Overexpression of YAP2, coding for a new yAP protein, and YAP1 in Saccharomyces cerevisiae alleviates growth inhibition caused by 1,10-phenanthroline

Transcription factors and ABC transporters: from pleiotropic drug resistance to cellular signaling in yeast

Budding yeast Saccharomyces cerevisiae survives in microenvironments utilizing networks of regulators and ATP-binding cassette (ABC) transporters to circumvent toxins and a variety of drugs. Our understanding of transcriptional regulation of ABC transporters in yeast is mainly derived from the study of multidrug resistance protein networks. Over the past two decades, this research has not only expanded the role of transcriptional regulators in pleiotropic drug resistance (PDR) but evolved to include the role that regulators play in cellular signaling and environmental adaptation. Inspection of the gene networks of the transcriptional regulators and characterization of the ABC transporters has clarified that they also contribute to environmental adaptation by controlling plasma membrane composition, toxic-metal sequestration, and oxidative stress adaptation. Additionally, ABC transporters and their regulators appear to be involved in cellular signaling for adaptation of S. cerevisiae populations to nutrient availability. In this review, we summarize the current understanding of the S. cerevisiae transcriptional regulatory networks and highlight recent work in other notable fungal organisms, underlining the expansion of the study of these gene networks across the kingdom fungi.

Fluorescence polarization assay for the identification and evaluation of inhibitors at YAP-TEAD protein-protein interface 3

The Hippo signaling pathway controls cell-cell contact, cell proliferation, as well as organ size by integrating changes in the cellular microenvironment. In recent years, the pivotal role of Hippo signaling in cancers has been well recognized. Inhibition of the pathway promotes the translocation of the major Hippo pathway effectors, the yes-associated protein (YAP) and its paralog TAZ, to the nucleus, where they interact with the transcription factor family transcriptional enhancer associate domain (TEAD), thus coactivating the expression of downstream genes, leading to cell transformation, tissue overgrowth, and tumor development. Therefore, the interruption of the YAP-TEAD transcriptional complex represents a novel opportunity for the treatment of cancer. Here, we established a fluorescence polarization (FP)-based assay for the identification and evaluation of YAP-TEAD protein-protein interface (PPI) inhibitors at the YAP Ω-loop binding region of TEAD, which is also called interface 3 at the YAP-TEAD binding surface. Furthermore, a patented small molecule (Patent-22) was evaluated by the FP assay, which confirmed that it was a YAP-TEAD PPI inhibitor at interface 3. Possessing great application value, this FP method is reliable, robust, and economical for inhibitor assessment and drug discovery.

The birth of enthusiasm and passion for science

In this paper I describe the main aspects of my career and focus on the retrospective on my life and my work.

Retrospective on the life and work of Claudina Rodrigues-Pousada.

Functional study of the Hap4-like genes suggests that the key regulators of carbon metabolism HAP4 and oxidative stress response YAP1 in yeast diverged from a common ancestor

The transcriptional regulator HAP4, induced by respiratory substrates, is involved in the balance between fermentation and respiration in S. cerevisiae. We identified putative orthologues of the Hap4 protein in all ascomycetes, based only on a conserved sixteen amino acid-long motif. In addition to this motif, some of these proteins contain a DNA-binding motif of the bZIP type, while being nonetheless globally highly divergent. The genome of the yeast Hansenula polymorpha contains two HAP4-like genes encoding the protein HpHap4-A which, like ScHap4, is devoid of a bZIP motif, and HpHap4-B which contains it. This species has been chosen for a detailed examination of their respective properties. Based mostly on global gene expression studies performed in the S. cerevisiae HAP4 disruption mutant (ScΔhap4), we show here that HpHap4-A is functionally equivalent to ScHap4, whereas HpHap4-B is not. Moreover HpHAP4-B is able to complement the H₂O₂ hypersensitivity of the ScYap1 deletant, YAP1 being, in S. cerevisiae, the main regulator of oxidative stress. Finally, a transcriptomic analysis performed in the ScΔyap1 strain overexpressing HpHAP4-B shows that HpHap4-B acts both on oxidative stress response and carbohydrate metabolism in a manner different from both ScYap1 and ScHap4. Deletion of these two genes in their natural host, H. polymorpha, confirms that HpHAP4-A participates in the control of the fermentation/respiration balance, while HpHAP4-B is involved in oxidative stress since its deletion leads to hypersensitivity to H₂O₂. These data, placed in an evolutionary context, raise new questions concerning the evolution of the HAP4 transcriptional regulation function and suggest that Yap1 and Hap4 have diverged from a unique regulatory protein in the fungal ancestor.

Multidrug resistance in fungi: regulation of transporter-encoding gene expression

A critical risk to the continued success of antifungal chemotherapy is the acquisition of resistance; a risk exacerbated by the few classes of effective antifungal drugs. Predictably, as the use of these drugs increases in the clinic, more resistant organisms can be isolated from patients. A particularly problematic form of drug resistance that routinely emerges in the major fungal pathogens is known as multidrug resistance. Multidrug resistance refers to the simultaneous acquisition of tolerance to a range of drugs via a limited or even single genetic change. This review will focus on recent progress in understanding pathways of multidrug resistance in fungi including those of most medical relevance. Analyses of multidrug resistance in Saccharomyces cerevisiae have provided the most detailed outline of multidrug resistance in a eukaryotic microorganism. Multidrug resistant isolates of S. cerevisiae typically result from changes in the activity of a pair of related transcription factors that in turn elicit overproduction of several target genes. Chief among these is the ATP-binding cassette (ABC)-encoding gene PDR5. Interestingly, in the medically important Candida species, very similar pathways are involved in acquisition of multidrug resistance. In both C. albicans and C. glabrata, changes in the activity of transcriptional activator proteins elicits overproduction of a protein closely related to S. cerevisiae Pdr5 called Cdr1. The major filamentous fungal pathogen, Aspergillus fumigatus, was previously thought to acquire resistance to azole compounds (the principal antifungal drug class) via alterations in the azole drug target-encoding gene cyp51A. More recent data indicate that pathways in addition to changes in the cyp51A gene are important determinants in A. fumigatus azole resistance. We will discuss findings that suggest azole resistance in A. fumigatus and Candida species may share more mechanistic similarities than previously thought.

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.

