Coexistence of the Entner-Doudoroff and Embden-Meyerhof-Parnas pathways enhances glucose consumption of ethanol-producing Corynebacterium glutamicum.

BACKGROUND: It is interesting to modify sugar metabolic pathways to improve the productivity of biocatalysts that convert sugars to value-added products. However, this attempt often fails due to the tight control of the sugar metabolic pathways. Recently, activation of the Entner-Doudoroff (ED) pathway in Escherichia coli has been shown to enhance glucose consumption, though the mechanism underlying this phenomenon is poorly understood. In the present study, we investigated the effect of a functional ED pathway in metabolically engineered Corynebacterium glutamicum that metabolizes glucose via the Embden-Meyerhof-Parnas (EMP) pathway to produce ethanol under oxygen deprivation. This study aims to provide further information on metabolic engineering strategies that allow the Entner-Doudoroff and Embden-Meyerhof-Parnas pathways to coexist. RESULTS: Three genes (zwf, edd, and eda) encoding glucose-6-phosphate dehydrogenase, 6-phosphogluconate dehydratase, and 2-keto-3-deoxy-6-phosphogluconate aldolase from Zymomonas mobilis were expressed in a genetically modified strain, C. glutamicum CRZ2e, which produces pyruvate decarboxylase and alcohol dehydrogenase from Z. mobilis. A 13C-labeling experiment using [1-13C] glucose indicated a distinctive 13C distribution of ethanol between the parental and the ED-introduced strains, which suggested an alteration of carbon flux as a consequence of ED pathway introduction. The ED-introduced strain, CRZ2e-ED, consumed glucose 1.5-fold faster than the parental strain. A pfkA deletion mutant of CRZ2e-ED (CRZ2e-EDΔpfkA) was also constructed to evaluate the effects of EMP pathway inactivation, which showed an almost identical rate of glucose consumption compared to that of the parental CRZ2e strain. The introduction of the ED pathway did not alter the intracellular NADH/NAD+ ratio, whereas it resulted in a slight increase in the ATP/ADP ratio. The recombinant strains with simultaneous overexpression of the genes for the EMP and ED pathways exhibited the highest ethanol productivity among all C. glutamicum strains ever constructed. CONCLUSIONS: The increased sugar consumption observed in ED-introduced strains was not a consequence of cofactor balance alterations, but rather the crucial coexistence of two active glycolytic pathways for enhanced glucose consumption. Coexistence of the ED and EMP pathways is a good strategy for improving biocatalyst productivity even when NADPH supply is not a limiting factor for fermentation.

Analysis of the Relationship of Microbial Contamination with Temperature and Cleaning Frequency and Method of Domestic Refrigerators in Japan.

ABSTRACT: Hygiene management of domestic refrigerators is an important aspect of food poisoning prevention. The aim of the present study was to confirm the relationship between microbial contamination and hygiene management by measuring microbial levels and investigating temperature and cleaning frequency and method of domestic refrigerators in Japan. We analyzed three internal sections (the egg compartment, bottom shelf, and vegetable drawer) of 100 domestic refrigerators in Japan. Salmonella, Listeria monocytogenes, and Yersinia enterocolitica were not found in any of the refrigerators, but coliforms and Escherichia coli were detected in more than one household, and Staphylococcus aureus was the most frequently isolated pathogen. The prevalences of these microorganisms had similar tendencies in all three sections sampled and were highest in the vegetable drawer. The temperature distribution in the refrigerators was also investigated, and a temperature >6.1°C (improper temperature) was found in 46.2% of the areas surveyed. Only 17% of the respondents cleaned their refrigerators monthly or more often, and this frequency was lower than that reported in other countries. Fifty percent of the respondents used only water to clean the refrigerator, 10% used only an alcohol or disinfecting wipe, and 8% used only a dry cloth. Although no significant correlations were found between microbial contamination and temperatures in refrigerators, correlations were found between microbial contamination and refrigerator cleaning frequency and/or method. To our knowledge, this is the first detailed survey concerning relationships between microbial contamination and hygiene management in domestic refrigerators in Japan. The data obtained can be used to promote food poisoning management in Japanese households.

Isobutanol production in Corynebacterium glutamicum: Suppressed succinate by-production by pckA inactivation and enhanced productivity via the Entner-Doudoroff pathway.

