Production of geranylgeraniol on overexpression of a prenyl diphosphate synthase fusion gene in Saccharomyces cerevisiae

An acyclic diterpene alcohol, (E,E,E)-geranylgeraniol (GGOH), is one of the important compounds used as perfume and pharmacological agents. A deficiency of squalene (SQ) synthase activity allows yeasts to accumulate an acyclic sesquiterpene alcohol, (E,E)-farnesol, in their cells. Since sterols are essential for the growth of yeasts, a deficiency of SQ synthase activity makes the addition of supplemental sterols to the culture media necessary. To develop a GGOH production method not requiring any supplemental sterols, we overexpressed HMG1 encoding hydroxymethylglutaryl-CoA reductase and the genes of two prenyl diphosphate synthases, ERG20 and BTS1, in Saccharomyces cerevisiae. A prototrophic diploid coexpressing HMG1 and the ERG20-BTS1 fusion accumulated GGOH with neither disruption of the SQ synthase gene nor the addition of any supplemental sterols. The GGOH content on the diploid cultivation in a 5-l jar fermenter reached 138.8 mg/l under optimal conditions.     

代谢工程改造酿酒酵母提高法尼醇产量

【目的】法尼醇(FOH,C_(15)H_(26)O)是一种具有芳香气味的非环状倍半萜醇,被广泛应用于化妆品和医学药物的工业化生产,也可作为航空燃料的理想替代品。具有食品级安全性的酿酒酵母细胞能够合成内源性法尼醇,但其产量很低,无法满足工业生产的需要。因此,需要采用代谢工程手段,改造法尼醇合成途径,以有效提高法尼醇在酿酒酵母中的产量。【方法】以酿酒酵母工业菌株CEN.PK2-1D为底盘细胞,强化甲羟戊酸途径中关键酶的表达水平和弱化麦角固醇合成分支途径,以提高法尼醇合成所需的直接前体物质法尼基焦磷酸(FPP);并分别表达催化FPP合成法尼醇的五种内源磷酸酶和两种异源合酶,筛选能高效合成法尼醇的磷酸酶或合酶。【结果】通过在CEN.PK2-1D(法尼醇产量0.1mg/L)中强化表达甲羟戊酸途径中截短形式的HMG-CoA还原酶(tHMGR1)和FPP合酶(ERG20),使法尼醇产量提高约50.8倍,达到5.08 mg/L;使用HXT1启动子替换鲨烯合酶编码基因ERG9启动子以下调其表达水平,使法尼醇产量进一步提升47.1倍,达到239.17 mg/L。在此基础上,筛选发现,表达酿酒酵母内源性磷酸酶PAH1时,获得最高产量法尼醇,达到393.13 mg/L。【结论】采用代谢工程策略对酿酒酵母法尼醇合成途径进行改造,有效提高法尼醇产量至393.13 mg/L,为目前报道的在酿酒酵母中摇瓶培养条件下的最高产量。     

Overproduction of Geranylgeraniol by Metabolically Engineered Saccharomyces cerevisiae

