The Phosphotransferase System of Corynebacterium glutamicum: Features of Sugar Transport and Carbon Regulation

Production of L-serine and its derivative L-cysteine from renewable feedstocks using Corynebacterium glutamicum: advances and perspectives

L-serine and its derivative L-cysteine have broad industrial applications, and their direct fermentative production from renewable biomass is gaining increasing attention. Corynebacterium glutamicum is an extensively studied and well-established industrial microorganism, which is a predominant microbial host for producing amino acids. In this review, updated information on the genetics and molecular mechanisms underlying L-serine and L-cysteine production using C. glutamicum is presented, including their synthesis and degradation pathways, and other intracellular processes related to their production, as well as the mechanisms underlying substrate import and product export are also analyzed. Furthermore, metabolic strategies for strain improvement are systematically discussed, and conclusions and future perspectives for bio-based L-serine and L-cysteine production using C. glutamicum are presented. This review can provide a thorough understanding of L-serine and L-cysteine metabolic pathways to facilitate metabolic engineering modifications of C. glutamicum and development of more efficient industrial fermentation processes for L-serine and L-cysteine production.

Metabolic engineering of Thermoanaerobacterium aotearoense strain SCUT27 for biofuels production from sucrose and molasses

BACKGROUND

Sucrose-rich sugarcane trash surpasses 28 million tons globally per year. Effective biorefinery systems could convert these biomasses to bioproducts, such as bioethanol from sugarcane sucrose in Brazil. Thermophilic microbes for biofuels have attracted great attention due to their higher fermentation temperature and wide substrate spectrum. However, few thermophiles using sucrose or molasses for biofuels production was reported. Thermoanaerobacterium aotearoense SCUT27 has been considered as an efficient ethanol producer, but it cannot directly utilize sucrose. In this study, various sucrose metabolic pathways were introduced and analyzed in Thermoanaerobaterium.

RESULTS

The sucrose-6-phosphate hydrolase (scrB), which was from a screened strain Thermoanaerobacterium thermosaccharolyticum G3-1 was overexpressed in T. aotearoense SCUT27 and endowed this strain with the ability to utilize sucrose. In addition, overexpression of the sucrose-specific PTS system (scrA) from Clostridium acetobutylicum accelerated the sucrose transport. To strengthen the alcohols production and substrates metabolism, the redox-sensing transcriptional repressor (rex) in T. aotearoense was further knocked out. Moreover, with the gene arginine repressor (argR) deleted, the ethanologenic mutant P8S10 showed great inhibitors-tolerance and finally accumulated ~ 34 g/L ethanol (a yield of 0.39 g/g sugars) from pretreated cane molasses in 5 L tank by fed-batch fermentation. When introducing butanol synthetic pathway, 3.22 g/L butanol was produced by P8SB4 with a yield of 0.44 g alcohols/g sugars at 50℃. This study demonstrated the potential application of T. aotearoense SCUT27 for ethanol and butanol production from low cost cane molasses.

CONCLUSIONS

Our work provided strategies for sucrose utilization in thermophiles and improved biofuels production as well as stress tolerances of T. aotearoense SCUT27, demonstrating the potential application of the strain for cost-effective biofuels production from sucrose-based feedstocks.

Metabolic engineering of Corynebacterium glutamicum for the high-level production of valerolactam, a nylon-5 monomer

Valerolactam (VL) is an important precursor chemical for nylon-5 and nylon 6,5. It has been produced by petroleum-based route involving harsh reaction conditions and generating toxic wastes. Here, we report the complete biosynthesis of VL by metabolically engineered Corynebacterium glutamicum overproducing L-lysine. The pathway comprising L-lysine monooxygenase (davB) and 5-aminovaleramide amidohydrolase (davA) from Pseudomonas putida, and β-alanine CoA transferase (act) from Clostridium propionicum was introduced into the C. glutamicum GA16 strain. To increase the VL flux, competitive pathways predicted from sRNA knockdown target screening were deleted. This engineered C. glutamicum strain produced VL as a major product, but still secreted significant amount of its precursor, 5-aminovaleric acid (5AVA). To circumvent this problem, putative 5AVA transporter genes were screened and engineered in the genome, thereby reuptaking 5AVA excreted. Also, multiple copies of the act gene were integrated into the genome to strengthen the conversion of 5AVA to VL. The final VL10 (pVL1) strain was constructed by enhancing glucose uptake system, which produced 9.68 g/L of VL in flask culture. Fed-batch fermentation of the VL10 (pVL1) strain produced 76.1 g/L of VL from glucose with the yield and productivity of 0.28 g/g and 0.99 g/L/h, respectively, showcasing a high potential for bio-based production of VL from renewable resources.

De novo tryptophanase-based indole production by metabolically engineered Corynebacterium glutamicum

Indole has an increasing interest in the flavor and fragrance industry. It is used in dairy products, tea drinks, and fine fragrances due to its distinct floral odor typical of jasmine blossoms. The current production of indole based on isolation from coal tar is non-sustainable and its isolation from plants is often unprofitable due to low yields. To offer an alternative to the conventional production, biosynthesis of indole has been studied recently. A glucose-based indole production was achieved by employing the Corynebacterium glutamicum tryptophan synthase α-subunit (TrpA) or indole-3-glycerol phosphate lyase (IGL) from wheat Triticum aestivum in a genetically-engineered C. glutamicum strain. In addition, a highly efficient bioconversion process using C. glutamicum heterologously expressing tryptophanase gene (tnaA) from Providencia rettgeri as a biocatalyst was developed. In this work, de novo indole production from glucose was enabled by expressing the P. rettgeri tnaA in a tryptophan-producing C. glutamicum strain. By metabolic engineering of a C. glutamicum shikimate accumulating base strain, tryptophan production of 2.14 ± 0.02 g L^-1 was achieved. Introduction of the tryptophanase form P. rettgeri enabled indole production, but to low titers, which could be improved by sequestering indole into the water-immiscible solvent tributyrin during fermentation and a titer of 1.38 ± 0.04 g L^-1 was achieved. The process was accelerated by decoupling growth from production increasing the volumetric productivity about 4-fold to 0.08 g L^-1 h^-1. KEY POINTS: • Efficient de novo indole production via tryptophanases from glucose • Increased indole titers by product sequestration and improved precursor supply • Decoupling growth from production accelerated indole production.

