Engineering of Saccharomyces cerevisiae for the production of L-glycerol 3-phosphate

L-glycerol 3-phosphate (L-G3P) was accumulated in Saccharomyces cerevisiae by pathway engineering. Intracellular concentration of this metabolic intermediate could be increased more than 20 times compared to the wild type by overexpressing GPD1 encoding the glycerol 3-phosphate dehydrogenase in a gpp1 Delta gpp2 Delta mutant which lacks both isoenzymes of glycerol 3-phosphatase. Investigation of cellular pattern of triacylglycerols and glycerophospholipids did not reveal considerable changes due to accumulation of their precursor L-G3P. Hyperosmotic stress did not affect the L-G3P pool in the gpp1 Delta gpp2 Delta mutant overexpressing GPD1 despite an about 4-fold increase of specific GPD activity. In contrast, oxygen limitation improved intracellular L-G3P concentration by enhancing the availability of cytosolic NADH. The reduction of pyruvate decarboxylase activity by deleting PDC2 led to an additional increase. In fact, the triple mutant gpp1 Delta gpp2 Delta pdc2 Delta overexpressing GPD1 accumulated 17 mg L-G3P/g dry weight during glucose batch fermentation under oxygen limitation. This value corresponds to an about 100-fold increase compared to that found in the wild type.     

Glycerol-3-phosphatase of Corynebacterium glutamicum

Formation of glycerol as by-product of amino acid production by Corynebacterium glutamicum has been observed under certain conditions, but the enzyme(s) involved in its synthesis from glycerol-3-phosphate were not known. It was shown here that cg1700 encodes an enzyme active as a glycerol-3-phosphatase (GPP) hydrolyzing glycerol-3-phosphate to inorganic phosphate and glycerol. GPP was found to be active as a homodimer. The enzyme preferred conditions of neutral pH and requires Mg2? or Mn2? for its activity. GPP dephosphorylated both L- and D-glycerol-3-phosphate with a preference for the D-enantiomer. The maximal activity of GPP was estimated to be 31.1 and 1.7 U mg?1 with K(M) values of 3.8 and 2.9 mM for DL- and L-glycerol-3-phosphate, respectively. For physiological analysis a gpp deletion mutant was constructed and shown to lack the ability to produce detectable glycerol concentrations. Vice versa, gpp overexpression increased glycerol accumulation during growth in fructose minimal medium. It has been demonstrated previously that intracellular accumulation of glycerol-3-phosphate is growth inhibitory as shown for a recombinant C. glutamicum strain overproducing glycerokinase and glycerol facilitator genes from E. coli in media containing glycerol. In this strain, overexpression of gpp restored growth in the presence of glycerol as intracellular glycerol-3-phosphate concentrations were reduced to wild-type levels. In C. glutamicum wild type, GPP was shown to be involved in utilization of DL-glycerol-3-phosphate as source of phosphorus, since growth with DL-glycerol-3-phosphate as sole phosphorus source was reduced in the gpp deletion strain whereas it was accelerated upon gpp overexpression. As GPP homologues were found to be encoded in the genomes of many other bacteria, the gpp homologues of Escherichia coli (b2293) and Bacillus subtilis (BSU09240, BSU34970) as well as gpp1 from the plant Arabidosis thaliana were overexpressed in E. coli MG1655 and shown to significantly increase GPP activity.     

Glycerol Export and Glycerol-3-phosphate Dehydrogenase, but Not Glycerol Phosphatase, Are Rate Limiting for Glycerol Production in Saccharomyces cerevisiae