On the basis of our previous studies of microbial L-valine production under oxygen deprivation, we developed isobutanol-producing Corynebacterium glutamicum strains. The artificial isobutanol synthesis pathway was composed of the first three steps of the L-valine synthesis pathway; and the subsequent Ehrlich Pathway: pyruvate was converted to 2-ketoisovalerate in the former reactions; and the 2-keto acid was decarboxylated into isobutyraldehyde, and subsequently reduced into isobutanol in the latter reactions. Although there exists redox cofactor imbalance in the overall reactions, i.e., NADH is generated via glycolysis whereas NADPH is required to synthesize isobutanol, it was resolved by taking advantage of the NAD-preferring mutant acetohydroxy acid isomeroreductase encoded by ilvCTM and the NAD-specific alcohol dehydrogenase encoded by adhA. Each enzyme activity to synthesize isobutanol was finely tuned by using two kinds of lac promoter derivatives. Efficient suppression of succinate by-production and improvement of isobutanol yield resulted from inactivation of pckA, which encodes phosphoenolpyruvate carboxykinase, whereas glucose consumption and isobutanol production rates decreased because of the elevated intracellular NADH/NAD+ ratio. On the other hand, introduction of the exogenous Entner-Doudoroff pathway effectively enhanced glucose consumption and productivity. Overexpression of phosphoenolpyruvate:carbohydrate phosphotransferase system specific to glucose and deletion of ilvE, which encodes branched-chain amino acid transaminase, further suppressed by-products and improved isobutanol productivity. Finally, the produced isobutanol concentration reached 280 mM at a yield of 84% (mol/mol glucose) in 24 h.

Metabolic engineering of Corynebacterium glutamicum for hyperproduction of polymer-grade L- and D-lactic acid.

Strain development is critical for microbial production of bio-based chemicals. The stereo-complex form of polylactic acid, a complex of poly-L- and poly-D-lactic acid, is a promising polymer candidate due to its high thermotolerance. Here, we developed Corynebacterium glutamicum strains producing high amounts of L- and D-lactic acid through intensive metabolic engineering. Chromosomal overexpression of genes encoding the glycolytic enzymes, glucokinase, glyceraldehyde-3-phosphate dehydrogenase, phosphofructokinase, triosephosphate isomerase, and enolase, increased L- and D-lactic acid concentration by 146% and 56%, respectively. Chromosomal integration of two genes involved in the Entner-Doudoroff pathway (6-phosphogluconate dehydratase and 2-dehydro-3-deoxyphosphogluconate aldolase), together with a gene encoding glucose-6-phosphate dehydrogenase from Zymomonas mobilis, to bypass the carbon flow from glucose, further increased L- and D-lactic acid concentration by 11% and 44%, respectively. Finally, additional chromosomal overexpression of a gene encoding NADH dehydrogenase to modulate the redox balance resulted in the production of 212 g/L L-lactic acid with a 97.9% yield and 264 g/L D-lactic acid with a 95.0% yield. The optical purity of both L- and D-lactic acid was 99.9%. Because the constructed metabolically engineered strains were devoid of plasmids and antibiotic resistance genes and were cultivated in mineral salts medium, these strains could contribute to the cost-effective production of the stereo-complex form of polylactic acid in practical scale.

Evaluation of the influence of sprinkling powdered slaked lime on microorganisms for the prevention of domestic animal infectious diseases.

When infectious diseases arise in domestic animals, a large amount of slaked lime is sprinkled on cattle sheds and their surroundings for disinfection and prevention. However, optimal sprinkling methods, standard and upper limit of slaked lime, and influence of slaked lime on non-target microorganisms remain unclear. In this study, we clarified detailed microbicidal effects of slaked lime via in vitro experiments and the influence of sprinkling powdered slaked lime (PSL) in field soil on microorganisms. In vitro disinfection tests assessing the appropriate amount of water and ventilation conditions were also performed in sterilized glass bottles with soil and Salmonella enterica subsp. enterica serovar Typhimurium. Under conditions with a small amount of water relative to the amount of PSL, the bactericidal effect and sustainability of powdered slaked lime (PSL) tended to be lower than those without spraying water. Moreover, the sterilization effect markedly decreased after 7 days under conditions with abundant water. These results indicate that the amount of sprayed water is very important for the bactericidal effect and persistence of PSL. A field experiment showed that the pH and exchange calcium (Ca) content of the soil sprinkled with over 1000 g m-2 PSL remained high even after a long period (≥1 year), with values of approximately 0.5-1.0 and approximately 3-11 times the level without PSL, respectively. However, sprinkling PSL did not influence viable microbial counts at any concentration.

