Xylose isomerase from Escherichia coli. Characterization of the protein and the structural gene

Engineering xylose utilization in Cupriavidus necator for enhanced poly(3-hydroxybutyrate) production from mixed sugars

Lignocellulosic biomass is a promising renewable feedstock for biodegradable plastics like polyhydroxyalkanoates (PHAs). Cupriavidus necator, a versatile microbial host that synthesizes poly(3-hydroxybutyrate) (PHB), the most abundant type of PHA, has been studied to expand its carbon source utilization. Since C. necator NCIMB11599 cannot metabolize xylose, we developed xylose-utilizing strains by introducing synthetic xylose metabolic pathways, including the xylose isomerase, Weimberg, and Dahms pathways. Through rational and evolutionary engineering, the RXI22 and RXW62 strains were able to efficiently utilize xylose as the sole carbon source, producing 64.2 wt% (wt%) and 61.4 wt% PHB, respectively. Among the engineered strains, the xylose isomerase-based RXI22 strain demonstrated the most efficient co-fermentation performance, with a PHB content of 75.7 wt% and a yield of 0.32 (g PHB/g glucose and xylose) from mixed sugars. The strains developed in this study represent an enhanced PHA producer, offering a sustainable route for converting lignocellulosic biomass into bioplastics.

Biotechnological production of lipid and terpenoid from thraustochytrids

As fungus-like protists, thraustochytrids have been increasingly studied for their faster growth rates and high lipid content. In the 1990s, thraustochytrids were used as docosahexaenoic acid (DHA) producers for the first time. Thraustochytrids genera, such as Thraustochytrium, Schizochytrium, and Aurantiochytrium have been developed and patented as industrial strains for DHA production. The high DHA yield is attributed to its unique and efficient polyketide-like synthase (PKS) pathway. Moreover, thraustochytrids possess a completed mevalonate (MVA) pathway, so it can be used as host for terpenoid production. In order to improve strain performance, the metabolic engineering strategies have been applied to promote or disrupt intracellular metabolic pathways, such as genetic engineering and addition of chemical activators. However, it is difficult to realize industrialization only by improving strain performance. Various operation strategies were developed to enlarge the production quantities from the laboratory-scale, including two-stage cultivation strategies, scale-up technologies and bioreactor design. Moreover, an economical and effective downstream process is also an important consideration for the industrial application of thraustochytrids. Downstream costs accounts for 20-60% of the overall process costs, which represents an attractive target for increasing the cost-competitiveness of thraustochytrids, including how to improve the efficiency of lipid extraction and the further application of biomass residues. This review aims to overview the whole lipid biotechnology of thraustochytrids to provide the background information for researchers.

Engineering of xylose metabolism in Escherichia coli for the production of valuable compounds

The lignocellulosic sugar d-xylose has recently gained prominence as an inexpensive alternative substrate for the production of value-added compounds using genetically modified organisms. Among the prokaryotes, Escherichia coli has become the de facto host for the development of engineered microbial cell factories. The favored status of E. coli resulted from a century of scientific explorations leading to a deep understanding of its systems. However, there are limited literature reviews that discuss engineered E. coli as a platform for the conversion of d-xylose to any target compounds. Additionally, available critical review articles tend to focus on products rather than the host itself. This review aims to provide relevant and current information about significant advances in the metabolic engineering of d-xylose metabolism in E. coli. This focusses on unconventional and synthetic d-xylose metabolic pathways as several review articles have already discussed the engineering of native d-xylose metabolism. This paper, in particular, is essential to those who are working on engineering of d-xylose metabolism using E. coli as the host.

Production, purification and physicochemical characterization of D-xylose/glucose isomerase from Escherichia coli strain BL21

Cell lysate of Escherichia coli strain BL21 showed significant D-glucose isomerase activity. The rate of glucose conversion was increased up to 40% when cells were induced with 1% D-xylose. E. coli BL21 xylose isomerase (ECXI-BL21) was purified to homogeneity, up to 1.9-fold with overall 10.88% enzyme yield by heat shock, salting out and electro-elution. The molecular mass of ECXI-BL21 was estimated as 43.9 kDa on SDS-PAGE. pH_opt. and T_opt. of the enzyme were calculated as 7.0 and 50 °C, respectively. Activation energy (E _a) of ECXI-BL21 was 45 kJ/mol. Enzyme was stable from 30 to 55 °C and at pH range 6.0-8.0. ECXI-BL21_(holo) was activated by 10 mM magnesium (35%), 0.5 mM cobalt (20%) and manganese (25%), and 0.5/10 mM Mn²⁺/Mg²⁺ (50%) and Co²⁺/Mg²⁺ (30%) as compared to ECXI-BL21_(apo). Catalytic affinity (K _m) of ECXI-BL21 for D-glucose was calculated as 0.82 mM, while maximum velocity (V _max) of the reaction D-glucose_(aldo) ⇌ D-fructose_(keto) was 108 μmol/mg/min. D-fructose formed was identified on silica gel plate. This thermophilic enzyme, T _m = 75 °C, has great potential for high fructose syrup production used in food and soft drink industries.

Enhancing the Co-utilization of Biomass-Derived Mixed Sugars by Yeasts

Plant biomass is a promising carbon source for producing value-added chemicals, including transportation biofuels, polymer precursors, and various additives. Most engineered microbial hosts and a select group of wild-type species can metabolize mixed sugars including oligosaccharides, hexoses, and pentoses that are hydrolyzed from plant biomass. However, most of these microorganisms consume glucose preferentially to non-glucose sugars through mechanisms generally defined as carbon catabolite repression. The current lack of simultaneous mixed-sugar utilization limits achievable titers, yields, and productivities. Therefore, the development of microbial platforms capable of fermenting mixed sugars simultaneously from biomass hydrolysates is essential for economical industry-scale production, particularly for compounds with marginal profits. This review aims to summarize recent discoveries and breakthroughs in the engineering of yeast cell factories for improved mixed-sugar co-utilization based on various metabolic engineering approaches. Emphasis is placed on enhanced non-glucose utilization, discovery of novel sugar transporters free from glucose repression, native xylose-utilizing microbes, consolidated bioprocessing (CBP), improved cellulase secretion, and creation of microbial consortia for improving mixed-sugar utilization. Perspectives on the future development of biorenewables industry are provided in the end.

