Effects of the changes in enzyme activities on metabolic flux redistribution around the 2-oxoglutarate branch in glutamate production by Corynebacterium glutamicum

Biotin concentration affects anaplerotic reactions functioning in glutamic acid production in Corynebacterium glutamicum

The study investigates the effect of biotin concentration on the role of anaplerotic reactions catalysed by pyruvate carboxylase (PC) and phosphoenolpyruvate carboxylase (PEPC) in glutamic acid production by Corynebacterium glutamicum. C. glutamicum requires biotin for its growth, and its glutamic acid production can be induced by the addition of Tween 40 or penicillin or by biotin limitation. The biotin enzyme PC and the non-biotin enzyme PEPC catalyse two anaplerotic reactions to supply oxaloacetic acid to the TCA cycle in C. glutamicum. Therefore, they are crucial for glutamic acid production in this bacterium. In this study, we investigated the contribution of each anaplerotic reaction to Tween 40- and penicillin-induced glutamic acid production using disruptants of PEPC and PC. In the presence of 20 µg l-1 biotin, which is sufficient for growth, the PEPC-catalysed anaplerotic reaction mainly contributed to Tween 40- and penicillin-induced glutamic acid production. However, when increasing biotin concentration 10-fold (i.e. 200 µg l-1), both PC- and PEPC-catalysed reactions could function in glutamic acid production. Western blotting revealed that the amount of biotin-bound PC was reduced by the addition of Tween 40 and penicillin in the presence of 20 µg l-1. However, these induction treatments did not change the amount of biotin-bound PC in the presence of 200 µg l-1 biotin. These results indicate that both anaplerotic reactions are functional during glutamic acid production in C. glutamicum and that biotin concentration mainly affects which anaplerotic reactions function during glutamic acid production.

Revealing Corynebacterium glutamicum proteoforms through top-down proteomics

Corynebacterium glutamicum is a bacterium widely employed in the industrial production of amino acids as well as a broad range of other biotechnological products. The present study describes the characterization of C. glutamicum proteoforms, and their post-translational modifications (PTMs) employing top-down proteomics. Despite previous evidence of PTMs having roles in the regulation of C. glutamicum metabolism, this is the first top-down proteome analysis of this organism. We identified 1125 proteoforms from 273 proteins, with 60% of proteins presenting at least one mass shift, suggesting the presence of PTMs, including several acetylated, oxidized and formylated proteoforms. Furthermore, proteins relevant to amino acid production, protein secretion, and oxidative stress were identified with mass shifts suggesting the presence of uncharacterized PTMs and proteoforms that may affect biotechnologically relevant processes in this industrial workhorse. For instance, the membrane proteins mepB and SecG were identified as a cleaved and a formylated proteoform, respectively. While in the central metabolism, OdhI was identified as two proteoforms with potential biological relevance: a cleaved proteoform and a proteoform with PTMs corresponding to a 70 Da mass shift.

Induction of glutamic acid production by copper in Corynebacterium glutamicum

From the previous transcriptome analysis (Hirasawa et al. Biotechnol J 13:e1700612, 2018), it was found that expression of genes whose expression is regulated by stress-responsive transcriptional regulators was altered during penicillin-induced glutamic acid production in Corynebacterium glutamicum. Therefore, we investigated whether stress treatments, such as copper and iron addition, could induce glutamic acid production in C. glutamicum and found that the addition of copper did induce glutamic acid production in this species. Moreover, we also determined that glutamic acid production levels upon copper addition in a gain-of-function mutant strain of the mechanosensitive channel, NCgl1221, involved in glutamic acid export, were comparable to glutamic acid levels produced upon penicillin addition and biotin limitation in the wild-type strain. Furthermore, disruption of the odhI gene, which encodes a protein responsible for the decreased activity of the 2-oxoglutarate dehydrogenase complex during glutamic acid production, significantly diminished glutamic acid production induced by copper. These results indicate that copper can induce glutamic acid production and this induction requires OdhI like biotin limitation and penicillin addition, but a gain-of-function mutation in the NCgl1221 mechanosensitive channel is necessary for its high-level glutamic acid production. However, a significant increase in odhI transcription was not observed with copper addition in both wild-type and NCgl1221 gain-of-function mutant strains. In addition, disruption of the csoR gene encoding a copper-responsive transcriptional repressor enhanced copper-induced glutamic acid production in the NCgl1221 gain-of-function mutant, indicating that unidentified CsoR-regulated genes may contribute to copper-induced glutamic acid production in C. glutamicum. KEY POINTS: • Copper can induce glutamic acid production by Corynebacterium glutamicum. • Copper-induced glutamic acid production requires OdhI protein. • Copper-induced glutamic acid production requires a gain-of-function mutation in the mechanosensitive channel NCgl1221, which is responsible for the production of glutamic acid.

Characterization of lysine acetylation in the peripheral subunit-binding domain of the E2 subunit of the pyruvate dehydrogenase-2-oxoglutarate dehydrogenase hybrid complex from Corynebacterium glutamicum

In Corynebacterium glutamicum, pyruvate dehydrogenase (PDH) and 2-oxoglutarate dehydrogenase (ODH) form a unique hybrid complex in which CgE1p and CgE1o are associated with the CgE2-CgE3 subcomplex. We analyzed the role of a lysine acetylation site in the peripheral subunit-binding domain of CgE2 in PDH and ODH functions. Acetylation-mimic substitution at Lys391 of CgE2 severely reduced the interaction of CgE2 with CgE1p and CgE3, but not with CgE1o, indicating the critical role of this residue in the assembly of CgE1p and CgE3 into the complex. It also suggested that Lys391 acetylation inhibited the binding of CgE1p and CgE3 to CgE2, thereby affecting PDH and ODH activities. Interestingly, the CgE2-K391R variant strain showed increased l-glutamate production and reduced pyruvate accumulation. Kinetic analysis suggested that the increased affinity of the K391R variant toward pyruvate might be advantageous for l-glutamate production.

Enhancing the production of poly-γ-glutamate in Bacillus subtilis ZJS18 by the heat- and osmotic shock and its mechanism

Poly-γ-glutamate (γ-PGA) is a natural macromolecule peptide, and is widely used in the food, medicine, and pharmaceutical industries. In this study, heat- and osmotic shock were used to improve the production of γ-PGA in Bacillus subtilis ZJS18, and its molecular mechanism was explored. The results indicated that the heat- and osmotic shock significantly promoted the production of γ-PGA owing to the stress response of B. subtilis cells to adverse environment. The highest concentrations of γ-PGA reached 14.53 and 15.98 g/l under heat- and osmotic shock, respectively. The activities of five enzymes related to the metabolism of the endogenous glutamate were determined and analyzed. It was found that the activities of glucose-6-phosphate dehydrogenase, isocitrate dehydrogenase, glutamate dehydrogenase and glutamate synthase were significantly altered during heat- and osmotic shock, while the activity of α-ketoglutarate dehydrogenase only showed a little alteration. This study provides a basis for the industrial production and use of γ-PGA, and for understanding its biosynthetic mechanism in B. subtilis ZJS18.

