D-arabitol metabolism in Candida albicans: studies of the biosynthetic pathway and the gene that encodes NAD-dependent D-arabitol dehydrogenase

d-Arabitol production by a high arabitol-producing yeast, Zygosaccharomyces sp. Gz-5 isolated from miso

d-Arabitol, an alternative sweetener to sugar, has low calorie content, high sweetness, low glycemic index, and insulin resistance-improving ability. In this study, d-arabitol-producing yeast strains were isolated from various commercial types of miso, and strain Gz-5 was selected among these strains. Phylogenetic tree analysis of the internal transcribed spacer sequence revealed that strain Gz-5 was distinct from Zygosaccharomyces rouxii, a major fermenting yeast of miso. The strain, identified as Zygosaccharomyces sp. Gz-5, grew better than other Z. rouxii in 150 g/L NaCl and produced 114 g/L d-arabitol from 295 g/L glucose in a batch culture for 8 days (0.386 g/g-consumed glucose). In a fed-batch culture, the yeast produced 133 g/L d-arabitol for 14 days, and the total d-arabitol amount increased by 1.75-fold. These results indicated that Zygosaccharomyces sp. Gz-5, a non-genetically modified strain, has excellent potential for the industrial production of d-arabitol.

Isolation of Zygosaccharomyces siamensis kiy1 as a novel arabitol-producing yeast and its arabitol production

Arabitol is gaining attention in the food industry as an alternative sweetener owing to its low-caloric and non-cariogenic characteristics. The yeast strain kiy1 was newly isolated from unpasteurized honey for arabitol production. Based on internal transcribed spacer sequence analysis, the isolated strain was identified as Zygosaccharomyces siamensis. In this study, the effects of different substrates and sugar concentrations on arabitol production were investigated. When three types of carbon sources (glycerol, fructose, and glucose) were used, glucose was the most suitable substrate for arabitol production (68.7 g/L). Maximum arabitol production (101.4 g/L) was observed at a glucose concentration of 30%, and the highest arabitol production yield was 0.34 g/g of initial glucose. In the time-course production of sugar alcohols by strain kiy1, glucose was completely consumed for 8 days. The concentration of arabitol exceeded that of glycerol after 3 days, and the final arabitol concentration reached 83.6 g/L after 10 days. The maximum production rate was 16.7 g/L/day. The yeast produced glycerol as an intracellular sugar alcohol in the early stage of culture and switched its metabolism to arabitol production after the middle stage. Z. siamensis kiy1 possessed an NADP⁺-dependent arabitol dehydrogenase, which indicated that it probably produces arabitol via ribulose from glucose. These results suggest that the novel yeast strain, Z. siamensis kiy1, is promising for arabitol production. The proposed arabitol production approach can contribute toward its production at the industrial scale.

Metabolic preference assay for rapid diagnosis of bloodstream infections

Bloodstream infections (BSIs) cause >500,000 infections and >80,000 deaths per year in North America. The length of time between the onset of symptoms and administration of appropriate antimicrobials is directly linked to mortality rates. It currently takes 2–5 days to identify BSI pathogens and measure their susceptibility to antimicrobials – a timeline that directly contributes to preventable deaths. To address this, we demonstrate a rapid metabolic preference assay (MPA) that uses the pattern of metabolic fluxes observed in ex-vivo microbial cultures to identify common pathogens and determine their antimicrobial susceptibility profiles. In a head-to-head race with a leading platform (VITEK 2, BioMérieux) used in diagnostic laboratories, MPA decreases testing timelines from 40 hours to under 20. If put into practice, this assay could reduce septic shock mortality and reduce the use of broad spectrum antibiotics.

It is currently slow to identify bloodstream infection pathogens. Here the authors report a rapid metabolic preference assay that uses the pattern of metabolic fluxes observed in ex-vivo microbial cultures to identify common pathogens and determine their antimicrobial susceptibility profiles.

Integrating transcriptomic and metabolomic analysis of the oleaginous yeast Rhodosporidium toruloides IFO0880 during growth under different carbon sources

Rhodosporidium toruloides is an oleaginous yeast capable of producing a variety of biofuels and bioproducts from diverse carbon sources. Despite numerous studies showing its promise as a platform microorganism, little is known about its metabolism and physiology. In this work, we investigated the central carbon metabolism in R. toruloides IFO0880 using transcriptomics and metabolomics during growth on glucose, xylose, acetate, or soybean oil. These substrates were chosen because they can be derived from plants. Significant changes in gene expression and metabolite concentrations were observed during growth on these four substrates. We mapped these changes onto the governing metabolic pathways to better understand how R. toruloides reprograms its metabolism to enable growth on these substrates. One notable finding concerns xylose metabolism, where poor expression of xylulokinase induces a bypass leading to arabitol production. Collectively, these results further our understanding of central carbon metabolism in R. toruloides during growth on different substrates. They may also help guide the metabolic engineering and development of better models of metabolism for R. toruloides.Key points• Gene expression and metabolite concentrations were significantly changed.• Reduced expression of xylulokinase induces a bypass leading to arabitol production.• R. toruloides reprograms its metabolism to allow growth on different substrates.

