A zinc-finger protein, Rst2p, regulates transcription of the fission yeast ste11(+) gene, which encodes a pivotal transcription factor for sexual development

Live-cell fluorescence imaging and optogenetic control of PKA kinase activity in fission yeast Schizosaccharomyces pombe

The cAMP-PKA signaling pathway plays a crucial role in sensing and responding to nutrient availability in the fission yeast Schizosaccharomyces pombe. This pathway monitors external glucose levels to control cell growth and sexual differentiation. However, the temporal dynamics of the cAMP-PKA pathway in response to external stimuli remains unclear mainly due to the lack of tools to quantitatively visualize the activity of the pathway. Here, we report the development of the kinase translocation reporter (KTR)-based biosensor spPKA-KTR1.0, which allows us to measure the dynamics of PKA activity in fission yeast cells. The spPKA-KTR1.0 is derived from the transcription factor Rst2, which translocates from the nucleus to the cytoplasm upon PKA activation. We found that spPKA-KTR1.0 translocates between the nucleus and cytoplasm in a cAMP-PKA pathway-dependent manner, indicating that the spPKA-KTR1.0 is a reliable indicator of the PKA activity in fission yeast cells. In addition, we implemented a system that simultaneously visualizes and manipulates the cAMP-PKA signaling dynamics by introducing bPAC, a photoactivatable adenylate cyclase, in combination with spPKA-KTR1.0. This system offers an opportunity for investigating the role of the signaling dynamics of the cAMP-PKA pathway in fission yeast cells with higher temporal resolution.

Regulation of sexual differentiation initiation in Schizosaccharomyces pombe

The fission yeast Schizosaccharomyces pombe is an excellent model organism to explore cellular events owing to rich tools in genetics, molecular biology, cellular biology, and biochemistry. Schizosaccharomyces pombe proliferates continuously when nutrients are abundant but arrests in G1 phase upon depletion of nutrients such as nitrogen and glucose. When cells of opposite mating types are present, cells conjugate, fuse, undergo meiosis, and finally form 4 spores. This sexual differentiation process in S. pombe has been studied extensively. To execute sexual differentiation, the glucose-sensing cAMP-PKA (cyclic adenosine monophosphate-protein kinase A) pathway, nitrogen-sensing TOR (target of rapamycin) pathway, and SAPK (stress-activating protein kinase) pathway are crucial, and the MAPK (mitogen-activating protein kinase) cascade is essential for pheromone sensing. These signals regulate ste11 at the transcriptional and translational levels, and Ste11 is modified in multiple ways. This review summarizes the initiation of sexual differentiation in S. pombe based on results I have helped to obtain, including the work of many excellent researchers.

Effects of ScRgt1-Like DNA-binding transcription factor SpRgt1 (SPCC320.03) on Hexose transporters gene expression in Schizosaccharomyces pombe

Glucose, which plays an essential role in carbon and energy metabolism in eukaryotes, is vital in directing various energy-consuming cellular processes. In S. cerevisiae, transcription factors involved in regulating hexose transporters and their mechanisms of action under different carbon sources were revealed in detail. However, there is limited information on these processes in S. pombe. In this study, the effect of SPCC320.03 (named SpRgt1), the ortholog of ScRgt1 whose molecular mechanism is known in detail in S. cerevisiae, on the transcriptional regulation of hexose transporters (ght1-8) dependent on different carbon sources was investigated. We measured the transcript levels of ght1-8 using the qPCR technique and performed relative evaluation in S. pombe strains (parental, rgt1 deleted mutant, rgt1 overexpressed, and vectoral rgt1 carrying mutant). We aimed to investigate the transcriptional changes caused by the protein product of the rgt1 (SPCC320.03) gene in terms of ght1-8 genes in strains that are grown in different carbon sources (2% glucose, 2% glycerol + 0.1% glucose, and 2% gluconate). Here, we show that SpRgt1 is involved in the regulation of the ght3, ght4, ght6, and ght7 genes but that the ght1, ght2, ght5, and ght8 gene expression vary depending on carbon sources, independently of SpRgt1.

A role for the carbon source of the cell and protein kinase A in regulating the S. pombe septation initiation network

The septation initiation network (SIN) is a conserved signal transduction network, which is important for cytokinesis in Schizosaccharomyces pombe. The SIN component Etd1p is required for association of some SIN proteins with the spindle pole body (SPB) during anaphase and for contractile ring formation. We show that tethering of Cdc7p or Sid1p to the SIN scaffold Cdc11p at the SPB, rescues etd1-Δ. Analysis of a suppressor of the mutant etd1-M9 revealed that SIN signalling is influenced by the carbon source of the cell. Growth on a non-fermentable carbon source glycerol reduces the requirement for SIN signalling but does not bypass it. The decreased need for SIN signalling is mediated largely by reduction of protein kinase A activity, and it is phenocopied by deletion of pka1 on glucose medium. We conclude that protein kinase A is an important regulator of the SIN, and that SIN signalling is regulated by the carbon source of the cell.

A dominant negative 14-3-3 mutant in Schizosaccharomyces pombe distinguishes the binding proteins involved in sexual differentiation and check point

The homothallic fission yeast Schizosaccharomyces pombe undergoes sexual differentiation when starved, but sam (skips the requirement of starvation for mating) mutants such as those carrying mutations in adenylate cyclase (cyr1) or protein kinase A (pka1) mate without starvation. Here, we identified sam3, a dominant negative allele of rad24, encoding one of two 14-3-3 proteins. Genetic mapping and whole-genome sequencing showed that the sam3 mutation comprises a change in nucleotide at position 959 from guanine to adenine, which switches the amino acid at position 185 from glutamic acid to lysine (E185K). We generated the rad24-E185K integrated mutant and its phenotype was similar to that of the sam3 mutant, including calcium sensitivity and UV non-sensitivity, but the phenotype is different from that of the Δrad24 strain. While the UV-sensitive phenotype was observed in the Δrad24 mutant, it was not observed in the sam3 and rad24-E185K mutants. The expression of the rad24-E185K gene in wild type cells induced spore formation in the nutrient rich medium, confirming rad24-E185K is dominant. This dominant effect of rad24-E185K was also observed in Δras1 and Δbyr2 diploid mutants, indicating that rad24-E185K generate stronger phenotype than rad24 null mutants. Ste11, the key transcription factor for sexual differentiation was expressed in sam3 mutants without starvation and it predominantly localized to the nucleus. The Rad24-E185K mutant protein retained its interaction with Check point kinase1 (Chk1), whereas it reduced interaction with Ste11, an RNA binding protein Mei2, and a MAPKKK Byr2, freeing these proteins from negative regulation by Rad24, that account for the sam phenotype and UV non-sensitive phenotype. Glucose depletion in rad24-E185K or Δpka1 Δrad24 double mutation induced haploid meiosis, leading to the formation of spores in haploid. The position of glutamic acid 185 is conserved in all major 14-3-3s; hence, our finding of a dominant negative allele of 14-3-3 is useful for understanding 14-3-3s in other organisms.

