Production of l-Lysine from starch by Corynebacterium glutamicum displaying α-amylase on its cell surface

Recent Advances in Metabolic Engineering for the Biosynthesis of Phosphoenol Pyruvate-Oxaloacetate-Pyruvate-Derived Amino Acids

The phosphoenol pyruvate-oxaloacetate-pyruvate-derived amino acids (POP-AAs) comprise native intermediates in cellular metabolism, within which the phosphoenol pyruvate-oxaloacetate-pyruvate (POP) node is the switch point among the major metabolic pathways existing in most living organisms. POP-AAs have widespread applications in the nutrition, food, and pharmaceutical industries. These amino acids have been predominantly produced in Escherichia coli and Corynebacterium glutamicum through microbial fermentation. With the rapid increase in market requirements, along with the global food shortage situation, the industrial production capacity of these two bacteria has encountered two bottlenecks: low product conversion efficiency and high cost of raw materials. Aiming to push forward the update and upgrade of engineered strains with higher yield and productivity, this paper presents a comprehensive summarization of the fundamental strategy of metabolic engineering techniques around phosphoenol pyruvate-oxaloacetate-pyruvate node for POP-AA production, including L-tryptophan, L-tyrosine, L-phenylalanine, L-valine, L-lysine, L-threonine, and L-isoleucine. Novel heterologous routes and regulation methods regarding the carbon flux redistribution in the POP node and the formation of amino acids should be taken into consideration to improve POP-AA production to approach maximum theoretical values. Furthermore, an outlook for future strategies of low-cost feedstock and energy utilization for developing amino acid overproducers is proposed.

Biosystem design of Corynebacterium glutamicum for bioproduction

Corynebacterium glutamicum, a natural glutamate-producing bacterium adopted for industrial production of amino acids, has been extensively explored recently for high-level biosynthesis of amino acid derivatives, bulk chemicals such as organic acids and short-chain alcohols, aromatics, and natural products, including polyphenols and terpenoids. Here, we review the recent advances with a focus on biosystem design principles, metabolic characterization and modeling, omics analysis, utilization of nonmodel feedstock, emerging CRISPR (Clustered Regularly Interspaced Short Palindromic Repeats) tools for Corynebacterium strain engineering, biosensors, and novel strains of C. glutamicum. Future research directions for developing C. glutamicum cell factories are also discussed.

Genome-Scale Mining of Novel Anchor Proteins of Corynebacterium glutamicum

The display of recombinant proteins on the surfaces of bacteria is a research topic with many possible biotechnology applications—among which, the choice of host cell and anchoring motif is the key for efficient display. Corynebacterium glutamicum is a promising host for surface display due to its natural advantages, while single screening methods and fewer anchor proteins restrict its application. In this study, the subcellular localization (SCL) predictor LocateP and tied-mixture hidden Markov models were used to analyze all five known endogenous anchor proteins of C. glutamicum and test the accuracy of the predictions. Using these two tools, the SCLs of all proteins encoded by the genome of C. glutamicum 13032 were predicted, and 14 potential anchor proteins were screened. Compared with the positive controls NCgl1221 and NCgl1337, three anchoring proteins—NCgl1307, NCgl2775, and NCgl0717—performed better. This study also discussed the applicability of the anchor protein screening method used in this experiment to other bacteria.

Application of Corynebacterium glutamicum engineering display system in three generations of biorefinery

The fermentation production of platform chemicals in biorefineries is a sustainable alternative to the current petroleum refining process. The natural advantages of Corynebacterium glutamicum in carbon metabolism have led to C. glutamicum being used as a microbial cell factory that can use various biomass to produce value-added platform chemicals and polymers. In this review, we discussed the use of C. glutamicum surface display engineering bacteria in the three generations of biorefinery resources, and analyzed the C. glutamicum engineering display system in degradation, transport, and metabolic network reconstruction models. These engineering modifications show that the C. glutamicum engineering display system has great potential to become a cell refining factory based on sustainable biomass, and further optimizes the inherent properties of C. glutamicum as a whole-cell biocatalyst. This review will also provide a reference for the direction of future engineering transformation.

