Recombinant production of the lantibiotic nisin using Corynebacterium glutamicum in a two-step process.

BACKGROUND: The bacteriocin nisin is naturally produced by Lactococcus lactis as an inactive prepeptide that is modified posttranslationally resulting in five (methyl-)lanthionine rings characteristic for class Ia bacteriocins. Export and proteolytic cleavage of the leader peptide results in release of active nisin. By targeting the universal peptidoglycan precursor lipid II, nisin has a broad target spectrum including important human pathogens such as Listeria monocytogenes and methicillin-resistant Staphylococcus aureus strains. Industrial nisin production is currently performed using natural producer strains resulting in rather low product purity and limiting its application to preservation of dairy food products. RESULTS: We established heterologous nisin production using the biotechnological workhorse organism Corynebacterium glutamicum in a two-step process. We demonstrate successful biosynthesis and export of fully modified prenisin and its activation to mature nisin by a purified, soluble variant of the nisin protease NisP (sNisP) produced in Escherichia coli. Active nisin was detected by a L. lactis sensor strain with strictly nisin-dependent expression of the fluorescent protein mCherry. Following activation by sNisP, supernatants of the recombinant C. glutamicum producer strain cultivated in standard batch fermentations contained at least 1.25 mg/l active nisin. CONCLUSIONS: We demonstrate successful implementation of a two-step process for recombinant production of active nisin with C. glutamicum. This extends the spectrum of bioactive compounds that may be produced using C. glutamicum to a bacteriocin harboring complex posttranslational modifications. Our results provide a basis for further studies to optimize product yields, transfer production to sustainable substrates and purification of pharmaceutical grade nisin.

Garvicin Q: characterization of biosynthesis and mode of action.

Bacteriocins are ribosomally synthesized antimicrobial peptides, that either kill target bacteria or inhibit their growth. Bacteriocins are used in food preservation and are of increasing interest as potential alternatives to conventional antibiotics. In the present study, we show that Lactococcus petauri B1726, a strain isolated from fermented balsam pear, produces a heat-stable and protease-sensitive compound. Following genome sequencing, a gene cluster for production of a class IId bacteriocin was identified consisting of garQ (encoding for the bacteriocin garvicin Q), garI (for a putative immunity protein), garC, and garD (putative transporter proteins). Growth conditions were optimized for increased bacteriocin activity in supernatants of L. petauri B1726 and purification and mass spectrometry identified the compound as garvicin Q. Further experiments suggest that garvicin Q adsorbs to biomass of various susceptible and insusceptible bacteria and support the hypothesis that garvicin Q requires a mannose-family phosphotransferase system (PTSMan) as receptor to kill target bacteria by disruption of membrane integrity. Heterologous expression of a synthetic garQICD operon was established in Corynebacterium glutamicum demonstrating that genes garQICD are responsible for biosynthesis and secretion of garvicin Q. Moreover, production of garvicin Q by the recombinant C. glutamicum strain was improved by using a defined medium yet product levels were still considerably lower than with the natural L. petauri B1726 producer strain.Collectively, our data identifies the genetic basis for production of the bacteriocin garvicin Q by L. petauri B1726 and provides insights into the receptor and mode of action of garvicin Q. Moreover, we successfully performed first attempts towards biotechnological production of this interesting bacteriocin using natural and heterologous hosts.

Transforming Escherichia coli Proteomembranes into Artificial Chloroplasts Using Molecular Photocatalysis.

During the light-dependent reaction of photosynthesis, green plants couple photoinduced cascades of redox reactions with transmembrane proton translocations to generate reducing equivalents and chemical energy in the form of NADPH (nicotinamide adenine dinucleotide phosphate) and ATP (adenosine triphosphate), respectively. We mimic these basic processes by combining molecular ruthenium polypyridine-based photocatalysts and inverted vesicles derived from Escherichia coli. Upon irradiation with visible light, the interplay of photocatalytic nicotinamide reduction and enzymatic membrane-located respiration leads to the simultaneous formation of two biologically active cofactors, NADH (nicotinamide adenine dinucleotide) and ATP, respectively. This inorganic-biologic hybrid system thus emulates the cofactor delivering function of an active chloroplast.

Establishing recombinant production of pediocin PA-1 in Corynebacterium glutamicum.