cAMP-Protein kinase A and stress-activated MAP kinase signaling mediate transcriptional control of autophagy in fission yeast during glucose limitation or starvation

Macroautophagy/autophagy is an essential adaptive physiological response in eukaryotes induced during nutrient starvation, including glucose, the primary immediate carbon and energy source for most cells. Although the molecular mechanisms that induce autophagy during glucose starvation have been extensively explored in the budding yeast Saccharomyces cerevisiae, little is known about how this coping response is regulated in the evolutionary distant fission yeast Schizosaccharomyces pombe. Here, we show that S. pombe autophagy in response to glucose limitation relies on mitochondrial respiration and the electron transport chain (ETC), but, in contrast to S. cerevisiae, the AMP-activated protein kinase (AMPK) and DNA damage response pathway components do not modulate fission yeast autophagic flux under these conditions. In the presence of glucose, the cAMP-protein kinase A (PKA) signaling pathway constitutively represses S. pombe autophagy by downregulating the transcription factor Rst2, which promotes the expression of respiratory genes required for autophagy induction under limited glucose availability. Furthermore, the stress-activated protein kinase (SAPK) signaling pathway, and its central mitogen-activated protein kinase (MAPK) Sty1, positively modulate autophagy upon glucose limitation at the transcriptional level through its downstream effector Atf1 and by direct in vivo phosphorylation of Rst2 at S292. Thus, our data indicate that the signaling pathways that govern autophagy during glucose shortage or starvation have evolved differently in S. pombe and uncover the existence of sophisticated and multifaceted mechanisms that control this self-preservation and survival response.

Myosin II regulatory light chain phosphorylation and formin availability modulate cytokinesis upon changes in carbohydrate metabolism

Cytokinesis, the separation of daughter cells at the end of mitosis, relies in animal cells on a contractile actomyosin ring (CAR) composed of actin and class II myosins, whose activity is strongly influenced by regulatory light chain (RLC) phosphorylation. However, in simple eukaryotes such as the fission yeast Schizosaccharomyces pombe, RLC phosphorylation appears dispensable for regulating CAR dynamics. We found that redundant phosphorylation at Ser35 of the S. pombe RLC homolog Rlc1 by the p21-activated kinases Pak1 and Pak2, modulates myosin II Myo2 activity and becomes essential for cytokinesis and cell growth during respiration. Previously, we showed that the stress-activated protein kinase pathway (SAPK) MAPK Sty1 controls fission yeast CAR integrity by downregulating formin For3 levels (Gómez-Gil et al., 2020). Here, we report that the reduced availability of formin For3-nucleated actin filaments for the CAR is the main reason for the required control of myosin II contractile activity by RLC phosphorylation during respiration-induced oxidative stress. Thus, the restoration of For3 levels by antioxidants overrides the control of myosin II function regulated by RLC phosphorylation, allowing cytokinesis and cell proliferation during respiration. Therefore, fine-tuned interplay between myosin II function through Rlc1 phosphorylation and environmentally controlled actin filament availability is critical for a successful cytokinesis in response to a switch to a respiratory carbohydrate metabolism.

Multi-Layered Regulations on the Chromatin Architectures: Establishing the Tight and Specific Responses of Fission Yeast fbp1 Gene Transcription

Transcriptional regulation is pivotal for all living organisms and is required for adequate response to environmental fluctuations and intercellular signaling molecules. For precise regulation of transcription, cells have evolved regulatory systems on the genome architecture, including the chromosome higher-order structure (e.g., chromatin loops), location of transcription factor (TF)-binding sequences, non-coding RNA (ncRNA) transcription, chromatin configuration (e.g., nucleosome positioning and histone modifications), and the topological state of the DNA double helix. To understand how these genome-chromatin architectures and their regulators establish tight and specific responses at the transcription stage, the fission yeast fbp1 gene has been analyzed as a model system for decades. The fission yeast fbp1 gene is tightly repressed in the presence of glucose, and this gene is induced by over three orders of magnitude upon glucose starvation with a cascade of multi-layered regulations on various levels of genome and chromatin architecture. In this review article, we summarize the multi-layered transcriptional regulatory systems revealed by the analysis of the fission yeast fbp1 gene as a model system.

Zfp1, a Cys2His2 zinc finger protein is required for meiosis initiation in Tetrahymena thermophila

Meiosis is an important and highly conserved process that occurs during eukaryotic sexual reproduction. Diverse mechanisms are responsible for meiosis initiation among eukaryotes, and transcription factors have been established to have an important role in many species. However, the specific function of transcription factors in initiating meiosis in ciliates is unknown. Here we show that a putative Cys2His2 zinc finger-containing transcription factor encoded by the ZFP1 gene is specifically expressed during sexual reproduction in Tetrahymena thermophila. Meiosis is not initiated in the cells lacking ZFP1. Transcriptome sequencing analyses reveal that Zfp1 is required for the expression of many meiosis-specific genes. Our results indicate that Zfp1 could be a transcriptional activator required for meiosis initiation in T. thermophila.

