Molecular genetic characterization of the L-lactate dehydrogenase gene (ldhL) of Lactobacillus helveticus and biochemical characterization of the enzyme

Engineering the methylotrophic yeast Ogataea polymorpha for lactate production from methanol

Introduction: Lactate has gained increasing attention as a platform chemical, particularly for the production of the bioplastic poly-lactic acid (PLA). While current microbial lactate production processes primarily rely on the use of sugars as carbon sources, it is possible to envision a future where lactate can be produced from sustainable, non-food substrates. Methanol could be such a potential substrate, as it can be produced by (electro)chemical hydrogenation from CO₂. Methods: In this study, the use of the methylotrophic yeast Ogataea polymorpha as a host organism for lactate production from methanol was explored. To enable lactate production in Ogataea polymorpha, four different lactate dehydrogenases were expressed under the control of the methanol-inducible MOX promoter. The L-lactate dehydrogenase of Lactobacillus helveticus performed well in the yeast, and the lactate production of this engineered strain could additionally be improved by conducting methanol fed-batch experiments in shake flasks. Further, the impact of different nitrogen sources and the resulting pH levels on production was examined more closely. In order to increase methanol assimilation of the lactate-producing strain, an adaptive laboratory evolution experiment was performed. Results and Discussion: The growth rate of the lactate-producing strain on methanol was increased by 55%, while at the same time lactate production was preserved. The highest lactate titer of 3.8 g/L in this study was obtained by cultivating this evolved strain in a methanol fed-batch experiment in shake flasks with urea as nitrogen source. This study provides a proof of principle that Ogataea polymorpha is a suitable host organism for the production of lactate using methanol as carbon source. In addition, it offers guidance for the engineering of methylotrophic organisms that produce platform chemicals from CO₂-derived substrates. With reduced land use, this technology will promote the development of a sustainable industrial biotechnology in the future.

Genome and fermentation analyses of Enterococcus faecalis DB-5 isolated from Japanese Mandarin orange: An assessment of potential application in lactic acid production

Enterococcus faecalis strain DB-5 is a lactic acid bacterium newly isolated from the Japanese mandarin orange (mikan). The DB-5 strain produces organic acid from various carbohydrate sources including glycerol and starch. To gain deeper insights into its potential application in lactic acid fermentation (LAF), the genome and fermentation analyses of E. faecalis DB-5 were performed. Whole genome sequencing was carried out using the DNBSEQ platform. After trimming and assembly, the total size of the assembled genome was revealed to be 3,048,630 bp, distributed into 63 contigs with an N₅₀ value of 203,673. The genome has 37.2% GC content, 2928 coding DNA sequences, and 54 putative RNA genes. The DB-5 strain harbored two l-lactate dehydrogenases (L-LDHs), both of which conserved the catalytic domain sequences. The optical purity measurement showed that strain DB-5 is homofermentative and produced only l-lactic acid (LA), which correlated with genome-based pathway analysis. To confirm its LA productivity at high temperatures, open repeated batch fermentation was performed at 45 °C using sucrose as a carbon source. The volumetric LA productivity of DB-5 was averaged at 3.66 g L^-1 h^-1 for 24 h during the 3rd to 11th fermentation cycles. E. faecalis DB-5 could efficiently convert around 94% of sucrose to LA throughout the fermentation cycles at 45 °C. These genomic characteristics and fermentation properties of E. faecalis DB-5 provide beneficial information for a deeper understanding of the functional properties of future high-temperature LAFs from biomass resources.

