Sensitivity of polyamine-deficient Saccharomyces cerevisiae to elevated temperatures

A link between urease and polyamine metabolism in Cryptococcus neoformans

The urease enzyme of Cryptococcus neoformans is linked to different metabolic pathways within the yeast cell, several of which are involved in polyamine metabolism. Cryptococcal biogenic amine production is, however, largely unexplored and is yet to be investigated in relation to urease. The aim of this study was therefore to explore and compare polyamine metabolism in wild-type, urease-negative and urease-reconstituted strains of C. neoformans. Mass spectrometry analysis showed that agmatine and spermidine were the major extra- and intracellular polyamines of C. neoformans and significant differences were observed between 26 and 37 °C. In addition, compared to the wild-type, the relative percentages of extracellular putrescine and spermidine were found to be lower and agmatine higher in cultures of the urease-deficient mutant. The inverse was true for intracellular spermidine and agmatine. Cyclohexylamine was a more potent polyamine inhibitor compared to DL-α-difluoromethylornithine and inhibitory effects were more pronounced at 37 °C than at 26 °C. At both temperatures, the urease-deficient mutant was less susceptible to cyclohexylamine treatment compared to the wild-type. For both inhibitors, growth inhibition was alleviated with polyamine supplementation. This study has provided novel insight into the polyamine metabolism of C. neoformans, highlighting the involvement of urease in biogenic amine production.

Introduction to the Thematic Minireview Series: Sixty plus years of polyamine research

Polyamines have a long history in biochemistry and physiology, dating back to 1678 when Leeuwenhoek first reported crystals that were composed of spermine phosphate in seminal fluid. Their quantification and biosynthetic pathway were first described by Herb and Celia Tabor in collaboration with Sanford Rosenthal in the late 1950s. This work led to immense interest in their physiological functions. The 11 Minireviews in this collection illustrate many of the wide-ranging biochemical effects of the polyamines. This series provides a fitting tribute to Herb Tabor on the occasion of his 100th birthday, demonstrating clearly the importance and growth of the research field that he pioneered in the late 1950s and has contributed to for many years. His studies of the synthesis, function, and toxicity of polyamines have yielded multiple insights into fundamental biochemical processes and formed the basis of successful and continuing drug development. This Minireview series reviews the highly diverse properties of polyamines in bacteria, protozoa, and mammals, highlighting the importance of these molecules in growth, development, and response to the environment, and their involvement in diseases, including cancer, and those caused by parasitic protozoans.

Enhanced tolerance of Saccharomyces cerevisiae to multiple lignocellulose-derived inhibitors through modulation of spermidine contents

Fermentation inhibitors present in lignocellulose hydrolysates are inevitable obstacles for achieving economic production of biofuels and biochemicals by industrial microorganisms. Here we show that spermidine (SPD) functions as a chemical elicitor for enhanced tolerance of Saccharomyces cerevisiae against major fermentation inhibitors. In addition, the feasibility of constructing an engineered S. cerevisiae strain capable of tolerating toxic levels of the major inhibitors without exogenous addition of SPD was explored. Specifically, we altered expression levels of the genes in the SPD biosynthetic pathway. Also, OAZ1 coding for ornithine decarboxylase (ODC) antizyme and TPO1 coding for the polyamine transport protein were disrupted to increase intracellular SPD levels through alleviation of feedback inhibition on ODC and prevention of SPD excretion, respectively. Especially, the strain with combination of OAZ1 and TPO1 double disruption and overexpression of SPE3 not only contained spermidine content of 1.1mg SPD/g cell, which was 171% higher than that of the control strain, but also exhibited 60% and 33% shorter lag-phase period than that of the control strain under the medium containing furan derivatives and acetic acid, respectively. While we observed a positive correlation between intracellular SPD contents and tolerance phenotypes among the engineered strains accumulating different amounts of intracellular SPD, too much SPD accumulation is likely to cause metabolic burden. Therefore, genetic perturbations for intracellular SPD levels should be optimized in terms of metabolic burden and SPD contents to construct inhibitor tolerant yeast strains. We also found that the genes involved in purine biosynthesis and cell wall and chromatin stability were related to the enhanced tolerance phenotypes to furfural. The robust strains constructed in this study can be applied for producing chemicals and advanced biofuels from cellulosic hydrolysates.