Bacteriocins are antimicrobial peptides produced by bacteria to inhibit competitors in their natural environments. Some of these peptides have emerged as commercial food preservatives and, due to the rapid increase in antibiotic resistant bacteria, are also discussed as interesting alternatives to antibiotics for therapeutic purposes. Currently, commercial bacteriocins are produced exclusively with natural producer organisms on complex substrates and are sold as semi-purified preparations or crude fermentates. To allow clinical application, efficacy of production and purity of the product need to be improved. This can be achieved by shifting production to recombinant microorganisms. Here, we identify Corynebacterium glutamicum as a suitable production host for the bacteriocin pediocin PA-1. C. glutamicum CR099 shows resistance to high concentrations of pediocin PA-1 and the bacteriocin was not inactivated when spiked into growing cultures of this bacterium. Recombinant C. glutamicum expressing a synthetic pedACDCgl operon releases a compound that has potent antimicrobial activity against Listeria monocytogenes and Listeria innocua and matches size and mass:charge ratio of commercial pediocin PA-1. Fermentations in shake flasks and bioreactors suggest that low levels of dissolved oxygen are favorable for production of pediocin. Under these conditions, however, reduced activity of the TCA cycle resulted in decreased availability of the important pediocin precursor l-asparagine suggesting options for further improvement. Overall, we demonstrate that C. glutamicum is a suitable host for recombinant production of bacteriocins of the pediocin family.

Switching the Mechanism of NADH Photooxidation by Supramolecular Interactions.

A series of three Ru(II) polypyridine complexes was investigated for the selective photocatalytic oxidation of NAD(P)H to NAD(P)+ in water. A combination of (time-resolved) spectroscopic studies and photocatalysis experiments revealed that ligand design can be used to control the mechanism of the photooxidation: For prototypical Ru(II) complexes a 1 O2 pathway was found. Rudppz ([(tbbpy)2 Ru(dppz)]Cl2 , tbbpy=4,4'-di-tert-butyl-2,2'-bipyridine, dppz=dipyrido[3,2-a:2',3'-c]phenazine), instead, initiated the cofactor oxidation by electron transfer from NAD(P)H enabled by supramolecular binding between substrate and catalyst. Expulsion of the photoproduct NAD(P)+ from the supramolecular binding site in Rudppz allowed very efficient turnover. Therefore, Rudppz permits repetitive selective assembly and oxidative conversion of reduced naturally occurring nicotinamides by recognizing the redox state of the cofactor under formation of H2 O2 as additional product. This photocatalytic process can fuel discontinuous photobiocatalysis.

Exploiting Hydrogenophaga pseudoflava for aerobic syngas-based production of chemicals.

Gasification is a suitable technology to generate energy-rich synthesis gas (syngas) from biomass or waste streams, which can be utilized in bacterial fermentation processes for the production of chemicals and fuels. Established microbial processes currently rely on acetogenic bacteria which perform an energetically inefficient anaerobic CO oxidation and acetogenesis potentially hampering the biosynthesis of complex and ATP-intensive products. Since aerobic oxidation of CO is energetically more favorable, we exploit in this study the Gram-negative ß-proteobacterium Hydrogenophaga pseudoflava DSM1084 as novel host for the production of chemicals from syngas. We sequenced and annotated the genome of H. pseudoflava and established a genetic engineering toolbox, which allows markerless chromosomal modification via the pk19mobsacB system and heterologous gene expression on pBBRMCS2-based plasmids. The toolbox was extended by identifying strong endogenous promotors such as PgapA2 which proved to yield high expression under heterotrophic and autotrophic conditions. H. pseudoflava showed relatively fast heterotrophic growth in complex and minimal medium with sugars and organic acids which allows convenient handling in lab routines. In autotrophic bioreactor cultivations with syngas, H. pseudoflava exhibited a growth rate of 0.06 h-1 and biomass specific uptakes rates of 14.2 ±â€¯0.3 mmol H2 gCDW-1 h-1, 73.9 ±â€¯1.8 mmol CO gCDW-1 h-1, and 31.4 ±â€¯0.3 mmol O2 gCDW-1 h-1. As proof of concept, we engineered the carboxydotrophic bacterium for the aerobic production of the C15 sesquiterpene (E)-α-bisabolene from the C1 carbon source syngas by heterologous expression of the (E)-α-bisabolene synthase gene agBIS. The resulting strain H. pseudoflava (pOCEx1:agBIS) produced 59 ±â€¯8 µg (E)-α-bisabolene L-1 with a volumetric productivity Qp of 1.2 ±â€¯0.2 µg L-1 h-1 and a biomass-specific productivity qp of 13.1 ±â€¯0.6 µg gCDW-1 h-1. The intrinsic properties and the genetic repertoire of H. pseudoflava make this carboxydotrophic bacterium a promising candidate for future aerobic production processes to synthesize more complex or ATP-intensive chemicals from syngas.