Recombinant production, purification, and biochemical characterization of a novel L-lactate dehydrogenase from Bacillus cereus NRC1 and inhibition study of mangiferin

Lactate dehydrogenase (LDH, EC 1.1.1.27) is one of the vital glycolytic conditions, especially during anaerobic conditions. It is a significant diagnostic, prognostic, and monitoring biomarker parameter. A 950-bp DNA fragment containing the gene (LDH) encoding LDH was amplified from Bacillus cereus NRC1. The deduced amino acid sequence reveals that B. cereus LDH (Bc-LDH) is highly homologous to the LDHs of Bacillus organisms. All LDH enzymes have a significant degree of conservation in their active site and several additional domains with unidentified functions. The gene for LDH, which catalyzes lactate synthesis, was cloned, sequenced (accession number: LC706200.1), and expressed in Escherichia coli BL21 (DE3). In this investigation, Bc-LDH was purified to homogeneity with a specific activity of 22.7 units/mg protein and a molecular weight of 35 kDa. It works optimally at pH 8.0. The purified enzyme was inhibited by FeCl2, CuCl2, ZnCl2, and NiCl, whereas CoCl2 was found to boost the activity of Bc-LDH. The molecular docking of the 3D model of the Bc-LDH structure with a natural inhibitor, mangiferin, demonstrated excellent LDH inhibition, with a free binding energy of −10.2 kcal/mol. Moreover, mangiferin is a potent Bc-LDH inhibitor that inhibits Bc-LDH competitively and has one binding site with a Ki value of 0.075 mM. The LDH-mangiferin interaction exhibits a low RMSF value (>1.5 Å), indicating a stable contact at the residues. This study will pave the way for more studies to improve the understanding of mangiferin, which could be considered an intriguing candidate for creating novel and improved LDH inhibitors.

Toward "homolactic" fermentation of glucose and xylose by engineered Saccharomyces cerevisiae harboring a kinetically efficient l-lactate dehydrogenase within pdc1-pdc5 deletion background

l-Lactic acid is an important platform chemical and its production from the lignocellulosic sugars glucose and xylose is, therefore, of high interest. Tolerance to low pH and a generally high robustness make Saccharomyces cerevisiae a promising host for l-lactic acid fermentation but strain development for effective utilization of both sugars is an unsolved problem. The herein used S. cerevisiae strain IBB10B05 incorporates a NADH-dependent pathway for oxidoreductive xylose assimilation within CEN.PK113-7D background and was additionally evolved for accelerated xylose-to-ethanol fermentation. Selecting the Plasmodium falciparum l-lactate dehydrogenase (pfLDH) for its high kinetic efficiency, strain IBB14LA1 was derived from IBB10B05 by placing the pfldh gene at the pdc1 locus under control of the pdc1 promotor. Strain IBB14LA1_5 additionally had the pdc5 gene disrupted. With both strains, continued l-lactic acid formation from glucose or xylose, each at 50 g/L, necessitated stabilization of pH. Using calcium carbonate (11 g/L), anaerobic shaken bottle fermentations at pH ≥ 5 resulted in l-lactic acid yields (Y_LA ) of 0.67 g/g glucose and 0.80 g/g xylose for strain IBB14LA1_5. Only little xylitol was formed (≤0.08 g/g) and no ethanol. In pH stabilized aerobic conversions of glucose, strain IBB14LA1_5 further showed excellent l-lactic acid productivities (1.8 g/L/h) without losses in Y_LA (0.69 g/g glucose). In strain IBB14LA1, the Y_LA was lower (≤0.18 g/g glucose; ≤0.27 g/g xylose) due to ethanol as well as xylitol formation. Therefore, this study shows that a S. cerevisiae strain originally optimized for xylose-to-ethanol fermentation was useful to implement l-lactic acid production from glucose and xylose; and with the metabolic engineering strategy applied, advance toward homolactic fermentation of both sugars was made. Biotechnol. Bioeng. 2017;114: 163-171. © 2016 Wiley Periodicals, Inc.

Colonization of the Intestinal Tract of the Polyphagous Pest Spodoptera littoralis with the GFP-Tagged Indigenous Gut Bacterium Enterococcus mundtii

The alkaline gut of Lepidopterans plays a crucial role in shaping communities of bacteria. Enterococcus mundtii has emerged as one of the predominant gut microorganisms in the gastrointestinal tract of the major agricultural pest, Spodoptera littoralis. Therefore, it was selected as a model bacterium to study its adaptation to harsh alkaline gut conditions in its host insect throughout different stages of development (larvae, pupae, adults, and eggs). To date, the mechanism of bacterial survival in insects' intestinal tract has been unknown. Therefore, we have engineered a GFP-tagged species of bacteria, E. mundtii, to track how it colonizes the intestine of S. littoralis. Three promoters of different strengths were used to control the expression of GFP in E. mundtii. The promoter ermB was the most effective, exhibiting the highest GFP fluorescence intensity, and hence was chosen as our main construct. Our data show that the engineered fluorescent bacteria survived and proliferated in the intestinal tract of the insect at all life stages for up to the second generation following ingestion.

