Heat shock factor-independent heat control of transcription of the CTT1 gene encoding the cytosolic catalase T of Saccharomyces cerevisiae

The effect of DMSO on Saccharomyces cerevisiae yeast with different energy metabolism and antioxidant status

We studied the effect of dimethyl sulfoxide (DMSO) on the biochemical and physiological parameters of S. cerevisiae yeast cells with varied energy metabolism and antioxidant status. The wild-type cells of varied genetic backgrounds and their isogenic mutants with impaired antioxidant defences (Δsod mutants) or response to environmental stress (ESR) (Δmsn2, Δmsn4 and double Δmsn2msn4 mutants) were used. Short-term exposure to DMSO even at a wide range of concentrations (2-20%) had little effect on the metabolic activity of the yeast cells and the stability of their cell membranes, but induced free radicals production and clearly altered their proliferative activity. Cells of the Δsod1 mutant showed greater sensitivity to DMSO in these conditions. DMSO at concentrations from 4 to 10-14% (depending on the strain and genetic background) activated the ESR programme. The effects of long-term exposure to DMSO were mainly depended on the type of energy metabolism and antioxidant system efficiency. Yeast cells with reduced antioxidant system efficiency and/or aerobic respiration were more susceptible to the toxic effects of DMSO than cells with a wild-type phenotype and respiro-fermentative or fully fermentative metabolism. These studies suggest a key role of stress response programs in both the processes of cell adaptation to small doses of this xenobiotic and the processes related to its toxicity resulting from large doses or chronic exposure to DMSO.

Deletion of the transcription factors Hsf1, Msn2 and Msn4 in yeast uncovers transcriptional reprogramming in response to proteotoxic stress

The response to proteotoxic stresses such as heat shock allows organisms to maintain protein homeostasis under changing environmental conditions. We asked what happens if an organism can no longer react to cytosolic proteotoxic stress. To test this, we deleted or depleted, either individually or in combination, the stress-responsive transcription factors Msn2, Msn4, and Hsf1 in Saccharomyces cerevisiae. Our study reveals a combination of survival strategies, which together protect essential proteins. Msn2 and 4 broadly reprogram transcription, triggering the response to oxidative stress, as well as biosynthesis of the protective sugar trehalose and glycolytic enzymes, while Hsf1 mainly induces the synthesis of molecular chaperones and reverses the transcriptional response upon prolonged mild heat stress (adaptation).

Differential Hsp90-dependent gene expression is strain-specific and common among yeast strains

Enhanced phenotypic diversity increases a population's likelihood of surviving catastrophic conditions. Hsp90, an essential molecular chaperone and a central network hub in eukaryotes, has been observed to suppress or enhance the effects of genetic variation on phenotypic diversity in response to environmental cues. Because many Hsp90-interacting genes are involved in signaling transduction pathways and transcriptional regulation, we tested how common Hsp90-dependent differential gene expression is in natural populations. Many genes exhibited Hsp90-dependent strain-specific differential expression in five diverse yeast strains. We further identified transcription factors (TFs) potentially contributing to variable expression. We found that on Hsp90 inhibition or environmental stress, activities or abundances of Hsp90-dependent TFs varied among strains, resulting in differential strain-specific expression of their target genes, which consequently led to phenotypic diversity. We provide evidence that individual strains can readily display specific Hsp90-dependent gene expression, suggesting that the evolutionary impacts of Hsp90 are widespread in nature.

Neuronal temperature perception induces specific defenses that enable C. elegans to cope with the enhanced reactivity of hydrogen peroxide at high temperature

Hydrogen peroxide is the most common reactive chemical that organisms face on the microbial battlefield. The rate with which hydrogen peroxide damages biomolecules required for life increases with temperature, yet little is known about how organisms cope with this temperature-dependent threat. Here, we show that Caenorhabditis elegans nematodes use temperature information perceived by sensory neurons to cope with the temperature-dependent threat of hydrogen peroxide produced by the pathogenic bacterium Enterococcus faecium. These nematodes preemptively induce the expression of specific hydrogen peroxide defenses in response to perception of high temperature by a pair of sensory neurons. These neurons communicate temperature information to target tissues expressing those defenses via an insulin/IGF1 hormone. This is the first example of a multicellular organism inducing their defenses to a chemical when they sense an inherent enhancer of the reactivity of that chemical.

Yeast Protein Kinase A Isoforms: A Means of Encoding Specificity in the Response to Diverse Stress Conditions?

Eukaryotic cells have developed a complex circuitry of signalling molecules which monitor changes in their intra- and extracellular environments. One of the most widely studied signalling pathways is the highly conserved cyclic AMP (cAMP)/protein kinase A (PKA) pathway, which is a major glucose sensing circuit in the yeast Saccharomyces cerevisiae. PKA activity regulates diverse targets in yeast, positively activating the processes that are associated with rapid cell growth (e.g., fermentative metabolism, ribosome biogenesis and cell division) and negatively regulating the processes that are associated with slow growth, such as respiratory growth, carbohydrate storage and entry into stationary phase. As in higher eukaryotes, yeast has evolved complexity at the level of the PKA catalytic subunit, and Saccharomyces cerevisiae expresses three isoforms, denoted Tpk1-3. Despite evidence for isoform differences in multiple biological processes, the molecular basis of PKA signalling specificity remains poorly defined, and many studies continue to assume redundancy with regards to PKA-mediated regulation. PKA has canonically been shown to play a key role in fine-tuning the cellular response to diverse stressors; however, recent studies have now begun to interrogate the requirement for individual PKA catalytic isoforms in coordinating distinct steps in stress response pathways. In this review, we discuss the known non-redundant functions of the Tpk catalytic subunits and the evolving picture of how these isoforms establish specificity in the response to different stress conditions.

Trehalose accumulation and radiation resistance due to prior heat stress in Saccharomyces cerevisiae

In this study, we examined the accumulation of trehalose, a stress-responsive substance, upon gamma-ray irradiation by evaluating the cause of trehalose accumulation and the development of gamma-ray resistance through intracellular trehalose accumulation. Saccharomyces cerevisiae cells cultured to the logarithmic growth phase were irradiated with gamma rays, and the intracellular trehalose content was measured. However, trehalose was not detectable. The yeast cells with trehalose accumulation caused by pre-treatment at 40 °C were irradiated with gamma rays, and the resistance of these cells to gamma radiation was compared with that of cells without heat treatment. Trehalose accumulation resulted in gamma-ray resistance and suppressed the increase in reactive oxygen species, lipid peroxidation, and DNA double-strand break production in yeast cells. The tests were also performed with a trehalose-6-phosphate-synthase (TPS1)-deficient mutant strain (Δtps1) unable to synthesize trehalose, and the results revealed that TPS1 was involved in protection against oxidative stress.