Differential oxidant tolerance determined by the key transcription factor Yap1 is controlled by levels of the Yap1-binding protein, Ybp1

The Saccharomyces cerevisiae transcription factor Yap1 is a central determinant of oxidative stress tolerance. This protein is found primarily in the cytoplasm in the absence of oxidative stress but, upon exposure to oxidants, rapidly translocates to the nucleus and activates expression of target genes. Although both diamide and H(2)O(2) have been used to impose oxidative stress on cells, these different oxidants trigger Yap1 nuclear localization in distinctly different ways. Diamide appears to oxidize particular cysteine residues on Yap1, leading to inhibition of association of Yap1 with the nuclear exportin Crm1. Crm1 would normally transport Yap1 out of the nucleus. H(2)O(2) activation of Yap1 nuclear localization requires the participation of the glutathione peroxidase Gpx3 and the Yap1-binding protein Ybp1. H(2)O(2) exposure triggers formation of a dual disulfide bonded Yap1 that is catalyzed by the presence of Gpx3 and Ybp1. In the current study, we have determined that two distinct pools of Yap1 exist in the cell. These pools are designated by the level of Ybp1. Ybp1 interacts directly with Yap1 and these proteins form a stable complex in vivo. Genetic and biochemical experiments indicate that Ybp1 is rate-limiting for Yap1 oxidative folding during H(2)O(2) stress. The fungal pathogen Candida glabrata expresses a protein homologous to Ybp1 called CgYbp1. Overproduction of CgYbp1 elevated H(2)O(2) tolerance in this pathogen indicating that the determinative role of Ybp1 in setting the level of H(2)O(2) resistance has been evolutionarily conserved.

Genome-wide functional profiling reveals genes required for tolerance to benzene metabolites in yeast

Benzene is a ubiquitous environmental contaminant and is widely used in industry. Exposure to benzene causes a number of serious health problems, including blood disorders and leukemia. Benzene undergoes complex metabolism in humans, making mechanistic determination of benzene toxicity difficult. We used a functional genomics approach to identify the genes that modulate the cellular toxicity of three of the phenolic metabolites of benzene, hydroquinone (HQ), catechol (CAT) and 1,2,4-benzenetriol (BT), in the model eukaryote Saccharomyces cerevisiae. Benzene metabolites generate oxidative and cytoskeletal stress, and tolerance requires correct regulation of iron homeostasis and the vacuolar ATPase. We have identified a conserved bZIP transcription factor, Yap3p, as important for a HQ-specific response pathway, as well as two genes that encode putative NAD(P)H:quinone oxidoreductases, PST2 and YCP4. Many of the yeast genes identified have human orthologs that may modulate human benzene toxicity in a similar manner and could play a role in benzene exposure-related disease.

The Sko1 protein represses the yeast-to-hypha transition and regulates the oxidative stress response in Candida albicans

Cells respond to environmental changes triggering adaptive responses which are, in part, mediated by a transcriptional response. These responses are complex and are dependent on different transcription factors. The present work reports the implication of the Sko1 protein in several processes relevant to the physiology of Candida albicans. First, Sko1 acts as transcriptional repressor of genes involved in pathogenesis and hyphal formation, which results in increased expression of the hyphal related genes ECE1 and HWP1 without significant changes in the virulence using a mouse model of systemic infection. Second Sko1 is involved in the response to oxidative stress and sko1 mutants increase the sensitivity of hog1 to the myelomonocytic cell line HL-60. Genome-wide transcriptional analysis after hydrogen peroxide treatment revealed that sko1 mutants were able to generate an adaptive response similar to wild type strains, although important differences were detected in the magnitude of the transcriptional response. Collectively, these results implicate Sko1 as an important mediator of the oxidative stress response in C. albicans.

The S. cerevisiae Yap1 and Yap2 transcription factors share a common cadmium-sensing domain

Towards elucidating the function of Yap2, which remains unclear, we have taken advantage of the C-terminal homology between Yap1 and Yap2. Swapping domains experiments show that the Yap2 C-terminal domain functionally substitutes for the homologous Yap1 domain in the response to Cd, but not to H2O2. We conclude that specificity determinants of the Cd response are encoded within both Yap1 and Yap2 C-terminus, whereas those required for H2O2 response are only present in the Yap1 C-terminus. Furthermore, our results identify FRM2 as Cd-responsive Yap2 target and indicate a possible role of this protein in regulating a metal stress response.

Isolation of theYAP1 homologue ofCandida utilis and its use as an efficient selection marker

The industrially important yeast Candida utilis is widely used in the production of food and medical materials, but its practical host-vector system has not been well developed. In order to construct a food-grade host-vector system, we isolated the YAP1 homologue, CuYAP1, of C. utilis IAM4264 and evaluated its use as a selection marker in transformation. A DNA probe was obtained by PCR using degenerate primers and the CuYAP1-encoding 438 amino acid protein was isolated by hybridization. Although the amino acid identity of Yap1 and CuYap1 was 28.7% as a whole, the characteristic bZip region and two cysteine-rich domains (CRDs) showed a higher homology. CuYAP1 was inserted in a CuGAP1 expression cassette of the C. utilis ARS vector pRI177, and C. utilis AHU3053 was transformed with this plasmid. A number of transformant colonies grew in the presence of cycloheximide, which indicated that CuGAP1-CuYAP1 is an effective selection marker. The transformant also showed higher resistance to other agents, including cadmium and fluconazole. The overexpression of CuYAP1 in S. cerevisiae also resulted in increased resistance to various types of drugs.

Rck1 and Rck2 MAPKAP kinases and the HOG pathway are required for oxidative stress resistance

We demonstrate a role in oxidative and metal stress resistance for the MAPK-activated protein kinases Rck1 and Rck2 in Saccharomyces cerevisiae. We show that Hog1 is robustly phosphorylated in a Pbs2-dependent way during oxidative stress, and that Rck2 also is phosphorylated under these circumstances. Hog1 concentrates in the nucleus in oxidative stress. Hog1 localization is partially dependent on Rck2, as rck2 cells have more nuclear Hog1 than wild-type cells. We find several proteins with a role in oxidative stress resistance using Rck1 or Rck2 as baits in a two-hybrid screen. We identify the transcription factor Yap2 as a putative target for Rck1, and the Zn2+ transporter Zrc1 as a target for Rck2. Yap2 is normally cytoplasmic, but rapidly migrates to the nucleus upon exposure to oxidative stress agents. In a fraction of untreated pbs2 cells, Yap2 is nuclear. Zrc1 co-immunoprecipitates with Rck2, and ZRC1 is genetically downstream of RCK2. These data connect activation of the Hog1 MAPK cascade with effectors having a role in oxidative stress resistance.

