Development of screening strategies for the identification of paramylon-degrading enzymes

Enzymatic degradation of the β-1,3-glucan paramylon could enable the production of bioactive compounds for healthcare and renewable substrates for biofuels. However, few enzymes have been found to degrade paramylon efficiently and their enzymatic mechanisms remain poorly understood. Thus, the aim of this work was to find paramylon-degrading enzymes and ways to facilitate their identification. Towards this end, a Euglena gracilis-derived cDNA expression library was generated and introduced into Escherichia coli. A flow cytometry-based screening assay was developed to identify E. gracilis enzymes that could hydrolyse the fluorogenic substrate fluorescein di-β-D-glucopyranoside in combination with time-saving auto-induction medium. In parallel, four amino acid sequences of potential E. gracilis β-1,3-glucanases were identified from proteomic data. The open reading frame encoding one of these candidate sequences (light_m.20624) was heterologously expressed in E. coli. Finally, a Congo Red dye plate assay was developed for the screening of enzyme preparations potentially able to degrade paramylon. This assay was validated with enzymes assumed to have paramylon-degrading activity and then used to identify four commercial preparations with previously unknown paramylon degradation ability.

Low Temperature Stress Alters the Expression of Phytoene Desaturase Genes (crtP1 and crtP2) and the ζ-Carotene Desaturase Gene (crtQ) Together with the Cellular Carotenoid Content of Euglena gracilis

Carotenoids participate in photosynthesis and photoprotection in oxygenic phototrophs. Euglena gracilis, a eukaryotic phytoflagellate, synthesizes several carotenoids: β-carotene, neoxanthin, diadinoxanthin and diatoxanthin. Temperature is one of the most striking external stimuli altering carotenoid production. In the present study, to elucidate the regulation of carotenoid synthesis of E. gracilis in response to environmental stimuli, we functionally identified phytoene desaturase genes (crtP1 and crtP2) and the ζ-carotene desaturase gene (crtQ) of this alga and analyzed expression of those genes and the composition of major carotenoids in cells grown under cold (20�C) and high-intensity light (HL; 240 �mol photon m-2 s-1) conditions. 20�C-HL treatment increased the transcriptional level of the phytoene synthase gene (crtB), and crtP1 and crtP2, whose products catalyze the early steps of carotenoid biosynthesis in this alga. Cultivation at 20�C under illumination at 55 �mol photon m-2 s-1 (low-intensity light; LL) decreased the cell concentration, Chl and total major carotenoid content by 61, 75 and 50%, respectively, relative to control (25�C-LL) cells. When grown at 20�C-HL, the cells showed a greater decrease in cell concentration and photosynthetic pigment contents than those in 20�C-LL. β-Carotene, neoxanthin and diadinoxanthin contents were decreased by more than half in 20�C-LL and 20�C-HL treatments. On the other hand, when subjected to 20�C-LL and 20�C-HL, the cells retained a diatoxanthin content comparable with control cells. Our findings suggested that diatoxanthin plays crucial roles in the acclimation to cold and intense light condition. To the best of our knowledge, this is the first report on a photosynthetic organism possessing dual crtP genes.

A major isoform of mitochondrial trans-2-enoyl-CoA reductase is dispensable for wax ester production in Euglena gracilis under anaerobic conditions

Under anaerobic conditions, Euglena gracilis produces a large amount of wax ester through mitochondrial fatty acid synthesis from storage polysaccharides termed paramylon, to generate ATP. Trans-2-enoyl-CoA reductases (TERs) in mitochondria have been considered to play a key role in this process, because the enzymes catalyze the reduction of short chain length CoA-substrates (such as crotonyl-CoA). A TER enzyme (EgTER1) has been previously identified and enzymologically characterized; however, its physiological significance remained to be evaluated by genetic analysis. We herein generated EgTER1-knockdown Euglena cells, in which total crotonyl-CoA reductase activity was decreased to 10% of control value. Notably, the knockdown cells showed a severe bleaching phenotype with deficiencies in chlorophylls and glycolipids, but grew normally under heterotrophic conditions (with glucose supplementation). Moreover, the knockdown cells accumulated much greater quantities of wax ester than control cells before and after transfer to anaerobic conditions, which was accompanied by a large metabolomic change. Furthermore, we failed to find any contribution of other potential TER genes in wax ester production. Our findings propose a novel role of EgTER1 in the greening process and demonstrate that this enzyme is dispensable for wax ester production under anaerobic conditions.

Physiological role of β-carotene monohydroxylase (CYP97H1) in carotenoid biosynthesis in Euglena gracilis

Some carotenoids are found in the Euglena gracilis, including β-carotene, diadinoxanthin, diatoxanthins, and neoxanthin as the major species; however, the molecular mechanism underlying carotenoid biosynthesis in E. gracilis is not well understood. To clarify the pathway and regulation of carotenoid biosynthesis in this alga, we functionally characterized the cytochrome P450 (CYP)-type carotene hydroxylase gene EgCYP97H1. Heterologous in vivo enzyme assay in E. coli indicated that EgCYP97H1 hydroxylated β-carotene to β-cryptoxanthin. E. gracilis cells suppressing EgCYP97H1 resulted in marked growth inhibition and reductions in total carotenoid and chlorophyll contents. Analysis of carotenoid composition revealed that suppression of EgCYP97H1 resulted in higher level of β-carotene, suggesting that EgCYP97H1 is physiologically essential for carotenoid biosynthesis and thus normal cell growth. To our knowledge, this is the first time EgCYP97H1 has been suggested to be β-carotene monohydroxylase, but not β-carotene dihydroxylase. Moreover, during light adaptation of dark-grown E. gracilis, transcript levels of the carotenoid biosynthetic genes (EgCYP97H1, geranylgeranyl pyrophosphate synthase EgcrtE, and phytoene synthase EgcrtB) remained virtually unchanged. In contrast, carotenoid accumulation in E. gracilis grown under the same conditions was inhibited by treatment with a translational inhibitor but not a transcriptional inhibitor, indicating that photo-responsive carotenoid biosynthesis is regulated post-transcriptionally in this alga.

Mode of action and specificity of a chitinase from unicellular microalgae, Euglena gracilis

Euglena gracilis is a unicellular microalga showing characteristics of both plants and animals, and extensively used as a model organism in the research works of biochemistry and molecular biology. Biotechnological applications of E. gracilis have been conducted for production of numerous important compounds. However, chitin-mediated defense system intensively studied in higher plants remains to be investigated in this microalga. Recently, Taira et al. (Biosci Biotechnol Biochem 82:1090-1100, 2018) isolated a unique chitinase gene, comprising two catalytic domains almost homologous to each other (Cat1 and Cat2) and two chitin-binding domains (CBD1 and CBD2), from E. gracilis. We herein examined the mode of action and the specificity of the recombinant Cat2 by size exclusion chromatography and NMR spectroscopy. Both Cat1 and Cat2 appeared to act toward chitin substrate with non-processive/endo-splitting mode, recognizing two contiguous N-acetylglucosamine units at subsites - 2 and - 1. This is the first report on a chitinase having two endo-splitting catalytic domains. A cooperative action of two different endo-splitting domains may be advantageous for defensive action of the E. gracilis chitinase. The unicellular alga, E. gracilis, produces a chitinase consisting of two GH18 catalytic domains (Cat1 and Cat2) and two CBM18 chitin-binding domains (CBD1 and CBD2). Here, we produced a recombinant protein of the Cat2 domain to examine its mode of action as well as specificity. Cat2 hydrolyzed N-acetylglucosamine (A) oligomers (A_n, n = 4, 5, and 6) and partially N-acetylated chitosans with a non-processive/endo-splitting mode of action. NMR analysis of the product mixture from the enzymatic digestion of chitosan revealed that the reducing ends were exclusively A-unit, and the nearest neighbors of the reducing ends were mostly A-unit but not exclusively. Both A-unit and D-unit were found at the non-reducing ends and the nearest neighbors. These results indicated strong and absolute specificities for subsites - 2 and - 1, respectively, and no preference for A-unit at subsites + 1 and + 2. The same results were obtained from sugar sequence analysis of the individual enzymatic products from the chitosans. The subsite specificities of Cat2 are similar to those of GH18 human chitotriosidase, but differ from those of plant GH18 chitinases. Since the structures of Cat1 and Cat2 resemble to each other (99% similarity in amino acid sequences), Cat1 may hydrolyze the substrate with the same mode of action. Thus, the E. gracilis chitinase appears to act toward chitin polysaccharide chain through a cooperative action of the two endo-splitting catalytic domains, recognizing two contiguous A-units at subsites - 2 and - 1.