Physiological and Molecular Characterization of an Oxidative Stress-Resistant Saccharomyces cerevisiae Strain Obtained by Evolutionary Engineering

Oxidative stress is a major stress type observed in yeast bioprocesses, resulting in a decrease in yeast growth, viability, and productivity. Thus, robust yeast strains with increased resistance to oxidative stress are in highly demand by the industry. In addition, oxidative stress is also associated with aging and age-related complex conditions such as cancer and neurodegenerative diseases. Saccharomyces cerevisiae, as a model eukaryote, has been used to study these complex eukaryotic processes. However, the molecular mechanisms underlying oxidative stress responses and resistance are unclear. In this study, we have employed evolutionary engineering (also known as adaptive laboratory evolution – ALE) strategies to obtain an oxidative stress-resistant and genetically stable S. cerevisiae strain. Comparative physiological, transcriptomic, and genomic analyses of the evolved strain were then performed with respect to the reference strain. The results show that the oxidative stress-resistant evolved strain was also cross-resistant against other types of stressors, including heat, freeze-thaw, ethanol, cobalt, iron, and salt. It was also found to have higher levels of trehalose and glycogen production. Further, comparative transcriptomic analysis showed an upregulation of many genes associated with the stress response, transport, carbohydrate, lipid and cofactor metabolic processes, protein phosphorylation, cell wall organization, and biogenesis. Genes that were downregulated included those related to ribosome and RNA processing, nuclear transport, tRNA, and cell cycle. Whole genome re-sequencing analysis of the evolved strain identified mutations in genes related to the stress response, cell wall organization, carbohydrate metabolism/transport, which are in line with the physiological and transcriptomic results, and may give insight toward the complex molecular mechanisms of oxidative stress resistance.

Combined hypoxia and high temperature affect differentially the response of antioxidant enzymes, glutathione and hydrogen peroxide in the white shrimp Litopenaeus vannamei

Low oxygen concentration in water (hypoxia) and high temperature are becoming more frequent due to climate change, forcing animals to endure stress or decease. Hypoxia and high temperature stress can lead to reactive oxygen species (ROS) accumulation and oxidative damage to the organisms. The shrimp Litopenaeus vannamei is the most cultivated crustacean worldwide. The aim of this study was to evaluate the expression and enzymatic activity of glutathione peroxidase (GPx), catalase (CAT) and cytosolic manganese superoxide dismutase (cMnSOD) in gills and hepatopancreas from L. vannamei in response to two combined stressors: hypoxia and reoxygenation at control and high temperature (28 vs 35 °C, respectively). In addition, glutathione and hydrogen peroxide content were analyzed. The changes were mainly tissue-specific. In gills, cMnSOD expression and enzymatic activity increased in response to the interactions between oxygen variation and thermal stress, while GPx and CAT were maintained. More changes occurred in GPx, CAT and MnSOD in hepatopancreas than in gills, mainly due to the effect of the individual stress factors of thermal stress or oxygen variations. On the other hand, the redox state of glutathione indicated that during high temperature, changes in the GSH/GSSG ratio occurred due to the fluctuations of GSSG. Hydrogen peroxide concentration was not affected by thermal stress or oxygen variations in hepatopancreas, whereas in gills, it was not detected. Altogether, these results indicate a complex pattern of antioxidant response to hypoxia, reoxygenation, high temperature and their combinations.

A white-to-opaque-like phenotypic switch in the yeast Torulaspora microellipsoides

Torulaspora microellipsoides is an under-characterized budding yeast of the Saccharomycetaceae family that is primarily associated with viticulture. Here we report for the first time to our knowledge that T. microellipsoides undergoes a low-frequency morphological switch from small budding haploid (white) yeast to larger, higher ploidy (opaque) yeast. Comparison of transcriptomes by mRNA-seq revealed 511 differentially regulated genes, with white cells having greater expression of genes involved in stress resistance and complex carbohydrate utilization, and opaque cells up-regulating genes involved in ribosome biogenesis. Growth assays showed that white cells are physiologically more resistant to stationary-phase conditions and oxidative stress, whereas opaque cells exhibited greater cold tolerance. We propose that phenotypic switching in T. microellipsoides is an ecological adaptation, as has been suggested for similar morphological switching in distantly related species like Candida albicans, and we propose that this switching is a more broadly utilized biological strategy among yeasts than previously thought.

Thoughts on the evolution of Core Environmental Responses in yeasts

The model yeasts, Saccharomyces cerevisiae and Schizosaccharomyces pombe, display Core Environmental Responses (CERs) that include the induction of a core set of stress genes in response to diverse environmental stresses. CERs underlie the phenomenon of stress cross-protection, whereby exposure to one type of stress can provide protection against subsequent exposure to a second type of stress. CERs have probably arisen through the accumulation, over evolutionary time, of protective anticipatory responses (“adaptive prediction”). CERs have been observed in other evolutionarily divergent fungi but, interestingly, not in the pathogenic yeast, Candida albicans. We argue that this is because we have not looked in the right place. In response to specific host inputs, C. albicans does activate anticipatory responses that protect it against impending attack from the immune system. Therefore, we suggest that C. albicans has evolved a CER that reflects the environmental challenges it faces in host niches.

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We review Core Environmental Responses (CERs) in domesticated and pathogenic yeasts.

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CERs probably evolved through the accumulation of protective anticipatory responses.

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Evolutionarily diverse yeasts display CERs, but the pathogen, Candida albicans, does not.

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C. albicans has evolved an alternative CER that protects against immune clearance.

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This has implications for the investigation of CERs in other fungi.

