Analyses of enzyme II gene mutants for sugar transport and heterologous expression of fructokinase gene in Corynebacterium glutamicum ATCC 13032

Physiological changes and growth behavior of Corynebacterium glutamicum cells in biofilm

Biofilm cells are well-known for their increased survival and metabolic capabilities and have been increasingly implemented in industrial and biotechnological processes. Corynebacterium glutamicum is one of the most widely used microorganisms in the fermentation industry. However, C. glutamicum biofilm has been rarely reported and little is known about its cellular basis. Here, the physiological changes and characteristics of C. glutamicum biofilm cells during long-term fermentation were studied for the first time. Results showed that the biofilm cells maintained stable metabolic activity and cell size was enlarged after repeated-batch of fermentation. Cell division was slowed, and chromosome content and cell proliferation efficiency were reduced during long-term fermentation. Compared to free cells, more biofilm cells were stained by the apoptosis indicator dyes Annexin V-FITC and propidium iodide (PI). Overall, these results suggested slow-growing, long-lived cells of C. glutamicum biofilm during fermentation, which could have important industrial implications. This study presents first insights into the physiological changes and growth behavior of C. glutamicum biofilm cell population, which would be valuable for understanding and developing biofilm-based processes.

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.

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.

Metabolic engineering of carbohydrate metabolism systems in Corynebacterium glutamicum for improving the efficiency of L-lysine production from mixed sugar

The efficiency of industrial fermentation process mainly depends on carbon yield, final titer and productivity. To improve the efficiency of l-lysine production from mixed sugar, we engineered carbohydrate metabolism systems to enhance the effective use of sugar in this study. A functional metabolic pathway of sucrose and fructose was engineered through introduction of fructokinase from Clostridium acetobutylicum. l-lysine production was further increased through replacement of phosphoenolpyruvate-dependent glucose and fructose uptake system (PTS^Glc and PTS^Fru) by inositol permeases (IolT1 and IolT2) and ATP-dependent glucokinase (ATP-GlK). However, the shortage of intracellular ATP has a significantly negative impact on sugar consumption rate, cell growth and l-lysine production. To overcome this defect, the recombinant strain was modified to co-express bifunctional ADP-dependent glucokinase (ADP-GlK/PFK) and NADH dehydrogenase (NDH-2) as well as to inactivate SigmaH factor (SigH), thus reducing the consumption of ATP and increasing ATP regeneration. Combination of these genetic modifications resulted in an engineered C. glutamicum strain K-8 capable of producing 221.3 ± 17.6 g/L l-lysine with productivity of 5.53 g/L/h and carbon yield of 0.71 g/g glucose in fed-batch fermentation. As far as we know, this is the best efficiency of l-lysine production from mixed sugar. This is also the first report for improving the efficiency of l-lysine production by systematic modification of carbohydrate metabolism systems.

Impact of CO2/HCO3 - Availability on Anaplerotic Flux in Pyruvate Dehydrogenase Complex-Deficient Corynebacterium glutamicum Strains

