Strain improvement ofArthrobacter simplex by protoplast fusion

Anl-tryptophan auxotroph and milky mutants were derived from an inducible cholesterol oxidase-producing bacterium,Arthrobacter simplex USA18, via UV-mutagenesis. Protoplasts of these mutants and a constitutive cholesterol oxidase producer, strain US3011, were prepared by growing cells in the presence of ampicillin (20g ml^–1) followed by digestion with lysozyme. Protoplast fusion between tested strains with complementary characteristics was achieved in the presence of 20–40% polyethylene glycol 6000. The fusion frequency was about 1.5–1.7×10^–3. The cholesterol oxidase activity of four fusants in a cholesterol-containing medium was 20–60% higher than that of parental strains. This study demonstrated that protoplast fusion is applicable to strain improvement ofArthrobacter strains for enzyme production.     

Characterization of salt-tolerant glutaminase from Stenotrophomonas maltophilia NYW-81 and its application in Japanese soy sauce fermentation

Glutaminase from Stenotrophomonas maltophilia NYW-81 was purified to homogeneity with a final specific activity of 325 U/mg. The molecular mass of the native enzyme was estimated to be 41 kDa by gel filtration. A subunit molecular mass of 36 kDa was measured with SDS-PAGE, thus indicating that the native enzyme is a monomer. The N-terminal amino acid sequence of the enzyme was determined to be KEAETQQKLANVVILATGGTIA. Besides l-glutamine, which was hydrolyzed with the highest specific activity (100%), l-asparagine (74%), d-glutamine (75%), and d-asparagine (67%) were also hydrolyzed. The pH and temperature optima were 9.0 and approximately 60°C, respectively. The enzyme was most stable at pH 8.0 and was highly stable (relative activities from 60 to 80%) over a wide pH range (5.0–10.0). About 70 and 50% of enzyme activity was retained even after treatment at 60 and 70°C, respectively, for 10 min. The enzyme showed high activity (86% of the original activity) in the presence of 16% NaCl. These results indicate that this enzyme has a higher salt tolerance and thermal stability than bacterial glutaminases that have been reported so far. In a model reaction of Japanese soy sauce fermentation, glutaminase from S. maltophilia exhibited high ability in the production of glutamic acid compared with glutaminases from Aspergillus oryzae, Escherichia coli, Pseudomonas citronellolis, and Micrococcus luteus, indicating that this enzyme is suitable for application in Japanese soy sauce fermentation.     

Introduction of a stress-responsive gene, yggG, enhances the yield of l-phenylalanine with decreased acetic acid production in a recombinant Escherichia coli

A stress-responsive gene, yggG, was introduced into an l-phenylalanine producer, Escherichia coli AJ12741. In shake-flask culture, the yggG-containing recombinant strain (named AJ12741/pHYGG) produced 6.4 g l-phenylalanine l⁻¹ at the end of culture and its yield on glucose was 0.16 g l-phenylalanine g glucose⁻¹. These values are much higher than those of the original AJ12741 strain (3.7 g l-phenylalanine l⁻¹ and 0.09 g l-phenylalanine g glucose⁻¹, respectively). On the other hand, AJ12741/pHYGG strain produced only 4.5 g acetic acid l⁻¹ and its yield on glucose was about a half of that of the AJ12741 culture. Analysis of gene expression revealed that in late growth phase, the expression levels of genes involved in acetic acid production (pta, ackA, and poxB) were relatively low in AJ12741/pHYGG cells. In particular, the level of poxB expression in AJ12741/pHYGG strains was one-seventh of that of the original strain. These results suggest that the formation of a bottleneck for acetic acid production brings about a metabolic flow favorable to l-phenylalanine synthesis in the recombinant strain over-expressing the yggG gene. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Enzymatic production of <Emphasis Type="SmallCaps">l</Emphasis>-amino acids from the corresponding <Emphasis Type="SmallCaps">dl</Emphasis>-5-substituted hydantoins by <Emphasis Type="Italic">Bacillus fordii</Emphasis> MH602