Acid Stress Triggers Resistance to Acetic Acid-Induced Regulated Cell Death through Hog1 Activation Which Requires RTG2 in Yeast

Acid stress causes resistance to acetic acid-induced regulated cell death (AA-RCD) in budding yeast, resulting in catalase activation. In order to explore the molecular determinants of evasion of AA-RCD triggered by acid stress adaptation, we studied the involvement and the possible interplay of the master regulator of transcription high-osmolarity glycerol 1 (HOG1) and RTG2, a positive regulator of the RTG-dependent mitochondrial retrograde signaling. Viability, DNA fragmentation, and ROS accumulation have been analyzed in wild-type and mutant cells lacking HOG1 and/or RTG2. Catalase activity and transcription of CTT1 and CTA1, coding the cytosolic and peroxisomal/mitochondrial catalase, respectively, as well as Hog1 phosphorylation, were also analyzed. Our results show that HOG1 is essential for resistance to AA-RCD and its activation results in the upregulation of CTT1, but not CTA1, transcription during acid stress adaptation. RTG2 is required for Hog1-dependent CTT1 upregulation upon acid stress, despite failure of RTG pathway activation. We give evidence that Rtg2 has a cytoprotective role and can act as a general cell stress sensor independent of Rtg1/3-dependent transcription.

A constitutive active allele of the transcription factor Msn2 mimicking low PKA activity dictates metabolic remodeling in yeast

In yeast, protein kinase A (PKA) adjusts transcriptional profiles, metabolic rates, and cell growth in accord with carbon source availability. PKA affects gene expression mostly via the transcription factors Msn2 and Msn4, two key regulators of the environmental stress response. Here we analyze the role of the PKA-Msn2 signaling module using an Msn2 allele that harbors serine-to-alanine substitutions at six functionally important PKA motifs (Msn2A6) . Expression of Msn2A6 mimics low PKA activity, entails a transcription profile similar to that of respiring cells, and prevents formation of colonies on glucose-containing medium. Furthermore, Msn2A6 leads to high oxygen consumption and hence high respiratory activity. Substantially increased intracellular concentrations of several carbon metabolites, such as trehalose, point to a metabolic adjustment similar to diauxic shift. This partial metabolic switch is the likely cause for the slow-growth phenotype in the presence of glucose. Consistently, Msn2A6 expression does not interfere with growth on ethanol and tolerated is to a limited degree in deletion mutant strains with a gene expression signature corresponding to nonfermentative growth. We propose that the lethality observed in mutants with hampered PKA activity resides in metabolic reprogramming that is initiated by Msn2 hyperactivity.

Rational synthetic combination genetic devices boosting high temperature ethanol fermentation

The growth and production of yeast in the industrial fermentation are seriously restrained by heat stress and exacerbated by heat induced oxidative stress. In this study, a novel synthetic biology approach was developed to globally boost the viability and production ability of S. cerevisiae at high temperature through rationally designing and combing heat shock protein (HSP) and superoxide dismutase (SOD) genetic devices to ultimately synergistically alleviate both heat stress and oxidative stress. HSP and SOD from extremophiles were constructed to be different genetic devices and they were preliminary screened by heat resistant experiments and anti-oxidative experiments, respectively. Then in order to customize and further improve thermotolerance of S. cerevisiae, the HSP genetic device and SOD genetic device were rationally combined. The results show the simply assemble of the same function genetic devices to solve heat stress or oxidative stress could not enhance the thermotolerance considerably. Only S. cerevisiae with the combination genetic device (FBA1p-sod-MB4-FBA1p-shsp-HB8) solving both stress showed 250% better thermotolerance than the control and displayed further 55% enhanced cell density compared with the strains with single FBA1p-sod-MB4 or FBA1p-shsp-HB8 at 42 °C. Then the most excellent combination genetic device was introduced into lab S. cerevisiae and industrial S. cerevisiae for ethanol fermentation. The ethanol yields of the two strains were increased by 20.6% and 26.3% compared with the control under high temperature, respectively. These results indicate synergistically defensing both heat stress and oxidative stress is absolutely necessary to enhance the thermotolerance and production of S. cerevisiae.

CTT1 overexpression increases the replicative lifespan of MMS-sensitive Saccharomyces cerevisiae deficient in KSP1

Ksplp is a nuclear-localized Ser/Thr kinase that is not essential for the vegetative growth of yeast. A global gene function analysis in yeast suggested that Ksplp was involved in the oxidative stress response; however, the underlying mechanism remains unclear. Here, we showed that KSP1-deficient yeast cells exhibit hypersensitivity to the DNA alkylating agent methyl methanesulphonate (MMS), and treatment of the KSP1-deficient strain with MMS could trigger abnormal mitochondrial membrane potential and up-regulate reactive oxygen species (ROS) production. In addition, the mRNA expression level of the catalase gene CTT1 (which encodes cytosolic catalase) and total catalase activity were strongly down-regulated in the KSP1-deleted strain compared with those in wild-type cells. Moreover, the KSP1 deficiency also leads to a shortened replicative lifespan, which could be restored by the increased expression of CTT1. On the other hand, KSP1-overexpressed (KSP1OX) yeast cells exhibited increased resistance towards MMS, an effect that was, at least in part, CTT1 independent. Collectively, these findings highlight the involvement of Ksplp in the DNA damage response and implicate Ksplp as a modulator of the replicative lifespan.

Effect of temperature on replicative aging of the budding yeast Saccharomyces cerevisiae

The use of the budding yeast Saccharomyces cerevisiae in gerontological studies was based on the assumption that the reproduction limit of a single cell (replicative aging) is a consequence of accumulation of a hypothetical universal "senescence factor" within the mother cell. However, some evidence suggests that molecules or structures proposed as the "aging factor", such as rDNA circles, oxidatively damaged proteins (with carbonyl groups) or mitochondria, have little effect on replicative lifespan of yeast cells. Our results also suggest that protein aggregates associated with Hsp104, treated as a marker of yeast aging, do not seem to affect the numeric value of replicative lifespan of yeast. What these results indicate, however, is the need for finding a different way of expressing age and longevity of yeast cells instead of the commonly used number of daughters produced over units of time, as in the case of other organisms. In this paper, we show that the temperature has a stronger influence on the time of life (the total lifespan) than on the reproductive potential of yeast cells.

Improved activity of the Cel5A endoglucanase in Saccharomyces cerevisiae deletion mutants defective in oxidative stress defense mechanisms

OBJECTIVES

Developing a Saccharomyces cerevisiae system for optimizing the expression of recombinant eukaryotic proteins.

RESULTS

Two deletion mutants, which were hypersensitive to H2O2, were obtained by knocking out CTT1 and SOD2, respectively. The mutation rate of the mutants was up to over 4000 times of the spontaneous mutation rate when treated with H2O2. Endoglucanase Cel5A was used as a model enzyme to evaluate the system for improving the expression of the recombinant protein. Sixteen mutants of the RDKY3615 (ctt1∆) transformant and seven mutants of the RDKY3615 (sod2∆) transformant had significantly high Cel5A activity, while none mutants of the RDKY3615 transformant had significantly high enzyme activity.