The pyruvate dehydrogenase complex (PDHC) catalyzes the oxidative decarboxylation of pyruvate, yielding acetyl coenzyme A (acetyl-CoA) and CO₂ The PDHC-deficient Corynebacterium glutamicum ΔaceE strain therefore lacks an important decarboxylation step in its central metabolism. Additional inactivation of pyc, encoding pyruvate carboxylase, resulted in a >15-h lag phase in the presence of glucose, while no growth defect was observed on gluconeogenetic substrates, such as acetate. Growth was successfully restored by deletion of ptsG, encoding the glucose-specific permease of the phosphotransferase system (PTS), thereby linking the observed phenotype to the increased sensitivity of the ΔaceE Δpyc strain to glucose catabolism. In this work, the ΔaceE Δpyc strain was used to systematically study the impact of perturbations of the intracellular CO₂/HCO₃ ^- pool on growth and anaplerotic flux. Remarkably, all measures leading to enhanced CO₂/HCO₃ ^- levels, such as external addition of HCO₃ ^-, increasing the pH, or rerouting metabolic flux via the pentose phosphate pathway, at least partially eliminated the lag phase of the ΔaceE Δpyc strain on glucose medium. In accordance with these results, inactivation of the urease enzyme, lowering the intracellular CO₂/HCO₃ ^- pool, led to an even longer lag phase, accompanied by the excretion of l-valine and l-alanine. Transcriptome analysis, as well as an adaptive laboratory evolution experiment with the ΔaceE Δpyc strain, revealed the reduction of glucose uptake as a key adaptive measure to enhance growth on glucose-acetate mixtures. Taken together, our results highlight the significant impact of the intracellular CO₂/HCO₃ ^- pool on metabolic flux distribution, which becomes especially evident in engineered strains exhibiting low endogenous CO₂ production rates, as exemplified by PDHC-deficient strains.IMPORTANCE CO₂ is a ubiquitous product of cellular metabolism and an essential substrate for carboxylation reactions. The pyruvate dehydrogenase complex (PDHC) catalyzes a central metabolic reaction contributing to the intracellular CO₂/HCO₃ ^- pool in many organisms. In this study, we used a PDHC-deficient strain of Corynebacterium glutamicum, which additionally lacked pyruvate carboxylase (ΔaceE Δpyc). This strain featured a >15-h lag phase during growth on glucose-acetate mixtures. We used this strain to systematically assess the impact of alterations in the intracellular CO₂/HCO₃ ^- pool on growth in glucose-acetate medium. Remarkably, all measures enhancing CO₂/HCO₃ ^- levels successfully restored growth. These results emphasize the strong impact of the intracellular CO₂/HCO₃ ^- pool on metabolic flux, especially in strains exhibiting low endogenous CO₂ production rates.

Synthetic Escherichia coli-Corynebacterium glutamicum consortia for l-lysine production from starch and sucrose

In the biorefinery concept renewable feedstocks are converted to a multitude of value-added compounds irrespective of seasonal or other variations of the complex biomass substrates. Conceptionally, this can be realized by specialized single microbial strains or by co-culturing various strain combinations. In the latter approach strains for substrate conversion and for product formation can be combined. This study addressed the construction of binary microbial consortia based on starch- and sucrose-based production of l-lysine and derived value-added compounds. A commensalism-based synthetic consortium for l-lysine production from sucrose was developed combining an l-lysine auxotrophic, naturally sucrose-negative E. coli strain with a C. glutamicum strain able to produce l-lysine that secretes fructose when grown with sucrose due to deletion of the fructose importer gene ptsF. Mutualistic synthetic consortia with an l-lysine auxotrophic, α-amylase secreting E. coli strain and naturally amylase-negative C. glutamicum strains was implemented for production of valuable fine chemicals from starch.

Searching whole genome sequences for biochemical identification features of emerging and reemerging pathogenic Corynebacterium species

Biochemical tests are traditionally used for bacterial identification at the species level in clinical microbiology laboratories. While biochemical profiles are generally efficient for the identification of the most important corynebacterial pathogen Corynebacterium diphtheriae, their ability to differentiate between biovars of this bacterium is still controversial. Besides, the unambiguous identification of emerging human pathogenic species of the genus Corynebacterium may be hampered by highly variable biochemical profiles commonly reported for these species, including Corynebacterium striatum, Corynebacterium amycolatum, Corynebacterium minutissimum, and Corynebacterium xerosis. In order to identify the genomic basis contributing for the biochemical variabilities observed in phenotypic identification methods of these bacteria, we combined a comprehensive literature review with a bioinformatics approach based on reconstruction of six specific biochemical reactions/pathways in 33 recently released whole genome sequences. We used data retrieved from curated databases (MetaCyc, PathoSystems Resource Integration Center (PATRIC), The SEED, TransportDB, UniProtKB) associated with homology searches by BLAST and profile Hidden Markov Models (HMMs) to detect enzymes participating in the various pathways and performed ab initio protein structure modeling and molecular docking to confirm specific results. We found a differential distribution among the various strains of genes that code for some important enzymes, such as beta-phosphoglucomutase and fructokinase, and also for individual components of carbohydrate transport systems, including the fructose-specific phosphoenolpyruvate-dependent sugar phosphotransferase (PTS) and the ribose-specific ATP-binging cassette (ABC) transporter. Horizontal gene transfer plays a role in the biochemical variability of the isolates, as some genes needed for sucrose fermentation were seen to be present in genomic islands. Noteworthy, using profile HMMs, we identified an enzyme with putative alpha-1,6-glycosidase activity only in some specific strains of C. diphtheriae and this may aid to understanding of the differential abilities to utilize glycogen and starch between the biovars.