An Integrated Analysis of Intracellular Metabolites and Virulence Gene Expression during Biofilm Development of a Clinical Isolate of Candida tropicalis on Distinct Surfaces

Emergence of Candida tropicalis, which causes potential life-threatening invasive candidiasis, is often associated with colonization of medical devices as biofilm. Biofilm plays an important role in the virulence of the pathogen because of its complex structure, which provides resistance to conventional antimicrobials. In this study, the metabolic response of a clinical strain of C. tropicalis colonizing three distinct surfaces (polytetrafluoroethylene (PTFE), polystyrene, and polycarbonate) as well as the expression of virulence and stress related genes (ALS3, Hsp21, SAP1, SAP2, SAP3, and CYR1), were explored. Our results showed that lesser biofilm was developed on PTFE compared to polystyrene and polycarbonate. GS-MS metabolic analysis identified a total of 36 metabolites in the intracellular extract of cells grown on polystyrene, polycarbonate, and PTFE, essentially belonging to central carbon metabolism, amino acids, and lipids metabolism. The metabolic analysis showed that saturated and unsaturated fatty acids are preferentially produced during biofilm development on polycarbonate, whereas trehalose and vitamin B6, known as cellular protectors against a variety of stressors, were characteristic of biofilm on PTFE. The results of the transcriptomic analysis consider the different degrees of colonization of the three substrates, being CYR1, which encodes the component of signaling pathway of hyphal formation-cAMP-PKA, downregulated in PTFE biofilm compared to polycarbonate or polystyrene biofilms, while Hsp21 was upregulated in concomitance with the potential unfavorable conditions for biofilm formation on PTFE. Overall, this work provides new insights into the knowledge of C. tropicalis biofilm development on surfaces of medical relevance in the perspective of improving the management of Candida infections.

Antifungal activity of polymethyl methacrylate/Si3N4 composites against Candida albicans

Previous studies using gram-positive and -negative bacteria demonstrated that hydrolysis of silicon nitride (Si₃N₄) in aqueous suspensions elutes nitrogen and produces gaseous ammonia while buffering pH. According to immunochemistry assays, fluorescence imaging, and in situ Raman spectroscopy, we demonstrate here that the antipathogenic surface chemistry of Si₃N₄ can be extended to polymethylmethacrylate (PMMA) by compounding it with a minor fraction (~8 vol.%) of Si₃N₄ particles without any tangible loss in bulk properties. The hydrolytic products, which were eluted from partly exposed Si₃N₄ particles at the composite surface, exhibited fungicidal action against Candida albicans. Using a specific nitrative stress sensing dye and highly resolved fluorescence micrographs, we observed in situ congestion of peroxynitrite (ONOO^-) radicals in the mitochondria of the Candida cells exposed to the PMMA/Si₃N₄ composite, while these radicals were absent in the mitochondria of identical cells exposed to monolithic PMMA. These in situ observations suggest that the surface chemistry of Si₃N₄ mimics the antifungal activity of macrophages, which concurrently produce NO radicals and superoxide anions (O₂•^-) resulting in the formation of candidacidal ONOO^-. The fungicidal properties of PMMA/Si₃N₄ composites could be used in dental appliances to inhibit the uncontrolled growth of Candida albicans and ensuing candidiasis while being synergic with chemoprophylaxis. STATEMENT OF SIGNIFICANCE: In a follow-up of previous studies of gram-positive and gram-negative bacteria, we demonstrate here that the antipathogenic surface chemistry of Si₃N₄ could be extended to polymethylmethacrylate (PMMA) containing a minor fraction (~8 vol.%) of Si₃N₄ particles without tangible loss in bulk properties. Hydrolytic products eluted from Si₃N₄ particles at the composite surface exhibited fungicidal action against Candida albicans. Highly resolved fluorescence microscopy revealed congestion of peroxynitrite (ONOO^-) radicals in the mitochondria of the Candida cells exposed to the PMMA/Si₃N₄ composite, while radicals were absent in the mitochondria of identical cells exposed to monolithic PMMA. The fungicidal properties of PMMA/Si₃N₄ composites could be used in dental appliances to inhibit uncontrolled growth of Candida albicans and ensuing candidiasis in synergy with chemoprophylaxis.

Torulaspora delbrueckii produces high levels of C5 and C6 polyols during wine fermentations

Non-Saccharomyces yeasts impact wine fermentations and can diversify the flavor profiles of wines. However, little information is available on the metabolic networks of most of these species. Here we show that unlike the main wine yeast Saccharomyces cerevisiae, Torulaspora delbrueckii and to a lesser extent Lachancea thermotolerans produce significant concentrations of C5 and C6 polyols under wine fermentation conditions. In particular, D-arabitol, D-sorbitol and D-mannitol were produced at significant levels. Their release into the extracellular matrix started when that of glycerol ceased. The data also show that polyol production is influenced by initial sugar concentration, repressed by acetic acid and induced in ethanol supplemented media. Moreover, unlike glycerol and sorbitol, mannitol was partially re-assimilated when populations started to decline. The findings suggest that polyol synthesis is a physiological adaptation to stressful conditions characteristic of alcoholic fermentation and that these polyols may serve a similar purpose as glycerol production in S. cerevisiae, including osmoadaptation and redox balancing.

