Aca1 and Aca2, ATF/CREB activators in Saccharomyces cerevisiae, are important for carbon source utilization but not the response to stress

The MAP kinase FvHog1 regulates FB1 synthesis and Ca2+ homeostasis in Fusarium verticillioides

The high osmolarity glycerol 1 mitogen-activated protein kinase (Hog1-MAPK) cascade genes are important for diverse biological processes. The activated Hog1 upon multiple environmental stress stimuli enters into the nucleus where it directly phosphorylates transcription factors to regulate various physiological processes in phytopathogenic fungi. However, their roles have not been well-characterized in Fusarium verticillioides. In this study, FvHog1 is identified and functionally analyzed. The findings reveal that the phosphorylation level and nuclear localization of FvHog1 are increased in Fumonisin B1 (FB1)-inducing condition to regulate the expression of FB1 biosynthesis FUM genes. More importantly, the deletion mutants of Hog1-MAPK pathway show increased sensitivity to Ca2+ stress and elevated intracellular Ca2+ content. The phosphorylation level and nuclear localization of FvHog1 are increased with Ca2+ treatment. Furthermore, our results show that FvHog1 can directly phosphorylate Ca2+-responsive zinc finger transcription factor 1 (FvCrz1) to regulate Ca2+ homeostasis. In conclusion, our findings indicate that FvHog1 is required for FB1 biosynthesis, pathogenicity and Ca2+ homeostasis in F. verticillioides. It provides a theoretical basis for effective prevention and control maize ear and stalk rot disease.

A Special Phenotype of Aconidial Aspergillus niger SH2 and Its Mechanism of Formation via CRISPRi

The complex morphological structure of Aspergillus niger influences its production of proteins, metabolites, etc., making the genetic manipulation and clonal purification of this species increasingly difficult, especially in aconidial Aspergillus niger. In this study, we found that N-acetyl-D-glucosamine (GlcNAc) could induce the formation of spore-like propagules in the aconidial Aspergillus niger SH2 strain. The spore-like propagules possessed life activities such as drug resistance, genetic transformation, and germination. Transcriptomic analysis indicated that the spore-like propagules were resting conidia entering dormancy and becoming more tolerant to environmental stresses. The Dac1 gene and the metabolic pathway of GlcNAc converted to glycolysis are related to the formation of the spore-like propagules, as evidenced by the CRISPRi system, qPCR, and semi-quantitative RT-PCR. Moreover, a method based on the CRISPR-Cas9 tool to rapidly recycle screening tags and recover genes was suitable for Aspergillus niger SH2. To sum up, this suggests that the spore-like propagules are resting conidia and the mechanism of their formation is the metabolic pathway of GlcNAc converted to glycolysis, particularly the Dac1 gene. This study can improve our understanding of the critical factors involved in mechanisms of phenotypic change and provides a good model for researching phenotypic change in filamentous fungi.

Cowpea NAC Transcription Factors Positively Regulate Cellular Stress Response and Balance Energy Metabolism in Yeast via Reprogramming of Biosynthetic Pathways

Yeast is a dominant host for recombinant production of heterologous proteins, high-value biochemical compounds, and microbial fermentation. During bioprocess operations, pH fluctuations, organic solvents, drying, starvation, osmotic pressure, and often a combination of these stresses cause growth inhibition or death, markedly limiting its industrial use. Thus, stress-tolerant yeast strains with balanced energy-bioenergetics are highly desirous for sustainable improvement of quality biotechnological production. We isolated two NAC transcription factors (TFs), VuNAC1 and VuNAC2, from a wild cowpea genotype, improving both stress tolerance and growth when expressed in yeast. The GFP-fused proteins were localized to the nucleus. Y2H and reporter assay demonstrated the dimerization and transactivation abilities of the VuNAC proteins having structural folds similar to rice SNAC1. The gel-shift assay indicated that the TFs recognize an "ATGCGTG" motif for DNA-binding shared by several native TFs in yeast. The heterologous expression of VuNAC1/2 in yeast improved growth, biomass, lifespan, fermentation efficiency, and altered cellular composition of biomolecules. The transgenic strains conferred tolerance to multiple stresses such as high salinity, osmotic stress, freezing, and aluminum toxicity. Analysis of the metabolome revealed reprogramming of major pathways synthesizing nucleotides, vitamin B complex, amino acids, antioxidants, flavonoids, and other energy currencies and cofactors. Consequently, the transcriptional tuning of stress signaling and biomolecule metabolism improved the survival of the transgenic strains during starvation and stress recovery. VuNAC1/2-based synthetic gene expression control may contribute to designing robust industrial yeast strains with value-added productivity.

Promoter Architecture and Promoter Engineering in Saccharomyces cerevisiae

Promoters play an essential role in the regulation of gene expression for fine-tuning genetic circuits and metabolic pathways in Saccharomyces cerevisiae (S. cerevisiae). However, native promoters in S. cerevisiae have several limitations which hinder their applications in metabolic engineering. These limitations include an inadequate number of well-characterized promoters, poor dynamic range, and insufficient orthogonality to endogenous regulations. Therefore, it is necessary to perform promoter engineering to create synthetic promoters with better properties. Here, we review recent advances related to promoter architecture, promoter engineering and synthetic promoter applications in S. cerevisiae. We also provide a perspective of future directions in this field with an emphasis on the recent advances of machine learning based promoter designs.

Stress sensor Ire1 deploys a divergent transcriptional program in response to lipid bilayer stress

Membrane integrity at the endoplasmic reticulum (ER) is tightly regulated, and its disturbance is implicated in metabolic diseases. Using an engineered sensor that activates the unfolded protein response (UPR) exclusively when normal ER membrane lipid composition is compromised, we identified pathways beyond lipid metabolism that are necessary to maintain ER integrity in yeast and in C. elegans. To systematically validate yeast mutants that disrupt ER membrane homeostasis, we identified a lipid bilayer stress (LBS) sensor in the UPR transducer protein Ire1, located at the interface of the amphipathic and transmembrane helices. Furthermore, transcriptome and chromatin immunoprecipitation analyses pinpoint the UPR as a broad-spectrum compensatory response wherein LBS and proteotoxic stress deploy divergent transcriptional UPR programs. Together, these findings reveal the UPR program as the sum of two independent stress responses, an insight that could be exploited for future therapeutic intervention.

Improved recovery of cell-cycle gene expression in Saccharomyces cerevisiae from regulatory interactions in multiple omics data

BACKGROUND

Gene expression is regulated by DNA-binding transcription factors (TFs). Together with their target genes, these factors and their interactions collectively form a gene regulatory network (GRN), which is responsible for producing patterns of transcription, including cyclical processes such as genome replication and cell division. However, identifying how this network regulates the timing of these patterns, including important interactions and regulatory motifs, remains a challenging task.

RESULTS

We employed four in vivo and in vitro regulatory data sets to investigate the regulatory basis of expression timing and phase-specific patterns cell-cycle expression in Saccharomyces cerevisiae. Specifically, we considered interactions based on direct binding between TF and target gene, indirect effects of TF deletion on gene expression, and computational inference. We found that the source of regulatory information significantly impacts the accuracy and completeness of recovering known cell-cycle expressed genes. The best approach involved combining TF-target and TF-TF interactions features from multiple datasets in a single model. In addition, TFs important to multiple phases of cell-cycle expression also have the greatest impact on individual phases. Important TFs regulating a cell-cycle phase also tend to form modules in the GRN, including two sub-modules composed entirely of unannotated cell-cycle regulators (STE12-TEC1 and RAP1-HAP1-MSN4).