(E, E, E)-Geranylgeraniol (GGOH) is a valuable starting material for perfumes and pharmaceutical products. In the yeast Saccharomyces cerevisiae, GGOH is synthesized from the end products of the mevalonate pathway through the sequential reactions of farnesyl diphosphate synthetase (encoded by the ERG20 gene), geranylgeranyl diphosphate synthase (the BTS1 gene), and some endogenous phosphatases. We demonstrated that overexpression of the diacylglycerol diphosphate phosphatase (DPP1) gene could promote GGOH production. We also found that overexpression of a BTS1-DPP1 fusion gene was more efficient for producing GGOH than coexpression of these genes separately. Overexpression of the hydroxymethylglutaryl-coenzyme A reductase (HMG1) gene, which encodes the major rate-limiting enzyme of the mevalonate pathway, resulted in overproduction of squalene (191.9 mg liter⁻¹) rather than GGOH (0.2 mg liter⁻¹) in test tube cultures. Coexpression of the BTS1-DPP1 fusion gene along with the HMG1 gene partially redirected the metabolic flux from squalene to GGOH. Additional expression of a BTS1-ERG20 fusion gene resulted in an almost complete shift of the flux to GGOH production (228.8 mg liter⁻¹ GGOH and 6.5 mg liter⁻¹ squalene). Finally, we constructed a diploid prototrophic strain coexpressing the HMG1, BTS1-DPP1, and BTS1-ERG20 genes from multicopy integration vectors. This strain attained 3.31 g liter⁻¹ GGOH production in a 10-liter jar fermentor with gradual feeding of a mixed glucose and ethanol solution. The use of bifunctional fusion genes such as the BTS1-DPP1 and ERG20-BTS1 genes that code sequential enzymes in the metabolic pathway was an effective method for metabolic engineering.(E,E,E)-Geranylgeraniol (GGOH) can be used as an important ingredient for perfumes and as a desirable raw material for synthesizing vitamins A and E (4, 13). It is also known to induce apoptosis in various cancer and tumor cell lines (24, 36). GGOH is the dephosphorylated derivative of (E,E,E)-geranylgeranyl diphosphate (GGPP) (Fig. ​(Fig.1).1). GGPP is a significant intermediate of ubiquinone and carotenoid biosyntheses, especially in carotenoid-producing microorganisms and plant cells. It is also utilized as the lipid anchor of geranylgeranylated proteins. In the yeast Saccharomyces cerevisiae, GGPP is synthesized by GGPP synthase (GGPS), encoded by the BTS1 gene, which catalyzes the condensation of farnesyl diphosphate (FPP) and isopentenyl diphosphate (IPP) rather than the successive addition of IPP molecules to dimethylallyl diphosphate, geranyl diphosphate, and FPP that is detected in mammalian tissues (14). Biologically synthesized GGOH comprises only (E,E,E)-geometric isomers, and only the (E,E,E)-isomers have significant biological activities (23). The chemically synthesized form is usually obtained as mixtures of (E)- and (Z)-isomers and thus has lower potency. Therefore, there is a greater possibility of attaining efficient production of (E,E,E)-GGOH through fermentative production.Open in a separate windowFIG. 1.Biosynthetic pathway for GGOH in S. cerevisiae. The solid arrows indicate the one-step conversions in the biosynthesis, and the dashed arrows indicate the several steps. Intermediates: HMG-CoA, 3-hydroxy-3-methylflutaryl coenzyme A; DMAPP, dimethylallyl diphosphate. Enzymes: HMG-R, HMG-coenzyme A reductase (encoded by the HMG1 gene); FPS, FPP synthase (ERG20).Some yeast strains accumulate ergosterol up to 4.6% dry mass (1). Thus, yeasts have the potential to produce large amounts of GGOH if it is possible to enhance and redirect the metabolic flux to GGOH synthesis. The enzyme 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMG-R), encoded by the HMG1 gene has been shown to be the major rate-limiting enzyme in the mevalonate pathway in S. cerevisiae (12). Overproduction of the catalytic domain of HMG-R in an S. cerevisiae strain resulted in squalene accumulation of up to 1% (27) and 2% (8) dry mass but did not cause any difference in the contents of isoprenoid alcohols such as farnesol (FOH) and geraniol (27). These results suggest that squalene is preferably accumulated rather than GGOH when the mevalonate pathway is enhanced by overexpression of the HMG1 gene. Squalene is synthesized through the condensation of two molecules of FPP catalyzed by squalene synthase (SQS) encoded by the ERG9 gene in S. cerevisiae (Fig. ​(Fig.1).1). The addition of an SQS inhibitor to cultures of S. cerevisiae strains resulted in the production of considerable amounts of FOH (∼77.5 mg liter⁻¹) and relatively small amounts of GGOH (∼2.2 mg liter⁻¹) (20). It has also been reported that SQS-deficient (Δerg9) S. cerevisiae strains, which are sterol auxotrophic, accumulated FPP in their cells (35) and excreted 1.3 mg liter⁻¹ of FOH into the culture medium (5). Therefore, inactivation of SQS seems to enhance FOH rather than GGOH production. This is probably because of the low GGPS activity in S. cerevisiae. Indeed, a carotenoid-producing Rhodotorula yeast strain showed higher GGOH (24.4 mg liter⁻¹) than FOH (4.4 mg liter⁻¹) production on cultivation with an SQS inhibitor (20). Our group previously found that GGOH production could be enhanced by overexpression of the BTS1 gene in S. cerevisiae without SQS inhibition. In addition, coexpression of a fusion of the BTS1 and farnesyl diphosphate synthetase (ERG20) genes along with the HMG1 gene resulted in the production of a substantial amount of GGOH with only a small amount of FOH (C. Ohto, M. Muramatsu, E. Sakuradani, S. Shimizu, and S. Obata, submitted for publication).These results suggest that GGOH can be produced from GGPP through some endogenous phosphatase activities when GGPP synthesis is enhanced. We therefore hypothesized that enhancement of the phosphatase activity could increase the productivity of GGOH. However, it is not clear what kind of phosphatase enhances the GGOH production. It has been reported that the products of the diacylglycerol diphosphate phosphatase (DPP1) gene and lipid phosphate phosphatase (LPP1) gene account for most of the FPP and GGPP phosphatase activities in a particulate (membrane associated) fraction of S. cerevisiae (9). In this study, we found that GGOH production could be enhanced by overexpression of these phosphatase genes. We also demonstrated that overexpression of the BTS1-DPP1 and BTS1-ERG20 fusion genes along with the HMG1 gene further increased GGOH production. Finally, we constructed a high-level GGOH-producing yeast available for industrial processes involving multicopy integration vectors. The productivity of GGOH was evaluated in test tube cultures and 10-liter jar fermentors.     