Rational reformation of Corynebacterium glutamicum for producing L-lysine by one-step fermentation from raw corn starch

This article focuses on engineering Corynebacterium glutamicum to produce L-lysine efficiently from starch using combined method of "classical breeding" and "genome breeding." Firstly, a thermo-tolerable L-lysine-producing C. glutamicum strain KT_45-6 was obtained after multi-round of acclimatization at high temperature. Then, amylolytic enzymes were introduced into strain KT_45-6, and the resultant strains could use starch for cell growth and L-lysine production except the strain with expression of isoamylase. In addition, co-expression of amylolytic enzymes showed a good performance in starch degradation, cell growth and L-lysine production, especially co-expression of α-amylase (AA) and glucoamylase (GA). Moreover, L-lysine yield was increased by introducing AA-GA fusion protein (i.e., strain KT_45-6S-5), and finally reached to 23.9 ± 2.3 g/L in CgXII^IPM-medium. It is the first report of an engineered L-lysine-producing strain with maximum starch utilization that may be used as workhorse for producing amino acid using starch as the main feedstock. KEY POINTS: • Thermo-tolerable C. glutamicum was obtained by temperature-induced adaptive evolution. • The fusion order between AA and GA affects the utilization efficiency of starch. • C. glutamicum with starch utilization was constructed by optimizing amylases expression.

Utilization of a Wheat Sidestream for 5-Aminovalerate Production in Corynebacterium glutamicum

Production of plastics from petroleum-based raw materials extensively contributes to global pollution and CO2 emissions. Biotechnological production of functionalized monomers can reduce the environmental impact, in particular when using industrial sidestreams as feedstocks. Corynebacterium glutamicum, which is used in the million-ton-scale amino acid production, has been engineered for sustainable production of polyamide monomers. In this study, wheat sidestream concentrate (WSC) from industrial starch production was utilized for production of l-lysine-derived bifunctional monomers using metabolically engineered C. glutamicum strains. Growth of C. glutamicum on WSC was observed and could be improved by hydrolysis of WSC. By heterologous expression of the genes xylA Xc B Cg (xylA from Xanthomonas campestris) and araBAD Ec from E. coli, xylose, and arabinose in WSC hydrolysate (WSCH), in addition to glucose, could be consumed, and production of l-lysine could be increased. WSCH-based production of cadaverine and 5-aminovalerate (5AVA) was enabled. To this end, the lysine decarboxylase gene ldcC Ec from E. coli was expressed alone or for conversion to 5AVA cascaded either with putrescine transaminase and dehydrogenase genes patDA Ec from E. coli or with putrescine oxidase gene puo Rq from Rhodococcus qingshengii and patD Ec . Deletion of the l-glutamate dehydrogenase-encoding gene gdh reduced formation of l-glutamate as a side product for strains with either of the cascades. Since the former cascade (ldcC Ec -patDA Ec ) yields l-glutamate, 5AVA production is coupled to growth by flux enforcement resulting in the highest 5AVA titer obtained with WSCH-based media.

Cascaded valorization of brown seaweed to produce l-lysine and value-added products using Corynebacterium glutamicum streamlined by systems metabolic engineering