Yeast activator proteins and stress response: an overview

Yeast, and especially Saccharomyces cerevisiae, are continuously exposed to rapid and drastic changes in their external milieu. Therefore, cells must maintain their homeostasis, which is achieved through a highly coordinated gene expression involving a plethora of transcription factors, each of them performing specific functions. Here, we discuss recent advances in our understanding of the function of the yeast activator protein family of eight basic-leucine zipper trans-activators that have been implicated in various forms of stress response.

Transcriptional activation of metalloid tolerance genes in Saccharomyces cerevisiae requires the AP-1-like proteins Yap1p and Yap8p

All organisms are equipped with systems for detoxification of the metalloids arsenic and antimony. Here, we show that two parallel pathways involving the AP-1-like proteins Yap1p and Yap8p are required for acquisition of metalloid tolerance in the budding yeast S. cerevisiae. Yap8p is demonstrated to reside in the nucleus where it mediates enhanced expression of the arsenic detoxification genes ACR2 and ACR3. Using chromatin immunoprecipitation assays, we show that Yap8p is associated with the ACR3 promoter in untreated as well as arsenic-exposed cells. Like for Yap1p, specific cysteine residues are critical for Yap8p function. We further show that metalloid exposure triggers nuclear accumulation of Yap1p and stimulates expression of antioxidant genes. Yap1p mutants that are unable to accumulate in the nucleus during H(2)O(2) treatment showed nearly normal nuclear retention in response to metalloid exposure. Thus, our data are the first to demonstrate that Yap1p is being regulated by metalloid stress and to indicate that this activation of Yap1p operates in a manner distinct from stress caused by chemical oxidants. We conclude that Yap1p and Yap8p mediate tolerance by controlling separate subsets of detoxification genes and propose that the two AP-1-like proteins respond to metalloids through distinct mechanisms.

Transcriptional control of multidrug resistance in the yeast Saccharomyces

A major problem in chemotherapeutic treatment of many pathological conditions including cancer and fungal infections is the development of a multidrug-resistant state in the target cell. Saccharomyces cerevisiae cells can be isolated that have single genetic alterations that cause the resulting mutant strains to become tolerant of a wide range of compounds that would otherwise be toxic. These mutant cells are referred to as having a pleiotropic drug-resistant (Pdr) phenotype. Studies of these Pdr cells have demonstrated that mutations either within genes encoding transcriptional regulators or in their regulatory inputs lead to overexpression of downstream transporter proteins with associated multidrug resistance. This review is aimed at providing a framework for understanding the networks modulating expression of PDR genes in S. cerevisiae.

Exploratory and confirmatory gene expression profiling of mac1Delta

Exploratory outlier identification methods and confirmatory gene expression studies showed induction of the iron regulon in Saccharomyces cerevisiae lacking Mac1p, a copper-responsive transcription factor. The Aft1p/Aft2p binding motif was the most discriminating motif between up- and down-regulated genes, and we identified new genes potentially regulated by Aft1p/Aft2p. In addition, multiple genes encoding proteins containing Fe-S clusters were down-regulated suggesting metabolic reorganization to conserve iron in mac1Delta. Null mutants of each of the differentially expressed genes were characterized for copper- or iron-related phenotypes. New or additional support for a role in copper and iron homeostasis is provided in this study for the gene products of AKR1, MRS4, PCA1, SSU1, TIS11, YBR047W, YHL035C, YHR045W, YLR047C, YLR126C, and YTP1.

Response of a strict anaerobe to oxygen: survival strategies in Desulfovibrio gigas

The biochemical response to oxygen of the strictly anaerobic sulfate-reducing bacterium Desulfovibrio gigas was studied with the goal of elucidating survival strategies in oxic environments. Cultures of D. gigas on medium containing lactate and sulfate were exposed to oxygen (concentration 5-120 micro M). Growth was fully inhibited by oxygen, but the cultures resumed growth as soon as they were shifted back to anoxic conditions. Following 24 h exposure to oxygen the growth rate was as high as 70 % of the growth rates observed before oxygenation. Catalase levels and activity were enhanced by exposure to oxygen whereas superoxide-scavenging and glutathione reductase activities were not affected. The general pattern of cellular proteins as analysed by two-dimensional electrophoresis was altered in the presence of oxygen, the levels of approximately 12 % of the detected proteins being markedly increased. Among the induced proteins, a homologue of a 60 kDa eukaryotic heat-shock protein (Hsp60) was identified by immunoassay analysis. In the absence of external substrates, the steady-state levels of nucleoside triphosphates detected by in vivo (31)P-NMR under saturating concentrations of oxygen were 20 % higher than under anoxic conditions. The higher energy levels developed under oxygen correlated with a lower rate of substrate (glycogen) mobilization, but no experimental evidence for a contribution from oxidative phosphorylation was found. The hypothesis that oxygen interferes with ATP dissipation processes is discussed.

The Hog1 mitogen-activated protein kinase is essential in the oxidative stress response and chlamydospore formation in Candida albicans

Candida albicans mutants with mutations in mitogen-activated protein (MAP) kinase HOG1 displayed an increased sensitivity to agents producing reactive oxygen species, such as oxidants (menadione, hydrogen peroxide, or potassium superoxide), and UV light. Consistent with this finding, C. albicans Hog1 was activated not only in response to an increase in external osmolarity, as happens with its Saccharomyces cerevisiae homologue, but also in response to hydrogen peroxide. The Hog1-mediated response to oxidative stress was different from that of transcription factor Cap1, the homologue of S. cerevisiae Yap1, as shown by the different sensitivities to oxidants and the kinetics of cell death of cap1Delta, hog1, and hog1 cap1Delta mutants. Deletion of CAP1 did not influence the level of Hog1 phosphorylation, and deletion of HOG1 did not affect Cap1 nuclear localization. Moreover, we show that the HOG1 gene plays a role in chlamydospore formation, another oxygen-related morphogenetic event, as demonstrated by the fact that hog1 cells were unable to generate these thick-walled structures in several media through a mechanism different from that of the EFG1 regulator. This is the first demonstration of the role of the Hog1-mediated MAP kinase pathway in resistance to oxidative stress in pathogenic fungi, and it allows us to propose a molecular model for the oxidative stress response in C. albicans.