Hot isopropanol quenching procedure for automated microtiter plate scale 13C-labeling experiments

Background

Currently, the generation of genetic diversity for microbial cell factories outpaces the screening of strain variants with omics-based phenotyping methods. Especially isotopic labeling experiments, which constitute techniques aimed at elucidating cellular phenotypes and supporting rational strain design by growing microorganisms on substrates enriched with heavy isotopes, suffer from comparably low throughput and the high cost of labeled substrates.

Results

We present a miniaturized, parallelized, and automated approach to ¹³C-isotopic labeling experiments by establishing and validating a hot isopropanol quenching method on a robotic platform coupled with a microbioreactor cultivation system. This allows for the first time to conduct automated labeling experiments at a microtiter plate scale in up to 48 parallel batches. A further innovation enabled by the automated quenching method is the analysis of free amino acids instead of proteinogenic ones on said microliter scale. Capitalizing on the latter point and as a proof of concept, we present an isotopically instationary labeling experiment in Corynebacterium glutamicum ATCC 13032, generating dynamic labeling data of free amino acids in the process.

Conclusions

Our results show that a robotic liquid handler is sufficiently fast to generate informative isotopically transient labeling data. Furthermore, the amount of biomass obtained from a sub-milliliter cultivation in a microbioreactor is adequate for the detection of labeling patterns of free amino acids. Combining the innovations presented in this study, isotopically stationary and instationary automated labeling experiments can be conducted, thus fulfilling the prerequisites for ¹³C-metabolic flux analyses in high-throughput.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12934-022-01806-4.

From Brown Seaweed to a Sustainable Microbial Feedstock for the Production of Riboflavin

The increasing global demand for food and energy production encourages the development of new production strategies focused on sustainability. Often, microbial bioprocesses rely on food or feed competitive feedstocks; hence, there is a trending need for green substrates. Here, we have proven the potential of brown seaweed biomass as microbial feedstock on account of its content of mannitol and the glucose polymer laminarin. Our host, Corynebacterium glutamicum, was engineered to enable access to mannitol as a carbon source through the heterologous expression of the mannitol-specific phosphotransferase system and the mannitol-1-phosphate-5-dehydrogenase from Bacillus subtilis. Overproduction of riboflavin was coupled with mannitol and glucose consumption via constitutive overexpression of the biosynthetic riboflavin operon ribGCAH from C. glutamicum. Brown seaweed extract and brown seaweed hydrolysate from Laminaria hyperborea, containing mannitol and glucose, were used as a carbon source for flask and bioreactor fermentations. In a seaweed-based fed-batch fermentation, the riboflavin final titer, yield, and volumetric productivity values of 1,291.2 mg L⁻¹, 66.1 mg g⁻¹, and 17.2 mg L⁻¹ h⁻¹, respectively, were achieved.

Carbon catabolite regulation of secondary metabolite formation, an old but not well-established regulatory system

Secondary microbial metabolites have various functions for the producer microorganisms, which allow them to interact and survive in adverse environments. In addition to these functions, other biological activities may have clinical relevance, as diverse as antimicrobial, anticancer and hypocholesterolaemic effects. These metabolites are usually formed during the idiophase of growth and have a wide diversity in their chemical structures. Their synthesis is under the impact of the type and concentration of the culture media nutrients. Some of the molecular mechanisms that affect the synthesis of secondary metabolites in bacteria (Gram-positive and negative) and fungi are partially known. Moreover, all microorganisms have their peculiarities in the control mechanisms of carbon sources, even those belonging to the same genus. This regulatory knowledge is necessary to establish culture conditions and manipulation methods for genetic improvement and product fermentation. As the carbon source is one of the essential nutritional factors for antibiotic production, its study has been imperative both at the industrial and research levels. This review aims to draw the utmost recent advances performed to clarify the molecular mechanisms of the negative effect exerted by the carbon source on the secondary metabolite formation, emphasizing those found in Streptomyces, one of the genera most profitable antibiotic producers.

Exploring the Potential of Corynebacterium glutamicum to Produce the Compatible Solute Mannosylglycerate