Seaweeds emerge as promising third-generation renewable for sustainable bioproduction. In the present work, we valorized brown seaweed to produce l-lysine, the world's leading feed amino acid, using Corynebacterium glutamicum, which was streamlined by systems metabolic engineering. The mutant C. glutamicum SEA-1 served as a starting point for development because it produced small amounts of l-lysine from mannitol, a major seaweed sugar, because of the deletion of its arabitol repressor AtlR and its engineered l-lysine pathway. Starting from SEA-1, we systematically optimized the microbe to redirect excess NADH, formed on the sugar alcohol, towards NADPH, required for l-lysine synthesis. The mannitol dehydrogenase variant MtlD D75A, inspired by 3D protein homology modelling, partly generated NADPH during the oxidation of mannitol to fructose, leading to a 70% increased l-lysine yield in strain SEA-2C. Several rounds of strain engineering further increased NADPH supply and l-lysine production. The best strain, SEA-7, overexpressed the membrane-bound transhydrogenase pntAB together with codon-optimized gapN, encoding NADPH-dependent glyceraldehyde 3-phosphate dehydrogenase, and mak, encoding fructokinase. In a fed-batch process, SEA-7 produced 76 g L^-1l-lysine from mannitol at a yield of 0.26 mol mol^-1 and a maximum productivity of 2.1 g L^-1 h^-1. Finally, SEA-7 was integrated into seaweed valorization cascades. Aqua-cultured Laminaria digitata, a major seaweed for commercial alginate, was extracted and hydrolyzed enzymatically, followed by recovery and clean-up of pure alginate gum. The residual sugar-based mixture was converted to l-lysine at a yield of 0.27 C-mol C-mol^-1 using SEA-7. Second, stems of the wild-harvested seaweed Durvillaea antarctica, obtained as waste during commercial processing of the blades for human consumption, were extracted using acid treatment. Fermentation of the hydrolysate using SEA-7 provided l-lysine at a yield of 0.40 C-mol C-mol^-1. Our findings enable improvement of the efficiency of seaweed biorefineries using tailor-made C. glutamicum strains.

Improved fermentative γ-aminobutyric acid production by secretory expression of glutamate decarboxylase by Corynebacterium glutamicum

Fermentative production of γ-aminobutyric acid by the glutamate overproducing Corynebacterium glutamicum from cheap sugar feedstock is generally regarded as one of the most promising methods to reduce the production cost. However, the intracellularly expressed glutamate decarboxylase in C. glutamicum often showed feeble catalysis activity to convert glutamate into γ-aminobutyric acid. Here we tried to secretory express glutamate decarboxylase to achieve efficient extracellular decarboxylation of glutamate, thus improving the γ-aminobutyric acid production by C. glutamicum. We first tested glutamate decarboxylases from different sources, and the mutated glutamate decarboxylase GadBmut from E. coli with better catalytic performance was selected. Then, a signal peptide of the SEC translocation pathway directed the successful secretion of glutamate decarboxylase in C. glutamicum. The extracellular catalysis by secreted glutamate decarboxylase increased the γ-aminobutyric acid generation by three-fold, compared with intracellular catalysis. Enhancing glutamate decarboxylase expression and decreasing γ-aminobutyric acid degradation further increased γ-aminobutyric acid production by 39 %. The fed-batch fermentation of the engineered C. glutamicum strain reached the record high titer (77.6 ± 0.0 g /L), overall yield (0.44 ± 0.00 g/g glucose), and productivity (1.21 ± 0.00 g/L/h). This study demonstrated a unique design of extracellular catalysis for efficient γ-aminobutyric acid production by C. glutamicum.

Cell Surface Display of Thermomyces lanuginosus Lipase in Pichia pastoris

A cell surface displayed system in Pichia pastoris GS115 was developed by using GCW61, a glycosylphosphatidylinositol-modified cell wall protein from P. pastoris, as the anchor protein. Thermomyces lanuginosus lipase (TLL) was successfully displayed on the P. pastoris cell wall by fusing GCW61 gene with TLL2 gene (NCBI Accession: O59952) that was optimized with codon bias and synthesized. Cell surface displayed TLL2 was confirmed by the immunofluorescence microscopy. Flask fermentation was performed for 144 h with lipase activity up to 1964.76 U/g. Enzymatic properties of cell surface displayed TLL2 were also investigated. Displayed TLL2 occurred the maximum activity at pH 9 and 55°C and demonstrated characteristics of wide thermal adaptability and alkaline pH resistance. The optimum substrate was p-nitrophenyl hexanoate. Bivalent metal ions Ca²⁺, Mn²⁺, and Zn²⁺ had the activation effect on displayed TLL2, while Cu²⁺, Fe²⁺, Fe³⁺, K⁺, Li⁺, Na⁺, and Co²⁺ ions had the inhibitory effect on it. Since cell surface displayed TLL2 required less purification steps compared with free enzyme and showed high enzyme activities, it would be able to be further applied in various potential applications.