Sporulation: A response to starvation in the fission yeast Schizosaccharomyces pombe

The fission yeast Schizosaccharomyces pombe employs two main strategies to adapt to the environment and survive when starved for nutrients. The strategies employ sporulation via sexual differentiation and extension of the chronological lifespan. When a cell is exposed to nutrient starvation in the presence of a cell of the opposite sex, the cells undergo fusion through conjugation and sporulation through meiosis. S. pombe spores are highly resistant to diverse stresses and may survive for a very long time. In this minireview, among the various sexual differentiation processes induced by starvation, we focused on and summarized the findings of the molecular mechanisms of spore formation in fission yeast. Furthermore, comparative measurements of the chronological lifespan of stationary phase cells and G0 cells and the survival period of spore cells revealed that the spore cells survived for a long period, indicating the presence of an effective mechanism for survival. Currently, many molecules involved in sporulation and their functions are being discovered; however, our understanding of these is not complete. Further understanding of spores may not only deepen our comprehension of sexual differentiation but may also provide hints for sustaining life.

Meiosis initiation: a story of two sexes in all creatures great and small

Meiosis facilitates diversity across individuals and serves as a major driver of evolution. However, understanding how meiosis begins is complicated by fundamental differences that exist between sexes and species. Fundamental meiotic research is further hampered by a current lack of human meiotic cells lines. Consequently, much of what we know relies on data from model organisms. However, contextualising findings from yeast, worms, flies and mice can be challenging, due to marked differences in both nomenclature and the relative timing of meiosis. In this review, we set out to combine current knowledge of signalling and transcriptional pathways that control meiosis initiation across the sexes in a variety of organisms. Furthermore, we highlight the emerging links between meiosis initiation and oncogenesis, which might explain the frequent re-expression of normally silent meiotic genes in a variety of human cancers.

Reciprocal stabilization of transcription factor binding integrates two signaling pathways to regulate fission yeast fbp1 transcription

Transcriptional regulation, a pivotal biological process by which cells adapt to environmental fluctuations, is achieved by the binding of transcription factors to target sequences in a sequence-specific manner. However, how transcription factors recognize the correct target from amongst the numerous candidates in a genome has not been fully elucidated. We here show that, in the fission-yeast fbp1 gene, when transcription factors bind to target sequences in close proximity, their binding is reciprocally stabilized, thereby integrating distinct signal transduction pathways. The fbp1 gene is massively induced upon glucose starvation by the activation of two transcription factors, Atf1 and Rst2, mediated via distinct signal transduction pathways. Atf1 and Rst2 bind to the upstream-activating sequence 1 region, carrying two binding sites located 45 bp apart. Their binding is reciprocally stabilized due to the close proximity of the two target sites, which destabilizes the independent binding of Atf1 or Rst2. Tup11/12 (Tup-family co-repressors) suppress independent binding. These data demonstrate a previously unappreciated mechanism by which two transcription-factor binding sites, in close proximity, integrate two independent-signal pathways, thereby behaving as a hub for signal integration.

Long Non-Coding RNAs in the Control of Gametogenesis: Lessons from Fission Yeast

Long non-coding RNAs (lncRNAs) contribute to cell fate decisions by modulating genome expression and stability. In the fission yeast Schizosaccharomyces pombe, the transition from mitosis to meiosis results in a marked remodeling of gene expression profiles, which ultimately ensures gamete production and inheritance of genetic information to the offspring. This key developmental process involves a set of dedicated lncRNAs that shape cell cycle-dependent transcriptomes through a variety of mechanisms, including epigenetic modifications and the modulation of transcription, post-transcriptional and post-translational regulations, and that contribute to meiosis-specific chromosomal events. In this review, we summarize the biology of these lncRNAs, from their identification to mechanism of action, and discuss their regulatory role in the control of gametogenesis.

Expression of Mug14 is regulated by the transcription factor Rst2 through the cAMP-dependent protein kinase pathway in Schizosaccharomyces pombe

The cAMP-dependent protein kinase (Pka1) regulates many cellular events, including sexual development and glycogenesis, and response to the limitation of glucose, in Schizosaccharomyces pombe. Despite its importance in many cellular events, the targets of the cAMP/PKA pathway have not been fully investigated. Here, we demonstrate that the expression of mug14 is induced by downregulation of the cAMP/PKA pathway and limitation of glucose. This regulation is dependent on the function of Rst2, a transcription factor that regulates transition from mitosis to meiosis. The loss of the C2H2-type zinc finger domain in Rst2, termed Rst2 (C2H2∆), abolished the induction of Mug14 expression. Upon deletion of the stress starvation response element of the S. pombe (STREP: CCCCTC) sequence, which is a potential binding site of Rst2 on mug14, in the pka1∆ strain, its induction was abolished. The expression of Mug14 was significantly reduced and delayed by the limitation of glucose and also by nitrogen starvation in the rst2∆ strain. Mug14 is known to share a common function with Mde1 and Mta3 in the methionine salvage pathway, but the expression of mde1 and mta3 mRNAs was not enhanced by pka1 deletion and limitation of glucose. We conclude that the expression of Mug14 is upregulated by Rst2 under the control of the cAMP/PKA signaling pathway, which senses the limitation of glucose.

Role of mitochondrial complex III/IV in the activation of transcription factor Rst2 in Schizosaccharomyces pombe

Mitochondria play essential roles in eukaryotic cells for glucose metabolism to produce ATP. In Schizosaccharomyces pombe, transcription factor Rst2 can be activated upon glucose deprivation. However, the link between Rst2 and mitochondrial function remains elusive. Here, we monitored Rst2 transcriptional activity in living cells using a Renilla luciferase reporter system, and found that inhibition of mitochondrial complex III/IV caused cells to produce reactive oxygen species (ROS) and nitric oxide (NO), which in turn activated Rst2. Furthermore, Rst2-GFP was observed to translocate from cytoplasm to nucleus upon mitochondrial complex III/IV inhibitors treatment, and deletion of genes associated with complex III/IV resulted in delayed process of Rst2-GFP nuclear exportation under glucose-rich condition. In particular, we found that Rst2 was phosphorylated following the treatment of complex III/IV inhibitors or SNAP. Altogether, our findings suggest that mitochondrial complex III/IV participates in the activation of Rst2 through ROS and NO generation in Schizosaccharomyces pombe.