Efficient construction of xenogeneic genomic libraries by circumventing restriction-modification systems that restrict methylated DNA.

An efficient method to construct xenogeneic genomic libraries with low errors and bias by circumventing restriction-modification systems that restrict methylated DNA was developed. Un-methylated genomic DNA of Escherichia coli prepared by ϕ29 DNA polymerase was introduced to Corynebacterium glutamicum R after ligation with un-methylated vector plasmids.

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

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

Engineering the glycolytic pathway: A potential approach for improvement of biocatalyst performance.

The glycolytic pathway is a main driving force in the fermentation process as it produces energy, cell component precursors, and fermentation products. Given its importance, the glycolytic pathway can be considered as an attractive target for the metabolic engineering of industrial microorganisms. However, many attempts to enhance glycolytic flux, by overexpressing homologous or heterologous genes encoding glycolytic enzymes, have been unsuccessful. In contrast, significant enhancement in glycolytic flux has been observed in studies with bacteria, specifically, Corynebacterium glutamicum. Although there has been a recent increase in the number of successful applications of this technology, little is known about the mechanisms leading to the enhancement of glycolytic flux. To explore the rational applications of glycolytic pathway engineering in biocatalyst development, this review summarizes recent successful studies as well as past attempts.

Thermal and solvent stress cross-tolerance conferred to Corynebacterium glutamicum by adaptive laboratory evolution.

Reinforcing microbial thermotolerance is a strategy to enable fermentation with flexible temperature settings and thereby to save cooling costs. Here, we report on adaptive laboratory evolution (ALE) of the amino acid-producing bacterium Corynebacterium glutamicum under thermal stress. After 65 days of serial passage of the transgenic strain GLY3, in which the glycolytic pathway is optimized for alanine production under oxygen deprivation, three strains adapted to supraoptimal temperatures were isolated, and all the mutations they acquired were identified by whole-genome resequencing. Of the 21 mutations common to the three strains, one large deletion and two missense mutations were found to promote growth of the parental strain under thermal stress. Additive effects on thermotolerance were observed among these mutations, and the combination of the deletion with the missense mutation on otsA, encoding a trehalose-6-phosphate synthase, allowed the parental strain to overcome the upper limit of growth temperature. Surprisingly, the three evolved strains acquired cross-tolerance for isobutanol, which turned out to be partly attributable to the genomic deletion associated with the enhanced thermotolerance. The deletion involved loss of two transgenes, pfk and pyk, encoding the glycolytic enzymes, in addition to six native genes, and elimination of the transgenes, but not the native genes, was shown to account for the positive effects on thermal and solvent stress tolerance, implying a link between energy-producing metabolism and bacterial stress tolerance. Overall, the present study provides evidence that ALE can be a powerful tool to refine the phenotype of C. glutamicum and to investigate the molecular bases of stress tolerance.

Metabolic engineering for improved production of ethanol by Corynebacterium glutamicum.

Recombinant Corynebacterium glutamicum harboring genes for pyruvate decarboxylase (pdc) and alcohol dehydrogenase (adhB) can produce ethanol under oxygen deprivation. We investigated the effects of elevating the expression levels of glycolytic genes, as well as pdc and adhB, on ethanol production. Overexpression of four glycolytic genes (pgi, pfkA, gapA, and pyk) in C. glutamicum significantly increased the rate of ethanol production. Overexpression of tpi, encoding triosephosphate isomerase, further enhanced productivity. Elevated expression of pdc and adhB increased ethanol yield, but not the rate of production. Fed-batch fermentation using an optimized strain resulted in ethanol production of 119 g/L from 245 g/L glucose with a yield of 95% of the theoretical maximum. Further metabolic engineering, including integration of the genes for xylose and arabinose metabolism, enabled consumption of glucose, xylose, and arabinose, and ethanol production (83 g/L) at a yield of 90 %. This study demonstrated that C. glutamicum has significant potential for the production of cellulosic ethanol.

