Production of ethanol from L-arabinose by Saccharomyces cerevisiae containing a fungal L-arabinose pathway

Microbial conversion of sugars from plant biomass to lactic acid or ethanol

Concerns for our environment and unease with our dependence on foreign oil have renewed interest in converting plant biomass into fuels and 'green' chemicals. The volume of plant matter available makes lignocellulose conversion desirable, although no single isolated organism has been shown to depolymerize lignocellulose and efficiently metabolize the resulting sugars into a specific product. This work reviews selected chemicals and fuels that can be produced from microbial fermentation of plant-derived cell-wall sugars and directed engineering for improvement of microbial biocatalysts. Lactic acid and ethanol production are highlighted, with a focus on engineered Escherichia coli.

A short review on SSF - an interesting process option for ethanol production from lignocellulosic feedstocks

Simultaneous saccharification and fermentation (SSF) is one process option for production of ethanol from lignocellulose. The principal benefits of performing the enzymatic hydrolysis together with the fermentation, instead of in a separate step after the hydrolysis, are the reduced end-product inhibition of the enzymatic hydrolysis, and the reduced investment costs. The principal drawbacks, on the other hand, are the need to find favorable conditions (e.g. temperature and pH) for both the enzymatic hydrolysis and the fermentation and the difficulty to recycle the fermenting organism and the enzymes. To satisfy the first requirement, the temperature is normally kept below 37 degrees C, whereas the difficulty to recycle the yeast makes it beneficial to operate with a low yeast concentration and at a high solid loading. In this review, we make a brief overview of recent experimental work and development of SSF using lignocellulosic feedstocks. Significant progress has been made with respect to increasing the substrate loading, decreasing the yeast concentration and co-fermentation of both hexoses and pentoses during SSF. Presently, an SSF process for e.g. wheat straw hydrolyzate can be expected to give final ethanol concentrations close to 40 g L-1 with a yield based on total hexoses and pentoses higher than 70%.

Altering the coenzyme preference of xylose reductase to favor utilization of NADH enhances ethanol yield from xylose in a metabolically engineered strain of Saccharomyces cerevisiae

Background

Metabolic engineering of Saccharomyces cerevisiae for xylose fermentation into fuel ethanol has oftentimes relied on insertion of a heterologous pathway that consists of xylose reductase (XR) and xylitol dehydrogenase (XDH) and brings about isomerization of xylose into xylulose via xylitol. Incomplete recycling of redox cosubstrates in the catalytic steps of the NADPH-preferring XR and the NAD⁺-dependent XDH results in formation of xylitol by-product and hence in lowering of the overall yield of ethanol on xylose. Structure-guided site-directed mutagenesis was previously employed to change the coenzyme preference of Candida tenuis XR about 170-fold from NADPH in the wild-type to NADH in a Lys²⁷⁴→Arg Asn²⁷⁶→Asp double mutant which in spite of the structural modifications introduced had retained the original catalytic efficiency for reduction of xylose by NADH. This work was carried out to assess physiological consequences in xylose-fermenting S. cerevisiae resulting from a well defined alteration of XR cosubstrate specificity.

Results

An isogenic pair of yeast strains was derived from S. cerevisiae Cen.PK 113-7D through chromosomal integration of a three-gene cassette that carried a single copy for C. tenuis XR in wild-type or double mutant form, XDH from Galactocandida mastotermitis, and the endogenous xylulose kinase (XK). Overexpression of each gene was under control of the constitutive TDH3 promoter. Measurement of intracellular levels of XR, XDH, and XK activities confirmed the expected phenotypes. The strain harboring the XR double mutant showed 42% enhanced ethanol yield (0.34 g/g) compared to the reference strain harboring wild-type XR during anaerobic bioreactor conversions of xylose (20 g/L). Likewise, the yields of xylitol (0.19 g/g) and glycerol (0.02 g/g) were decreased 52% and 57% respectively in the XR mutant strain. The xylose uptake rate per gram of cell dry weight was identical (0.07 ± 0.02 h^-1) in both strains.