Discrimination between paralogs using microarray analysis: application to the Yap1p and Yap2p transcriptional networks

Ohno [Ohno, S. (1970) in Evolution by Gene Duplication, Springer, New York] proposed that gene duplication with subsequent divergence of paralogs could be a major force in the evolution of new gene functions. In practice the functional differences between closely related homologues produced by duplications can be subtle and difficult to separate experimentally. Here we show that DNA microarrays can distinguish the functions of two closely related homologues from the yeast Saccharomyces cerevisiae, Yap1p and Yap2p. Although Yap1p and Yap2p are both bZIP transcription factors involved in multiple stress responses and are 88% identical in their DNA binding domains, our work shows that these proteins activate nonoverlapping sets of genes. Yap1p controls a set of genes involved in detoxifying the effects of reactive oxygen species, whereas Yap2p controls a set of genes over represented for the function of stabilizing proteins. In addition we show that the binding sites in the promoters of the Yap1p-dependent genes differ from the sites in the promoters of Yap2p-dependent genes and we validate experimentally that these differences are important for regulation by Yap1p. We conclude that while Yap1p and Yap2p may have some overlapping functions they are clearly not redundant and, more generally, that DNA microarray analysis will be an important tool for distinguishing the functions of the large numbers of highly conserved genes found in all eukaryotic genomes.

Transcription factors regulating the response to oxidative stress in yeast

A main avenue of defense against fungal infection uses oxidative killing of these and other microorganisms. Consequently, the ability of fungi to withstand an oxidative challenge has important implications for their ultimate pathogenicity in a host organism. Fungi also serve as an excellent model system for handling of reactive oxygen species in eukaryotic cells. For these reasons, a great deal of work has been invested in analyzing pathways involved in and the mechanisms regulating oxidative stress tolerance in fungi. The goal of this review is to discuss the current state of knowledge underlying the ability of fungal cells to mount a response to oxidative stress via activation of transcription factors. Studies in Saccharomyces cerevisiae have identified multiple transcriptional regulatory proteins that mediate tolerance to oxidative stress. Experiments focused on the fission yeast Schizosaccharomyces pombe have led to the discovery of protein kinase cascades highly related to mammalian stress-activated protein kinases. Recent studies on the pathogenic yeast Candida albicans have allowed analysis of the role of a critical oxidant-regulated transcription factor in this important human pathogen. Further understanding of oxidative stress resistance pathways in fungi is an important step toward understanding the molecular pathogenesis of these microorganisms.

Yap1 overproduction restores arsenite resistance to the ABC transporter deficient mutant ycf1 by activating ACR3 expression

Ycf1 and Acr3 are transporters that have been previously shown to protect Saccharomyces cerevisiae cells from the toxic effects of arsenite. Ycf1 and Acr3 are positively regulated by distinct, but related bZIP transcriptional activators, Yap1 and Yap8, respectively. In this study, we show that overexpression of Yap1 complemented the arsenite hypersensitivity of the ycf1 null mutant, but only if the ACR3 gene is functional. We further show that the expression of either an ACR3-lacZ promoter fusion reporter or the endogenous ACR3 gene was stimulated by the overproduction of Yap1 upon exposure to arsenite. These data suggest that Yap1 confers arsenite resistance to the ycf1 null mutant by activating expression of the Yap8-dependent target gene, ACR3. Our data also show Yap8-dependent ACR3-lacZ expression was greatly stimulated by arsenite in a dose-dependent manner in the parental strain. However, overproduction of Yap1 in the parental strain severely limited dose-dependent activation of the reporter by arsenite. We conclude that Yap1 may compete with Yap8 for binding to the ACR3 promoter, but is unable to act as a potent activator.Key words: arsenite, ABC transporters, AP-1 factors, overproduction, element, yeast.

Identification and characterization of a low oxygen response element involved in the hypoxic induction of a family of Saccharomyces cerevisiae genes. Implications for the conservation of oxygen sensing in eukaryotes

An organism's ability to respond to changes in oxygen tension depends in large part on alterations in gene expression. The oxygen sensing and signaling mechanisms in eukaryotic cells are not fully understood. To further define these processes, we have studied the Delta9 fatty acid desaturase gene OLE1 in Saccharomyces cerevisiae. We have confirmed previous data showing that the expression of OLE1 mRNA is increased in hypoxia and in the presence of certain transition metals. OLE1 expression was also increased in the presence of the iron chelator 1,10-phenanthroline. A 142-base pair (bp) region 3' to the previously identified fatty acid response element was identified as critical for the induction of OLE1 in response to these stimuli using OLE1 promoter-lacZ reporter constructs. Electromobility shift assays confirmed the presence of an inducible band shift in response to hypoxia and cobalt. Mutational analysis defined the nonameric sequence ACTCAACAA as necessary for transactivation. A 20-base pair oligonucleotide containing this nonamer confers up-regulation by hypoxia and inhibition by unsaturated fatty acids when placed upstream of a heterologous promoter in a lacZ reporter construct. Additional yeast genes were identified which respond to hypoxia and cobalt in a manner similar to OLE1. A number of mammalian genes are also up-regulated by hypoxia, cobalt, nickel, and iron chelators. Hence, the identification of a family of yeast genes regulated in a similar manner has implications for understanding oxygen sensing and signaling in eukaryotes.

Transcriptional activation of FLR1 gene during Saccharomyces cerevisiae adaptation to growth with benomyl: role of Yap1p and Pdr3p

The adaptation of Saccharomyces cerevisiae to growth in the presence of the antimitotic fungicide benomyl involves the dramatic activation of FLR1 transcription, taking place during benomyl-induced latency following sudden exposure to the fungicide. FLR1 gene encodes a plasma membrane transporter of the major facilitator superfamily (MFS) conferring resistance to multiple drugs, in particular to benomyl. FLR1 activation is completely abolished in a mutant devoided of YAP1 gene being exerted by Yap1p either directly or via Pdr3p. YAP1 gene was proved to be a determinant of benomyl resistance; the duration of the adaptation period preceding cell division under benomyl stress was longer for the Deltayap1 population, presumably due to the abolishment of FLR1 activation during latency. Although benomyl resistance mediated by Yap1p is reduced in a FLR1 deletion mutant, results also indicate that Yap1p may have other target genes that confer benomyl resistance in yeast.