Specificity of the osmotic stress response in Candida albicans highlighted by quantitative proteomics

Stress adaptation is critical for the survival of microbes in dynamic environments, and in particular, for fungal pathogens to survive in and colonise host niches. Proteomic analyses have the potential to significantly enhance our understanding of these adaptive responses by providing insight into post-transcriptional regulatory mechanisms that contribute to the outputs, as well as testing presumptions about the regulation of protein levels based on transcript profiling. Here, we used label-free, quantitative mass spectrometry to re-examine the response of the major fungal pathogen of humans, Candida albicans, to osmotic stress. Of the 1,262 proteins that were identified, 84 were down-regulated in response to 1M NaCl, reflecting the decrease in ribosome biogenesis and translation that often accompanies stress. The 64 up-regulated proteins included central metabolic enzymes required for glycerol synthesis, a key osmolyte for this yeast, as well as proteins with functions during stress. These data reinforce the view that adaptation to salt stress involves a transient reduction in ribosome biogenesis and translation together with the accumulation of the osmolyte, glycerol. The specificity of the response to salt stress is highlighted by the small proportion of quantified C. albicans proteins (5%) whose relative elevated abundances were statistically significant.

Molecular Identification of d-Ribulokinase in Budding Yeast and Mammals

Proteomes of even well characterized organisms still contain a high percentage of proteins with unknown or uncertain molecular and/or biological function. A significant fraction of those proteins is predicted to have catalytic properties. Here we aimed at identifying the function of the Saccharomyces cerevisiae Ydr109c protein and its human homolog FGGY, both of which belong to the broadly conserved FGGY family of carbohydrate kinases. Functionally identified members of this family phosphorylate 3- to 7-carbon sugars or sugar derivatives, but the endogenous substrate of S. cerevisiae Ydr109c and human FGGY has remained unknown. Untargeted metabolomics analysis of an S. cerevisiae deletion mutant of YDR109C revealed ribulose as one of the metabolites with the most significantly changed intracellular concentration as compared with a wild-type strain. In human HEK293 cells, ribulose could only be detected when ribitol was added to the cultivation medium, and under this condition, FGGY silencing led to ribulose accumulation. Biochemical characterization of the recombinant purified Ydr109c and FGGY proteins showed a clear substrate preference of both kinases for d-ribulose over a range of other sugars and sugar derivatives tested, including l-ribulose. Detailed sequence and structural analyses of Ydr109c and FGGY as well as homologs thereof furthermore allowed the definition of a 5-residue d-ribulokinase signature motif (TCSLV). The physiological role of the herein identified eukaryotic d-ribulokinase remains unclear, but we speculate that S. cerevisiae Ydr109c and human FGGY could act as metabolite repair enzymes, serving to re-phosphorylate free d-ribulose generated by promiscuous phosphatases from d-ribulose 5-phosphate. In human cells, FGGY can additionally participate in ribitol metabolism.

Modulation of Candida albicans Biofilm by Different Carbon Sources

In the present investigation, the role of carbon sources (glucose, lactate, sucrose, and arabinose) on Candida albicans biofilm development and virulence factors was studied on polystyrene microtiter plates. Besides this, structural changes in cell wall component β-glucan in presence of different carbon sources have also been highlighted. Biofilm formation was analyzed by XTT (2,3-bis[2-Methoxy-4-nitro-5-sulfophenyl]-2H-tetrazolium-5-carboxanilide) reduction assay. Glucose-grown cells exhibited the highest metabolic activity during adhesion among all carbon sources tested (p < 0.05). However, cells exposed to sucrose exhibited highest biofilm formation and matrix polysaccharides secretion after 48 h. The results also correlated with the biofilm height and roughness measurements by atomic force microscopy. Exposure to lactate induced hyphal structures with the highest proteinase activity while arabinose-grown cells formed pseudohyphal structures possessing the highest phospholipase activity. Structural changes in β-glucan characterized by Fourier transform infrared (FTIR) spectroscopy displayed characteristic band of β-glucan at 892 cm(-1) in all carbon sources tested. The β(1→6) to β(1→3) glucan ratio calculated as per the band area of the peak was less in lactate (1.15) as compared to glucose (1.73), sucrose (1.62), and arabinose (2.85). These results signify that carbon sources influence C. albicans biofilm development and modulate virulence factors and structural organization of cell wall component β-glucan.