Holistic bioengineering: rewiring central metabolism for enhanced bioproduction

What does it take to convert a living organism into a truly productive biofactory? Apart from optimizing biosynthesis pathways as standalone units, a successful bioengineering approach must bend the endogenous metabolic network of the host, and especially its central metabolism, to support the bioproduction process. In practice, this usually involves three complementary strategies which include tuning-down or abolishing competing metabolic pathways, increasing the availability of precursors of the desired biosynthesis pathway, and ensuring high availability of energetic resources such as ATP and NADPH. In this review, we explore these strategies, focusing on key metabolic pathways and processes, such as glycolysis, anaplerosis, the TCA (tricarboxylic acid) cycle, and NADPH production. We show that only a holistic approach for bioengineering — considering the metabolic network of the host organism as a whole, rather than focusing on the production pathway alone — can truly mold microorganisms into efficient biofactories.

Transcription of Sialic Acid Catabolism Genes in Corynebacterium glutamicum Is Subject to Catabolite Repression and Control by the Transcriptional Repressor NanR

Corynebacterium glutamicum metabolizes sialic acid (Neu5Ac) to fructose-6-phosphate (fructose-6P) via the consecutive activity of the sialic acid importer SiaEFGI, N-acetylneuraminic acid lyase (NanA), N-acetylmannosamine kinase (NanK), N-acetylmannosamine-6P epimerase (NanE), N-acetylglucosamine-6P deacetylase (NagA), and glucosamine-6P deaminase (NagB). Within the cluster of the three operons nagAB, nanAKE, and siaEFGI for Neu5Ac utilization a fourth operon is present, which comprises cg2936, encoding a GntR-type transcriptional regulator, here named NanR. Microarray studies and reporter gene assays showed that nagAB, nanAKE, siaEFGI, and nanR are repressed in wild-type (WT) C. glutamicum but highly induced in a ΔnanR C. glutamicum mutant. Purified NanR was found to specifically bind to the nucleotide motifs A[AC]G[CT][AC]TGATGTC[AT][TG]ATGT[AC]TA located within the nagA-nanA and nanR-sialA intergenic regions. Binding of NanR to promoter regions was abolished in the presence of the Neu5Ac metabolism intermediates GlcNAc-6P and N-acetylmannosamine-6-phosphate (ManNAc-6P). We observed consecutive utilization of glucose and Neu5Ac as well as fructose and Neu5Ac by WT C. glutamicum, whereas the deletion mutant C. glutamicum ΔnanR simultaneously consumed these sugars. Increased reporter gene activities for nagAB, nanAKE, and nanR were observed in cultivations of WT C. glutamicum with Neu5Ac as the sole substrate compared to cultivations when fructose was present. Taken together, our findings show that Neu5Ac metabolism in C. glutamicum is subject to catabolite repression, which involves control by the repressor NanR.