Glycerol, one of the most important by-products of alcoholic fermentation, has positive effects on the sensory properties of fermented beverages. It was recently shown that the most direct approach for increasing glycerol formation is to overexpress GPD1, which encodes the glycerol-3-phosphate dehydrogenase (GPDH) isoform Gpd1p. We aimed to identify other steps in glycerol synthesis or transport that limit glycerol flux during glucose fermentation. We showed that the overexpression of GPD2, encoding the other isoform of glycerol-3-phosphate dehydrogenase (Gpd2p), is equally as effective as the overexpression of GPD1 in increasing glycerol production (3.3-fold increase compared to the wild-type strain) and has similar effects on yeast metabolism. In contrast, overexpression of GPP1, encoding glycerol 3-phosphatase (Gpp1p), did not enhance glycerol production. Strains that simultaneously overexpress GPD1 and GPP1 did not produce higher amounts of glycerol than a GPD1-overexpressing strain. These results demonstrate that GPDH, but not the glycerol 3-phosphatase, is rate-limiting for glycerol production. The channel protein Fps1p mediates glycerol export. It has recently been shown that mutants lacking a region in the N-terminal domain of Fps1p constitutively release glycerol. We showed that cells producing truncated Fps1p constructs during glucose fermentation compensate for glycerol loss by increasing glycerol production. Interestingly, the strain with a deregulated Fps1 glycerol channel had a different phenotype to the strain overexpressing GPD genes and showed poor growth during fermentation. Overexpression of GPD1 in this strain increased the amount of glycerol produced but led to a pronounced growth defect.     

Heterologous expression of Zygosaccharomyces rouxii glycerol 3-phosphate dehydrogenase gene (ZrGPD1) and glycerol dehydrogenase gene (ZrGCY1) in Saccharomyces cerevisiae

We examined the effects of heterologous expression of the open reading frames (ORF) of two genes on salt tolerance and glycerol production in a Saccharomyces cerevisiae strain deficient in glycerol synthesis (gpd1Deltagpd2Delta). When the ORF of the Zygosaccharomyces rouxii glycerol 3-phosphate dehydrogenase gene (ZrGPD1) was expressed under the control of the GAL10 promoter, salt tolerance and glycerol production increased; when the ORF of the glycerol dehydrogenase gene (ZrGCY1) was expressed under the control of the GAL1 promoter, no such changes were observed. Zrgcy1p had a weak effect on glycerol production. These results suggest that Zrgpd1p is the primary enzyme involved in Z. rouxii glycerol production, following a mechanism similar to that of S. cerevisiae (Gpd1p). When the ORFs of the S. cerevisiae glycerol 3-phosphatase gene (GPP2) and ZrGPD1 were simultaneously expressed, glycerol production increased, compared with that in yeast expressing only ZrGPD1.     

Thermosensitivity of the <Emphasis Type="Italic">Saccharomyces cerevisiae gpp1gpp2</Emphasis> double deletion strain can be reduced by overexpression of genes involved in cell wall maintenance

A Saccharomyces cerevisiae strain in which the GPP1 and GPP2 genes, both encoding glycerol-3-phosphate phosphatase isoforms, are deleted, displays both osmo- and thermosensitive (ts) phenotypes. We isolated genes involved in cell wall maintenance as multicopy suppressors of the gpp1gpp2 ts phenotype. We found that the gpp1gpp2 strain is hypersensitive to cell wall stress such as treatment with β-1,3-glucanase containing cocktail Zymolyase and chitin-binding dye Calcofluor-white (CFW). Sensitivity to Zymolyase was rescued by overexpression of SSD1, while CFW sensitivity was rescued by SSD1, FLO8 and WSC3—genes isolated as multicopy suppressors of the gpp1gpp2 ts phenotype. Some of the isolated suppressor genes (SSD1, FLO8) also rescued the lytic phenotype of slt2 deletion strain. Additionally, the sensitivity to CFW was reduced when the cells were supplied with glycerol. Both growth on glycerol-based medium and overexpression of SSD1, FLO8 or WSC3 had additive suppressing effect on CFW sensitivity of the gpp1gpp2 mutant strain. We also confirmed that the internal glycerol level changed in cells exposed to cell wall perturbation.     

GPD1, which encodes glycerol-3-phosphate dehydrogenase, is essential for growth under osmotic stress in Saccharomyces cerevisiae, and its expression is regulated by the high-osmolarity glycerol response pathway.