Identification of Euglena gracilis β-1,3-glucan phosphorylase and establishment of a new glycoside hydrolase (GH) family GH149

Glycoside phosphorylases (EC 2.4.x.x) carry out the reversible phosphorolysis of glucan polymers, producing the corresponding sugar 1-phosphate and a shortened glycan chain. β-1,3-Glucan phosphorylase activities have been reported in the photosynthetic euglenozoan Euglena gracilis, but the cognate protein sequences have not been identified to date. Continuing our efforts to understand the glycobiology of E. gracilis, we identified a candidate phosphorylase sequence, designated EgP1, by proteomic analysis of an enriched cellular protein lysate. We expressed recombinant EgP1 in Escherichia coli and characterized it in vitro as a β-1,3-glucan phosphorylase. BLASTP identified several hundred EgP1 orthologs, most of which were from Gram-negative bacteria and had 37-91% sequence identity to EgP1. We heterologously expressed a bacterial metagenomic sequence, Pro_7066 in E. coli and confirmed it as a β-1,3-glucan phosphorylase, albeit with kinetics parameters distinct from those of EgP1. EgP1, Pro_7066, and their orthologs are classified as a new glycoside hydrolase (GH) family, designated GH149. Comparisons between GH94, EgP1, and Pro_7066 sequences revealed conservation of key amino acids required for the phosphorylase activity, suggesting a phosphorylase mechanism that is conserved between GH94 and GH149. We found bacterial GH149 genes in gene clusters containing sugar transporter and several other GH family genes, suggesting that bacterial GH149 proteins have roles in the degradation of complex carbohydrates. The Bacteroidetes GH149 genes located to previously identified polysaccharide utilization loci, implicated in the degradation of complex carbohydrates. In summary, we have identified a eukaryotic and a bacterial β-1,3-glucan phosphorylase and uncovered a new family of phosphorylases that we name GH149.

Wax Ester Synthase/Diacylglycerol Acyltransferase Isoenzymes Play a Pivotal Role in Wax Ester Biosynthesis in Euglena gracilis

Wax ester fermentation is a unique energy gaining pathway for a unicellular phytoflagellated protozoan, Euglena gracilis, to survive under anaerobiosis. Wax esters produced in E. gracilis are composed of saturated fatty acids and alcohols, which are the major constituents of myristic acid and myristyl alcohol. Thus, wax esters can be promising alternative biofuels. Here, we report the identification and characterization of wax ester synthase/diacylglycerol acyltrasferase (WSD) isoenzymes as the terminal enzymes of wax ester production in E. gracilis. Among six possible Euglena WSD orthologs predicted by BLASTX search, gene expression analysis and in vivo evaluation for enzyme activity with yeast expressing individual recombinant WSDs indicated that two of them (EgWSD2 and EgWSD5) predominantly function as wax ester synthase. Furthermore, experiments with gene silencing demonstrated a pivotal role of both EgWSD2 and EgWSD5 in wax ester synthesis, as evidenced by remarkably reduced wax ester contents in EgWSD2/5-double knockdown E. gracilis cells treated with anaerobic conditions. Interestingly, the decreased ability to produce wax ester did not affect adaptation of E. gracilis to anaerobiosis. Lipid profile analysis suggested allocation of metabolites to other compounds including triacylglycerol instead of wax esters.

Suppression of the phytoene synthase gene (EgcrtB) alters carotenoid content and intracellular structure of Euglena gracilis

BACKGROUND

Photosynthetic organisms utilize carotenoids for photoprotection as well as light harvesting. Our previous study revealed that high-intensity light increases the expression of the gene for phytoene synthase (EgcrtB) in Euglena gracilis (a unicellular phytoflagellate), the encoded enzyme catalyzes the first committed step of the carotenoid biosynthesis pathway. To examine carotenoid synthesis of E. gracilis in response to light stress, we analyzed carotenoid species and content in cells grown under various light intensities. In addition, we investigated the effect of suppressing EgcrtB with RNA interference (RNAi) on growth and carotenoid content.

RESULTS

After cultivation for 7 days under continuous light at 920 μmol m^-2 s^-1, β-carotene, diadinoxanthin (Ddx), and diatoxanthin (Dtx) content in cells was significantly increased compared with standard light intensity (55 μmol m^-2 s^-1). The high-intensity light (920 μmol m^-2 s^-1) increased the pool size of diadinoxanthin cycle pigments (i.e., Ddx + Dtx) by 1.2-fold and the Dtx/Ddx ratio from 0.05 (control) to 0.09. In contrast, the higher-intensity light treatment caused a 58% decrease in chlorophyll (a + b) content and diminished the number of thylakoid membranes in chloroplasts by approximately half compared with control cells, suggesting that the high-intensity light-induced accumulation of carotenoids is associated with an increase in both the number and size of lipid globules in chloroplasts and the cytoplasm. Transient suppression of EgcrtB in this alga by RNAi resulted in significant decreases in cell number, chlorophyll, and total major carotenoid content by 82, 82 and 86%, respectively, relative to non-electroporated cells. Furthermore, suppression of EgcrtB decreased the number of chloroplasts and thylakoid membranes and increased the Dtx/Ddx ratio by 1.6-fold under continuous illumination even at the standard light intensity, indicating that blocking carotenoid synthesis increased the susceptibility of cells to light stress.

CONCLUSIONS

Our results indicate that suppression of EgcrtB causes a significant decrease in carotenoid and chlorophyll content in E. gracilis accompanied by changes in intracellular structures, suggesting that Dtx (de-epoxidized form of diadinoxanthin cycle pigments) contributes to photoprotection of this alga during the long-term acclimation to light-induced stress.

Identification and functional analysis of the geranylgeranyl pyrophosphate synthase gene (crtE) and phytoene synthase gene (crtB) for carotenoid biosynthesis in Euglena gracilis

BACKGROUND

Euglena gracilis, a unicellular phytoflagellate within Euglenida, has attracted much attention as a potential feedstock for renewable energy production. In outdoor open-pond cultivation for biofuel production, excess direct sunlight can inhibit photosynthesis in this alga and decrease its productivity. Carotenoids play important roles in light harvesting during photosynthesis and offer photoprotection for certain non-photosynthetic and photosynthetic organisms including cyanobacteria, algae, and higher plants. Although, Euglenida contains β-carotene and xanthophylls (such as zeaxanthin, diatoxanthin, diadinoxanthin and 9'-cis neoxanthin), the pathway of carotenoid biosynthesis has not been elucidated.

RESULTS

To clarify the carotenoid biosynthetic pathway in E. gracilis, we searched for the putative E. gracilis geranylgeranyl pyrophosphate (GGPP) synthase gene (crtE) and phytoene synthase gene (crtB) by tblastn searches from RNA-seq data and obtained their cDNAs. Complementation experiments in Escherichia coli with carotenoid biosynthetic genes of Pantoea ananatis showed that E. gracilis crtE (EgcrtE) and EgcrtB cDNAs encode GGPP synthase and phytoene synthase, respectively. Phylogenetic analyses indicated that the predicted proteins of EgcrtE and EgcrtB belong to a clade distinct from a group of GGPP synthase and phytoene synthase proteins, respectively, of algae and higher plants. In addition, we investigated the effects of light stress on the expression of crtE and crtB in E. gracilis. Continuous illumination at 460 or 920 μmol m(-2) s(-1) at 25 °C decreased the E. gracilis cell concentration by 28-40 % and 13-91 %, respectively, relative to the control light intensity (55 μmol m(-2) s(-1)). When grown under continuous light at 920 μmol m(-2) s(-1), the algal cells turned reddish-orange and showed a 1.3-fold increase in the crtB expression. In contrast, EgcrtE expression was not significantly affected by the light-stress treatments examined.

CONCLUSIONS

We identified genes encoding CrtE and CrtB in E. gracilis and found that their protein products catalyze the early steps of carotenoid biosynthesis. Further, we found that the response of the carotenoid biosynthetic pathway to light stress in E. gracilis is controlled, at least in part, by the level of crtB transcription. This is the first functional analysis of crtE and crtB in Euglena.