IMPORTANCE

Neu5Ac utilization is currently regarded as a common trait of both pathogenic and commensal bacteria. Interestingly, the nonpathogenic soil bacterium C. glutamicum efficiently utilizes Neu5Ac as a substrate for growth. Expression of genes for Neu5Ac utilization in C. glutamicum is here shown to depend on the transcriptional regulator NanR, which is the first GntR-type regulator of Neu5Ac metabolism not to use Neu5Ac as effector but relies instead on the inducers GlcNAc-6P and ManNAc-6P. The identification of conserved NanR-binding sites in intergenic regions within the operons for Neu5Ac utilization in pathogenic Corynebacterium species indicates that the mechanism for the control of Neu5Ac catabolism in C. glutamicum by NanR as described in this work is probably conserved within this genus.

Regulation of Streptococcus mutans PTS Bio by the transcriptional repressor NigR

Streptococcus mutans is implicated in human dental caries, and the carbohydrate metabolism of this organism plays an important role in the formation of this disease. Carbohydrate transport and metabolism are essential for the survival of S. mutans in the oral cavity. It is known that a unique phosphoenolpyruvate-sugar phosphotransferase system PTS(B) (io) of S. mutans UA159 is expressed in sucrose-grown biofilms (Mol Oral Microbiol 28: 2013; 114). In this study we analyzed the transcriptional regulation of the operon (O(B) (io) ) encoding the PTS(B) (io) and showed that it was repressed by NigR, a LacI-like transcriptional regulator. Using electro-mobility shift assay, we described two operators to which NigR bound with different affinities. We also identified the transcriptional start site and showed that one of the operators overlaps with the promoter and presumably represses initiation of transcription. Mutational analyses revealed the key nucleotides in the operators required for high-affinity binding of NigR. PTS(B) (io) is expressed in S. mutans biofilms so understanding its regulation may provide improved strategies for caries treatment and prevention.

Increasing available NADH supply during succinic acid production by Corynebacterium glutamicum

A critical factor in the biotechnological production of succinic acid with Corynebacterium glutamicum is the sufficient supply of NADH. It is conceivable that cofactor availability and the proportion of cofactor in the active form may play an important role in dictating the succinic acid yield. PntAB genes from Escherichia coli can directly catalyze the reversible hydride transfer and adjust the dynamic balance between NADP(H) and NAD(H). Hence, we studied the physiological effect of coenzyme systems by expressing the membrane-bound transhydrogenase pntAB genes. We have shown experimentally that the pntAB genes could function as an alternative source of NADH. In an anaerobic fermentation with C. glutamicum NC-3-pntAB, a 16% higher succinic acid yield and a 57% higher production from glucose were obtained by pntAB expression. Moreover, the formation of by-products was significantly decreased. The concomitant increase in the consumption of intracellular NADPH from 0.6 to 1.2 mmol/g CDW and the increased NADH/NAD(+) ratio resulted from introduction of pntAB, suggesting that the membrane-bound transhydrogenase converted excess NADPH to NADH for succinic acid production. Finally, we explored whether the transhydrogenase had different effects on the succinic acid formation on different carbon sources. The succinic acid yield was increased in the presence of pntAB by 16% on glucose, 7% on sucrose, and without large influence on fructose and xylose. The results of this study demonstrated that the effectiveness of cofactor manipulation could be a promising strategy applied in metabolic engineering.

Improvement of cell growth and L-lysine production by genetically modified Corynebacterium glutamicum during growth on molasses