Stress and polyamine metabolism in fungi

Fungi, as well as the rest of living organisms must deal with environmental challenges such as stressful stimuli. Fungi are excellent models to study the general mechanisms of the response to stress, because of their simple, but conserved, signal-transduction and metabolic pathways that are often equivalent to those present in other eukaryotic systems. A factor that has been demonstrated to be involved in these responses is polyamine metabolism, essentially of the three most common polyamines: putrescine, spermidine and spermine. The gathered evidences on this subject suggest that polyamines are able to control cellular signal transduction, as well as to modulate protein-protein interactions. In the present review, we will address the recent advances on the study of fungal metabolism of polyamines, ranging from mutant characterization to potential mechanism of action during different kinds of stress in selected fungal models.

Novel chimeric spermidine synthase-saccharopine dehydrogenase gene (SPE3-LYS9) in the human pathogen Cryptococcus neoformans

The Cryptococcus neoformans LYS9 gene (encoding saccharopine dehydrogenase) was cloned and found to be part of an evolutionarily conserved chimera with SPE3 (encoding spermidine synthase). spe3-lys9, spe3-LYS9, and SPE3-lys9 mutants were constructed, and these were auxotrophic for lysine and spermidine, spermidine, and lysine, respectively. Thus, SPE3-LYS9 encodes functional spermidine synthase and saccharopine dehydrogenase gene products. In contrast to Saccharomyces cerevisiae spe3 mutants, the polyamine auxotrophy of C. neoformans spe3-LYS9 mutants was not satisfied by spermine. In vitro phenotypes of spe3-LYS9 mutants included reduced capsule and melanin production and growth rate, while SPE3-lys9 mutants grew slowly at 30 degrees C, were temperature sensitive in rich medium, and died upon lysine starvation. Consistent with the importance of saccharopine dehydrogenase and spermidine synthase in vitro, spe3-lys9 mutants were avirulent and unable to survive in vivo and both functions individually contributed to virulence. SPE3-LYS9 mRNA levels showed little evidence of being influenced by exogenous spermidine or lysine or starvation for spermidine or lysine; thus, any regulation is likely to be posttranscriptional. Expression in S. cerevisiae of the full-length C. neoformans SPE3-LYS9 cDNA complemented a lys9 mutant but not a spe3 mutant. However, expression in S. cerevisiae of a truncated gene product, consisting of only C. neoformans SPE3, complemented a spe3 mutant, suggesting possible modes of regulation. Therefore, we identified and describe a novel chimeric SPE3-LYS9 gene, which may link spermidine and lysine biosynthesis in C. neoformans.

Abrogation of upstream open reading frame-mediated translational control of a plant S-adenosylmethionine decarboxylase results in polyamine disruption and growth perturbations

S-Adenosylmethionine decarboxylase (AdoMetDC) is a key enzyme in polyamine biosynthesis. We show that the plant AdoMetDC activity is subject to post-transcriptional control by polyamines. A highly conserved small upstream open reading frame (uORF) in the AdoMetDC mRNA 5' leader is responsible for translational repression of a downstream beta-glucuronidase reporter cistron in transgenic tobacco plants. Elimination of the small uORF from an AdoMetDC cDNA led to increased relative translational efficiency of the AdoMetDC proenzyme in transgenic plants. The resulting increased activity of AdoMetDC caused disruption to polyamine levels with depletion of putrescine, reduction of spermine levels, and a more than 400-fold increase in the level of decarboxylated S-adenosylmethionine. These changes were associated with severe growth and developmental defects. The high level of decarboxylated S-adenosylmethionine was not associated with any change in 5'-methylcytosine content in genomic DNA and S-adenosylmethionine levels were more or less normal, indicating a highly efficient system for maintenance of S-adenosylmethionine levels in plants. This work demonstrates that uORF-mediated translational control of AdoMetDC is essential for polyamine homeostasis and for normal growth and development.