The yeast Saccharomyces cerevisiae responds to osmotic stress, i.e., an increase in osmolarity of the growth medium, by enhanced production and intracellular accumulation of glycerol as a compatible solute. We have cloned a gene encoding the key enzyme of glycerol synthesis, the NADH-dependent cytosolic glycerol-3-phosphate dehydrogenase, and we named it GPD1. gpd1 delta mutants produced very little glycerol, and they were sensitive to osmotic stress. Thus, glycerol production is indeed essential for the growth of yeast cells during reduced water availability. hog1 delta mutants lacking a protein kinase involved in osmostress-induced signal transduction (the high-osmolarity glycerol response [HOG] pathway) failed to increase glycerol-3-phosphate dehydrogenase activity and mRNA levels when osmotic stress was imposed. Thus, expression of GPD1 is regulated through the HOG pathway. However, there may be Hog1-independent mechanisms mediating osmostress-induced glycerol accumulation, since a hog1 delta strain could still enhance its glycerol content, although less than the wild type. hog1 delta mutants are more sensitive to osmotic stress than isogenic gpd1 delta strains, and gpd1 delta hog1 delta double mutants are even more sensitive than either single mutant. Thus, the HOG pathway most probably has additional targets in the mechanism of adaptation to hypertonic medium.     

The control of intracellular glycerol in Saccharomyces cerevisiae influences osmotic stress response and resistance to increased temperature

Glycerol has been demonstrated to serve as the major osmolyte of Saccharomyces cerevisiae. Consistently, mutant strains gpd1gpd2 and gpp1gpp2, which are devoid of the main glycerol biosynthesis pathway, have been shown to be osmosensitive. In addition, the primary hyperosmotic stress response is affected in these strains. Hog1p phosphorylation turned out to be prolonged and osmostress-induced gene expression is delayed compared with the kinetics observed in wild-type cells. A hog1 deletion strain was previously found to contain lower internal glycerol and therefore displays an osmosensitive phenotype. Here, we show that the osmosensitivity of hog1 is suppressed by growth at 37 degrees C. We reasoned that this temperature-remedial osmoresistance might be caused by a higher intracellular glycerol level at the elevated temperature. This hypothesis was confirmed by measurement of the glycerol concentration, which was shown to be similar for wild type and hog1 cells only at elevated growth temperatures. In agreement with this finding, hog1 cells containing an fps1 allele, encoding a constitutively open glycerol channel, have lost their temperature-remedial osmoresistance. Furthermore, gpd1gpd2 and gpp1gpp2 strains were found to be temperature sensitive. The growth defect of these strains could be suppressed by adding external glycerol. In conclusion, the ability to control glycerol levels influences proper osmostress-induced signalling and the cellular potential to grow at elevated temperatures. These data point to an important, as yet unidentified, role of glycerol in cellular functioning.     

Accumulation and release of the osmolyte glycerol is independent of the putative MIP channel Spac977.17p in Schizosaccharomyces pombe

Schizosaccharomyces pombe accumulates glycerol as an osmotic regulatory solute in response to hyper-osmotic conditions. Upon a decrease in the external osmolarity, the intracellular glycerol levels should be adjusted in order to attain osmotic homeostasis. In this study, the patterns and kinetics of glycerol export from S. pombe were investigated. Upon a decrease in external osmolarity, glycerol was rapidly exported from cells to the external medium. The amount of glycerol released from the cells was proportional to the degree of change in the external osmolarity. The export process was well controlled and was not affected by reduced temperature. This points to S. pombe controlling glycerol export using specialized facilitating proteins as has been found in Saccharomyces cerevisiae where a MIP family channel protein Fps1p is involved. Analysis of the S. pombe databases revealed a putative transport protein (Spac977.17p) with homology to glycerol channel proteins of the MIP family. However, expression of the gene into the S. cerevisiae strain lacking a glycerol channel protein (fps1Delta mutant), did not complement the defect in glycerol export during hypo-osmotic stress. Deletion of spac977.17, did not affect glycerol accumulation or release in S. pombe. The patterns and kinetics of glycerol release in the mutant were similar to those of the wild type strains suggesting that the export process is independent of Spac977.17p, the only putative MIP family glycerol channel homologue in S. pombe. While the process of glycerol export in response to hypo-osmotic stress is similar to budding yeast, the underlying molecular mechanism in S. pombe appears distinct from that described in S. cerevisiae. Further studies are needed to elucidate the physiological role of the Spac977.17p channel.     