A step forward: Compatible and dual-inducible expression vectors for gene co-expression in Corynebacterium glutamicum.

The Gram-positive bacterium Corynebacterium glutamicum represents a promising platform for the production of amino acids, organic acids, and other bio-products. However, the availability of only few expression vectors limits its use for production purposes, using metabolic engineering approaches when co-expression of several target genes is desired. To widen the scope for co-expression, the pCG1/p15A and pBL1/colE1 replicons were employed to construct the two differentially-inducible and compatible expression vectors pRG_Duet1 and pRG_Duet2. To functionally validate these newly constructed expression vectors, target genes for easily measurable enzymes were cloned and over-expression of these genes was investigated using respective enzyme assays. Furthermore, functionality and co-existence of the pCG1-based C. glutamicum - E. coli shuttle vector pRG_Duet1 were confirmed with pBL1-based expression vectors pRG_Duet2 and pEKEx2, using co-transformation and enzyme assays. The novel shuttle expression vectors pRG_Duet1 and pRG_Duet2 are attractive additions to the existing set of vectors for co-expression studies and metabolic engineering of C. glutamicum.

A simple dual-inducible CRISPR interference system for multiple gene targeting in Corynebacterium glutamicum.

The development of CRISPR interference (CRISPRi) technology has dramatically increased the pace and the precision of target identification during platform strain development. In order to develop a simple, reliable, and dual-inducible CRISPRi system for the industrially relevant Corynebacterium glutamicum, we combined two different inducible repressor systems in a single plasmid to separately regulate the expression of dCas9 (anhydro-tetracycline-inducible) and a given single guide RNA (IPTG-inducible). The functionality of the resulting vector was demonstrated by targeting the l-arginine biosynthesis pathway in C. glutamicum. By co-expressing dCas9 and a specific single guide RNA targeting the 5'-region of the argininosuccinate lyase gene argH, the specific activity of the target enzyme was down-regulated and in a l-arginine production strain, l-arginine formation was shifted towards citrulline formation. The system was also employed for down-regulation of multiple genes by concatenating sgRNA sequences encoded on one plasmid. Simultaneous down-regulated expression of both argH and the phosphoglucose isomerase gene pgi proved the potential of the system for multiplex targeting. The system can be a promising tool for further pathway engineering in C. glutamicum. Cumulative effects on targeted genes can be rapidly evaluated avoiding tedious and time-consuming traditional gene knockout approaches.

The RamA regulon: complex regulatory interactions in relation to central metabolism in Corynebacterium glutamicum.

Corynebacterium glutamicum is an industrial workhorse used for the production of amino acids and a variety of other chemicals and fuels. Within its regulatory repertoire, C. glutamicum possesses RamA which was initially identified as essential transcriptional regulator of acetate metabolism. Further studies revealed its relevance for ethanol and propionate catabolism and also identified RamA to function as global regulator in the metabolism of C. glutamicum. Thereby, RamA acts as transcriptional activator or repressor of genes encoding enzymes which are involved in carbon uptake, central carbon metabolism, and cell wall synthesis. RamA controls the expression of target genes either directly and/or indirectly by constituting feed-forward loop type of transcriptional motifs with other regulators such as GlxR, SugR, RamB, and GntR1. In this review, we summarize the current knowledge on RamA, its regulon, and its regulatory interplay with other transcriptional regulators coordinating the metabolism of C. glutamicum.

Stereospecificity of Corynebacterium glutamicum 2,3-butanediol dehydrogenase and implications for the stereochemical purity of bioproduced 2,3-butanediol.