Production of L-lactic acid by the yeast Candida sonorensis expressing heterologous bacterial and fungal lactate dehydrogenases

BACKGROUND

Polylactic acid is a renewable raw material that is increasingly used in the manufacture of bioplastics, which offers a more sustainable alternative to materials derived from fossil resources. Both lactic acid bacteria and genetically engineered yeast have been implemented in commercial scale in biotechnological production of lactic acid. In the present work, genes encoding L-lactate dehydrogenase (LDH) of Lactobacillus helveticus, Bacillus megaterium and Rhizopus oryzae were expressed in a new host organism, the non-conventional yeast Candida sonorensis, with or without the competing ethanol fermentation pathway.

RESULTS

Each LDH strain produced substantial amounts of lactate, but the properties of the heterologous LDH affected the distribution of carbon between lactate and by-products significantly, which was reflected in extra-and intracellular metabolite concentrations. Under neutralizing conditions C. sonorensis expressing L. helveticus LDH accumulated lactate up to 92 g/l at a yield of 0.94 g/g glucose, free of ethanol, in minimal medium containing 5 g/l dry cell weight. In rich medium with a final pH of 3.8, 49 g/l lactate was produced. The fermentation pathway was modified in some of the strains studied by deleting either one or both of the pyruvate decarboxylase encoding genes, PDC1 and PDC2. The deletion of both PDC genes together abolished ethanol production and did not result in significantly reduced growth characteristic to Saccharomyces cerevisiae deleted of PDC1 and PDC5.

CONCLUSIONS

We developed an organism without previous record of genetic engineering to produce L-lactic acid to a high concentration, introducing a novel host for the production of an industrially important metabolite, and opening the way for exploiting C. sonorensis in additional biotechnological applications. Comparison of metabolite production, growth, and enzyme activities in a representative set of transformed strains expressing different LDH genes in the presence and absence of a functional ethanol pathway, at neutral and low pH, generated a comprehensive picture of lactic acid production in this yeast. The findings are applicable in generation other lactic acid producing yeast, thus providing a significant contribution to the field of biotechnical production of lactic acid.

Rational improvement of the engineered isobutanol-producing Bacillus subtilis by elementary mode analysis

Background

Isobutanol is considered as a leading candidate for the replacement of current fossil fuels, and expected to be produced biotechnologically. Owing to the valuable features, Bacillus subtilis has been engineered as an isobutanol producer, whereas it needs to be further optimized for more efficient production. Since elementary mode analysis (EMA) is a powerful tool for systematical analysis of metabolic network structures and cell metabolism, it might be of great importance in the rational strain improvement.

Results

Metabolic network of the isobutanol-producing B. subtilis BSUL03 was first constructed for EMA. Considering the actual cellular physiological state, 239 elementary modes (EMs) were screened from total 11,342 EMs for potential target prediction. On this basis, lactate dehydrogenase (LDH) and pyruvate dehydrogenase complex (PDHC) were predicted as the most promising inactivation candidates according to flux flexibility analysis and intracellular flux distribution simulation. Then, the in silico designed mutants were experimentally constructed. The maximal isobutanol yield of the LDH- and PDHC-deficient strain BSUL05 reached 61% of the theoretical value to 0.36 ± 0.02 C-mol isobutanol/C-mol glucose, which was 2.3-fold of BSUL03. Moreover, this mutant produced approximately 70 % more isobutanol to the maximal titer of 5.5 ± 0.3 g/L in fed-batch fermentations.