Cloning and characterization of a NAD+-dependent glycerol-3-phosphate dehydrogenase gene from Candida glycerinogenes, an industrial glycerol producer.

The osmotolerant yeast Candida glycerinogenes produces glycerol as a major metabolite on an industrial scale, but the underlying molecular mechanisms are poorly understood. We cloned and characterized a 4900-bp genomic fragment containing the CgGPD gene encoding a glycerol-3-phosphate dehydrogenase homologous to GPD genes in other yeasts using degenerate primers in conjunction with inverse PCR. Sequence analysis revealed a 1167-bp open reading frame encoding a putative peptide of 388 deduced amino acids with a molecular mass of 42 695 Da. The CgGPD gene consisted of an N-terminal NAD(+)-binding domain and a central catalytic domain, whereas seven stress response elements were found in the upstream region. Functional analysis revealed that Saccharomyces cerevisiae gpd1Delta and gpd1Delta/gpd2Delta osmosensitive mutants transformed with CgGPD were restored to the wild-type phenotype when cultured in high osmolarity media, suggesting that it is a functional GPD protein. Transformants also accumulated glycerol intracellularly and GPD-specific activity increased significantly when stressed with NaCl, whereas the S. cerevisiae mutants transformed with the empty plasmid showed only slight increases. The full-length CgGPD gene sequence including upstream and downstream regions has been deposited in GenBank under accession no. EU186536.     

The glycerol channel Fps1p mediates the uptake of arsenite and antimonite in Saccharomyces cerevisiae

The Saccharomyces cerevisiae FPS1 gene encodes a glycerol channel protein involved in osmoregulation. We present evidence that Fps1p mediates influx of the trivalent metalloids arsenite and antimonite in yeast. Deletion of FPS1 improves tolerance to arsenite and potassium antimonyl tartrate. Under high osmolarity conditions, when the Fps1p channel is closed, wild-type cells show the same degree of As(III) and Sb(III) tolerance as the fps1Delta mutant. Additional deletion of FPS1 in mutants defective in arsenite and antimonite detoxification partially suppresses their hypersensitivity to metalloid salts. Cells expressing a constitutively open form of the Fps1p channel are highly sensitive to both arsenite and antimonite. We also show by direct transport assays that arsenite uptake is mediated by Fps1p. Yeast cells appear to control the Fps1p-mediated pathway of metalloid uptake, as expression of the FPS1 gene is repressed upon As(III) and Sb(III) addition. To our knowledge, this is the first report describing a eukaryotic uptake mechanism for arsenite and antimonite and its involvement in metalloid tolerance.     

Distinct intracellular localization of Gpd1p and Gpd2p, the two yeast isoforms of NAD+-dependent glycerol-3-phosphate dehydrogenase, explains their different contributions to redox-driven glycerol production