Engineering xylose metabolism in thraustochytrid T18

BACKGROUND

Thraustochytrids are heterotrophic, oleaginous, marine protists with a significant potential for biofuel production. High-value co-products can off-set production costs; however, the cost of raw materials, and in particular carbon, is a major challenge to developing an economical viable production process. The use of hemicellulosic carbon derived from agricultural waste, which is rich in xylose and glucose, has been proposed as a sustainable and low-cost approach. Thraustochytrid strain T18 is a commercialized environmental isolate that readily consumes glucose, attaining impressive biomass, and oil production levels. However, neither thraustochytrid growth capabilities in the presence of xylose nor a xylose metabolic pathway has been described. The aims of this study were to identify and characterize the xylose metabolism pathway of T18 and, through genetic engineering, develop a strain capable of growth on hemicellulosic sugars.

RESULTS

Characterization of T18 performance in glucose/xylose media revealed diauxic growth and copious extracellular xylitol production. Furthermore, T18 did not grow in media containing xylose as the only carbon source. We identified, cloned, and functionally characterized a xylose isomerase. Transcriptomics indicated that this xylose isomerase gene is upregulated when xylose is consumed by the cells. Over-expression of the native xylose isomerase in T18, creating strain XI 16, increased xylose consumption from 5.2 to 7.6 g/L and reduced extracellular xylitol from almost 100% to 68%. Xylose utilization efficiency of this strain was further enhanced by over-expressing a heterologous xylulose kinase to reduce extracellular xylitol to 20%. Moreover, the ability to grow in media containing xylose as a sole sugar was dependent on the copy number of both xylose isomerase and xylulose kinase present. In fed-batch fermentations, the best xylose metabolizing isolate, XI-XK 7, used 137 g of xylose versus 39 g by wild type and produced more biomass and fatty acid.

CONCLUSIONS

The presence of a typically prokaryotic xylose isomerase and xylitol production through a typically eukaryotic xylose reductase pathway in T18 is the first report of an organism naturally encoding enzymes from two native xylose metabolic pathways. Our newly engineered strains pave the way for the growth of T18 on waste hemicellulosic feedstocks for biofuel production.

Microbial conversion of xylose into useful bioproducts

Microorganisms can produce a number of different bioproducts from the sugars in plant biomass. One challenge is devising processes that utilize all of the sugars in lignocellulosic hydrolysates. D-xylose is the second most abundant sugar in these hydrolysates. The microbial conversion of D-xylose to ethanol has been studied extensively; only recently, however, has conversion to bioproducts other than ethanol been explored. Moreover, in the case of yeast, D-xylose may provide a better feedstock for the production of bioproducts other than ethanol, because the relevant pathways are not subject to glucose-dependent repression. In this review, we discuss how different microorganisms are being used to produce novel bioproducts from D-xylose. We also discuss how D-xylose could be potentially used instead of glucose for the production of value-added bioproducts.

Metabolic Engineering and Comparative Performance Studies of Synechocystis sp. PCC 6803 Strains for Effective Utilization of Xylose

Wood sugars such as xylose can be used as an inexpensive carbon source for biotechnological applications. The model cyanobacterium Synechocystis sp. PCC 6803 lacks the ability to catabolize wood sugars as an energy source. Here, we generated four Synechocystis strains that heterologously expressed XylAB enzymes, which mediate xylose catabolism, either in combination with or without one of three xylose transporters, namely XylE, GalP, or Glf. Except for glf, which is derived from the bacterium Zymomonas mobilis ZM4, the heterologous genes were sourced from Escherichia coli K-12. All of the recombinant strains were able to utilize xylose in the absence of catabolite repression. When xylose was the lone source of organic carbon, strains possessing the XylE and Glf transporters were most efficient in terms of dry biomass production and xylose consumption and the strain lacking a heterologous transporter was the least efficient. However, in the presence of a xylose-glucose mixed sugar source, the strains exhibited similar levels of growth and xylose consumption. This study demonstrates that various bacterial xylose transporters can boost xylose catabolism in transgenic Synechocystis strains, and paves the way for the sustainable production of bio-compounds and green fuels from lignocellulosic biomass.

Molecular mechanisms underlying the close association between soil Burkholderia and fungi

Bacterial species belonging to the genus Burkholderia have been repeatedly reported to be associated with fungi but the extent and specificity of these associations in soils remain undetermined. To assess whether associations between Burkholderia and fungi are widespread in soils, we performed a co-occurrence analysis in an intercontinental soil sample collection. This revealed that Burkholderia significantly co-occurred with a wide range of fungi. To analyse the molecular basis of the interaction, we selected two model fungi frequently co-occurring with Burkholderia, Alternaria alternata and Fusarium solani, and analysed the proteome changes caused by cultivation with either fungus in the widespread soil inhabitant B. glathei, whose genome we sequenced. Co-cultivation with both fungi led to very similar changes in the B. glathei proteome. Our results indicate that B. glathei significantly benefits from the interaction, which is exemplified by a lower abundance of several starvation factors that were highly expressed in pure culture. However, co-cultivation also gave rise to stress factors, as indicated by the increased expression of multidrug efflux pumps and proteins involved in oxidative stress response. Our data suggest that the ability of Burkholderia to establish a close association with fungi mainly lies in the capacities to utilize fungal-secreted metabolites and to overcome fungal defense mechanisms. This work indicates that beneficial interactions with fungi might contribute to the survival strategy of Burkholderia species in environments with sub-optimal conditions, including acidic soils.

Carbohydrate metabolism in Archaea: current insights into unusual enzymes and pathways and their regulation

The metabolism of Archaea, the third domain of life, resembles in its complexity those of Bacteria and lower Eukarya. However, this metabolic complexity in Archaea is accompanied by the absence of many "classical" pathways, particularly in central carbohydrate metabolism. Instead, Archaea are characterized by the presence of unique, modified variants of classical pathways such as the Embden-Meyerhof-Parnas (EMP) pathway and the Entner-Doudoroff (ED) pathway. The pentose phosphate pathway is only partly present (if at all), and pentose degradation also significantly differs from that known for bacterial model organisms. These modifications are accompanied by the invention of "new," unusual enzymes which cause fundamental consequences for the underlying regulatory principles, and classical allosteric regulation sites well established in Bacteria and Eukarya are lost. The aim of this review is to present the current understanding of central carbohydrate metabolic pathways and their regulation in Archaea. In order to give an overview of their complexity, pathway modifications are discussed with respect to unusual archaeal biocatalysts, their structural and mechanistic characteristics, and their regulatory properties in comparison to their classic counterparts from Bacteria and Eukarya. Furthermore, an overview focusing on hexose metabolic, i.e., glycolytic as well as gluconeogenic, pathways identified in archaeal model organisms is given. Their energy gain is discussed, and new insights into different levels of regulation that have been observed so far, including the transcript and protein levels (e.g., gene regulation, known transcription regulators, and posttranslational modification via reversible protein phosphorylation), are presented.