Conclusions

EMA was employed as a guiding tool to direct rational improvement of the engineered isobutanol-producing B. subtilis. The consistency between model prediction and experimental results demonstrates the rationality and accuracy of this EMA-based approach for target identification. This network-based rational strain improvement strategy could serve as a promising concept to engineer efficient B. subtilis hosts for isobutanol, as well as other valuable products.

Characterization of the major dehydrogenase related to d-lactic acid synthesis in Leuconostoc mesenteroides subsp. mesenteroides ATCC 8293

Leuconostoc mesenteroides subsp. mesenteroides ATCC 8293 is a lactic acid bacterium that converts pyruvate mainly to d-(-)-lactic acid by using d-(-)-lactate dehydrogenase (ldhD). The aim of this study was to identify the gene responsible for d-lactic acid formation in this organism and to characterize the enzyme to facilitate the production of optically pure d-lactic acid. A genomic analysis of L. mesenteroides ATCC 8293 revealed that 7 genes encode lactate-related dehydrogenase. According to transcriptomic, proteomic, and phylogenetic analyses, LEUM_1756 was the major gene responsible for the production of d-lactic acid. The LEUM_1756 gene, of 996bp and encoding 332 amino acids (36.5kDa), was cloned and overexpressed in Escherichia coli BL21(DE3) Star from an inducible pET-21a(+) vector. The enzyme was purified by Ni-NTA column chromatography and showed a specific activity of 4450U/mg, significantly higher than those of other previously reported ldhDs. The gel permeation chromatography analysis showed that the purified enzyme exists as tetramers in solution and this was the first report among lactic acid bacteria. The pH and temperature optima were pH 8.0 and 30°C, respectively, for the pyruvate reduction reaction, and pH 11.0 and 20°C, respectively, for the lactate oxidation reaction. The K(m) kinetic parameters for pyruvate and lactate were 0.58mM and 260mM, respectively. In addition, the k(cat) values for pyruvate and lactate were 2900s(-1) and 2280s(-1), respectively. The enzyme was not inhibited by Ca(2+), Co(2+), Cu(2+), Mg(2+), Mn(2+), Na(+), or urea, but was inhibited by 1mM Zn(2+) and 1mM SDS.

Recent research on 3-phenyllactic acid, a broad-spectrum antimicrobial compound

3-Phenyllactic acid (PLA), which is an organic acid widely existing in honey and lactic acid bacteria fermented food, can be produced by many microorganisms, especially lactic acid bacteria. It was proved as an ideal antimicrobial compound with broad and effective antimicrobial activity against both bacteria and fungi. In addition, it could be used as feed additives to replace antibiotics in livestock feeds. This article presented a review of recent studies on the existing resource, antimicrobial activity, and measurement of PLA. In addition, microorganism strains and dehydrogenases producing PLA were reviewed in detail, the metabolic pathway and regulation of PLA synthesis in LAB strains were discussed, and high-level bioproduction of PLA by microorganism fermentation was also summarized.

Bioconversion of phenylpyruvate to phenyllactate: gene cloning, expression, and enzymatic characterization of D- and L1-lactate dehydrogenases from Lactobacillus plantarum SK002

Two DNA fragments containing the entire coding sequences of lactate dehydrogenase (LDH; ldhL1 and ldhD), whose enzymes have high activity for bioconversion of phenylpyruvate (PPA) to phenyllactate (PLA), were amplified from Lactobacillus plantarum SK002 using PCR. Sequencing showed open reading frames of 963 bp (ldhL1) and 999 bp (ldhD) encoding putative proteins of 320 and 332 amino acid residues, respectively. The LDH genes were cloned into an expression vector pET-22b(+) and expressed in Escherichia coli BL21(DE3). The purified recombinant L1-LDH and D-LDH had approximate (SDS-PAGE) molecular weights of 35 and 40 kDa, respectively. L1-LDH and D-LDH had PPA bioconversion specific activities of 71.06 and 215.84 U/mg with K (m) values of 3.96 and 5.4 mM, respectively. The rL1-LDH and rD-LDH showed maximum enzyme activity at 30 and 40 degrees C while both had optimum activity at pH 6.0. L1-LDH exhibited a higher pH and temperature stability than D-LDH. The results show that the his-tagged L. plantarum SK002 D- and L1-LDHs are efficient catalysts for bioconversion of PPA to PLA.