Bacillus fordii MH602 was newly screened from soil at 45 °C and exhibited high activities of hydantoinase and carbamoylase, efficiently yielding l-amino acids including phenylalanine, phenylglycine and tryptophan with the bioconversion yield of 60–100% from the corresponding dl-5-substituted hydantoins. Hydantoinase activity was found to be cell-associated and inducible. The optimal inducer was dl-5-methylhydantoin with concentration of 0.014 mol L⁻¹ and added to the fermentation medium in the exponential phase of growth. In the production of optically pure amino acids from dl-5-benylhydantoin, the optimal temperature and pH of this reaction were 45–50 °C and 7.5 respectively. The hydantoinase was non-stereoselective, while carmbamoylase was l-selective. The hydantoinase activity was not subject to substrate inhibition, or product inhibition by ammonia. In addition, The activities of both enzymes from crude extract of the strain were thermostable; the hydantoinase and carbamoylase retained about 90% and 60% activity after 6 h at 50 °C, respectively. Since reaction at higher temperature is advantageous for enhancement of solubility and for racemization of dl-5-substituted hydantoins, the relative paucity of l-selective hydantoinase systems, together with the high level of hydantoinase and carbamoylase activity and unusual substrate selectivity of the strain MH602, suggest that it has significant potential applications.     

Effect of pyruvate dehydrogenase complex deficiency on <Emphasis Type="SmallCaps">l</Emphasis>-lysine production with <Emphasis Type="Italic">Corynebacterium glutamicum</Emphasis>

Intracellular precursor supply is a critical factor for amino acid productivity of Corynebacterium glutamicum. To test for the effect of improved pyruvate availability on l-lysine production, we deleted the aceE gene encoding the E1p enzyme of the pyruvate dehydrogenase complex (PDHC) in the l-lysine-producer C. glutamicum DM1729 and characterised the resulting strain DM1729-BB1 for growth and l-lysine production. Compared to the host strain, C. glutamicum DM1729-BB1 showed no PDHC activity, was acetate auxotrophic and, after complete consumption of the available carbon sources glucose and acetate, showed a more than 50% lower substrate-specific biomass yield (0.14 vs 0.33 mol C/mol C), an about fourfold higher biomass-specific l-lysine yield (5.27 vs 1.23 mmol/g cell dry weight) and a more than 40% higher substrate-specific l-lysine yield (0.13 vs 0.09 mol C/mol C). Overexpression of the pyruvate carboxylase or diaminopimelate dehydrogenase genes in C. glutamicum DM1729-BB1 resulted in a further increase in the biomass-specific l-lysine yield by 6 and 56%, respectively. In addition to l-lysine, significant amounts of pyruvate, l-alanine and l-valine were produced by C. glutamicum DM1729-BB1 and its derivatives, suggesting a surplus of precursor availability and a further potential to improve l-lysine production by engineering the l-lysine biosynthetic pathway. This study is dedicated to Prof. Dr. Hermann Sahm on the occasion of his 65th birthday.     

Improvement of the cell volume ofCandida blankii through protoplast fusion

Summary The yeastCandida blankii ESP-94, capable of utilizing xylose as substrate, was isolated for the production of single-cell protein (SCP) on bagasse hydrolysates. However, the small cell volume of strain ESP-94 would complicate harvesting of the cells during a continuous fermentation process. Auxotrophic mutants of strain ESP-94 were generated and intraspecific protoplast fusion experiments performed in an attempt to increase the cell volume of strain ESP-94. The fusion products were characterised with respect to cell volume, DNA content and genetic stability. Six genetically stable fusants with bigger cell volumes and higher DNA contents were obtained. One such fusant, fusant F17, had a cell volume 3-times that of strain ESP-94, while exhibiting similar growth rates to strain ESP-94 ond-xylose as carbon source.     