During anaerobiosis Saccharomyces cerevisiae strongly increases glycerol production to provide for non-respiratory oxidation of NADH to NAD(+). We here report that respiratory-deficient cells become strictly dependent on the Gpd2p isoform of the NAD(+)-linked glycerol-3-phosphate dehydrogenase (Gpd). The growth inhibition of respiratory incompetent cox18Delta cells lacking GPD2 is reversed by the addition of acetoin, an alternative sink for NADH oxidation. Growth is also restored by addition of lysine or glutamic acid/glutamine, the synthesis of which involves production of mitochondrial NADH. Lysine produced a stronger growth stimulating effect than glutamic acid consistent with an upregulated expression of the IDP3 gene for peroxisomal synthesis of the glutamate precursor alpha-ketoglutarate. Gpd2p is known to be a cytosolic protein but possesses a classical mitochondrial presequence, which we show is sufficient for mitochondrial targeting. A partial mitochondrial localization of Gpd2p will provide for establishment of intramitochondrial redox balance under non-respiratory conditions. Gpd1p, the other Gpd isoform, is partly cytosolic and partly peroxisomal and becomes more strictly peroxisomal in respiratory-deficient mutants. The different cellular distribution of Gpd1p and Gpd2p thus appears to be the main reason Gpd1p cannot substitute for Gpd2p in cox18Deltagpd2Delta cells, despite similar kinetic characteristics of the two iso-enzymes.     

Novel fermentation strategy for enhancing glycerol production by Candida krusei

During the later stage of glycerol production by fermentation of Candida krusei, glycerol consumption by the strain was observed, although there was residual sugar in the medium. To enhance the final glycerol accumulation, a new fermentation strategy was developed by maintaining high activities of glycerol synthetic enzymes (i.e., glycerol-3-phosphate dehydrogenase (ctGPD) and glycerol-3-phosphatase (GPP)) for a relatively long period while conducting oxygen limitation at a later stage to inhibit the increase of another enzyme activity related to glycerol degradation (i.e., mitochondrial glycerol-3-phosphate dehydrogenase (mtGPD)). With oxygen limitation performed from 88 h, when ctGPD and GPP activities were already at a low level while mtGPD activity was increasing, the glycerol dissimilation was efficiently reduced. The final glycerol concentration reached 55.6 g/L, which was about 18% (96 h) and 30% (104 h) higher than control, and its productivity increased to 0.54 g/(L h). The proposed strategy based on cell physiology was proved useful to the glycerol fermentation process.     

Opi1p,Ume6p and Sin3p control expression from the promoter of the INO2 regulatory gene via a novel regulatory cascade

The INO2 gene of Saccharomyces cerevisiae is required for expression of most of the phospholipid biosynthetic genes. INO2 expression is regulated by a complex cascade that includes autoregulation, Opi1p-mediated repression and Ume6p-mediated activation. To screen for mutants with altered INO2 expression directly, we constructed an INO2-HIS3 reporter that provides a plate assay for INO2 promoter activity. This reporter was used to isolate mutants (dim1) that fail to repress expression of the INO2 gene in an otherwise wild-type strain. The dim1 mutants contain mutations in the OPI1 gene. To define further the mechanism for Ume6p regulation of INO2 expression, we isolated suppressors (rum1, 2, 3) of the ume6Delta mutation that overexpress the INO2-HIS3 gene. Two of the rum mutant groups contain mutations in the OPI1 and SIN3 genes showing that opi1 and sin3 mutations are epistatic to the ume6Delta mutation. These results are surprising given that Ume6p, Sin3p and Rpd3p are known to form a complex that represses the expression of a diverse set of yeast genes. This prompted us to examine the effect of sin3Delta and rpd3Delta mutants on INO2-cat expression. Surprisingly, the sin3Delta allele overexpressed INO2-cat, whereas the rpd3Delta mutant had no effect. We also show that the UME6 gene does not affect the expression of an OPI1-cat reporter. This suggests that Ume6p does not regulate INO2 expression indirectly by regulating OPI1 expression.     