Conclusion

Integration of enzyme and strain engineering to enhance utilization of NADH in the XR-catalyzed conversion of xylose results in notably improved fermentation capabilities of recombinant S. cerevisiae.

Use of in vivo 13C nuclear magnetic resonance spectroscopy to elucidate L-arabinose metabolism in yeasts

Candida arabinofermentans PYCC 5603(T) and Pichia guilliermondii PYCC 3012 were shown to grow well on L-arabinose, albeit exhibiting distinct features that justify an in-depth comparative study of their respective pentose catabolism. Carbon-13 labeling experiments coupled with in vivo nuclear magnetic resonance (NMR) spectroscopy were used to investigate L-arabinose metabolism in these yeasts, thereby complementing recently reported physiological and enzymatic data. The label supplied in L-[2-(13)C]arabinose to nongrowing cells, under aerobic conditions, was found on C-1 and C-2 of arabitol and ribitol, on C-2 of xylitol, and on C-1, C-2, and C-3 of trehalose. The detection of labeled arabitol and xylitol constitutes additional evidence for the operation in yeast of the redox catabolic pathway, which is widespread among filamentous fungi. Furthermore, labeling at position C-1 of trehalose and arabitol demonstrates that glucose-6-phosphate is recycled through the oxidative pentose phosphate pathway (PPP). This result was interpreted as a metabolic strategy to regenerate NADPH, the cofactor essential for sustaining l-arabinose catabolism at the level of L-arabinose reductase and L-xylulose reductase. Moreover, the observed synthesis of D-arabitol and ribitol provides a route with which to supply NAD(+) under oxygen-limiting conditions. In P. guilliermondii PYCC 3012, the strong accumulation of L-arabitol (intracellular concentration of up to 0.4 M) during aerobic L-arabinose metabolism indicates the existence of a bottleneck at the level of L-arabitol 4-dehydrogenase. This report provides the first experimental evidence for a link between L-arabinose metabolism in fungi and the oxidative branch of the PPP and suggests rational guidelines for the design of strategies for the production of new and efficient L-arabinose-fermenting yeasts.

l-Arabinose transport and catabolism in yeast

Two yeasts, Candida arabinofermentans PYCC 5603(T) and Pichia guilliermondii PYCC 3012, which show rapid growth on L-arabinose and very high rates of L-arabinose uptake on screening, were selected for characterization of L-arabinose transport and the first steps of intracellular L-arabinose metabolism. The kinetics of L-arabinose uptake revealed at least two transport systems with distinct substrate affinities, specificities, functional mechanisms and regulatory properties. The L-arabinose catabolic pathway proposed for filamentous fungi also seems to operate in the yeasts studied. The kinetic parameters of the initial L-arabinose-metabolizing enzymes were determined. Reductases were found to be mostly NADPH-dependent, whereas NAD was the preferred cofactor of dehydrogenases. The differences found between the two yeasts agree with the higher efficiency of L-arabinose metabolism in C. arabinofermentans. This is the first full account of the initial steps of L-arabinose catabolism in yeast including the biochemical characterization of a specific L-arabinose transporter.

Engineering of Saccharomyces cerevisiae for efficient anaerobic alcoholic fermentation of L-arabinose