CONCLUSION

Our findings illustrate the importance of integrating both multiple omics data and regulatory motifs in order to understand the significance regulatory interactions involved in timing gene expression. This integrated approached allowed us to recover both known cell-cycles interactions and the overall pattern of phase-specific expression across the cell-cycle better than any single data set. Likewise, by looking at regulatory motifs in the form of TF-TF interactions, we identified sets of TFs whose co-regulation of target genes was important for cell-cycle expression, even when regulation by individual TFs was not. Overall, this demonstrates the power of integrating multiple data sets and models of interaction in order to understand the regulatory basis of established biological processes and their associated gene regulatory networks.

Sumoylation of DNA-bound transcription factor Sko1 prevents its association with nontarget promoters

Sequence-specific transcription factors (TFs) represent one of the largest groups of proteins that is targeted for SUMO post-translational modification, in both yeast and humans. SUMO modification can have diverse effects, but recent studies showed that sumoylation reduces the interaction of multiple TFs with DNA in living cells. Whether this relates to a general role for sumoylation in TF binding site selection, however, has not been fully explored because few genome-wide studies aimed at studying such a role have been reported. To address this, we used genome-wide analysis to examine how sumoylation regulates Sko1, a yeast bZIP TF with hundreds of known binding sites. We find that Sko1 is sumoylated at Lys 567 and, although many of its targets are osmoresponse genes, the level of Sko1 sumoylation is not stress-regulated and the modification does not depend or impinge on its phosphorylation by the osmostress kinase Hog1. We show that Sko1 mutants that cannot bind DNA are not sumoylated, but attaching a heterologous DNA binding domain restores the modification, implicating DNA binding as a major determinant for Sko1 sumoylation. Genome-wide chromatin immunoprecipitation (ChIP-seq) analysis shows that a sumoylation-deficient Sko1 mutant displays increased occupancy levels at its numerous binding sites, which inhibits the recruitment of the Hog1 kinase to some induced osmostress genes. This strongly supports a general role for sumoylation in reducing the association of TFs with chromatin. Extending this result, remarkably, sumoylation-deficient Sko1 binds numerous additional promoters that are not normally regulated by Sko1 but contain sequences that resemble the Sko1 binding motif. Our study points to an important role for sumoylation in modulating the interaction of a DNA-bound TF with chromatin to increase the specificity of TF-DNA interactions.

Disruption of the transcription factors Thi2p and Nrm1p alleviates the post-glucose effect on xylose utilization in Saccharomyces cerevisiae

BACKGROUND

The recombinant Saccharomyces cerevisiae strains that acquired the ability to utilize xylose through metabolic and evolutionary engineering exhibit good performance when xylose is the sole carbon source in the medium (designated the X stage in the present work). However, the xylose consumption rate of strains is generally low after glucose depletion during glucose-xylose co-fermentation, despite the presence of xylose in the medium (designated the GX stage in the present work). Glucose fermentation appears to reduce the capacity of these strains to "recognize" xylose during the GX stage, a phenomenon termed the post-glucose effect on xylose metabolism.

RESULTS

Two independent xylose-fermenting S. cerevisiae strains derived from a haploid laboratory strain and a diploid industrial strain were used in the present study. Their common characteristics were investigated to reveal the mechanism underlying the post-glucose effect and to develop methods to alleviate this effect. Both strains showed lower growth and specific xylose consumption rates during the GX stage than during the X stage. Glycolysis, the pentose phosphate pathway, and translation-related gene expression were reduced; meanwhile, genes in the tricarboxylic acid cycle and glyoxylic acid cycle demonstrated higher expression during the GX stage than during the X stage. The effects of 11 transcription factors (TFs) whose expression levels significantly differed between the GX and X stages in both strains were investigated. Knockout of THI2 promoted ribosome synthesis, and the growth rate, specific xylose utilization rate, and specific ethanol production rate of the strain increased by 17.4, 26.8, and 32.4%, respectively, in the GX stage. Overexpression of the ribosome-related genes RPL9A, RPL7B, and RPL7A also enhanced xylose utilization in a corresponding manner. Furthermore, the overexpression of NRM1, which is related to the cell cycle, increased the growth rate by 8.7%, the xylose utilization rate by 30.0%, and the ethanol production rate by 76.6%.

CONCLUSIONS

The TFs Thi2p and Nrm1p exerted unexpected effects on the post-glucose effect, enhancing ribosome synthesis and altering the cell cycle, respectively. The results of this study will aid in maintaining highly efficient xylose metabolism during glucose-xylose co-fermentation, which is utilized for lignocellulosic bioethanol production.

CO2 sensing in fungi: at the heart of metabolic signaling

Adaptation to the changing environmental CO₂ levels is essential for all living cells. In particular, microorganisms colonizing and infecting the human body are exposed to highly variable concentrations, ranging from atmospheric 0.04 to 5% and more in blood and specific host niches. Carbonic anhydrases are highly conserved metalloenzymes that enable fixation of CO₂ by its conversion into bicarbonate. This process is not only crucial to ensure the supply of adequate carbon amounts for cellular metabolism, but also contributes to several signaling processes in fungi, including morphology and communication. The fungal specific carbonic anhydrase gene NCE103 is transcribed in response to CO₂ availability. As recently shown, this regulation relies on the ATF/CREB transcription factor Cst6 and the AGC family protein kinase Sch9. Here, we review the regulatory mechanisms which control NCE103 expression in the model organism Saccharomyces cerevisiae and the pathogenic yeasts Candida albicans and Candida glabrata and discuss which additional factors might contribute in this novel CO₂ sensing cascade.

Lipid Signaling via Pkh1/2 Regulates Fungal CO2 Sensing through the Kinase Sch9

Adaptation to alternating CO₂ concentrations is crucial for all organisms. Carbonic anhydrases—metalloenzymes that have been found in all domains of life—enable fixation of scarce CO₂ by accelerating its conversion to bicarbonate and ensure maintenance of cellular metabolism. In fungi and other eukaryotes, the carbonic anhydrase Nce103 has been shown to be essential for growth in air (~0.04% CO₂). Expression of NCE103 is regulated in response to CO₂ availability. In Saccharomyces cerevisiae, NCE103 is activated by the transcription factor ScCst6, and in Candida albicans and Candida glabrata, it is activated by its homologues CaRca1 and CgRca1, respectively. To identify the kinase controlling Cst6/Rca1, we screened an S. cerevisiae kinase/phosphatase mutant library for the ability to regulate NCE103 in a CO₂-dependent manner. We identified ScSch9 as a potential ScCst6-specific kinase, as the sch9Δ mutant strain showed deregulated NCE103 expression on the RNA and protein levels. Immunoprecipitation revealed the binding capabilities of both proteins, and detection of ScCst6 phosphorylation by ScSch9 in vitro confirmed Sch9 as the Cst6 kinase. We could show that CO₂-dependent activation of Sch9, which is part of a kinase cascade, is mediated by lipid/Pkh1/2 signaling but not TORC1. Finally, we tested conservation of the identified regulatory cascade in the pathogenic yeast species C. albicans and C. glabrata. Deletion of SCH9 homologues of both species impaired CO₂-dependent regulation of NCE103 expression, which indicates a conservation of the CO₂ adaptation mechanism among yeasts. Thus, Sch9 is a Cst6/Rca1 kinase that links CO₂ adaptation to lipid signaling via Pkh1/2 in fungi.