Structure-function studies of the staphylococcal methicillin resistance antirepressor MecR2

Methicillin resistance in Staphylococcus aureus is elicited by the MecI-MecR1-MecA axis encoded by the mec locus. Recently, MecR2 was also identified as a regulator of mec through binding of the methicillin repressor, MecI. Here we show that plasmid-encoded full-length MecR2 restores resistance in a sensitive S. aureus mecR2 deletion mutant of the resistant strain N315. The crystal structure of MecR2 reveals an N-terminal DNA-binding domain, an intermediate scaffold domain, and a C-terminal dimerization domain that contributes to oligomerization. The protein shows structural similarity to ROK (repressors, open reading frames, and kinases) family proteins, which bind DNA and/or sugar molecules. We found that functional cell-based assays of three point mutants affecting residues participating in sugar binding in ROK proteins had no effect on the resistance phenotype. By contrast, MecR2 bound short double-stranded DNA oligonucleotides nonspecifically, and a deletion mutant affecting the N-terminal DNA-binding domain showed a certain effect on activity, thus contributing to resistance less than the wild-type protein. Similarly, a deletion mutant, in which a flexible segment of intermediate scaffold domain had been replaced by four glycines, significantly reduced MecR2 function, thus indicating that this domain may likewise be required for activity. Taken together, these results provide the structural basis for the activity of a methicillin antirepressor, MecR2, which would sequester MecI away from its cognate promoter region and facilitate its degradation.

Hyperexpression of Escherichia coli Xylose Isomerase

The xylose isomerase (xylA) structural gene was cloned under the control of the tac promoter and expressed in a xyl(+) E. coli strain. Xylose isomerase accounted for approximately 28% of the total cell protein when this tac-xylA fusion was induced with isopropylthio beta-D-galactopyranoside. Hyperexpression of the xylA gene inhibited xylose utilization. E. coli carrying this tac-xylA fusion was encapsulated in calcium-alginate beads and used to isomerase xylose in a column reactor. Conversion of xylose to xylulose was 3-4% with a residence time in the column of 2 minutes and a maximum of 12% upon recycling.

Adaptation yields a highly efficient xylose-fermenting Zymomonas mobilis strain

Zymomonas mobilis is a superb ethanol producer with productivity exceeding yeast strains by several fold. Although metabolic engineering was successfully applied to expand its substrate range to include xylose, xylose fermentation lagged far behind glucose. In addition, xylose fermentation was often incomplete when its initial concentration was higher than 5%. Improvement of xylose fermentation is therefore necessary. In this work, we applied adaptation to improve xylose fermentation in metabolically engineered strains. As a result of adaptation over 80 days and 30 serial transfers in a medium containing high concentration of xylose, a strain, referred as A3, with markedly improved xylose metabolism was obtained. The strain was able to grow on 10% (w/v) xylose and rapidly ferment xylose to ethanol within 2 days and retained high ethanol yield. Similarly, in mixed glucose-xylose fermentation, a total of 9% (w/v) ethanol was obtained from two doses of 5% glucose and 5% xylose (or a total of 10% glucose and 10% xylose). Further investigation reveals evidence for an altered xylitol metabolism in A3 with reduced xylitol formation. Additionally xylitol tolerance in A3 was increased. Furthermore, xylose isomerase activity was increased by several times in A3, allowing cells to channel more xylose to ethanol than to xylitol. Taken together, these results strongly suggest that altered xylitol metabolism is key to improved xylose metabolism in adapted A3 strain. This work further demonstrates that adaptation and metabolic engineering can be used synergistically for strain improvement.

Purification and characterization of a highly thermostable glucose isomerase produced by the extremely thermophilic eubacterium, Thermotoga maritima

Thermotoga maritima, among the most thermophilic eubacteria currently known, produces glucose isomerase when grow in the presence of xylose. The purified enzyme is a homotetramer with submit molecular Wight of about 45,000. It has a number of features in common with previously described glucose isomerases-pH optimum of 6.5 to 7.5, presence of active-site histidine, requirement for metal cations such as Co(2+) and Mg(2+), and preference for xylose as substrate. In addition, it has significant sequence/structural homology with other glucose isomerases, as shown by both N-terminal sequencing and immunological crossreactivity. The T. maritima enzyme is distinguished by its extreme thermostability-a temperature optimum of 105 to 110 degrees C, and an estimated half-life of 10 minutes at 120 degrees C, pH 7.0. The high degree of thermostability, coupled with a neutral to slightly acid pH optimum, reveal this enzyme to be a promising candidate for improvement of the industrial glucose isomerization process.

Reconstruction of xylose utilization pathway and regulons in Firmicutes

Background

Many Firmicutes bacteria, including solvent-producing clostridia such as Clostridium acetobutylicum, are able to utilize xylose, an abundant carbon source in nature. Nevertheless, homology searches failed to recognize all the genes for the complete xylose and xyloside utilization pathway in most of them. Moreover, the regulatory mechanisms of xylose catabolism in many Firmicutes except Bacillus spp. still remained unclear.

Results

A comparative genomic approach was used to reconstruct the xylose and xyloside utilization pathway and analyze its regulatory mechanisms in 24 genomes of the Firmicutes. A novel xylose isomerase that is not homologous to previously characterized xylose isomerase, was identified in C. acetobutylicum and several other Clostridia species. The candidate genes for the xylulokinase, xylose transporters, and the transcriptional regulator of xylose metabolism (XylR), were unambiguously assigned in all of the analyzed species based on the analysis of conserved chromosomal gene clustering and regulons. The predicted functions of these genes in C. acetobutylicum were experimentally confirmed through a combination of genetic and biochemical techniques. XylR regulons were reconstructed by identification and comparative analysis of XylR-binding sites upstream of xylose and xyloside utilization genes. A novel XylR-binding DNA motif, which is exceptionally distinct from the DNA motif known for Bacillus XylR, was identified in three Clostridiales species and experimentally validated in C. acetobutylicum by an electrophoretic mobility shift assay.

Conclusions

This study provided comprehensive insights to the xylose catabolism and its regulation in diverse Firmicutes bacteria especially Clostridia species, and paved ways for improving xylose utilization capability in C. acetobutylicum by genetic engineering in the future.