Integration and Validation of the Genome-Scale Metabolic Models of Pichia pastoris: A Comprehensive Update of Protein Glycosylation Pathways, Lipid and Energy Metabolism

Motivation

Genome-scale metabolic models (GEMs) are tools that allow predicting a phenotype from a genotype under certain environmental conditions. GEMs have been developed in the last ten years for a broad range of organisms, and are used for multiple purposes such as discovering new properties of metabolic networks, predicting new targets for metabolic engineering, as well as optimizing the cultivation conditions for biochemicals or recombinant protein production. Pichia pastoris is one of the most widely used organisms for heterologous protein expression. There are different GEMs for this methylotrophic yeast of which the most relevant and complete in the published literature are iPP668, PpaMBEL1254 and iLC915. However, these three models differ regarding certain pathways, terminology for metabolites and reactions and annotations. Moreover, GEMs for some species are typically built based on the reconstructed models of related model organisms. In these cases, some organism-specific pathways could be missing or misrepresented.

Results

In order to provide an updated and more comprehensive GEM for P. pastoris, we have reconstructed and validated a consensus model integrating and merging all three existing models. In this step a comprehensive review and integration of the metabolic pathways included in each one of these three versions was performed. In addition, the resulting iMT1026 model includes a new description of some metabolic processes. Particularly new information described in recently published literature is included, mainly related to fatty acid and sphingolipid metabolism, glycosylation and cell energetics. Finally the reconstructed model was tested and validated, by comparing the results of the simulations with available empirical physiological datasets results obtained from a wide range of experimental conditions, such as different carbon sources, distinct oxygen availability conditions, as well as producing of two different recombinant proteins. In these simulations, the iMT1026 model has shown a better performance than the previous existing models.

Metabolic engineering of the chloroplast genome reveals that the yeast ArDH gene confers enhanced tolerance to salinity and drought in plants

Osmoprotectants stabilize proteins and membranes against the denaturing effect of high concentrations of salts and other harmful solutes. In yeast, arabitol dehydrogenase (ArDH) reduces D-ribulose to D-arabitol where D-ribulose is derived by dephosphorylating D-ribulose-5-PO4 in the oxidized pentose pathway. Osmotolerance in plants could be developed through metabolic engineering of chloroplast genome by introducing genes encoding polyols since chloroplasts offer high level transgene expression and containment. Here, we report that ArDH expression in tobacco chloroplasts confers tolerance to NaCl (up to 400 mM). Transgenic plants compared to wild type (WT) survived for only 4-5 weeks on 400 mM NaCl whereas plants remained green and grew normal on concentrations up to 350 mM NaCl. Further, a-week-old seedlings were also challenged with poly ethylene glycol (PEG, up to 6%) in the liquid medium, considering that membranes and proteins are protected under stress conditions due to accumulation of arabitol in chloroplasts. Seedlings were tolerant to 6% PEG, suggesting that ARDH enzyme maintains integrity of membranes in chloroplasts under drought conditions via metabolic engineering. Hence, the gene could be expressed in agronomic plants to withstand abiotic stresses.

Linear Discriminant Analysis Identifies Mitochondrially Localized Proteins in Neurospora crassa

Besides their role as powerhouses, mitochondria play a pivotal role in the spatial organization of numerous enzymatic functions. They are connected to the ER, and many pathways are organized through the mitochondrial membranes. Thus, the precise definition of mitochondrial proteomes remains a challenging task. Here, we have established a proteomic strategy to accurately determine the mitochondrial localization of proteins from the fungal model organism Neurospora crassa. This strategy relies on both highly pure mitochondria as well as the quantitative monitoring of mitochondrial components along their consecutive enrichment. Pure intact mitochondria were obtained by a multistep approach combining differential and density Percoll (ultra) centrifugations. When compared with three other intermediate enrichment stages, peptide sequencing and quantitative profiling of pure mitochondrial fractions revealed prototypic regulatory profiles of per se mitochondrial components. These regulatory profiles constitute a distinct cluster defining the mitochondrial compartment and support linear discriminant analyses, which rationalized the annotation process. In total, this approach experimentally validated the mitochondrial localization of 512 proteins including 57 proteins that had not been reported for N. crassa before.

Metal accumulation and differentially expressed proteins in gill of oyster (Crassostrea hongkongensis) exposed to long-term heavy metal-contaminated estuary