All living organisms have to cope with alternating CO₂ concentrations as CO₂ levels range from very low in the atmosphere (0.04%) to high (5% and more) in other niches, including the human body. In fungi, CO₂ is sensed via two pathways. The first regulates virulence in pathogenic yeast by direct activation of adenylyl cyclase. The second pathway, although playing a fundamental role in fungal metabolism, is much less understood. Here the transcription factor Cst6/Rca1 controls carbon homeostasis by regulating carbonic anhydrase expression. Upstream signaling in this pathway remains elusive. We identify Sch9 as the kinase controlling Cst6/Rca1 activity in yeast and demonstrate that this pathway is conserved in pathogenic yeast species, which highlights identified key players as potential pharmacological targets. Furthermore, we provide a direct link between adaptation to changing CO₂ conditions and lipid/Pkh1/2 signaling in yeast, thus establishing a new signaling cascade central to metabolic adaptation.

Genome-Wide Mapping of Binding Sites Reveals Multiple Biological Functions of the Transcription Factor Cst6p in Saccharomyces cerevisiae

In the model eukaryote Saccharomyces cerevisiae, the transcription factor Cst6p has been reported to play important roles in several biological processes. However, the genome-wide targets of Cst6p and its physiological functions remain unknown. Here, we mapped the genome-wide binding sites of Cst6p at high resolution. Cst6p binds to the promoter regions of 59 genes with various biological functions when cells are grown on ethanol but hardly binds to the promoter at any gene when cells are grown on glucose. The retarded growth of the CST6 deletion mutant on ethanol is attributed to the markedly decreased expression of NCE103, encoding a carbonic anhydrase, which is a direct target of Cst6p. The target genes of Cst6p have a large overlap with those of stress-responsive transcription factors, such as Sko1p and Skn7p. In addition, a CST6 deletion mutant growing on ethanol shows hypersensitivity to oxidative stress and ethanol stress, assigning Cst6p as a new member of the stress-responsive transcriptional regulatory network. These results show that mapping of genome-wide binding sites can provide new insights into the function of transcription factors and highlight the highly connected and condition-dependent nature of the transcriptional regulatory network in S. cerevisiae.

Transcription factors regulate the activity of various biological processes through binding to specific DNA sequences. Therefore, the determination of binding positions is important for the understanding of the regulatory effects of transcription factors. In the model eukaryote Saccharomyces cerevisiae, the transcription factor Cst6p has been reported to regulate several biological processes, while its genome-wide targets remain unknown. Here, we mapped the genome-wide binding sites of Cst6p at high resolution. We show that the binding of Cst6p to its target promoters is condition dependent and explain the mechanism for the retarded growth of the CST6 deletion mutant on ethanol. Furthermore, we demonstrate that Cst6p is a new member of a stress-responsive transcriptional regulatory network. These results provide deeper understanding of the function of the dynamic transcriptional regulatory network in S. cerevisiae.

Biofilm formation in Candida glabrata: What have we learnt from functional genomics approaches?

Biofilms are a source of therapeutic failures because of their intrinsic tolerance to antimicrobials. Candida glabrata is one of the pathogenic yeasts that is responsible for life-threatening disseminated infections and able to form biofilms on medical devices such as vascular and urinary catheters. Recent progresses in the functional genomics of C. glabrata have been applied to the study of biofilm formation, revealing the contribution of an array of genes to this process. In particular, the Yak1 kinase and the Swi/Snf chromatin remodeling complex have been shown to relieve the repression exerted by subtelomeric silencing on the expression of the EPA6 and EPA7 genes, thus allowing the encoded adhesins to exert their key roles in biofilm formation. This provides a framework to evaluate the contribution of other genes that have been genetically linked to biofilm development and, based on the function of their orthologs in Saccharomyces cerevisiae, appear to have roles in adaptation to nutrient deprivation, calcium signaling, cell wall remodeling and adherence. Future studies combining the use of in vitro and animal models of biofilm formation, omics approaches and forward or reverse genetics are needed to expand the current knowledge of C. glabrata biofilm formation and reveal the mechanisms underlying their antifungal tolerance.

Condition-specific genetic interaction maps reveal crosstalk between the cAMP/PKA and the HOG MAPK pathways in the activation of the general stress response

Cells must quickly respond and efficiently adapt to environmental changes. The yeast Saccharomyces cerevisiae has multiple pathways that respond to specific environmental insults, as well as a generic stress response program. The later is regulated by two transcription factors, Msn2 and Msn4, that integrate information from upstream pathways to produce fast, tunable, and robust response to different environmental changes. To understand this integration, we employed a systematic approach to genetically dissect the contribution of various cellular pathways to Msn2/4 regulation under a range of stress and growth conditions. We established a high-throughput liquid handling and automated flow cytometry system and measured GFP levels in 68 single-knockout and 1,566 double-knockout strains that carry an HSP12-GFP allele as a reporter for Msn2/4 activity. Based on the expression of this Msn2/4 reporter in five different conditions, we identified numerous genetic and epistatic interactions between different components in the network upstream to Msn2/4. Our analysis gains new insights into the functional specialization of the RAS paralogs in the repression of stress response and identifies a three-way crosstalk between the Mediator complex, the HOG MAPK pathway, and the cAMP/PKA pathway.

A genetic screen for high copy number suppressors of the synthetic lethality between elg1Δ and srs2Δ in yeast

Elg1 and Srs2 are two proteins involved in maintaining genome stability in yeast. After DNA damage, the homotrimeric clamp PCNA, which provides stability and processivity to DNA polymerases and serves as a docking platform for DNA repair enzymes, undergoes modification by the ubiquitin-like molecule SUMO. PCNA SUMOylation helps recruit Srs2 and Elg1 to the replication fork. In the absence of Elg1, both SUMOylated PCNA and Srs2 accumulate at the chromatin fraction, indicating that Elg1 is required for removing SUMOylated PCNA and Srs2 from DNA. Despite this interaction, which suggests that the two proteins work together, double mutants elg1Δ srs2Δ have severely impaired growth as haploids and exhibit synergistic sensitivity to DNA damage and a synergistic increase in gene conversion. In addition, diploid elg1Δ srs2Δ double mutants are dead, which implies that an essential function in the cell requires at least one of the two gene products for survival. To gain information about this essential function, we have carried out a high copy number suppressor screen to search for genes that, when overexpressed, suppress the synthetic lethality between elg1Δ and srs2Δ. We report the identification of 36 such genes, which are enriched for functions related to DNA- and chromatin-binding, chromatin packaging and modification, and mRNA export from the nucleus.

Fission yeast hotspot sequence motifs are also active in budding yeast

In most organisms, including humans, meiotic recombination occurs preferentially at a limited number of sites in the genome known as hotspots. There has been substantial progress recently in elucidating the factors determining the location of meiotic recombination hotspots, and it is becoming clear that simple sequence motifs play a significant role. In S. pombe, there are at least five unique sequence motifs that have been shown to produce hotspots of recombination, and it is likely that there are more. In S. cerevisiae, simple sequence motifs have also been shown to produce hotspots or show significant correlations with hotspots. Some of the hotspot motifs in both yeasts are known or suspected to bind transcription factors (TFs), which are required for the activity of those hotspots. Here we show that four of the five hotspot motifs identified in S. pombe also create hotspots in the distantly related budding yeast S. cerevisiae. For one of these hotspots, M26 (also called CRE), we identify TFs, Cst6 and Sko1, that activate and inhibit the hotspot, respectively. In addition, two of the hotspot motifs show significant correlations with naturally occurring hotspots. The conservation of these hotspots between the distantly related fission and budding yeasts suggests that these sequence motifs, and others yet to be discovered, may function widely as hotspots in many diverse organisms.