Cell cycle-dependent and independent mating blocks ensure fungal zygote survival and ploidy maintenance

To ensure genome stability, sexually reproducing organisms require that mating brings together exactly 2 haploid gametes and that meiosis occurs only in diploid zygotes. In the fission yeast Schizosaccharomyces pombe, fertilization triggers the Mei3-Pat1-Mei2 signaling cascade, which represses subsequent mating and initiates meiosis. Here, we establish a degron system to specifically degrade proteins postfusion and demonstrate that mating blocks not only safeguard zygote ploidy but also prevent lysis caused by aberrant fusion attempts. Using long-term imaging and flow-cytometry approaches, we identify previously unrecognized and independent roles for Mei3 and Mei2 in zygotes. We show that Mei3 promotes premeiotic S-phase independently of Mei2 and that cell cycle progression is both necessary and sufficient to reduce zygotic mating behaviors. Mei2 not only imposes the meiotic program and promotes the meiotic cycle, but also blocks mating behaviors independently of Mei3 and cell cycle progression. Thus, we find that fungi preserve zygote ploidy and survival by at least 2 mechanisms where the zygotic fate imposed by Mei2 and the cell cycle reentry triggered by Mei3 synergize to prevent zygotic mating.

During sexual reproduction, fertilization must happen between exactly two gametes to ensure genome stability. This study shows that two mechanisms – establishment of zygotic fate and re-entry to the cell cycle – combine to prevent fission yeast zygotes fusing with further gametes.

The S. pombe CDK5 ortholog Pef1 regulates sexual differentiation through control of the TORC1 pathway and autophagy

In Schizosaccharomyces pombe, a general strategy for survival in response to environmental changes is sexual differentiation, which is triggered by TORC1 inactivation. However, mechanisms of TORC1 regulation in fission yeast remain poorly understood. In this study, we found that Pef1, which is an ortholog of mammalian CDK5, regulates the initiation of sexual differentiation through positive regulation of TORC1 activity. Conversely, deletion of pef1 leads to activation of autophagy and subsequent excessive TORC1 reactivation during the early phases of the nitrogen starvation response. This excessive TORC1 reactivation results in the silencing of the Ste11-Mei2 pathway and mating defects. Additionally, we found that pef1 genetically interacts with tsc1 and tsc2 for TORC1 regulation, and physically interacts with three cyclins, Clg1, Pas1 and Psl1. The double deletion of clg1 and pas1 promotes activation of autophagy and TORC1 during nitrogen starvation, similar to what is seen in pef1Δ cells. Overall, our work suggests that Pef1-Clg1 and Pef1-Pas1 complexes regulate initiation of sexual differentiation through control of the TSC-TORC1 pathway and autophagy.

Adaptation to Industrial Stressors Through Genomic and Transcriptional Plasticity in a Bioethanol Producing Fission Yeast Isolate

Schizosaccharomyces pombe is a model unicellular eukaryote with ties to the basic research, oenology and industrial biotechnology sectors. While most investigations into S. pombe cell biology utilize Leupold’s 972h^- laboratory strain background, recent studies have described a wealth of genetic and phenotypic diversity within wild populations of S. pombe including stress resistance phenotypes which may be of interest to industry. Here we describe the genomic and transcriptomic characterization of Wilmar-P, an S. pombe isolate used for bioethanol production from sugarcane molasses at industrial scale. Novel sequences present in Wilmar-P but not in the laboratory S. pombe genome included multiple coding sequences with near-perfect nucleotide identity to Schizosaccharomyces octosporus sequences. Wilmar-P also contained a ∼100kb duplication in the right arm of chromosome III, a region harboring ght5⁺, the predominant hexose transporter encoding gene. Transcriptomic analysis of Wilmar-P grown in molasses revealed strong downregulation of core environmental stress response genes and upregulation of hexose transporters and drug efflux pumps compared to laboratory S. pombe. Finally, examination of the regulatory network of Scr1, which is involved in the regulation of several genes differentially expressed on molasses, revealed expanded binding of this transcription factor in Wilmar-P compared to laboratory S. pombe in the molasses condition. Together our results point to both genomic plasticity and transcriptomic adaptation as mechanisms driving phenotypic adaptation of Wilmar-P to the molasses environment and therefore adds to our understanding of genetic diversity within industrial fission yeast strains and the capacity of this strain for commercial scale bioethanol production.

A genome-wide analysis of carbon catabolite repression in Schizosaccharomyces pombe

BACKGROUND

Optimal glucose metabolism is central to the growth and development of cells. In microbial eukaryotes, carbon catabolite repression (CCR) mediates the preferential utilization of glucose, primarily by repressing alternate carbon source utilization. In fission yeast, CCR is mediated by transcriptional repressors Scr1 and the Tup/Ssn6 complex, with the Rst2 transcription factor important for activation of gluconeogenesis and sexual differentiation genes upon derepression. Through genetic and genome-wide methods, this study aimed to comprehensively characterize CCR in fission yeast by identifying the genes and biological processes that are regulated by Scr1, Tup/Ssn6 and Rst2, the core CCR machinery.

RESULTS

The transcriptional response of fission yeast to glucose-sufficient or glucose-deficient growth conditions in wild type and CCR mutant cells was determined by RNA-seq and ChIP-seq. Scr1 was found to regulate genes involved in carbon metabolism, hexose uptake, gluconeogenesis and the TCA cycle. Surprisingly, a role for Scr1 in the suppression of sexual differentiation was also identified, as homothallic scr1 deletion mutants showed ectopic meiosis in carbon and nitrogen rich conditions. ChIP-seq characterised the targets of Tup/Ssn6 and Rst2 identifying regulatory roles within and independent of CCR. Finally, a subset of genes bound by all three factors was identified, implying that regulation of certain loci may be modulated in a competitive fashion between the Scr1, Tup/Ssn6 repressors and the Rst2 activator.

CONCLUSIONS

By identifying the genes directly and indirectly regulated by Scr1, Tup/Ssn6 and Rst2, this study comprehensively defined the gene regulatory networks of CCR in fission yeast and revealed the transcriptional complexities governing this system.