The glucose uptake systems in Corynebacterium glutamicum: a review

The phosphoenolpyruvate-dependent glucose phosphotransferase system (PTS^Glc) is the major uptake system responsible for transporting glucose, and is involved in glucose translocation and phosphorylation in Corynebacterium glutamicum. For the longest time, the PTS^Glc was considered as the only uptake system for glucose. However, some PTS-independent glucose uptake systems (non-PTS^Glc) were discovered in recent years, such as the coupling system of inositol permeases and glucokinases (IPGS) and the coupling system of β-glucoside-PTS permease and glucokinases (GPGS). The products (e.g. lysine, phenylalanine and leucine) will be increased because of the increasing intracellular level of phosphoenolpyruvate (PEP), while some by-products (e.g. lactic acid, alanine and acetic acid) will be reduced when this system become the main uptake pathway for glucose. In this review, we survey the uptake systems for glucose in C. glutamicum and their composition. Furthermore, we summarize the latest research of the regulatory mechanisms among these glucose uptake systems. Detailed strategies to manipulate glucose uptake system are addressed based on this knowledge.

Metabolic engineering advances and prospects for amino acid production

Amino acid fermentation is one of the major pillars of industrial biotechnology. The multi-billion USD amino acid market is rising steadily and is diversifying. Metabolic engineering is no longer focused solely on strain development for the bulk amino acids L-glutamate and L-lysine that are produced at the million-ton scale, but targets specialty amino acids. These demands are met by the development and application of new metabolic engineering tools including CRISPR and biosensor technologies as well as production processes by enabling a flexible feedstock concept, co-production and co-cultivation schemes. Metabolic engineering advances are exemplified for specialty proteinogenic amino acids, cyclic amino acids, omega-amino acids, and amino acids functionalized by hydroxylation, halogenation and N-methylation.

Recombinant Protein Expression System in Corynebacterium glutamicum and Its Application

Corynebacterium glutamicum, a soil-derived gram-positive actinobacterium, has been widely used for the production of biochemical molecules such as amino acids (i.e., L-glutamate and L-lysine), nucleic acids, alcohols, and organic acids. The metabolism of the bacterium has been engineered to increase the production of the target biochemical molecule, which requires a cytosolic enzyme expression. As recent demand for new proteinaceous biologics (such as antibodies, growth factors, and hormones) increase, C. glutamicum is attracting industrial interest as a recombinant protein expression host for therapeutic protein production due to the advantages such as low protease activity without endotoxin activity. In this review, we have summarized the recent studies on the heterologous expression of the recombinant protein in C. glutamicum for metabolic engineering, expansion of substrate availability, and recombinant protein secretion. We have also outlined the advances in genetic components such as promoters, surface anchoring systems, and secretory signal sequences in C. glutamicum for effective recombinant protein expression.

Current status on metabolic engineering for the production of l-aspartate family amino acids and derivatives

The l-aspartate amino acids (AFAAs) are constituted of l-aspartate, l-lysine, l-methionine, l-threonine and l-isoleucine. Except for l-aspartate, AFAAs are essential amino acids that cannot be synthesized by humans and most farm animals, and thus possess wide applications in food, animal feed, pharmaceutical and cosmetics industries. To date, a number of amino acids, including AFAAs have been industrially produced by microbial fermentation. However, the overall metabolic and regulatory mechanisms of the synthesis of AFAAs and the recent progress on strain construction have rarely been reviewed. Aiming to promote the establishment of strains of Corynebacterium glutamicum and Escherichia coli, the two industrial amino acids producing bacteria, that are capable of producing high titers of AFAAs and derivatives, this paper systematically summarizes the current progress on metabolic engineering manipulations in both central metabolic pathways and AFAA synthesis pathways based on the category of the five-word strain breeding strategies: enter, flow, moderate, block and exit.