Response to hyperosmotic stress

An appropriate response and adaptation to hyperosmolarity, i.e., an external osmolarity that is higher than the physiological range, can be a matter of life or death for all cells. It is especially important for free-living organisms such as the yeast Saccharomyces cerevisiae. When exposed to hyperosmotic stress, the yeast initiates a complex adaptive program that includes temporary arrest of cell-cycle progression, adjustment of transcription and translation patterns, and the synthesis and retention of the compatible osmolyte glycerol. These adaptive responses are mostly governed by the high osmolarity glycerol (HOG) pathway, which is composed of membrane-associated osmosensors, an intracellular signaling pathway whose core is the Hog1 MAP kinase (MAPK) cascade, and cytoplasmic and nuclear effector functions. The entire pathway is conserved in diverse fungal species, while the Hog1 MAPK cascade is conserved even in higher eukaryotes including humans. This conservation is illustrated by the fact that the mammalian stress-responsive p38 MAPK can rescue the osmosensitivity of hog1Δ mutations in response to hyperosmotic challenge. As the HOG pathway is one of the best-understood eukaryotic signal transduction pathways, it is useful not only as a model for analysis of osmostress responses, but also as a model for mathematical analysis of signal transduction pathways. In this review, we have summarized the current understanding of both the upstream signaling mechanism and the downstream adaptive responses to hyperosmotic stress in yeast.

m:Explorer: multinomial regression models reveal positive and negative regulators of longevity in yeast quiescence

We developed m:Explorer for identifying process-specific transcription factors (TFs) from multiple genome-wide sources, including transcriptome, DNA-binding and chromatin data. m:Explorer robustly outperforms similar techniques in finding cell cycle TFs in Saccharomyces cerevisiae. We predicted and experimentally tested regulators of quiescence (G0), a model of ageing, over a six-week time-course. We validated nine of top-12 predictions as novel G0 TFs, including Δmga2, Δcst6, Δbas1 with higher viability and G0-essential TFs Tup1, Swi3. Pathway analysis associates longevity to reduced growth, reprogrammed metabolism and cell wall remodeling. m:Explorer (http://biit.cs.ut.ee/mexplorer/) is instrumental in interrogating eukaryotic regulatory systems using heterogeneous data.

A framework for mapping, visualisation and automatic model creation of signal-transduction networks

An intuitive formalism for reconstructing cellular networks from empirical data is presented, and used to build a comprehensive yeast MAP kinase network. The accompanying rxncon software tool can convert networks to a range of standard graphical formats and mathematical models.

Network mapping at the granularity of empirical data that largely avoids combinatorial complexity

Automatic visualisation and model generation with the rxncon open source software tool

Visualisation in a range of formats, including all three SBGN formats, as well as contingency matrix or regulatory graph

Comprehensive and completely references map of the yeast MAP kinase network in the rxncon format

Intracellular signalling systems are highly complex. This complexity makes handling, analysis and visualisation of available knowledge a major challenge in current signalling research. Here, we present a novel framework for mapping signal-transduction networks that avoids the combinatorial explosion by breaking down the network in reaction and contingency information. It provides two new visualisation methods and automatic export to mathematical models. We use this framework to compile the presently most comprehensive map of the yeast MAP kinase network. Our method improves previous strategies by combining (I) more concise mapping adapted to empirical data, (II) individual referencing for each piece of information, (III) visualisation without simplifications or added uncertainty, (IV) automatic visualisation in multiple formats, (V) automatic export to mathematical models and (VI) compatibility with established formats. The framework is supported by an open source software tool that facilitates integration of the three levels of network analysis: definition, visualisation and mathematical modelling. The framework is species independent and we expect that it will have wider impact in signalling research on any system.

The bZIP transcription factor Rca1p is a central regulator of a novel CO₂ sensing pathway in yeast

Like many organisms the fungal pathogen Candida albicans senses changes in the environmental CO₂ concentration. This response involves two major proteins: adenylyl cyclase and carbonic anhydrase (CA). Here, we demonstrate that CA expression is tightly controlled by the availability of CO₂ and identify the bZIP transcription factor Rca1p as the first CO₂ regulator of CA expression in yeast. We show that Rca1p upregulates CA expression during contact with mammalian phagocytes and demonstrate that serine 124 is critical for Rca1p signaling, which occurs independently of adenylyl cyclase. ChIP-chip analysis and the identification of Rca1p orthologs in the model yeast Saccharomyces cerevisiae (Cst6p) point to the broad significance of this novel pathway in fungi. By using advanced microscopy we visualize for the first time the impact of CO₂ build-up on gene expression in entire fungal populations with an exceptional level of detail. Our results present the bZIP protein Rca1p as the first fungal regulator of carbonic anhydrase, and reveal the existence of an adenylyl cyclase independent CO₂ sensing pathway in yeast. Rca1p appears to regulate cellular metabolism in response to CO₂ availability in environments as diverse as the phagosome, yeast communities or liquid culture.

Skin infection, oral and vaginal thrush, or bloodstream candidiasis are some of the diseases caused by the human pathogen Candida albicans. The high versatility of infection niches reflects the capacity of this yeast to respond to strong variations in its environment such as CO₂ concentration. This molecule initiates the regulation of an essential protein: carbonic anhydrase, not through the known adenylyl cyclase CO₂ sensor but as we discovered via a novel fungal CO₂ sensing pathway involving the transcriptional regulator Rca1p. This protein is additionally implicated in growth, yeast-to-hyphae morphological switch and cell wall stability of C. albicans. The ortholog of Rca1p in Saccharomyces cerevisiae demonstrated a conserved function in the induction of the carbonic anhydrase in low CO₂ concentration atmospheres pointing to the broad significance of Rca1p in fungal CO₂ sensing.

The Cyc8-Tup1 complex inhibits transcription primarily by masking the activation domain of the recruiting protein

The yeast Tup1-Cyc8 corepressor complex is recruited to promoters by DNA-binding repressors, but the mechanisms by which it inhibits expression of genes involved in various stress pathways are poorly understood. Conditional and rapid depletion of Tup1 from the nucleus leads to concurrent nucleosome depletion and histone acetylation, recruitment of coactivators (Swi/Snf, SAGA, and Mediator), and increased transcriptional activity. Conversely, coactivator dissociation occurs rapidly upon rerepression by Cyc8-Tup1, although coactivator association and transcription can be blocked even in the absence of nucleosomes. The coactivators are recruited to the sites where Tup1 was located prior to depletion, indicating that the repressor proteins that recruit Tup1 function as activators in its absence. Last, Cyc8-Tup1 can interact with activation domains in vivo. Thus, Cyc8-Tup1 regulates transcription primarily by masking and inhibiting the transcriptional activation domains of the recruiting proteins, not by acting as a corepressor. We suggest that the corepressor function of Cyc8-Tup1 makes only a modest contribution to expression of target genes, specifically to keep expression levels below the nonactivated state.