Engineering of a xylose metabolic pathway in Corynebacterium glutamicum

The aerobic microorganism Corynebacterium glutamicum was metabolically engineered to broaden its substrate utilization range to include the pentose sugar xylose, which is commonly found in agricultural residues and other lignocellulosic biomass. We demonstrated the functionality of the corynebacterial xylB gene encoding xylulokinase and constructed two recombinant C. glutamicum strains capable of utilizing xylose by cloning the Escherichia coli gene xylA encoding xylose isomerase, either alone (strain CRX1) or in combination with the E. coli gene xylB (strain CRX2). These genes were provided on a high-copy-number plasmid and were under the control of the constitutive promoter trc derived from plasmid pTrc99A. Both recombinant strains were able to grow in mineral medium containing xylose as the sole carbon source, but strain CRX2 grew faster on xylose than strain CRX1. We previously reported the use of oxygen deprivation conditions to arrest cell replication in C. glutamicum and divert carbon source utilization towards product production rather than towards vegetative functions (M. Inui, S. Murakami, S. Okino, H. Kawaguchi, A. A. Vertès, and H. Yukawa, J. Mol. Microbiol. Biotechnol. 7:182-196, 2004). Under these conditions, strain CRX2 efficiently consumed xylose and produced predominantly lactic and succinic acids without growth. Moreover, in mineral medium containing a sugar mixture of 5% glucose and 2.5% xylose, oxygen-deprived strain CRX2 cells simultaneously consumed both sugars, demonstrating the absence of diauxic phenomena relative to the new xylA-xylB construct, albeit glucose-mediated regulation still exerted a measurable influence on xylose consumption kinetics.

Kinetic modeling to optimize pentose fermentation inZymomonas mobilis

Zymomonas mobilis engineered to express four heterologous enzymes required for xylose utilization ferments xylose along with glucose. A network of pentose phosphate (PP) pathway enzymatic reactions interacting with the native glycolytic Entner Doudoroff (ED) pathway has been hypothesized. We have investigated this putative reaction network by developing a kinetic model incorporating all of the enzymatic reactions of the PP and ED pathways, including those catalyzed by the heterologous enzymes. Starting with the experimental literature on in vitro characterization of each enzymatic reaction, we have developed a kinetic model to enable dynamic simulation of intracellular metabolite concentrations along the network of interacting PP and ED metabolic pathways. This kinetic model is useful for performing in silico simulations to predict how varying the different enzyme concentrations will affect intracellular metabolite concentrations and ethanol production rate during continuous fermentation of glucose and xylose mixtures. Among the five enzymes whose concentrations were varied as inputs to the model, ethanol production in the continuous fermentor was optimized when xylose isomerase (XI) was present at the highest level, followed by transaldolase (TAL). Predictions of the model that the interconnecting enzyme phosphoglucose isomerase (PGI) does not need to be overexpressed were recently confirmed through experimental investigations. Through such systematic analysis, we can develop efficient strategies for maximizing the fermentation of both glucose and xylose, while minimizing the expression of heterologous enzymes.

Expression of genes encoding xylose isomerases from and in the methylotrophic yeast

The thermotolerant methylotrophic yeast Hansenula polymorpha is able to ferment xylose to ethanol at high temperatures. H. polymorpha xylose reductase and xylitol dehydrogenase are involved during the first steps of this fermentation. In this article, expression of bacterial xylA genes coding for xylose isomerases from Escherichia coli or Streptomyces coelicolor in the yeast H. polymorpha was shown. The expression was achieved by integration of the xylA genes driven by the promoter of the H. polymorpha glyceraldehyde-3-phosphate dehydrogenase gene ( HpGAP) into the H. polymorpha genome. Expression of the bacterial xylose isomerase genes restored the ability of the H. polymorpha Deltaxyl1 mutant to grow in a medium with xylose as the sole carbon source. This mutant has a deletion of the XYL1 gene encoding xylose reductase and is not able to grow in the xylose medium. The H. polymorpha Deltaxyl1(xylA) transformants displayed xylose isomerase activities, which were near 20% of that of the bacterial host strain. The transformants did not differ from the yeast wild-type strain with respect to ethanol production in xylose medium.

d-Mannitol formation from d-glucose in a whole-cell biotransformation with recombinant Escherichia coli

Recently, we reported on the construction of a whole-cell biotransformation system in Escherichia coli for the production of D: -mannitol from D: -fructose. Supplementation of this strain with extracellular glucose isomerase resulted in the formation of 800 mM D: -mannitol from 1,000 mM D: -glucose. Co-expression of the xylA gene of E. coli in the biotransformation strain resulted in a D: -mannitol concentration of 420 mM from 1,000 mM D: -glucose. This is the first example of conversion of D: -glucose to D: -mannitol with direct coupling of a glucose isomerase to the biotransformation system.

Engineering a homo-ethanol pathway in Escherichia coli: increased glycolytic flux and levels of expression of glycolytic genes during xylose fermentation

Replacement of the native fermentation pathway in Escherichia coli B with a homo-ethanol pathway from Zymomonas mobilis (pdc and adhB genes) resulted in a 30 to 50% increase in growth rate and glycolytic flux during the anaerobic fermentation of xylose. Gene array analysis was used as a tool to investigate differences in expression levels for the 30 genes involved in xylose catabolism in the parent (strain B) and the engineered strain (KO11). Of the 4,290 total open reading frames, only 8% were expressed at a significantly higher level in KO11 (P < 0.05). In contrast, over half of the 30 genes involved in the catabolism of xylose to pyruvate were expressed at 1.5-fold- to 8-fold-higher levels in KO11. For 14 of the 30 genes, higher expression was statistically significant at the 95% confidence level (xylAB, xylE, xylFG, xylR, rpiA, rpiB, pfkA, fbaA, tpiA, gapA, pgk, and pykA) during active fermentation (6, 12, and 24 h). Values at single time points for only four of these genes (eno, fbaA, fbaB, and talA) were higher in strain B than in KO11. The relationship between changes in mRNA (cDNA) levels and changes in specific activities was verified for two genes (xylA and xylB) with good agreement. In KO11, expression levels and activities were threefold higher than in strain B for xylose isomerase (xylA) and twofold higher for xylulokinase (xylB). Increased expression of genes involved in xylose catabolism is proposed as the basis for the increase in growth rate and glycolytic flux in ethanologenic KO11.