The stereochemistry of 2,3-butanediol (2,3-BD) synthesis in microbial fermentations is important for many applications. In this work, we showed that Corynebacterium glutamicum endowed with the Lactococcus lactis genes encoding α-acetolactate synthase and decarboxylase activities produced meso-2,3-BD as the major end product, meaning that (R)-acetoin is a substrate for endogenous 2,3-butanediol dehydrogenase (BDH) activity. This is curious in view of the reported absolute stereospecificity of C. glutamicum BDH for (S)-acetoin (Takusagawa et al. Biosc Biotechnol Biochem 65:1876-1878, 2001). To resolve this discrepancy, the enzyme encoded by butA Cg was produced in Escherichia coli and purified, and the stereospecific properties of the pure protein were examined. Activity assays monitored online by 1H-NMR using racemic acetoin and an excess of NADH showed an initial, fast production of (2S,3S)-2,3-BD, followed by a slow (∼20-fold lower apparent rate) formation of meso-2,3-BD. Kinetic parameters for (S)-acetoin, (R)-acetoin, meso-2,3-BD and (2S,3S)-BD were determined by spectrophotometric assays. V max values for (S)-acetoin and (R)-acetoin were 119 ± 15 and 5.23 ± 0.06 µmol min-1 mg protein-1, and K m values were 0.23 ± 0.02 and 1.49 ± 0.07 mM, respectively. We conclude that C. glutamicum BDH is not absolutely specific for (S)-acetoin, though this is the preferred substrate. Importantly, the low activity of BDH with (R)-acetoin was sufficient to support high yields of meso-2,3-BD in the engineered strain C. glutamicum ΔaceEΔpqoΔldhA(pEKEx2-als,aldB,butA Cg ). Additionally, we found that the BDH activity was nearly abolished upon inactivation of butA Cg (from 0.30 ± 0.03 to 0.004 ± 0.001 µmol min-1 mg protein-1), indicating that C. glutamicum expresses a single BDH under the experimental conditions examined.

The α-glucan phosphorylase MalP of Corynebacterium glutamicum is subject to transcriptional regulation and competitive inhibition by ADP-glucose.

UNLABELLED: α-Glucan phosphorylases contribute to degradation of glycogen and maltodextrins formed in the course of maltose metabolism in bacteria. Accordingly, bacterial α-glucan phosphorylases are classified as either glycogen or maltodextrin phosphorylase, GlgP or MalP, respectively. GlgP and MalP enzymes follow the same catalytic mechanism, and thus their substrate spectra overlap; however, they differ in their regulation: GlgP genes are constitutively expressed and the enzymes are controlled on the activity level, whereas expression of MalP genes are transcriptionally controlled in response to the carbon source used for cultivation. We characterize here the modes of control of the α-glucan phosphorylase MalP of the Gram-positive Corynebacterium glutamicum. In accordance to the proposed function of the malP gene product as MalP, we found transcription of malP to be regulated in response to the carbon source. Moreover, malP transcription is shown to depend on the growth phase and to occur independently of the cell glycogen content. Surprisingly, we also found MalP activity to be tightly regulated competitively by the presence of ADP-glucose, an intermediate of glycogen synthesis. Since the latter is considered a typical feature of GlgPs, we propose that C. glutamicum MalP acts as both maltodextrin and glycogen phosphorylase and, based on these findings, we question the current system for classification of bacterial α-glucan phosphorylases. IMPORTANCE: Bacterial α-glucan phosphorylases have been classified conferring to their purpose as either glycogen or maltodextrin phosphorylases. We found transcription of malP in C. glutamicum to be regulated in response to the carbon source, which is recognized as typical for maltodextrin phosphorylases. Surprisingly, we also found MalP activity to be tightly regulated competitively by the presence of ADP-glucose, an intermediate of glycogen synthesis. The latter is considered a typical feature of GlgPs. These findings, taken together, suggest that C. glutamicum MalP is the first α-glucan phosphorylase that does not fit into the current system for classification of bacterial α-glucan phosphorylases and exemplifies the complex mechanisms underlying the control of glycogen content and maltose metabolism in this model organism.

Engineering Corynebacterium glutamicum for the production of 2,3-butanediol.