High-level production of poly-γ-glutamic acid from untreated molasses by Bacillus siamensis IR10

Background

Poly-γ-glutamic acid (γ-PGA) is a promising biopolymer and has been applied in many fields. Bacillus siamensis SB1001 was a newly isolated poly-γ-glutamic acid producer with sucrose as its optimal carbon source. To improve the utilization of carbon source, and then molasses can be effectively used for γ-PGA production, ⁶⁰cobalt gamma rays was used to mutate the genes of B. siamensis SB1001.

Results

Bacillus siamensis IR10 was screened for the production of γ-PGA from untreated molasses. In batch fermentation, 17.86 ± 0.97 g/L γ-PGA was obtained after 15 h, which is 52.51% higher than that of its parent strain. Fed-batch fermentation was performed to further improve the yield of γ-PGA with untreated molasses, yielding 41.40 ± 2.01 g/L of γ-PGA with a productivity of 1.73 ± 0.08 g/L/h. An average γ-PGA productivity of 1.85 g/L/h was achieved in the repeated fed-batch fermentation. This is the first report of such a high γ-PGA productivity. The analysis of the enzyme activities showed that they were affected by the carbon sources, enhanced ICDH and GDH, and decreased ODHC, which are important for γ-PGA production.

Conclusion

These results suggest that untreated molasses can be used for economical and industrial-scale production of γ-PGA by B. siamensis IR10.

Deciphering metabolic responses of biosurfactant lichenysin on biosynthesis of poly-γ-glutamic acid

Poly-γ-glutamic acid (γ-PGA) is an extracellularly produced biodegradable polymer, which has been widely used as agricultural fertilizer, mineral fortifier, cosmetic moisturizer, and drug carrier. This study firstly discovered that lichenysin, as a biosurfactant, showed the capability to enhance γ-PGA production in Bacillus licheniformis. The exogenous addition of lichenysin improved the γ-PGA yield up to 17.9% and 21.9%, respectively, in the native strain B. licheniformis WX-02 and the lichenysin-deficient strain B. licheniformis WX02-ΔlchAC. The capability of intracellular biosynthesis of lichenysin was positively correlated with γ-PGA production. The yield of γ-PGA increased by 25.1% in the lichenysin-enhanced strain B. licheniformis WX02-Psrflch and decreased by 12.2% in the lichenysin-deficient strain WX02-ΔlchAC. Analysis of key enzyme activities and gene expression in the TCA cycle, precursor glutamate synthesis, and γ-PGA synthesis pathway revealed that the existence of lichenysin led to increased γ-PGA via shifting the carbon flux in the TCA cycle towards glutamate and γ-PGA biosynthetic pathways, minimizing by-product formation, and facilitating the uptake of extracellular substrates and the polymerization of glutamate to γ-PGA. Insight into the mechanisms of enhanced production of γ-PGA by lichenysin would define the essential parameters involved in γ-PGA biosynthesis and provide the basis for large-scale production of γ-PGA.

Reconstruction of tricarboxylic acid cycle in Corynebacterium glutamicum with a genome-scale metabolic network model for trans-4-hydroxyproline production

trans-4-Hydroxy- l-proline (Hyp) is an abundant component of mammalian collagen and functions as a chiral synthon for the syntheses of anti-inflammatory drugs in the pharmaceutical industry. Proline 4-hydroxylase (P4H) can catalyze the conversion of l-proline to Hyp; however, it is still challenging for the fermentative production of Hyp from glucose using P4H due to the low yield and productivity. Here, we report the metabolic engineering of Corynebacterium glutamicum for the fermentative production of Hyp by reconstructing tricarboxylic acid (TCA) cycle together with heterologously expressing the p4h gene from Dactylosporangium sp. strain RH1. In silico model-based simulation showed that α-ketoglutarate was redirected from the TCA cycle toward Hyp synthetic pathway driven by P4H when the carbon flux from succinyl-CoA to succinate descended to zero. The interruption of the TCA cycle by the deletion of sucCD-encoding the succinyl-CoA synthetase (SUCOAS) led to a 60% increase in Hyp production and had no obvious impact on the growth rate. Fine-tuning of plasmid-borne ProB* and P4H abundances led to a significant increase in the yield of Hyp on glucose. The final engineered Hyp-7 strain produced up to 21.72 g/L Hyp with a yield of 0.27 mol/mol (Hyp/glucose) and a volumetric productivity of 0.36 g·L ^-1 ·hr ^-1 in the shake flask fermentation. To our knowledge, this is the highest yield and productivity achieved by microbial fermentation in a glucose-minimal medium for Hyp production. This strategy provides new insights into engineering C. glutamicum by flux coupling for the fermentative production of Hyp and related products.

Rich biotin content in lignocellulose biomass plays the key role in determining cellulosic glutamic acid accumulation by Corynebacterium glutamicum

BACKGROUND

Lignocellulose is one of the most promising alternative feedstocks for glutamic acid production as commodity building block chemical, but the efforts by the dominant industrial fermentation strain Corynebacterium glutamicum failed for accumulating glutamic acid using lignocellulose feedstock.

RESULTS

We identified the existence of surprisingly high biotin concentration in corn stover hydrolysate as the determining factor for the failure of glutamic acid accumulation by Corynebacterium glutamicum. Under excessive biotin content, induction by penicillin resulted in 41.7 ± 0.1 g/L of glutamic acid with the yield of 0.50 g glutamic acid/g glucose. Our further investigation revealed that corn stover contained 353 ± 16 μg of biotin per kg dry solids, approximately one order of magnitude greater than the biotin in corn grain. Most of the biotin remained stable during the biorefining chain and the rich biotin content in corn stover hydrolysate almost completely blocked the glutamic acid accumulation. This rich biotin existence was found to be a common phenomenon in the wide range of lignocellulose biomass and this may be the key reason why the previous studies failed in cellulosic glutamic acid fermentation from lignocellulose biomass. The extended recording of the complete members of all eight vitamin B compounds in lignocellulose biomass further reveals that the major vitamin B members were also under the high concentration levels even after harsh pretreatment.