Promiscuous activity of (S,S)-butanediol dehydrogenase is responsible for glycerol production from 1,3-dihydroxyacetone in Corynebacterium glutamicum under oxygen-deprived conditions.

Corynebacterium glutamicum can consume glucose to excrete glycerol under oxygen deprivation. Although glycerol synthesis from 1,3-dihydroxyacetone (DHA) has been speculated, no direct evidence has yet been provided in C. glutamicum. Enzymatic and genetic investigations here indicate that the glycerol is largely produced from DHA and, unexpectedly, the reaction is catalyzed by (S,S)-butanediol dehydrogenase (ButA) that inherently catalyzes the interconversion between S-acetoin and (S,S)-2,3-butanediol. Consequently, the following pathway for glycerol biosynthesis in the bacterium emerges: dihydroxyacetone phosphate is dephosphorylated by HdpA to DHA, which is subsequently reduced to glycerol by ButA. This study emphasizes the importance of promiscuous activity of the enzyme in vivo.

Engineering of Corynebacterium glutamicum for high-yield L-valine production under oxygen deprivation conditions.

We previously demonstrated efficient L-valine production by metabolically engineered Corynebacterium glutamicum under oxygen deprivation. To achieve the high productivity, a NADH/NADPH cofactor imbalance during the synthesis of l-valine was overcome by engineering NAD-preferring mutant acetohydroxy acid isomeroreductase (AHAIR) and using NAD-specific leucine dehydrogenase from Lysinibacillus sphaericus. Lactate as a by-product was largely eliminated by disrupting the lactate dehydrogenase gene ldhA. Nonetheless, a few other by-products, particularly succinate, were still produced and acted to suppress the L-valine yield. Eliminating these by-products therefore was deemed key to improving theL-valine yield. By additionally disrupting the phosphoenolpyruvate carboxylase gene ppc, succinate production was effectively suppressed, but both glucose consumption and L-valine production dropped considerably due to the severely elevated intracellular NADH/NAD(+) ratio. In contrast, this perturbed intracellular redox state was more than compensated for by deletion of three genes associated with NADH-producing acetate synthesis and overexpression of five glycolytic genes, including gapA, encoding NADH-inhibited glyceraldehyde-3-phosphate dehydrogenase. Inserting feedback-resistant mutant acetohydroxy acid synthase and NAD-preferring mutant AHAIR in the chromosome resulted in higher L-valine yield and productivity. Deleting the alanine transaminase gene avtA suppressed alanine production. The resultant strain produced 1,280 mM L-valine at a yield of 88% mol mol of glucose(-1) after 24 h under oxygen deprivation, a vastly improved yield over our previous best.

Identification of a HAD superfamily phosphatase, HdpA, involved in 1,3-dihydroxyacetone production during sugar catabolism in Corynebacterium glutamicum.

Corynebacterium glutamicum produces 1,3-dihydroxyacetone (DHA) as metabolite of sugar catabolism but the responsible enzyme is yet to be identified. Using a transposon mutant library, the gene hdpA (cgR_2128) was shown to encode a haloacid dehalogenase superfamily member that catalyzes dephosphorylation of dihydroxyacetone phosphate to produce DHA. Inactivation of hdpA led to a drastic decrease in DHA production from each of glucose, fructose, and sucrose, indicating that HdpA is the main enzyme responsible for DHA production from sugars in C. glutamicum. Confirmation of DHA production via dihydroxyacetone phosphatase finally confirms a long-speculated route through which bacteria produce DHA.

Overexpression of genes encoding glycolytic enzymes in Corynebacterium glutamicum enhances glucose metabolism and alanine production under oxygen deprivation conditions.