Comparison of engineered <Emphasis Type="Italic">Saccharomyces cerevisiae</Emphasis> and engineered <Emphasis Type="Italic">Escherichia coli</Emphasis> for the production of an optically pure keto alcohol

In this study, the production of enantiomerically pure (1R,4S,6S)-6-hydroxy-bicyclo[2.2.2]octane-2-one ((−)-2) through stereoselective bioreduction was used as a model reaction for the comparison of engineered Saccharomyces cerevisiae and engineered Escherichia coli as biocatalysts. For both microorganisms, over-expression of the gene encoding the NADPH-dependent aldo-keto reductase YPR1 resulted in high purity of the keto alcohol (−)-2 (>99% ee, 97–98% de). E. coli had three times higher initial reduction rate but S. cerevisiae continued the reduction reaction for a longer time period, thus reaching a higher conversion of the substrate (95%). S. cerevisiae was also more robust than E. coli, as demonstrated by higher viability during bioreduction. It was also investigated whether the NADPH regeneration rate was sufficient to supply the over-expressed reductase with NADPH. Five strains of each microorganism with varied carbon flux through the NADPH regenerating pentose phosphate pathway were genetically constructed and compared. S. cerevisiae required an increased NADPH regeneration rate to supply YPR1 with co-enzyme while the native NADPH regeneration rate was sufficient for E. coli. Nádia Skorupa Parachin and Magnus Carlquist have contributed equally to the paper.     

Stereoselective synthesis of (<Emphasis Type="Italic">R</Emphasis>)-3-quinuclidinol through asymmetric reduction of 3-quinuclidinone with 3-quinuclidinone reductase of <Emphasis Type="Italic">Rhodotorula rubra</Emphasis>

A novel nicotinamide adenine dinucleotide phosphate-dependent carbonyl reductase, 3-quinuclidinone reductase, was isolated from Rhodotorula rubra JCM3782. The enzyme catalyzes the asymmetric reduction of 3-quinuclidinone to (R)-3-quinuclidinol. The gene encoding the enzyme was also cloned and sequenced. A 819-bp nucleotide fragment was confirmed to be the gene encoding the 3-quinuclidinone reductase by agreement of the internal amino acid sequences of the purified enzyme. The gene encodes a total of 272 amino acid residues, and the deduced amino acid sequence shows similarity to those of several short-chain dehydrogenase/reductase family proteins. An expression vector, pWKLQ, which contains the full length 3-quinuclidinone reductase gene was constructed. Using Escherichia coli cells coexpressing the 3-quinuclidinone reductase and glucose dehydrogenase (cofactor regeneration enzyme) genes, 618 mM 3-quinuclidinone was almost stiochiometrically converted to (R)-3-quinuclidinol with an >99.9% enantiomeric excess within 21 h of reaction.     