For cost-effective and efficient ethanol production from lignocellulosic fractions of plant biomass, the conversion of not only major constituents, such as glucose and xylose, but also less predominant sugars, such as l-arabinose, is required. Wild-type strains of Saccharomyces cerevisiae, the organism used in industrial ethanol production, cannot ferment xylose and arabinose. Although metabolic and evolutionary engineering has enabled the efficient alcoholic fermentation of xylose under anaerobic conditions, the conversion of l-arabinose into ethanol by engineered S. cerevisiae strains has previously been demonstrated only under oxygen-limited conditions. This study reports the first case of fast and efficient anaerobic alcoholic fermentation of l-arabinose by an engineered S. cerevisiae strain. This fermentation was achieved by combining the expression of the structural genes for the l-arabinose utilization pathway of Lactobacillus plantarum, the overexpression of the S. cerevisiae genes encoding the enzymes of the nonoxidative pentose phosphate pathway, and extensive evolutionary engineering. The resulting S. cerevisiae strain exhibited high rates of arabinose consumption (0.70 g h(-1) g [dry weight](-1)) and ethanol production (0.29 g h(-1) g [dry weight](-1)) and a high ethanol yield (0.43 g g(-1)) during anaerobic growth on l-arabinose as the sole carbon source. In addition, efficient ethanol production from sugar mixtures containing glucose and arabinose, which is crucial for application in industrial ethanol production, was achieved.

Engineering primary metabolic pathways of industrial micro-organisms

Metabolic engineering is a powerful tool for the optimisation and the introduction of new cellular processes. This is mostly done by genetic engineering. Since the introduction of this multidisciplinary approach, the success stories keep accumulating. The primary metabolism of industrial micro-organisms has been studied for long time and most biochemical pathways and reaction networks have been elucidated. This large pool of biochemical information, together with data from proteomics, metabolomics and genomics underpins the strategies for design of experiments and choice of targets for manipulation by metabolic engineers. These targets are often located in the primary metabolic pathways, such as glycolysis, pentose phosphate pathway, the TCA cycle and amino acid biosynthesis and mostly at major branch points within these pathways. This paper describes approaches taken for metabolic engineering of these pathways in bacteria, yeast and filamentous fungi.

Towards industrial pentose-fermenting yeast strains

Production of bioethanol from forest and agricultural products requires a fermenting organism that converts all types of sugars in the raw material to ethanol in high yield and with a high rate. This review summarizes recent research aiming at developing industrial strains of Saccharomyces cerevisiae with the ability to ferment all lignocellulose-derived sugars. The properties required from the industrial yeast strains are discussed in relation to four benchmarks: (1) process water economy, (2) inhibitor tolerance, (3) ethanol yield, and (4) specific ethanol productivity. Of particular importance is the tolerance of the fermenting organism to fermentation inhibitors formed during fractionation/pretreatment and hydrolysis of the raw material, which necessitates the use of robust industrial strain background. While numerous metabolic engineering strategies have been developed in laboratory yeast strains, only a few approaches have been realized in industrial strains. The fermentation performance of the existing industrial pentose-fermenting S. cerevisiae strains in lignocellulose hydrolysate is reviewed. Ethanol yields of more than 0.4 g ethanol/g sugar have been achieved with several xylose-fermenting industrial strains such as TMB 3400, TMB 3006, and 424A(LNF-ST), carrying the heterologous xylose utilization pathway consisting of xylose reductase and xylitol dehydrogenase, which demonstrates the potential of pentose fermentation in improving lignocellulosic ethanol production.

Metabolic engineering for pentose utilization in Saccharomyces cerevisiae

The introduction of pentose utilization pathways in baker's yeast Saccharomyces cerevisiae is summarized together with metabolic engineering strategies to improve ethanolic pentose fermentation. Bacterial and fungal xylose and arabinose pathways have been expressed in S. cerevisiae but do not generally convey significant ethanolic fermentation traits to this yeast. A large number of rational metabolic engineering strategies directed among others toward sugar transport, initial pentose conversion, the pentose phosphate pathway, and the cellular redox metabolism have been exploited. The directed metabolic engineering approach has often been combined with random approaches including adaptation, mutagenesis, and hybridization. The knowledge gained about pentose fermentation in S. cerevisiae is primarily limited to genetically and physiologically well-characterized laboratory strains. The translation of this knowledge to strains performing in an industrial context is discussed.