The D-2-hydroxyacid dehydrogenase incorrectly annotated PanE is the sole reduction system for branched-chain 2-keto acids in Lactococcus lactis

Hydroxyacid dehydrogenases of lactic acid bacteria, which catalyze the stereospecific reduction of branched-chain 2-keto acids to 2-hydroxyacids, are of interest in a variety of fields, including cheese flavor formation via amino acid catabolism. In this study, we used both targeted and random mutagenesis to identify the genes responsible for the reduction of 2-keto acids derived from amino acids in Lactococcus lactis. The gene panE, whose inactivation suppressed hydroxyisocaproate dehydrogenase activity, was cloned and overexpressed in Escherichia coli, and the recombinant His-tagged fusion protein was purified and characterized. The gene annotated panE was the sole gene responsible for the reduction of the 2-keto acids derived from leucine, isoleucine, and valine, while ldh, encoding L-lactate dehydrogenase, was responsible for the reduction of the 2-keto acids derived from phenylalanine and methionine. The kinetic parameters of the His-tagged PanE showed the highest catalytic efficiencies with 2-ketoisocaproate, 2-ketomethylvalerate, 2-ketoisovalerate, and benzoylformate (V(max)/K(m) ratios of 6,640, 4,180, 3,300, and 2,050 U/mg/mM, respectively), with NADH as the exclusive coenzyme. For the reverse reaction, the enzyme accepted d-2-hydroxyacids but not l-2-hydroxyacids. Although PanE showed the highest degrees of identity to putative NADP-dependent 2-ketopantoate reductases (KPRs), it did not exhibit KPR activity. Sequence homology analysis revealed that, together with the d-mandelate dehydrogenase of Enterococcus faecium and probably other putative KPRs, PanE belongs to a new family of D-2-hydroxyacid dehydrogenases which is unrelated to the well-described D-2-hydroxyisocaproate dehydrogenase family. Its probable physiological role is to regenerate the NAD(+) necessary to catabolize branched-chain amino acids, leading to the production of ATP and aroma compounds.

Efficient production of optically pure D-lactic acid from raw corn starch by using a genetically modified L-lactate dehydrogenase gene-deficient and alpha-amylase-secreting Lactobacillus plantarum strain

In order to achieve direct and efficient fermentation of optically pure D-lactic acid from raw corn starch, we constructed L-lactate dehydrogenase gene (ldhL1)-deficient Lactobacillus plantarum and introduced a plasmid encoding Streptococcus bovis 148 alpha-amylase (AmyA). The resulting strain produced only D-lactic acid from glucose and successfully expressed amyA. With the aid of secreting AmyA, direct D-lactic acid fermentation from raw corn starch was accomplished. After 48 h of fermentation, 73.2 g/liter of lactic acid was produced with a high yield (0.85 g per g of consumed sugar) and an optical purity of 99.6%. Moreover, a strain replacing the ldhL1 gene with an amyA-secreting expression cassette was constructed. Using this strain, direct D-lactic acid fermentation from raw corn starch was accomplished in the absence of selective pressure by antibiotics. This is the first report of direct D-lactic acid fermentation from raw starch.