Identification and enzymatic characterization of an endo-1,3-β-glucanase from Euglena gracilis

Euglena produces paramylon as a storage polysaccharide, and is thought to require β-1,3-glucan degrading enzymes to release and utilize the accumulated carbohydrate. To investigate β-1,3-glucan degradation in Euglena, endo-1,3-β-glucanases were partially purified from Euglena gracilis by hydrophobic, gel filtration and anion-exchange chromatography. Tryptic digests and mass-spectrometric analysis identified three proteins in the purified fraction as a member of glycoside hydrolase family (GH) 17 and two members of GH81. These genes were cloned from an Euglena cDNA pool by PCR. EgCel17A fused with a histidine-tag at the carboxy terminus was heterologously produced by Aspergillus oryzae and purified by immobilized metal affinity chromatography. Purified EgCel17A had a molecular weight of about 40kDa by SDS-PAGE, which was identical to that deduced from its amino acid sequence. The enzyme showed hydrolytic activity towards β-1,3-glucans such as laminarin and paramylon. Maximum activity of laminarin degradation by EgCel17A was attained at pH 4.0-5.5 and 60°C after 1h incubation or 50°C after 20h incubation. The enzyme had a Km of 0.21mg/ml and a Vmax of 40.5units/mg protein for laminarin degradation at pH 5.0 and 50°C. Furthermore, EgCel17A catalyzed a transglycosylation reaction by which reaction products with a higher molecular weight than the supplied substrates were initially generated; however, ultimately the substrates were degraded into glucose, laminaribiose and laminaritriose. EgCel17A effectively produced soluble β-1,3-glucans from alkaline-treated Euglena freeze-dried powder containing paramylon. Thus, EgCel17 is the first functional endo-1,3-β-glucanase to be identified from E. gracilis.

[Reconstitution of polyunsaturated fatty acid synthesis enzymes in mammalian cells to convert LA to DHA]

DHA (22:6n-3) is a Ω-3 polyunsaturated fatty acid with 22 carbon atoms and 6 double bonds, which has important biological functions in human body. Human and other mammals synthesize only limited amounts of DHA, more requirements must be satisfied from food resources. However, the natural resources of DHA (Mainly deep-sea fish and other marine products) are prone to depletion. New resources development is still insufficient to satisfy the growing market demand. Previous studies have revealed that the mammals can increase the synthesis of DHA and other long-chain polyunsaturated fatty acids after transgenic procedures. In this study, mammalian cells were transfected with Δ6, Δ5 desaturase, Δ6, Δ5 elongase, Δ15 desaturase (Isolated from nematode Caenorhabditis elegans) and Δ4 desaturase (Isolated from Euglena gracilis), simultaneously. Results show that the expression or overexpression of these 6 enzymes is capable of conversion of the o-6 linoleic acid (LA, 18:2n-6) in DHA (22:6n-3). DHA content has increased from 16.74% in the control group to 25.3% in the experimental group. The strategy and related technology in our research provided important data for future production the valuable DHA (22:6n-3) by using genetically modified animals.

Ecotoxicological studies of micro- and nanosized barium titanate on aquatic photosynthetic microorganisms

The interaction between live organisms and micro- or nanosized materials has become a current focus in toxicology. As nanosized barium titanate has gained momentum lately in the medical field, the aims of the present work are: (i) to assess BT toxicity and its mechanisms on the aquatic environment, using two photosynthetic organisms (Anabaena flos-aquae, a colonial cyanobacteria, and Euglena gracilis, a flagellated euglenoid); (ii) to study and correlate the physicochemical properties of BT with its toxic profile; (iii) to compare the BT behavior (and Ba(2+) released ions) and the toxic profile in synthetic (Bold's Basal, BB, or Mineral Medium, MM) and natural culture media (Seine River Water, SRW); and (iv) to address whether size (micro, BT MP, or nano, BT NP) is an issue in BT particles toxicity. Responses such as growth inhibition, cell viability, superoxide dismutase (SOD) activity, adenosine-5-triphosphate (ATP) content and photosynthetic efficiency were evaluated. The main conclusions are: (i) BT have statistically significant toxic effects on E. gracilis growth and viability even in small concentrations (1μgmL(-1)), for both media and since the first 24 h; on the contrary of on A. flos-aquae, to whom the effects were noticeable only for the higher concentrations (after 96 h: ≥75 μg mL(-1) for BT NP and =100 μg mL(-1) for BT MP, in BB; and ≥75 μg mL(-1) for both materials in SRW), in spite of the viability being affected in all concentrations; (ii) the BT behaviors in synthetic and natural culture media were slightly different, being the toxic effects more pronounced when grown in SRW - in this case, a worse physiological state of the organisms in SRW can occur and account for the lower resistance, probably linked to a paucity of nutrients or even a synergistic effect with a contaminant from the river; and (iii) the effects seem to be mediated by induced stress without a direct contact in A. flos-aquae and by direct endocytosis in E. gracilis, but in both organisms the contact with both BT MP and BT NP increased SOD activity and decreased photosynthetic efficiency and intracellular ATP content; and (iv) size does not seem to be an issue in BT particles toxicity since micro- and nano-particles produced significant toxic for the model-organisms.

Detoxification Effect of Iron-encaging Zeolite-processed Water in Tributyltin-intoxicated Euglena gracilis Z

In our previous paper, we reported the restoration promoting effects of mineral-encaging zeolite-processed water, especially of a Fe-encaging one, on tributyltin chloride (TBTCl)-intoxicated Euglena gracilis. This present study extends the investigation on the behavior of TBTCl and a xenobiotic enzyme, cytochrome P-450, in Euglena cells incubated with or without Fe-encaging zeolite-processed water (FeZW). Subcellular fractionation of TBTCl-intoxicated Euglena cells, atomic absorption spectrophotometry, and GC analyses showed that TBTCl was rapidly incorporated into the cells to halt cell motility. GC-MS showed that FeZW promoted conversion of TBTCl to dibutyltin (DBT) as the major metabolite in the microsomal fraction of the cells. An in vitro incubation system with heat-treated microsomes did not convert TBTCl to DBT. The contribution of cytochrome P-450 in the microsomal fraction was suggested by an immunochemical method. The results suggest that the improvement of detoxification by FeZW in the TBT-intoxicated Euglena cells should be due to activation of biotransformation system of the Euglena cells by FeZW.

A more desirable balanced polyunsaturated fatty acid composition achieved by heterologous expression of Δ15/Δ4 desaturases in mammalian cells

Arachidonic (ARA), eicosapentaenoic (EPA) and docosahexaenoic (DHA) acids are the most biologically active polyunsaturated fatty acids, but their biosyntheses in mammals are very limited. The biosynthesis of DHA is the most difficult, because this undergoes the Sprecher pathway--a further elongation step from docosapentaenoic acid (DPA), a Δ6-desaturase acting on a C24 fatty acid substrate followed by a peroxisomal chain shortening step. This paper reports the successful heterologous expression of two non-mammalian genes (with modification of codon usage), coding for Euglena gracilis Δ4-desaturase and Siganus canaliculatus Δ4-desaturase respectively, in mammalian cells (HEK293 cell line). Both of the Δ4-desaturases can efficiently function, directly converting DPA into DHA. Moreover, the cooperation of the E. gracilis Δ4-desaturase with C. elegans Δ15-desaturase (able to convert a number of n-6 PUFAs to their corresponding n-3 PUFAs) in transgenic HEK293 cells made a more desirable fatty acid composition--a drastically reduced n-6/n-3 PUFAs ratio and a high level of DHA as well as EPA and ARA. Our findings provide a basis for potential applications of the gene constructs for expression of Δ15/Δ4-desaturases in transgenic livestock to produce such a fatty acid profile in the related products, which certainly will bring benefit to human health.

Removal, accumulation and resistance to chromium in heterotrophic Euglena gracilis

The removal, uptake and toxicity of chromium in Euglena gracilis cultured in absence and presence of malate with Cr(VI) or Cr(III) was evaluated. The malate extrusion and the extra- and intracellular Cr(VI) reduction capacity were determined and the contents of molecules with thiol group and ascorbate were also evaluated. Absence of malate in the medium decreased cell growth, increased Cr(III) toxicity, induced faster Cr(VI) disappearance from medium, and increased intracellular and intramitochondrial chromium accumulation. Both chromium species induced soluble and particulate ascorbate-dependent chromate reductase activities. Cells also secreted large amounts of malate and increased intracellular contents of thiol-molecules to bind extracellular and intracellular Cr(III), respectively. The former process was supported by significant increase in malate-producing enzyme activities and the assessment of the Cr-complexes indicated the in situ formation with thiol-molecules. The present results establish new paradigms regarding chromium stress on algae-like microorganisms: (i) Cr(III) may be more toxic than Cr(VI), depending on the culture (or environmental) conditions; (ii) several simultaneous mechanisms are turned on to inactivate chromium species and their toxic effects. These mechanisms, now well understood may further optimize, by genetically modifying E. gracilis, and facilitate the development of strategies for using this protist as potential bio-remediator of chromium-polluted water systems.