A Rox1-independent hypoxic pathway in yeast. Antagonistic action of the repressor Ord1 and activator Yap1 for hypoxic expression of the SRP1/TIR1 gene

Hypoxic SRP1/TIR1 gene expression depends on the absence of haem but is independent of Rox1-mediated repression. We have found a new hypoxic pathway involving an antagonistic interaction between the Ixr1/Ord1 repressor and the Yap1 factor, a transcriptional activator involved in oxidative stress response. Here, we show that Ord1 repressed SRP1 gene expression under normoxia and hypoxia, whereas Yap1 activated it. Ord1 and Yap1 have been shown to bind the SRP1 promoter in a region extending from -299 to -156 bp upstream of the start codon. A typical AP-1 responsive element lying from -247 to -240 bp allows Yap1 binding. Internal deletion of sequences within the SRP1 promoter were introduced. Two regions were characterized at positions -299/-251 and -218/-156 that, once removed, resulted in a constitutive expression of SRP1 in a wild-type strain under normoxic conditions. Deletion of both these two sequences allowed the bypass of YAP1 requirement in a Deltayap1 strain, whereas these two internal deletions did not yield increased expression in a Deltaord1 strain compared with the full-length promoter. Both a single Deltaord1 mutant and a doubly disrupted Deltayap1 Deltaord1 strain yielded normoxic constitutive SRP1 expression and increased hypoxic SRP1 induction, thereby demonstrating that ord1 is epistatic to yap1. Thus, Yap1 is not directly involved in SRP1 induction by hypoxia, but is necessary to counteract the Ord1 effect.

Stress-controlled transcription factors, stress-induced genes and stress tolerance in budding yeast

The transcriptional response to environmental changes is a major topic in both basic and applied research. From a basic point of view, to understand this response includes unravelling how the stress signal is sensed and transduced to the nucleus, to identify which genes are induced under each stress condition and, finally, to establish the phenotypic consequences of this induction in stress tolerance. The possibility of using genetic approaches has made the yeast Saccharomyces cerevisiae a compelling model to study stress response at a molecular level. Moreover, this information can be used to isolate and characterise stress-related proteins in higher eukaryotes and to design strategies to increase stress resistance in organisms of industrial interest. In this review the progress made in recent years is discussed.

A large-scale study of Yap1p-dependent genes in normal aerobic and H2O2-stress conditions: the role of Yap1p in cell proliferation control in yeast

Yeast genes regulated by the transcriptional activator Yap1p were screened by two independent methods: (i) use of a LacZ-fused gene library and (ii) high-density membrane hybridization. Changes in transcriptome profile were examined in the presence and in the absence of Yap1p, as well as under normal and H2O2-mediated stress conditions. Both approaches gave coherent results, leading to the identification of many genes that appear to be directly or indirectly regulated by Yap1p. Promoter sequence analysis of target genes revealed that this regulatory effect is not always dependent upon the presence of a Yap1p binding site. The results show that the regulatory role of Yap1p is not restricted to the activation of stress response but that this factor can act as a positive or a negative regulator, both under normal and oxidative stress conditions. Among the targets, a few genes participating in growth control cascades were detected. In particular, the RPI1 gene, a repressor of the ras-cAMP pathway, was found to be downregulated by Yap1p during the early phase of growth, but upregulated in the stationary phase or after oxidative stress.

A novel nuclear export signal sensitive to oxidative stress in the fission yeast transcription factor Pap1

Pap1, a fission yeast AP-1-like transcription factor, is negatively regulated by CRM1/exportin 1, the nuclear export factor. Pap1 was localized normally in the cytoplasm but was accumulated in the nucleus when Crm1 was inactivated by a temperature-sensitive mutation or by treatment with leptomycin B, a specific export inhibitor. Deletion of the C-terminal cysteine-rich domain (CRD) resulted in nuclear accumulation of Pap1, while a glutathione S-transferase-green fluorescent protein-CRD fusion protein was localized in the cytoplasm in a Crm1-dependent manner. Deletion and mutational analyses identified several important amino acids in a 19-amino acid region in the CRD as a nuclear export signal (NES). Strikingly, a cysteine residue (Cys-532), in addition to two leucines and an isoleucine, was important for the NES function and the presence of at least one of the two cysteine residues was essential. Unlike classical NESs such as the human immunodeficiency virus Rev NES, the Pap1 NES lost the function upon treatment with oxidants such as diethyl maleate. The oxidative stress response is conserved through evolution, as green fluorescent protein-fused proteins bearing the Pap1 NES expressed in mammalian cells responded to diethyl maleate. These results show that the hydrophobic amino acid-rich region containing two important cysteines in Pap1 serves as a novel NES, which is sensitive to oxidative stress.

AP-1 transcription factors in yeast

In the past two years, the completion of the Saccharomyces cerevisiae genome project and molecular analysis of other fungal species has resulted in the identification of a growing number of yeast AP-1 transcription factors. Characterisation of these factors indicates that, like their mammalian counterparts, they activate gene expression in response to a variety of extracellular stimuli. In particular, these factors are required for the response to oxidative stress and for surviving exposure to a variety of cytotoxic agents. Much progress has also been made in understanding how members of this family of proteins are regulated. These studies promise to further our awareness of eukaryotic stress responses and are likely to have implications for the study of mammalian AP-1.

The bZip transcription factor Cap1p is involved in multidrug resistance and oxidative stress response in Candida albicans