Development of a Potential Protein Display Platform in Corynebacterium glutamicum Using Mycolic Acid Layer Protein, NCgl1337, as an Anchoring Motif

In the cell surface display, the choice of host cell and anchoring motif are the most crucial for the efficient display of passenger proteins. Corynebacterium glutamicum has mycolic acid layer in outer membrane and the use of protein in the mycolic acid layer as an anchoring motif can provide a potential platform for surface display in C. glutamicum. All 19 mycolic acid layer proteins of C. glutamicum are analyzed, and two proteins, NCgl0535 and NCgl1337, which have a signal peptide and predicted O-mycoloylation site, are selected as anchoring motifs candidates. Among them, NCgl1337, which shows better expression with higher display efficiency, is chosen as a potential anchoring motif. Two forms of the NCgl1337 anchoring motif, a full-length (1-324 amino acids) and a short-length (1-50 amino acids) containing only signal peptide and O-mycoloylation site, are constructed and their abilities for surface display are examined using two protein models, endoxylanase from Streptomyces coelicolor and α-amylase from Streptococcus bovis. For both model proteins, the short-length NCgl1337 anchoring motif exhibits higher yield of protein display on the surface of C. glutamicum than the full-length NCgl1337. Finally, with C. glutamicum displaying α-amylase, a batch fermentation is performed for the production of l-lysine from starch degradation, and a production of l-lysine as high as 10.8 ± 0.92 g L^-1 was achieved after 18 h of culture.

Corynebacterium glutamicum possesses β-N-acetylglucosaminidase

BACKGROUND

In Gram-positive Corynebacterium glutamicum and other members of the suborder Corynebacterianeae, which includes mycobacteria, cell elongation and peptidoglycan biosynthesis is mainly due to polar growth. C. glutamicum lacks an uptake system for the peptidoglycan constituent N-acetylglucosamine (GlcNAc), but is able to catabolize GlcNAc-6-phosphate. Due to its importance in white biotechnology and in order to ensure more sustainable processes based on non-food renewables and to reduce feedstock costs, C. glutamicum strains have previously been engineered to produce amino acids from GlcNAc. GlcNAc also is a constituent of chitin, but it is unknown if C. glutamicum possesses chitinolytic enzymes.

RESULTS

Chitin was shown here not to be growth substrate for C. glutamicum. However, its genome encodes a putative N-acetylglucosaminidase. The nagA 2 gene product was active as β-N-acetylglucosaminidase with 0.27 mM 4-nitrophenyl N,N'-diacetyl-β-D-chitobioside as substrate supporting half-maximal activity. NagA2 was secreted into the culture medium when overproduced with TAT and Sec dependent signal peptides, while it remained cytoplasmic when overproduced without signal peptide. Heterologous expression of exochitinase gene chiB from Serratia marcescens resulted in chitinolytic activity and ChiB secretion was enhanced when a signal peptide from C. glutamicum was used. Colloidal chitin did not support growth of a strain secreting exochitinase ChiB and β-N-acetylglucosaminidase NagA2.

CONCLUSIONS

C. glutamicum possesses β-N-acetylglucosaminidase. In the wild type, β-N-acetylglucosaminidase activity was too low to be detected. However, overproduction of the enzyme fused to TAT or Sec signal peptides led to secretion of active β-N-acetylglucosaminidase. The finding that concomitant secretion of endogenous NagA2 and exochitinase ChiB from S. marcescens did not entail growth with colloidal chitin as sole or combined carbon source, may indicate the requirement for higher or additional enzyme activities such as processive chitinase or endochitinase activities.