Multiple roles and diverse regulation of the Ras/cAMP/protein kinase A pathway in Candida albicans

Candida albicans is a major fungal pathogen of humans, causing both superficial and life-threatening systemic infections in immunocompromised people. The conserved Ras/cAMP/PKA pathway plays a key role in regulating multiple traits important for the virulence of C. albicans such as cell growth, yeast-hyphal transition, white-opaque switching, sexual reproduction and biofilm development. Diverse external signals influence cell physiology by activating this signaling pathway. The key components of the Ras/cAMP/PKA pathway include two Ras GTPases (Ras1 and Ras2), an adenylyl cyclase (Cyr1, also known as Cdc35), two cyclic nucleotide phosphodiesterases (Pde1 and Pde2) and the catalytic (Tpk1 and Tpk2) and regulatory (Bcy1) subunits of PKA kinase. Activation of this pathway dramatically alters the gene expression profile via several transcription factors, leading to the activation of specific biological processes. Here, we review the progress made in the past two decades to elucidate the molecular mechanisms by which the Ras/cAMP/PKA pathway senses diverse environmental cues and controls specific cellular responses and its connection with other signaling pathways in C. albicans.

Histone Chaperone Asf1 Is Required for the Establishment of Repressive Chromatin in Schizosaccharomyces pombe fbp1 Gene Repression

The arrangement of nucleosomes in chromatin plays a role in transcriptional regulation by restricting the accessibility of transcription factors and RNA polymerase II to cis-acting elements and promoters. For gene activation, the chromatin structure is altered to an open configuration. The mechanism for this process has been extensively analyzed. However, the mechanism by which repressive chromatin is reconstituted to terminate transcription has not been fully elucidated. Here, we investigated the mechanisms by which chromatin is reconstituted in the fission yeast Schizosaccharomyces pombefbp1 gene, which is robustly induced upon glucose starvation but tightly repressed under glucose-rich conditions. We found that the chromatin structure in the region upstream from fbp1 is closed by a two-step process. When cells are returned to glucose-rich medium following glucose starvation, changes in the nucleosome pattern alter the chromatin configuration at the transcription factor binding site to an inaccessible state, after which the nucleosome density upstream from fbp1 gradually increases via histone loading. Interestingly, this histone loading was observed in the absence of the Tup family corepressors Tup11 and Tup12. Analysis of strains carrying either gene disruptions or mutations affecting nine fission yeast histone chaperone genes demonstrated that the histone chaperone Asf1 induces nucleosome loading during glucose repression. These data establish a previously unappreciated chromatin reconstitution mechanism in fbp1 repression.

Computational modelling of meiotic entry and commitment

In response to developmental and environmental conditions, cells exit the mitotic cell cycle and enter the meiosis program to generate haploid gametes from diploid germ cells. Once cells decide to enter the meiosis program they become irreversibly committed to the completion of meiosis irrespective of the presence of cue signals. How meiotic entry and commitment occur due to the dynamics of the regulatory network is not well understood. Therefore, we constructed a mathematical model of the regulatory network that controls the transition from mitosis to meiosis in Schizosaccharomyces pombe. Upon nitrogen starvation, yeast cells exit mitosis and undergo conjugation and meiotic entry. The model includes the regulation of Mei2, an RNA binding protein required for conjugation and meiotic entry, by multiple feedback loops involving Pat1, a kinase that keeps cells in mitosis, and Ste11, a transcription activator required for the sexual differentiation. The model accounts for various experimental observations and demonstrates that the activation of Mei2 is bistable, which ensures the irreversible commitment to meiosis. Further, we show by integrating the meiosis-specific regulation with a cell cycle model, the dynamics of cell cycle exit, G1 arrest and entry into meiosis under nitrogen starvation.

cAMP-dependent protein kinase involves calcium tolerance through the regulation of Prz1 in Schizosaccharomyces pombe

The cAMP-dependent protein kinase Pka1 is known as a regulator of glycogenesis, meiosis, and stress responses in Schizosaccharomyces pombe. We demonstrated that Pka1 is responsible for calcium tolerance. Loss of functional components of the PKA pathway such as Git3, Gpa2, Cyr1, and Pka1 yields a CaCl2-sensitive phenotype, while loss of Cgs1, a regulatory subunit of PKA, results in CaCl2 tolerance. Cytoplasmic distribution of Cgs1 and Pka1 is increased by the addition of CaCl2, suggesting that CaCl2 induces dissociation of Cgs1 and Pka1. The expression of Prz1, a transcriptional regulator in calcium homeostasis, is elevated in a pka1∆ strain and in a wild type strain under glucose-limited conditions. Accordingly, higher expression of Prz1 in the wild type strain results in a CaCl2-sensitive phenotype. These findings suggest that Pka1 is essential for tolerance to exogenous CaCl2, probably because the expression level of Prz1 needs to be properly regulated by Pka1.

Genome-wide Analysis of Phosphoenolpyruvate Carboxylase Gene Family and Their Response to Abiotic Stresses in Soybean

Phosphoenolpyruvate carboxylase (PEPC) plays an important role in assimilating atmospheric CO2 during C4 and crassulacean acid metabolism photosynthesis, and also participates in various non-photosynthetic processes, including fruit ripening, stomatal opening, supporting carbon-nitrogen interactions, seed formation and germination, and regulation of plant tolerance to stresses. However, a comprehensive analysis of PEPC family in Glycine max has not been reported. Here, a total of ten PEPC genes were identified in soybean and denominated as GmPEPC1-GmPEPC10. Based on the phylogenetic analysis of the PEPC proteins from 13 higher plant species including soybean, PEPC family could be classified into two subfamilies, which was further supported by analyses of their conserved motifs and gene structures. Nineteen cis-regulatory elements related to phytohormones, abiotic and biotic stresses were identified in the promoter regions of GmPEPC genes, indicating their roles in soybean development and stress responses. GmPEPC genes were expressed in various soybean tissues and most of them responded to the exogenously applied phytohormones. GmPEPC6, GmPEPC8 and GmPEPC9 were significantly induced by aluminum toxicity, cold, osmotic and salt stresses. In addition, the enzyme activities of soybean PEPCs were also up-regulated by these treatments, suggesting their potential roles in soybean response to abiotic stresses.