Xylose transport studies with xylose-utilizing Saccharomyces cerevisiae strains expressing heterologous and homologous permeases

In the present study, we modified xylose uptake properties of a recombinant xylose-utilizing yeast Saccharomyces cerevisiae by expression of heterologous and homologous permease-encoding genes. In a mutant yeast strain with the main seven hexose transporter genes deleted, and engineered for xylose utilization, we screened an expression cDNA library of the filamentous fungus Trichoderma reesei (Hypocrea jecorina) for enhanced growth on xylose plates. One cDNA clone with significant homology to fungal sugar transporters was obtained, but when the clone was retransformed into the host, it did not support significant growth on xylose. However, during a long liquid culture of the strain carrying the cDNA clone, adaptive mutations apparently occurred in the host, which led to growth on xylose but not on glucose. The new transporter homologue, Trxlt1 thus appears to code for a protein specific for xylose uptake. In addition, xylose-transporting properties of some homologous hexose transporters were studied. All of them, i.e., Hxt1, Hxt2, Hxt4, and Hxt7 were capable of xylose uptake. Their affinities for xylose varied, K (m) values between 130 and 900 mM were observed. The single-Hxt strains showed a biphasic growth mode on xylose, alike the Trxlt1 harboring strain. The initial, slow growth was followed by a long lag and finally by exponential growth.

Selection and characterisation of a xylitol-derepressed Aspergillus niger mutant that is apparently impaired in xylitol transport

Aspergillus niger is known for its biotechnological applications, such as the use of xylanase enzyme for the degradation of hemicellulose. Depending on culture conditions, several polyols may also be accumulated, such as xylitol during D: -xylose oxidation. Also during industrial fermentation of xylose for the production of fuel ethanol by recombinant yeast, xylitol is a by-product. We studied xylitol metabolism by isolating mutants that have impaired xylitol-mediated repression. Genetic and biochemical characterisation revealed that one of these mutants was affected not only in xylitol-mediated carbon repression, but also had impaired xylitol transport.

Bio-ethanol--the fuel of tomorrow from the residues of today

The increased concern for the security of the oil supply and the negative impact of fossil fuels on the environment, particularly greenhouse gas emissions, has put pressure on society to find renewable fuel alternatives. The most common renewable fuel today is ethanol produced from sugar or grain (starch); however, this raw material base will not be sufficient. Consequently, future large-scale use of ethanol will most certainly have to be based on production from lignocellulosic materials. This review gives an overview of the new technologies required and the advances achieved in recent years to bring lignocellulosic ethanol towards industrial production. One of the major challenges is to optimize the integration of process engineering, fermentation technology, enzyme engineering and metabolic engineering.

Alcoholic fermentation of carbon sources in biomass hydrolysates by Saccharomyces cerevisiae: current status

Fuel ethanol production from plant biomass hydrolysates by Saccharomyces cerevisiae is of great economic and environmental significance. This paper reviews the current status with respect to alcoholic fermentation of the main plant biomass-derived monosaccharides by this yeast. Wild-type S. cerevisiae strains readily ferment glucose, mannose and fructose via the Embden-Meyerhof pathway of glycolysis, while galactose is fermented via the Leloir pathway. Construction of yeast strains that efficiently convert other potentially fermentable substrates in plant biomass hydrolysates into ethanol is a major challenge in metabolic engineering. The most abundant of these compounds is xylose. Recent metabolic and evolutionary engineering studies on S. cerevisiae strains that express a fungal xylose isomerase have enabled the rapid and efficient anaerobic fermentation of this pentose. L: -Arabinose fermentation, based on the expression of a prokaryotic pathway in S. cerevisiae, has also been established, but needs further optimization before it can be considered for industrial implementation. In addition to these already investigated strategies, possible approaches for metabolic engineering of galacturonic acid and rhamnose fermentation by S. cerevisiae are discussed. An emerging and major challenge is to achieve the rapid transition from proof-of-principle experiments under 'academic' conditions (synthetic media, single substrates or simple substrate mixtures, absence of toxic inhibitors) towards efficient conversion of complex industrial substrate mixtures that contain synergistically acting inhibitors.