The compatible solute mannosylglycerate (MG) has exceptional properties in terms of protein stabilization and protection under salt, heat, and freeze-drying stresses as well as against protein aggregation. Due to these characteristics, MG possesses large potential for clinical and biotechnological applications. To achieve efficient MG production, Corynebacterium glutamicum was equipped with a bifunctional MG synthase (encoded by mgsD and catalyzing the condensation of 3-phosphoglycerate and GDP-mannose to MG) from Dehalococcoides mccartyi. The resulting strain C. glutamicum (pEKEx3 mgsD) intracellularly accumulated about 111 mM MG (60 ± 9 mg gCDW -1) with 2% glucose as a carbon source. To enable efficient mannose metabolization, the native manA gene, encoding mannose 6-phosphate isomerase, was overexpressed. Combined overexpression of manA and mgsD from two plasmids in C. glutamicum resulted in intracellular MG accumulation of up to ca. 329 mM [corresponding to 177 mg g cell dry weight (CDW) -1] with glucose, 314 mM (168 mg gCDW -1) with glucose plus mannose, and 328 mM (176 mg gCDW -1) with mannose as carbon source(s), respectively. The product was successfully extracted from cells by using a cold water shock, resulting in up to 5.5 mM MG (1.48 g L-1) in supernatants. The two-plasmid system was improved by integrating the mgsD gene into the manA-bearing plasmid and the resulting strain showed comparable production but faster growth. Repeated cycles of growth/production and extraction of MG in a bacterial milking-like experiment showed that cells could be recycled, which led to a cumulative MG production of 19.9 mM (5.34 g L-1). The results show that the newly constructed C. glutamicum strain produces MG from glucose and mannose and that a cold water shock enables extraction of MG from the cytosol into the medium.

Engineering of microbial cells for L-valine production: challenges and opportunities

L-valine is an essential amino acid that has wide and expanding applications with a suspected growing market demand. Its applicability ranges from animal feed additive, ingredient in cosmetic and special nutrients in pharmaceutical and agriculture fields. Currently, fermentation with the aid of model organisms, is a major method for the production of L-valine. However, achieving the optimal production has often been limited because of the metabolic imbalance in recombinant strains. In this review, the constrains in L-valine biosynthesis are discussed first. Then, we summarize the current advances in engineering of microbial cell factories that have been developed to address and overcome major challenges in the L-valine production process. Future prospects for enhancing the current L-valine production strategies are also discussed.

Synergistic improvement of N-acetylglucosamine production by engineering transcription factors and balancing redox cofactors

The regulation of single gene transcription level in the metabolic pathway is often failed to significantly improve the titer of the target product, and even leads to the imbalance of carbon/nitrogen metabolic network and cofactor network. Global transcription machinery engineering (gTME) can activate or inhibit the synergistic expression of multiple genes in specific metabolic pathways, so transcription factors with specific functions can be expressed according to different metabolic regulation requirements, thus effectively increasing the synthesis of target metabolites. In addition, maintaining intracellular redox balance through cofactor engineering can realize the self-balance of cofactors and promote the efficient synthesis of target products. In this study, we rebalanced the central carbon/nitrogen metabolism and redox metabolism of Corynebacterium glutamicum S9114 by gTME and redox cofactors engineering to promote the production of the nutraceutical N-acetylglucosamine (GlcNAc). Firstly, it was found that the overexpression of the transcription factor RamA can promote GlcNAc synthesis, and the titer was further improved to 16 g/L in shake flask by using a mutant RamA (RamAM). Secondly, a CRISPR interference (CRISPRi) system based on dCpf1 was developed and used to inhibit the expression of global negative transcriptional regulators of GlcNAc synthesis, which promoted the GlcNAc titer to 27.5 g/L. Thirdly, the cofactor specificity of the key enzymes in GlcNAc synthesis pathway was changed by rational protein engineering, and the titer of GlcNAc in shake flask was increased to 36.9 g/L. Finally, the production of GlcNAc was scaled up in a 50-L fermentor, and the titer reached 117.1 ± 1.9 g/L, which was 6.62 times that of the control group (17.7 ± 0.4 g/L), and the yield was increased from 0.19 g/g to 0.31 g/g glucose. The results obtained here highlight the importance of engineering the global regulation of central carbon/nitrogen metabolism and redox metabolism to improve the production performance of microbial cell factories.

Evolving a New Efficient Mode of Fructose Utilization for Improved Bioproduction in Corynebacterium glutamicum

Fructose utilization in Corynebacterium glutamicum starts with its uptake and concomitant phosphorylation via the phosphotransferase system (PTS) to yield intracellular fructose 1-phosphate, which enters glycolysis upon ATP-dependent phosphorylation to fructose 1,6-bisphosphate by 1-phosphofructokinase. This is known to result in a significantly reduced oxidative pentose phosphate pathway (oxPPP) flux on fructose (∼10%) compared to glucose (∼60%). Consequently, the biosynthesis of NADPH demanding products, e.g., L-lysine, by C. glutamicum is largely decreased when fructose is the only carbon source. Previous works reported that fructose is partially utilized via the glucose-specific PTS presumably generating fructose 6-phosphate. This closer proximity to the entry point of the oxPPP might increase oxPPP flux and, consequently, NADPH availability. Here, we generated deletion strains lacking either the fructose-specific PTS or 1-phosphofructokinase activity. We used these strains in short-term evolution experiments on fructose minimal medium and isolated mutant strains, which regained the ability of fast growth on fructose as a sole carbon source. In these fructose mutants, the deletion of the glucose-specific PTS as well as the 6-phosphofructokinase gene, abolished growth, unequivocally showing fructose phosphorylation via glucose-specific PTS to fructose 6-phosphate. Gene sequencing revealed three independent amino acid substitutions in PtsG (M260V, M260T, and P318S). These three PtsG variants mediated faster fructose uptake and utilization compared to native PtsG. In-depth analysis of the effects of fructose utilization via these PtsG variants revealed significantly increased ODs, reduced side-product accumulation, and increased L-lysine production by 50%.