TORC1 Regulates Developmental Responses to Nitrogen Stress via Regulation of the GATA Transcription Factor Gaf1

The TOR (target of rapamycin [sirolimus]) is a universally conserved kinase that couples nutrient availability to cell growth. TOR complex 1 (TORC1) in Schizosaccharomyces pombe positively regulates growth in response to nitrogen availability while suppressing cellular responses to nitrogen stress. Here we report the identification of the GATA transcription factor Gaf1 as a positive regulator of the nitrogen stress-induced gene isp7⁺, via three canonical GATA motifs. We show that under nitrogen-rich conditions, TORC1 positively regulates the phosphorylation and cytoplasmic retention of Gaf1 via the PP2A-like phosphatase Ppe1. Under nitrogen stress conditions when TORC1 is inactivated, Gaf1 becomes dephosphorylated and enters the nucleus. Gaf1 was recently shown to negatively regulate the transcription induction of ste11⁺, a major regulator of sexual development. Our findings support a model of a two-faceted role of Gaf1 during nitrogen stress. Gaf1 positively regulates genes that are induced early in the response to nitrogen stress, while inhibiting later responses, such as sexual development. Taking these results together, we identify Gaf1 as a novel target for TORC1 signaling and a step-like mechanism to modulate the nitrogen stress response.

TOR complex 1 (TORC1) is an evolutionary conserved protein complex that positively regulates growth and proliferation, while inhibiting starvation responses. In fission yeast, the activity of TORC1 is downregulated in response to nitrogen starvation, and cells reprogram their transcriptional profile and prepare for sexual development. We identify Gaf1, a GATA-like transcription factor that regulates transcription and sexual development in response to starvation, as a downstream target for TORC1 signaling. Under nitrogen-rich conditions, TORC1 positively regulates the phosphorylation and cytoplasmic retention of Gaf1 via the PP2A-like phosphatase Ppe1. Under nitrogen stress conditions when TORC1 is inactivated, Gaf1 becomes dephosphorylated and enters the nucleus. Budding yeast TORC1 regulates GATA transcription factors via the phosphatase Sit4, a structural homologue of Ppe1. Thus, the TORC1-GATA transcription module appears to be conserved in evolution and may also be found in higher eukaryotes.

Genome-wide identification and transcriptional expression analysis of mitogen-activated protein kinase and mitogen-activated protein kinase kinase genes in Capsicum annuum

The tripartite mitogen-activated protein kinase (MAPK) signaling cascades have been implicated in plant growth, development, and environment adaptation, but a comprehensive understanding of MAPK signaling at genome-wide level is limited in Capsicum annuum. Herein, genome-wide identification and transcriptional expression analysis of MAPK and MAPK kinase (MAPKK) were performed in pepper. A total of 19 pepper MAPK (CaMAPKs) genes and five MAPKK (CaMAPKKs) genes were identified. Phylogenetic analysis indicated that CaMAPKs and CaMAPKKs could be classified into four groups and each group contains similar exon-intron structures. However, significant divergences were also found. Notably, five members of the pepper MAPKK family were much less conserved than those found in Arabidopsis, and 9 Arabidopsis MAPKs did not have orthologs in pepper. Additionally, 7 MAPKs in Arabidopsis had either two or three orthologs in the pepper genome, and six pepper MAPKs and one MAPKK differing in sequence were found in three pepper varieties. Quantitative real-time RT-PCR analysis showed that the majority of MAPK and MAPKK genes were ubiquitously expressed and transcriptionally modified in pepper leaves after treatments with heat, salt, and Ralstonia solanacearum inoculation as well as exogenously applied salicylic acid, methyl jasmonate, ethephon, and abscisic acid. The MAPKK-MAPK interactome was tested by yeast two-hybrid assay, the results showed that one MAPKK might interact with multiple MAPKs, one MAPK might also interact with more than one MAPKKs, constituting MAPK signaling networks which may collaborate in transmitting upstream signals into appropriate downstream cellular responses and processes. These results will facilitate future functional characterization of MAPK cascades in pepper.

Antagonistic controls of chromatin and mRNA start site selection by Tup family corepressors and the CCAAT-binding factor

The Tup family corepressors contribute to critical cellular responses, such as the stress response and differentiation, presumably by inducing repressive chromatin, though the precise repression mechanism remains to be elucidated. The Schizosaccharomyces pombe fission yeast Tup family corepressors Tup11 and Tup12 (Tup11/12), which are orthologs of Tup1 in Saccharomyces cerevisiae budding yeast and Groucho in Drosophila, negatively control chromatin and the transcriptional activity of some stress-responsive genes. Here, we demonstrate that Tup11/12 repress transcription of a gluconeogenesis gene, fbp1⁺, by three distinct mechanisms. First, Tup11/12 inhibit chromatin remodeling in the fbp1⁺ promoter region where the Atf1 and Rst2 transcriptional activators bind. Second, they repress the formation of an open chromatin configuration at the fbp1⁺ TATA box. Third, they repress mRNA transcription per se by regulating basic transcription factors. These inhibitory actions of Tup11/12 are antagonized by three different types of transcriptional activators: CREB/ATF-type Atf1, C₂H₂zinc finger-type Rst2, and CBF/NF-Y-type Php5 proteins. We also found that impaired chromatin remodeling and fbp1⁺ mRNA transcription in php5Δ strains are rescued by the double deletions of tup11⁺ and tup12⁺, although the distribution of the transcription start sites becomes broader than that in wild-type cells. These data reveal a new mechanism of precise determination of the mRNA start site by Tup family corepressors and CBF/NF-Y proteins.

Sexual development of Schizosaccharomyces pombe is induced by zinc or iron limitation through Ecl1 family genes

Ecl1 family genes (ecl1 (+), ecl2 (+), and ecl3 (+)) have been identified as extenders of the chronological lifespan in Schizosaccharomyces pombe. Here, we found that the triple-deletion mutant (∆ecl1/2/3) had a defect in sexual development after entry into the stationary phase, although the mutant essentially showed normal mating and sporulation under nitrogen starvation or carbon limitation. In this study, we showed that limitation of zinc or iron can be a signal for sexual development of S. pombe cells grown in Edinburgh minimal medium until the stationary phase and that Ecl1 family genes are important for this process. Because the ∆ecl1/2/3 mutant diminishes the zinc depletion-dependent gene expression, Ecl1 family proteins may function as zinc sensors in the process of sexual development.