Candida albicans is an opportunistic pathogenic yeast which frequently develops resistance to the antifungal agent fluconazole (FCZ) in patients undergoing long-term therapy. FCZ-resistant strains often display a reduced intracellular FCZ accumulation which correlates with the overexpression of the ATP-binding cassette transporters CDR1 and CDR2 or the major facilitator (MF) MDR1. We have recently cloned a C. albicans gene, named CAP1, which codes for a bZip transcription factor of the AP-1 family homologous to the Yap1 protein involved in multidrug resistance and response to oxidative stress in Saccharomyces cerevisiae. CAP1 was found to confer FCZ resistance in S. cerevisiae by transcriptionally activating FLR1, a gene coding for an MF homologous to the C. albicans MDR1 gene product (A.-M. Alarco, I. Balan, D. Talibi, N. Mainville, and M. Raymond, J. Biol. Chem. 272:19304-19313, 1997). To study the role of CAP1 in C. albicans, we constructed a CAI4-derived mutant strain carrying a homozygous deletion of the CAP1 gene (CJD21). We found that deletion of CAP1 did not affect the susceptibility of CJD21 cells to FCZ, cerulenin, brefeldin A, and diamide but caused hypersensitivity to cadmium, 4-nitroquinoline N-oxide, 1,10-phenanthroline, and hydrogen peroxide, an effect which was reverted by reintroduction of the CAP1 gene in these cells. Introduction of a hyperactive truncated allele of CAP1 (CAP1-TR) in CJD21 resulted in resistance of the cells to all of the above compounds except hydrogen peroxide. The hyperresistant phenotype displayed by the CJD21 CAP1-TR transformants was found to correlate with the overexpression of a number of potential CAP1 transcriptional targets such as MDR1, CaYCF1, CaGLR1, and CaTRR1. Taken together, our results demonstrate that CAP1 is involved in multidrug resistance and oxidative stress response in C. albicans. Finally, disruption of CAP1 in strain FR2, selected in vitro for FCZ resistance and constitutively overexpressing MDR1, did not suppress but rather increased the levels of MDR1 expression, demonstrating that CAP1 acts as a negative transcriptional regulator of the MDR1 gene in FR2 and is not responsible for MDR1 overexpression in this strain.

Isolation of a putative Candida albicans transcriptional regulator involved in pleiotropic drug resistance by functional complementation of a pdr1 pdr3 mutation in Saccharomyces cerevisiae

Three Candida albicans genes, designated FCR (for fluconazole resistance), have been isolated by their ability to complement the fluconazole (FCZ) hypersensitivity of a Saccharomyces cerevisiae mutant lacking the transcription factors Pdr1p and Pdr3p. Overexpression of any of the three FCR genes in the pdr1 pdr3 mutant resulted in increased resistance of the cells to FCZ and cycloheximide and in increased expression of PDR5, a gene coding for a drug efflux transporter of the ATP-binding cassette superfamily and whose transcription is under the control of Pdr1p and Pdr3p. Deletion of PDR5 in the pdr1 pdr3 strain completely abrogated the ability of the three FCR genes to confer FCZ resistance, demonstrating that PDR5 is required for FCR-mediated FCZ resistance in S. cerevisiae. The FCR1 gene encodes a putative 517-amino-acid protein with an N-terminal Zn2C6-type zinc finger motif homologous to that found in fungal zinc cluster proteins, including S. cerevisiae Pdr1p and Pdr3p. We have constructed a C. albicans CAI4-derived mutant strain carrying a homozygous deletion of the FCR1 gene and analyzed its ability to grow in the presence of FCZ. We found that the fcr1Delta/fcr1Delta mutant displays hyperresistance to FCZ and other antifungal drugs compared to the parental CAI4 strain. This hyperresistance could be reversed to wild-type levels by reintroduction of a plasmid-borne copy of FCR1 into the fcr1Delta/fcr1Delta mutant. Taken together, our results indicate that the FCR1 gene behaves as a negative regulator of drug resistance in C. albicans and constitute the first evidence that FCZ resistance can result from the inactivation of a regulatory factor such as Fcr1p.

Oxidative stress responses of the yeast Saccharomyces cerevisiae

All aerobically growing organisms suffer exposure to oxidative stress, caused by partially reduced forms of molecular oxygen, known as reactive oxygen species (ROS). These are highly reactive and capable of damaging cellular constituents such as DNA, lipids and proteins. Consequently, cells from many different organisms have evolved mechanisms to protect their components against ROS. This review concentrates on the oxidant defence systems of the budding yeast Saccharomyces cerevisiae, which appears to have a number of inducible adaptive stress responses to oxidants, such as H2O2, superoxide anion and lipid peroxidation products. The oxidative stress responses appear to be regulated, at least in part, at the level of transcription and there is considerable overlap between them and many diverse stress responses, allowing the yeast cell to integrate its response towards environmental stress.

Posttranscriptional control of gene expression in yeast

Studies of the budding yeast Saccharomyces cerevisiae have greatly advanced our understanding of the posttranscriptional steps of eukaryotic gene expression. Given the wide range of experimental tools applicable to S. cerevisiae and the recent determination of its complete genomic sequence, many of the key challenges of the posttranscriptional control field can be tackled particularly effectively by using this organism. This article reviews the current knowledge of the cellular components and mechanisms related to translation and mRNA decay, with the emphasis on the molecular basis for rate control and gene regulation. Recent progress in characterizing translation factors and their protein-protein and RNA-protein interactions has been rapid. Against the background of a growing body of structural information, the review discusses the thermodynamic and kinetic principles that govern the translation process. As in prokaryotic systems, translational initiation is a key point of control. Modulation of the activities of translational initiation factors imposes global regulation in the cell, while structural features of particular 5' untranslated regions, such as upstream open reading frames and effector binding sites, allow for gene-specific regulation. Recent data have revealed many new details of the molecular mechanisms involved while providing insight into the functional overlaps and molecular networking that are apparently a key feature of evolving cellular systems. An overall picture of the mechanisms governing mRNA decay has only very recently begun to develop. The latest work has revealed new information about the mRNA decay pathways, the components of the mRNA degradation machinery, and the way in which these might relate to the translation apparatus. Overall, major challenges still to be addressed include the task of relating principles of posttranscriptional control to cellular compartmentalization and polysome structure and the role of molecular channelling in these highly complex expression systems.

Stress-activated signalling pathways in yeast

Eukaryotic cells have developed response mechanisms to combat the harmful effects of a variety of stress conditions. In the majority of cases, such responses involve changes in the gene expression pattern of the cell, leading to increased levels and activities of proteins that have stress-protective functions. Over the last few years, considerable progress has been made in understanding how stress-dependent transcriptional changes are brought about, and it transpires that the underlying mechanisms are highly conserved, being similar in organisms ranging from yeast to man. Many of the stress signals derive from the extracellular environment and accordingly these signals require transduction from the cell surface to the nucleus. This is accomplished through stress-activated signalling pathways, key amongst which are the highly conserved stress-activated MAP kinase pathways. Stimulation of these pathways leads to the increased activity of specific transcription factors and consequently the increased expression of certain stress-related genes. In this review, we focus on the progress that has been made in understanding these stress responses in yeast.