CONCLUSION

The combination of deletion mutagenesis and H2O2 treatment greatly accelerate the generation of genetic variants and will be a useful tool in improving the heterologous expression in S. cerevisiae.

tRNA thiolation links translation to stress responses in Saccharomyces cerevisiae

The URM1 pathway functions in a tRNA thiolation reaction that is required for synthesis of the mcm⁵s²U₃₄ nucleoside found in tRNAs. Growth of Saccharomyces cerevisiae cells at an elevated temperature results in altered levels of modification enzymes, and this leads to decreased levels of tRNA thiolation. tRNA thiolation is tied to cellular stress responses.

Although tRNA modifications have been well catalogued, the precise functions of many modifications and their roles in mediating gene expression are still being elucidated. Whereas tRNA modifications were long assumed to be constitutive, it is now apparent that the modification status of tRNAs changes in response to different environmental conditions. The URM1 pathway is required for thiolation of the cytoplasmic tRNAs tGlu^UUC, tGln^UUG, and tLys^UUU in Saccharomyces cerevisiae. We demonstrate that URM1 pathway mutants have impaired translation, which results in increased basal activation of the Hsf1-mediated heat shock response; we also find that tRNA thiolation levels in wild-type cells decrease when cells are grown at elevated temperature. We show that defects in tRNA thiolation can be conditionally advantageous, conferring resistance to endoplasmic reticulum stress. URM1 pathway proteins are unstable and hence are more sensitive to changes in the translational capacity of cells, which is decreased in cells experiencing stresses. We propose a model in which a stress-induced decrease in translation results in decreased levels of URM1 pathway components, which results in decreased tRNA thiolation levels, which further serves to decrease translation. This mechanism ensures that tRNA thiolation and translation are tightly coupled and coregulated according to need.

Stress adaptation in a pathogenic fungus

Candida albicans is a major fungal pathogen of humans. This yeast is carried by many individuals as a harmless commensal, but when immune defences are perturbed it causes mucosal infections (thrush). Additionally, when the immune system becomes severely compromised, C. albicans often causes life-threatening systemic infections. A battery of virulence factors and fitness attributes promote the pathogenicity of C. albicans. Fitness attributes include robust responses to local environmental stresses, the inactivation of which attenuates virulence. Stress signalling pathways in C. albicans include evolutionarily conserved modules. However, there has been rewiring of some stress regulatory circuitry such that the roles of a number of regulators in C. albicans have diverged relative to the benign model yeasts Saccharomyces cerevisiae and Schizosaccharomyces pombe. This reflects the specific evolution of C. albicans as an opportunistic pathogen obligately associated with warm-blooded animals, compared with other yeasts that are found across diverse environmental niches. Our understanding of C. albicans stress signalling is based primarily on the in vitro responses of glucose-grown cells to individual stresses. However, in vivo this pathogen occupies complex and dynamic host niches characterised by alternative carbon sources and simultaneous exposure to combinations of stresses (rather than individual stresses). It has become apparent that changes in carbon source strongly influence stress resistance, and that some combinatorial stresses exert non-additive effects upon C. albicans. These effects, which are relevant to fungus–host interactions during disease progression, are mediated by multiple mechanisms that include signalling and chemical crosstalk, stress pathway interference and a biological transistor.

Protein folding and secretion: mechanistic insights advancing recombinant protein production in S. cerevisiae

The emergence of genomic approaches coupled to recombinant DNA technologies have identified the quality control systems that regulate proteostasis - biological pathways that modulate protein biogenesis, maturation, trafficking, and degradation. The elucidation of these pathways has become of growing importance in therapeutics as loss of proteostasis has been suggested to lead to a number of human diseases including Alzheimer's, Parkinson's Disease and Type II Diabetes. We anticipate that the most successful strategies for protein expression and therapeutics development may involve integration of protein engineering strategies with host manipulation, to exploit the cell's native stress response pathways and trafficking mechanisms. This review will highlight recent findings and mechanistic detail correlated to quality control in the early secretory pathway of Saccharomyces cerevisiae.

Catalase activity is stimulated by H(2)O(2) in rich culture medium and is required for H(2)O(2) resistance and adaptation in yeast

Catalases are efficient scavengers of H₂O₂ and protect cells against H₂O₂ stress. Examination of the H₂O₂ stimulon in Saccharomyces cerevisiae revealed that the cytosolic catalase T (Ctt1) protein level increases 15-fold on H₂O₂ challenge in synthetic complete media although previous work revealed that deletion of the CCT1 or CTA1 genes (encoding peroxisomal/mitochondrial catalase A) does not increase the H₂O₂ sensitivity of yeast challenged in phosphate buffer (pH 7.4). This we attributed to our observation that catalase activity is depressed when yeast are challenged with H₂O₂ in nutrient-poor media. Hence, we performed a systematic comparison of catalase activity and cell viability of wild-type yeast and of the single catalase knockouts, ctt1∆ and cta1∆, following H₂O₂ challenge in nutrient-rich medium (YPD) and in phosphate buffer (pH 7.4). Ctt1 but not Cta1 activity is strongly induced by H₂O₂ when cells are challenged in YPD but suppressed when cells are challenged in buffer. Consistent with the activity results, exponentially growing ctt1∆ cells in YPD are more sensitive to H₂O₂ than wild-type or cta1∆ cells, whereas in buffer all three strains exhibit comparable H₂O₂ hypersensitivity. Furthermore, catalase activity is increased during adaptation to sublethal H₂O₂ concentrations in YPD but not in buffer. We conclude that induction of cytosolic Ctt1 activity is vital in protecting yeast against exogenous H₂O₂ but this activity is inhibited by H₂O₂ when cells are challenged in nutrient-free media.

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Ctt1 activity increases on H₂O₂ challenge in nutrient-rich medium (YPD) but not in nutrient-free buffer.

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Upregulation of Ctt1 is critical for yeast survival on H₂O₂ challenge.

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The role of inducible Ctt1 activity in stress response is masked in nutrient-free medium.

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To confirm their role in stress response, antioxidant enzyme activities should be compared for wild-type and knockout cells.