CONCLUSIONS

The high content of biotin in wide range of lignocellulose biomass feedstocks and the corresponding hydrolysates was discovered and it was found to be the key factor in determining the cellulosic glutamic acid accumulation. The highly reserved biotin and the high content of their other vitamin B compounds in biorefining process might act as the potential nutrients to biorefining fermentations. This study creates a new insight that lignocellulose biorefining not only generates inhibitors, but also keeps nutrients for cellulosic fermentations.

Integrated Analysis of the Transcriptome and Metabolome of Corynebacterium glutamicum during Penicillin-Induced Glutamic Acid Production

Corynebacterium glutamicum is known for its ability to produce glutamic acid and has been utilized for the fermentative production of various amino acids. Glutamic acid production in C. glutamicum is induced by penicillin. In this study, the transcriptome and metabolome of C. glutamicum is analyzed to understand the mechanism of penicillin-induced glutamic acid production. Transcriptomic analysis with DNA microarray revealed that expression of some glycolysis- and TCA cycle-related genes, which include those encoding the enzymes involved in conversion of glucose to 2-oxoglutaric acid, is upregulated after penicillin addition. Meanwhile, expression of some TCA cycle-related genes, encoding the enzymes for conversion of 2-oxoglutaric acid to oxaloacetic acid, and the anaplerotic reactions decreased. In addition, expression of NCgl1221 and odhI, encoding proteins involved in glutamic acid excretion and inhibition of the 2-oxoglutarate dehydrogenase, respectively, is upregulated. Functional category enrichment analysis of genes upregulated and downregulated after penicillin addition revealed that genes for signal transduction systems are enriched among upregulated genes, whereas those for energy production and carbohydrate and amino acid metabolisms are enriched among the downregulated genes. As for the metabolomic analysis using capillary electrophoresis time-of-flight mass spectrometry, the intracellular content of most metabolites of the glycolysis and the TCA cycle decreased dramatically after penicillin addition. Overall, these results indicate that the cellular metabolism and glutamic acid excretion are mainly optimized at the transcription level during penicillin-induced glutamic acid production by C. glutamicum.

Effect of lysine succinylation on the regulation of 2-oxoglutarate dehydrogenase inhibitor, OdhI, involved in glutamate production in Corynebacterium glutamicum

In Corynebacterium glutamicum, the activity of the 2-oxoglutarate dehydrogenase (ODH) complex is negatively regulated by the unphosphorylated form of OdhI protein, which is critical for L-glutamate overproduction. We examined the potential impact of protein acylation at lysine (K)-132 of OdhI in C. glutamicum ATCC13032. The K132E succinylation-mimic mutation reduced the ability of OdhI to bind OdhA, the catalytic subunit of the ODH complex, which reduced the inhibition of ODH activity. In vitro succinylation of OdhI protein also reduced the ability to inhibit ODH, and the K132R mutation blocked the effect. These results suggest that succinylation at K132 may attenuate the OdhI function. Consistent with these results, the C. glutamicum mutant strain with OdhI-K132E showed decreased L-glutamate production. Our results indicated that not only phosphorylation but also succinylation of OdhI protein may regulate L-glutamate production in C. glutamicum.

Beyond topology: coevolution of structure and flux in metabolic networks

Interactions between the structure of a metabolic network and its functional properties underlie its evolutionary diversification, but the mechanism by which such interactions arise remains elusive. Particularly unclear is whether metabolic fluxes that determine the concentrations of compounds produced by a metabolic network, are causally linked to a network's structure or emerge independently of it. A direct empirical study of populations where both structural and functional properties vary among individuals' metabolic networks is required to establish whether changes in structure affect the distribution of metabolic flux. In a population of house finches (Haemorhous mexicanus), we reconstructed full carotenoid metabolic networks for 442 individuals and uncovered 11 structural variants of this network with different compounds and reactions. We examined the consequences of this structural diversity for the concentrations of plumage-bound carotenoids produced by flux in these networks. We found that concentrations of metabolically derived, but not dietary carotenoids, depended on network structure. Flux was partitioned similarly among compounds in individuals of the same network structure: within each network, compound concentrations were closely correlated. The highest among-individual variation in flux occurred in networks with the strongest among-compound correlations, suggesting that changes in the magnitude, but not the distribution of flux, underlie individual differences in compound concentrations on a static network structure. These findings indicate that the distribution of flux in carotenoid metabolism closely follows network structure. Thus, evolutionary diversification and local adaptations in carotenoid metabolism may depend more on the gain or loss of enzymatic reactions than on changes in flux within a network structure.

RETRACTED ARTICLE: Comparative analysis of the Corynebacterium glutamicum transcriptome in response to changes in dissolved oxygen levels

The dissolved oxygen (DO) level of a culture of Corynebacterium glutamicum (C. glutamicum) in a bioreactor has a significant impact on the cellular redox potential and the distribution of energy and metabolites. In this study, to gain a deeper understanding of the effects of DO on the metabolism of C. glutamicum, we sought to systematically explore the influence of different DO concentrations on genetic regulation and metabolism through transcriptomic analysis. The results revealed that after 20 h of fermentation, oxygen limitation enhanced the glucose metabolism, pyruvate metabolism and carbon overflow, and restricted NAD+ availability. A high oxygen supply enhanced the TCA cycle and reduced glyoxylate metabolism. Several key genes involved in response of C. glutamicum to different oxygen concentrations were examined, which provided suggestions for target site modifications in developing optimized oxygen supply strategies. These data provided new insights into the relationship between oxygen supply and metabolism of C. glutamicum.

Ciprofloxacin triggered glutamate production by Corynebacterium glutamicum

BACKGROUND

Corynebacterium glutamicum is a well-studied bacterium which naturally overproduces glutamate when induced by an elicitor. Glutamate production is accompanied by decreased 2-oxoglutatate dehydrogenase activity. Elicitors of glutamate production by C. glutamicum analyzed to molecular detail target the cell envelope.

RESULTS

Ciprofloxacin, an inhibitor of bacterial DNA gyrase and topoisomerase IV, was shown to inhibit growth of C. glutamicum wild type with concomitant excretion of glutamate. Enzyme assays showed that 2-oxoglutarate dehydrogenase activity was decreased due to ciprofloxacin addition. Transcriptome analysis revealed that this inhibitor of DNA gyrase increased RNA levels of genes involved in DNA synthesis, repair and modification. Glutamate production triggered by ciprofloxacin led to glutamate titers of up to 37 ± 1 mM and a substrate specific glutamate yield of 0.13 g/g. Even in the absence of the putative glutamate exporter gene yggB, ciprofloxacin effectively triggered glutamate production. When C. glutamicum wild type was cultivated under nitrogen-limiting conditions, 2-oxoglutarate rather than glutamate was produced as consequence of exposure to ciprofloxacin. Recombinant C. glutamicum strains overproducing lysine, arginine, ornithine, and putrescine, respectively, secreted glutamate instead of the desired amino acid when exposed to ciprofloxacin.