Stress tolerance: the key to effective strains of industrial baker's yeast

Application of yeasts in traditional biotechnologies such as baking, brewing, distiller's fermentations, and wine making, involves them in exposure to numerous environmental stresses. These can be encountered in concert and sequentially. Yeast exhibit a complex array of stress responses when under conditions that are less than physiologically ideal. These responses involve aspects of cell sensing, signal transduction, transcriptional and posttranslational control, protein-targeting to organelles, accumulation of protectants, and activity of repair functions. The efficiency of these processes in a given yeast strain determines its robustness, and to a large extent, whether it is able to perform to necessary commercial standards in industrial processes. This article reviews aspects of stress and stress response in the context of baker's yeast manufacturing and applications, and discusses the potential for improving the general robustness of industrial baker's yeast strains, in relation to physiological and genetic manipulations.

Active efflux by multidrug transporters as one of the strategies to evade chemotherapy and novel practical implications of yeast pleiotropic drug resistance

Mankind is faced by the increasing emergence of resistant pathogens, including cancer cells. An overview of the different strategies adopted by a variety of cells to evade chemotherapy is presented, with a focus on the mechanisms of multidrug transport. In particular, we analyze the yeast network for pleiotropic drug resistance and assess the potentiality of this system for further understanding of the mechanism of broad specificity and for development of novel practical applications.

Saccharomyces cerevisiae basic region-leucine zipper protein regulatory networks converge at the ATR1 structural gene

Saccharomyces cerevisiae cells express a family of transcription factors belonging to the basic region-leucine zipper family. Two of these proteins, yAP-1 and Gcn4p, are known to be involved in oxidative stress tolerance and general control of amino acid biosynthesis, respectively. Strains lacking the YAP1 or GCN4 structural gene have very different phenotypes, which have been taken as evidence that these transcriptional regulatory proteins control separate batteries of target genes. In this study, we provide evidence that both yAP-1 and Gcn4p control the expression of a putative integral membrane protein, Atr1p. Both yAP-1 and Gcn4p can elevate resistance to 3-amino-1,2,4-triazole and 4-nitroquinoline-N-oxide but only if the ATR1 gene is intact. Expression of ATR1 is enhanced in the presence of constitutively active alleles of YAP1 and GCN4. Regulation of ATR1 transcription by yAP-1 and Gcn4p occurs through a common DNA element related to the yAP-1 recognition element found upstream of other yAP-1-regulated genes. These data provide the first indication of overlap between the regulatory networks defined by yAP-1 and Gcn4p.

AP1-mediated multidrug resistance in Saccharomyces cerevisiae requires FLR1 encoding a transporter of the major facilitator superfamily

We have isolated a Candida albicans gene that confers resistance to the azole derivative fluconazole (FCZ) when overexpressed in Saccharomyces cerevisiae. This gene encodes a protein highly homologous to S. cerevisiae yAP-1, a bZip transcription factor known to mediate cellular resistance to toxicants such as cycloheximide (CYH), 4-nitroquinoline N-oxide (4-NQO), cadmium, and hydrogen peroxide. The gene was named CAP1, for C. albicans AP-1. Cap1 and yAP-1 are functional homologues, since CAP1 expression in a yap1 mutant strain partially restores the ability of the cells to grow on toxic concentrations of cadmium or hydrogen peroxide. We have found that the expression of YBR008c, an open reading frame identified in the yeast genome sequencing project and predicted to code for a multidrug transporter of the major facilitator superfamily, is dramatically induced in S. cerevisiae cells overexpressing CAP1. Overexpression of either CAP1 or YAP1 in a wild-type strain results in resistance to FCZ, CYH, and 4-NQO, whereas such resistance is completely abrogated (FCZ and CYH) or strongly reduced (4-NQO) in a ybr008c deletion mutant, demonstrating that YBR008c is involved in YAP1- and CAP1-mediated multidrug resistance. YBR008c has been renamed FLR1, for fluconazole resistance 1. The expression of an FLR1-lacZ reporter construct is strongly induced by the overexpression of either CAP1 or YAP1, indicating that the FLR1 gene is transcriptionally regulated by the Cap1 and yAP-1 proteins. Taken collectively, our results demonstrate that FLR1 represents a new YAP1-controlled multidrug resistance molecular determinant in S. cerevisiae. A similar detoxification pathway is also likely to operate in C. albicans.

Disruption of ribosomal scanning on the 5'-untranslated region, and not restriction of translational initiation per se, modulates the stability of nonaberrant mRNAs in the yeast Saccharomyces cerevisiae

Translation and mRNA decay constitute key players in the post-transcriptional control of gene expression. We examine the mechanisms by which the 5'-untranslated region (UTR) of nonaberrant mRNAs acts to modulate both these processes in Saccharomyces cerevisiae. Two classes of functional relationship between ribosome-5'-UTR interactions and mRNA decay are identifiable. In the first of these, elements in the main open reading frame (ORF) dictate how the decay process reacts to inhibitory structures in the 5'-UTR. The same types of stability modulation can be elicited by trans-regulation of translation via inducible binding of the iron-regulatory protein to an iron-responsive element located 9 nucleotides from the 5' cap. A eukaryotic translational repressor can therefore modulate mRNA decay via the 5'-UTR. In contrast, translational regulation mediated via changes in the activity of the cap-binding eukaryotic translation initiation factor eIF-4E bypasses translation-dependent pathways of mRNA degradation. Thus modulation of mRNA stability via the 5'-UTR depends on disruption of the scanning process, rather than changes in translational initiation efficiency per se. In the second class of pathway, an upstream ORF (uORF) functions as a powerful destabilizing element, inducing termination-dependent degradation that is apparently independent of any main ORF determinants but influenced by the efficiencies of ribosomal recognition of the uORF start and stop codons. This latter mechanism provides a regulatable means to modulate the stability of nonaberrant mRNAs via a UPF-dependent pathway.

Regulation of genes encoding subunits of the trehalose synthase complex in Saccharomyces cerevisiae: novel variations of STRE-mediated transcription control?