BACKGROUND: 2,3-Butanediol is an important bulk chemical with a wide range of applications. In bacteria, this metabolite is synthesised from pyruvate via a three-step pathway involving α-acetolactate synthase, α-acetolactate decarboxylase and 2,3-butanediol dehydrogenase. Thus far, the best producers of 2,3-butanediol are pathogenic strains, hence, the development of more suitable organisms for industrial scale fermentation is needed. Herein, 2,3-butanediol production was engineered in the Generally Regarded As Safe (GRAS) organism Corynebacterium glutamicum. A two-stage fermentation process was implemented: first, cells were grown aerobically on acetate; in the subsequent production stage cells were used to convert glucose into 2,3-butanediol under non-growing and oxygen-limiting conditions. RESULTS: A gene cluster, encoding the 2,3-butanediol biosynthetic pathway of Lactococcus lactis, was assembled and expressed in background strains, C. glutamicum ΔldhA, C. glutamicum ΔaceEΔpqoΔldhA and C. glutamicum ΔaceEΔpqoΔldhAΔmdh, tailored to minimize pyruvate-consuming reactions, i.e., to prevent carbon loss in lactic, acetic and succinic acids. Producer strains were characterized in terms of activity of the relevant enzymes in the 2,3-butanediol forming pathway, growth, and production of 2,3-butanediol under oxygen-limited conditions. Productivity was maximized by manipulating the aeration rate in the production phase. The final strain, C. glutamicum ΔaceEΔpqoΔldhAΔmdh(pEKEx2-als,aldB,Ptuf butA), under optimized conditions produced 2,3-butanediol with a 0.66 mol mol(-1) yield on glucose, an overall productivity of 0.2 g L(-1) h(-1) and a titer of 6.3 g L(-1). CONCLUSIONS: We have successfully developed C. glutamicum into an efficient cell factory for 2,3-butanediol production. The use of the engineered strains as a basis for production of acetoin, a widespread food flavour, is proposed.

Carbon flux analysis by 13C nuclear magnetic resonance to determine the effect of CO2 on anaerobic succinate production by Corynebacterium glutamicum.

Wild-type Corynebacterium glutamicum produces a mixture of lactic, succinic, and acetic acids from glucose under oxygen deprivation. We investigated the effect of CO2 on the production of organic acids in a two-stage process: cells were grown aerobically in glucose, and subsequently, organic acid production by nongrowing cells was studied under anaerobic conditions. The presence of CO2 caused up to a 3-fold increase in the succinate yield (1 mol per mol of glucose) and about 2-fold increase in acetate, both at the expense of l-lactate production; moreover, dihydroxyacetone formation was abolished. The redistribution of carbon fluxes in response to CO2 was estimated by using (13)C-labeled glucose and (13)C nuclear magnetic resonance (NMR) analysis of the labeling patterns in end products. The flux analysis showed that 97% of succinate was produced via the reductive part of the tricarboxylic acid cycle, with the low activity of the oxidative branch being sufficient to provide the reducing equivalents needed for the redox balance. The flux via the pentose phosphate pathway was low (~5%) regardless of the presence or absence of CO2. Moreover, there was significant channeling of carbon to storage compounds (glycogen and trehalose) and concomitant catabolism of these reserves. The intracellular and extracellular pools of lactate and succinate were measured by in vivo NMR, and the stoichiometry (H(+):organic acid) of the respective exporters was calculated. This study shows that it is feasible to take advantage of natural cellular regulation mechanisms to obtain high yields of succinate with C. glutamicum without genetic manipulation.

Application of metabolic engineering for the biotechnological production of L-valine.

The branched chain amino acid L-valine is an essential nutrient for higher organisms, such as animals and humans. Besides the pharmaceutical application in parenteral nutrition and as synthon for the chemical synthesis of e.g. herbicides or anti-viral drugs, L-valine is now emerging into the feed market, and significant increase of sales and world production is expected. In accordance, well-known microbial production bacteria, such as Escherichia coli and Corynebacterium glutamicum strains, have recently been metabolically engineered for efficient L-valine production under aerobic or anaerobic conditions, and the respective cultivation and production conditions have been optimized. This review summarizes the state of the art in L-valine biosynthesis and its regulation in E. coli and C. glutamicum with respect to optimal metabolic network for microbial L-valine production, genetic strain engineering and bioprocess development for L-valine production, and finally, it will shed light on emerging technologies that have the potential to accelerate strain and bioprocess engineering in the near future.