Bio-accumulation and bio-transmission of toxic metals and the toxicological responses of organisms exposed to toxic metals have been focused, due to heavy metal contaminations have critically threatened the ecosystem and food security. However, still few investigations focused on the responses of certain organisms exposed to the long term and severe heavy metal contamination in specific environments. In present investigation, the Hong Kong oyster, Crassostrea hongkongensis were obtained from 3 sites which were contaminated by different concentrations of heavy metals (such as zinc, copper, manganese and lead etc.), respectively. Heavy metal concentrations in the sea water samples collected from the 3 sites and the dissected tissues of the oysters with blue visceral mass were determinated to estimate the metal contamination levels in environments and the bio-accumulation ratios of the heavy metals in the different tissues of oysters. Moreover, Proteomic methods were employed to analyze the differentially expressed proteins in the gills of oysters exposed to long-term heavy metal contaminations. Results indicated that the Jiulong River estuary has been severely contaminated by Cu, Zn and slightly with Cr, Ni, Mn, etc, moreover, Zn and Cu were the major metals accumulated by oysters to phenomenally high concentrations (more than 3.0% of Zn and about 2.0% of Cu against what the dry weight of tissues were accumulated), and Cr, Ni, Mn, etc were also significantly accumulated. The differentially expressed proteins in the gills of oysters exposed to heavy metals participate in several cell processes, such as metal binding, transporting and saving, oxidative-reduction balance maintaining, stress response, signal transduction, etc. Significantly up-regulated expression (about 10 folds) of an important metal binding protein, metallothionein (MT) and granular cells was observed in the gills of oysters exposed to long-term and severely heavy-metal-contaminated estuary, it suggested that binding toxic metals with metallothionein-like proteins (MTLP) and storing toxic metals in metal-rich granules (MRG) with insoluble forms were the important strategies of oyster to detoxify the toxic metals and adapt to the high level of metal-contaminated environment. Most of the stress and immunity responsive proteins, such as heat shock proteins (HSP), extracellular superoxide dismutase (ECSOD) and cavortin, and the cellular redox reaction relative proteins such as 20G-Fe (II) oxygenase family oxidoreductase, aldehyde dehydrogenase and retinal dehydrogenase 2, were detected significantly down-regulated in the gills of oysters exposed to long term heavy metal contaminated environments, it indicated that long term exposure different from emergent exposure to heavy metal contamination may significantly suppress the stress and immunity response system of oysters. Moreover, Formin homology 2 domain containing protein (FH2). The only protein domain to directly nucleate actin monomers into unbranched filament polymers, by which will subsequently control gene expression and chromatin remodelling complexes, was also detected greatly up-regulated in the gills of oysters exposed to long-term heavy metal contaminations. It indicated that nuclear activity regulation may also be important for oyster to adapt to the long-term heavy-metal-contaminated environment.

Properties of recombinant Strep-tagged and untagged hyperthermophilic D-arabitol dehydrogenase from Thermotoga maritima

The first hyperthermophilic D-arabitol dehydrogenase from Thermotoga maritima was heterologously purified from Escherichia coli. The protein was purified with and without a Strep-tag. The enzyme exclusively catalyzed the NAD(H)-dependent oxidoreduction of D-arabitol, D-xylitol, D-ribulose, or D-xylulose. A twofold increase of catalytic rates was observed upon addition of Mg(2+) or K(+). Interestingly, only the tag-less protein was thermostable, retaining 90% of its activity after 90 min at 85 °C. However, the tag-less form of D-arabitol dehydrogenase had similar kinetic parameters compared to the tagged enzyme, demonstrating that the Strep-tag was not deleterious to protein function but decreased protein stability. A single band at 27.6 kDa was observed on SDS-PAGE and native PAGE revealed that the protein formed a homohexamer and a homododecamer. The enzyme catalyzed oxidation of D-arabitol to D: -ribulose and therefore belongs to the class of D-arabitol 2-dehydrogenases, which are typically observed in yeast and not bacteria. The product D-ribulose is a rare ketopentose sugar that has numerous industrially applications. Given its thermostability and specificity, D-arabitol 2-dehydrogenase is a desirable biocatalyst for the production of rare sugar precursors.

Cloning, purification and characterization of an NAD-Dependent D-Arabitol dehydrogenase from acetic acid bacterium, Acetobacter suboxydans

D-Xylulose-forming D-arabitol dehydrogenase (aArDH) is a key enzyme in the bio-conversion of D-arabitol to xylitol. In this study, we cloned the NAD-dependent D-xylulose-forming D-arabitol dehydrogenase gene from an acetic acid bacterium, Acetobacter suboxydans sp. The enzyme was purified from A. suboxydans sp. and was heterogeneously expressed in Escherichia coli. The native or recombinant enzyme was preferred NAD(H) to NADP(H) as coenzyme. The active recombinant aArDH expressed in E. coli is a homodimer, whereas the native aArDH in A. suboxydans is a homotetramer. On SDS-PAGE, the recombinant and native aArDH give one protein band at the position corresponding to 28 kDa. The optimum pH of polyol oxidation and ketone reduction is found to be pH 8.5 and 5.5 respectively. The highest reaction rate is observed when D-arabitol is used as the substrate (K (m) = 4.5 mM) and the product is determined to be D-xylulose by HPLC analysis.