The involvement of a protein kinase in phototaxis and gravitaxis of Euglena gracilis

The unicellular flagellate Euglena gracilis shows positive phototaxis at low-light intensities (<10 W/m(2)) and a negative one at higher irradiances (>10 W/m(2)). Phototaxis is based on blue light-activated adenylyl cyclases, which produce cAMP upon irradiation. In the absence of light the cells swim upward in the water column (negative gravitaxis). The results of sounding rocket campaigns and of a large number of ground experiments led to the following model of signal perception and transduction in gravitaxis of E. gracilis: The body of the cell is heavier than the surrounding medium, sediments and thereby exerts a force onto the lower membrane. Upon deviation from a vertical swimming path mechano-sensitive ion channels are activated. Calcium is gated inwards which leads to an increase in the intracellular calcium concentration and causes a change of the membrane potential. After influx, calcium activates one of several calmodulins found in Euglena, which in turn activates an adenylyl cyclase (different from the one involved in phototaxis) to produce cAMP from ATP. One further element in the sensory transduction chain of both phototaxis and gravitaxis is a specific protein kinase A. We found five different protein kinases A in E. gracilis. The blockage of only one of these (PK.4, accession No. EU935859) by means of RNAi inhibited both phototaxis and gravitaxis, while inhibition of the other four affected neither phototaxis nor gravitaxis. It is assumed that cAMP directly activates this protein kinase A which may in turn phosphorylate a protein involved in the flagellar beating mechanism.

Toxic effects of Cr(VI) and Cr(III) on energy metabolism of heterotrophic Euglena gracilis

To assess the toxic effect of Cr on energy metabolism, heterotrophic Euglena gracilis was grown in a medium that prompts high yield biomass and in the presence of different Cr(VI) or Cr(III) concentrations. The cell growth IC₅₀ value was 12 and >250μM for Cr(VI) and Cr(III), respectively; in these cells chromium was accumulated and a fraction compartmentalized into mitochondria, and synthesis of cysteine and glutathione was induced. Respiration of control isolated mitochondria was strongly inhibited by added Cr(VI) or Cr(III) with L-lactate or succinate as substrates. In turn, cellular and mitochondrial respiration, respiratory Complexes I, III and IV, glycolysis and cytosolic NAD(+)-alcohol and -lactate dehydrogenases from cells cultured with Cr(VI) were significantly lower than control, whereas AOX and external NADH dehydrogenase activities were unaltered or increased, respectively. Addition of Cr(VI) or Cr(III) to isolated mitochondria or cytosol from control- or Cr(VI)-grown cells induced inhibition of respiration, respiratory Complexes III, IV and AOX, and glycolytic pyruvate kinase; whereas Complex I, external NADH dehydrogenase, and other glycolytic enzymes were unaffected. Protein contents of mitochondrial Complexes I, III, IV and V, and ANT were diminished in Cr(VI)-grown cells. Decreased respiration and glycolysis induced by Cr(VI) resulted in lower cellular ATP content. Results suggested that Cr(VI) cytotoxicity altered gene expression (as widely documented) and hence enzyme content, and induced oxidative stress, but it was also related with direct enzyme inhibition; Cr(III) was also cytotoxic although at higher concentrations. These findings establish new paradigms for chromium toxicity: Cr(VI) direct enzyme inhibition and non-innocuous external Cr(III) toxicity.

[Cloning of delta8-fatty acid desaturase gene from Euglena gracilis and its expression in Saccharomyces cerevisiae]

Delta8 desaturase pathway, different from common delta6 desaturase pathway, is an alternate pathway of polyunsaturated fatty acids biosynthesis. Delta8-fatty acid desaturase is one of the key enzymes in delta8 desaturase pathway. Two specific fragments were separately cloned from genomic DNA and cDNA of Euglena gracilis by PCR with the primers designed according to the reported sequence. Comparison of the genomic and cDNA sequences revealed that there wasn't intron in this delta8-fatty acid desaturase gene. This gene has an open reading frame of 1 266 bp that encodes 421 amino acids. It is 6 bp longer than the reported gene sequence, and also showed certain difference from the reported sequence in the N-terminal. The recombinant expression plasmid pYEFD by subcloning delta8-fatty acid desaturase gene into the yeast-E. coli shuttle vector pYES2.0 was constructed and was transformed into the defective mutant INVSc1 of Saccharomyces cerevisiae by electrotransformation. The resulting strain YD8 harboring plasmid pYEFD was selected and was cultured in the induction medium with exogenous substrates omega6-eicosadienoic acid and omega3-eicosatrienoic acid for the expression of delta8-fatty acid desaturase gene. The results indicated that high level expressed As-fatty acid desaturase could convert omega6-eicosadienoic acid and omega3-eicosatrienoic acid to dihomo-gamma-linolenic acid and eicosatetraenoic acid with substrate conversion ratio 31.2% and 46.3%, respectively.

Transketolase from Cyanophora paradoxa: in vitro import into cyanelles and pea chloroplasts and a complex history of a gene often, but not always, transferred in the context of secondary endosymbiosis

The glaucocystophyte Cyanophora paradoxa is an obligatorily photoautotrophic biflagellated protist containing cyanelles, peculiar plastids surrounded by a peptidoglycan layer between their inner and outer envelope membranes. Although the 136-kb cyanelle genome surpasses higher plant chloroplast genomes in coding capacity by about 50 protein genes, these primitive plastids still have to import >2,000 polypeptides across their unique organelle wall. One such protein is transketolase, an essential enzyme of the Calvin cycle. We report the sequence of the pre-transketolase cDNA from C. paradoxa and in vitro import experiments of precursor polypeptides into cyanelles and into pea chloroplasts. The transit sequence clearly indicates the localization of the gene product to cyanelles and is more similar to the transit sequences of the plant homologues than to transit sequences of other cyanelle precursor polypeptides with the exception of a cyanelle consensus sequence at the N-terminus. The mature sequence reveals conservation of the thiamine pyrophosphate binding site. A neighbor-net planar graph suggests that Cyanophora, higher plants, and the photosynthetic protist Euglena gracilis acquired their nuclear-encoded transketolase genes via endosymbiotic gene transfer from the cyanobacterial ancestor of plastids; in the case of Euglena probably entailing two transfers, once from the plastid in the green algal lineage and once again in the secondary endosymbiosis underlying the origin of Euglena's plastids. By contrast, transketolase genes in some eukaryotes with secondary plastids of red algal origin, such as Thalassiosira pseudonana, have retained the pre-existing transketolase gene germane to their secondary host.

Genotoxicity of organic pollutants in source of drinking water on microalga Euglena gracilis

The potential toxicities of organic pollutants in the drinking water source at Meiliang Bay of Lake Taihu were investigated by comet assay and antioxidant enzyme approach on microalgae Euglena gracilis. The organic extracts of the water samples could induce DNA damage on microalgae cells. Statistically significant differences (P < 0.05) were observed at groups of 0.3x, 3x and 10x concentrations compared with the control and a solvent control (DMSO). The organic extracts also affected antioxidant enzyme activity and induced lipid peroxidation in the microalga. In the high dose group, there was an obvious increase in SOD content (P < 0.05). The results suggest that the concentrated organics from water sample extracts have adversary effects on E. gracilis and could possibly damage the ecosystem.

The pathway via D-galacturonate/L-galactonate is significant for ascorbate biosynthesis in Euglena gracilis: identification and functional characterization of aldonolactonase

We have previously proposed that Euglena gracilis possesses a pathway for the production of ascorbate (AsA) through d-galacturonate/L-galactonate as representative intermediates ( Shigeoka, S., Nakano, Y., and Kitaoka, S. (1979) J. Nutr. Sci. Vitaminol. 25, 299-307 ). However, genetic evidence proving that the pathway exists has not been obtained yet. We report here the identification of a gene encoding aldonolactonase, which catalyzes a penultimate step of the biosynthesis of AsA in Euglena. By a BLAST search, we identified one candidate for the enzyme having significant sequence identity with rat gluconolactonase, a key enzyme for the production of AsA via d-glucuronate in animals. The purified recombinant aldonolactonase expressed in Escherichia coli catalyzed the reversible reaction of L-galactonate and L-galactono-1,4-lactone with zinc ion as a cofactor. The apparent K(m) values for L-galactonate and L-galactono-1,4-lactone were 1.55 +/- 0.3 and 1.67 +/- 0.39 mm, respectively. The cell growth of Euglena was arrested by silencing the expression of aldonolactonase through RNA interference and then restored to the normal state by supplementation with L-galactono-1,4-lactone. Euglena cells accumulated more AsA on supplementation with d-galacturonate than d-glucuronate. The present results indicate that aldonolactonase is significant for the biosynthesis of AsA in Euglena cells, which predominantly utilize the pathwayviad-galacturonate/L-galactonate. The identification of aldonolactonase provides the first insight into the biosynthesis of AsA via uronic acids as the intermediate in photosynthetic algae including Euglena.