Simultaneous utilization of D-cellobiose, D-glucose, and D-xylose by recombinant Corynebacterium glutamicum under oxygen-deprived conditions

Corynebacterium glutamicum R was metabolically engineered to broaden its sugar utilization range to d-xylose and d-cellobiose contained in lignocellulose hydrolysates. The resultant recombinants expressed Escherichia coli xylA and xylB genes, encoding d-xylose isomerase and xylulokinase, respectively, for d-xylose utilization and expressed C. glutamicum R bglF ^317A and bglA genes, encoding phosphoenolpyruvate:carbohydrate phosphotransferase system (PTS) β-glucoside-specific enzyme IIBCA component and phospho-β-glucosidase, respectively, for d-cellobiose utilization. The genes were fused to the non-essential genomic regions distributed around the C. glutamicum R chromosome and were under the control of their respective constitutive promoter trc and tac that permitted their expression even in the presence of d-glucose. The enzyme activities of resulting recombinants increased with the increase in the number of respective integrated genes. Maximal sugar utilization was realized with strain X5C1 harboring five xylA–xylB clusters and one bglF ^317A –bglA cluster. In both d-cellobiose and d-xylose utilization, the sugar consumption rates by genomic DNA-integrated strain were faster than those by plasmid-bearing strain, respectively. In mineral medium containing 40 g l⁻¹ d-glucose, 20 g l⁻¹ d-xylose, and 10 g l⁻¹ d-cellobiose, strain X5C1 simultaneously and completely consumed these sugars within 12 h and produced predominantly lactic and succinic acids under growth-arrested conditions.     

Increase in d-tagatose Production Rate by Site-directed Mutagenesis of l-arabinose Isomerase from Geobacillus thermodenitrificans

Among single-site mutations of l-arabinose isomerase derived from Geobacillus thermodenitrificans, two mutants were produced having the lowest and highest activities of d-tagatose production. Site-directed mutagenesis at these sites showed that the aromatic ring at amino acid 164 and the size of amino acid 475 were important for d-tagatose production. Among double-site mutations, one mutant converted d-galactose into d-tagatose with a yield of 58% whereas the wild type gave 46% d-tagatose conversion after 300 min at 65 °C. Received 31 August 2005; Revisions requested 27 September 2005; Revisions received 8 November 2005; Accepted 8 November 2005     

The alternative<Emphasis Type="SmallCaps"> d</Emphasis>-galactose degrading pathway of<Emphasis Type="Italic"> Aspergillus nidulans</Emphasis> proceeds via<Emphasis Type="SmallCaps"> l</Emphasis>-sorbose

The catabolism of d-galactose in yeast depends on the enzymes of the Leloir pathway. In contrast, Aspergillus nidulans mutants in galactokinase (galE) can still grow on d-galactose in the presence of ammonium—but not nitrate—ions as nitrogen source. A. nidulans galE mutants transiently accumulate high (400 mM) intracellular concentrations of galactitol, indicating that the alternative d-galactose degrading pathway may proceed via this intermediate. The enzyme degrading galactitol was identified as l-arabitol dehydrogenase, because an A. nidulans loss-of-function mutant in this enzyme (araA1) did not show NAD⁺-dependent galactitol dehydrogenase activity, still accumulated galactitol but was unable to catabolize it thereafter, and a double galE/araA1 mutant was unable to grow on d-galactose or galactitol. The product of galactitol oxidation was identified as l-sorbose, which is a substrate for hexokinase, as evidenced by a loss of l-sorbose phosphorylating activity in an A. nidulans hexokinase (frA1) mutant. l-Sorbose catabolism involves a hexokinase step, indicated by the inability of the frA1 mutant to grow on galactitol or l-sorbose, and by the fact that a galE/frA1 double mutant of A. nidulans was unable to grow on d-galactose. The results therefore provide evidence for an alternative pathway of d-galactose catabolism in A. nidulans that involves reduction of the d-galactose to galactitol and NAD⁺-dependent oxidation of galactitol by l-arabitol dehydrogenase to l-sorbose.     