Identification of glycolaldehyde as the key inhibitor of bioethanol fermentation by yeast and genome-wide analysis of its toxicity

Degradation of lignocellulose with pressurised hot water is an efficient method of bioethanol production. However, the resultant solution inhibits ethanol fermentation by Saccharomyces cerevisiae. Here, we first report that glycolaldehyde, which is formed when lignocellulose is treated with pressurised hot water, inhibits ethanol fermentation. The final concentration of glycolaldehyde formed by the treatment of lignocellulose with pressurised hot water ranges from 1 to 24 mM, and 1–10 mM glycolaldehyde was sufficient to inhibit fermentation. This result indicates that glycolaldehyde is one of the main substances responsible for inhibiting fermentation after pressurised hot water degradation of lignocellulose. Genome-wide screening of S. cerevisiae revealed that genes encoding alcohol dehydrogenase, methylglyoxal reductase, polysomes, and the ubiquitin ligase complex are required for glycolaldehyde tolerance. These novel findings will provide new perspectives on breeding yeast for bioethanol production from biomass treated with pressurised hot water.     

Heterologous expression of the<Emphasis Type="Italic"> Saccharomyces cerevisiae</Emphasis> alcohol acetyltransferase genes in<Emphasis Type="Italic"> Clostridium acetobutylicum</Emphasis> and<Emphasis Type="Italic"> Escherichia coli</Emphasis> for the production of isoamyl acetate

Esters are formed by the condensation of acids with alcohols. The esters isoamyl acetate and butyl butyrate are used for food and beverage flavorings. Alcohol acetyltransferase is one enzyme responsible for the production of esters from acetyl-CoA and different alcohol substrates. The genes ATF1 and ATF2, encoding alcohol acetyltransferases from the yeast Saccharomyces cerevisiae have been sequenced and characterized. The production of acids and alcohols in mass quantities by the industrially important Clostridium acetobutylicum makes it a potential organism for exploitation of alcohol acetyltransferase activity. This report focuses on the heterologous expression of the alcohol acetyltransferases in Escherichia coli and C. acetobutylicum. ATF1 and ATF2 were cloned and expressed in E. coli and ATF2 was expressed in C. acetobutylicum. Isoamyl acetate production from the substrate isoamyl alcohol in E. coli and C. acetobutylicum cultures was determined by head-space gas analysis. Alcohol acetyltransferase I produced more than twice as much isoamyl acetate as alcohol acetyltransferase II when expressed from a high-copy expression vector. The effect of substrate levels on ester production was explored in the two bacterial hosts to demonstrate the efficacy of utilizing ATF1and ATF2 in bacteria for ester production.     

Effects of expression of <Emphasis Type="Italic">lcb1/lcb2</Emphasis> and <Emphasis Type="Italic">lac1/lag1</Emphasis> genes on the biosynthesis of ceramides

Ceramides are important signal messenger molecules due to their role in regulating diverse responses such as cell cycle arrest, apoptosis, and senescence. Yeasts are more suitable for ceramides production than any other microorganisms since they grow fast and are non-pathogenic. However, regulations of the expression of genes involved in sphingolipid synthesis pathway are required to enhance ceramides production. In this study, we investigated the effects of co-expression of two pairs of genes that encode for serine palmitoyltransferase and ceramide synthase, respectively. Effect of other genes of the enzymes associated with the sphingolipid synthesis pathway, 3-ketosphinganine reductase and sphinganine C-4 hydroxylase were also studied and compared. The genes were cloned in to pESC-URA vector. Saccharomyces cerevisiae was cultivated aerobically in YPDG medium at 30°C. Ceramides were seperated from cell extracts by solvent extraction and quantified by HPLC with ELSD. The highest ceramides production (10.52 mg ceramides/g cell) was obtained when 3-ketosphinganine reductase, which is encoded by tsc10 gene, was overexpressed. Also, S. cerevisiae SCEL2,1 overexpressing serine palmitoyltransferase encoded by lcb2 and lcb1 genes, and S. cerevisiae SCEG1C1 overexpressing ceramide synthase encode by lag1 and lac1 genes, showed a high level of ceramides production (10.08 mg ceramides/g cell and 9.88 mg ceramides/g cell, respectively).     