CONCLUSIONS

Ciprofloxacin induced DNA synthesis and repair genes, reduced 2-oxoglutarate dehydrogenase activity and elicited glutamate production by C. glutamicum. Production of 2-oxoglutarate could be triggered by ciprofloxacin under nitrogen-limiting conditions.

Genome-Based Genetic Tool Development for Bacillus methanolicus: Theta- and Rolling Circle-Replicating Plasmids for Inducible Gene Expression and Application to Methanol-Based Cadaverine Production

Bacillus methanolicus is a thermophilic methylotroph able to overproduce amino acids from methanol, a substrate not used for human or animal nutrition. Based on our previous RNA-seq analysis a mannitol inducible promoter and a putative mannitol activator gene mtlR were identified. The mannitol inducible promoter was applied for controlled gene expression using fluorescent reporter proteins and a flow cytometry analysis, and improved by changing the -35 promoter region and by co-expression of the mtlR regulator gene. For independent complementary gene expression control, the heterologous xylose-inducible system from B. megaterium was employed and a two-plasmid gene expression system was developed. Four different replicons for expression vectors were compared with respect to their copy number and stability. As an application example, methanol-based production of cadaverine was shown to be improved from 6.5 to 10.2 g/L when a heterologous lysine decarboxylase gene cadA was expressed from a theta-replicating rather than a rolling-circle replicating vector. The current work on inducible promoter systems and compatible theta- or rolling circle-replicating vectors is an important extension of the poorly developed B. methanolicus genetic toolbox, valuable for genetic engineering and further exploration of this bacterium.

Glutamate Fermentation-2: Mechanism of L-Glutamate Overproduction in Corynebacterium glutamicum

The nonpathogenic coryneform bacterium, Corynebacterium glutamicum, was isolated as an L-glutamate-overproducing microorganism by Japanese researchers and is currently utilized in various amino acid fermentation processes. L-Glutamate production by C. glutamicum is induced by limitation of biotin and addition of fatty acid ester surfactants and β-lactam antibiotics. These treatments affect the cell surface structures of C. glutamicum. After the discovery of C. glutamicum, many researchers have investigated the underlying mechanism of L-glutamate overproduction with respect to the cell surface structures of this organism. Furthermore, metabolic regulation during L-glutamate overproduction by C. glutamicum, particularly, the relationship between central carbon metabolism and L-glutamate biosynthesis, has been investigated. Recently, the role of a mechanosensitive channel protein in L-glutamate overproduction has been reported. In this chapter, mechanisms of L-glutamate overproduction by C. glutamicum have been reviewed.

Altered acetylation and succinylation profiles in Corynebacterium glutamicum in response to conditions inducing glutamate overproduction

The bacterium Corynebacterium glutamicum is utilized during industrial fermentation to produce amino acids such as L-glutamate. During L-glutamate fermentation, C. glutamicum changes the flux of central carbon metabolism to favor L-glutamate production, but the molecular mechanisms that explain these flux changes remain largely unknown. Here, we found that the profiles of two major lysine acyl modifications were significantly altered upon glutamate overproduction in C. glutamicum; acetylation decreased, whereas succinylation increased. A label-free semi-quantitative proteomic analysis identified 604 acetylated proteins with 1328 unique acetylation sites and 288 succinylated proteins with 651 unique succinylation sites. Acetylation and succinylation targeted enzymes in central carbon metabolic pathways that are directly related to glutamate production, including the 2-oxoglutarate dehydrogenase complex (ODHC), a key enzyme regulating glutamate overproduction. Structural mapping revealed that several critical lysine residues in the ODHC components were susceptible to acetylation and succinylation. Furthermore, induction of glutamate production was associated with changes in the extent of acetylation and succinylation of lysine, suggesting that these modifications may affect the activity of enzymes involved in glutamate production. Deletion of phosphotransacetylase decreased the extent of protein acetylation in nonproducing condition, suggesting that acetyl phosphate-dependent acetylation is active in C. glutamicum. However, no effect was observed on the profiles of acetylation and succinylation in glutamate-producing condition upon disruption of acetyl phosphate metabolism or deacetylase homologs. It was considered likely that the reduced acetylation in glutamate-producing condition may reflect metabolic states where the flux through acid-producing pathways is very low, and substrates for acetylation do not accumulate in the cell. Succinylation would occur more easily than acetylation in such conditions where the substrates for both acetylation and succinylation are limited. This is the first study investigating the acetylome and succinylome of C. glutamicum, and it provides new insight into the roles of acyl modifications in C. glutamicum biology.

Control of carbon flux to glutamate excretion in Klebsiella pneumoniae: the role of the indigenous plasmid and its encoded isocitrate dehydrogenase

Klebsiella pneumoniae (NCTC, CL687/80) harbors a large indigenous plasmid (p(C3)), which in addition to encoding for citrate utilization, proline synthesis and glutamate excretion, it uniquely carries the structural gene (icd); encoding isocitrate dehydrogenase (ICDH). Flux analysis revealed that ICDH, despite its role in the generation of NADPH required for glutamate dehydrogenase, is not rate-limiting (controlling) in central metabolism as evidenced by a negative flux control coefficient and an adverse effect of overexpression (14-fold) on glutamate excretion. More significantly, however, this paper presents, for the first time, clear evidence that the accumulation of glutamate and its subsequent excretion is associated with the C3 plasmid-encoded regulatory elements, which trigger a shift-down in the activity of α-ketoglutarate dehydrogenase, both in the K. pneumoniae parental strain as well as in the E. coli exconjugants strains. This finding opens the door for the exploitation of regulatory elements as a tool for manipulating flux in microbial cell factories.

Effect of Tween 40 and DtsR1 on L-arginine overproduction in Corynebacterium crenatum

Background

l-Glutamate is an important precursor in the l-arginine (l-Arg) biosynthetic pathway. Various methods, including polyoxyethylene sorbitan monopalmitate (Tween 40) addition and dtsR1 disruption, have been widely used to induce l-glutamate overproduction in Corynebacterium glutamicum. In this study, a novel strategy for l-Arg overproduction through Tween 40 trigger and ΔdtsR1 mutant were proposed in Corynebacterium crenatum.