The MAP kinase Hog1p differentially regulates stress-induced production and accumulation of glycerol and d-arabitol in Candida albicans

Candida albicansproduces and accumulates large amounts of the polyolsd-arabitol and glycerol in culture, and/or in infected mammalian tissues. However, the effects of environmental stresses on production and accumulation of these polyols, and the means by which polyol production and accumulation are regulated have not been studied.C. albicansgrown in glucose at 30 °C (i) produced maximal amounts of glycerol within 6 h, (ii) produced maximal amounts ofd-arabitol and ribitol within 12 h, and (iii) released most of these polyols into the extracellular environment.C. albicansresponded to osmotic and citric acid stress by producing and accumulating more glycerol, and to temperature and oxidative stresses by producing mored-arabitol. The increase in intracellular glycerol was proportional to extracellular osmolarity, suggesting that glycerol functions as an osmolyte. The MAP kinase Hog1p is required for wild-type glycerol production in several fungal species subjected to osmotic stress, but it is not known if Hog1p plays a role in regulatingd-arabitol production. Therefore, twoC. albicans hog1null mutants were constructed and tested for the ability to produce glycerol andd-arabitol in response to environmental stresses. The ability to grow and produce glycerol when exposed to osmotic or citric acid stresses, and to produced-arabitol when exposed to oxidative stress, was partially dependent on Hog1p, but the ability to produced-arabitol when exposed to temperature stress was Hog1p independent. These results imply that multiple pathways regulate glycerol andd-arabitol synthesis inC. albicans.

Characterization of a novel NADP(+)-dependent D-arabitol dehydrogenase from the plant pathogen Uromyces fabae

We have identified and characterized a novel NADP(+)-dependent D-arabitol dehydrogenase and the corresponding gene from the rust fungus Uromyces fabae, a biotrophic plant pathogen on broad bean (Vicia faba). The new enzyme was termed ARD1p (D-arabitol dehydrogenase 1). It recognizes D-arabitol and mannitol as substrates in the forward reaction, and D-xylulose, D-ribulose and D-fructose as substrates in the reverse reaction. Co-factor specificity was restricted to NADP(H). Kinetic data for the major substrates and co-factors are presented. A detailed analysis of the organization and expression pattern of the ARD1 gene are also given. Immunocytological data indicate a localization of the gene product predominantly in haustoria, the feeding structures of these fungi. Analyses of metabolite levels during pathogenesis indicate that the D-arabitol concentration rises dramatically as infection progresses, and D-arabitol was shown in an in vitro system to be capable of quenching reactive oxygen species involved in host plant defence reactions. ARD1p may therefore play an important role in carbohydrate metabolism and in establishing and/or maintaining the biotrophic interaction in U. fabae.

Polyol accumulation by Aspergillus oryzae at low water activity in solid-state fermentation

Polyol accumulation and metabolism were examined in Aspergillus oryzae cultured on whole wheat grains or on wheat dough as a model for solid-state culture. In solid-state fermentation (SSF), water activity (a(w)) is typically low resulting in osmotic stress. In addition to a high level of mannitol, which is always present in the cells, A. oryzae accumulated high concentrations of glycerol, erythritol and arabitol at relatively low a(w) (0.96-0.97) in SSF. Accumulation of such a mixture of polyols is rather unusual and might be typical for SSF. A. oryzae mycelium accumulating various polyols at low a(w) contained at least four distinct polyol dehydrogenases with highest activities toward glycerol, erythritol, D-arabitol and mannitol. NADP(+)-dependent glycerol dehydrogenase activity correlated very well with glycerol accumulation. A similar correlation was observed for erythritol and NADP(+)-erythritol dehydrogenase suggesting that NADP(+)-dependent glycerol and erythritol dehydrogenases are involved in biosynthesis of glycerol and erythritol, respectively, and that these enzymes are induced by osmotic stress.

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.

A rapid, automated enzymatic fluorometric assay for determination of D-arabinitol in serum

A rapid enzymatic fluorometric assay for measuring D-arabinitol in serum was developed using recombinant D-arabinitol dehydrogenase from Candida albicans (rArDH). rArDH was produced in Escherichia coli and purified by dye-ligand affinity chromatography. rArDH was highly specific for D-arabinitol, cross-reacting only with xylitol (4.9%) among all polyols tested. A Cobas Fara II centrifugal autoanalyzer (Roche) was used to measure NADH fluorometrically when rArDH and NAD were added to serum extracts, and D-arabinitol concentrations were calculated from standard curves derived from pooled human serum containing known amounts of D-arabinitol. The method was precise (mean intra-assay coefficients of variation [CVs], 0.8%, and mean interassay CVs, 1.6%) and rapid (3.5 min per assay) and showed excellent recovery of added D-arabinitol in serum (mean recovery rate, 101%). The mean and median D-arabinitol/creatinine ratios were 2.74 and 2.23 microM/mg/dl, respectively, for the 11 patients with candidemia compared to 1.14 and 1.23 microM/mg/dl, respectively, for 10 healthy controls (P < 0.01). These results confirm earlier studies showing that serum D-arabinitol measurement may help to promptly diagnose invasive candidiasis. The technique shows a significant improvement in terms of accuracy, cost, simplicity, specificity, and speed compared with gas chromatography, mass spectrometry, and earlier enzymatic assays.

