXMJ19-xynA-mediated expression in the original strain, respectively. In fed-batch cultivation, the highest XynA accumulation (1.77 g/L) was achieved in the culture supernatant after 44 h of cultivation.

CONCLUSION

Adaptation between the expression elements and the host is crucial for XynA production in C. glutamicum CGMCC1.15647. Strategies including host optimization, chromosomal integration, and coexistence of plasmids were useful for efficient protein production in C. glutamicum.