Fructose-1,6-bisphosphatase (FBPase) and fructokinase (ScrK) have important roles in regenerating glucose-6-phosphate in the pentose phosphate pathway (PPP), and thus increasing L-lysine production. This article focuses on the development of L-lysine high-producing strains by heterologous expression of FBPase gene fbp and ScrK gene scrK in C. glutamicum lysC (fbr) with molasses as the sole carbon source. Heterologous expression of fbp and scrK lead to a decrease of residual sugar in fermentation broth, and heterologous expression of scrK prevents the fructose efflux. Heterologous expression of fbp and scrK not only increases significantly the activity of corresponding enzymes but also improves cell growth during growth on molasses. FBPase activities are increased tenfold by heterologous expression of fbp, whereas the FBPase activity is only increase fourfold during co-expression of scrK and fbp. Compared with glucose, the DCW of heterologous expression strains are higher on molasses except co-expression of fbp and scrK strain. In addition, heterologous expression of fbp and scrK can strongly increase the L-lysine production with molasses as the sole carbon source. The highest increase (88.4 %) was observed for C. glutamicum lysC (fbr) pDXW-8-fbp-scrK, but the increase was also significant for C. glutamicum lysC (fbr) pDXW-8-fbp (47.2 %) and C. glutamicum lysC (fbr) pDXW-8-scrK (36.8 %). By-products, such as glycerol and dihydroxyacetone, are decreased by heterologous expression of fbp and scrK, whereas trehalose is only slightly increased. The strategy for enhancing L-lysine production by regeneration of glucose-6-phosphate in PPP may provide a reference to enhance the production of other amino acids during growth on molasses or starch.

Maltose uptake by the novel ABC transport system MusEFGK2I causes increased expression of ptsG in Corynebacterium glutamicum

The Gram-positive Corynebacterium glutamicum efficiently metabolizes maltose by a pathway involving maltodextrin and glucose formation by 4-α-glucanotransferase, glucose phosphorylation by glucose kinases, and maltodextrin degradation via maltodextrin phosphorylase and α-phosphoglucomutase. However, maltose uptake in C. glutamicum has not been investigated. Interestingly, the presence of maltose in the medium causes increased expression of ptsG in C. glutamicum by an unknown mechanism, although the ptsG-encoded glucose-specific EII permease of the phosphotransferase system itself is not required for maltose utilization. We identified the maltose uptake system as an ABC transporter encoded by musK (cg2708; ATPase subunit), musE (cg2705; substrate binding protein), musF (cg2704; permease), and musG (cg2703; permease) by combination of data obtained from characterization of maltose uptake and reanalyses of transcriptome data. Deletion of the mus gene cluster in C. glutamicum Δmus abolished maltose uptake and utilization. Northern blotting and reverse transcription-PCR experiments revealed that musK and musE are transcribed monocistronically, whereas musF and musG are part of an operon together with cg2701 (musI), which encodes a membrane protein of unknown function with no homologies to characterized proteins. Characterization of growth and [(14)C]maltose uptake in the musI insertion strain C. glutamicum IMcg2701 showed that musI encodes a novel essential component of the maltose ABC transporter of C. glutamicum. Finally, ptsG expression during cultivation on different carbon sources was analyzed in the maltose uptake-deficient strain C. glutamicum Δmus. Indeed, maltose uptake by the novel ABC transport system MusEFGK2I is required for the positive effect of maltose on ptsG expression in C. glutamicum.

Accelerated pentose utilization by Corynebacterium glutamicum for accelerated production of lysine, glutamate, ornithine and putrescine

Because of their abundance in hemicellulosic wastes arabinose and xylose are an interesting source of carbon for biotechnological production processes. Previous studies have engineered several Corynebacterium glutamicum strains for the utilization of arabinose and xylose, however, with inefficient xylose utilization capabilities. To improve xylose utilization, different xylose isomerase genes were tested in C. glutamicum. The gene originating from Xanthomonas campestris was shown to have the highest effect, resulting in growth rates of 0.14 h⁻¹, followed by genes from Bacillus subtilis, Mycobacterium smegmatis and Escherichia coli. To further increase xylose utilization different xylulokinase genes were expressed combined with X. campestris xylose isomerase gene. All combinations further increased growth rates of the recombinant strains up to 0.20 h⁻¹ and moreover increased biomass yields. The gene combination of X. campestris xylose isomerase and C. glutamicum xylulokinase was the fastest growing on xylose and compared with the previously described strain solely expressing E. coli xylose isomerase gene delivered a doubled growth rate. Productivity of the amino acids glutamate, lysine and ornithine, as well as the diamine putrescine was increased as well as final titres except for lysine where titres remained unchanged. Also productivity in medium containing rice straw hydrolysate as carbon source was increased.