Efficient production of L-lactic acid from xylose by Pichia stipitis

Microbial conversion of renewable raw materials to useful products is an important objective in industrial biotechnology. Pichia stipitis, a yeast that naturally ferments xylose, was genetically engineered for l-(+)-lactate production. We constructed a P. stipitis strain that expressed the l-lactate dehydrogenase (LDH) from Lactobacillus helveticus under the control of the P. stipitis fermentative ADH1 promoter. Xylose, glucose, or a mixture of the two sugars was used as the carbon source for lactate production. The constructed P. stipitis strain produced a higher level of lactate and a higher yield on xylose than on glucose. Lactate accumulated as the main product in xylose-containing medium, with 58 g/liter lactate produced from 100 g/liter xylose. Relatively efficient lactate production also occurred on glucose medium, with 41 g/liter lactate produced from 94 g/liter glucose. In the presence of both sugars, xylose and glucose were consumed simultaneously and converted predominantly to lactate. Lactate was produced at the expense of ethanol, whose production decreased to approximately 15 to 30% of the wild-type level on xylose-containing medium and to 70 to 80% of the wild-type level on glucose-containing medium. Thus, LDH competed efficiently with the ethanol pathway for pyruvate, even though the pathway from pyruvate to ethanol was intact. Our results show, for the first time, that lactate production from xylose by a yeast species is feasible and efficient. This is encouraging for further development of yeast-based bioprocesses to produce lactate from lignocellulosic raw material.

Differential activities of four Lactobacillus casei promoters during bacterial transit through the gastrointestinal tracts of human-microbiota-associated mice

In a previous study using fusion of the deregulated lactose promoter lacTp* and reporter genes, we suggested that Lactobacillus casei could initiate de novo protein synthesis during intestinal transit. In order to confirm this finding and extend it to other promoters, we adopted a reverse transcriptase quantitative PCR (RT-QPCR) approach combined with a transcriptional fusion system consisting of luciferase genes under the control of four promoters (ccpA, dlt, ldh, and lacT*) from L. casei DN-114 001. Promoter expression was monitored during cell growth, and variable luciferase activities were detected. In 3-day cultures, all the genetically modified strains survived but without exhibiting luciferase activity. Luciferase mRNA levels determined by RT-QPCR analysis (RNA/CFU) were not significant. The cultures were administered to human-microbiota-associated mice, and the feces were collected 6 h later. L. casei promoters lacTp* and ldhp initiated mRNA synthesis during gastrointestinal transit. The promoters, ccpAp and dltp, exhibited no luciferase activity, nor was de novo-synthesized luciferase mRNA detected in the feces. L. casei seems to adapt its physiology to the gastrointestinal tract environment by modulating promoter activities. The approach (fecal transcriptional analysis) described herein may, moreover, be of value in studying gene expression of transiting bacteria in human fecal specimens.

Molecular characterization of two lactate dehydrogenase genes with a novel structural organization on the genome of Lactobacillus sp. strain MONT4

Two lactate dehydrogenase (ldh) genes from Lactobacillus sp. strain MONT4 were cloned by complementation in Escherichia coli DC1368 (ldh pfl) and were sequenced. The sequence analysis revealed a novel genomic organization of the ldh genes. Subcloning of the individual ldh genes and their Northern blot analyses indicated that the genes are monocistronic.

Cloning and expression of theClostridium thermocellumL-lactate dehydrogenase gene inEscherichia coliand enzyme characterization

The structural gene for L-lactate dehydrogenase (LDH) (EC.1.1.1.27) from Clostridium thermocellum 27405 was cloned in Escherichia coli by screening the Lambda Zap II phage library of C. thermocellum genomic DNA. In one positive clone, an open reading frame of 948 base pairs corresponded to C. thermocellum ldh gene encoding for the predicted 315-residue protein. The ldh gene was successfully expressed in E. coli FMJ39 (ldh mutant) under the lac promoter. The recombinant enzyme was partially purified from E. coli cell extracts and its kinetic properties were determined. Clostridium thermocellum LDH was shown to catalyze a highly reversible reaction and to be an allosteric enzyme that is activated by fructose-1,6-diphosphate (FDP). For pyruvate, partially purified LDH had Kmand Vmaxvalues of 7.3 mmol/L and 87 µmol/min, respectively, and in the presence of FDP, a 24-fold decrease in Kmand a 5.7-fold increase in Vmaxwere recorded. The enzyme exhibited no marked catalytic activity for lactate in the absence of FDP, whereas Kmand Vmaxvalues were 59.5 mmol/L and 52 µmol/min, respectively, in its presence. The enzyme did not lose activity when incubated at 65 °C for 5 min.Key words: L-lactate dehydrogenase purification, thermophilic bacteria.