Elongation factor P is required for EIIGlc translation in Corynebacterium glutamicum due to an essential polyproline motif

Translating ribosomes require elongation factor P (EF-P) to incorporate consecutive prolines (XPPX) into nascent peptide chains. The proteome of Corynebacterium glutamicum ATCC 13032 contains a total of 1,468 XPPX motifs, many of which are found in proteins involved in primary and secondary metabolism. We show here that synthesis of EII^Glc , the glucose-specific permease of the phosphoenolpyruvate (PEP): sugar phosphotransferase system (PTS) encoded by ptsG, is strongly dependent on EF-P, as an efp deletion mutant grows poorly on glucose as sole carbon source. The amount of EII^Glc is strongly reduced in this mutant, which consequently results in a lower rate of glucose uptake. Strikingly, the XPPX motif is essential for the activity of EII^Glc , and substitution of the prolines leads to inactivation of the protein. Finally, translation of GntR2, a transcriptional activator of ptsG, is also dependent on EF-P. However, its reduced amount in the efp mutant can be compensated for by other regulators. These results reveal for the first time a translational bottleneck involving production of the major glucose transporter EII^Glc , which has implications for future strain engineering strategies.

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.

Substrate-dependent cluster density dynamics of Corynebacterium glutamicum phosphotransferase system permeases

Many bacteria take up carbohydrates by membrane‐integral sugar specific phosphoenolpyruvate‐dependent carbohydrate:phosphotransferase systems (PTS). Although the PTS is centrally involved in regulation of carbon metabolism in different bacteria, little is known about localization and putative oligomerization of the permease subunits (EII). Here, we analyzed localization of the fructose specific PtsF and the glucose specific PtsG transporters, as well as the general components EI and HPr from Corynebacterium glutamicum using widefield and single molecule localization microscopy. PtsF and PtsG form membrane embedded clusters that localize in a punctate pattern. Size, number and fluorescence of the membrane clusters change upon presence or absence of the transported substrate, and a direct influence of EI and HPr was not observed. In presence of the transport substrate, EII clusters significantly increased in size. Photo‐activated localization microscopy data revealed that, in presence of different carbon sources, the number of EII proteins per cluster remains the same, however, the density of these clusters reduces. Our work reveals a simple mechanism for efficient membrane occupancy regulation. Clusters of PTS EII transporters are densely packed in absence of a suitable substrate. In presence of a transported substrate, the EII proteins in individual clusters occupy larger membrane areas.

Understanding the high L-valine production in Corynebacterium glutamicum VWB-1 using transcriptomics and proteomics

Toward the elucidation of the advanced mechanism of l-valine production by Corynebacterium glutamicum, a highly developed industrial strain VWB-1 was analyzed, employing the combination of transcriptomics and proteomics methods. The transcriptional level of 1155 genes and expression abundance of 96 proteins were changed significantly by the transcriptome and proteome comparison of VWB-1 and ATCC 13869. It was indicated that the key genes involved in the biosynthesis of l-valine, ilvBN, ilvC, ilvD, ilvE were up-regulated in VWB-1, which together made prominent contributions in improving the carbon flow towards l-valine. The l-leucine and l-isoleucine synthesis ability were weakened according to the down-regulation of leuB and ilvA. The up-regulation of the branched chain amino acid transporter genes brnFE promoted the l-valine secretion capability of VWB-1. The NADPH and ATP generation ability of VWB-1 were strengthened through the up-regulation of the genes involved in phosphate pentose pathway and TCA pathway. Pyruvate accumulation was achieved through the weakening of the l-lactate, acetate and l-alanine pathways. The up-regulation of the genes coding for elongation factors and ribosomal proteins were beneficial for l-valine synthesis in C. glutamicum. All information acquired were useful for the genome breeding of better industrial l-valine producing strains.

Enhanced Glucose Consumption and Organic Acid Production by Engineered Corynebacterium glutamicum Based on Analysis of a pfkB1 Deletion Mutant

In the analysis of a carbohydrate metabolite pathway, we found interesting phenotypes in a mutant strain of Corynebacterium glutamicum deficient in pfkB1, which encodes fructose-1-phosphate kinase. After being aerobically cultivated with fructose as a carbon source, this mutant consumed glucose and produced organic acid, predominantly l-lactate, at a level more than 2-fold higher than that of the wild-type grown with glucose under conditions of oxygen deprivation. This considerably higher fermentation capacity was unique for the combination of pfkB1 deletion and cultivation with fructose. In the metabolome and transcriptome analyses of this strain, marked intracellular accumulation of fructose-1-phosphate and significant upregulation of several genes related to the phosphoenolpyruvate:carbohydrate phosphotransferase system, glycolysis, and organic acid synthesis were identified. We then examined strains overexpressing several of the identified genes and demonstrated enhanced glucose consumption and organic acid production by these engineered strains, whose values were found to be comparable to those of the model pfkB1 deletion mutant grown with fructose. l-Lactate production by the ppc deletion mutant of the engineered strain was 2,390 mM (i.e., 215 g/liter) after 48 h under oxygen deprivation, which was a 2.7-fold increase over that of the wild-type strain with a deletion of ppc IMPORTANCE: Enhancement of glycolytic flux is important for improving microbiological production of chemicals, but overexpression of glycolytic enzymes has often resulted in little positive effect. That is presumably because the central carbon metabolism is under the complex and strict regulation not only transcriptionally but also posttranscriptionally, for example, by the ATP/ADP ratio. In contrast, we studied a mutant strain of Corynebacterium glutamicum that showed markedly enhanced glucose consumption and organic acid production and, based on the findings, identified several genes whose overexpression was effective in enhancing glycolytic flux under conditions of oxygen deprivation. These results will further understanding of the regulatory mechanisms of glycolytic flux and can be widely applied to the improvement of the microbial production of useful chemicals.