Metabolic engineering of Corynebacterium glutamicum for 2-ketoisocaproate production.

2-Ketoisocaproate (KIC) is used as a therapeutic agent, and a KIC-producing organism may serve as a platform for products deriving from this 2-keto acid. We engineered Corynebacterium glutamicum for the production of KIC from glucose by deletion of ltbR and ilvE, encoding the transcriptional repressor LtbR and transaminase B, respectively, and additional overexpression of ilvBNCD, encoding acetohydroxyacid synthase, acetohydroxyacid isomeroreductase, and dihydroxyacid dehydratase. The KIC-producing strain was improved by deletion of the methylcitrate synthase genes and by decreasing citrate synthase activity by exchange of the native promoter of the citrate synthase gene. In shake-flask fermentations under L-leucine limitation, the newly constructed strain C. glutamicum VB (pJC4ilvBNCD) produced 31 ± 2 mM (4.0 ± 0.3 g l(-1)) KIC and showed a product yield of about 0.26 ± 0.02 mol per mole (0.19 ± 0.01 g per gram) of glucose. As by-product, the strain formed about 33 mM 2-ketoisovalerate, which is a precursor of KIC. KIC production was further improved by additional expression of an isopropylmalate synthase allele (leuA (EC-G462D)), encoding an enzyme resistant towards L-leucine inhibition, and by addition of acetate as additional substrate. With glucose and acetate, the newly constructed strain produced 71 ± 3.2 mM (9.2 ± 0.4 g l(-1)) KIC with a yield of 0.24 ± 0.01 mol C (KIC) per mole C (in both substrates) and with nearly no 2-ketoisovalerate by-product formation (<2 mM). Investigating the activities and regulation of the native isopropylmalate synthase and dehydratase of C. glutamicum, we observed competitive and noncompetitive inhibition, respectively, by KIC.

Recombinant Production of Pseudomonas aeruginosa Rhamnolipids in P. putida KT2440 on Acetobacterium woodii Cultures Grown Chemo-Autotrophically with Carbon Dioxide and Hydrogen.

The establishment of sustainable processes for the production of commodity chemicals is one of today's central challenges for biotechnological industries. The chemo-autotrophic fixation of CO2 and the subsequent production of acetate by acetogenic bacteria via anaerobic gas fermentation represents a promising platform for the ecologically sustainable production of high-value biocommodities via sequential fermentation processes. In this study, the applicability of acetate-containing cell-free spent medium of the gas-fermenting acetogenic bacterium A. woodii WP1 as the feeder strain for growth and the recombinant production of P. aeruginosa PAO1 mono-rhamnolipids in the well-established nonpathogenic producer strain P. putida KT2440 were investigated. Additionally, the potential possibility of a simplified production process without the necessary separation of feeder strain cells was elucidated via the cultivation of P. putida in cell-containing A. woodii culture broth. For these cultures, the content of both strains was investigated by examining the relative quantification of strain-exclusive genes via qPCR. The recombinant production of mono-rhamnolipids was successfully achieved with maximum titers of approximately 360-400 mg/L for both cell-free and cell-containing A. woodii spent medium. The reported processes therefore represent a successful proof of principle for gas fermentation-derived acetate as a potential sustainable carbon source for future recombinant rhamnolipid production processes by P. putida KT2440.

Complex regulation of the phosphoenolpyruvate carboxykinase gene pck and characterization of its GntR-type regulator IolR as a repressor of myo-inositol utilization genes in Corynebacterium glutamicum.

DNA affinity chromatography with the promoter region of the Corynebacterium glutamicum pck gene, encoding phosphoenolpyruvate carboxykinase, led to the isolation of four transcriptional regulators, i.e., RamA, GntR1, GntR2, and IolR. Determination of the phosphoenolpyruvate carboxykinase activity of the ΔramA, ΔgntR1 ΔgntR2, and ΔiolR deletion mutants indicated that RamA represses pck during growth on glucose about 2-fold, whereas GntR1, GntR2, and IolR activate pck expression about 2-fold irrespective of whether glucose or acetate served as the carbon source. The DNA binding sites of the four regulators in the pck promoter region were identified and their positions correlated with the predicted functions as repressor or activators. The iolR gene is located upstream and in a divergent orientation with respect to a iol gene cluster, encoding proteins involved in myo-inositol uptake and degradation. Comparative DNA microarray analysis of the ΔiolR mutant and the parental wild-type strain revealed strongly (>100-fold) elevated mRNA levels of the iol genes in the mutant, indicating that the primary function of IolR is the repression of the iol genes. IolR binding sites were identified in the promoter regions of iolC, iolT1, and iolR. IolR therefore is presumably subject to negative autoregulation. A consensus DNA binding motif (5'-KGWCHTRACA-3') which corresponds well to those of other GntR-type regulators of the HutC family was identified. Taken together, our results disclose a complex regulation of the pck gene in C. glutamicum and identify IolR as an efficient repressor of genes involved in myo-inositol catabolism of this organism.