Biology of the heat shock response and protein chaperones: budding yeast (Saccharomyces cerevisiae) as a model system

The eukaryotic heat shock response is an ancient and highly conserved transcriptional program that results in the immediate synthesis of a battery of cytoprotective genes in the presence of thermal and other environmental stresses. Many of these genes encode molecular chaperones, powerful protein remodelers with the capacity to shield, fold, or unfold substrates in a context-dependent manner. The budding yeast Saccharomyces cerevisiae continues to be an invaluable model for driving the discovery of regulatory features of this fundamental stress response. In addition, budding yeast has been an outstanding model system to elucidate the cell biology of protein chaperones and their organization into functional networks. In this review, we evaluate our understanding of the multifaceted response to heat shock. In addition, the chaperone complement of the cytosol is compared to those of mitochondria and the endoplasmic reticulum, organelles with their own unique protein homeostasis milieus. Finally, we examine recent advances in the understanding of the roles of protein chaperones and the heat shock response in pathogenic fungi, which is being accelerated by the wealth of information gained for budding yeast.

Transcriptional regulation of small HSP-HSF1 and beyond

The members of the small heat shock protein (sHSP) family are molecular chaperones that play major roles in development, stress responses, and diseases, and have been envisioned as targets for therapy, particularly in cancer. The molecular mechanisms that regulate their transcription, in normal, stress, or pathological conditions, are characterized by extreme complexity and subtlety. Although historically linked to the heat shock transcription factors (HSFs), the stress-induced or developmental expression of the diverse members, including HSPB1/Hsp27/Hsp25, αA-crystallin/HSPB4, and αB-crystallin/HSPB5, relies on the combinatory effects of many transcription factors. Coupled with remarkably different cis-element architectures in the sHsp regulatory regions, they confer to each member its developmental expression or stress-inducibility. For example, multiple regulatory pathways coordinate the spatio-temporal expression of mouse αA-, αB-crystallin, and Hsp25 genes during lens development, through the action of master genes, like the large Maf family proteins and Pax6, but also HSF4. The inducibility of Hsp27 and αB-crystallin transcription by various stresses is exerted by HSF-dependent mechanisms, by which concomitant induction of Hsp27 and αB-crystallin expression is observed. In contrast, HSF-independent pathways can lead to αB-crystallin expression, but not to Hsp27 induction. Not surprisingly, deregulation of the expression of sHSP is associated with various pathologies, including cancer, neurodegenerative, or cardiac diseases. However, many questions remain to be addressed, and further elucidation of the developmental mechanisms of sHsp gene transcription might help to unravel the tissue- and stage-specific functions of this fascinating class of proteins, which might prove to be crucial for future therapeutic strategies. This article is part of a Directed Issue entitled: Small HSPs in physiology and pathology.

Molecular characterization of a catalase from Hydra vulgaris

Catalase, an antioxidant and hydroperoxidase enzyme protects the cellular environment from harmful effects of hydrogen peroxide by facilitating its degradation to oxygen and water. Molecular information on a cnidarian catalase and/or peroxidase is, however, limited. In this work an apparent full length cDNA sequence coding for a catalase (HvCatalase) was isolated from Hydra vulgaris using 3'- and 5'- (RLM) RACE approaches. The 1859 bp HvCatalase cDNA included an open reading frame of 1518 bp encoding a putative protein of 505 amino acids with a predicted molecular mass of 57.44 kDa. The deduced amino acid sequence of HvCatalase contained several highly conserved motifs including the heme-ligand signature sequence RLFSYGDTH and the active site signature FXRERIPERVVHAKGXGA. A comparative analysis showed the presence of conserved catalytic amino acids [His(71), Asn(145), and Tyr(354)] in HvCatalase as well. Homology modeling indicated the presence of the conserved features of mammalian catalase fold. Hydrae exposed to thermal, starvation, metal and oxidative stress responded by regulating its catalase mRNA transcription. These results indicated that the HvCatalase gene is involved in the cellular stress response and (anti)oxidative processes triggered by stressor and contaminant exposure.

The response to heat shock and oxidative stress in Saccharomyces cerevisiae

A common need for microbial cells is the ability to respond to potentially toxic environmental insults. Here we review the progress in understanding the response of the yeast Saccharomyces cerevisiae to two important environmental stresses: heat shock and oxidative stress. Both of these stresses are fundamental challenges that microbes of all types will experience. The study of these environmental stress responses in S. cerevisiae has illuminated many of the features now viewed as central to our understanding of eukaryotic cell biology. Transcriptional activation plays an important role in driving the multifaceted reaction to elevated temperature and levels of reactive oxygen species. Advances provided by the development of whole genome analyses have led to an appreciation of the global reorganization of gene expression and its integration between different stress regimens. While the precise nature of the signal eliciting the heat shock response remains elusive, recent progress in the understanding of induction of the oxidative stress response is summarized here. Although these stress conditions represent ancient challenges to S. cerevisiae and other microbes, much remains to be learned about the mechanisms dedicated to dealing with these environmental parameters.

Histone H2B ubiquitylation and H3 lysine 4 methylation prevent ectopic silencing of euchromatic loci important for the cellular response to heat

In Saccharomyces cerevisiae, ubiquitylation of histone H2B signals methylation of histone H3 at lysine residues 4 (K4) and 79. These modifications occur at active genes but are believed to stabilize silent chromatin by limiting movement of silencing proteins away from heterochromatin domains. In the course of studying atypical phenotypes associated with loss of H2B ubiquitylation/H3K4 methylation, we discovered that these modifications are also required for cell wall integrity at high temperatures. We identified the silencing protein Sir4 as a dosage suppressor of loss of H2B ubiquitylation, and we showed that elevated Sir4 expression suppresses cell wall integrity defects by inhibiting the function of the Sir silencing complex. Using comparative transcriptome analysis, we identified a set of euchromatic genes-enriched in those required for the cellular response to heat-whose expression is attenuated by loss of H2B ubiquitylation but restored by disruption of Sir function. Finally, using DNA adenine methyltransferase identification, we found that Sir3 and Sir4 associate with genes that are silenced in the absence of H3K4 methylation. Our data reveal that H2B ubiquitylation/H3K4 methylation play an important role in limiting ectopic association of silencing proteins with euchromatic genes important for cell wall integrity and the response to heat.

Oxidative stress during aging of the yeast in a stationary culture and its attenuation by antioxidants

Oxidative stress during aging of Saccharomyces cerevisiae in stationary culture was documented by demonstration of progressive increase in the formation of superoxide, decrease in the content of acid-soluble thiols and of acid-soluble antioxidant capacity of cell extracts, and accumulation of aldehydes and protein carbonyl groups in two yeast strains and decreases in activities of antioxidant enzymes. Cells of a CuZn-SOD (superoxide dismutase)-1-deficient strain showed a higher loss of viability than cells of an isogenic wild-type strain. Cell survival was augmented, and changes in biochemical parameters were ameliorated, by addition of exogenous antioxidants (ascorbic acid, glutathione and melatonin) in both strains.