Deletion of a subgroup of ribosome-related genes minimizes hypoxia-induced changes and confers hypoxia tolerance

Hypoxia is a widely occurring condition experienced by diverse organisms under numerous physiological and disease conditions. To probe the molecular mechanisms underlying hypoxia responses and tolerance, we performed a genome-wide screen to identify mutants with enhanced hypoxia tolerance in the model eukaryote, the yeast Saccharomyces cerevisiae. Yeast provides an excellent model for genomic and proteomic studies of hypoxia. We identified five genes whose deletion significantly enhanced hypoxia tolerance. They are RAI1, NSR1, BUD21, RPL20A, and RSM22, all of which encode functions involved in ribosome biogenesis. Further analysis of the deletion mutants showed that they minimized hypoxia-induced changes in polyribosome profiles and protein synthesis. Strikingly, proteomic analysis by using the iTRAQ profiling technology showed that a substantially fewer number of proteins were changed in response to hypoxia in the deletion mutants, compared with the parent strain. Computational analysis of the iTRAQ data indicated that the activities of a group of regulators were regulated by hypoxia in the wild-type parent cells, but such regulation appeared to be diminished in the deletion strains. These results show that the deletion of one of the genes involved in ribosome biogenesis leads to the reversal of hypoxia-induced changes in gene expression and related regulators. They suggest that modifying ribosomal function is an effective mechanism to minimize hypoxia-induced specific protein changes and to confer hypoxia tolerance. These results may have broad implications in understanding hypoxia responses and tolerance in diverse eukaryotes ranging from yeast to humans.

Redox regulation in respiring Saccharomyces cerevisiae

BACKGROUND

In biological systems, redox reactions are central to most cellular processes and the redox potential of the intracellular compartment dictates whether a particular reaction can or cannot occur. Indeed the widespread use of redox reactions in biological systems makes their detailed description outside the scope of one review.

SCOPE OF THE REVIEW

Here we will focus on how system-wide redox changes can alter the reaction and transcriptional landscape of Saccharomyces cerevisiae. To understand this we explore the major determinants of cellular redox potential, how these are sensed by the cell and the dynamic responses elicited.

MAJOR CONCLUSIONS

Redox regulation is a large and complex system that has the potential to rapidly and globally alter both the reaction and transcription landscapes. Although we have a basic understanding of many of the sub-systems and a partial understanding of the transcriptional control, we are far from understanding how these systems integrate to produce coherent responses. We argue that this non-linear system self-organises, and that the output in many cases is temperature-compensated oscillations that may temporally partition incompatible reactions in vivo.

GENERAL SIGNIFICANCE

Redox biochemistry impinges on most of cellular processes and has been shown to underpin ageing and many human diseases. Integrating the complexity of redox signalling and regulation is perhaps one of the most challenging areas of biology. This article is part of a Special Issue entitled Systems Biology of Microorganisms.

Integrated phosphoproteomics analysis of a signaling network governing nutrient response and peroxisome induction

Phosphorylation of proteins is a key posttranslational modification in cellular signaling, regulating many aspects of cellular responses. We used a quantitative, integrated, phosphoproteomics approach to characterize the cellular responses of the yeast Saccharomyces cerevisiae to the fatty acid oleic acid, a molecule with broad human health implications and a potent inducer of peroxisomes. A combination of cryolysis and urea solubilization was used to minimize the opportunity for reorientation of the phosphoproteome, and hydrophilic interaction liquid chromatography and IMAC chemistries were used to fractionate and enrich for phosphopeptides. Using these approaches, numerous phosphorylated peptides specific to oleate-induced and glucose-repressed conditions were identified and mapped to known signaling pathways. These include several transcription factors, two of which, Pip2p and Cst6p, must be phosphorylated for the normal transcriptional response of fatty acid-responsive loci encoding peroxisomal proteins. The phosphoproteome data were integrated with results from genome-wide assays studying the effects of signaling molecule deletions and known protein-protein interactions to generate a putative fatty acid-responsive signaling network. In this network, the most highly connected nodes are those with the largest effects on cellular responses to oleic acid. These properties are consistent with a scale-free topology, demonstrating that scale-free properties are conserved in condition-specific networks.

Multilayered control of gene expression by stress-activated protein kinases

Stress-activated protein kinases (SAPKs) are key elements for intracellular signalling networks that serve to respond and adapt to extracellular changes. Exposure of yeast to high osmolarity results in the activation of p38-related SAPK, Hog1, which is essential for reprogramming the gene expression capacity of the cell by regulation of several steps of the transcription process. At initiation, active Hog1 not only directly phosphorylates several transcription factors to alter their activities, but also associates at stress-responsive promoters through such transcription factors. Once at the promoters, Hog1 serves as a platform to recruit general transcription factors, chromatin-modifying activities and RNA Pol II. In addition, the SAPK pathway has a role in elongation. At the stress-responsive ORFs, Hog1 recruits the RSC chromatin-remodelling complex to modify nucleosome organization. Several SAPKs from yeast to mammals have maintained some of the regulatory abilities of Hog1. Thus, elucidating the control of gene expression by the Hog1 SAPK should help to understand how eukaryotic cells implement a massive and rapid change on their transcriptional capacity in response to adverse conditions.

CaZF, a plant transcription factor functions through and parallel to HOG and calcineurin pathways in Saccharomyces cerevisiae to provide osmotolerance

Salt-sensitive yeast mutants were deployed to characterize a gene encoding a C2H2 zinc finger protein (CaZF) that is differentially expressed in a drought-tolerant variety of chickpea (Cicer arietinum) and provides salinity-tolerance in transgenic tobacco. In Saccharomyces cerevisiae most of the cellular responses to hyper-osmotic stress is regulated by two interconnected pathways involving high osmolarity glycerol mitogen-activated protein kinase (Hog1p) and Calcineurin (CAN), a Ca(2+)/calmodulin-regulated protein phosphatase 2B. In this study, we report that heterologous expression of CaZF provides osmotolerance in S. cerevisiae through Hog1p and Calcineurin dependent as well as independent pathways. CaZF partially suppresses salt-hypersensitive phenotypes of hog1, can and hog1can mutants and in conjunction, stimulates HOG and CAN pathway genes with subsequent accumulation of glycerol in absence of Hog1p and CAN. CaZF directly binds to stress response element (STRE) to activate STRE-containing promoter in yeast. Transactivation and salt tolerance assays of CaZF deletion mutants showed that other than the transactivation domain a C-terminal domain composed of acidic and basic amino acids is also required for its function. Altogether, results from this study suggests that CaZF is a potential plant salt-tolerance determinant and also provide evidence that in budding yeast expression of HOG and CAN pathway genes can be stimulated in absence of their regulatory enzymes to provide osmotolerance.

Selective inhibition of yeast regulons by daunorubicin: a transcriptome-wide analysis

Background

The antitumor drug daunorubicin exerts some of its cytotoxic effects by binding to DNA and inhibiting the transcription of different genes. We analysed this effect in vivo at the transcriptome level using the budding yeast Saccharomyces cerevisiae as a model and sublethal (IC₄₀) concentrations of the drug to minimise general toxic effects.

Results

Daunorubicin affected a minor proportion (14%) of the yeast transcriptome, increasing the expression of 195 genes and reducing expression of 280 genes. Daunorubicin down-regulated genes included essentially all genes involved in the glycolytic pathway, the tricarboxylic acid cycle and alcohol metabolism, whereas transcription of ribosomal protein genes was not affected or even slightly increased. This pattern is consistent with a specific inhibition of glucose usage in treated cells, with only minor effects on proliferation or other basic cell functions. Analysis of promoters of down-regulated genes showed that they belong to a limited number of transcriptional regulatory units (regulons). Consistently, data mining showed that daunorubicin-induced changes in expression patterns were similar to those observed in yeast strains deleted for some transcription factors functionally related to the glycolysis and/or the cAMP regulatory pathway, which appeared to be particularly sensitive to daunorubicin.

Conclusion

The effects of daunorubicin treatment on the yeast transcriptome are consistent with a model in which this drug impairs binding of different transcription factors by competing for their DNA binding sequences, therefore limiting their effectiveness and affecting the corresponding regulatory networks. This proposed mechanism might have broad therapeutic implications against cancer cells growing under hypoxic conditions.