Regulation of entry into gametogenesis by Ste11: the endless game

Sexual reproduction is a fundamental aspect of eukaryotic cells, and a conserved feature of gametogenesis is its dependency on a master regulator. The ste11 gene was isolated more than 20 years ago by the Yamamoto laboratory as a suppressor of the uncontrolled meiosis driven by a pat1 mutant. Numerous studies from this laboratory and others have established the role of the Ste11 transcription factor as the master regulator of the switch between proliferation and differentiation in fission yeast. The transcriptional and post-transcriptional controls of ste11 expression are intricate, but most are not redundant. Whereas the transcriptional controls ensure that the gene is transcribed at a high level only when nutrients are rare, the post-transcriptional controls restrict the ability of Ste11 to function as a transcription factor to the G1-phase of the cell cycle from where the differentiation programme is initiated. Several feedback loops ensure that the cell fate decision is irreversible. The complete panel of molecular mechanisms operating to warrant the timely expression of the ste11 gene and its encoded protein basically mirrors the advances in the understanding of the numerous ways by which gene expression can be modulated.

Klf1, a C2H2 zinc finger-transcription factor, is required for cell wall maintenance during long-term quiescence in differentiated G0 phase

Fission yeast, Schizoaccharomyces pombe, is a model for studying cellular quiescence. Shifting to a medium that lacks a nitrogen-source induces proliferative cells to enter long-term G0 quiescence. Klf1 is a Krüppel-like transcription factor with a 7-amino acid Cys2His2-type zinc finger motif. The deletion mutant, ∆klf1, normally divides in vegetative medium, but proliferation is not restored after long-term G0 quiescence. Cell biologic, transcriptomic, and metabolomic analyses revealed a unique phenotype of the ∆klf1 mutant in quiescence. Mutant cells had diminished transcripts related to signaling molecules for switching to differentiation; however, proliferative metabolites for cell-wall assembly and antioxidants had significantly increased. Further, the size of ∆klf1 cells increased markedly during quiescence due to the aberrant accumulation of Calcofluor-positive, chitin-like materials beneath the cell wall. After 4 weeks of quiescence, reversible proliferation ability was lost, but metabolism was maintained. Klf1 thus plays a role in G0 phase longevity by enhancing the differentiation signal and suppressing metabolism for growth. If Klf1 is lost, S. pombe fails to maintain a constant cell size and normal cell morphology during quiescence.

A network of HMG-box transcription factors regulates sexual cycle in the fungus Podospora anserina

High-mobility group (HMG) B proteins are eukaryotic DNA-binding proteins characterized by the HMG-box functional motif. These transcription factors play a pivotal role in global genomic functions and in the control of genes involved in specific developmental or metabolic pathways. The filamentous ascomycete Podospora anserina contains 12 HMG-box genes. Of these, four have been previously characterized; three are mating-type genes that control fertilization and development of the fruit-body, whereas the last one encodes a factor involved in mitochondrial DNA stability. Systematic deletion analysis of the eight remaining uncharacterized HMG-box genes indicated that none were essential for viability, but that seven were involved in the sexual cycle. Two HMG-box genes display striking features. PaHMG5, an ortholog of SpSte11 from Schizosaccharomyces pombe, is a pivotal activator of mating-type genes in P. anserina, whereas PaHMG9 is a repressor of several phenomena specific to the stationary phase, most notably hyphal anastomoses. Transcriptional analyses of HMG-box genes in HMG-box deletion strains indicated that PaHMG5 is at the hub of a network of several HMG-box factors that regulate mating-type genes and mating-type target genes. Genetic analyses revealed that this network also controls fertility genes that are not regulated by mating-type transcription factors. This study points to the critical role of HMG-box members in sexual reproduction in fungi, as 11 out of 12 members were involved in the sexual cycle in P. anserina. PaHMG5 and SpSte11 are conserved transcriptional regulators of mating-type genes, although P. anserina and S. pombe diverged 550 million years ago. Two HMG-box genes, SOX9 and its upstream regulator SRY, also play an important role in sex determination in mammals. The P. anserina and S. pombe mating-type genes and their upstream regulatory factor form a module of HMG-box genes analogous to the SRY/SOX9 module, revealing a commonality of sex regulation in animals and fungi.

Podospora anserina, a coprophilous fungus, is used extensively as a model organism to address questions of sexual development and mating-type functions. Its mating-type locus contains three HMGB genes that encode transcription factors involved in fertilization and fruit-body development. We present the functional characterization of the remaining HMGB genes, which revealed that 11 of 12 HMGB genes were involved in sexual development. An analysis of the relationships between these genes uncovered a regulatory network governing the expression of mating-type genes. PaHMG5 is a key transcription factor that operates upstream of mating-type genes in this network. A homolog of PaHMG5 performs a similar function in the fission yeast Schizosaccharomyces pombe, which diverged from P. anserina 550 million years ago. The conservation of a regulatory circuit over such a prolonged timeframe is a striking exception to the general observation that sex developmental pathways are highly variable, even across closely related lineages. A module consisting of two HMGB transcription factors (Sry and Sox9) is a key regulator of sex determination in mammals. We propose that the module containing PaHMG5 and mating-type HMGB genes is the fungal counterpart of the mammalian module, revealing a commonality of sex regulation in animals and fungi.

The fission yeast GATA factor, Gaf1, modulates sexual development via direct down-regulation of ste11+ expression in response to nitrogen starvation

Gaf1 is the first GATA family zinc-finger transcription factor identified in Schizosaccharomyces pombe. Here, we report that Gaf1 functions as a negatively acting transcription factor of ste11⁺, delaying the entrance of cells exposed to transient nitrogen starvation into the meiotic cycle. gaf1Δ strains exhibited accelerated G₁-arrest upon nitrogen starvation. Moreover, gaf1Δ mutation caused increased mating and sporulation frequency under both nitrogen-starved and unstarved conditions, while overexpression of gaf1⁺ led to a significant impairment of sporulation. By microarray analysis, we found that approximately 63% (116 genes) of the 183 genes up-regulated in unstarved gaf1Δ cells were nitrogen starvation-responsive genes, and furthermore that 25 genes among the genes up-regulated by gaf1Δ mutation are Ste11 targets (e.g., gpa1⁺, ste4⁺, spk1⁺, ste11⁺, and mei2⁺). The phenotype caused by gaf1Δ mutation was masked by ste11Δ mutation, indicating that ste11⁺ is epistatic to gaf1⁺ with respect to sporulation efficiency, and accordingly that gaf1⁺ functions upstream of ste11⁺ in the signaling pathway governing sexual development. gaf1Δ strains showed accelerated ste11⁺ expression under nitrogen starvation and increased ste11⁺ expression even under normal conditions. Electrophoretic mobility shift assay analysis demonstrated that Gaf1 specifically binds to the canonical GATA motif (5′-HGATAR-3′) spanning from −371 to −366 in ste11⁺ promoter. Consequently, Gaf1 provides the prime example for negative regulation of ste11⁺ transcription through direct binding to a cis-acting motif of its promoter.