Kinetic modelling reveals current limitations in the production of ethanol from xylose by recombinant Saccharomyces cerevisiae

Saccharomyces cerevisiae lacks the ability to ferment the pentose sugar xylose that is the second most abundant sugar in nature. Therefore two different xylose catabolic pathways have been heterologously expressed in S. cerevisiae. Whereas the xylose reductase (XR)-xylitol dehydrogenase (XDH) pathway leads to the production of the by-product xylitol, the xylose isomerase (XI) pathway results in significantly lower xylose consumption. In this study, kinetic models including the reactions ranging from xylose transport into the cell to the phosphorylation of xylulose to xylulose 5-P were constructed. They were used as prediction tools for the identification of putative targets for the improvement of xylose utilization in S. cerevisiae strains engineered for higher level of the non-oxidative pentose phosphate pathway (PPP) enzymes, higher xylulokinase and inactivated GRE3 gene encoding an endogenous NADPH-dependent aldose reductase. For both pathways, the in silico analyses identified a need for even higher xylulokinase (XK) activity. In a XR-XDH strain expressing an integrated copy of the Escherichia coli XK encoding gene xylB about a six-fold reduction of xylitol formation was confirmed under anaerobic conditions. Similarly overexpression of the xylB gene in a XI strain increased the aerobic growth rate on xylose by 21%. In contrast to the in silico predictions, the aerobic growth also increased 24% when the xylose transporter gene GXF1 from Candida intermedia was overexpressed together with xylB in the XI strain. Under anaerobic conditions, the XI strains overexpressing xylB gene and the combination of xylB and GFX1 genes consumed 27% and 37% more xylose than the control strain.     

Design of a secondary alcohol degradation pathway from <Emphasis Type="Italic">Pseudomonas fluorescens</Emphasis> DSM 50106 in an engineered <Emphasis Type="Italic">Escherichia coli</Emphasis>

The genes encoding an alcohol dehydrogenase, Baeyer–Villiger monooxygenase and an esterase from P. fluorescens DSM 50106, which seemed to be metabolically connected based on the sequence of the corresponding open reading frames, were cloned into one vector (pABE) and functionally expressed in Escherichia coli. Overall expression levels were quite low, however, using whole cells of E. coli JM109 pABE expressing the three recombinant enzymes, conversion of secondary alcohols (C_n) to the corresponding primary alcohols (C_n−2) and acetic acid via ketone and ester was possible. In this way, 2-decanol was almost completely converted within 20 h at 30°C. Thus, it could be shown that the three enzymes are metabolically connected and that they are most probably involved in alkane degradation via sub-terminal oxidation of the acyclic aliphatic hydrocarbons.     

Roles of rat and human aldo-keto reductases in metabolism of farnesol and geranylgeraniol

Farnesol (FOH) and geranylgeraniol (GGOH) with multiple biological actions are produced from the mevalonate pathway, and catabolized into farnesoic acid and geranylgeranoic acid, respectively, via the aldehyde intermediates (farnesal and geranylgeranial). We investigated the intracellular distribution, sequences and properties of the oxidoreductases responsible for the metabolic steps in rat tissues. The oxidation of FOH and GGOH into their aldehyde intermediates were mainly mediated by alcohol dehydrogenases 1 (in the liver and colon) and 7 (in the stomach and lung), and the subsequent step into the carboxylic acids was catalyzed by a microsomal aldehyde dehydrogenase. In addition, high reductase activity catalyzing the aldehyde intermediates into FOH (or GGOH) was detected in the cytosols of the extra-hepatic tissues, where the major reductase was identified as aldo-keto reductase (AKR) 1C15. Human reductases with similar specificity were identified as AKR1B10 and AKR1C3, which most efficiently reduced farnesal and geranylgeranial among seven enzymes in the AKR1A-1C subfamilies. The overall metabolism from FOH to farnesoic acid in cultured cells was significantly decreased by overexpression of AKR1C15, and increased by addition of AKR1C3 inhibitors, tolfenamic acid and R-flurbiprofen. Thus, AKRs (1C15 in rats, and 1B10 and 1C3 in humans) may play an important role in controlling the bioavailability of FOH and GGOH.     