Oxidative stress in industrial fungi

Fungi are amongst the most industrially important microorganisms in current use within the biotechnology industry. Most such fungal cultures are highly aerobic in nature, a character that has been frequently referred to in both reactor design and fungal physiology. The most fundamentally significant outcome of the highly aerobic growth environment in fermenter vessels is the need for the fungal culture to effectively combat in the intracellular environment the negative consequences of high oxygen transfer rates. The use of oxygen as the respiratory substrate is frequently reported to lead to the development of oxidative stress, mainly due to oxygen-derived free radicals, which are collectively termed as reactive oxygen species (ROS). Recently, there has been extensive research on the occurrence, extent, and consequences of oxidative stress in microorganisms, and the underlying mechanisms through which cells prevent and repair the damage caused by ROS. In the present study, we critically review the current understanding of oxidative stress events in industrially relevant fungi. The review first describes the current state of knowledge of ROS concisely, and then the various antioxidant strategies employed by fungal cells to counteract the deleterious effects, together with their implications in fungal bioprocessing are also discussed. Finally, some recommendations for further research are made.

Comparison of the transcriptomic "stress response" evoked by antimycin A and oxygen deprivation in Saccharomyces cerevisiae

BACKGROUND

Acute changes in environmental parameters (e.g., O2, pH, UV, osmolarity, nutrients, etc.) evoke a common transcriptomic response in yeast referred to as the "environmental stress response" (ESR) or "common environmental response" (CER). Why such a diverse array of insults should elicit a common transcriptional response remains enigmatic. Previous functional analyses of the networks involved have found that, in addition to up-regulating those for mitigating the specific stressor, the majority appear to be involved in balancing energetic supply and demand and modulating progression through the cell cycle. Here we compared functional and regulatory aspects of the stress responses elicited by the acute inhibition of respiration with antimycin A and oxygen deprivation under catabolite non-repressed (galactose) conditions.

RESULTS

Gene network analyses of the transcriptomic responses revealed both treatments result in the transient (10 - 60 min) down-regulation of MBF- and SBF-regulated networks involved in the G1/S transition of the cell cycle as well as Fhl1 and PAC/RRPE-associated networks involved in energetically costly programs of ribosomal biogenesis and protein synthesis. Simultaneously, Msn2/4 networks involved in hexose import/dissimilation, reserve energy regulation, and autophagy were transiently up-regulated. Interestingly, when cells were treated with antimycin A well before experiencing anaerobiosis these networks subsequently failed to respond to oxygen deprivation. These results suggest the transient stress response is elicited by the acute inhibition of respiration and, we postulate, changes in cellular energetics and/or the instantaneous growth rate, not oxygen deprivation per se. After a considerable delay (> or = 1 generation) under anoxia, predictable changes in heme-regulated gene networks (e.g., Hap1, Hap2/3/4/5, Mot3, Rox1 and Upc2) were observed both in the presence and absence of antimycin A.

CONCLUSION

This study not only differentiates between the gene networks that respond to respiratory inhibition and those that respond to oxygen deprivation but suggests the function of the ESR or CER is to balance energetic supply/demand and coordinate growth with the cell cycle, whether in response to perturbations that disrupt catabolic pathways or those that require rapidly up-regulating energetically costly programs for combating specific stressors.

Silencing of mitochondrial NADP+-dependent isocitrate dehydrogenase by small interfering RNA enhances heat shock-induced apoptosis

Heat shock may increase oxidative stress due to increased production of reactive oxygen species and/or the promotion of cellular oxidation events. Mitochondrial NADP(+)-dependent isocitrate dehydrogenase (IDPm) produces NADPH, an essential reducing equivalent for the antioxidant system. In this report, we demonstrate that silencing of IDPm expression in HeLa cells greatly enhances apoptosis induced by heat shock. Transfection of HeLa cells with an IDPm small interfering RNA (siRNA) markedly decreased activity of IDPm, enhancing the susceptibility of heat shock-induced apoptosis reflected by morphological evidence of apoptosis, DNA fragmentation, cellular redox status, mitochondria redox status and function, and the modulation of apoptotic marker proteins. These results indicate that IDPm may play an important role in regulating the apoptosis induced by heat shock and the sensitizing effect of IDPm siRNA on the apoptotic cell death of HeLa cells offers the possibility of developing a modifier of cancer therapy.

Yeast responses to stresses associated with industrial brewery handling: Figure 1

During brewery handling, production strains of yeast must respond to fluctuations in dissolved oxygen concentration, pH, osmolarity, ethanol concentration, nutrient supply and temperature. Fermentation performance of brewing yeast strains is dependent on their ability to adapt to these changes, particularly during batch brewery fermentation which involves the recycling (repitching) of a single yeast culture (slurry) over a number of fermentations (generations). Modern practices, such as the use of high-gravity worts and preparation of dried yeast for use as an inoculum, have increased the magnitude of the stresses to which the cell is subjected. The ability of yeast to respond effectively to these conditions is essential not only for beer production but also for maintaining the fermentation fitness of yeast for use in subsequent fermentations. During brewery handling, cells inhabit a complex environment and our understanding of stress responses under such conditions is limited. The advent of techniques capable of determining genomic and proteomic changes within the cell is likely vastly to improve our knowledge of yeast stress responses during industrial brewery handling.

Oxalomalate, a competitive inhibitor of NADP+-dependent isocitrate dehydrogenase, regulates heat shock-induced apoptosis

Heat shock may increase oxidative stress due to increased production of reactive oxygen species and/or the promotion of cellular oxidation events. Recently, we demonstrated that the control of cytosolic and mitochondrial redox balance and the cellular defense against oxidative damage is one of the primary functions of NADP(+)-dependent isocitrate dehydrogenase (ICDH) by supplying NADPH for antioxidant systems. The protective role of ICDH against heat shock-induced apoptosis in U937 cells was investigated in the control and the cells pre-treated with oxalomalate, a competitive inhibitor of ICDH. Upon exposure to heat shock, there was a distinct difference between the control cells and the cells pre-treated with 3mM oxalomalate for 3h in regard to apoptotic parameters, cellular redox status, and mitochondrial function. The oxalomalate pre-treated cells showed significant enhancement of apoptotic features such as activation of caspase-3, up-regulation of Bax, and down-regulation of Bcl-2 compared to the control cells upon exposure to heat shock. This study indicates that ICDH may play an important role in regulating the apoptosis induced by heat shock presumably through maintaining the cellular redox status.