Enhanced alternative oxidase and antioxidant enzymes under Cd2+ stress in Euglena

To identify some of the mechanisms involved in the high resistance to Cd(2+) in the protist Euglena gracilis, we studied the effect of Cd(2+) exposure on its energy and oxidative stress metabolism as well as on essential heavy metals homeostasis. In E. gracilis heterotrophic cells, as in other organisms, CdCl(2) (50 microM) induced diminution in cell growth, severe oxidative stress accompanied by increased antioxidant enzyme activity and strong perturbation of the heavy metal homeostasis. However, Cd(2+) exposure did not substantially modify the cellular respiratory rate or ATP intracellular level, although the activities of respiratory complexes III and IV were strongly decreased. In contrast, an enhanced capacity of the alternative oxidase (AOX) in both intact cells and isolated mitochondria was determined under Cd(2+) stress; in fact, AOX activity accounted for 69-91% of total respiration. Western blotting also revealed an increased AOX content in mitochondria from Cd(2+)-exposed cells. Moreover, AOX was more resistant to Cd(2+) inhibition than cytochrome c oxidase in mitochondria from control and Cd(2+)-exposed cells. Therefore, an enhanced AOX seems to be a relevant component of the resistance mechanism developed by E. gracilis against Cd(2+)-stress, in addition to the usual increased antioxidant enzyme activity, that enabled cells to maintain a relatively unaltered the energy status.

Heterologous expression of photoactivated adenylyl cyclase (PAC) genes from the flagellate Euglena gracilis in insect cells

The unicellular, green flagellate wild-type Euglena gracilis (strain Z) possesses two genes of the photoactivated adenylyl cyclase (PAC) family. The corresponding gene products were found to be responsible for step-up (but not step-down) photophobic responses as well as both positive and negative phototaxis. The proteins consist of two PACalpha (Mr 105 kDa) and two PACbeta (90 kDa) subunits. In an effort to produce sufficient amounts of PAC proteins, several routes of over-expression have been tried including homologous expression in Euglena and heterologous expression in Escherichia coli. All these approaches were hampered by low yield or formation of inclusion bodies. Therefore we decided to attempt a heterologous expression in an insect cell line. PACalpha and PACbeta were separately cloned in the transfer vector pBacPAK9 with a His tag attached. The transfer vector was subsequently cotransfected via baculovirus into the insect cells and amplified. For the expression both recombinant viruses (containing PACbeta and PACbeta, respectively) were cotransfected simultaneously into insect cells. The expressed proteins were analyzed in Western blots using PACalpha and PACbeta antibodies. Most of the proteins were found to be in soluble form in high yield. The recombinant PAC proteins were purified via their attached His tag on an anti-His resin. Adenylyl cyclase activity was quantified after blue-light excitation using a cAMP enzyme immunoassay kit.

Fast manipulation of cellular cAMP level by light in vivo

The flagellate Euglena gracilis contains a photoactivated adenylyl cyclase (PAC), consisting of the flavoproteins PACalpha and PACbeta. Here we report functional expression of PACs in Xenopus laevis oocytes, HEK293 cells and in Drosophila melanogaster, where neuronal expression yields light-induced changes in behavior. The activity of PACs is strongly and reversibly enhanced by blue light, providing a powerful tool for light-induced manipulation of cAMP in animal cells.

Functional characterization of D-galacturonic acid reductase, a key enzyme of the ascorbate biosynthesis pathway, from Euglena gracilis

D-Galacturonic acid reductase, a key enzyme in ascorbate biosynthesis, was purified to homogeneity from Euglena gracilis. The enzyme was a monomer with a molecular mass of 38-39 kDa, as judged by SDS-PAGE and gel filtration. Apparently it utilized NADPH with a Km value of 62.5+/-4.5 microM and uronic acids, such as D-galacturonic acid (Km=3.79+/-0.5 mM) and D-glucuronic acid (Km=4.67+/-0.6 mM). It failed to catalyze the reverse reaction with L-galactonic acid and NADP(+). The optimal pH for the reduction of D-galacturonic acid was 7.2. The enzyme was activated 45.6% by 0.1 mM H(2)O(2), suggesting that enzyme activity is regulated by cellular redox status. No feedback regulation of the enzyme activity by L-galactono-1,4-lactone or ascorbate was observed. N-terminal amino acid sequence analysis revealed that the enzyme is closely related to the malate dehydrogenase families.

Localization of pyruvate:NADP+ oxidoreductase in sporozoites of Cryptosporidium parvum

Cryptosporidium parvum contains a unique fusion protein pyruvate:NADP+ oxidoreductase (CpPNO) that is composed of two distinct, conserved domains, an N-terminal pyruvate:ferredoxin oxidoreductase (PFO) and a C-terminal cytochrome P450 reductase (CPR). Unlike a similar fusion protein that localizes to the mitochondrion of the photosynthetic protist Euglena gracilis, CpPNO lacks an N-terminal mitochondrial targeting sequence. Using two distinct polyclonal antibodies raised against CpPFO and one polyclonal antibody against CpCPR, Western blot analysis has shown that sporozoites of C. parvum express the entire CpPNO fusion protein. Furthermore, confocal immunofluorescence and transmission electron microscopy confirm that CpPNO is localized within the cytosol rather than the relict mitochondrion of C. parvum. The distribution of this protein is not, however, strictly confined to the cytosol. CpPNO also appears to localize posteriorly within the crystalloid body.

Euglena gracilis Ribonucleotide Reductase

Ribonucleotide reductases provide the building blocks for DNA synthesis. Three classes of enzymes are known, differing widely in amino acid sequence but with similar structural motives and allosteric regulation. Class I occurs in eukaryotes and aerobic prokaryotes, class II occurs in aerobic and anaerobic prokaryotes, and class III occurs in anaerobic prokaryotes. The eukaryote Euglena gracilis contains a class II enzyme (Gleason, F. K., and Hogenkamp, H. P. (1970) J. Biol. Chem. 245, 4894-4899) and, thus, forms an exception. Class II enzymes depend on vitamin B(12) for their activity. We purified the reductase from Euglena cells, determined partial peptide sequences, identified its cDNA, and purified the recombinant enzyme. Its amino acid sequence and general properties, including its allosteric behavior, were similar to the class II reductase from Lactobacillus leichmannii. Both enzymes belong to a distinct small group of reductases that unlike all other homodimeric reductases are monomeric. They compensate the loss of the second polypeptide of dimeric enzymes by a large insertion in the monomeric chain. Data base searching and sequence comparison revealed a homolog from the eukaryote Dictyostelium discoideum as the closest relative to the Euglena reductase, suggesting that the class II enzyme was present in a common, B(12)-dependent, eukaryote ancestor.

Characterization of an Aldehyde Dehydrogenase from Euglena gracilis

The free-living protist Euglena gracilis showed an enhanced growth when cultured in the dark with high concentrations of ethanol as carbon source. In a medium containing glutamate/malate plus 1% ethanol, E. gracilis reached a density of 3 x 10(7) cells/ml after 100 h of culture, which was 5 times higher than that attained with glutamate/malate or ethanol separately. This observation suggested the involvement of a highly active aldehyde dehydrogenase in the metabolism of ethanol. Purification of the E. gracilis aldehyde dehydrogenase from the mitochondrial fraction by affinity chromatography yielded an enrichment of 34 times and recovery of 33% of the total mitochondrial activity. SDS-PAGE and molecular exclusion chromatography revealed a native tetrameric protein of 160 kDa. Kinetic analysis showed Km values of 5 and 50 microM for propionaldehyde and NAD(+), respectively, and a Vm value of 1,300 nmol (min x mg protein)(-1). NAD(+) and NADH stimulated the esterase activity of the purified aldehyde dehydrogenase. The present data indicated that the E. gracilis aldehyde dehydrogenase has kinetic and structural properties similar to those of human aldehyde dehydrogenases class 1 and 2.

Physiological role of rhodoquinone in Euglena gracilis mitochondria

Rhodoquinone (RQ) participates in fumarate reduction under anaerobiosis in some bacteria and some primitive eukaryotes. Euglena gracilis, a facultative anaerobic protist, also possesses significant rhodoquinone-9 (RQ9) content. Growth under low oxygen concentration induced a decrease in cytochromes and ubiquinone-9 (UQ9) content, while RQ9 and fumarate reductase (FR) activity increased. However, in cells cultured under aerobic conditions, a relatively high RQ9 content was also attained together with significant FR activity. In addition, RQ9 purified from E. gracilis mitochondria was able to trigger the activities of cytochrome bc1 complex, bc1-like alternative component and alternative oxidase, although with lower efficiency (higher Km, lower Vm) than UQ9. Moreover, purified E. gracilis mitochondrial NAD+-independent D-lactate dehydrogenase (D-iLDH) showed preference for RQ9 as electron acceptor, whereas L-iLDH and succinate dehydrogenase preferred UQ9. These results indicated a physiological role for RQ9 under aerobiosis and microaerophilia in E. gracilis mitochondria, in which RQ9 mediates electron transfer between D-iLDH and other respiratory chain components, including FR.