Co-utilization of L-arabinose and D-xylose by laboratory and industrial Saccharomyces cerevisiae strains

Background

Fermentation of lignocellulosic biomass is an attractive alternative for the production of bioethanol. Traditionally, the yeast Saccharomyces cerevisiae is used in industrial ethanol fermentations. However, S. cerevisiae is naturally not able to ferment the pentose sugars D-xylose and L-arabinose, which are present in high amounts in lignocellulosic raw materials.

Results

We describe the engineering of laboratory and industrial S. cerevisiae strains to co-ferment the pentose sugars D-xylose and L-arabinose. Introduction of a fungal xylose and a bacterial arabinose pathway resulted in strains able to grow on both pentose sugars. Introduction of a xylose pathway into an arabinose-fermenting laboratory strain resulted in nearly complete conversion of arabinose into arabitol due to the L-arabinose reductase activity of the xylose reductase. The industrial strain displayed lower arabitol yield and increased ethanol yield from xylose and arabinose.

Conclusion

Our work demonstrates simultaneous co-utilization of xylose and arabinose in recombinant strains of S. cerevisiae. In addition, the co-utilization of arabinose together with xylose significantly reduced formation of the by-product xylitol, which contributed to improved ethanol production.

The influence of different cultivation conditions on the metabolome of Fusarium oxysporum

The two most widespread pentose sugars found in the biosphere are d-xylose and l-arabinose. They are both potential substrates for ethanol production. The purpose of this study was to better understand the redox constraints imposed to Fusarium oxysporum during utilization of pentoses. In order to increase ethanol yield and decrease by-product formation, nitrate was used as nitrogen source. The use of NADH, the cofactor in denitrification process when using nitrate as a nitrogen source, improved the ethanol yield on xylose to 0.89 mol mol(-1) compared to the ethanol yield achieved using ammonium as nitrogen source 0.44 mol mol(-1). The improved ethanol yield was followed by a 28% decrease in yield of the by-product xylitol. In order to investigate the metabolic pathway of arabinose and the metabolic limitations for the efficient ethanol production from this sugar, the extracellular and intracellular metabolite profiles were determined under aerobic and anaerobic cultivation conditions. The results of this study clearly show difficulties in channelling of glucose-1-P (G1P) to pentose phosphate pathway (PPP) and reduced NADPH regeneration, suggesting that NADPH becomes a limiting factor for arabinose conversion, resulting in excessive acetate production. Variations of the fungus intracellular amino and non-amino acid pool, under different culture conditions, were evaluated using principal component analysis (PCA). PCA projection of the metabolome data collected from F. oxysporum subjected to environmental perturbations succeeded to visualize different physiological states and the conclusions of this study were that the metabolite profile is unique according to: (1) the carbon source and (2) the oxygen supply, and to a lesser extent to the cultivation phase.

Ethanol production from hexoses, pentoses, and dilute-acid hydrolyzate by

Consumption of hexoses and pentoses and production of ethanol by Mucor indicus were investigated in both synthetic media and dilute-acid hydrolyzates. The fungus was able to grow in a poor medium containing only carbon, nitrogen, phosphate, potassium, and magnesium sources. However, the cultivation took more than a week and the ethanol yield was only 0.2 gg(-1). Enrichment of the medium by addition of trace metals, particularly zinc and yeast extract, improved the growth rate and yield, such that the cultivation was completed in less than 24 h and the ethanol and biomass yields were increased to 0.40 and 0.20 gg(-1), respectively. The fungus was able to assimilate glucose, galactose, mannose, and xylose, and produced ethanol with yields of 0.40, 0.34, 0.39, and 0.18 gg(-1), respectively. However, arabinose was poorly consumed and no formation of ethanol was detected. Glycerol was the major by-product in the cultivation on the hexoses, while formation of glycerol and xylitol were detected in the cultivation of the fungus on xylose. The fungus was able to take up the sugars present in dilute-acid hydrolyzate as well as the inhibitors, acetic acid, furfural, and hydroxymethyl furfural. M. indicus was able to grow under anaerobic conditions when glucose was the sole carbon source, but not on xylose or the hydrolyzate. The yield of ethanol in anaerobic cultivation on glucose was 0.46 g g(-1).