Isolation and heterologous expression of PHA synthesising genes from <Emphasis Type="Italic">Bacillus thuringiensis</Emphasis> R1

The polyhydroxyalkanoate biosynthesis gene locus from Bacillus thuringiensis R1 was isolated, cloned and analyzed at the molecular level. We found that a ∼5 kb SacI–ClaI digested fragment of genomic DNA from B. thuringiensis R1 encoding the PHA synthesising genes, conferred PHA producing ability to E. coli. The fragment was sequenced and found to be of 4787 bp with five open reading frames. Sequence alignment with closely related species of Bacillus in the existing database revealed that the ORFs correspond to phaP, phaQ, phaR, phaB and phaC genes. However, E. coli harboring phaP, phaQ, phaR, phaB and phaC locus produced very low PHA. Furthermore, complementation of the locus with phaA from Ralstonia eutropha increased the PHA production in the recombinant E. coli from 3.0% to 24% of cell dry mass. The putative promoter regions and ribosome binding sites were identified for each of the gene. Conserved domains for PHA synthase and aceto-acetyl-coA reductase were also identified. We hence conclude that the PHA operon of Bacillus thuringiensis R1 consists of phaP, phaQ, phaR, phaB, phaC and complementation of the same with phaA is accountable for its high PHA production.     

A genetic overhaul of <Emphasis Type="Italic">Saccharomyces cerevisiae</Emphasis> 424A(LNH-ST) to improve xylose fermentation

Robust microorganisms are necessary for economical bioethanol production. However, such organisms must be able to effectively ferment both hexose and pentose sugars present in lignocellulosic hydrolysate to ethanol. Wild type Saccharomyces cerevisiae can rapidly ferment hexose, but cannot ferment pentose sugars. Considerable efforts were made to genetically engineer S. cerevisiae to ferment xylose. Our genetically engineered S cerevisiae yeast, 424A(LNH-ST), expresses NADPH/NADH xylose reductase (XR) that prefer NADPH and NAD⁺-dependent xylitol dehydrogenase (XD) from Pichia stipitis, and overexpresses endogenous xylulokinase (XK). This strain is able to ferment glucose and xylose, as well as other hexose sugars, to ethanol. However, the preference for different cofactors by XR and XD might lead to redox imbalance, xylitol excretion, and thus might reduce ethanol yield and productivity. In the present study, genes responsible for the conversion of xylose to xylulose with different cofactor specificity (1) XR from N. crassa (NADPH-dependent) and C. parapsilosis (NADH-dependent), and (2) mutant XD from P. stipitis (containing three mutations D207A/I208R/F209S) were overexpressed in wild type yeast. To increase the NADPH pool, the fungal GAPDH enzyme from Kluyveromyces lactis was overexpressed in the 424A(LNH-ST) strain. Four pentose phosphate pathway (PPP) genes, TKL1, TAL1, RKI1 and RPE1 from S. cerevisiae, were also overexpressed in 424A(LNH-ST). Overexpression of GAPDH lowered xylitol production by more than 40%. However, other strains carrying different combinations of XR and XD, as well as new strains containing the overexpressed PPP genes, did not yield any significant improvement in xylose fermentation.     