Saccharomyces cerevisiae cells show under suboptimal growth conditions a complex response that leads to the acquisition of tolerance to different types of environmental stress. This response is characterised by enhanced expression of a number of genes which contain so-called stress-responsive elements (STREs) in their promoters. In addition, the cells accumulate under suboptimal conditions the putative stress protectant trehalose. In this work, we have examined the expression of four genes encoding subunits of the trehalose synthase complex, GGS1/TPS1, TPS2, TPS3 and TSL1. We show that expression of these genes is coregulated under stress conditions. Like for many other genes containing STREs, expression of the trehalose synthase genes is also induced by heat and osmotic stress and by nutrient starvation, and negatively regulated by the Ras-cAMP pathway. However, during fermentative growth only TSL1 shows an expression pattern like that of the STRE-controlled genes CTT1 and SSA3, while expression of the three other trehalose synthase genes is only transiently down-regulated. This difference in expression might be related to the known requirement of trehalose biosynthesis for the control of yeast glycolysis and hence for fermentative growth. We conclude that the mere presence in the promoter of (an) active STRE(s) does not necessarily imply complete coregulation of expression. Additional mechanisms appear to fine tune the activity of STREs in order to adapt the expression of the downstream genes to specific requirements.

Molecular and genetic characterization of SLC1, a putative Saccharomyces cerevisiae homolog of the metazoan cytoplasmic dynein light chain 1

Cytoplasmic dynein is a multisubunit, microtubule-dependent motor enzyme that has been proposed to function in a variety of intracellular movements. As part of an effort to understand the evolution and the biological roles of cytoplasmic dynein, we have identified the first non-metazoan dynein light chain 1, SLC1, in the yeast Saccharomyces cerevisiae. The amino acid sequence of the SLC1 protein is similar to those of the human, Drosophila and Caenorhabditis cytoplasmic dynein light chains 1. The SLC1 gene lies adjacent to the YAP2 (= CAD1) transcription unit. The SLC1 coding sequence is split by two introns and its mRNA is detectable throughout the cell cycle. Tetrad analysis of heterozygotes harboring a TRP insertion in the SLC1 coding region indicate that SLC1 function is not essential for cell viability. Furthermore, we demonstrate that double mutants, defective for SLC1 and the kinesin-related CIN8 genes are non-lethal. The redundancy of SLC1 function in yeast contrasts with the cell death caused by loss-of-function mutations in the dynein light chain 1 gene in Drosophila melanogaster.

ROD1, a novel gene conferring multiple resistance phenotypes in Saccharomyces cerevisiae

Glutathione-dependent detoxification reactions are catalyzed by the enzyme glutathione S-transferase and are important in drug resistance in organisms ranging from bacteria to humans. The yeast Issatchenkia orientalis expresses a glutathione S-transferase (GST) protein that is induced when the GST substrate o-dinitrobenzene (o-DNB) is added to the culture. In this study, we show that overproduction of the I. orientalis GST in Saccharomyces cerevisiae leads to an increase in o-dinitrobenzene resistance in S. cerevisiae cells. To recover genes that influence o-DNB resistance in S. cerevisiae, a high copy plasmid library was screened for loci that elevate o-DNB tolerance. One gene was recovered and designated ROD1 (resistance to o-dinitrobenzene). This locus was found to encode a novel protein with no significant sequence similarity with proteins of known function in the data base. An epitope-tagged version of Rod1p was produced in S. cerevisiae and shown to function properly. Subcellular fractionation experiments indicated that this factor was found in the particulate fraction by differential centrifugation. Overproduction of Rod1p leads to resistance to not only o-DNB but also zinc and calcium. Strains that lack the ROD1 gene are hypersensitive to these same compounds. Rod1p represents a new type of molecule influencing drug tolerance in eukaryotes.

Transcriptional regulators of oxidative stress-inducible genes in prokaryotes and eukaryotes

It appears that redox regulation is an important mechanism for the control of transcription factor activation. The role of oxidation-reduction is probably determined in part by the structure of the transcription factors. For example, the presence of cysteine residues within the DNA binding sites may sensitize a transcription factor to ROS. The ROS-mediated regulation of transcription factors is specific, some ROS are more efficient than other ROS in activating defined regulators. While the protective antioxidant responses induced by ROS in prokaryotes and eukaryotes are rather conserved (for example, SOD, HSP...), the regulators for these genes do not appear to be conserved. Further studies designed to fully characterize these regulators and understand the subtle mechanisms involved in redox gene regulation are ongoing, and should provide the theoretical basis for clinical approaches using antioxidant therapies in human diseases in which oxidative stress is implicated.

Identification of regulatory proteins that might be involved in carbon catabolite repression of the aminopeptidase I gene of the yeast Saccharomyces cerevisiae

Transcription of the vacuolar aminopeptidase yscI (APE1) gene in Saccharomyces cerevisiae has previously been suggested to require the participation of a cis upstream activation sequence (UAS) involved in carbon catabolite repression that responds to glucose. To determine the structure of the APE1 UAS element, we used the 18-bp sequence 5'-ATGAATTAGTCAGCTTCT-3' as the DNA-binding site. Using gel mobility shift assays, we have identified a 78 kDa protein from yeast that binds specifically to both single and double-stranded forms of the UAS DNA-binding site. We have also identified a 48 kDa heterodimer from yeast that binds specifically to the single-stranded form of the UAS and whose DNA binding activity is remarkably heat stable. Even though the APE1 UAS contains a consensus sequence for the binding of the yeast activator protein yAP1, the two DNA-protein complexes could still be detected in a strain bearing a deletion in the YAP1 gene.

Arabidopsis thaliana NADPH oxidoreductase homologs confer tolerance of yeasts toward the thiol-oxidizing drug diamide

To isolate new plant genes involved in the defense against oxidative stress, an Arabidopsis cDNA library in a yeast expression vector was transformed into a yeast strain deficient in the YAP1 gene, which encodes a b-Zip transcription factor and regulates general stress response in yeasts. Cells from approximately 10(5) primary transformants were subjected to a tolerance screen toward the thiol-oxidizing drug diamide, which depletes the reduced glutathione in the cell. Four types of Arabidopsis cDNAs were isolated. Three of these cDNAs (P1, P2, and P4) belong to a plant zeta-crystallin family and P3 is an Arabidopsis homolog of isoflavonoid reductases. As such, all four isolated cDNAs are homologous to NADPH oxidoreductases. P1, P2, and P3 steady-state mRNAs accumulated rapidly in Arabidopsis plants under various oxidative stress conditions, such as treatment with paraquat, t-butylhydroperoxide, diamide, and menadione. The data suggested that proteins encoded by the isolated cDNAs play a distinct role in plant antioxidant defense and are possibly involved in NAD(P)/NAD(P)H homeostasis.