Fermentation performance of engineered and evolved xylose-fermenting Saccharomyces cerevisiae strains

Lignocellulose hydrolysate is an abundant substrate for bioethanol production. The ideal microorganism for such a fermentation process should combine rapid and efficient conversion of the available carbon sources to ethanol with high tolerance to ethanol and to inhibitory components in the hydrolysate. A particular biological problem are the pentoses, which are not naturally metabolized by the main industrial ethanol producer Saccharomyces cerevisiae. Several recombinant, mutated, and evolved xylose fermenting S. cerevisiae strains have been developed recently. We compare here the fermentation performance and robustness of eight recombinant strains and two evolved populations on glucose/xylose mixtures in defined and lignocellulose hydrolysate-containing medium. Generally, the polyploid industrial strains depleted xylose faster and were more resistant to the hydrolysate than the laboratory strains. The industrial strains accumulated, however, up to 30% more xylitol and therefore produced less ethanol than the haploid strains. The three most attractive strains were the mutated and selected, extremely rapid xylose consumer TMB3400, the evolved C5 strain with the highest achieved ethanol titer, and the engineered industrial F12 strain with by far the highest robustness to the lignocellulosic hydrolysate.

Molecular basis for anaerobic growth of Saccharomyces cerevisiae on xylose, investigated by global gene expression and metabolic flux analysis

Yeast xylose metabolism is generally considered to be restricted to respirative conditions because the two-step oxidoreductase reactions from xylose to xylulose impose an anaerobic redox imbalance. We have recently developed, however, a Saccharomyces cerevisiae strain that is at present the only known yeast capable of anaerobic growth on xylose alone. Using transcriptome analysis of aerobic chemostat cultures grown on xylose-glucose mixtures and xylose alone, as well as a combination of global gene expression and metabolic flux analysis of anaerobic chemostat cultures grown on xylose-glucose mixtures, we identified the distinguishing characteristics of this unique phenotype. First, the transcript levels and metabolic fluxes throughout central carbon metabolism were significantly higher than those in the parent strain, and they were most pronounced in the xylose-specific, pentose phosphate, and glycerol pathways. Second, differential expression of many genes involved in redox metabolism indicates that increased cytosolic NADPH formation and NADH consumption enable a higher flux through the two-step oxidoreductase reaction of xylose to xylulose in the mutant. Redox balancing is apparently still a problem in this strain, since anaerobic growth on xylose could be improved further by providing acetoin as an external NADH sink. This improved growth was accompanied by an increased ATP production rate and was not accompanied by higher rates of xylose uptake or cytosolic NADPH production. We concluded that anaerobic growth of the yeast on xylose is ultimately limited by the rate of ATP production and not by the redox balance per se, although the redox imbalance, in turn, limits ATP production.

Metabolic engineering of a phosphoketolase pathway for pentose catabolism in Saccharomyces cerevisiae

Low ethanol yields on xylose hamper economically viable ethanol production from hemicellulose-rich plant material with Saccharomyces cerevisiae. A major obstacle is the limited capacity of yeast for anaerobic reoxidation of NADH. Net reoxidation of NADH could potentially be achieved by channeling carbon fluxes through a recombinant phosphoketolase pathway. By heterologous expression of phosphotransacetylase and acetaldehyde dehydrogenase in combination with the native phosphoketolase, we installed a functional phosphoketolase pathway in the xylose-fermenting Saccharomyces cerevisiae strain TMB3001c. Consequently the ethanol yield was increased by 25% because less of the by-product xylitol was formed. The flux through the recombinant phosphoketolase pathway was about 30% of the optimum flux that would be required to completely eliminate xylitol and glycerol accumulation. Further overexpression of phosphoketolase, however, increased acetate accumulation and reduced the fermentation rate. By combining the phosphoketolase pathway with the ald6 mutation, which reduced acetate formation, a strain with an ethanol yield 20% higher and a xylose fermentation rate 40% higher than those of its parent was engineered.