Co-expression of endoglucanase and β-glucosidase in Corynebacterium glutamicum DM1729 towards direct lysine fermentation from cellulose

The aim of the present study is the development of a consolidated bioprocess for the production of lysine with recombinant Corynebacterium glutamicum DM1729 strains expressing endoglucanase and β-glucosidase genes. Here, the endoglucanase genes from Xanthomonas campestris XCC3521 and XCC2387 and betaglucosidase gene from Saccharophagus degradans Sde1394 were cloned in C. glutamicum DM1729 and expressed either extracellularly or on cell surface. The highest β-glucosidase activity of 9±0.5U/OD600 of 1 and endoglucanase activity of 5.5±0.8U was obtained in C. glutamicum DM 1729 (pVWEx1-TATXCC2387) (pEKEx3-PorC-Sde1394) when cellobiose (20g/L) alone or in combination with carboxymethyl cellulose (20g/L) was used as the carbon sources respectively. The overall efforts resulted in a lysine titre of 5.9±0.5mM. The ability of the constructs to utilize carboxymethyl cellulose and cellobiose for growth and amino acid production proves the concept of utilization of C. glutamicum as a biocatalyst in the lignocellulosic biorefinery.

Advances and needs for endotoxin-free production strains

The choice of an appropriate microbial host cell and suitable production conditions is crucial for the downstream processing of pharmaceutical- and food-grade products. Although Escherichia coli serves as a highly valuable leading platform for the production of value-added products, like most Gram-negative bacteria, this bacterium contains a potent immunostimulatory lipopolysaccharide (LPS), referred to as an endotoxin. In contrast, Gram-positive bacteria, notably Bacillus, lactic acid bacteria (LAB), Corynebacterium, and yeasts have been extensively used as generally recognized as safe (GRAS) endotoxin-free platforms for the production of a variety of products. This review summarizes the currently available knowledge on the utilization of these representative Gram-positive bacteria for the production of eco- and bio-friendly products, particularly natural polyesters, polyhydroxyalkanoates, bacteriocins, and membrane proteins. The successful case studies presented here serve to inspire the use of these microorganisms as a main-player or by-player depending on their individual properties for the industrial production of these desirable targets.

Cell surface engineering of industrial microorganisms for biorefining applications

In order to decrease carbon emissions and negative environmental impacts of various pollutants, biofuel/biochemical production should be promoted for replacing fossil-based industrial processes. Utilization of abundant lignocellulosic biomass as a feedstock has recently become an attractive option. In this review, we focus on recent efforts of cell surface display using industrial microorganisms such as Escherichia coli and yeast. Cell surface display is used primarily for endowing cellulolytic activity on the host cells, and enables direct fermentation to generate useful fuels and chemicals from lignocellulosic biomass. Cell surface display systems are systematically summarized, and the drawbacks/perspectives as well as successful application of surface display for industrial biotechnology are discussed.

Recent advances in the metabolic engineering of Corynebacterium glutamicum for the production of lactate and succinate from renewable resources

Recent increasing attention to environmental issues and the shortage of oil resources have spurred political and industrial interest in the development of environmental friendly and cost-effective processes for the production of bio-based chemicals from renewable resources. Thus, microbial production of commercially important chemicals is viewed as a desirable way to replace current petrochemical production. Corynebacterium glutamicum, a Gram-positive soil bacterium, is one of the most important industrial microorganisms as a platform for the production of various amino acids. Recent research has explored the use of C. glutamicum as a potential cell factory for producing organic acids such as lactate and succinate, both of which are commercially important bulk chemicals. Here, we summarize current understanding in this field and recent metabolic engineering efforts to develop C. glutamicum strains that efficiently produce l- and d-lactate, and succinate from renewable resources.

L-citrulline production by metabolically engineered Corynebacterium glutamicum from glucose and alternative carbon sources

L-citrulline plays an important role in human health and nutrition and is an intermediate of the L-arginine biosynthetic pathway. L-citrulline is a by-product of L-arginine production by Corynebacterium glutamicum. In this study, C. glutamicum was engineered for overproduction of L-citrulline as major product without L-arginine being produced as by-product. To this end, L-arginine biosynthesis was derepressed by deletion of the arginine repressor gene argR and conversion of L-citrulline towards L-arginine was avoided by deletion of the argininosuccinate synthetase gene argG. Moreover, to facilitate L-citrulline production the gene encoding a feedback resistant N-acetyl L-glutamate kinase argB^fbr as well as the gene encoding L-ornithine carbamoylphosphate transferase argF were overexpressed. The resulting strain accumulated 44.1 ± 0.5 mM L-citrulline from glucose minimal medium with a yield of 0.38 ± 0.01 g⋅g⁻¹ and a volumetric productivity of 0.32 ± 0.01 g⋅l⁻¹⋅h⁻¹. In addition, production of L-citrulline from the alternative carbon sources starch, xylose, and glucosamine could be demonstrated.