Production of <Emphasis Type="SmallCaps">d</Emphasis>-lactic acid by <Emphasis Type="Italic">Corynebacterium glutamicum</Emphasis> under oxygen deprivation

In mineral salts medium under oxygen deprivation, Corynebacterium glutamicum exhibits high productivity of l-lactic acid accompanied with succinic and acetic acids. In taking advantage of this elevated productivity, C. glutamicum was genetically modified to produce d-lactic acid. The modification involved expression of fermentative d-lactate dehydrogenase (d-LDH)-encoding genes from Escherichia coli and Lactobacillus delbrueckii in l-lactate dehydrogenase (l-LDH)-encoding ldhA-null C. glutamicum mutants to yield strains C. glutamicum ΔldhA/pCRB201 and C. glutamicum ΔldhA/pCRB204, respectively. The productivity of C. glutamicum ΔldhA/pCRB204 was fivefold higher than that of C. glutamicum ΔldhA/pCRB201. By using C. glutamicum ΔldhA/pCRB204 cells packed to a high density in mineral salts medium, up to 1,336 mM (120 g l⁻¹) of d-lactic acid of greater than 99.9% optical purity was produced within 30 h.     

Biochemical and genetic characterization of l-glutamate transport and utilization in Salmonella typhimurium LT-2 mutants

Two systems for l-glutamate transport were found in Salmonella typhimurium LT-2 GltU⁺ (glutamate utilization) mutants. The first one is similar to the glt system previously described in Escherichia coli; by transductional analysis the structural gene, gltS, coding for the transport protein was located at minute 80 of the chromosome as part of the operon gltC-gltS, and its regulator, the gltR gene, near minute 90; the gltS gene product transports both l-glutamate and l-aspartate, is sodium independent, and is -hydroxyaspartate sensitive. The second transport system, whose structural gene was called gltF and is located at minute 0, was l-glutamate specific, sodium independent, and -methylglutamate sensitive. Two aspartase activities occurred in S. typhimurium LT-2: the first one was present only in the GltU⁺ mutants, had a pH 6.4 optimum, was essential for both l-glutamate and l-aspartate metabolism, and mapped at minute 94, close to the ampC gene. The second one had a pH 7.2 optimum, could be induced by several amino acids, and thus may have a general role in nitrogen metabolism.     

Probing the antigenicity of E. coli l-asparaginase by mutational analysis

A strategy, termed alanine-scanning mutagenesis, was used to identify the amino acid residues which are critical to the antigenicity of Escherichia coli l-asparaginase (l-ASP). Three continuous alkaline residues, 195RKH197, were mutated to Ala selectively. Four mutant recombinant l-ASPs were constructed and expressed in E. coli, and then purified. The purified mutants showed a single band by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and were more than 95% pure by reverse high-perfomance liquid chromatography. The activities of wild-type and m l-ASPs in the fermentative medium were all about 130 U/mL. The change from 195RKH 197 to 195AAA 197 reduced the antigenicity ofhe enzyme greatly as shown in competition enzyme-linked immunosorbent assay using polyclonal antibodies raised against the wild-type l-ASP from rabbits. The results show that residues 195RKH197 of E. coli l-ASP are critical to its antigenicity. These authors contributed equally to this work.     

Effect of new rotenoid glycoside from the fruits of Amorpha fruticosa LINNE on the growth of human immune cells

A new compound, rotenoid isoflavone glycoside named, 6′-O-β-d-glucopyranosyl-12a-hydroxydalpanol was isolated from the methanolic (MeOH) fruit extract of Amorpha fruticosa LINNE by means of multi-stage column chromatography. Immuno-modulatory activities of this new glycoside were compared with the partitioned fractions of Amorpha fruticosa LINNE. Both of the fractions and purified single compound showed a 19% relatively low cytotoxicity at a maximum concentration of 1.0 g/L in a cultivated normal human lung cell line (HEL299). The purified single compound showed less cytotoxicity than the crude extracts, possibly because residual toxicants were eliminated during purification processes. Cell growth of human T cells was increased by about 15% by adding 0.5 g/L of the fractions compared to the control. Specific production rates of interleukin-6 (IL-6) and tumor necrosis factor (TNF-α) from T cell were higher as 1.16 × 10⁻⁴ and 1.86 × 10⁻⁴ pg/cell, respectively, in the purified compound, compared to 1.38 × 10⁻⁴ and 2.22 × 10⁻⁴ pg/cell, respectively, by adding 0.5 g/L of the dichloromethane fraction. Natural killer cell-92MI (NK-92MI) growth supplemented with the supernatant of human T cell was up to 19% higher with the dichloromethane fraction compared with a new single compound at a concentration of 0.5 g/L. Overall, the dichloromethane fraction showed relatively higher immuno-modulatory activities compared with a new single compound, probably due to the synergic effect given by other substances existing in the fractions.     