Increased glucose utilization and cell growth of Corynebacterium glutamicum by modifying the glucose-specific phosphotransferase system (PTSGlc) genes

The phosphoenolpyruvate:glucose phosphotransferase system (PTSGlc) is the major pathway of glucose uptake in Corynebacterium glutamicum. This study investigated glucose consumption rate, cell growth, and metabolite changes resulting from modification of PTSGlc. The classical l-lysine producer C. glutamicum XQ-8 exhibited low glucose consumption, cell growth, and l-lysine production rates, whereas these parameters were significantly increased during cultivating on glucose plus maltose, through inactivation of SugR, or by overexpression of PTSGlc genes. XQ-8sugR::cat/pDXW-8-ptsI exhibited the highest increase in glucose consumption, growth rate, and l-lysine production, followed by XQ-8sugR::cat/pDXW-8-ptsG. However, overexpression of ptsH had little effect on the above-mentioned factors. Although co-overexpression of ptsGHI led to the highest glucose consumption, growth rate, and final l-lysine production; the l-lysine production rate was lower than that of XQ-8sugR::cat/pDXW-8-ptsIH. In fed-batch fermentation, XQ-8sugR::cat/pDXW-8-ptsIH had a higher growth rate of 0.54 h−1 to a dry cell mass of 66 g·L−1 after 16 h, and had a higher l-lysine production rate of 159.2 g·L−1 after 36 h. These results indicate that modification of the sugar transport systems improves amino acid production, especially for mutants obtained by repeated physical and (or) chemical mutagenesis. However, modification of these systems needs to be performed on a case-by-case basis.

Elucidation of the regulatory role of the fructose operon reveals a novel target for enhancing the NADPH supply in Corynebacterium glutamicum

The performance of Corynebacterium glutamicum cell factories producing compounds which rely heavily on NADPH has been reported to depend on the sugar being metabolized. While some aspects of this phenomenon have been elucidated, there are still many unresolved questions as to how sugar metabolism is linked to redox and to the general metabolism. We here provide new insights into the regulation of the metabolism of this important platform organism by systematically characterizing mutants carrying various lesions in the fructose operon. Initially, we found that a strain where the dedicated fructose uptake system had been inactivated (KO-ptsF) was hampered in growth on sucrose minimal medium, and suppressor mutants appeared readily. Comparative genomic analysis in conjunction with enzymatic assays revealed that suppression was linked to inactivation of the pfkB gene, encoding a fructose-1-phosphate kinase. Detailed characterization of KO-ptsF, KO-pfkB and double knock-out (DKO) derivatives revealed a strong role for sugar-phosphates, especially fructose-1-phosphate (F1P), in governing sugar as well as redox metabolism due to effects on transcriptional regulation of key genes. These findings allowed us to propose a simple model explaining the correlation between sugar phosphate concentration, gene expression and ultimately the observed phenotype. To guide us in our analysis and help us identify bottlenecks in metabolism we debugged an existing genome-scale model onto which we overlaid the transcriptome data. Based on the results obtained we managed to enhance the NADPH supply and transform the wild-type strain into delivering the highest yield of lysine ever obtained on sucrose and fructose, thus providing a good example of how regulatory mechanisms can be harnessed for bioproduction.

A third glucose uptake bypass in Corynebacterium glutamicum ATCC 31833

In Corynebacterium glutamicum, the phosphoenolpyruvate-dependent sugar phosphotransferase system (PTS) has long been the only known glucose uptake system, but we recently found suppressor mutants emerging from a PTS-negative strain of C. glutamicum ATCC 31833 on glucose agar plates, and identified two alternative potential glucose uptake systems, the myo-inositol transporters encoded by iolT1 and iolT2. The expression of either gene renders the PTS-negative strain WTΔptsH capable of growing on glucose. In the present study, we found a suppressor strain that still grew on glucose even after the iolT1 and iolT2 genes were both disrupted under the PTS-negative background. Whole-genome sequencing of the suppressor strain SPH1 identified a G-to-T exchange at 134 bp upstream of the bglF gene encoding an EII component of the β-glucoside-PTS, which is found in limited wild-type strains of C. glutamicum. Introduction of the mutation into strain WTΔptsH allowed the PTS-negative strain to grow on glucose. Reverse transcription-quantitative PCR analysis revealed that the mutation upregulates the bglF gene by approximately 11-fold. Overexpression of bglF under the gapA promoter in strain WTΔptsH rendered the strain capable of growing on glucose, and deletion of bglF in strain SPH1 abolished the growth again, proving that bglF is responsible for glucose uptake in the suppressor strain. Simultaneous disruption of three glucokinase genes, glk (Cgl2185, NCgl2105), ppgK (Cgl1910, NCgl1835), and Cgl2647 (NCgl2558), in strain SPH1 resulted in no growth on glucose. Plasmid-mediated expression of any of the three genes in the triple-knockout mutant restored the growth on glucose. These results indicate that C. glutamicum ATCC 31833 has an additional non-PTS glucose uptake route consisting of the bglF-specified EII permease and native glucokinases.

Hyaluronic acid production with Corynebacterium glutamicum: effect of media composition on yield and molecular weight

AIMS

Corynebacterium glutamicum was tested as an alternative host for heterologous production of hyaluronic acid (HA).