Results

Corynebacterium crenatum mutant (CCM01) was selected as a host strain, whose argR was lethal via mutagenesis screening, the proB gene was knocked out, and argB was replaced by argB M4 (E19R, H26E, D311R, and D312R) to release l-Arg feedback resistance. After Tween 40 trigger in the logarithmic period, l-Arg production increased from 15.22 to 17.73 g/L in CCM01 strain. When NCgl1221 and dtsR1 disruption (CCM03), l-Arg production drastically increased to 27.45 g/L and then further to 29.97 g/L after Tween 40 trigger. Moreover, the specific activity of α-oxoglutarate dehydrogenase complex (ODHC) decreased, whereas the regeneration of NADP⁺/NADPH significantly increased after dtsR1 disruption and Tween 40 trigger. Results of real-time PCR showed that the transcriptional levels of odhA, sucB, and lpdA (encoding three subunits of the ODHC complex) were downregulated after Tween 40 trigger or dtsR1 disruption. By contrast, zwf transcription (encoding glucose-6-phosphate dehydrogenase) showed no significant difference among CCM01, CCM02 (ΔNCgl1221), and CCM03 (ΔNCgl1221ΔdtsR1) strains without Tween 40 trigger but evidently increased by 5.50 folds after Tween 40 trigger.

Conclusion

A novel strategy for l-Arg overproduction by dtsR1 disruption and Tween 40 trigger in C. crenatum was reported. Tween 40 addition exhibited a bifunctional mechanism for l-Arg overproduction, including reduced ODHC activity and enhanced NADPH pools accumulation by downregulated dtsR1 expression and upregulated zwf expression, respectively.

5-Aminolevulinic acid production in engineered Corynebacterium glutamicum via C5 biosynthesis pathway

ALA (5-aminolevulinic acid) is an important intermediate in the synthesis of tetrapyrroles and the use of ALA has been gradually increasing in many fields, including medicine and agriculture. In this study, improved biological production of ALA in Corynebacterium glutamicum was achieved by overexpressing glutamate-initiated C5 pathway. For this purpose, copies of the glutamyl t-RNA reductase HemA from several bacteria were mutated by site-directed mutagenesis of which a HemA version from Salmonella typhimurium exhibited the highest ALA production. Cultivation of the HemA-expressing strain produced approximately 204 mg/L of ALA, while co-expression with HemL (glutamate-1-semialdehyde aminotransferase) increased ALA concentration to 457 mg/L, representing 11.6- and 25.9-fold increases over the control strain (17 mg/L of ALA). Further effects of metabolic perturbation were investigated, leading to penicillin addition that further improves ALA production to 584 mg/L. In an optimized flask fermentation, engineered C. glutamicum strains expressing the HemA and hemAL operon produced up to 1.1 and 2.2g/L ALA, respectively, under glutamate-producing conditions. The final yields represent 10.7- and 22.0-fold increases over the control strain (0.1g/L of ALA). From these findings, ALA biosynthesis from glucose was successfully demonstrated and this study is the first to report ALA overproduction in C. glutamicum via metabolic engineering.

L-Glutamate secretion by the N-terminal domain of the Corynebacterium glutamicum NCgl1221 mechanosensitive channel

The Corynebacterium glutamicum NCgl1221 mechanosensitive channel mediates L-glutamate secretion by sensing changes in membrane tension caused by treatments such as biotin limitation and penicillin. The NCgl1221 protein has an N-terminal domain (1-286 a.a.) homologous to the Escherichia coli MscS and a long C-terminal domain (287-533 a.a.) of unknown function. In order to investigate the role of the C-terminal domain in L-glutamate secretion, we constructed a series of C-terminally truncated mutants of NCgl1221. We found that the N-terminal domain, homologous to E. coli MscS, retained the ability to cause L-glutamate secretion in response to the treatment. Electrophysiological analysis confirmed that the N-terminal domain mediated L-glutamate secretion. 3D homology modeling has suggested that the N-terminal domain of NCgl1221 has an extra loop structure (221-232 a.a.) that is not found in most other MscS proteins. The mutant NCgl1221, deleted for this loop structure, lost the ability to secrete L-glutamate. In addition, we found that mutant NCgl1221 lacking the C-terminal extracytoplasmic domain (420-533 a.a.) produced L-glutamate without any inducing treatment. These results suggest that the N-terminal domain is necessary and sufficient for the excretion of L-glutamate in response to inducing treatment, and that the C-terminal extracytoplasmic domain has a negative regulatory role in L-glutamate production.

High-level exogenous glutamic acid-independent production of poly-(γ-glutamic acid) with organic acid addition in a new isolated Bacillus subtilis C10

A new exogenous glutamic acid-independent γ-PGA producing strain was isolated and characterized as Bacillus subtilis C10. The factors influencing the endogenous glutamic acid supply and the biosynthesis of γ-PGA in this strain were investigated. The results indicated that citric acid and oxalic acid showed the significant capability to support the overproduction of γ-PGA. This stimulated increase of γ-PGA biosynthesis by citric acid or oxalic acid was further proved in the 10 L fermentor. To understand the possible mechanism contributing to the improved γ-PGA production, the activities of four key intracellular enzymes were measured, and the possible carbon fluxes were proposed. The result indicated that the enhanced level of pyruvate dehydrogenase (PDH) activity caused by oxalic acid was important for glutamic acid synthesized de novo from glucose. Moreover, isocitrate dehydrogenase (ICDH) and glutamate dehydrogenase (GDH) were the positive regulators of glutamic acid biosynthesis, while 2-oxoglutarate dehydrogenase complex (ODHC) was the negative one.

Investigation of phosphorylation status of OdhI protein during penicillin- and Tween 40-triggered glutamate overproduction by Corynebacterium glutamicum

Glutamate overproduction by Corynebacterium glutamicum is triggered by treatment with penicillin or Tween 40 and is accompanied by a decrease in 2-oxoglutarate dehydrogenase complex (ODHC) activity. We have reported that de novo synthesis of OdhI, which inhibits ODHC activity by interacting specifically with the E1o subunit of ODHC (OdhA), is induced by penicillin, and that odhI overexpression induces glutamate overproduction in the absence of any triggers for glutamate overproduction. In this study, to determine the function of OdhI in glutamate overproduction by C. glutamicum, changes in OdhI levels and phosphorylation status during penicillin- and Tween 40-induced glutamate overproduction were examined by western blot. The synthesis of both unphosphorylated and phosphorylated OdhI was increased by addition of Tween 40 or penicillin and the levels of unphosphorylated OdhI, which can inhibit ODHC activity, was significantly higher than those of phosphorylated OdhI, which is unable to inhibit ODHC activity. Meanwhile, the OdhA levels were maintained throughout the culture. These results indicate that OdhI synthesis is induced by additions of penicillin and Tween 40 and most synthesized OdhI is unphosphorylated, resulting in the decrease in ODHC activity and glutamate overproduction. Similarly, in the odhI-overexpressing strain, both unphosphorylated and phosphorylated OdhI were synthesized, while the levels of OdhA were nearly constant throughout culture. Our results suggest that high level of unphosphorylated OdhI regulates glutamate overproduction by C. glutamicum.