We previously reported that Corynebacterium glutamicum strain ΔldhAΔppc+alaD+gapA, overexpressing glyceraldehyde-3-phosphate dehydrogenase-encoding gapA, shows significantly improved glucose consumption and alanine formation under oxygen deprivation conditions (T. Jojima, M. Fujii, E. Mori, M. Inui, and H. Yukawa, Appl. Microbiol. Biotechnol. 87:159-165, 2010). In this study, we employ stepwise overexpression and chromosomal integration of a total of four genes encoding glycolytic enzymes (herein referred to as glycolytic genes) to demonstrate further successive improvements in C. glutamicum glucose metabolism under oxygen deprivation. In addition to gapA, overexpressing pyruvate kinase-encoding pyk and phosphofructokinase-encoding pfk enabled strain GLY2/pCRD500 to realize respective 13% and 20% improved rates of glucose consumption and alanine formation compared to GLY1/pCRD500. Subsequent overexpression of glucose-6-phosphate isomerase-encoding gpi in strain GLY3/pCRD500 further improved its glucose metabolism. Notably, both alanine productivity and yield increased after each overexpression step. After 48 h of incubation, GLY3/pCRD500 produced 2,430 mM alanine at a yield of 91.8%. This was 6.4-fold higher productivity than that of the wild-type strain. Intracellular metabolite analysis showed that gapA overexpression led to a decreased concentration of metabolites upstream of glyceraldehyde-3-phosphate dehydrogenase, suggesting that the overexpression resolved a bottleneck in glycolysis. Changing ratios of the extracellular metabolites by overexpression of glycolytic genes resulted in reduction of the intracellular NADH/NAD(+) ratio, which also plays an important role on the improvement of glucose consumption. Enhanced alanine dehydrogenase activity using a high-copy-number plasmid further accelerated the overall alanine productivity. Increase in glycolytic enzyme activities is a promising approach to make drastic progress in growth-arrested bioprocesses.

Improvement of the redox balance increases L-valine production by Corynebacterium glutamicum under oxygen deprivation conditions.

Production of L-valine under oxygen deprivation conditions by Corynebacterium glutamicum lacking the lactate dehydrogenase gene ldhA and overexpressing the L-valine biosynthesis genes ilvBNCDE was repressed. This was attributed to imbalanced cofactor production and consumption in the overall L-valine synthesis pathway: two moles of NADH was generated and two moles of NADPH was consumed per mole of L-valine produced from one mole of glucose. In order to solve this cofactor imbalance, the coenzyme requirement for L-valine synthesis was converted from NADPH to NADH via modification of acetohydroxy acid isomeroreductase encoded by ilvC and introduction of Lysinibacillus sphaericus leucine dehydrogenase in place of endogenous transaminase B, encoded by ilvE. The intracellular NADH/NAD(+) ratio significantly decreased, and glucose consumption and L-valine production drastically improved. Moreover, L-valine yield increased and succinate formation decreased concomitantly with the decreased intracellular redox state. These observations suggest that the intracellular NADH/NAD(+) ratio, i.e., reoxidation of NADH, is the primary rate-limiting factor for L-valine production under oxygen deprivation conditions. The L-valine productivity and yield were even better and by-products derived from pyruvate further decreased as a result of a feedback resistance-inducing mutation in the acetohydroxy acid synthase encoded by ilvBN. The resultant strain produced 1,470 mM L-valine after 24 h with a yield of 0.63 mol mol of glucose(-1), and the L-valine productivity reached 1,940 mM after 48 h.

Engineering of sugar metabolism of Corynebacterium glutamicum for production of amino acid L-alanine under oxygen deprivation.

Corynebacterium glutamicum was genetically engineered to produce L-alanine from sugar under oxygen deprivation. The genes associated with production of organic acids in C. glutamicum were inactivated and the alanine dehydrogenase gene (alaD) from Lysinibacillus sphaericus was overexpressed to direct carbon flux from organic acids to alanine. Although the alaD-expressing strain produced alanine from glucose under oxygen deprivation, its productivity was relatively low due to retarded glucose consumption. Homologous overexpression of the gapA gene encoding glyceraldehyde 3-phosphate dehydrogenase (GAPDH) in the alaD-expressing strain stimulated glucose consumption and consequently improved alanine productivity. In contrast gapA overexpression did not affect glucose consumption under aerobic conditions, indicating that oxygen deprivation engendered inefficient regeneration of NAD+ resulting in impaired GAPDH activity and reduced glucose consumption in the alanine-producing strains. Inactivation of the alanine racemase gene allowed production of L-alanine with optical purity greater than 99.5%. The resulting strain produced 98 g l(-1) of L-alanine after 32 h in mineral salts medium. Our results show promise for amino acid production under oxygen deprivation.