Funding Information No funding information provided.

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.

Glucosamine as carbon source for amino acid-producing Corynebacterium glutamicum

Corynebacterium glutamicum grows with a variety of carbohydrates and carbohydrate derivatives as sole carbon sources; however, growth with glucosamine has not yet been reported. We isolated a spontaneous mutant (M4) which is able to grow as fast with glucosamine as with glucose as sole carbon source. Glucosamine also served as a combined source of carbon, energy and nitrogen for the mutant strain. Characterisation of the M4 mutant revealed a significantly increased expression of the nagB gene encoding the glucosamine-6P deaminase NagB involved in degradation of glucosamine, as a consequence of a single mutation in the promoter region of the nagAB-scrB operon. Ectopic nagB overexpression verified that the activity of the NagB enzyme is in fact the growth limiting factor under these conditions. In addition, glucosamine uptake was studied, which proved to be unchanged in the wild-type and M4 mutant strains. Using specific deletion strains, we identified the PTS(Glc) transport system to be responsible for glucosamine uptake in C. glutamicum. The affinity of this uptake system for glucosamine was about 40-fold lower than that for its major substrate glucose. Because of this difference in affinity, glucosamine is efficiently taken up only if external glucose is absent or present at low concentrations. C. glutamicum was also examined for its suitability to use glucosamine as substrate for biotechnological purposes. Upon overexpression of the nagB gene in suitable C. glutamicum producer strains, efficient production of both the amino acid L-lysine and the diamine putrescine from glucosamine was demonstrated.

Phosphotransferase system-independent glucose utilization in corynebacterium glutamicum by inositol permeases and glucokinases

Phosphoenolpyruvate-dependent glucose phosphorylation via the phosphotransferase system (PTS) is the major path of glucose uptake in Corynebacterium glutamicum, but some growth from glucose is retained in the absence of the PTS. The growth defect of a deletion mutant lacking the general PTS component HPr in glucose medium could be overcome by suppressor mutations leading to the high expression of inositol utilization genes or by the addition of inositol to the growth medium if a glucokinase is overproduced simultaneously. PTS-independent glucose uptake was shown to require at least one of the inositol transporters IolT1 and IolT2 as a mutant lacking IolT1, IolT2, and the PTS component HPr could not grow with glucose as the sole carbon source. Efficient glucose utilization in the absence of the PTS necessitated the overexpression of a glucokinase gene in addition to either iolT1 or iolT2. IolT1 and IolT2 are low-affinity glucose permeases with K(s) values of 2.8 and 1.9 mM, respectively. As glucose uptake and phosphorylation via the PTS differs from glucose uptake via IolT1 or IolT2 and phosphorylation via glucokinase by the requirement for phosphoenolpyruvate, the roles of the two pathways for l-lysine production were tested. The l-lysine yield by C. glutamicum DM1729, a rationally engineered l-lysine-producing strain, was lower than that by its PTS-deficient derivate DM1729Δhpr, which, however, showed low production rates. The combined overexpression of iolT1 or iolT2 with ppgK, the gene for PolyP/ATP-dependent glucokinase, in DM1729Δhpr enabled l-lysine production as fast as that by the parent strain DM1729 but with 10 to 20% higher l-lysine yield.

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 the membrane protein SucE and its role in succinate transport in Corynebacterium glutamicum

Succinic acid is excreted during anaerobiosis by many bacteria, and manifold applications are known making the biotechnological production of succinate attractive. Although the pathways for succinate formation are known, succinate export is not understood in most of the succinate producing bacteria. Here, we present a bioinformatic approach for identification of a putative succinate export system in Corynebacterium glutamicum. The subsequent screening revealed that a mutant in the gene cg2425 is impaired in succinate production or transport under anaerobic conditions. A function of the Cg2425 protein as import system was excluded. In contrast, a role of the Cg2425 protein as succinate export system was indicated by accumulation of increased amounts of internal succinate under anaerobic conditions in a Cg2425-dependent manner and a concomitant impairment of external succinate accumulation. In conclusion, we propose that Cg2425 participates in succinate export in C. glutamicum and suggest the name SucE for the protein.