Engineering of nitrogen metabolism and its regulation in Corynebacterium glutamicum: influence on amino acid pools and production

Nitrogen is one of the macronutrients necessary for living cells, and consequently, assimilation of nitrogen is a crucial step for metabolism. To satisfy their nitrogen demand and to ensure a sufficient nitrogen supply even in situations of nitrogen limitation, microorganisms have evolved sophisticated uptake and assimilation mechanisms for different nitrogen sources. This mini-review focuses on nitrogen metabolism and its control in the biotechnology workhorse Corynebacterium glutamicum, which is used for the industrial production of more than 2 million tons of L: -amino acids annually. Ammonium assimilation and connected control mechanisms on activity and transcription level are summarized, and the influence of mutations on amino acid pools and production is described with emphasis on L: -glutamate, L: -glutamine, and L: -lysine.

Systems biology of industrial microorganisms

The field of industrial biotechnology is expanding rapidly as the chemical industry is looking towards more sustainable production of chemicals that can be used as fuels or building blocks for production of solvents and materials. In connection with the development of sustainable bioprocesses, it is a major challenge to design and develop efficient cell factories that can ensure cost efficient conversion of the raw material into the chemical of interest. This is achieved through metabolic engineering, where the metabolism of the cell factory is engineered such that there is an efficient conversion of sugars, the typical raw materials in the fermentation industry, into the desired product. However, engineering of cellular metabolism is often challenging due to the complex regulation that has evolved in connection with adaptation of the different microorganisms to their ecological niches. In order to map these regulatory structures and further de-regulate them, as well as identify ingenious metabolic engineering strategies that full-fill mass balance constraints, tools from systems biology can be applied. This involves both high-throughput analysis tools like transcriptome, proteome and metabolome analysis, as well as the use of mathematical modeling to simulate the phenotypes resulting from the different metabolic engineering strategies. It is in fact expected that systems biology may substantially improve the process of cell factory development, and we therefore propose the term Industrial Systems Biology for how systems biology will enhance the development of industrial biotechnology for sustainable chemical production.

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.

Development and experimental verification of a genome-scale metabolic model for Corynebacterium glutamicum

BACKGROUND

In silico genome-scale metabolic models enable the analysis of the characteristics of metabolic systems of organisms. In this study, we reconstructed a genome-scale metabolic model of Corynebacterium glutamicum on the basis of genome sequence annotation and physiological data. The metabolic characteristics were analyzed using flux balance analysis (FBA), and the results of FBA were validated using data from culture experiments performed at different oxygen uptake rates.

RESULTS

The reconstructed genome-scale metabolic model of C. glutamicum contains 502 reactions and 423 metabolites. We collected the reactions and biomass components from the database and literatures, and made the model available for the flux balance analysis by filling gaps in the reaction networks and removing inadequate loop reactions. Using the framework of FBA and our genome-scale metabolic model, we first simulated the changes in the metabolic flux profiles that occur on changing the oxygen uptake rate. The predicted production yields of carbon dioxide and organic acids agreed well with the experimental data. The metabolic profiles of amino acid production phases were also investigated. A comprehensive gene deletion study was performed in which the effects of gene deletions on metabolic fluxes were simulated; this helped in the identification of several genes whose deletion resulted in an improvement in organic acid production.

CONCLUSION

The genome-scale metabolic model provides useful information for the evaluation of the metabolic capabilities and prediction of the metabolic characteristics of C. glutamicum. This can form a basis for the in silico design of C. glutamicum metabolic networks for improved bioproduction of desirable metabolites.

Double deletion of dtsR1 and pyc induce efficient L: -glutamate overproduction in Corynebacterium glutamicum

Corynebacterium glutamicum strains are used for the fermentative production of L-glutamate. Five C. glutamicum deletion mutants were isolated by two rounds of selection for homologous recombination and identified by Southern blot analysis. The growth, glucose consumption and glutamate production of the mutants were analyzed and compared with the wild-type ATCC 13032 strain. Double disruption of dtsR1 (encoding a subunit of acetyl-CoA carboxylase complex) and pyc (encoding pyruvate carboxylase) caused efficient overproduction of L-glutamate in C. glutamicum; production was much higher than that of the wild-type strain and DeltadtsR1 strain under glutamate-inducing conditions. In the absence of any inducing conditions, the amount of glutamate produced by the double-deletion strain DeltadtsR1Deltapyc was more than that of the mutant DeltadtsR1. The activity of phosphoenolpyruvate carboxylase (PEPC) was found to be higher in the DeltadtsR1Deltapyc strain than in the DeltadtsR1 strain and the wild-type strain. Therefore, PEPC appears to be an important anaplerotic enzyme for glutamate synthesis in DeltadtsR1 derivatives. Moreover, this conclusion was confirmed by overexpression of ppc and pyc in the two double-deletion strains (DeltadtsR1Deltappc and DeltadtsR1Deltapyc), respectively. Based on the data generated in this investigation, we suggest a new method that will improve glutamate production strains and provide a better understanding of the interaction(s) between the anaplerotic pathway and fatty acid synthesis.

Effect of odhA overexpression and odhA antisense RNA expression on Tween-40-triggered glutamate production by Corynebacterium glutamicum

Recent studies have suggested that a decrease in the specific activity of the 2-oxoglutarate dehydrogenase complex (ODHC) is important for glutamate overproduction by Corynebacterium glutamicum. To further investigate the role of the odhA gene and its product in this process, we constructed the recombinant strains of C. glutamicum in which the expression of the odhA and its product could be controlled by odhA overexpression and odhA antisense RNA expression. We examined changes in glutamate production and ODHC specific activity of the constructed strains during glutamate production triggered by Tween 40 addition. The ODHC specific activity increased with odhA overexpression, resulting in dramatically reduced glutamate production despite Tween 40 addition, indicating that a decrease in the specific activity of ODHC is required for glutamate production induced by Tween 40 addition. However, odhA antisense RNA expression alone did not result in glutamate overproduction in spite of the decrease in ODHC specific activity. Rather, it enhanced glutamate production triggered by Tween 40 addition due to the additional decrease in ODHC specific activity, suggesting that odhA antisense RNA expression is effective in enhancing Tween-40-triggered glutamate overproduction. Our results suggest that a change in ODHC specific activity is critical but is not the only factor responsible for glutamate overproduction by C. glutamicum.