Isolation and characterization of a xylose-glucose isomerase from a new strain Streptomyces thermovulgaris 127, var. 7-86

A thermostable D-xylose–glucose isomerase was isolated from the thermophilic strain Streptomyces thermovulgaris 127, var. 7-86, as a result of mutagenic treatment by γ-irradiation of the parent strain, by precipitation and sequential chromatographies on DEAE–Sephadex A50, TSK-gel, FPLC-Mono Q/HR, and Superose 12™ columns. The N-terminal amino acid sequence and amino acid analysis shows 73–92% homology with xylose–glucose isomerases from other sources. The native molecular mass, determined by gel filtration on a Superose 12™ column, is 180 kDa, and 44.6 and 45 kDa were calculated, based on amino acid analysis and 10% SDS-PAGE, respectively. Both, the activity and stability of the enzyme were investigated toward pH, temperature, and denaturation with guanidine hydrochloride. The enzyme activity showed a clear pH optimum between pH 7.2 and 9.0 with D-glucose and 7.4 and 8.3 with D-xylose as substrates, respectively. The enzyme is active up to 60–85°C at pH 7.0, using D-glucose, and up to 50–60°C at pH 7.6, using D-xylose as substrates. The activation energy (Ea = 46 kJ·mol–1) and the critical temperature (Tc = 60°C) were determined by fluorescence spectroscopy. Tc is in close coincidence with the melting temperature of denaturation (Tm = 59°C), determined by circular dichroism (CD) spectroscopy. The free energy of stabilization in water after denaturation with Gdn.HCl was calculated to be 12 kJ·mol–1. The specific activity (km values) for D-xylose-glucose isomerase at 70°C toward different substrates, D-xylose, D-glucose, and D-ribose, were determined to be 4.4, 55.5, and 13.3 mM, recpectively.Key words: D-xylose-glucose isomerase, protein sequencing, protein stability, protein denaturation.

Glucose isomerase: insights into protein engineering for increased thermostability

Thermostable glucose isomerases are desirable for production of 55% fructose syrups at >90 degrees C. Current commercial enzymes operate only at 60 degrees C to produce 45% fructose syrups. Protein engineering to construct more stable enzymes has so far been relatively unsuccessful, so this review focuses on elucidation of the thermal inactivation pathway as a future guide. The primary and tertiary structures of 11 Class 1 and 20 Class 2 enzymes are compared. Within each class the structures are almost identical and sequence differences are few. Structural differences between Class 1 and Class 2 are less than previously surmised. The thermostabilities of Class 1 enzymes are essentially identical, in contrast to previous reports, but in Class 2 they vary widely. In each class, thermal inactivation proceeds via the tetrameric apoenzyme, so metal ion affinity dominates thermostability. In Class 1 enzymes, subunit dissociation is not involved, but there is an irreversible conformational change in the apoenzyme leading to a more thermostable inactive tetramer. This may be linked to reversible conformational changes in the apoenzyme at alkaline pH arising from electrostatic repulsions in the active site, which break a buried Arg-30-Asp-299 salt bridge and bring Arg-30 to the surface. There is a different salt bridge in Class 2 enzymes, which might explain their varying thermostability. Previous protein engineering results are reviewed in light of these insights.

A bifunctional beta-xylosidase-xylose isomerase from Streptomyces sp. EC 10

beta-Xylosidase (1,4-beta-D-xylan xylohydrolase EC 3.2.1.37) and xylose isomerase (D-xylose ketol-isomerase EC 5.3.1.5) produced by Streptomyces sp. strain EC 10, were cell-bound enzymes induced by xylan, straw, and xylose. Enzyme production was subjected to a form of carbon catabolite repression by glycerol. beta-Xylosidase and xylose isomerase copurified strictly, and the preparation was found homogeneous by gel electrophoresis after successive chromatography on DEAE-Sephacel and gel filtration on Biogel A. Streptomyces sp. produced apparently a bifunctional beta-xylosidase-xylose isomerase enzyme. The molecular weight of the enzyme was measured to be 163,000 by gel filtration and 42,000 by SDS-PAGE, indicating that the enzyme behaved as a tetramer of identical subunits. The Streptomyces sp. beta-xylosidase was a typical glycosidase acting as an exoenzyme on xylooligosaccharides, and working optimally at pH 7.5 and 45 degrees C. The xylose isomerase optimal temperature was 70 degrees C and maximal activity was observed in a broad range pH (5-8). Enhanced saccharification of arabinoxylan caused by the addition of the enzyme to endoxylanase suggested a cooperative enzyme action. The first 35 amino acids of the N-terminal sequence of the enzyme showed strong analogies with N-terminal sequences of xylose isomerase produced by other microorganisms but not with other published N-terminal sequences of beta-xylosidases.

Simultaneous bioconversion of cellulose and hemicellulose to ethanol

Lignocellulosic materials containing cellulose, hemicellulose, and lignin as their main constituents are the most abundant renewable organic resource present on Earth. The conversion of both cellulose and hemicellulose for production of fuel ethanol is being studied intensively with a view to develop a technically and economically viable bioprocess. The fermentation of glucose, the main constituent of cellulose hydrolyzate, to ethanol can be carried out efficiently. On the other hand, although bioconversion of xylose, the main pentose sugar obtained on hydrolysis of hemicellulose, to ethanol presents a biochemical challenge, especially if it is present along with glucose, it needs to be fermented to make the biomass-to-ethanol process economical. A lot of attention therefore has been focussed on the utilization of both glucose and xylose to ethanol. Accordingly, while describing the advancements that have taken place to get xylose converted efficiently to ethanol by xylose-fermenting organisms, the review deals mainly with the strategies that have been put forward for bioconversion of both the sugars to achieve high ethanol concentration, yield, and productivity. The approaches, which include the use of (1) xylose-fermenting yeasts alone, (2) xylose isomerase enzyme as well as yeast, (3) immobilized enzymes and cells, and (4) sequential fermentation and co-culture process are described with respect to their underlying concepts and major limitations. Genetic improvements in the cultures have been made either to enlarge the range of substrate utilization or to channel metabolic intermediates specifically toward ethanol. These contributions represent real significant advancements in the field and have also been adequately dealt with from the point of view of their impact on utilization of both cellulose and hemicellulose sugars to ethanol.