Phosphotransferase system-mediated glucose uptake is repressed in phosphoglucoisomerase-deficient Corynebacterium glutamicum strains.

Corynebacterium glutamicum is particularly known for its industrial application in the production of amino acids. Amino acid overproduction comes along with a high NADPH demand, which is covered mainly by the oxidative part of the pentose phosphate pathway (PPP). In previous studies, the complete redirection of the carbon flux toward the PPP by chromosomal inactivation of the pgi gene, encoding the phosphoglucoisomerase, has been applied for the improvement of C. glutamicum amino acid production strains, but this was accompanied by severe negative effects on the growth characteristics. To investigate these effects in a genetically defined background, we deleted the pgi gene in the type strain C. glutamicum ATCC 13032. The resulting strain, C. glutamicum Δpgi, lacked detectable phosphoglucoisomerase activity and grew poorly with glucose as the sole substrate. Apart from the already reported inhibition of the PPP by NADPH accumulation, we detected a drastic reduction of the phosphotransferase system (PTS)-mediated glucose uptake in C. glutamicum Δpgi. Furthermore, Northern blot analyses revealed that expression of ptsG, which encodes the glucose-specific EII permease of the PTS, was abolished in this mutant. Applying our findings, we optimized l-lysine production in the model strain C. glutamicum DM1729 by deletion of pgi and overexpression of plasmid-encoded ptsG. l-Lysine yields and productivity with C. glutamicum Δpgi(pBB1-ptsG) were significantly higher than those with C. glutamicum Δpgi(pBB1). These results show that ptsG overexpression is required to overcome the repressed activity of PTS-mediated glucose uptake in pgi-deficient C. glutamicum strains, thus enabling efficient as well as fast l-lysine production.

Platform engineering of Corynebacterium glutamicum with reduced pyruvate dehydrogenase complex activity for improved production of L-lysine, L-valine, and 2-ketoisovalerate.

Exchange of the native Corynebacterium glutamicum promoter of the aceE gene, encoding the E1p subunit of the pyruvate dehydrogenase complex (PDHC), with mutated dapA promoter variants led to a series of C. glutamicum strains with gradually reduced growth rates and PDHC activities. Upon overexpression of the l-valine biosynthetic genes ilvBNCE, all strains produced l-valine. Among these strains, C. glutamicum aceE A16 (pJC4 ilvBNCE) showed the highest biomass and product yields, and thus it was further improved by additional deletion of the pqo and ppc genes, encoding pyruvate:quinone oxidoreductase and phosphoenolpyruvate carboxylase, respectively. In fed-batch fermentations at high cell densities, C. glutamicum aceE A16 Δpqo Δppc (pJC4 ilvBNCE) produced up to 738 mM (i.e., 86.5 g/liter) l-valine with an overall yield (YP/S) of 0.36 mol per mol of glucose and a volumetric productivity (QP) of 13.6 mM per h [1.6 g/(liter × h)]. Additional inactivation of the transaminase B gene (ilvE) and overexpression of ilvBNCD instead of ilvBNCE transformed the l-valine-producing strain into a 2-ketoisovalerate producer, excreting up to 303 mM (35 g/liter) 2-ketoisovalerate with a YP/S of 0.24 mol per mol of glucose and a QP of 6.9 mM per h [0.8 g/(liter × h)]. The replacement of the aceE promoter by the dapA-A16 promoter in the two C. glutamicum l-lysine producers DM1800 and DM1933 improved the production by 100% and 44%, respectively. These results demonstrate that C. glutamicum strains with reduced PDHC activity are an excellent platform for the production of pyruvate-derived products.