Metabolic regulation and overproduction of primary metabolites

Overproduction of microbial metabolites is related to developmental phases of microorganisms. Inducers, effectors, inhibitors and various signal molecules play a role in different types of overproduction. Biosynthesis of enzymes catalysing metabolic reactions in microbial cells is controlled by well-known positive and negative mechanisms, e.g. induction, nutritional regulation (carbon or nitrogen source regulation), feedback regulation, etc. The microbial production of primary metabolites contributes significantly to the quality of life. Fermentative production of these compounds is still an important goal of modern biotechnology. Through fermentation, microorganisms growing on inexpensive carbon and nitrogen sources produce valuable products such as amino acids, nucleotides, organic acids and vitamins which can be added to food to enhance its flavour, or increase its nutritive values. The contribution of microorganisms goes well beyond the food and health industries with the renewed interest in solvent fermentations. Microorganisms have the potential to provide many petroleum-derived products as well as the ethanol necessary for liquid fuel. Additional applications of primary metabolites lie in their impact as precursors of many pharmaceutical compounds. The roles of primary metabolites and the microbes which produce them will certainly increase in importance as time goes on. In the early years of fermentation processes, development of producing strains initially depended on classical strain breeding involving repeated random mutations, each followed by screening or selection. More recently, methods of molecular genetics have been used for the overproduction of primary metabolic products. The development of modern tools of molecular biology enabled more rational approaches for strain improvement. Techniques of transcriptome, proteome and metabolome analysis, as well as metabolic flux analysis. have recently been introduced in order to identify new and important target genes and to quantify metabolic activities necessary for further strain improvement.

Changes in enzyme activities at the pyruvate node in glutamate-overproducing Corynebacterium glutamicum

Glutamate is industrially produced by fermentation using Corynebacterium glutamicum. The key factor for efficient glutamate production by this microorganism has been considered to be a metabolic change at the 2-oxoglutarate dehydrogenase (ODH) branch point caused by a decrease in ODH activity under glutamate-overproducing conditions. However, this change would be insufficient because the ODH branch is merely the final branch in the glutamate biosynthetic pathway, and efficient glutamate production requires a balanced supply of acetyl-CoA and oxaloacetate (OAA), which are condensed to form a precursor of glutamate, namely, citrate. Therefore, there must be another (other) change(s) in metabolic flux. In this study, we demonstrated that a decrease in pyruvate dehydrogenase (PDH) activity catalyzes the conversion of pyruvate to acetyl-CoA. It is speculated that carbon flux from pyruvate to acetyl-CoA decreases under glutamate-overproducing conditions. Furthermore, an increase in pyruvate carboxylase (PC) activity, which catalyzes the reaction of pyruvate to OAA, is evident under glutamate-overproducing conditions, except under biotin-limited condition, which may lead to an increase in carbon flux from pyruvate to OAA. These data suggest that a novel metabolic change occurs at the pyruvate node, leading to a high yield of glutamate through adequate partitioning of the carbon flux.

Study on roles of anaplerotic pathways in glutamate overproduction of Corynebacterium glutamicum by metabolic flux analysis

BACKGROUND

Corynebacterium glutamicum has several anaplerotic pathways (anaplerosis), which are essential for the productions of amino acids, such as lysine and glutamate. It is still not clear how flux changes in anaplerotic pathways happen when glutamate production is induced by triggers, such as biotin depletion and the addition of the detergent material, Tween 40. In this study, we quantitatively analyzed which anaplerotic pathway flux most markedly changes the glutamate overproduction induced by Tween 40 addition.

RESULTS

We performed a metabolic flux analysis (MFA) with [1-13C]- and [U-13C]-labeled glucose in the glutamate production phase of C. glutamicum, based on the analysis of the time courses of 13C incorporation into proteinogenic amino acids by gas chromatography-mass spectrometry (GC-MS). The flux from phosphoenolpyruvate (PEP) to oxaloacetate (Oxa) catalyzed by phosphoenolpyruvate carboxylase (PEPc) was active in the growth phase not producing glutamate, whereas that from pyruvate to Oxa catalyzed by pyruvate carboxylase (Pc) was inactive. In the glutamate overproduction phase induced by the addition of the detergent material Tween 40, the reaction catalyzed by Pc also became active in addition to the reaction catalyzed by PEPc.

CONCLUSION

It was clarified by a quantitative 13C MFA that the reaction catalyzed by Pc is most markedly increased, whereas other fluxes of PEPc and PEPck remain constant in the glutamate overproduction induced by Tween 40. This result is consistent with the previous results obtained in a comparative study on the glutamate productions of genetically recombinant Pc- and PEPc-overexpressing strains. The importance of a specific reaction in an anaplerotic pathway was elucidated at a metabolic level by MFA.

Glutamate production by Corynebacterium glutamicum: dependence on the oxoglutarate dehydrogenase inhibitor protein OdhI and protein kinase PknG

We recently showed that the activity of the 2-oxoglutarate dehydrogenase complex (ODHC) in Corynebacterium glutamicum is controlled by a novel regulatory mechanism that involves a 15-kDa protein called OdhI and serine/threonine protein kinase G (PknG). In its unphosphorylated state, OdhI binds to the E1 subunit (OdhA) of ODHC and, thereby, inhibits its activity. Inhibition is relieved by phosphorylation of OdhI at threonine-14 by PknG under conditions requiring high ODHC activity. In this work, evidence is provided that the dephosphorylation of phosphorylated OdhI is catalyzed by a phospho-Ser/Thr protein phosphatase encoded by the gene cg0062, designated ppp. As a decreased ODHC activity is important for glutamate synthesis, we investigated the role of OdhI and PknG for glutamate production under biotin limitation and after addition of Tween-40, penicillin, or ethambutol. A DeltaodhI mutant formed only 1-13% of the glutamate synthesized by the wild type. Thus, OdhI is essential for efficient glutamate production. The effect of a pknG deletion on glutamate synthesis was dependent on the induction conditions. Under strong biotin limitation and in the presence of ethambutol, the DeltapknG mutant showed significantly increased glutamate production, offering a new way to improve production strains.