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.

Mannheimia succiniciproducens phosphotransferase system for sucrose utilization

The succinic acid producer Mannheimia succiniciproducens can efficiently utilize sucrose as a carbon source, but its metabolism has not been understood. This study revealed that M. succiniciproducens uses a sucrose phosphotransferase system (PTS), sucrose 6-phosphate hydrolase, and a fructose PTS for the transport and utilization of sucrose.

Analysis and engineering of metabolic pathway fluxes in Corynebacterium glutamicum

The Gram-positive soil bacterium Corynebacterium glutamicum was discovered as a natural overproducer of glutamate about 50 years ago. Linked to the steadily increasing economical importance of this microorganism for production of glutamate and other amino acids, the quest for efficient production strains has been an intense area of research during the past few decades. Efficient production strains were created by applying classical mutagenesis and selection and especially metabolic engineering strategies with the advent of recombinant DNA technology. Hereby experimental and computational approaches have provided fascinating insights into the metabolism of this microorganism and directed strain engineering. Today, C. glutamicum is applied to the industrial production of more than 2 million tons of amino acids per year. The huge achievements in recent years, including the sequencing of the complete genome and efficient post genomic approaches, now provide the basis for a new, fascinating era of research - analysis of metabolic and regulatory properties of C. glutamicum on a global scale towards novel and superior bioprocesses.

Identification of a second β-glucoside phosphoenolpyruvate : carbohydrate phosphotransferase system in Corynebacterium glutamicum R

The phosphoenolpyruvate : carbohydrate phosphotransferase system (PTS) catalyses carbohydrate transport by coupling it to phosphorylation. Previously, we reported a Corynebacterium glutamicum R β-glucoside PTS encoded by bglF. Here we report that C. glutamicum R contains an additional β-glucoside PTS gene, bglF2, organized in a cluster with a putative phospho-β-glucosidase gene, bglA2, and a putative antiterminator, bglG2. While single gene disruption strains of either bglF or bglF2 were able to utilize salicin or arbutin as sole carbon sources, a double disruption strain exhibited defects in utilization of both carbon sources. Expression of both bglF and bglF2 was induced in the presence of either salicin or arbutin, although disruption of bglG2 affected only bglF2 expression. Moreover, in the presence of either salicin or arbutin, glucose completely repressed the expression of bglF but only slightly repressed that of bglF2. We conclude that BglF and BglF2 have a redundant role in β-glucoside transport even though the catabolite repression control of their encoding genes is different. We also show that expression of both bglF and bglF2 requires the general PTS.

Regulation of ldh expression during biotin-limited growth of Corynebacterium glutamicum

Corynebacterium glutamicum is a biotin-auxotrophic bacterium and some strains efficiently produce glutamic acid under biotin-limiting conditions. In an effort to understand C. glutamicum metabolism under biotin limitation, growth of the type strain ATCC 13032 was investigated in batch cultures and a time-course analysis was performed. A transient excretion of organic acids was observed and we focused our attention on lactate synthesis. Lactate synthesis was due to the ldh-encoded l-lactate dehydrogenase (Ldh). Features of Ldh activity and ldh transcription were analysed. The ldh gene was shown to be regulated at the transcriptional level by SugR, a pleiotropic transcriptional repressor also acting on most phosphotransferase system (PTS) genes. Electrophoretic mobility shift assays (EMSAs) and site-directed mutagenesis allowed the identification of the SugR-binding site. Effector studies using EMSAs and analysis of ldh expression in a ptsF mutant revealed fructose 1-phosphate as a highly efficient negative effector of SugR. Fructose 1,6-bisphosphate also affected SugR binding.