METHODS AND RESULTS

A set of expression vectors containing hasA, encoding HA synthase from Streptococcus equi subsp. zooepidemicus, alone or in combination with genes encoding enzymes for HA precursor production (hasB, hasC, glmU from Pseudomonas putida KT2440) or bacterial haemoglobin (vgb from Vitreoscilla sp.) was constructed. Recombinant Coryne. glutamicum strains were cultivated in two different minimal media, CGXII and MEK700. HA was isolated from the culture broth by ethanol precipitation or ultrafiltration. Analyses of the isolated HA revealed that overall production was higher in CGXII medium (1241 mg l(-1)) than in MEK700 medium (363 mg l(-1)), but molecular weight of the product was higher in MEK700 (>1·4 MDa) than in CGXII (<270 kDa). Coexpression of hasB, hasC or glmU had no effect on HA yield and did not improve molecular weight of the product. Coexpression of vgb lowered HA yield about 1·5-fold and did not affect molecular weight of the product. Microscopy of negative-stained cultures revealed that Coryne. glutamicum produces no distinct HA capsule.

CONCLUSIONS

Regulation of cell growth and gene expression level of hasA are reasonable starting points for controlling the molecular weight of HA produced by Coryne. glutamicum.

SIGNIFICANCE AND IMPACT OF THE STUDY

Corynebacterium glutamicum has a great potential as an alternative production host for HA. The fact that Coryne. glutamicum produces no distinct HA capsule facilitates HA isolation and improves overall yield.

Investigation of ptsG gene in response to xylose utilization in Corynebacterium glutamicum

Corynebacterium glutamicum strains NC-2 were able to grow on xylose as sole carbon sources in our previous work. Nevertheless, it exhibited the major shortcoming that the xylose consumption was repressed in the presence of glucose. So far, regarding C. glutamicum, there are a number of reports on ptsG gene, the glucose-specific transporter, involved in glucose metabolism. Recently, we found ptsG had influence on xylose utilization and investigated the ptsG gene in response to xylose utilization in C. glutamicum with the aim to improve xylose consumption and simultaneously utilized glucose and xylose. The ptsG-deficient mutant could grow on xylose, while exhibiting noticeably reduced growth on xylose as sole carbon source. A mutant deficient in ptsH, a general PTS gene, exhibited a similar phenomenon. When complementing ptsG gene, the mutant ΔptsG-ptsG restored the ability to grow on xylose similarly to NC-2. These indicate that ptsG gene is not only essential for metabolism on glucose but also important in xylose utilization. A ptsG-overexpressing recombinant strain could not accelerate glucose or xylose metabolism. When strains were aerobically cultured in a sugar mixture of glucose and xylose, glucose and xylose could not be utilized simultaneously. Interestingly, the ΔptsG strain could co-utilize glucose and xylose under oxygen-deprived conditions, though the consumption rate of glucose and xylose dramatically declined. It was the first report of ptsG gene in response to xylose utilization in C. glutamicum.

Sugar transport systems in Corynebacterium glutamicum: features and applications to strain development

Corynebacterium glutamicum uses the phosphoenolpyruvate-dependent sugar phosphotransferase system (PTS) to take up and phosphorylate glucose, fructose, and sucrose, the major sugars from agricultural crops that are used as the primary feedstocks for industrial amino acid fermentation. This means that worldwide amino acid production using this organism has depended exclusively on the PTS. Recently, a better understanding not only of PTS-mediated sugar uptake but also of global regulation associated with the PTS has permitted the correction of certain negative aspects of this sugar transport system for amino acid production. In addition, the recent identification of different glucose uptake systems in this organism has led to a strategy for the generation of C. glutamicum strains that express non-PTS routes instead of the original PTS. The potential practical advantages of the development of such strains are discussed.

Translation efficiency of antiterminator proteins is a determinant for the difference in glucose repression of two β-glucoside phosphotransferase system gene clusters in Corynebacterium glutamicum R

Corynebacterium glutamicum R has two β-glucoside phosphoenolpyruvate, carbohydrate phosphotransferase systems (PTS) encoded by bglF and bglF2 located in the respective clusters, bglF-bglA-bglG and bglF2-bglA2-bglG2. Previously, we reported that whereas β-glucoside-dependent induction of bglF is strongly repressed by glucose, glucose repression of bglF2 is very weak. Here, we reveal the mechanism behind the different effects of glucose on the two bgl genes. Deletion of the ribonucleic antiterminator sequence and transcriptional terminator located upstream of the translation initiation codon of bglF markedly relieved the glucose repression of a bglF-lacZ fusion, indicating that glucose affects the antitermination mechanism that is responsible for the β-glucoside-dependent induction of the bglF cluster. The glucose repression of bglF mRNA was also relieved by introducing a multicopy plasmid carrying the bglG gene encoding an antiterminator of the bglF cluster. Moreover, replacement of the GUG translation initiation codon of bglG with AUG was effective in relieving the glucose repression of bglF and bglG. Inversely, expression of bglF2 and bglG2 was subject to strict glucose repression in a mutant strain in which the AUG translation initiation codon of bglG2 encoding antiterminator of the bglF2 cluster was replaced with GUG. These results suggest that the translation initiation efficiency of the antiterminator proteins, at least in part, determines whether the target genes are subject to glucose repression. We also found that bglF expression was induced by glucose in the BglG-overexpressing strains, which may be explained by the ability of BglF to transport glucose.