Precise metabolic flux analysis of coryneform bacteria by gas chromatography-mass spectrometry and verification by nuclear magnetic resonance

Precise metabolic flux analysis (MFA) by gas chromatography-mass spectrometry (GC-MS) and computer calculation was performed, and the consistency of the estimated results was verified by independently performed nuclear magnetic resonance (NMR) analysis. The precise estimation of flux by the integration method of the mass isotopomer signal, defined as the coefficient of variance (CV) of multiple determination, was investigated, and the results estimated using different data sets with the same magnitude of error were confirmed. The CV of multiple determinations was sufficiently small to discuss and compare the fluxes of a metabolic pathway. The estimated fluxes using the GC-MS data were cross-validated with the NMR data that were independently measured and not used for MFA. The developed method was successfully applied to the MFA of the growth phase of two different glutamate-producing coryneform bacteria, Corynebacterium glutamicum and C. efficiens. The difference in the growth rate between these two bacterial species was discussed while considering the results of MFA, including forward and backward (exchange) fluxes.

Gene expression of Corynebacterium glutamicum in response to the conditions inducing glutamate overproduction

AIM

The ultimate aim is to elucidate the molecular mechanisms for glutamate overproduction by Corynebacterium glutamicum.

METHODS AND RESULTS

Gene expression in response to the conditions inducing glutamate overproduction was investigated by using a DNA microarray technique. Most genes involved in the EMP pathway, the PPP, and the TCA cycle were downregulated, while five genes that were highly upregulated (NCgl0917, NCgl2944, NCgl2945, NCgl2946, and NCgl2975) were identified under all the three conditions for overproduction that are studied here. Gene products of NCgl2944, NCgl2945, and NCgl2946 were highly homologous to each other, did not resemble any other protein, and have remained uncharacterized thus far. The product of NCgl0917 showed a similarity to a few hypothetical and uncharacterized proteins. NCgl2975 was homologous to metal-binding proteins.

CONCLUSIONS

The decrease in the activity of 2-oxoglutarate dehydrogenase complex, a key enzyme that is downregulated during glutamate overproduction, can be mainly attributed to the downregulation of odhA and sucB. Five highly upregulated genes were also identified.

SIGNIFICANCE AND IMPACT OF THE STUDY

Although fermentative production of glutamate has been carried out for more than 45 years, information on the molecular mechanisms of glutamate overproduction is still limited. This study further elucidates these mechanisms.

On-line optimization of glutamate production based on balanced metabolic control by RQ

In glutamate fermentations by Corynebacterium glutamicum, higher glutamate concentration could be achieved by constantly controlling dissolved oxygen concentration (DO) at a lower level; however, by-product lactate also severely accumulated. The results of analyzing activities changes of the two key enzymes, glutamate and lactate dehydrogenases involved with the fermentation, and the entire metabolic network flux analysis showed that the lactate overproduction was because the metabolic flux in TCA cycle was too low to balance the glucose glycolysis rate. As a result, the respiratory quotient (RQ) adaptive control based "balanced metabolic control" (BMC) strategy was proposed and used to regulate the TCA metabolic flux rate at an appropriate level to achieve the metabolic balance among glycolysis, glutamate synthesis, and TCA metabolic flux. Compared with the best results of various DO constant controls, the BMC strategy increased the maximal glutamate concentration by about 15% and almost completely repressed the lactate accumulation with competitively high glutamate productivity.

Triggering mechanism of L-glutamate overproduction by DtsR1 in coryneform bacteria

The mechanism of L-glutamate-overproduction by Corynebacterium glutamicum, a biotin auxotroph, is very unique and interesting. L-Glutamate overproduction by this bacterium is induced by biotin-limitation and suppressed by an excess of biotin. Addition of a surfactant or penicillin is also induces L-glutamate overproduction even under excess biotin. After the development of general molecular biological tools such as cloning vectors and DNA transfer techniques, genes encoding biosynthetic enzymes were isolated. With those genes and tools, recombinant DNA technology can be applied to the analysis of biosynthetic pathways and the construction of C. glutamicum strains. In this review, recent studies on the triggering mechanism of L-glutamate overproduction by C. glutamicum are discussed. Disruption of the dtsR1 gene, which encodes a putative component of a biotin-containing enzyme complex that is involved in fatty acid synthesis, causes constitutive overproduction of L-glutamate. As in the case of biotin-limitation, i.e., addition of a surfactant or penicillin, dtsR1-disruption also reduces the activity of the 2-oxoglutarate dehydrogense complex (ODHC). These results indicate that the DtsR1 level affects the activity of ODHC. In our recent studies, a novel regulatory factor that suppresses the expression of DtsR1 was determined. Based on these findings, the triggering mechanism of L-glutamate overproduction is expected to be clarified in more detail.

Comparative study of flux redistribution of metabolic pathway in glutamate production by two coryneform bacteria

In amino acid production by coryneform bacteria, study on relationship between change in enzyme activities and production of a target amino acid is important. In glutamate production, Kawahara et al. discovered that the effect of decrease in 2-oxoglutamate dehydrogenase complex (ODHC) on glutamate production is essential (Kawahara et al., Biosci. Biotechnol. Biochem. 61(7) (1997) 1109). Significant reduction of the ODHC activity was observed in the cells under the several glutamate-productive conditions in Corynebacterium glutamicum. Recent progress in metabolic engineering enables us to quantitatively compare the flux redistribution of the different strains after change in enzyme activity precisely. In this paper, relationship between flux redistribution and change in enzyme activities after biotin deletion and addition of detergent (Tween 40) was studied in two coryneform bacteria, C. glutamicum and a newly isolated strain, Corynebacterium efficiens (Fudou et al., Int. J. Syst. Evol. Microbiol. 52(Part 4) 1127), based on metabolic flux analysis (MFA). It was observed that in both species the specific activities of isocitrate dehydrogenase (ICDH) and glutamate dehydrogenase (GDH) did not significantly change throughout the fermentation, while that of the ODHC significantly decreased after biotin depletion and Tween 40 addition. Flux redistribution clearly occurred after the decrease in ODHC specific activity. The difference in glutamate production between C. glutamicum and C. efficiens was caused by the difference in the degree of decrease in ODHC specific activity. The difference in Michaelis-Menten constants or K(m) value between ICDH, GDH, and ODHC explained the mechanism of flux redistribution at the branch point of 2-oxoglutarate. It was found that the K(m) values of ICDH and ODHC were much lower than that of GDH for both strains. It was quantitatively proved that the ODHC plays the most important role in controlling flux distribution at the key branch point of 2-oxoglutarate in both coryneform bacteria. Flux redistribution mechanism was well simulated by a Michaelis-Menten-based model with kinetic parameters. The knowledge of the mechanism of flux redistribution will contribute to improvement of glutamate production in coryneform bacteria.