Sugar transporters in efficient utilization of mixed sugar substrates: current knowledge and outlook.

There is increasing interest in production of transportation fuels and commodity chemicals from lignocellulosic biomass, most desirably through biological fermentation. Considerable effort has been expended to develop efficient biocatalysts that convert sugars derived from lignocellulose directly to value-added products. Glucose, the building block of cellulose, is the most suitable fermentation substrate for industrial microorganisms such as Escherichia coli, Corynebacterium glutamicum, and Saccharomyces cerevisiae. Other sugars including xylose, arabinose, mannose, and galactose that comprise hemicellulose are generally less efficient substrates in terms of productivity and yield. Although metabolic engineering including introduction of functional pentose-metabolizing pathways into pentose-incompetent microorganisms has provided steady progress in pentose utilization, further improvements in sugar mixture utilization by microorganisms is necessary. Among a variety of issues on utilization of sugar mixtures by the microorganisms, recent studies have started to reveal the importance of sugar transporters in microbial fermentation performance. In this article, we review current knowledge on diversity and functions of sugar transporters, especially those associated with pentose uptake in microorganisms. Subsequently, we review and discuss recent studies on engineering of sugar transport as a driving force for efficient bioconversion of sugar mixtures derived from lignocellulose.

Xylitol production by recombinant Corynebacterium glutamicum under oxygen deprivation.

Wild-type Corynebacterium glutamicum produced 0.6 g l(-1) xylitol from xylose at a productivity of 0.01 g l(-1) h(-1) under oxygen deprivation. To increase this productivity, the pentose transporter gene (araE) from C. glutamicum ATCC31831 was integrated into the C. glutamicum R chromosome. Consequent disruption of its lactate dehydrogenase gene (ldhA), and expression of single-site mutant xylose reductase from Candida tenuis (CtXR (K274R)) resulted in recombinant C. glutamicum strain CtXR4 that produced 26.5 g l(-1) xylitol at 3.1 g l(-1) h(-1). To eliminate possible formation of toxic intracellular xylitol phosphate, genes encoding xylulokinase (XylB) and phosphoenolpyruvate-dependent fructose phosphotransferase (PTS(fru)) were disrupted to yield strain CtXR7. The productivity of strain CtXR7 increased 1.6-fold over that of strain CtXR4. A fed-batch 21-h CtXR7 culture in mineral salts medium under oxygen deprivation yielded 166 g l(-1) xylitol at 7.9 g l(-1) h(-1), representing the highest bacterial xylitol productivity reported to date.

Engineering of pentose transport in Corynebacterium glutamicum to improve simultaneous utilization of mixed sugars.

Corynebacterium glutamicum strains CRA1 and CRX2 are able to grow on L-arabinose and D-xylose, respectively, as sole carbon sources. Nevertheless, they exhibit the major shortcoming that their sugar consumption appreciably declines at lower concentrations of these substrates. To address this, the C. glutamicum ATCC31831 L-arabinose transporter gene, araE, was independently integrated into both strains. Unlike its parental strain, resultant CRA1-araE was able to aerobically grow at low (3.6 g.l(-1)) L-arabinose concentrations. Interestingly, strain CRX2-araE grew 2.9-fold faster than parental CRX2 at low (3.6 g.l(-1)) D-xylose concentrations. The corresponding substrate consumption rates of CRA1-araE and CRX2-araE under oxygen-deprived conditions were 2.8- and 2.7-fold, respectively, higher than those of their respective parental strains. Moreover, CRA1-araE and CRX2-araE utilized their respective substrates simultaneously with D-glucose under both aerobic and oxygen-deprived conditions. Based on these observations, a platform strain, ACX-araE, for C. glutamicum-based mixed sugar utilization was designed. It harbored araBAD for L-arabinose metabolism, xylAB for D-xylose metabolism, D-cellobiose permease-encoding bglF317A, beta-glucosidase-encoding bglA and araE in its chromosomal DNA. In mineral medium containing a sugar mixture of D-glucose, D-xylose, L-arabinose, and D-cellobiose under oxygen-deprived conditions, strain ACX-araE simultaneously and completely consumed all sugars.