Engineering of Corynebacterium glutamicum for growth and L-lysine and lycopene production from N-acetyl-glucosamine

Sustainable supply of feedstock has become a key issue in process development in microbial biotechnology. The workhorse of industrial amino acid production Corynebacterium glutamicum has been engineered towards utilization of alternative carbon sources. Utilization of the chitin-derived aminosugar N-acetyl-glucosamine (GlcNAc) for both cultivation and production with C. glutamicum has hitherto not been investigated. Albeit this organism harbors the enzymes N-acetylglucosamine-6-phosphatedeacetylase and glucosamine-6P deaminase of GlcNAc metabolism (encoded by nagA and nagB, respectively) growth of C. glutamicum with GlcNAc as substrate was not observed. This was attributed to the lack of a functional system for GlcNAc uptake. Of the 17 type strains of the genus Corynebacterium tested here for their ability to grow with GlcNAc, only Corynebacterium glycinophilum DSM45794 was able to utilize this substrate. Complementation studies with a GlcNAc-uptake deficient Escherichia coli strain revealed that C. glycinophilum possesses a nagE-encoded EII permease for GlcNAc uptake. Heterologous expression of the C. glycinophilum nagE in C. glutamicum indeed enabled uptake of GlcNAc. For efficient GlcNac utilization in C. glutamicum, improved expression of nagE with concurrent overexpression of the endogenous nagA and nagB genes was found to be necessary. Based on this strategy, C. glutamicum strains for the efficient production of the amino acid L-lysine as well as the carotenoid lycopene from GlcNAc as sole substrate were constructed.

Direct production of organic acids from starch by cell surface-engineered Corynebacterium glutamicum in anaerobic conditions

We produced organic acids, including lactate and succinate, directly from soluble starch under anaerobic conditions using high cell-density cultures of Corynebacterium glutamicum displaying α-amylase (AmyA) from Streptococcus bovis 148 on the cell surface. Notably, reactions performed under anaerobic conditions at 35 and 40°C, which are higher than the optimal growth temperature of 30°C, showed 32% and 19%, respectively, higher productivity of the organic acids lactate, succinate, and acetate compared to that at 30°C. However, α-amylase was not stably anchored and released into the medium from the cell surface during reactions at these higher temperatures, as demonstrated by the 61% and 85% decreases in activity, respectively, from baseline, compared to the only 8% decrease at 30°C. The AmyA-displaying C. glutamicum cells retained their starch-degrading capacity during five 10 h reaction cycles at 30°C, producing 107.8 g/l of total organic acids, including 88.9 g/l lactate and 14.0 g/l succinate. The applicability of cell surface-engineering technology for the production of organic acids from biomass by high cell-density cultures of C. glutamicum under anaerobic conditions was demonstrated.

Recent advances in recombinant protein expression by Corynebacterium, Brevibacterium, and Streptomyces: from transcription and translation regulation to secretion pathway selection

Gram-positive bacteria are widely used to produce recombinant proteins, amino acids, organic acids, higher alcohols, and polymers. Many proteins have been expressed in Gram-positive hosts such as Corynebacterium, Brevibacterium, and Streptomyces. The favorable and advantageous characteristics (e.g., high secretion capacity and efficient production of metabolic products) of these species have increased the biotechnological applications of bacteria. However, owing to multiplicity from genes encoding the proteins and expression hosts, the expression of recombinant proteins is limited in Gram-positive bacteria. Because there is a very recent review about protein expression in Bacillus subtilis, here we summarize recent strategies for efficient expression of recombinant proteins in the other three typical Gram-positive bacteria (Corynebacterium, Brevibacterium, and Streptomyces) and discuss future prospects. We hope that this review will contribute to the development of recombinant protein expression in Corynebacterium, Brevibacterium, and Streptomyces.