Purification and characterization of a feruloyl esterase from the intestinal bacterium Lactobacillus acidophilus

Dietary ferulic acid (FA), a significant antioxidant substance, is currently the subject of extensive research. FA in cereals exists mainly as feruloylated sugar ester. To release FA from food matrices, it is necessary to cleave ester cross-linking by feruloyl esterase (FAE) (hydroxycinnamoyl esterase; EC 3.1.1.73). In the present study, the FAE from a human typical intestinal bacterium, Lactobacillus acidophilus, was isolated, purified, and characterized for the first time. The enzyme was purified in successive steps including hydrophobic interaction chromatography and anion-exchange chromatography. The purified FAE appeared as a single band in sodium dodecyl sulfate-polyacrylamide gel electrophoresis, with an apparent molecular mass of 36 kDa. It has optimum pH and temperature characteristics (5.6 and 37 degrees C, respectively). The metal ions Cu(2+) and Fe(3+) (at a concentration of 5 mmol liter(-1)) inhibited FAE activity by 97.25 and 94.80%, respectively. Under optimum pH and temperature with 5-O-feruloyl-L-arabinofuranose (FAA) as a substrate, the enzyme exhibited a K(m) of 0.0953 mmol liter(-1) and a V(max) of 86.27 mmol liter(-1) min(-1) mg(-1) of protein. Furthermore, the N-terminal amino acid sequence of the purified FAE was found to be A R V E K P R K V I L V G D G A V G S T. The FAE released FA from O-(5-O-feruloyl-alpha-L-arabinofuranosyl)-(1-->3)-O-beta-D-xylopyranosyl-(1-->4)-D-xylopyranose (FAXX) and FAA obtained from refined corn bran. Moreover, it released two times more FA from FAXX in the presence of added xylanase.

Characterisation of the enzyme activities involved in the valine biosynthetic pathway in a valine-producing strain of Corynebacterium glutamicum

The enzyme activities of the valine biosynthetic pathway and their regulation have been studied in the valine-producing strain, Corynebacterium glutamicum 13032DeltailvApJC1ilvBNCD. In this micro-organism, this pathway might involve up to five enzyme activities: acetohydroxy acid synthase (AHAS), acetohydroxy acid isomeroreductase (AHAIR), dihydroxyacid dehydratase and transaminases B and C. For each enzyme, kinetic parameters (optimal temperature, optimal pH and affinity for substrates) were determined. The first enzyme of the pathway, AHAS, was shown to exhibit a weak affinity for pyruvate (K(m)=8.3 mM). It appeared that valine and leucine inhibited the three first steps of the pathway (AHAS, AHAIR and DHAD). Moreover, the AHAS activity was inhibited by isoleucine. Considering the kinetic data collected during this work, AHAS would be a key enzyme for further strain improvement intending to increase the valine production by C. glutamicum.

IS981-mediated adaptive evolution recovers lactate production by ldhB transcription activation in a lactate dehydrogenase-deficient strain of Lactococcus lactis

Lactococcus lactis NZ9010 in which the las operon-encoded ldh gene was replaced with an erythromycin resistance gene cassette displayed a stable phenotype when grown under aerobic conditions, and its main end products of fermentation under these conditions were acetate and acetoin. However, under anaerobic conditions, the growth of these cells was strongly retarded while the main end products of fermentation were acetate and ethanol. Upon prolonged subculturing of this strain under anaerobic conditions, both the growth rate and the ability to produce lactate were recovered after a variable number of generations. This recovery was shown to be due to the transcriptional activation of a silent ldhB gene coding for an Ldh protein (LdhB) with kinetic parameters different from those of the native las operon-encoded Ldh protein. Nevertheless, cells producing LdhB produced mainly lactate as the end product of fermentation. The mechanism underlying the ldhB gene activation was primarily studied in a single-colony isolate of the recovered culture, designated L. lactis NZ9015. Integration of IS981 in the upstream region of ldhB was responsible for transcription activation of the ldhB gene by generating an IS981-derived -35 promoter region at the correct spacing with a natively present -10 region. Subsequently, analysis of 10 independently isolated lactate-producing derivatives of L. lactis NZ9010 confirmed that the ldhB gene is transcribed in all of them. Moreover, characterization of the upstream region of the ldhB gene in these derivatives indicated that site-specific and directional IS981 insertion represents the predominant mechanism of the observed recovery of the ability to produce lactate.