Cellular defense against heat shock-induced oxidative damage by mitochondrial NADP+ -dependent isocitrate dehydrogenase

Heat shock may increase oxidative stress due to increased production of reactive oxygen species and/or the promotion of cellular oxidation events. Mitochondrial NADP+ -dependent isocitrate dehydrogenase (IDPm) produces NADPH, an essential reducing equivalent for the antioxidant system. The protective role of IDPm against heat shock in HEK293 cells, an embryonic kidney cell line, was investigated in control and cells transfected with the cDNA for IDPm, where IDPm activity was 6-7 fold higher than that in the control cells carrying the vector alone. Upon exposure to heat shock, the viability was lower and the protein oxidation, lipid peroxidation and oxidative DNA damage were higher in control cells as compared to HEK293 cells in which IDPm was over-expressed. We also observed the significant difference in the cellular redox status reflected by the endogenous production of reactive oxygen species, NADPH pool and GSH recycling between two cells. The results suggest that IDPm plays an important role as an antioxidant defense enzyme in cellular defense against heat shock through the removal of reactive oxygen species.

Caloric restriction augments ROS defense in S. cerevisiae, by a Sir2p independent mechanism

Aging is associated with increased production of reactive oxygen species (ROS) and oxidation-induced damage to intracellular structures and membranes. Caloric restriction (CR) has been demonstrated to delay aging in a variety of species. Although the mechanisms of CR remain to be clearly elucidated, reductions in oxidative damage have been shown to increase lifespan in several model systems. Contrary to the general belief that ROS production is reduced in CR, this article provides evidence that not only oxygen consumption but ROS production is enhanced in the calorie restricted condition. To understand the biological mechanism underlying the anti aging action of CR, the role of scavenging enzymes was studied. It was found that super oxide dismutase (SOD1 and SOD2), catalase and glutathione peroxidase (GPx) all are over expressed in CR. We further investigated the role of Sir2, a potential effector of CR response in the activation of scavenging enzymes. No marked difference was found in CR mediated activation of SOD and catalase in the absence of Sir2. Our results suggest that in CR scavenging enzymes are activated by a Sir2 independent manner.

Do mitochondria regulate the heat-shock response in Saccharomyces cerevisiae?

A mild heat shock induces the synthesis of heat-shock proteins (hsps), which protect cells from damage during more extreme heat exposure. The nature of the signals that induce transcription of heat shock-regulated genes remains conjectural. In this work we studied the role of mitochondria in regulating hsps synthesis in Saccharomyces cerevisiae. The results obtained clearly indicate that a mild heat shock elicits a hyperpolarization of the inner mitochondrial membrane and such an event is one of several signals triggering the chain of reactions that activates the expression of the HSP104 gene and probably the expression of other heat shock-regulated genes in S. cerevisiae. The uncouplers or mitochondrial inhibitors which are capable of dissipating the potential on the inner mitochondrial membrane under particular experimental conditions prevent the synthesis of Hsp104 induced by mild heat shock and thus inhibit the development of induced thermotolerance. It is suggested that cAMP-dependent protein kinase A is participating in the mitochondrial regulation of nuclear genes.

Cloning and expression analysis of two catalase genes from Aspergillus oryzae

Fungi contain distinct genes encoding the same class of enzyme that are differentially regulated according to conditions. We cloned two catalase genes, catA and catB, from Aspergillus oryzae. The catA gene predicts a 747-amino-acid polypeptide sharing 81% identity with Aspergillus fumigatus catalase (catA) and 77% with Aspergillus nidulans catalase (catA). The catB gene predicts a 725-amino-acid polypeptide sharing 82% identity with A. fumigatus catalase (catB) and 75% with A. nidulans catalase (catB). However, the catA and catB genes share little homology (41%) with one another, suggesting that each gene belongs to a distinct gene family. Overexpression studies demonstrated that both genes encode a functional catalase. Promoter assays indicated that the catA gene is developmentally regulated as it was preferentially expressed in solid-state cultures undergoing sporulation. However, its expression was not affected by hydrogen peroxide treatment. Conversely, the catB gene was highly expressed under all culture conditions tested, and it was induced by hydrogen peroxide treatment. These results suggest that the catB gene may be mainly used for detoxification of oxidative stress while the catA gene may have another role such as chaperoning proteins in the spore.

New roles for CDC25 in growth control, galactose regulation and cellular differentiation in Saccharomyces cerevisiae

Living organisms display large differences in stress resistance throughout their life cycles. To study the coordinated regulation of development and stress responses in exponentially growing yeast, mutants that displayed elevated heat-shock resistance at this stage were screened for. Here, two new mutant alleles of CDC25 in Saccharomyces cerevisiae, cdc25-21 and cdc25-22, are described. During exponential growth in glucose at 25 degrees C, these mutants are resistant to heat, oxidative, osmotic and ionic shock, accumulate stress-protein transcripts, show slow growth rates, thick cell walls and glycogen hyperaccumulation and lack cAMP signalling in response to glucose. Genetic and cellular analyses revealed that the stationary-phase phenotypes of cdc25-21 and cdc25-22 mutants are not due to entrance to a G(0) state during exponential growth, but are the result of a prolonged G(1) phase. It was found that, in the W303 background, CDC25 is dispensable for growth in glucose media. However, CDC25 is essential for growth in galactose, in non-fermentable carbon sources and under continuous incubation at 38 degrees C. In conclusion, the function of the catalytic, C-terminal domain of Cdc25p is not only important for fermentative growth, but also for growth in non-fermentable carbon sources and to trigger galactose derepression.

Enrichment of yeast thioredoxin by green tea extract through activation of Yap1 transcription factor in Saccharomyces cerevisiae

Thioredoxin (TRX) is an important antioxidant present in all types of organisms. Besides its role as an antioxidant, TRX protects the gastric mucosa due to its antiinflammatory effect. In addition, TRX decreases allergenicity; therefore, the oral administration of TRX is of considerable interest with respect to its clinical use as well as the development of functional foods containing TRX. We have attempted to enrich the cellular TRX content in Saccharomyces cerevisiae, and found that green tea extract (Sunphenon), which is rich in catechins (polyphenols), activates the Yap1 transcription factor, leading to the induction of TRX2, a target of Yap1. Production of yeast TRX was monitored by both a TRX2-lacZ reporter expression assay and Western blotting using an anti-yeast TRX antibody. Maximal production of TRX was achieved in a medium containing 0.1% green tea extract at pH 7.6. We discuss the underlying mechanism by which green tea extract activates Yap1.