Identification of mannose uptake and catabolism genes in Corynebacterium glutamicum and genetic engineering for simultaneous utilization of mannose and glucose

Here, focus is on Corynebacterium glutamicum mannose metabolic genes with the aim to improve this industrially important microorganism's ability to ferment mannose present in mixed sugar substrates. cgR_0857 encodes C. glutamicum's protein with 36% amino acid sequence identity to mannose 6-phosphate isomerase encoded by manA of Escherichia coli. Its deletion mutant did not grow on mannose and exhibited noticeably reduced growth on glucose as sole carbon sources. In effect, C. glutamicum manA is not only essential for growth on mannose but also important in glucose metabolism. A double deletion mutant of genes encoding glucose and fructose permeases (ptsG and ptsF, respectively) of the phosphoenolpyruvate-dependent phosphotransferase system (PTS) was not able to grow on mannose unlike the respective single deletion mutants with mannose utilization ability. A mutant deficient in ptsH, a general PTS gene, did not utilize mannose. These indicate that the glucose-PTS and fructose-PTS are responsible for mannose uptake in C. glutamicum. When cultured with a glucose and mannose mixture, mannose utilization of manA-overexpressing strain CRM1 was significantly higher than that of its wild-type counterpart, but with a strong preference for glucose. ptsF-overexpressing strain CRM2 co-utilized mannose and glucose, but at a total sugar consumption rate much lower than that of the wild-type strain and CRM1. Strain CRM3 overexpressing both manA and ptsF efficiently co-utilized mannose and glucose. Under oxygen-deprived conditions, high volumetric productivity of organic acids concomitant with the simultaneous consumption of the mixed sugars was achieved by the densely packed growth-arrested CRM3 cells.

Regulation of genes involved in sugar uptake, glycolysis and lactate production in Corynebacterium glutamicum

Corynebacterium glutamicum is a nonpathogenic, GC-rich, Gram-positive bacterium with a long history in the industrial production of amino acids. Recently, the species has become of increasing interest as a model bacterium for closely related, medically important pathogenic species such as Corynebacterium diphtheriae and Mycobacterium tuberculosis. In this article, recent advances in understanding of the C. glutamicum regulatory network of genes involved in carbohydrate metabolism are reviewed with regards to sugar uptake, glycolysis and lactate production.

The pstSCAB operon for phosphate uptake is regulated by the global regulator GlxR in Corynebacterium glutamicum

The pstSCAB operon of Corynebacterium glutamicum, which encodes a high affinity transport system for uptake of the phosphorus source inorganic phosphate, is induced upon phosphate starvation involving activation by the two-component regulatory system PhoS-PhoR. Partial phosphate starvation induction of the pstSCAB operon in a ΔphoRS mutant indicated the involvement of (an) additional transcriptional regulator(s). Here, GlxR, a global cAMP-dependent transcriptional regulator, was shown to bind to the pstS promoter -133 bp to -117 bp upstream of the transcriptional start site as shown by gel shift and mutation experiments. Transcriptional fusion analysis revealed that GlxR activates the pstSCAB operon under phosphate limiting conditions in a carbon source dependent manner. Commensurate with these findings, overexpression of glxR was shown to stimulate growth under phosphate limiting conditions with glucose, but not with acetate, as carbon source. Thus, in C. glutamicum pstSCAB expression is regulated in response to the availability of phosphorus and carbon sources.

RamA and RamB are global transcriptional regulators in Corynebacterium glutamicum and control genes for enzymes of the central metabolism

In Corynebacterium glutamicum, the transcriptional regulators of acetate metabolism RamA (encoded by cg2831) and RamB (encoded by cg0444) play an important role in expression control of genes involved in acetate and ethanol metabolism. Both regulators were speculated to have broader significance in expression control of further genes in the central metabolism of C. glutamicum. Here we investigated the RamA and RamB regulons by genome-wide transcriptome analysis with special emphasis on genes encoding enzymes of the central carbon metabolism. When compared to the parental wild-type, 253 genes and 81 genes showed different mRNA levels in defined RamA- and RamB-deficient C. glutamicum strains, respectively. Among these were genes involved in sugar uptake, glycolysis, gluconeogenesis, acetate, l-lactate or ethanol metabolism. The direct interaction of RamA and RamB proteins with the respective promoter/operator fragments was demonstrated in vitro by electrophoretic mobility shift assays. Taken together, we present evidence for an important role of RamA and RamB in global gene expression control in C. glutamicum.

Cg2091 encodes a polyphosphate/ATP-dependent glucokinase of Corynebacterium glutamicum

The Corynebacterium glutamicum gene cg2091 is encoding a polyphosphate (PolyP)/ATP-dependent glucokinase (PPGK). Previous work demonstrated the association of PPGK to PolyP granules. The deduced amino acid sequence of PPGK shows 45% sequence identity to PolyP/ATP glucomannokinase of Arthrobacter sp. strain KM and 50% sequence identity to PolyP glucokinase of Mycobacterium tuberculosis H37Rv. PPGK from C. glutamicum was purified from recombinant Escherichia coli. PolyP was highly preferred over ATP and other NTPs as substrate and with respect to the tested PolyPs differing in chain length; the protein was most active with PolyP(75). Gel filtration analysis revealed that PolyP supported the formation of homodimers of PPGK and that PPGK was active as a homodimer. A ppgK deletion mutant (Delta ppgK) showed slowed growth in minimal medium with maltose as sole carbon source. Moreover, in minimal medium containing 2 to 4% (w/v) glucose as carbon source, Delta ppgK grew to lower final biomass concentrations than the wild type. Under phosphate starvation conditions, growth of Delta ppgK was reduced, and growth of a ppgK overexpressing strain was increased as compared to wild type and empty vector control, respectively. Thus, under conditions of glucose excess, the presence of PPGK entailed a growth advantage.

Increased glucose utilization in Corynebacterium glutamicum by use of maltose, and its application for the improvement of L-valine productivity

Corynebacterium glutamicum efficiently utilizes maltose as a substrate. We show here that the presence of maltose increases glucose utilization by raising the expression of ptsG, which encodes the glucose-specific EII permease of the phosphotransferase system. Consequently, the L-valine productivity of a pyruvate dehydrogenase complex-deficient C. glutamicum strain was improved by the presence of maltose.