Suppression of S-adenosylmethionine decarboxylase activity is a major cause for high-temperature inhibition of pollen germination and tube growth in tomato (Lycopersicon esculentum Mill.)

Possible involvement of impaired polyamine biosynthesis in the poor performance of tomato pollen (Lycopersicon esculentum Mill.) at high temperatures was investigated. Incubation of pollen at 38 degrees C suppressed the increase of S-adenosylmethionine decarboxylase (SAMDC) activity in germinating pollen with little influence on arginine decarboxylase activity. Consequently, spermidine and spermine content in the pollen did not increase at 38 degrees C, while putrescine content increased at both 25 degrees C and 38 degrees C. High-temperature inhibition of pollen germination was alleviated by the addition of spermidine or spermine but not of putrescine to the germination medium. Cycloheximide inhibited SAMDC activity in parallel with pollen germination at 25 degrees C, whereas actinomycin D had no effect on either of them, indicating that enhanced SAMDC activity is associated with de novo protein synthesis. Incubation of crude enzyme extracts at 40 degrees C for 1 h did not affect SAMDC. In addition, high temperatures did not enhance protease activity in germinating pollen. These results indicate that low activity of SAMDC, probably due to impaired protein synthesis or functional enzyme formation, is a major cause for the poor performance of tomato pollen at high temperatures.

Acquisition of tolerance against oxidative damage in Saccharomyces cerevisiae

BACKGROUND

Living cells constantly sense and adapt to redox shifts by the induction of genes whose products act to maintain the cellular redox environment. In the eukaryote Saccharomyces cerevisiae, while stationary cells possess a degree of constitutive resistance towards oxidants, treatment of exponential phase cultures with sub-lethal stresses can lead to the transient induction of protection against subsequent lethal oxidant conditions. The sensors of oxidative stress and the corresponding transcription factors that activate gene expression under these conditions have not yet been completely identified.

RESULTS

We report the role of SOD1, SOD2 and TPS1 genes (which encode the cytoplasmic Cu/Zn-superoxide dismutase, the mitochondrial Mn-isoform and trehalose-6-phosphate synthase, respectively) in the development of resistance to oxidative stress. In all experimental conditions, the cultures were divided into two parts, one was immediately submitted to severe stress (namely: exposure to H2O2, heat shock or ethanol stress) while the other was initially adapted to 40 degrees C for 60 min. The deficiency in trehalose synthesis did not impair the acquisition of tolerance to H2O2, but this disaccharide played an essential role in tolerance against heat and ethanol stresses. We also verified that the presence of only one Sodp isoform was sufficient to improve cellular resistance to 5 mM H2O2. On the other hand, while the lack of Sod2p caused high cell sensitivity to ethanol and heat shock, the absence of Sod1p seemed to be beneficial to the process of acquisition of tolerance to these adverse conditions. The increase in oxidation-dependent fluorescence of crude extracts of sod1 mutant cells upon incubation at 40 degrees C was approximately 2-fold higher than in sod2 and control strain extracts. Furthermore, in Western blots, we observed that sod mutants showed a different pattern of Hsp104p and Hsp26p expression also different from that in their control strain.

CONCLUSIONS

Trehalose seemed not to be essential in the acquisition of tolerance to H2O2 stress, but its absence was strongly felt under water stress conditions such as heat and alcoholic stresses. On the other hand, Sod1p could be involved in the control of ROS production; these reactive molecules could signal the induction of genes implicated within cell tolerance to heat and ethanol. The effects of this deletion needs further investigation.