Aspergillus oryzae atfB encodes a transcription factor required for stress tolerance in conidia

Using an Aspergillus oryzae EST database, we identified a gene encoding a transcription factor (atfB), which is a member of the ATF/CREB family. Expression of atfB was barely detectable during vegetative growth, but was readily detected during conidiation in solid-state culture. Microarray analyses showed that expression of many other genes, including catalase (catA), were downregulated in an atfB-disruptant. The expression of most of these genes was upregulated in the wild-type strain during the conidiation phase in solid-state culture, and the expression pattern was similar to that of atfB itself. In the absence of stress, e.g. heat-shock or hydrogen peroxide, the conidial germination ratios for the DeltaatfB strain and the wild-type strain were similar, but the stress tolerance of conidia carrying the DeltaatfB deletion was less than that of the wild-type conidia. CRE-like DNA motifs, which are bound by ATF/CREB proteins, were found in the promoters of most of the downregulated genes in the DeltaatfB strain. Thus, atfB appears to encode a transcription factor required for stress tolerance in conidia.

Identification and purification of CREB like protein in Candida albicans

cAMP response element binding protein (CREB) belongs to ATF/CREB family of transcription factors, which are bonafide targets of cAMP-PKA signalling pathway in mammalian cells. CREB is known to regulate the genes involved in transcription, cell cycle, cell survival, neurotransmitter, growth factors and immune regulation. But there is no evidence of presence of ATF/CREB family members in Candida albicans. In the present study, CREB like transcription factor has been identified and purified in C. albicans. The putative CREB was observed to have different molecular mass (47 kDa) as compared to its mammalian counterpart (43 kDa). Both forms of CREB (CREB and phosphorylated CREB) were detected in C. albicans and phosphorylation of CREB was found to be a function of cAMP levels and protein kinase A activity within this organism. CREB protein was purified by sequence-specific CRE-DNA affinity chromatography. Purified CREB exhibited characteristic CRE binding activity as revealed by electrophoretic mobility shift assay and gave reactivity with CREB antibodies. CREB protein was phosphorylated by purified catalytic subunit of PKA under in vitro conditions. To the best of our knowledge, this study reports for the first time identification of CREB like protein as an important component of cAMP signalling pathway in C. albicans.

Connecting protein structure with predictions of regulatory sites

A common task posed by microarray experiments is to infer the binding site preferences for a known transcription factor from a collection of genes that it regulates and to ascertain whether the factor acts alone or in a complex. The converse problem can also be posed: Given a collection of binding sites, can the regulatory factor or complex of factors be inferred? Both tasks are substantially facilitated by using relatively simple homology models for protein-DNA interactions, as well as the rapidly expanding protein structure database. For budding yeast, we are able to construct reliable structural models for 67 transcription factors and with them redetermine factor binding sites by using a Bayesian Gibbs sampling algorithm and an extensive protein localization data set. For 49 factors in common with a prior analysis of this data set (based largely on phylogenetic conservation), we find that half of the previously predicted binding motifs are in need of some revision. We also solve the inverse problem of ascertaining the factors from the binding sites by assigning a correct protein fold to 25 of the 49 cases from a previous study. Our approach is easily extended to other organisms, including higher eukaryotes. Our study highlights the utility of enlarging current structural genomics projects that exhaustively sample fold structure space to include all factors with significantly different DNA-binding specificities.

Plc1p is required for SAGA recruitment and derepression of Sko1p-regulated genes

In Saccharomyces cerevisiae, many osmotically inducible genes are regulated by the Sko1p-Ssn6p-Tup1p complex. On osmotic shock, the MAP kinase Hog1p associates with this complex, phosphorylates Sko1p, and converts it into an activator that subsequently recruits Swi/Snf and SAGA complexes. We have found that phospholipase C (Plc1p encoded by PLC1) is required for derepression of Sko1p-Ssn6p-Tup1p-controlled osmoinducible genes upon osmotic shock. Although plc1Delta mutation affects the assembly of the preinitiation complex after osmotic shock, it does not affect the recruitment of Hog1p and Swi/Snf complex at these promoters. However, Plc1p facilitates osmotic shock-induced recruitment of the SAGA complex. Like plc1Delta cells, SAGA mutants are osmosensitive and display compromised expression of osmotically inducible genes. The reduced binding of SAGA to Sko1p-Ssn6p-Tup1p-repressed promoters in plc1Delta cells does not correlate with reduced histone acetylation. However, SAGA functions at these promoters to facilitate recruitment of the TATA-binding protein. The results thus provide evidence that Plc1p and inositol polyphosphates affect derepression of Sko1p-Ssn6p-Tup1p-controlled genes by a mechanism that involves recruitment of the SAGA complex and TATA-binding protein.

The nature of the nitrogen source added to nitrogen depleted vinifications conducted by a Saccharomyces cerevisiae strain in synthetic must affects gene expression and the levels of several volatile compounds

Nitrogen starvation may lead to stuck and sluggish fermentations. These undesirable situations result in wines with high residual sugar, longer vinification times, and risks of microbial contamination. The typical oenological method to prevent these problems is the early addition of ammonium salts to the grape juice, although excessive levels of these compounds may lead to negative consequences for the final product. This addition reduces the overall fermentation time, regardless of the time of addition, but the effect is more significant when nitrogen is added during the yeast exponential phase. In this work we analysed the effect of adding different nitrogen sources (ammonia, amino acids or a combination of both) under nitrogen depletion in order to understand yeast metabolic changes that lead to the adaptation to the new conditions. These studies were carried out in a synthetic must that mimics the composition of the natural must. Furthermore, we studied how this addition affects fermentative behaviour, the levels of several yeast volatile compounds in the final product, arginase activity, and the expression of several genes involved in stress response and nitrogen metabolism during vinification. We found that the nature of the nitrogen source added during yeast late exponential growth phase introduces changes to the volatile compounds profile and to the gene expression. On the other hand, arginase activity and the expression of the stress response gene ACA1 are useful to monitor nitrogen depletion/addition during growth of the wine yeast considered under our vinification conditions.

Cross-species annotation of basic leucine zipper factor interactions: Insight into the evolution of closed interaction networks

Dimeric basic leucine zipper (bZIP) factors constitute one of the most important classes of enhancer-type transcription factors. In vertebrates, bZIP factors are involved in many cellular processes, including cell survival, learning and memory, cancer progression, lipid metabolism, and a variety of developmental processes. These factors have the ability to homodimerize and heterodimerize in a specific and predictable manner, resulting in hundreds of dimers with unique effects on transcription. In recent years, several studies have described dimerization preferences for bZIP factors from different species, including Homo sapiens, Drosophila melanogaster, Arabidopsis thaliana, and Saccharomyces cerevisiae. Here, these findings are summarized as novel, graphical representations of closed, interacting protein networks. These representations combine phylogenetic information, DNA-binding properties, and dimerization preference. Beyond summarizing bZIP dimerization preferences within selected species, we have included annotation for a solitary bZIP factor found in the primitive eukaryote, Giardia lamblia, a possible evolutionary precursor to the complex networks of bZIP factors encoded by other genomes. Finally, we discuss the fundamental similarities and differences between dimerization networks within the context of bZIP factor evolution.