Metabolic engineering of Lactobacillus helveticus CNRZ32 for production of pure L-(+)-lactic acid

Expression of D-(-)-lactate dehydrogenase (D-LDH) and L-(+)-LDH genes (ldhD and ldhL, respectively) and production of D-(-)- and L-(+)-lactic acid were studied in Lactobacillus helveticus CNRZ32. In order to develop a host for production of pure L-(+)-isomer of lactic acid, two ldhD-negative L. helveticus CNRZ32 strains were constructed using gene replacement. One of the strains was constructed by deleting the promoter region of the ldhD gene, and the other was constructed by replacing the structural gene of ldhD with an additional copy of the structural gene (ldhL) of L-LDH of the same species. The resulting strains were designated GRL86 and GRL89, respectively. In strain GRL89, the second copy of the ldhL structural gene was expressed under the ldhD promoter. The two D-LDH-negative strains produced only L-(+)-lactic acid in an amount equal to the total lactate produced by the wild type. The maximum L-LDH activity was found to be 53 and 93% higher in GRL86 and GRL89, respectively, than in the wild-type strain. Furthermore, process variables for L-(+)-lactic acid production by GRL89 were optimized using statistical experimental design and response surface methodology. The temperature and pH optima were 41 degrees C and pH 5.9. At low pH, when the growth and lactic acid production are uncoupled, strain GRL89 produced approximately 20% more lactic acid than GRL86.

Disruption of the sole ldhL gene in Lactobacillus sakei prevents the production of both L- and D-lactate

A 7 kb DNA fragment was cloned from Lactobacillus sakei which contains the IdhL gene encoding the L(+)-lactate dehydrogenase (L-LDH). Analysis of the DNA sequence, Northern experiments and primer extension experiments showed that IdhL is transcribed from a single promoter, leading to a monocistronic 1.15 kb mRNA which yields the L-LDH. A stable mutant was constructed by chromosomal integration of a chloramphenicol cassette into IdhL by a double-crossover event. Both L- and D-lactate were produced by the wild-type strain whereas only residual amounts of both isomers were produced by the mutant. This demonstrates that L. sakei possesses an L-LDH producing L-lactate and a lactate racemase able to transform it to D-lactate, but is devoid of D-LDH activity. Moreover the ability to degrade L-lactate present in the medium that was observed with the mutant strain grown aerobically suggests that an L-lactate oxidase activity is also present in L. sakei.

Molecular characterization of a stress-inducible gene from Lactobacillus helveticus

A gene (htrA) coding for a stress-inducible HtrA-like protein from Lactobacillus helveticus CNRZ32 was cloned, sequenced, and characterized. The deduced amino acid sequence of the gene exhibited 30% identity with the HtrA protein from Escherichia coli; the putative catalytic triad and a PDZ domain that characterize the HtrA family of known bacterial serine proteases were also found in the sequence. Expression of the L. helveticus htrA gene in a variety of stress conditions was analyzed at the transcriptional level. The strongest induction, resulting in over an eightfold increase in the htrA transcription level, was found in growing CNRZ32 cells exposed to 4% (wt/vol) NaCl. Enhanced htrA mRNA expression was also seen in CNRZ32 cells after exposure to puromycin, ethanol, or heat. The reporter gene gusA was integrated in the Lactobacillus chromosome downstream of the htrA promoter by a double-crossover event which also interrupted the wild-type gene. The expression of gusA in the stress conditions tested was similar to that of htrA itself. In addition, the presence of an intact htrA gene facilitated growth under heat stress but not under salt stress.