Spe3, which encodes spermidine synthase, is required for full repression through NRE(DIT) in Saccharomyces cerevisiae

We previously identified a transcriptional regulatory element, which we call NRE(DIT), that is required for repression of the sporulation-specific genes, DIT1 and DIT2, during vegetative growth of Saccharomyces cerevisiae. Repression through this element is dependent on the Ssn6-Tup1 corepressor. In this study, we show that SIN4 contributes to NRE(DIT)-mediated repression, suggesting that changes in chromatin structure are, at least in part, responsible for regulation of DIT gene expression. In a screen for additional genes that function in repression of DIT (FRD genes), we recovered alleles of TUP1, SSN6, SIN4, and ROX3 and identified mutations comprising eight complementation groups of FRD genes. Four of these FRD genes appeared to act specifically in NRE(DIT)mediated repression, and four appeared to be general regulators of gene expression. We cloned the gene complementing the frd3-1 phenotype and found that it was identical to SPE3, which encodes spermidine synthase. Mutant spe3 cells not only failed to support complete repression through NRE(DIT) but also had modest defects in repression of some other genes. Addition of spermidine to the medium partially restored repression to spe3 cells, indicating that spermidine may play a role in vivo as a modulator of gene expression. We suggest various mechanisms by which spermidine could act to repress gene expression.

Deoxyhypusine synthase activity is essential for cell viability in the yeast Saccharomyces cerevisiae

Deoxyhypusine synthase catalyzes the first step in the posttranslational synthesis of an unusual amino acid, hypusine (N epsilon-(4-amino-2-hydroxybutyl)lysine), in the eukaryotic translation initiation factor 5A (eIF-5A) precursor protein. The null mutation in the single copy gene, yDHS, encoding deoxyhypusine synthase results in the loss of viability in the yeast Saccharomyces cerevisiae. Upon depletion of deoxyhypusine synthase, and consequently of eIF-5A, cessation of growth was accompanied by a marked enlargement of cells, suggesting a defect in cell cycle progression or in cell division. Two residues of the yeast enzyme, Lys308 and Lys350, corresponding to Lys287 and Lys329, respectively, known to be critical for the activity of the human enzyme, were targeted for site-directed mutagenesis. The chromosomal ydhs null mutation was complemented by the plasmid-borne yDHS wild-type gene, but not by mutated genes encoding inactive proteins, including that with Lys350-->Arg substitution or with substitutions at both Lys308 and Lys350. The mutated gene ydhs (K308R) encoding a protein with diminished activities (< 1% of wild type) could support growth but only to a very limited extent. These findings provide strong evidence that the hypusine modification is indeed essential for the survival of S. cerevisiae and imply a vital function for eIF-5A in all eukaryotes.

Spermidine biosynthesis in Saccharomyces cerevisae: polyamine requirement of a null mutant of the SPE3 gene (spermidine synthase)

The Saccharomyces cerevisiae SPE3 gene, coding for spermidine synthase, was cloned, sequenced, and localized on the right arm of chromosome XVI. The deduced amino acid sequence has a high similarity to mammalian spermidine synthases, and has putative S-adenosylmethionine binding motifs. To investigate the effect of total loss of the SPE3 gene, we constructed a null mutant of this gene, spe3delta, which has no spermidine synthase activity and has an absolute requirement for spermidine or spermine for the growth. This requirement is satisfied by a very low concentration of spermidine (10(-8) M) or a higher concentration of spermine (10(-6) M).

Polyamines in growth and dimorphism of Paracoccidioides brasiliensis

Putrescine and spermidine were the only polyamines found in Paracoccidioides brasiliensis, a dimorphic fungus pathogenic for humans. Free polyamines (putrescine > spermidine) increased during the first 24 h of yeast growth, with a second peak at 42 h, and also during the first 12 h of mycelium-to-yeast transition (spermidine > putrescine). Conjugated and bound polyamines were also quantified. 1, 4-Diamino-2-butanone decreased free putrescine and spermidine accumulation by inhibiting the activity of ornithine decarboxylase. The increase in free polyamines corresponds to bud emergence in yeast growth and to the mycelium-to-yeast transition of P. brasiliensis.