Distinct regulatory mechanism of yeastGPX2encoding phospholipid hydroperoxide glutathione peroxidase by oxidative stress and a calcineurin/Crz1-mediated Ca2+signaling pathway

The GPX2 gene encodes a homologue of mammalian phospholipid hydroperoxide glutathione peroxidase in Saccharomyces cerevisiae. Previously, we have reported that the oxidative stress-induced expression of GPX2 is strictly regulated by Yap1 and Skn7 transcription factors. Here, we found that the expression of GPX2 is induced by CaCl(2) in a calcineurin (CN)/Crz1-dependent manner, and the CN-dependent response element was specified in the GPX2 promoter. Neither Yap1 nor Skn7 was required for Ca(2+)-dependent induction of GPX2, therefore, distinct regulation for the oxidative stress response and Ca(2+) signal response for GPX2 exists in yeast cells.

The antioxidant systems in Toxoplasma gondii and the role of cytosolic catalase in defence against oxidative injury

Superoxide dismutase, catalase, glutathione peroxidase and peroxiredoxins form an antioxidant network protecting cells against reactive oxygen species (ROS). Catalase is a potent H2O2-detoxifying enzyme, which is unexpectedly absent in some members of the Kinetoplastida and Apicomplexa, but present in Toxoplasma gondii. In T. gondii, catalase appears to be cytosolic. In addition, T. gondii also possesses genes coding for other types of peroxidases, including glutathione/thioredoxin-like peroxidases and peroxiredoxins. This study presents a detailed analysis of the role of catalase in the parasite and reports the existence of antioxidant enzymes localized in the cytosol and the mitochondrion of T. gondii. The catalase gene was disrupted and, in addition, T. gondii cell lines overexpressing either catalase or a cytosolic 1-cys peroxiredoxin, TgPrx2, under the control of a strong promoter were created. Analysis of these mutants confirmed that the catalase activity is cytosolic and is encoded by a unique gene in T. gondii. Furthermore, the catalase confers protection against H2O2 exposure and contributes to virulence in mice. The overexpression of Prx2 also increases protection against H2O2 treatment, suggesting that catalase and other peroxidases function as a defence mechanism against endogenously produced reactive oxygen intermediates and the oxidative stress imposed by the host.

Elevated temperature effects on the oxidant/antioxidant balance in submerged batch cultures of the filamentous fungus Aspergillus niger B1-D

In the present study the relationship between oxidative stress and elevated culture temperature was examined in an industrially relevant fungal culture, Aspergillus niger B1-D. For the first time, both the intracellular levels of the main stressor species (superoxide radical [O(2) (.-)]) and activities of cellular defensive enzymes (superoxide dismutase [SOD], catalase [CAT], and glutathione peroxide [GPx]) were quantified at varying temperature (25, 30, 35, 40 degrees C) to more fully characterize culture response in different growth phases. Elevated culture temperature led to increased O(2) (.-) levels in various culture phases. In the exponential phase this was due to an enhanced generation of O(2) (.-), whereas in stationary phase a decreased dismutation rate may also have contributed. CAT activities generally increased with culture temperature, whereas GPx activity changed little as temperature rose, indicating that GPx played only a minor role in destroying H(2)O(2) in this A. niger. The combination of elevated temperature (35 degrees C) and increased O(2) supply (50% enrichment) led to decreased levels of O(2) (.-) compared to the cultivation at 35 degrees C gassed with air, probably due to enhanced activity of the alternative fungal respiratory pathway. Our findings indicate that while elevated cultivation temperature does clearly induce oxidative stress events, mechanistically, it does so by a rather more complex route than previous studies indicate. Elevated temperature caused a marked disparity in the activities of SOD and CAT, very distinct from the integrated increase in activity of these enzymes in response to oxidative stress.

Stress-induced gene expression in Candida albicans: absence of a general stress response

We used transcriptional profiling to investigate the response of the fungal pathogen Candida albicans to temperature and osmotic and oxidative stresses under conditions that permitted >60% survival of the challenged cells. Each stress generated the transient induction of a specific set of genes including classic markers observed in the stress responses of other organisms. We noted that the classical hallmarks of the general stress response observed in Saccharomyces cerevisiae are absent from C. albicans; no C. albicans genes were significantly induced in a common response to the three stresses. This observation is supported by our inability to detect stress cross-protection in C. albicans. Similarly, in C. albicans there is essentially no induction of carbohydrate reserves like glycogen and trehalose in response to a mild stress, unlike the situation in S. cerevisiae. Thus C. albicans lacks the strong general stress response exhibited by S. cerevisiae.

Yeast glycogen synthase kinase-3 activates Msn2p-dependent transcription of stress responsive genes

The yeast Saccharomyces cerevisiae has four genes, MCK1, MDS1 (RIM11), MRK1, and YOL128c, that encode homologues of mammalian glycogen synthase kinase 3 (GSK-3). A gsk-3 null mutant in which these four genes are disrupted showed growth defects on galactose medium. We isolated several multicopy suppressors of this growth defect. Two of them encoded Msn2p and phosphoglucomutase (PGM). Msn2p is a transcription factor that binds to the stress-response element (STRE). PGM is an enzyme that interconverts glucose-1 phosphate and glucose-6 phosphate and is regulated by Msn2p at the transcriptional level. Expression of the mRNAs of PGM2 and DDR2, whose promoter regions possess STRE sequences, on induction by heat shock or salt stress was reduced not only in an msn2 msn4 (msn2 homologue) double mutant but also in the gsk-3 null mutant. STRE-dependent transcription was greatly inhibited in the gsk-3 null mutant or mck1 mds1 double mutant, and this phenotype was suppressed by the expression of Mck1p but not of a kinase-inactive form of Mck1p. Although Msn2p accumulated in the nucleus of the gsk-3 null mutant as well as in the wild-type strain under various stress conditions, its STRE-binding activity was reduced in extracts prepared from the gsk-3 null mutant or mck1 mds1 double mutant. These results suggest that yeast GSK-3 promotes formation of a complex between Msn2p and DNA, which is required for the proper response to different forms of stress. Because neither Msn2p-GSK-3 complex formation nor GSK-3-dependent phosphorylation of Msn2p could be detected, the regulation of Msn2p by GSK-3 may be indirect.