The bacterial-like lactate shuttle components from heterotrophic Euglena gracilis

The structural and kinetic analyses of the components of the lactate shuttle from heterotrophic Euglena gracilis were carried out. Mitochondrial membrane-bound, NAD(+)-independent d-lactate dehydrogenase (d-iLDH) was purified by solubilization with CHAPS and heat treatment. The active enzyme was a 62-kDa monomer containing non-covalently bound FAD as cofactor. d-iLDH was specific for d-lactate and it was able to reduce quinones of different redox potential values. Oxalate and l-lactate were mixed-type inhibitors of d-iLDH. Mitochondrial l-iLDH also catalyzed the reduction of quinones, but it was inactivated during the extraction with detergents. Both l-iLDH and d-iLDH were inhibited by the specific flavoprotein-inhibitor diphenyleneiodonium, suggesting that l-iLDH was also a flavoprotein. Affinity chromatography revealed that the E. gracilis cytosolic fraction contained two types of NAD(+)-dependent LDH specific for the generation of d- and l-lactate (d-nLDH and l-nLDH, respectively). These two enzymes were tetramers of 126-132 kDa and showed an ordered bi-bi kinetic mechanism. Kinetic properties were different in both enzymes. Pyruvate reduction by d-nLDH was inhibited by its two products; the d-lactate oxidation was 40-fold lower than forward reaction. l-lactate oxidation by l-nLDH was not detected, whereas pyruvate reduction was activated by fructose-1, 6-bisphosphate, K(+) or NH(4)(+). Interestingly, membrane-bound l- and d-lactate dehydrogenases with quinone reductase activity have been only detected in bacteria, whereas the activity of soluble d-nLDH has been identified in bacteria and some yeast. Also, FBP-activated l-nLDH has been found solely in lactic bacteria. Based on their similar kinetic and structural characteristics, a possible common origin among bacterial and E. gracilis lactic dehydrogenase enzymes is discussed.

Photocycle features of heterologously expressed and assembled eukaryotic flavin-binding BLUF domains of photoactivated adenylyl cyclase (PAC), a blue-light receptor in Euglena gracilis

Photoactivated adenylyl cyclase (PAC) is a recently discovered blue-light photoreceptor that mediates photomovement in Euglena gracilis(Iseki et al., Nature, 2002, 415, 1047--1051). PAC appears to be a heterotetramer composed of two FAD-binding subunits (PACalpha and PACbeta). Both subunits have a pair of homologous regions (F1 and F2) which show homology with prokaryotic "sensors of blue-light using FAD"(BLUF) domains. The F1 and F2 domains of PAC are the only eukaryotic BLUF domains found thus far. We obtained soluble recombinant F1 and F2 proteins in PACalpha by heterologous expression with fused glutathione-S-transferase (GST) in E. coli. The expressed F1 samples did not bind flavins, but the F2 samples contained both FAD and FMN with trace amounts of riboflavin. We also assembled the histidine-tagged recombinant F2 (6His-F2) from inclusion bodies in E. coli with exogenous FAD or FMN. Blue-light-induced changes in absorption spectra of these assembled samples were highly similar to those reported for prokaryotic BLUF domains. The FAD- or FMN-assembled 6His-F2 photocycled with nearly the same rate constants of light-reaction and dark-relaxation, which were slightly lower than those of GST-cleaved F2. The estimated quantum efficiency for the phototransformation was 0.28--0.32, and the half-life was 34--44 s at 25 degrees C for the recombinant PACalpha F2, whereas that reported for prokaryotic BLUF domains varied from ca. 3.5 s (Tll0078) to ca. 900 s (AppA). The mutated recombinant Y472F and Q514G of PACalpha F2 and the F2 domain of the PACalpha homologue from Eutreptiella gymnastica, which lacks the Gln residue conserved in other BLUF domains, showed no photoinduced transformation.

Molecular characterization of a bifunctional glyoxylate cycle enzyme, malate synthase/isocitrate lyase, in Euglena gracilis

Euglena gracilis induced glyoxylate cycle enzymes when ethanol was fed as a sole carbon source. We purified, cloned and characterized a bifunctional glyoxylate cycle enzyme from E. gracilis (EgGCE). This enzyme consists of an N-terminal malate synthase (MS) domain fused to a C-terminal isocitrate lyase (ICL) domain in a single polypeptide chain. This domain order is inverted compared to the bifunctional glyoxylate cycle enzyme in Caenorhabditis elegans, an N-terminal ICL domain fused to a C-terminal MS domain. Purified EgGCE catalyzed the sequential ICL and MS reactions. ICL activity of purified EgGCE increased in the existence of acetyl-CoA at a concentration of micro-molar order. We discussed the physiological roles of the bifunctional glyoxylate cycle enzyme in these organisms as well as its molecular evolution.

Kinetic analysis of the activation of photoactivated adenylyl cyclase (PAC), a blue-light receptor for photomovements of Euglena

Photoactivated adenylyl cyclase (PAC) was first purified from a photosensing organelle (the paraflagellar body) of the unicellular flagellate Euglena gracilis, and is regarded as the photoreceptor for the step-up photophobic response. Here, we report the kinetic properties of photoactivation of PAC and a change in intracellular cAMP levels upon blue light irradiation. Activation of PAC was dependent both on photon fluence rate and duration of irradiation, between which reciprocity held well in the range of 2--50 micromol m(-2) s(-1)(total fluence of 1200 micromol m(-2)). Intermittent irradiation also caused activation of PAC in a photon fluence-dependent manner irrespective of cycle periods. Wavelength dependency of PAC activation showed prominent peaks in the UV-B/C, UV-A and blue regions of the spectrum. The time course of the changes in intracellular cAMP levels corresponded well with that of the step-up photophobic response. From this and the kinetic properties of PAC photoactivation, we concluded that an increase in intracellular cAMP levels evoked by photoactivation of PAC is a key event of the step-up photophobic response.

Photoactivated adenylyl cyclase (PAC) genes in the flagellate Euglena gracilis mutant strains

The unicellular, green flagellate wild-type Euglena gracilis(strain Z) and its colorless phototaxis-mutant strains as well as the non-photosynthetic close relative, Astasia longa, possess several genes of the photoactivated adenylyl cyclase (PAC) family. The corresponding gene products were found to be responsible for step-up (but not step-down) photophobic responses as well as both positive and negative phototaxis. The proteins consist of two PACalpha(M(r) 105 kDa) and two PACbeta(90 kDa) subunits. While the proteins were first believed all to be located in the paraxonemal body (PAB), confocal microscopy revealed that Astasia longa as well as some of the mutant strains do not contain a PAB. Immunofluorescence using PAC antibodies showed that the PAC proteins are also located along the total length of the flagellum at least in some of the strains. In order to determine if the genes responsible for the PAC proteins in the PAB and flagella are identical, sequences of all PAC proteins were analyzed in the Euglena and Astasia strains studied for PAC protein location. Full sequence analysis using PCR and 3' and 5' RACE indicated a substantial divergence between strains with a homology between strains of between 45 and 100%. Sequence alignment and sequence tree construction for the main functional groups (BLUF domain, which binds FAD, and adenylyl cyclase) showed that the pacalpha and the pacbeta gene products form clusters each with some of the mutants being closely related while others show a substantial degree of genetic diversity. The conclusion of these results is that there is a family of very dissimilar PAC proteins located in the PAB and the flagellum where they serve different functions in phototaxis and step-up photophobic reactions.

The Origin of Dihydroorotate Dehydrogenase Genes of Kinetoplastids, with Special Reference to Their Biological Significance and Adaptation to Anaerobic, Parasitic Conditions

Trypanosoma cruzi dihydroorotate dehydrogenase (DHOD), the fourth enzyme of the de novo pyrimidine biosynthetic pathway, is localized in the cytosol and utilizes fumarate as electron acceptor (fumarate reductase activity), while the enzyme from other various eukaryotes is mitochondrial membrane-linked. Here we report that DHOD-knockout T. cruzi did not express the enzyme protein and could not survive even in the presence of pyrimidine nucleosides, substrates for the potentially active salvage pathway, suggesting a vital role of fumarate reductase activity in the regulation of cellular redox balance. Cloning and phylogenetic analysis of euglenozoan DHOD genes showed that the euglenoid Euglena gracilis had a mitochondrial DHOD and that biflagellated bodonids, a sister group of trypanosomatids within kinetoplastids, harbor the cytosolic DHOD. Further, Bodo saliens, a bodonid, had an ACT/DHOD gene fusion encoding aspartate carbamoyltransferase (ACT), the second enzyme of the de novo pyrimidine pathway, and DHOD. This is the first report of this novel gene structure. These results are consistent with suggestions that an ancient common ancestor of Euglenozoa had a mitochondrial DHOD whose descendant exists in E. gracilis and that a common ancestor of kinetoplastids (bodonids and trypanosomatids) subsequently acquired a cytosolic DHOD by horizontal gene transfer. The cytosolic DHOD gene thus acquired may have contributed to adaptation to anaerobiosis in the kinetoplastid lineage and further contributed to the subsequent establishment of parasitism in a trypanosomatid ancestor. Different molecular strategies for anaerobic adaptation in pyrimidine biosynthesis, used by kinetoplastids and by euglenoids, are discussed. Evolutionary implications of the ACT/DHOD gene fusion are also discussed.