Genetic engineering for improved xylose fermentation by yeasts

Xylose utilization is essential for the efficient conversion of lignocellulosic materials to fuels and chemicals. A few yeasts are known to ferment xylose directly to ethanol. However, the rates and yields need to be improved for commercialization. Xylose utilization is repressed by glucose which is usually present in lignocellulosic hydrolysates, so glucose regulation should be altered in order to maximize xylose conversion. Xylose utilization also requires low amounts of oxygen for optimal production. Respiration can reduce ethanol yields, so the role of oxygen must be better understood and respiration must be reduced in order to improve ethanol production. This paper reviews the central pathways for glucose and xylose metabolism, the principal respiratory pathways, the factors determining partitioning of pyruvate between respiration and fermentation, the known genetic mechanisms for glucose and oxygen regulation, and progress to date in improving xylose fermentations by yeasts.

Independent signaling pathways regulate cellular turgor during hyperosmotic stress and appressorium-mediated plant infection by Magnaporthe grisea

The phytopathogenic fungus Magnaporthe grisea elaborates a specialized infection cell called an appressorium with which it mechanically ruptures the plant cuticle. To generate mechanical force, appressoria produce enormous hydrostatic turgor by accumulating molar concentrations of glycerol. To investigate the genetic control of cellular turgor, we analyzed the response of M. grisea to hyperosmotic stress. During acute and chronic hyperosmotic stress adaptation, M. grisea accumulates arabitol as its major compatible solute in addition to smaller quantities of glycerol. A mitogen-activated protein kinase-encoding gene OSM1 was isolated from M. grisea and shown to encode a functional homolog of HIGH-OSMOLARITY GLYCEROL1 (HOG1), which encodes a mitogen-activated protein kinase that regulates cellular turgor in yeast. A null mutation of OSM1 was generated in M. grisea by targeted gene replacement, and the resulting mutants were sensitive to osmotic stress and showed morphological defects when grown under hyperosmotic conditions. M. grisea deltaosm1 mutants showed a dramatically reduced ability to accumulate arabitol in the mycelium. Surprisingly, glycerol accumulation and turgor generation in appressoria were unaltered by the Deltaosm1 null mutation, and the mutants were fully pathogenic. This result indicates that independent signal transduction pathways regulate cellular turgor during hyperosmotic stress and appressorium-mediated plant infection. Consistent with this, exposure of M. grisea appressoria to external hyperosmotic stress induced OSM1-dependent production of arabitol.

Occurrence of multiple ethyl 4-chloro-3-oxobutanoate-reducing enzymes in Candida magnoliae

Multiple ethyl 4-chloro-3-oxobutanoate (COBE)-reducing enzymes were isolated from a cell-free extract of Candida magnoliae. A NADPH-dependent COBE-reducing enzyme, distinct from the carbonyl reductase and aldehyde reductase previously reported, was purified to homogeneity using five steps, including polyethylene glycol treatment. The relative molecular mass of the enzyme was estimated to be 86,000 on high performance gel-permeation chromatography and 29,000 on sodium dodecyl sulfate polyacrylamide gel electrophoresis. The enzyme catalyzed the reduction of COBE to the corresponding (S)-alcohol with a 51% enantiomeric excess. The substrate specificity of the enzyme was different from those of the other COBE-reducing enzymes of the same strain. The partial amino acid sequences of the enzyme showed that it belongs to the short chain alcohol dehydrogenase/reductase (SDR) family. This is the first report of multiple COBE-reducing enzymes with various stereoselectivities occurring in the same strain but belonging to different (super)families.

Monosomy of a specific chromosome determines L-sorbose utilization: a novel regulatory mechanism in Candida albicans

We report the identification of the gene, SOU1, required for L-sorbose assimilation in Candida albicans. The level of the expression of SOU1 is determined by the copy number of chromosome III (also denoted chromosome 5), such that monosomic strains assimilate L-sorbose, whereas disomic strains do not, in spite of the fact that SOU1 is not on this chromosome. We suggest that C. albicans contains a resource of potentially beneficial genes that are activated by changes in chromosome number, and that this elaborate mechanism regulates the utilization of food supplies and possibly other important functions, thus representing a novel general means for regulating gene expression in microbes.

Polyol metabolism of Rhodobacter sphaeroides: biochemical characterization of a short-chain sorbitol dehydrogenase