产甘油假丝酵母胞浆3-磷酸甘油脱氢酶基因（CgGPD）功能分析

【目的】从高产甘油生产菌株产甘油假丝酵母（Candida glycerinogenes）基因组中克隆了NAD+依赖3-磷酸甘油脱氢酶编码基因（CgGPD）,但是该基因及其上游调控序列具体的功能还是未知的。本文研究了CgGPD基因及其上游调控序列的功能。【方法】本文以酿酒酵母（Saccharomyces cerevisiae）及其渗透压敏感型突变株为宿主,构建3种不同的酵母表达载体导入酵母细胞,研究了不同酵母转化子在渗透压胁迫条件下CgGPD基因表达对细胞的耐高渗透压胁迫应答及其细胞的甘油合成能力的影响。【结果】实验结果表明无论是以来源于S. cerevisiae 的TPI启动子还是来源于CgGPD基因的启动子,过量表达CgGPD基因的转化子均能够显著加速葡萄糖消耗速度和提高甘油合成能力,在gpd1/gpd2突变株中表达CgGPD基因能够消除细胞对外界高渗透压的敏感性,同时转化子胞内甘油大量积累。【结论】CgGPD基因在野生型酵母S. cerevisiae W303-1A表达显著提高细胞的甘油合成能力,在gpd/1gpd2突变株中能够互补GPD1基因的功能,CgGPD基因表达受渗透压诱导 调控。     

Oxygen limitation improves glycerol production by Candida krusei in a bioreactor

An oxygen limitation strategy based on dynamic enzyme activity was applied to improve glycerol accumulation and decrease the residual sugar level in a fermentation of Candida krusei in a bioreactor. By applying oxygen limitation at 88 h when the activities of two glycerol synthetic enzymes cytosolic glycerol-3-phosphate dehydrogenase (ctGPD) and glycerol-3-phosphatase (GPP) were low and the activity of mitochondrial glycerol-3-phosphate dehydrogenase (mtGPD) which catalyzes the glycerol dissimilation was high, the glycerol dissimilation was efficiently reduced. The final glycerol concentration reached 51.8 g l⁻¹ at 96 h and 54.9 g l⁻¹ at 116 h, which was 18 and 60% higher than the control (without oxygen limitation), respectively. The residual sugar was consumed completely while it was 11.2 g l⁻¹ at the end of fermentation in the control. Under oxygen limitation, ethanol production was detected at a final concentration of 3.6 g l⁻¹. This work suggests a metabolic flux shift by oxygen limitation in the bioreactor.     

In vivo analysis of the mechanisms for oxidation of cytosolic NADH by Saccharomyces cerevisiae mitochondria

During respiratory glucose dissimilation, eukaryotes produce cytosolic NADH via glycolysis. This NADH has to be reoxidized outside the mitochondria, because the mitochondrial inner membrane is impermeable to NADH. In Saccharomyces cerevisiae, this may involve external NADH dehydrogenases (Nde1p or Nde2p) and/or a glycerol-3-phosphate shuttle consisting of soluble (Gpd1p or Gpd2p) and membrane-bound (Gut2p) glycerol-3-phosphate dehydrogenases. This study addresses the physiological relevance of these mechanisms and the possible involvement of alternative routes for mitochondrial oxidation of cytosolic NADH. Aerobic, glucose-limited chemostat cultures of a gut2Delta mutant exhibited fully respiratory growth at low specific growth rates. Alcoholic fermentation set in at the same specific growth rate as in wild-type cultures (0.3 h(-1)). Apparently, the glycerol-3-phosphate shuttle is not essential for respiratory glucose dissimilation. An nde1Delta nde2Delta mutant already produced glycerol at specific growth rates of 0.10 h(-1) and above, indicating a requirement for external NADH dehydrogenase to sustain fully respiratory growth. An nde1Delta nde2Delta gut2Delta mutant produced even larger amounts of glycerol at specific growth rates ranging from 0.05 to 0.15 h(-1). Apparently, even at a low glycolytic flux, alternative mechanisms could not fully replace the external NADH dehydrogenases and glycerol-3-phosphate shuttle. However, at low dilution rates, the nde1Delta nde2Delta gut2Delta mutant did not produce ethanol. Since glycerol production could not account for all glycolytic NADH, another NADH-oxidizing system has to be present. Two alternative mechanisms for reoxidizing cytosolic NADH are discussed: (i) cytosolic production of ethanol followed by its intramitochondrial oxidation and (ii) a redox shuttle linking cytosolic NADH oxidation to the internal NADH dehydrogenase.