N2-Succinylornithine in ornithine catabolism of Pseudomonas aeruginosa

Most Pseudomonas aeruginosa PAO mutants which were unable to utilize l-arginine as the sole carbon and nitrogen source (aru mutants) under aerobic conditions were also affected in l-ornithine utilization. These aru mutants were impaired in one or several enzymes involved in the conversion of N²-succinylornithine to glutamate and succinate, indicating that the latter steps of the arginine succinyltransferase pathway can be used for ornithine catabolism. Addition of aminooxyacetate, an inhibitor of the N²-succinylornithine 5-aminotransferase, to resting cells of P. aeruginosa in ornithine medium led to the accumulation of N²-succinylornithine. In crude extracts of P. aeruginosa an ornithine succinyltransferase (l-ornithine:succinyl-CoA N²-succinyltransferase) activity could be detected. An aru mutant having reduced arginine succinyltransferase activity also had correspondingly low levels of ornithine succinyltransferase. Thus, in P. aeruginosa, these two activities might be due to the same enzyme, which initiates aerobic arginine and ornithine catabolism.Abbreviations OAT ornithine 5-aminotransferase - SOAT N²-succinylornithine 5-aminotransferase - Oru ornithine utilization - Aru arginine utilization     

The three tricarboxylate synthase activities of <Emphasis Type="Italic">Corynebacterium glutamicum</Emphasis> and increase of <Emphasis Type="SmallCaps">l</Emphasis>-lysine synthesis

Corynebacterium glutamicum owns a citrate synthase and two methylcitrate synthases. Characterization of the isolated enzymes showed that the two methylcitrate synthases have comparable catalytic efficiency, k _cat/K _m, as the citrate synthase with acetyl-CoA as substrate, although these enzymes are only synthesized during growth on propionate-containing media. Thus, the methylcitrate synthases have a relaxed substrate specifity, as also demonstrated by their activity with butyryl-CoA, whereas the citrate synthase does not accept acyl donors other than acetyl-CoA. A double mutant deleted of the citrate synthase gene gltA and one of the methylcitrate synthase genes, prpC1, was made unable to grow on glucose. From this mutant, a collection of suppressor mutants could be isolated which were demonstrated to have regained citrate synthase activity due to the relaxed specificity of the methylcitrate synthase PrpC2. Molecular characterization of these mutants showed that the regulator PrpR (Cg0800) located downstream of prpC1 is mutated with mutations likely to effect the secondary structure of the regulator, thus, resulting in expression of prpC2. This expression results in a citrate synthase activity, which is lower than that due to gltA in the original strain and results in increased l-lysine accumulation.     

Isolation of <Emphasis Type="Italic">Aspergillus oryzae</Emphasis> mutants for heterologous protein production from a double proteinase gene disruptant