Chloroplast phosphoglycerate kinase from Euglena gracilis

Two chloroplast phosphoglycerate kinase isoforms from the photosynthetic flagellate Euglena gracilis were purified to homogeneity, partially sequenced, and subsequently cDNAs encoding phosphoglycerate kinase isoenzymes from both the chloroplast and cytosol of E. gracilis were cloned and sequenced. Chloroplast phosphoglycerate kinase, a monomeric enzyme, was encoded as a polyprotein precursor of at least four mature subunits that were separated by conserved tetrapeptides. In a Neighbor-Net analysis of sequence similarity with homologues from numerous prokaryotes and eukaryotes, cytosolic phosphoglycerate kinase of E. gracilis showed the highest similarity to cytosolic and glycosomal homologues from the Kinetoplastida. The chloroplast isoenzyme of E. gracilis did not show a close relationship to sequences from other photosynthetic organisms but was most closely related to cytosolic homologues from animals and fungi.

Modification of the proteolytic fragmentation pattern upon oxidation of cysteines from ribulose 1,5-bisphosphate carboxylase/oxygenase

The proteolytic susceptibility of the native CO(2)-fixing photosynthetic enzyme ribulose 1,5-bisphosphate carboxylase/oxygenase (EC 4.1.1.39, Rubisco) has been shown to increase in vitro after oxidative treatments that affect cysteine thiols. A limited incubation of oxidized (pretreated with the disulfide cystamine) Rubisco from Chlamydomonas reinhardtii with subtilisin or proteinase K generated fragments of molecular mass about 53 kDa (band I in SDS-PAGE) and 47 kDa (band II) derived from the large subunit (55 kDa) of the enzyme. In contrast, proteolysis of the reduced Rubisco (pretreated with the free thiol cysteamine) produced only the 53 kDa band. The same fragmentation pattern was reproduced with Rubiscos from other algae and higher plants, as well as with other chemical modifications of protein cysteines. N-terminal sequencing of the fragments showed that band I arised from clipping the unstructured N-terminal stretch of the large subunit up to Lys18. Band II was generated by a cleavage close to Val69. The increased susceptibility of the oxidized form resulted from proteases gaining access to a loop (from Ser61 to Thr68) located between stretches of secondary structure that form the N-terminal domain. Native electrophoresis and kinetic analysis of fragment accumulation during subtilisin digestion demonstrated that subunit dissociation was induced by the proteolytic processing at the Ser61-Thr68 loop, which is characteristic of the oxidized Rubisco. Holoenzyme dissasembly was readily followed by the full degradation of the released subunits. In contrast, the limited processing to band I observed with the reduced enzyme did not compromise the quaternary structure of the Rubisco hexadecamer, thus preventing further proteolysis.

Biosynthesis of docosahexaenoic acid in Euglena gracilis: biochemical and molecular evidence for the involvement of a Delta4-fatty acyl group desaturase

Docosahexaenoic acid (DHA) can be synthesized via alternative routes from which only the omega3/omega6-pathways involve the action of a Delta4-fatty acid desaturase. We examined the suitability of Euglena gracilis, Thraustochytrium sp., Schizochytrium sp., and Crypthecodinium cohnii to serve as sources for cloning a cDNA encoding a Delta4-fatty acid desaturase. For this purpose we carried out in vivo labeling studies with radiolabeled C22 polyunsaturated fatty acid substrates. Schizochytrium sp. was unable to convert exogenously supplied [2-(14)C]-docosapentaenoic acid (DPA, 22:5(Delta)(7,10,13,16,19)) to DHA, while E. gracilis and Thraustochytrium sp. carried out this desaturation very efficiently. Hydrogenation and alpha-oxidation of the labeled DHA isolated from these two organisms showed that it was the result of direct Delta4-desaturation and not of substrate breakdown and resynthesis. To clone the desaturase gene, a cDNA library of E. gracilis was subjected to mass sequencing. A full-length clone with highest homology to the Delta4-desaturase of Thraustochytrium sp. was isolated, and its function was verified by heterologous expression in yeast. The desaturase efficiently converted DPA to DHA. Analysis of the substrate specificity demonstrated that the enzyme activity was not limited to C22 fatty acids, since it also efficiently desaturated C16 fatty acids. The enzyme showed strict Delta4-regioselectivity and required the presence of a Delta7-double bond in the substrate. Positional analysis of phosphatidylcholine revealed that the proportion of the Delta4-desaturated products was up to 20 times higher in the sn-2 position than in the sn-1 position.

A comparative study of antioxidant protection in cryopreserved unicellular algae Euglena gracilis and Haematococcus pluvialis

Algal culture collections are required to develop robust and broadly applicable cryogenic storage methods for diverse taxonomic groups and understanding differential responses to cryoinjury helps to achieve this end. Antioxidant profiles were constructed for cryopreserved Euglena gracilis (Klebs CCAP 1224-5Z), a freeze sensitive alga, and Haematococcus pluvialis (Flotow CCAP 34-8), which is a highly freeze-tolerant. H. pluvialis had a coordinated antioxidant response with respect to catalase, superoxide dismutase (SOD) and glutathione reductase; it is postulated that this may contribute to freeze tolerance. Formation (via SOD) and removal (via catalase) of hydrogen perioxide were not fully coordinated in freeze-sensitive E. gracilis and this may exacerbate cryoinjury. Increased SOD activity in the absence of catalase thus compromises survival due to the formation of hydroxyl radicals (.OH) from hydrogen perioxide. Changes in sulfhydryl group (SH) status for non-Protein bound SH groups were greater in freeze-tolerant H. pluvialis. Therefore, the tolerant organism may have a range of coordinated protection mechanisms that ameliorate the deleterious effects of oxidative stress during cryopreservation.

Presence of glyoxylate cycle enzymes in the mitochondria of Euglena gracilis

Isocitrate lyase and malate synthase are specific enzymes of the glyoxylate cycle, used here as glyoxysomal markers. Both enzymes were found in the mitochondrial fraction after organelle fractionation by isopycnic centrifugation. Electron microscopy of this fraction indicated that mitochondria were the only recognizable organelles. Using an immunogold labeling method with anti-(malate synthase) antiserum, the only organelles stained in cells were the mitochondria. These results show that the glyoxylate cycle is present in mitochondria in Euglena.

Constitutive overexpression of immunoidentical forms of PCP-induced Euglena gracilis CYP-450

Environmental pollutants are classically associated with increased drug metabolism. In this report, antibodies that are able to detect mammalian CYP proteins, namely the CYP1A1, CYP1A2, CYP2B1/B2, and CYP3A4 proteins, were used to investigate the expression of CYP-related proteins in Euglena gracilis (EG) cells under normal and PCP-treated conditions and in a EG-cell line adapted to PCP. Compared to normal conditions, the presence of PCP in the culture medium induced elevated levels of EG CYP-like proteins. With the exception of CYP3A4, this overexpression was correlated with expression of additional forms of CYP proteins having, respectively, the same molecular weight but slightly different pIs. Even in EG cells which had lost their PCP-adapted property after having been cultured without PCP, these additional forms were continuously expressed. This observation raised the question about the definition of a biomarker of pollution.

Kinetic and thermodynamic characterization of adenylyl cyclase from Euglena gracilis

Some kinetic and thermodynamic properties of the plasma membrane adenylyl cyclase (AC) from the protist Euglena gracilis were examined. The AC kinetics for Mg-ATP was hyperbolic with a K(m) value of 0.33-0.43 mM, whereas the inhibition exerted by 2('),5(')-dideoxyadenosine was of the mixed type with a K(i) of 80-147 microM. The V(m) value (0.9 or 1.8 nmol(mg protein)(-1)min(-1)) changed, depending upon the carbon source in the growth medium (lactic acid or glutamate plus malate). Lactic acid membrane AC was slightly more thermolabile (from 28 to 40 degrees C) and showed higher activation energy (range 15-25 degrees C). With lactate, the total and saturated fatty acid percentage content in the plasma membrane was significantly greater than with glutamate plus malate, whereas the percentage content of polyunsaturated (n-3) fatty acids was lower. The data suggest that the fatty acid composition, as changed by the carbon source in the growth medium, may modulate the AC activity in Euglena.