The heme-Bach1 pathway in the regulation of oxidative stress response and erythroid differentiation

Heme--as a prosthetic group of proteins required for oxygen transport and storage, respiration, and biosynthetic pathways--is essential for practically all forms of life. Additionally, the degradation products of heme (i.e., carbon monoxide, biliverdin, and bilirubin) produced by the enzymatic actions of heme oxygenase (HO) and biliverdin reductase, possess various biological activities in vivo. In mammalian cells, heme also functions as an intracellular regulator of gene expression by virtue of its ability to bind to Bach1, a transcription factor that functions in association with small Maf proteins. Normally, such complexes function as repressors by binding to specific target sequences, the Maf recognition element (MARE), within enhancers of genes encoding proteins such as HO-1 and beta-globin. By binding to Bach1, heme induces selective removal of the repressor from the gene enhancers permitting subsequent occupancy of the MAREs by activators that, interestingly, also contain small Maf proteins. Thus small Maf proteins play dual functions in gene expression: complexes with Bach1 repress MARE-dependent gene expression, whereas heterodimers with NF-E2 p45 or related factors (Nrf1, Nrf2, and Nrf3) activate MARE-driven genes. By modulating the equilibrium of the small Maf heterodimer network, heme regulates expression of the cytoprotective enzyme HO-1 during the stress response and of beta-globin during erythroid differentiation. Implications of such heme-regulated gene expression in human diseases including atherosclerosis are discussed.

DhARO4, an amino acid biosynthetic gene, is stimulated by high salinity inDebaryomyces hansenii

The highly halotolerant yeast Debaryomyces hansenii when grown in the presence of 2M NaCl, increased the expression of ARO4 which is involved in the biosynthesis of aromatic amino acids. The function of the isolated gene was verified by complementation of a Saccharomyces cerevisiae null mutant, aro4Delta, restoring the specific activity of the enzyme (a 3-deoxy-D-arabino-heptulosonate-7-phosphate synthase) to wild-type levels. DhARO4 transcript expression under high salinity was stimulated at the beginning of the exponential growth phase. As the DhARO4 promoter region presents putative GCRE and CRE sequences, its expression was evaluated under conditions of NaCl stress and amino acid starvation, showing similar expression levels for either condition. The combined effect of both stressors resulted in a further increase in transcript levels over the singly added stressors, indicating independent stimulatory events. Our results support the hypothesis that high salinity and amino acid availability are physiologically interconnected.

Sequence elements necessary for transcriptional activation of BAD1 in the yeast phase of Blastomyces dermatitidis

Blastomyces dermatitidis is a dimorphic fungal pathogen that converts from mycelia or conidia to a host-adapted yeast morphotype upon infection. Conversion to the yeast form is accompanied by the production of the virulence factor BAD1. Yeast-phase-specific expression of BAD1 is transcriptionally regulated, and its promoter shares homology with that of the yeast-phase-specific gene YPS3 of Histoplasma capsulatum. Serial truncations of the BAD1 upstream region were fused to the lacZ reporter to define functional areas in the promoter. Examination of PBAD1-lacZ fusions in B. dermatitidis indicated that BAD1 transcription is upregulated in the yeast phase. The 63-nucleotide box A region conserved in the YPS3 upstream region was shown to be an essential component of the minimal BAD1 promoter. A matched PYPS3-lacZ construct indicated that this same region was needed for minimal YPS3 promoter activity in B. dermatitidis transformants. Reporter activity in H. capsulatum transformants similarly showed a requirement for box A in the minimal BAD1 promoter. Several putative transcription factor binding sites were identified within box A of BAD1. Replacement of two of these predicted sites within box A--a cAMP responsive element and a Myb binding site--sharply reduced transcriptional activity, indicating that these regions are critical in dictating the yeast-phase-specific expression of this crucial virulence determinant of B. dermatitidis.

Genomewide identification of Sko1 target promoters reveals a regulatory network that operates in response to osmotic stress in Saccharomyces cerevisiae

In Saccharomyces cerevisiae, the ATF/CREB transcription factor Sko1 (Acr1) regulates the expression of genes induced by osmotic stress under the control of the high osmolarity glycerol (HOG) mitogen-activated protein kinase pathway. By combining chromatin immunoprecipitation and microarrays containing essentially all intergenic regions, we estimate that yeast cells contain approximately 40 Sko1 target promoters in vivo; 20 Sko1 target promoters were validated by direct analysis of individual loci. The ATF/CREB consensus sequence is not statistically overrepresented in confirmed Sko1 target promoters, although some sites are evolutionarily conserved among related yeast species, suggesting that they are functionally important in vivo. These observations suggest that Sko1 association in vivo is affected by factors beyond the protein-DNA interaction defined in vitro. Sko1 binds a number of promoters for genes directly involved in defense functions that relieve osmotic stress. In addition, Sko1 binds to the promoters of genes encoding transcription factors, including Msn2, Mot3, Rox1, Mga1, and Gat2. Stress-induced expression of MSN2, MOT3, and MGA1 is diminished in sko1 mutant cells, while transcriptional regulation of ROX1 seems to be unaffected. Lastly, Sko1 targets PTP3, which encodes a phosphatase that negatively regulates Hog1 kinase activity, and Sko1 is required for osmotic induction of PTP3 expression. Taken together our results suggest that Sko1 operates a transcriptional network upon osmotic stress, which involves other specific transcription factors and a phosphatase that regulates the key component of the signal transduction pathway.

MAP kinases as structural adaptors and enzymatic activators in transcription complexes

Mitogen-activated protein kinase (MAPK) pathways regulate eukaryotic gene expression in response to extracellular stimuli. MAPKs and their downstream kinases phosphorylate transcription factors, co-regulators and chromatin proteins to initiate transcriptional changes. However, the spatial context in which the MAPKs operate in transcription complexes is poorly understood. Recent findings in budding yeast show that MAPKs can form integral components of transcription complexes and have novel structural functions in addition to phosphorylating local substrates. Hog1p MAPK is stably recruited to target promoters by specific transcription factors in response to osmotic stress, and acts as both a structural adaptor and enzymatic activator driving the assembly and activation of the transcription complex. We review the evidence that suggests a similar bifunctional role for MAPKs in mammalian transcription complexes.

Expression of the HXT1 Low Affinity Glucose Transporter Requires the Coordinated Activities of the HOG and Glucose Signalling Pathways

Expression of the HXT1 gene, which encodes a low affinity glucose transporter in Saccharomyces cerevisiae, is regulated positively in response to glucose by the general glucose induction pathway, involving the Snf3/Rgt2 membrane glucose sensors, the SCF-Grr1 ubiquitination complex and the Rgt1 transcription factor. In this study we show that, in addition to the glucose signaling pathway, regulation of HXT1 expression also requires the HOG pathway. Deletion of components in the glucose signaling pathway or in the HOG pathway results in impaired HXT1 expression. Genetic analyses showed that, whereas the glucose signaling pathway regulates HXT1 through modulation of the Rgt1 transcription factor, the HOG pathway modulates HXT1 through regulation of the Sko1-Tup1-Ssn6 complex. Coordinated regulation of the two signaling pathways is required for expression of HXT1 by glucose and in response to osmostress.

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.

Osmostress-induced transcription by Hot1 depends on a Hog1-mediated recruitment of the RNA Pol II

In budding yeast, the mitogen-activated protein kinase (MAPK) Hog1 coordinates the transcriptional program required for cell survival upon osmostress. The Hot1 transcription factor acts downstream of the MAPK and regulates a subset of Hog1-responsive genes. In response to high osmolarity, Hot1 targets Hog1 to specific osmostress-responsive promoters. Here, we show that assembly of the general transcription machinery at Hot1-dependent promoters depends on the presence of Hot1 and active Hog1 MAPK. Unexpectedly, recruitment of RNA polymerase (Pol) II complex to target promoters does not depend on the phosphorylation of the Hot1 activator by the MAPK. Hog1 interacts with the RNA Pol II and with general components of the transcription machinery. More over, when tethered to a promoter as a LexA fusion protein, Hog1 activates transcription in a stress- regulated manner. Thus, anchoring of active Hog1 to promoters by the Hot1 activator is essential for recruitment and activation of RNA Pol II. The mammalian p38 also interacts with the RNA Pol II, which might suggest a conserved mechanism for regulation of gene expression by SAPKs among eukaryotic cells.