The recombinant Pichia pastoris harboring an improved methionine adenosyltransferase (MAT) shuffled gene was employed to biosynthesize S-adenosyl-l-methionine (SAM). Two l-methionine (l-Met) addition strategies were used to supply the precursor: the batch addition strategy (l-Met was added separately at three time points) and the continuous feeding strategies (l-Met was fed continuously at the rate of 0.1, 0.2, and 0.5 g l⁻¹ h⁻¹, respectively). SAM accumulation, l-Met conversion rate, and SAM productivity with the continuous feeding strategies were all improved over the batch addition strategy, which reached 8.46 ± 0.31 g l⁻¹, 41.7 ± 1.4%, and 0.18 ± 0.01 g l⁻¹ h⁻¹ with the best continuous feeding strategy (0.2 g l⁻¹ h⁻¹), respectively. The bottleneck for SAM production with the low l-Met feeding rate (0.1 g L⁻¹ h⁻¹) was the insufficient l-Met supply. The analysis of the key enzyme activities indicated that the tricarboxylic acid cycle and glycolytic pathway were reduced with the increasing l-Met feeding rate, which decreased the adenosine triphosphate (ATP) synthesis. The MAT activity also decreased as the l-Met feeding rate rose. The reduced ATP synthesis and MAT activity were probably the reason for the low SAM accumulation when the l-Met feeding rate reached 0.5 g l⁻¹ h⁻¹.     

Synthesis of <Emphasis Type="SmallCaps">dl</Emphasis>-tryptophan by modified broad specificity amino acid racemase from <Emphasis Type="Italic">Pseudomonas putida</Emphasis> IFO 12996

Broad specificity amino acid racemase (E.C. 5.1.1.10) from Pseudomonas putida IFO 12996 (BAR) is a unique racemase because of its broad substrate specificity. BAR has been considered as a possible catalyst which directly converts inexpensive l-amino acids to dl-amino acid racemates. The gene encoding BAR was cloned to utilize BAR for the synthesis of d-amino acids, especially d-Trp which is an important intermediate of pharmaceuticals. The substrate specificity of cloned BAR covered all of the standard amino acids; however, the activity toward Trp was low. Then, we performed random mutagenesis on bar to obtain mutant BAR derivatives with high activity for Trp. Five positive mutants were isolated after the two-step screening of the randomly mutated BAR. After the determination of the amino acid substitutions in these mutants, it was suggested that the substitutions at Y396 and I384 increased the Trp specific racemization activity and the racemization activity for overall amino acids, respectively. Among the positive mutants, I384M mutant BAR showed the highest activity for Trp. l-Trp (20 mM) was successfully racemized, and the proportion of d-Trp was reached 43% using I384M mutant BAR, while wild-type BAR racemized only 6% of initial l-Trp.     

d-Mannitol dehydrogenase and d-mannitol-1-phosphate dehydrogenase in Platymonas subcordiformis: some characteristics and their role in osmotic adaptation

d-Mannitol-1-phosphate dehydrogenase (EC 1.1.1.17) and d-mannitol dehydrogenase (EC 1.1.1.67) were estimated in a cell-free extract of the unicellular alga Platymonas subcordiformis Hazen (Prasinophyceae), d-Mannitol dehydrogenase had two activity maxima at pH 7.0 and 9.5, and a substrate specifity for d-fructose and NADH or for d-mannitol and NAD⁺. The K _m values were 43 mM for d-fructose and 10 mM for d-mannitol. d-Mannitol-1-phosphate dehydrogenase had a maximum activity at pH 7.5 and was specific for d-fructose 6-phosphate and NADH. The K _m value for d-fructose 6-phosphate was 5.5 mM. The reverse reaction with d-mannitol 1-phosphate as substrate could not be detected in the extract. After the addition of NaCl (up to 800 mM) to the enzyme assay, the activity of d-mannitol dehydrogenase was strongly inhibited while the activity of d-mannitol-1-phosphate dehydrogenase was enhanced. Under salt stress the K _m values of the d-mannitol dehydrogenase were shifted to higher values. The K _m value for d-fructose 6-phosphate as substrate for d-mannitol-1-phosphate dehydrogenase remained constant. Hence, it is concluded that in Platymonas the d-mannitol pool is derectly regulated via alternative pathways with different activities dependent on the osmotic pressure.Abbreviations Fru6P d-fructose 6-phosphate - Mes 2-(N-morpholino)ethanesulfonic acid - MT-DH d-mannitol-dehydrogenase - MT1P-DH d-mannitol-1-phosphate dehydrogenase - Pipes 1,4-piperazinediethanesulfonic acid - Tris 2-amino-2-(hydroxymethyl)-1,3-propanediol     