Linkage map of Escherichia coli K-12, edition 10: the traditional map

This map is an update of the edition 9 map by Berlyn et al. (M. K. B. Berlyn, K. B. Low, and K. E. Rudd, p. 1715-1902, in F. C. Neidhardt et al., ed., Escherichia coli and Salmonella: cellular and molecular biology, 2nd ed., vol. 2, 1996). It uses coordinates established by the completed sequence, expressed as 100 minutes for the entire circular map, and adds new genes discovered and established since 1996 and eliminates those shown to correspond to other known genes. The latter are included as synonyms. An alphabetical list of genes showing map location, synonyms, the protein or RNA product of the gene, phenotypes of mutants, and reference citations is provided. In addition to genes known to correspond to gene sequences, other genes, often older, that are described by phenotype and older mapping techniques and that have not been correlated with sequences are included.

Primer design for a prokaryotic differential display RT-PCR

We have developed a primer set for a prokaryotic differential display of mRNA in the Enterobacteriaceae group. Each combination of ten 10mer and ten 11mer primers generates up to 85 bands from total Escherichia coli RNA, thus covering expressed sequences of a complete bacterial genome. Due to the lack of polyadenylation in prokaryotic RNA the type T11VN anchored oligonucleotides for the reverse transcriptase reaction had to be replaced with respect to the original method described by Liang and Pardee [ Science , 257, 967-971 (1992)]. Therefore, the sequences of both the 10mer and the new 11mer oligonucleotides were determined by a statistical evaluation of species-specific coding regions extracted from the EMBL database. The 11mer primers used for reverse transcription were selected for localization in the 3'-region of the bacterial RNA. The 10mer primers preferentially bind to the 5'-end of the RNA. None of the primers show homology to rRNA or other abundant small RNA species. Randomly sampled cDNA bands were checked for their bacterial origin either by re-amplification, cloning and sequencing or by re-amplification and direct sequencing with 10mer and 11mer primers after asymmetric PCR.

Cloning and expression of the gene for xylose isomerase from Thermus flavus AT62 in Escherichia coli

The gene encoding xylose isomerase (xylA) was cloned from Thermus flavus AT62 and the DNA sequence was determined. The xylA gene encodes the enzyme xylose isomerase (XI or xylA) consisting of 387 amino acids (calculated Mr of 44,941). Also, there was a partial xylulose kinase gene that was 4 bp overlapped in the end of XI gene. The XI gene was stably expressed in E. coli under the control of tac promoter. XI produced in E. coli was simply purified by heat treatment at 90 degrees C for 10 min and column chromatography of DEAE-Sephacel. The Mr of the purified enzyme was estimated to be 45 kDa on SDS-polyacrylamide gel electrophoresis. However, Mr of the cloned XI was 185 kDa on native condition, indicating that the XI consists of homomeric tetramer. The enzyme has an optimum temperature at 90 degrees C. Thermostability tests revealed that half life at 85 degrees C was 2 mo and 2 h at 95 degrees C. The optimum pH is around 7.0, close to where by-product formation is minimal. The isomerization yield of the cloned XI was about 55% from glucose, indicating that the yield is higher than those of reported enzymes. The K(m) values for various sugar substrates were calculated as 106 mM for glucose. Divalent cations such as Mn2+, Co2+, and Mg2+ are required for the enzyme activity and 100 mM EDTA completely inhibited the enzyme activity.

Cloning and expression of the genes for xylose isomerase and xylulokinase from Klebsiella pneumoniae 1033 in Escherichia coli K12

The genes xylA and xylB were cloned together with their promoter region from the chromosome of Klebsiella pneumoniae var. aerogenes 1033 and the DNA sequence (3225 bp) was determined. The gene xylA encodes the enzyme xylose isomerase (XI or XylA) consisting of 440 amino acids (calculated M(r) of 49,793). The gene xylB encodes the enzyme xylulokinase (XK or XylB) with a calculated M(r) of 51,783 (483 amino acids). The two genes successfully complemented xyl mutants of Escherichia coli K12, but no gene dosage effect was detected. E. coli wild-type cells which harbored plasmids with the intact xylAKp 5' upstream region in high copy number (but lacking an active xylB gene on the plasmids) were phenotypically xylose-negative and xylose isomerase and xylulokinase activities were drastically diminished. Deletion of 5' upstream regions of xylA on these plasmids and their substitution by a lac promoter resulted in a xylose-positive phenotype. This also resulted in overproduction of plasmid-encoded xylose isomerase and xylulokinase activities in recombinant E. coli cells.

Cloning and sequencing the Lactobacillus brevis gene encoding xylose isomerase

The gene (xylA) coding for the Lactobacillus brevis xylose isomerase (Xi) has been isolated and its complete nucleotide sequence determined. L. brevis Xi was purified and the N-terminal sequence determined. All attempts to directly clone the intact xylA using a degenerative primer deduced from amino acids (aa) 10-14 were not successful. A fragment coding for the first 462 bp from the 5' end of xylA was isolated by PCR with two primers, one coding for aa M36 to W43 and the second coding for an aa sequence (WGGREG) conserved in a number of Xi's isolated from other bacteria. From the sequence of this fragment, two additional PCR primers were synthesized, which were used in an 'outward' reaction to clone a 546-bp fragment including a region upstream from the N terminus. Finally, the complete xylA gene was cloned in a 0.43-kb NlaIII-SalI fragment and a 1.9-kb SalI-EcoRI fragment. The 449-aa sequence for the L. brevis Xi shows homology with Xis isolated from other bacteria, especially within the primary catalytic domains of the enzyme.

Organization and characterization of three genes involved in D-xylose catabolism in Lactobacillus pentosus

A cluster of three genes involved in D-xylose catabolism (viz. xylose genes) in Lactobacillus pentosus has been cloned in Escherichia coli and characterized by nucleotide sequence analysis. The deduced gene products show considerable sequence similarity to a repressor protein involved in the regulation of expression of xylose genes in Bacillus subtilis (58%), to E. coli and B. subtilis D-xylose isomerase (68% and 77%, respectively), and to E. coli D-xylulose kinase (58%). The cloned genes represent functional xylose genes since they are able to complement the inability of a L. casei strain to ferment D-xylose. NMR analysis confirmed that 13C-xylose was converted into 13C-acetate in L. casei cells transformed with L. pentosus xylose genes but not in untransformed L. casei cells. Comparison with the aligned amino acid sequences of D-xylose isomerases of different bacteria suggests that L. pentosus D-xylose isomerase belongs to the same similarity group as B. subtilis and E. coli D-xylose isomerase and not to a second similarity group comprising D-xylose isomerases of Streptomyces violaceoniger, Ampullariella sp. and Actinoplanes. The organization of the L. pentosus xylose genes, 5'-xylR (1167 bp, repressor) - xylA (1350 bp, D-xylose isomerase) - xylB (1506 bp, D-xylulose kinase) - 3' is similar to that in B. subtilis. In contrast to B. subtilis xylR, L. pentosus xylR is transcribed in the same direction as xylA and xylB.