Carbon fluxes of xylose-consuming <Emphasis Type="Italic">Saccharomyces cerevisiae</Emphasis> strains are affected differently by NADH and NADPH usage in HMF reduction

Industrial Saccharomyces cerevisiae strains able to utilize xylose have been constructed by overexpression of XYL1 and XYL2 genes encoding the NADPH-preferring xylose reductase (XR) and the NAD⁺-dependent xylitol dehydrogenase (XDH), respectively, from Pichia stipitis. However, the use of different co-factors by XR and XDH leads to NAD⁺ deficiency followed by xylitol excretion and reduced product yield. The furaldehydes 5-hydroxymethyl-furfural (HMF) and furfural inhibit yeast metabolism, prolong the lag phase, and reduce the ethanol productivity. Recently, genes encoding furaldehyde reductases were identified and their overexpression was shown to improve S. cerevisiae growth and fermentation rate in HMF containing media and in lignocellulosic hydrolysate. In the current study, we constructed a xylose-consuming S. cerevisiae strain using the XR/XDH pathway from P. stipitis. Then, the genes encoding the NADH- and the NADPH-dependent HMF reductases, ADH1-S110P-Y295C and ADH6, respectively, were individually overexpressed in this background. The performance of these strains, which differed in their co-factor usage for HMF reduction, was evaluated under anaerobic conditions in batch fermentation in absence or in presence of HMF. In anaerobic continuous culture, carbon fluxes were obtained for simultaneous xylose consumption and HMF reduction. Our results show that the co-factor used for HMF reduction primarily influenced formation of products other than ethanol, and that NADH-dependent HMF reduction influenced product formation more than NADPH-dependent HMF reduction. In particular, NADH-dependent HMF reduction contributed to carbon conservation so that biomass was produced at the expense of xylitol and glycerol formation.     

Improved glutathione production by gene expression in <Emphasis Type="Italic">Pichia pastoris</Emphasis>

To utilize Pichia pastoris to produce glutathione, an intracellular expression vector harboring two genes (gsh1 and gsh2) from Saccharomyces cerevisiae encoding enzymes involved in glutathione synthesis and regulated by the glyceraldehyde-3-phosphate dehydrogenase (GAP) promoter was transformed into P. pastoris GS115. Through Zeocin resistance and expression screening, a transformant that had higher glutathione yield (217 mg/L) in flask culture than the host strain was obtained. In fed-batch culture process, this recombinant strain displayed high activity for converting precursor amino acids into glutathione. The glutathione yield and biomass achieved 4.15 g/L and 98.15 g (dry cell weight, DCW)/L, respectively, after 50 h fermentation combined with addition of three amino acids (15 mmol/L glutamic acid, 15 mmol/L cysteine, and 15 mmol/L glycine).     

Double mutation of the <Emphasis Type="Italic">PDC1</Emphasis> and <Emphasis Type="Italic">ADH1</Emphasis> genes improves lactate production in the yeast <Emphasis Type="Italic">Saccharomyces cerevisiae</Emphasis> expressing the bovine lactate dehydrogenase gene

Expression of a heterologous l-lactate dehydrogenase (l-ldh) gene enables production of optically pure l-lactate by yeast Saccharomyces cerevisiae. However, the lactate yields with engineered yeasts are lower than those in the case of lactic acid bacteria because there is a strong tendency for ethanol to be competitively produced from pyruvate. To decrease the ethanol production and increase the lactate yield, inactivation of the genes that are involved in ethanol production from pyruvate is necessary. We conducted double disruption of the pyruvate decarboxylase 1 (PDC1) and alcohol dehydrogenase 1 (ADH1) genes in a S. cerevisiae strain by replacing them with the bovine l-ldh gene. The lactate yield was increased in the pdc1/adh1 double mutant compared with that in the single pdc1 mutant. The specific growth rate of the double mutant was decreased on glucose but not affected on ethanol or acetate compared with in the control strain. The aeration rate had a strong influence on the production rate and yield of lactate in this strain. The highest lactate yield of 0.75 g lactate produced per gram of glucose consumed was achieved at a lower aeration rate.     

4-Hydroxy-β-damascone and 4-Hydroxy-dihydro-β-damascone from Cigar Tobacco

The cDNA encoding a putative xylose reductase (xyrA) from Aspergillus oryzae was cloned and coexpressed in the yeast Saccharomyces cerevisiae with A. oryzae xylitol dehydrogenase cDNA (xdhA). XyrA exhibited NADPH-dependent xylose reductase activity. The S. cerevisiae strain, overexpressing the xyrA, xdhA, endogenous XKS1, and TAL1 genes, grew on xylose as sole carbon source, and produced ethanol.