Role of fission yeast Tup1-like repressors and Prr1 transcription factor in response to salt stress

In Schizosaccharomyces pombe, the Sty1 mitogen-activated protein kinase and the Atf1 transcription factor control transcriptional induction in response to elevated salt concentrations. Herein, we demonstrate that two repressors, Tup11 and Tup12, and the Prr1 transcription factor also function in the response to salt shock. We find that deletion of both tup genes together results in hypersensitivity to elevated cation concentrations (K(+) and Ca(2+)) and we identify cta3(+), which encodes an intracellular cation transporter, as a novel stress gene whose expression is positively controlled by the Sty1 pathway and negatively regulated by Tup repressors. The expression of cta3(+) is maintained at low levels by the Tup repressors, and relief from repression requires the Sty1, Atf1, and Prr1. Prr1 is also required for KCl-mediated induction of several other Sty1-dependent genes such as gpx1(+) and ctt1(+). Surprisingly, the KCl-mediated induction of cta3(+) expression occurs independently of Sty1 in a tup11Delta tup12Delta mutant and so the Tup repressors link induction to the Sty1 pathway. We also report that in contrast to a number of other Sty1- and Atf1-dependent genes, the expression of cta3(+) is induced only by high salt concentrations. However, in the absence of the Tup repressors this specificity is lost and a range of stresses induces cta3(+) expression.

Hog1 kinase converts the Sko1-Cyc8-Tup1 repressor complex into an activator that recruits SAGA and SWI/SNF in response to osmotic stress

The yeast ATF/CREB repressor Sko1(Acr1) regulates genes that are induced upon hyperosmotic stress by recruiting the Cyc8(Ssn6)-Tup1 corepressor complex to target promoters. During hyperosmotic stress, Hog1 MAP kinase associates with target promoters, phosphorylates Sko1, and converts Sko1 into a transcriptional activator. Unexpectedly, Tup1 remains bound to target promoters during osmotic stress. Sko1, Hog1, and Tup1 are all important for recruitment of SAGA histone acetylase and SWI/SNF nucleosome-remodeling complexes to osmotic-inducible promoters, and both complexes are important for activation upon osmotic stress. Thus, osmotic induction involves a switch of Sko1-Cyc8-Tup1 from a repressing to an activating state in a process that is triggered by Hog1 phosphorylation. Cyc8-Tup1 is not simply a corepressor but is also involved in recruiting SWI/SNF and SAGA during the transcriptional induction process.

Osmotic stress signaling and osmoadaptation in yeasts

The ability to adapt to altered availability of free water is a fundamental property of living cells. The principles underlying osmoadaptation are well conserved. The yeast Saccharomyces cerevisiae is an excellent model system with which to study the molecular biology and physiology of osmoadaptation. Upon a shift to high osmolarity, yeast cells rapidly stimulate a mitogen-activated protein (MAP) kinase cascade, the high-osmolarity glycerol (HOG) pathway, which orchestrates part of the transcriptional response. The dynamic operation of the HOG pathway has been well studied, and similar osmosensing pathways exist in other eukaryotes. Protein kinase A, which seems to mediate a response to diverse stress conditions, is also involved in the transcriptional response program. Expression changes after a shift to high osmolarity aim at adjusting metabolism and the production of cellular protectants. Accumulation of the osmolyte glycerol, which is also controlled by altering transmembrane glycerol transport, is of central importance. Upon a shift from high to low osmolarity, yeast cells stimulate a different MAP kinase cascade, the cell integrity pathway. The transcriptional program upon hypo-osmotic shock seems to aim at adjusting cell surface properties. Rapid export of glycerol is an important event in adaptation to low osmolarity. Osmoadaptation, adjustment of cell surface properties, and the control of cell morphogenesis, growth, and proliferation are highly coordinated processes. The Skn7p response regulator may be involved in coordinating these events. An integrated understanding of osmoadaptation requires not only knowledge of the function of many uncharacterized genes but also further insight into the time line of events, their interdependence, their dynamics, and their spatial organization as well as the importance of subtle effects.

ATF/CREB sites present in sub-telomeric regions of Saccharomyces cerevisiae chromosomes are part of promoters and act as UAS/URS of highly conserved COS genes

A highly conserved 48 bp DNA element was identified present at 26 chromosome ends of Saccharomyces cerevisiae. Each element harbours an ideal or a mutated ATF/CREB site, which is a well-known target sequence for bZip transcription factors. In all cases, the sub-telomeric ATF/CREB site element (SACE) is a direct extension of the respective sub-telomeric coreX element. Eight SACEs are part of very long quasi-identical regions of several kilobases, including a sub-telomeric COS open reading frame. Three of these eight SACEs harbour an ideal ATF/CREB site, four a triple-exchange variant (5'-ATGGTATCAT-3'; GTA variant), and one a single exchange variant with a C to G exchange at the left side of the center of symmetry. We analyzed the function of the SACE of the left arm of chromosome VIII in vivo and found its ATF/CREB site to act as UAS/URS of the COS8 promoter, effected by the yeast bZip proteins Sko1p, Aca1p, and Aca2p. Cos8 protein was found in proximity to the nuclear membrane, where it accumulated, especially during cell division. When the ATF/CREB site of the COS8 promoter was exchanged with the GTA variant, the regulation was changed. COS8 was then regulated by Hac1p, a bZip protein known to be involved in the unfolded protein response of S. cerevisiae, indicating, for the first time, a possible functional category for the Cos proteins of S. cerevisiae.

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.

Multiple levels of control regulate the yeast cAMP-response element-binding protein repressor Sko1p in response to stress

The Sko1p transcriptional repressor regulates a subset of osmoinducible stress defense genes in Saccharomyces cerevisiae by binding to cAMP-responsive elements. We have reported previously that in response to stress Sko1p is phosphorylated by the stress-activated Hog1p mitogen-activated protein kinase, which disrupts its interaction with the Ssn6p x Tup1p corepressor. Here we report that other mechanisms are essential for the regulation of the Sko1p repressor activity upon stress. The nuclear localization of Sko1p depends on the stress-inhibited protein kinase A (PKA). Sko1p is localized in the nucleus of unstressed cells, and it redistributes to the cytosol upon severe salt stress (1 m NaCl). Yeast mutants with low PKA activity localize Sko1p to the cytoplasm in the absence of stress and exhibit deregulated expression of cAMP-responsive element-regulated genes. The central part (315) of Sko1p, containing the PKA phosphorylation sites and the basic domain-leucine zipper domain, is essential for its nuclear localization. Salt-induced export of Sko1p from the nucleus is independent of Hog1p and of the Bcy1p regulatory subunit of PKA. Furthermore, phosphorylation by PKA slightly enhanced DNA binding affinity of Sko1p in vitro, whereas Sko1p dimerization in vivo is not regulated by stress. Sko1p repressor activity is associated to its binding to the Ssn6p x Tup1p complex. Interestingly, the Sko1p NH(2) terminus (1), containing the Hog1p phosphorylation sites, associates in vivo with Tup1p in the absence of Ssn6p, suggesting that Sko1p represses gene transcription by interacting directly with the Tup1p subunit of the Ssn6p x Tup1p complex.