Genetic organization and regulation of the xylose degradation genes in Streptomyces rubiginosus

The xylose isomerase (xylA) and the xylulose kinase (xylB) genes from Streptomyces rubiginosus were isolated, and their nucleotide sequences were determined. The xylA and xylB genes encode proteins of 388 and 481 amino acids, respectively. These two genes are transcribed divergently from within a 114-nucleotide sequence separating the coding regions. Regulation of the xyl genes in S. rubiginosus was examined by fusing their promoters to the Pseudomonas putida catechol dioxygenase gene and integrating the fusions into the minicircle integration site on the S. rubiginosus chromosome. The expression of catechol dioxygenase was then measured under a variety of conditions. The results indicated that transcription of the xyl genes was induced by D-xylose and repressed by glucose. Data from quantitative S1 mapping were consistent with this conclusion and suggested that xylA had one and xylB had two transcription initiation sites. The transcription initiation site of xylA was 40 bp upstream of the coding region. The two transcription initiation sites of xylB were 20 and 41 bp 5' of its translation initiation codon. Under control of appropriate regulatory elements, the cloned xyl genes are capable of complementing either Escherichia coli xylose isomerase- or xylulose kinase-deficient strains. The deduced amino acid sequence of the S. rubiginosus xylA protein is highly homologous to sequences of other microbial xylose isomerases.

Organization, promoter analysis and transcriptional regulation of the Staphylococcus xylosus xylose utilization operon

The Staphylococcus xylosus xyl genes were cloned in Staphylococcus carnosus by complementation to xylose utilization. Xylose isomerase assays under inducing (xylose present) and non-inducing (xylose absent) conditions indicated the presence of a regulated xylA gene on the recombinant plasmid. The nucleotide sequence (4520 bases) revealed three open reading frames with the same polarity. They were identified by sequence homologies as xylR, encoding the Xyl repressor, xylA, encoding xylose isomerase and xylB, encoding xylulokinase. Primer extension analyses indicated constitutive transcription of xylR and xylose-inducible transcription of xylA. Promoter consensus sequences were found upstream of both transcriptional start sites. A transcriptional terminator between xylR and xylA separates the different transcriptional units. Potential regulatory elements were identified by sequence analysis and suggest a repressor-operator mechanism for the regulation of xylAB expression.

Molecular cloning, structure, promoters and regulatory elements for transcription of the Bacillus megaterium encoded regulon for xylose utilization

The xylA and xylB genes of Bacillus subtilis BR151 encoding xylose isomerase and xylulokinase, respectively, were disrupted by gene replacement rendering the constructed mutant strain unable to grow on xylose as the sole carbon source. The Bacillus megaterium encoded xyl genes were cloned by complementation of this strain to xylose utilization. The nucleotide sequence of about 4 kbp of the insertion indicates the presence of the xylA and xylB genes on the complementing plasmid. Furthermore, a regulatory gene, xylR, is located upstream of xylA and has opposite polarity to it. The intergenic region between the divergently oriented reading frames of xylR and xylA contains palindromic sequences of 24 bp spaced by five central bp and 29 bp spaced by 11 bp, respectively, and two promoters with opposite orientation as determined by primer extension analysis. They overlap with one nucleotide of their--35 consensus boxes. Transcriptional fusions of lacZ to xylA, xylB and xylR were constructed and revealed that xylA and xylB are repressed in the absence and can be 200-fold induced in the presence of xylose. The increased level of xylAB mRNA in induced and its absence in repressed cells confirms that this regulation occurs on the level of transcription. Deletion of the xylR gene encoding the Xyl repressor results in constitutive expression of xylAB. The transcription of xylR is autoregulated and can be induced 9-fold by xylose. The mechanism of this regulation is not clear. While the apparent xyl operator palindrome is upstream of the xylR promoter, the potential recognition of another palindrome downstream of this promoter by Xyl repressor is discussed.

Molecular cloning, structure, promoters and regulatory elements for transcription of the Bacillus licheniformis encoded regulon for xylose utilization

In this article we describe the cloning of the xyl regulon encoding xylose utilization from Bacillus licheniformis by complementation of a xyl mutant of B. subtilis. The xylose isomerase encoding gene, xylA, was sequenced and identified by its extensive homology to other xylose isomerases. The expression of xylA is regulated on the level of transcription by a repressor protein encoded by xylR. Its gene has the opposite orientation of xylA and the start codons are 181 bp apart. A deletion of xylR renders xylA expression constitutive. The xylR sequence was determined and is discussed with respect to its homology to other xylR structures. Primer extension analyses of the xylA and xylR transcripts under repressing and including conditions define their promoters and confirm the regulation of xylA transcription. Furthermore, some induction of the xylR transcript by xylose is also observed. The regulatory sequence of both genes consists of a bipolar promoter system and contains three palindromic sequence elements. Their potential functions with respect to xylA and xylR regulation are discussed. The primary structures of the genes, promoters and regulatory sequences are compared to the xyl regulons encoded by B. subtilis, B. megaterium, Staphylococcus xylosus and E. coli. Homology is greatest between the B. subtilis and B. megaterium encoded xyl genes while the B. licheniformis borne genes are clearly more distant. The next greater differences are found to the S. xylosus and the greatest to the E. coli encoded genes. These results are discussed with respect to the taxonomic relations of these bacteria.

Xylose (glucose) isomerase gene from the thermophile Thermus thermophilus: cloning, sequencing, and comparison with other thermostable xylose isomerases

The xylose isomerase gene from the thermophile Thermus thermophilus was cloned by using a fragment of the Streptomyces griseofuscus gene as a probe. The complete nucleotide sequence of the gene was determined. T. thermophilus is the most thermophilic organism from which a xylose isomerase gene has been cloned and characterized. The gene codes for a polypeptide of 387 amino acids with a molecular weight of 44,000. The Thermus xylose isomerase is considerably more thermostable than other described xylose isomerases. Production of the enzyme in Escherichia coli, by using the tac promoter, increases the xylose isomerase yield 45-fold compared with production in T. thermophilus. Moreover, the enzyme from E. coli can be purified 20-fold by simply heating the cell extract at 85 degrees C for 10 min. The characteristics of the enzyme made in E. coli are the same as those of enzyme made in T. thermophilus. Comparison of the Thermus xylose isomerase amino acid sequence with xylose isomerase sequences from other organisms showed that amino acids involved in substrate binding and isomerization are well conserved. Analysis of amino acid substitutions that distinguish the Thermus xylose isomerase from other thermostable xylose isomerases suggests that the further increase in thermostability in T. thermophilus is due to substitution of amino acids which react during irreversible inactivation and results also from increased hydrophobicity.