A sorbitol dehydrogenase (SDH; L-iditol:NAD+ 2-oxidoreductase; EC 1.1.1.14) was isolated from the phototrophic bacterium Rhodobacter sphaeroides strain M22, a transposon mutant of R. sphaeroides Si4 with the transposon inserted in the mannitol dehydrogenase (MDH) gene. SDH was purified 470-fold to apparent homogeneity by ammonium sulfate precipitation, chromatography on Phenyl-Sepharose, Q-Sepharose and Matrex Gel Red-A, and by gel filtration on Superdex 200. The relative molecular mass (M(r)) of the native SDH was 61000 as calculated from its Stokes' radius (rs = 3.5 nm) and sedimentation coefficient (S20,w = 4.23S). SDS-PAGE resulted in one single band representing a polypeptide with a M(r) of 29,000, indicating that the native protein is a dimer. The isoelectric point of SDH was determined to be pH 4.8. The enzyme was specific for NAD+ and catalysed the oxidation of D-glucitol (sorbitol) to D-fructose, galactitol to D-tagatose and of L-iditol. The apparent Km values were NAD+, 0.06 mM; D-glucitol, 6.2 mM; galactitol, 1.5 mM; NADH, 0.13 mM; D-fructose, 160 mM; and D-tagatose, 13 mM. The pH-optimum of substrate oxidation was 11.0 and that of substrate reduction 6.0-7.2. It was demonstrated that SDH is expressed in the wild-type strain R. sphaeroides Si4 together with MDH during growth on D-glucitol. Forty-four amino acids of the SDH N terminus were sequenced. This sequence exhibited 45-55% identity to the N-terminal sequence of 10 enzymes belonging to the short-chain alcohol dehydrogenase family.

A short-chain dehydrogenase gene from Pichia stipitis having D-arabinitol dehydrogenase activity

An NAD(+)-dependent D-arabinitol dehydrogenase (polyol dehydrogenase) gene was isolated from Pichia stipitis CBS 6054 and cloned in Saccharomyces cerevisiae. The gene was isolated by screening of a lambda-cDNA library with a zymogram technique. D-Arabinitol, xylitol, D-glucitol and galactitol are substrates for the recombinant protein. With D-arabinitol as substrate the reaction product is D-ribulose. The molecular weight of the native tetramer enzyme is 110,000 Da and the monomer is 30,000 Da. The amino acid sequence is homologous to the short-chain dehydrogenase family. It is 85.5% identical to a D-arabinitol dehydrogenase from Candida albicans. The gene in P. stipitis was induced by D-arabinitol and P. stipitis was able to grow on D-arabinitol. The physiological role of D-arabinitol metabolism is discussed.

D-arabitol metabolism in Candida albicans: construction and analysis of mutants lacking D-arabitol dehydrogenase

Candida albicans produces large amounts of the acyclic pentitol D-arabitol in culture and in infected animals and humans, and most strains also grow on minimal D-arabitol medium. An earlier study showed that the major metabolic precursor of D-arabitol in C. albicans was D-ribulose-5-PO4 from the pentose pathway, that C. albicans contained an NAD-dependent D-arabitol dehydrogenase (ArDH), and that the ArDH structural gene (ARD) encoded a 31-kDa short-chain dehydrogenase that catalyzed the reaction D-arabitol + NAD <=> D-ribulose + NADH. In the present study, we disrupted both ARD chromosomal alleles in C. albicans and analyzed the resulting mutants. The ard null mutation was verified by Southern hybridization, and the null mutant's inability to produce ArDH was verified by Western immunoblotting. The ard null mutant grew well on minimal glucose medium, but it was unable to grow on minimal D-arabitol or D-arabinose medium. Thus, ArDH catalyzes the first step in D-arabitol utilization and a necessary intermediate step in D-arabinose utilization. Unexpectedly, the ard null mutant synthesized D-arabitol from glucose. Moreover, 13C nuclear magnetic resonance studies showed that the ard null mutant and its wild-type parent synthesized D-arabitol via the same pathway. These results imply that C. albicans synthesizes and utilizes D-arabitol via separate metabolic pathways, which was not previously suspected for fungi.

Isolation, characterization and expression of the gene that encodes D-arabinitol dehydrogenase in Candida tropicalis

The gene (ARD) that encodes NAD-dependent D-arabinitol dehydrogenase (ArDH) in the pathogenic fungus Candida tropicalis (Ct) was cloned by transforming Escherichia coli (Ec) BW31M (araCc) with a plasmid library of Ct genomic DNA and selecting for D-arabinitol-utilizing (D-arab+) clones. Plasmid DNA from a D-arab+ clone retransformed fresh Ec BW31M cells to D-arab+; these cells produced both ArDH catalytic activity and a 31-kDa protein recognized by antibodies to native Ct ArDH. The plasmid contained an 846-bp open reading frame (ORF) that encoded a deduced protein of 282 amino acids (aa) (30,748 Da). Four partial aa sequences from Ct ArDH were present in the deduced aa sequence, thus verifying that Ct ARD had been cloned. Ct ArDH was 95% identical to ArDH from Candida albicans (Ca), 85% identical to a xylitol dehydrogenase (XDH) from Pichia stipitis (Ps) and 20-25% identical to many other short-chain dehydrogenases. Ct ArDH, Ca ArDH and Ps XDH were typical short-chain dehydrogenases except that they lacked an N-terminal Gly that is conserved in other members of this family. Thus, these enzymes may represent a subclass of closely-related fungal pentitol dehydrogenases. Large amounts of recombinant ArDH (re-ArDH) were produced in Ec and purified by dye ligand affinity chromatography. The physical and catalytic properties of re-ArDH were similar to those of native Ct ArDH, and re-ArDH and native ArDH performed similarly in an automated enzymatic assay for D-arabinitol in human serum.