The global repressor SugR controls expression of genes of glycolysis and of the L-lactate dehydrogenase LdhA in Corynebacterium glutamicum

The transcriptional regulator SugR from Corynebacterium glutamicum represses genes of the phosphoenolpyruvate-dependent phosphotransferase system (PTS). Growth experiments revealed that the overexpression of sugR not only perturbed the growth of C. glutamicum on the PTS sugars glucose, fructose, and sucrose but also led to a significant growth inhibition on ribose, which is not taken up via the PTS. Chromatin immunoprecipitation combined with DNA microarray analysis and gel retardation experiments were performed to identify further target genes of SugR. Gel retardation analysis confirmed that SugR bound to the promoter regions of genes of the glycolytic enzymes 6-phosphofructokinase (pfkA), fructose-1,6-bisphosphate aldolase (fba), enolase (eno), pyruvate kinase (pyk), and NAD-dependent L-lactate dehydrogenase (ldhA). The deletion of sugR resulted in increased mRNA levels of eno, pyk, and ldhA in acetate medium. Enzyme activity measurements revealed that SugR-mediated repression affects the activities of PfkA, Fba, and LdhA in vivo. As the deletion of sugR led to increased LdhA activity under aerobic and under oxygen deprivation conditions, L-lactate production by C. glutamicum was determined. The overexpression of sugR reduced L-lactate production by about 25%, and sugR deletion increased L-lactate formation under oxygen deprivation conditions by threefold. Thus, SugR functions as a global repressor of genes of the PTS, glycolysis, and fermentative L-lactate dehydrogenase in C. glutamicum.

Structural and functional characterization of the LldR from Corynebacterium glutamicum: a transcriptional repressor involved in L-lactate and sugar utilization

LldR (CGL2915) from Corynebacterium glutamicum is a transcription factor belonging to the GntR family, which is typically involved in the regulation of oxidized substrates associated with amino acid metabolism. In the present study, the crystal structure of LldR was determined at 2.05-A resolution. The structure consists of N- and C-domains similar to those of FadR, but with distinct domain orientations. LldR and FadR dimers achieve similar structures by domain swapping, which was first observed in dimeric assembly of transcription factors. A structural feature of Zn(2+) binding in the regulatory domain was also observed, as a difference from the FadR subfamily. DNA microarray and DNase I footprint analyses suggested that LldR acts as a repressor regulating cgl2917-lldD and cgl1934-fruK-ptsF operons, which are indispensable for l-lactate and fructose/sucrose utilization, respectively. Furthermore, the stoichiometries and affinities of LldR and DNAs were determined by isothermal titration calorimetry measurements. The transcriptional start site and repression of LldR on the cgl2917-lldD operon were analysed by primer extension assay. Mutation experiments showed that residues Lys4, Arg32, Arg42 and Gly63 are crucial for DNA binding. The location of the putative ligand binding cavity and the regulatory mechanism of LldR on its affinity for DNA were proposed.

Engineering of a glycerol utilization pathway for amino acid production by Corynebacterium glutamicum

The amino acid-producing organism Corynebacterium glutamicum cannot utilize glycerol, a stoichiometric by-product of biodiesel production. By heterologous expression of Escherichia coli glycerol utilization genes, C. glutamicum was engineered to grow on glycerol. While expression of the E. coli genes for glycerol kinase (glpK) and glycerol 3-phosphate dehydrogenase (glpD) was sufficient for growth on glycerol as the sole carbon and energy source, additional expression of the aquaglyceroporin gene glpF from E. coli increased growth rate and biomass formation. Glutamate production from glycerol was enabled by plasmid-borne expression of E. coli glpF, glpK, and glpD in C. glutamicum wild type. In addition, a lysine-producing C. glutamicum strain expressing E. coli glpF, glpK, and glpD was able to produce lysine from glycerol as the sole carbon substrate as well as from glycerol-glucose mixtures.