Transcriptional control of the succinate dehydrogenase operon sdhCAB of Corynebacterium glutamicum by the cAMP-dependent regulator GlxR and the LuxR-type regulator RamA

In experiments performed to identify transcriptional regulators of the tricarboxylic acid cycle of Corynebacterium glutamicum, the cAMP-dependent regulator GlxR and the regulators of acetate metabolism RamA and RamB were enriched by DNA affinity chromatography with the promoter region of the sdhCAB operon encoding succinate dehydrogenase. The binding of purified GlxR, RamA and RamB was verified by electrophoretic mobility shift assays and the regulatory effects of these proteins on sdhCAB gene expression were tested by promoter activity assays and SDH activity measurements. Evidence was obtained that GlxR functions as a repressor and RamA as an activator of sdhCAB expression, whereas RamB had no obvious influence under the conditions tested.

Characterization of an adenylate cyclase gene (cyaB) deletion mutant of Corynebacterium glutamicum ATCC 13032

Genome analysis of C. glutamicum ATCC 13032 has showed one putative adenylate cyclase gene, cyaB (cg0375) which encodes membrane protein belonging to class III adenylate cyclases. To characterize the function of cyaB, a deletion mutant was constructed, and the mutant showed decreased level of intracellular cyclic AMP compared to that of wild-type. Interestingly, the cyaB mutant displayed growth defect on acetate medium, and this effect was reversed by complementation with cyaB gene. Similarly, it showed growth defect on glucose-acetate mixture minimal medium, and the utilization of glucose was retarded in the presence of acetate. The deletion mutant retained the activity of glyoxylate bypass enzymes. Additionally, the mutant could grow on ethanol but not on propionate medium. The data obtained from this study suggests that adenylate cyclase plays an essential role in the acetate metabolism of C. glutamicum, even though detailed regulatory mechanisms involving cAMP are not yet clearly defined. The observation that glyoxylate bypass enzymes are derepressed in cyaB mutant indicates the involvement of cAMP in the repression of aceB and aceA.

Ins and outs of glucose transport systems in eubacteria

Glucose is the classical carbon source that is used to investigate the transport, metabolism, and regulation of nutrients in bacteria. Many physiological phenomena like nutrient limitation, stress responses, production of antibiotics, and differentiation are inextricably linked to nutrition. Over the years glucose transport systems have been characterized at the molecular level in more than 20 bacterial species. This review aims to provide an overview of glucose uptake systems found in the eubacterial kingdom. In addition, it will highlight the diverse and sophisticated regulatory features of glucose transport systems.

The DeoR-type transcriptional regulator SugR acts as a repressor for genes encoding the phosphoenolpyruvate:sugar phosphotransferase system (PTS) in Corynebacterium glutamicum

BACKGROUND

The major uptake system responsible for the transport of fructose, glucose, and sucrose in Corynebacterium glutamicum ATCC 13032 is the phosphoenolpyruvate:sugar phosphotransferase system (PTS). The genes encoding PTS components, namely ptsI, ptsH, and ptsF belong to the fructose-PTS gene cluster, whereas ptsG and ptsS are located in two separate regions of the C. glutamicum genome. Due to the localization within and adjacent to the fructose-PTS gene cluster, two genes coding for DeoR-type transcriptional regulators, cg2118 and sugR, are putative candidates involved in the transcriptional regulation of the fructose-PTS cluster genes.

RESULTS

Four transcripts of the extended fructose-PTS gene cluster that comprise the genes sugR-cg2116, ptsI, cg2118-fruK-ptsF, and ptsH, respectively, were characterized. In addition, it was shown that transcription of the fructose-PTS gene cluster is enhanced during growth on glucose or fructose when compared to acetate. Subsequently, the two genes sugR and cg2118 encoding for DeoR-type regulators were mutated and PTS gene transcription was found to be strongly enhanced in the presence of acetate only in the sugR deletion mutant. The SugR regulon was further characterized by microarray hybridizations using the sugR mutant and its parental strain, revealing that also the PTS genes ptsG and ptsS belong to this regulon. Binding of purified SugR repressor protein to a 21 bp sequence identified the SugR binding site as an AC-rich motif. The two experimentally identified SugR binding sites in the fructose-PTS gene cluster are located within or downstream of the mapped promoters, typical for transcriptional repressors. Effector studies using electrophoretic mobility shift assays (EMSA) revealed the fructose PTS-specific metabolite fructose-1-phosphate (F-1-P) as a highly efficient, negative effector of the SugR repressor, acting in the micromolar range. Beside F-1-P, other sugar-phosphates like fructose-1,6-bisphosphate (F-1,6-P) and glucose-6-phosphate (G-6-P) also negatively affect SugR-binding, but in millimolar concentrations.

CONCLUSION

In C. glutamicum ATCC 13032 the DeoR-type regulator SugR acts as a pleiotropic transcriptional repressor of all described PTS genes. Thus, in contrast to most DeoR-type repressors described, SugR is able to act also on the transcription of the distantly located genes ptsG and ptsS of C. glutamicum. Transcriptional repression of the fructose-PTS gene cluster is observed during growth on acetate and transcription is derepressed in the presence of the PTS sugars glucose and fructose. This derepression of the fructose-PTS gene cluster is mainly modulated by the negative effector F-1-P, but reduced sensitivity to the other effectors, F-1,6-P or G-6-P might cause differential transcriptional regulation of genes of the general part of the PTS (ptsI, ptsH) and associated genes encoding sugar-specific functions (ptsF, ptsG, ptsS).