Pathways of d-fructose catabolism in species of Pseudomonas

Cell-free extracts of d-fructose grown cells of Pseudomonas putida, P. fluorescens, P. aeruginosa, P. stutzeri, P. mendocina, P. acidovorans and P. maltophila catalyzed a P-enolpyruvate-dependent phosphorylation of d-fructose and contained 1-P-fructokinase activity suggesting that in these species fructuse-1-P and fructose-1,6-P₂ were intermediates of d-fructose catabolism. Neither the 1-P-fructokinase nor the activity catalyzing a P-enolpyruvate-dependent phosphorylation of d-fructose was present in significant amounts in succinate-grown cells indicating that both activities were inducible. Cell-free extracts also contained activities of fructose-1,6-P₂ aldolase, fructose-1,6-P₂ phosphatase, and P-hexose isomerase which could convert fructose-1,6-P₂ to intermediates of either the Embden-Meyerhof pathway or Entner-Doudoroff pathway. Radiolabeling experiments with 1-¹⁴C-d-fructose suggested that in P. putida, P. aeruginosa, P. stutzeri, and P. acidovorans most of the alanine was made via the Entner-Doudoroff pathway with a minor portion being made via the Embden-meyerhof pathway. An edd ^- mutant of P. putida which lacked a functional Entner-Doudoroff pathway but was able to grow on d-fructose appeared to make alanine solely via the Embden-Meyerhof pathway.Non-Standard Abbreviations cpm counts per min - edd ^- mutant lacking Entner-Doudoroff dehydrase (6-PGA dehydrase) - EDP Entner-Doudoroff pathway - EMP Embden-Meyerhof pathway - FDP fructose-1,6-P₂ - FDPase FDP phosphatase - F-1-P fructose-1-P - F-6-P fructose-6-P - FPTs PEP: d-fructose phosphotransferase system - G-6-P glucose-6-P - KDPG 2-keto-3-deoxy-6-P-gluconate - PEP P-enolpyruvate - 1-PFK 1-P-fructokinase - 6-PFK 6-P-fructokinase - 6-PGA 6-P-gluconate     

Cloning,sequencing, overexpression and characterization of <Emphasis Type="SmallCaps">l</Emphasis>-rhamnose isomerase from <Emphasis Type="Italic">Bacillus pallidus</Emphasis> Y25 for rare sugar production

The l-rhamnose isomerase gene (L -rhi) encoding for l-rhamnose isomerase (l-RhI) from Bacillus pallidus Y25, a facultative thermophilic bacterium, was cloned and overexpressed in Escherichia coli with a cooperation of the 6×His sequence at a C-terminal of the protein. The open reading frame of L -rhi consisted of 1,236 nucleotides encoding 412 amino acid residues with a calculated molecular mass of 47,636 Da, showing a good agreement with the native enzyme. Mass-produced l-RhI was achieved in a large quantity (470 mg/l broth) as a soluble protein. The recombinant enzyme was purified to homogeneity by a single step purification using a Ni-NTA affinity column chromatography. The purified recombinant l-RhI exhibited maximum activity at 65°C (pH 7.0) under assay conditions, while 90% of the initial enzyme activity could be retained after incubation at 60°C for 60 min. The apparent affinity (K _m) and catalytic efficiency (k _cat/K _m) for l-rhamnose (at 65°C) were 4.89 mM and 8.36 × 10⁵ M⁻¹ min⁻¹, respectively. The enzyme demonstrated relatively low levels of amino acid sequence similarity (42 and 12%), higher thermostability, and different substrate specificity to those of E. coli and Pseudomonas stutzeri, respectively. The enzyme has a good catalyzing activity at 50°C, for d-allose, l-mannose, d-ribulose, and l-talose from d-psicose, l-fructose, d-ribose and l-tagatose with a conversion yield of 35, 25, 16 and 10%, respectively, without a contamination of by-products. These findings indicated that the recombinant l-RhI from B. pallidus is appropriate for use as a new source of rare sugar producing enzyme on a mass scale production.