Occurrence of a novel NADP(+)-linked alcohol dehydrogenase in Euglena gracilis

An NADP(+)-dependent alcohol dehydrogenase was found in Euglena gracilis Z grown on 1-hexanol, while it was detected at low activity in cells grown on ethanol or glucose as a carbon source, indicating that the enzyme is induced by the addition of 1-hexanol into the medium as a carbon source. This enzyme was extremely unstable, even at 4 degrees C, unless 20% ethylene glycol was added. The optimal pH was 8.8-9.0 for oxidation reaction. The apparent K(m) values for 1-hexanol and NADP(+) were found to be 6.79 mM and 46.7 microM for this enzyme, respectively. The substrate specificity of this enzyme was very different from that of already purified NAD(+)-specific ethanol dehydrogenase by showing the highest activity with 1-hexanol as a substrate, followed by 1-pentanol and 1-butanol, and there was very little activity with ethanol and 1-propanol. This enzyme was active towards the primary alcohols but not secondary alcohols. Accordingly, since the NADP(+)-specific enzyme was separated on DEAE cellulose column, Euglena was confirmed to contain a novel enzyme to be active towards middle and long-chain length of fatty alcohols.

Identification and comparative analysis of the chloroplast alpha-subunit gene of DNA-dependent RNA polymerase from seven Euglena species

When the sequence of the Euglena gracilis chloroplast genome was reported in 1993 the alpha-subunit gene (rpoA) of RNA polymerase appeared to be missing, based on a comparison of all putative reading frames to the then known rpoA loci. Since there has been a large increase in known rpoA sequences, the question of a Euglena chloroplast rpoA gene was re-examined. A previously described unknown reading frame of 161 codons was found to be part of an rpoA gene split by a single group III intron. This rpoA gene, which is highly variable from species to species, was then isolated and characterized in five other euglenoid species, Euglena anabaena, Euglena granulata, Euglena myxocylindracea, Euglena stellata and Euglena viridis, and in the Astasia longa plastid genome. All seven Euglena rpoA genes have either one or three group III introns. The rpoA gene products in Euglena spp. appear to be the most variable in this gene family when compared to the rpoA gene in other species of bacteria, algae and plants. Additionally, Euglena rpoA proteins lack a C-terminal domain required for interaction with some regulatory proteins, a feature shared only with some chlorophyte green algae. The E.gracilis rpoA gene is the distal cistron of a multigene cluster that includes genes for carbohydrate biosynthesis, photosynthetic electron transport, an antenna complex and ribosomal proteins. This study provides new insights into the transcription system of euglenoid plastids, the organization of the plastid genome, group III intron evolution and euglenoid phylogeny.

Deoxyribonuclease II purified from Euglena gracilis SM-ZK, a chloroplast-lacking mutant: comparison with porcine spleen deoxyribonuclease II

An acid deoxyribonuclease was extracted from Euglena gracilis SM-ZK, a chloroplast-lacking strain, by homogenizing the cells in 50 mM sodium acetate (pH 4.6). The enzyme was then purified by heat treatment and a series of chromatographic separations. The molecular mass of the Euglena acid DNase was estimated to be 45 kDa by sensitive activity staining in an SDS-polyacrylamide gel using SYBR Green. Treatment of the Euglena enzyme with a reducing agent prior to electrophoresis destroyed its DNase activity in the gel, indicating that disulfide bridging is essential for its enzyme activity. Nucleolytic properties of this enzyme are essentially the same as to those of porcine DNase II. The Euglena enzyme acts on both double-stranded (ds) and single-stranded DNA, but acts preferentially on dsDNA with an optimum pH at approximately 5.3. EDTA did not inhibit its enzyme activity. Euglena DNase makes double-strand breaks in circular DNA substrate and generates a terminus with 3'-phosphate and 5'-OH. These results indicate that the Euglena acid DNase is in fact a member of the DNase II family.

The Membrane-bound L- and D-lactate dehydrogenase activities in mitochondria from Euglena gracilis

The activity of the pyridine nucleotide-independent lactate dehydrogenase (iLDH) was characterized in mitochondria isolated from the protist Euglena gracilis. The dissociation constants for L- and D-lactate were similar, but the V(max) was higher with the d isomer. A ping-pong kinetic mechanism was displayed with 2,4-dichlorophenol-indolphenol (DCPIP), or coenzyme Q(1), reacting as the second substrate with the modified, reduced enzyme. Oxamate was a competitive inhibitor against both L- and D-lactate. Oxalate exerted a mixed-type inhibition regarding L- or D-lactate and also against DCPIP. The rate of L-lactate uptake was partially inhibited by mersalyl and lower than the rate of dehydrogenation, which was mersalyl-insensitive. These data suggested that the active site of L-iLDH was orientated toward the intermembrane space. The following observations indicated the existence of two stereo-specific iLDH enzymes in the inner membrane of Euglena mitochondria: a greater affinity of the D-iLDH for both inhibitors, D-iLDH thermo-stability at 70 degrees C and denaturation of L-iLDH, opposite signs in the enthalpy change for the association reaction of the isomers to the enzyme, differential solubilization of both activities with detergents, and different molecular mass.

Pyruvate : NADP+ oxidoreductase from the mitochondrion of Euglena gracilis and from the apicomplexan Cryptosporidium parvum: a biochemical relic linking pyruvate metabolism in mitochondriate and amitochondriate protists

Most eukaryotes perform the oxidative decarboxylation of pyruvate in mitochondria using pyruvate dehydrogenase (PDH). Eukaryotes that lack mitochondria also lack PDH, using instead the O(2)-sensitive enzyme pyruvate : ferredoxin oxidoreductase (PFO), which is localized either in the cytosol or in hydrogenosomes. The facultatively anaerobic mitochondria of the photosynthetic protist Euglena gracilis constitute a hitherto unique exception in that these mitochondria oxidize pyruvate with the O(2)-sensitive enzyme pyruvate : NADP oxidoreductase (PNO). Cloning and analysis of Euglena PNO revealed that the cDNA encodes a mitochondrial transit peptide followed by an N-terminal PFO domain that is fused to a C-terminal NADPH-cytochrome P450 reductase (CPR) domain. Two independent 5.8-kb full-size cDNAs for Euglena mitochondrial PNO were isolated; the gene was expressed in cultures supplied with 2% CO(2) in air and with 2% CO(2) in N(2). The apicomplexan Cryptosporidium parvum was also shown to encode and express the same PFO-CPR fusion, except that, unlike E. gracilis, no mitochondrial transit peptide for C. parvum PNO was found. Recombination-derived remnants of PNO are conserved in the genomes of Saccharomyces cerevisiae and Schizosaccharomyces pombe as proteins involved in sulfite reduction. Notably, Trypanosoma brucei was found to encode homologs of both PFO and all four PDH subunits. Gene organization and phylogeny revealed that eukaryotic nuclear genes for mitochondrial, hydrogenosomal, and cytosolic PFO trace to a single eubacterial acquisition. These findings suggest a common ancestry of PFO in amitochondriate protists with Euglena mitochondrial PNO and Cryptosporidium PNO. They are also consistent with the view that eukaryotic PFO domains are biochemical relics inherited from a facultatively anaerobic, eubacterial ancestor of mitochondria and hydrogenosomes.

Evidence for the existence of two soluble NAD(+) kinase isoenzymes in Euglena gracilis Z

Two soluble NAD(+) kinase isoenzymes (isoenzymes 1 and 2) from Euglena gracilis were separated by preparative electrophoresis and characterized. They display several similar properties: both have an identical apparent molecular weight of 68 kDa and their activities are independent on calmodulin, insensitive to 2-mercaptoethanol but inhibited by p-chloromercurybenzoate, 5, 5'-dithiobis(2-nitrobenzoate) and, surprisingly, by low dithiothreitol concentrations, the inhibition by dithiothreitol being irreversible for isoenzyme 1 but reversible for isoenzyme 2. Nevertheless, the two isoenzymes mainly differ by their specificities towards triphosphate nucleotides and their catalytic mechanisms. Isoenzyme 1 is as active in the presence of ATP as of GTP and acts by a ping-pong mechanism with a k(M) for NAD(+) of 0.26 mM and a k(M) for low MgATP(2-)concentrations of 0.03 mM. Isoenzyme 2 is three-fold more active in the presence of GTP than of ATP and operates by a sequential mechanism with k(M)s for NAD(+) and MgGTP(2-) of 1.03 and 0.20 mM, respectively. This study shows the evidence for the existence of two structurally similar but catalytically different NAD(+) kinase isoenzymes in E. gracilis. One resembles the enzyme previously described in bacteria. The other displays a catalytic mechanism identical to that of NAD(+) kinase from other organisms but remains unique among all the NAD(+) kinases studied to-date regarding its specificity towards GTP.