Direct L-lysine production from cellobiose by Corynebacterium glutamicum displaying beta-glucosidase on its cell surface

We constructed beta-glucosidase (BGL)-displaying Corynebacterium glutamicum, and direct L-lysine fermentation from cellobiose was demonstrated. After screening active BGLs, Sde1394, which is a BGL from Saccharophagus degradans, was successfully displayed on the C. glutamicum cell surface using porin as an anchor protein, and cellobiose was directly assimilated as a carbon source. The optical density at 600 nm of BGL-displaying C. glutamicum grown on cellobiose as a carbon source reached 23.5 after 48 h of cultivation, which was almost the same as that of glucose after 24 h of cultivation. Finally, Sde1394-displaying C. glutamicum produced 1.08 g/l of L-lysine from 20 g/l of cellobiose after 4 days of cultivation, which was about threefold higher than the amount of produced L-lysine using BGL-secretory C. glutamicum strains (0.38 g/l after 5 days of cultivation). This is the first report on amino acid production using cellobiose as a carbon source by BGL-expressing C. glutamicum.

Metabolic engineering of industrial platform microorganisms for biorefinery applications--optimization of substrate spectrum and process robustness by rational and evolutive strategies

Bio-based production promises a sustainable route to myriads of chemicals, materials and fuels. With regard to eco-efficiency, its future success strongly depends on a next level of bio-processes using raw materials beyond glucose. Such renewables, i.e., polymers, complex substrate mixtures and diluted waste streams, often cannot be metabolized naturally by the producing organisms. This particularly holds for well-known microorganisms from the traditional sugar-based biotechnology, including Escherichia coli, Corynebacterium glutamicum and Saccharomyces cerevisiae which have been engineered successfully to produce a broad range of products from glucose. In order to make full use of their production potential within the bio-refinery value chain, they have to be adapted to various feed-stocks of interest. This review focuses on the strategies to be applied for this purpose which combine rational and evolutive approaches. Hereby, the three industrial platform microorganisms, E. coli, C. glutamicum and S. cerevisiae are highlighted due to their particular importance.

A review of enzymes and microbes for lignocellulosic biorefinery and the possibility of their application to consolidated bioprocessing technology

The biorefinery manufacturing process for producing chemicals and liquid fuels from biomass is a promising approach for securing energy and resources. To establish cost-effective fermentation of lignocellulosic biomass, the consolidation of sacccharification and fermentation processes is a desirable strategy, but requires the development of microorganisms capable of cellulose/hemicellulose hydrolysis and target chemical production. Such an endeavor requires a large number of prerequisites to be realized, including engineering microbial strains with high cellulolytic activity, high product yield, productivities, and titers, ability to use many carbon sources, and resistance to toxic compounds released during the pretreatment of lignocellulosic biomass. Researchers have focused on either engineering naturally cellulolytic microorganisms to improve product-related properties or modifying non-cellulolytic organisms with high product yields to become cellulolytic. This article reviews recent advances in the development of microorganisms for the production of renewable chemicals and advanced biofuels, as well as ethanol, from lignocellulosic materials through consolidated bioprocessing.

Development of a secretion system for the production of heterologous proteins in Corynebacterium glutamicum using the Porin B signal peptide

Corynebacterium glutamicum is one of the useful hosts for the secretory production of heterologous proteins because of intrinsic attributes such as the presence of few endogenous proteins and proteases in culture medium. Here, we report the development of a new secretory system for the production of heterologous proteins by using the porin B (PorB) signal peptide in C. glutamicum. We examined two different endoxylanases and an antibody fragment (scFv) as model proteins for secretory production. In the flask cultivations, all the examined proteins were successfully produced as active forms into the culture medium with high efficiency. For the high-level production of endoxylanase, fed-batch cultivation was also performed in a lab-scale (5L) bioreactor, and the endoxylanases were efficiently secreted in the culture medium at levels as high as 615mg/L. From the culture supernatant, the secreted endoxylanases could be purified with high purity via one-step affinity column chromatography.