Minimal metabolic engineering of Saccharomyces cerevisiae for efficient anaerobic xylose fermentation: a proof of principle

When xylose metabolism in yeasts proceeds exclusively via NADPH-specific xylose reductase and NAD-specific xylitol dehydrogenase, anaerobic conversion of the pentose to ethanol is intrinsically impossible. When xylose reductase has a dual specificity for both NADPH and NADH, anaerobic alcoholic fermentation is feasible but requires the formation of large amounts of polyols (e.g., xylitol) to maintain a closed redox balance. As a result, the ethanol yield on xylose will be sub-optimal. This paper demonstrates that anaerobic conversion of xylose to ethanol, without substantial by-product formation, is possible in Saccharomyces cerevisiae when a heterologous xylose isomerase (EC 5.3.1.5) is functionally expressed. Transformants expressing the XylA gene from the anaerobic fungus Piromyces sp. E2 (ATCC 76762) grew in synthetic medium in shake-flask cultures on xylose with a specific growth rate of 0.005 h(-1). After prolonged cultivation on xylose, a mutant strain was obtained that grew aerobically and anaerobically on xylose, at specific growth rates of 0.18 and 0.03 h(-1), respectively. The anaerobic ethanol yield was 0.42 g ethanol x g xylose(-1) and also by-product formation was comparable to that of glucose-grown anaerobic cultures. These results illustrate that only minimal genetic engineering is required to recruit a functional xylose metabolic pathway in Saccharomyces cerevisiae. Activities and/or regulatory properties of native S. cerevisiae gene products can subsequently be optimised via evolutionary engineering. These results provide a gateway towards commercially viable ethanol production from xylose with S. cerevisiae.

A novel NADH-linked l-xylulose reductase in the l-arabinose catabolic pathway of yeast

An NADH-dependent l-xylulose reductase and the corresponding gene were identified from the yeast Ambrosiozyma monospora. The enzyme is part of the yeast pathway for l-arabinose catabolism. A fungal pathway for l-arabinose utilization has been described previously for molds. In this pathway l-arabinose is sequentially converted to l-arabinitol, l-xylulose, xylitol, and d-xylulose and enters the pentose phosphate pathway as d-xylulose 5-phosphate. In molds the reductions are NADPH-linked, and the oxidations are NAD(+)-linked. Here we show that in A. monospora the pathway is similar, i.e. it has the same two reduction and two oxidation reactions, but the reduction by l-xylulose reductase is not performed by a strictly NADPH-dependent enzyme as in molds but by a strictly NADH-dependent enzyme. The ALX1 gene encoding the NADH-dependent l-xylulose reductase is strongly expressed during growth on l-arabinose as shown by Northern analysis. The gene was functionally overexpressed in Saccharomyces cerevisiae and the purified His-tagged protein characterized. The reversible enzyme converts l-xylulose to xylitol. It also converts d-ribulose to d-arabinitol but has no activity with l-arabinitol or adonitol, i.e. it is specific for sugar alcohols where, in a Fischer projection, the hydroxyl group of the C-2 is in the l-configuration and the hydroxyl group of C-3 is in the d-configuration. It also has no activity with C-6 sugars or sugar alcohols. The K(m) values for l-xylulose and d-ribulose are 9.6 and 4.7 mm, respectively. To our knowledge this is the first report of an NADH-linked l-xylulose reductase.