Isolation and Characterization of Pichia heedii Mutants Defective in Xylose Uptake

To investigate the role of xylose uptake in xylose metabolism in yeasts, we isolated a series of mutated strains of the yeast Pichia heedii which are defective in xylose utilization. Four of these demonstrated defects in xylose uptake. Overlaps between the functional or regulatory mechanisms for glucose and xylose uptake may exist in this yeast since some of the mutants defective in xylose uptake were also defective in glucose transport. None of the mutants were defective in xylose reductase or xylitol dehydrogenase activities.

Structures of D-xylose isomerase from Arthrobacter strain B3728 containing the inhibitors xylitol and D-sorbitol at 2.5 A and 2.3 A resolution, respectively

The structures of D-xylose isomerase from Arthrobacter strain B3728 containing the polyol inhibitors xylitol and D-sorbitol have been solved at 2.5 A and 2.3 A, respectively. The structures have been refined using restrained least-squares refinement methods. The final crystallographic R-factors for the D-sorbitol (xylitol) bound molecules, for 43,615 (32,989) reflections are 15.6 (14.7). The molecule is a tetramer and the asymmetric unit of the crystal contains a dimer, the final model of which, incorporates a total of 6086 unique protein, inhibitor and magnesium atoms together with 535 bound solvent molecules. Each subunit of the enzyme contains two domains: the main domain is a parallel-stranded alpha-beta barrel, which has been reported in 14 other enzymes. The C-terminal domain is a loop structure consisting of five helical segments and is involved in intermolecular contacts between subunits that make up the tetramer. The structures have been analysed with respect to molecular symmetry, intersubunit contacts, inhibitor binding and active site geometry. The refined model shows the two independent subunits to be similar apart from local deviations due to solvent contacts in the solvent-exposed helices. The enzyme is dependent on a divalent cation for catalytic activity. Two metal ions are required per monomer, and the high-affinity magnesium(II) site has been identified from the structural results presented here. The metal ion is complexed, at the high-affinity site, by four carboxylate side-chains of the conserved residues, Glu180, Glu216, Asp244 and Asp292. The inhibitor polyols are bound in the active site in an extended open chain conformation and complete an octahedral co-ordination shell for the magnesium cation via their oxygen atoms O-2 and O-4. The active site lies in a deep pocket near the C-terminal ends of the beta-strands of the barrel domain and includes residues from a second subunit. The tetrameric molecule can be considered to be a dimer of "active" dimers, the active sites being composed of residues from both subunits. The analysis has revealed the presence of several internal salt-bridges stabilizing the tertiary and quaternary structure. One of these, between Asp23 and Arg139, appears to play a key role in stabilizing the active dimer and is conserved in the known sequences of this enzyme.(ABSTRACT TRUNCATED AT 400 WORDS)

Conversion of pentoses to ethanol by yeasts and fungi

Fermentation of D-xylose is of interest in enhancing the yield of ethanol obtainable from lignocellulosic hydrolysates. Such hydrolysates can contain both pentoses and hexoses, and while technology to convert hexoses to ethanol is well established, the fermentation of pentoses had been problematical. To overcome the difficulty, yeasts and fungi have been sought and identified in recent years that can convert D-xylose into ethanol. However, operation of their cultures in the presence of the pentose to obtain rapid and efficient ethanol production is somewhat more complex than in the archetype alcoholic fermentation, Saccharomyces cerevisiae on D-glucose. The complexity stems, in part, from the association of ethanol accumulation in cultures where D-xylose is the sole carbon source with conditions that limit growth, by oxygen in particular, although limitation by other nutrients might also be implicated. Aspects of screening for appropriate organisms and of the parameters that play a role in determining culture variables, especially those associated with ethanol productivity, are reviewed. Performance with D-xylose as sole carbon source, in sugar mixtures, and in lignocellulosic hydrolysates is discussed. A model that involves biochemical considerations of D-xylose metabolism is presented that rationalizes the effects of oxygen on cultures where D-xylose is the sole carbon source, notably effects of the specific rate of oxygen use on the rate and extent of ethanol accumulation. Alternate methods to direct fermentation of D-xylose have been developed that depend on its prior isomerization to D-xylose, followed by fermentation of the pentulose by certain yeasts and fungi. Factors involved in the biochemistry, use, and performance of these methods, which with some organisms involves sensitivity to oxygen, are reviewed.

A novel xylB-based positive selection vector

Expression of a plasmid-borne Escherichia coli xylulokinase gene (xylB) under the control of the lac promoter yields constitutively high levels of xylulokinase activity. When a plasmid containing this lac-xylB fusion (pLEK100) is transformed into a xylB- mutant the Xyl+ phenotype is restored on xylose-containing media. When the same transformants are plated on xylitol medium, growth inhibition is observed. Positive selection is achieved by cloning DNA into the unique restriction sites of pLEK100, to disrupt xylB expression, transforming E. coli, and then plating transformants on xylitol medium. With this protocol only transformants with insert containing plasmids will be obtained. This results in a considerable reduction in the time and effort needed to construct genomic libraries or perform routine DNA cloning experiments. Three unique sites are available which are suitable for positive selection of DNA fragments, via the disruption of translation (BglII) or transcription (HindIII, SalI, and BglII) of the xylB gene.

Expression of the Escherichia coli xylose isomerase gene in Saccharomyces cerevisiae

Transformation of Saccharomyces cerevisiae by yeast expression plasmids bearing the Escherichia coli xylose isomerase gene leads to production of the protein. Western blotting (immunoblotting) experiments show that immunoreactive protein chains which comigrate with the E. coli enzyme are made in the transformant strains and that the amount produced parallels the copy number of the plasmid. When comparable amounts of immunologically cross-reactive xylose isomerase protein made in E. coli or S. cerevisiae were assayed for enzymatic activity, however, the yeast protein was at least 10(3)-fold less active.