AMPK phosphorylation by Ssp1 is required for proper sexual differentiation in fission yeast

The AMP-activated protein kinase (AMPK) is a central regulator of cellular energy homeostasis, which, in response to a fall in intracellular ATP levels, activates energy-producing pathways and inhibits energy-consuming processes. Here, we report that fission yeast cells lacking AMPK activity are unable to advance entry into mitosis in response to nitrogen starvation and cannot undergo proper G1 arrest and cell differentiation. We also show that AMPK is important in the promotion of the nuclear localization and accumulation of the Ste11 transcription factor. As in animal cells, the fission yeast CaMKK ortholog (Ssp1) phosphorylates and activates the catalytic subunit of AMPK (Ssp2) in its activation loop (Thr189) when cells are starved for nitrogen or glucose. Interestingly, we found that the phosphorylation of Ssp2 on Thr189 is required for nuclear accumulation of AMPK. Our data demonstrate the existence of a signal transduction pathway activated by nutrient starvation that triggers Ssp2 phosphorylation and AMPK redistribution from the cytoplasm to the nucleus. This pathway is important to advance fission cells into mitosis and to establish a timely pre-Start G1 cell cycle arrest for mating.

Genetic screening for regulators of Prz1, a transcriptional factor acting downstream of calcineurin in fission yeast

Calcineurin phosphatase plays crucial roles in a wide variety of cell types and organisms. Dephosphorylation of the nuclear factor of activated T-cell (NFAT) family of transcriptional factors by calcineurin is essential for activating immune-responsive genes in mammals. NFAT activity is also regulated by diverse signaling pathways, which affect NFAT kinases and nuclear partner proteins. In fission yeast, calcineurin dephosphorylates and activates Prz1, a C2H2-type zinc finger transcriptional factor. Calcineurin-Prz1 signaling regulates the expression of the Pmc1 Ca(2+) pump. Prz1-overexpressing cells showed extremely slow growth and high transcriptional activity of Prz1 in the absence of stimulation. Here, we isolated seven genes as dosage-dependent suppressors of this slow growth phenotype. These seven genes encode Rad24, Rad25, Pka1, Msn5 (SPAC328.01c), Pac1, Ape2, and Tfs1. All of them decreased the high transcriptional activity caused by Prz1 overexpression. Overexpression of Pka1, Rad24, and Rad25 also repressed the Ca(2+)-induced transcriptional activity in cells with Prz1 expressed at wild-type levels. Knock-out of rad24 or rad25 significantly enhanced the transcriptional activity of Prz1, whereas knock-out or mutation of other genes did not enhance the activity. The 14-3-3 proteins, Rad24 and Rad25, bound Prz1 and the Rad24-binding site located at residues 421-426 of Prz1. In msn5 deletion mutants, GFP-Prz1 localized at nucleus in the absence of Ca(2+) stimulation, suggesting that Msn5 functions as an exportin for Prz1. In summary, our data suggest that Rad24 and Rad25 negatively regulate Prz1 and that Pka1, Msn5, Pac1, Tfs1, and Ape2 also regulate Prz1.

Gene-specific requirement of RNA polymerase II CTD phosphorylation

The largest subunit of RNA polymerase II, Rpb1, contains an unusual C-terminal domain (CTD) composed of numerous repeats of the YSPTSPS consensus sequence. This sequence is the target of post-translational modifications such as phosphorylation, glycosylation, methylation and transitions between stereoisomeric states, resulting in a vast combinatorial potential referred to as the CTD code. In order to gain insight into the biological significance of this code, several studies recently reported the genome-wide distribution of some of these modified polymerases and associated factors in either fission yeast (Schizosaccharomyces pombe) or budding yeast (Saccharomyces cerevisiae). The resulting occupancy maps reveal that a general RNA polymerase II transcription complex exists and undergoes uniform transitions from initiation to elongation to termination. Nevertheless, CTD phosphorylation dynamics result in a gene-specific effect on mRNA expression. In this review, we focus on the gene-specific requirement of CTD phosphorylation and discuss in more detail the case of serine 2 phosphorylation (S2P) within the CTD, a modification that is dispensable for general transcription in fission yeast but strongly affects transcription reprogramming and cell differentiation in response to environmental cues. The recent discovery of Cdk12 as a genuine CTD S2 kinase and its requirement for gene-specific expression are discussed in the wider context of metazoa.

Regulation of entry into gametogenesis

Gametogenesis is a fundamental aspect of sexual reproduction in eukaryotes. In the unicellular fungi Saccharomyces cerevisiae (budding yeast) and Schizosaccharomyces pombe (fission yeast), where this developmental programme has been extensively studied, entry into gametogenesis requires the convergence of multiple signals on the promoter of a master regulator. Starvation signals and cellular mating-type information promote the transcription of cell fate inducers, which in turn initiate a transcriptional cascade that propels a unique type of cell division, meiosis, and gamete morphogenesis. Here, we will provide an overview of how entry into gametogenesis is initiated in budding and fission yeast and discuss potential conserved features in the germ cell development of higher eukaryotes.

The RNA binding protein Csx1 promotes sexual differentiation in Schizosaccharomyces pombe

Sexual differentiation is a highly regulated process in the fission yeast Schizosaccharomyces pombe and is triggered by nutrient depletion, mainly nitrogen source.

One of the key regulatory proteins in fission yeast sexual differentiation is the transcription factor Ste11. Ste11 regulates the transcription of many genes required for the initial steps of conjugation and meiosis, and its deficiency leads to sterility. Ste11 activity is mainly regulated at two levels: phosphorylation and abundance of its mRNA.

Csx1 is an RNA binding protein that we have previously described to bind and regulate the turnover rate of the mRNA encoding the transcription factor Atf1 in the presence of oxidative stress.

We have observed that Csx1-deficient cells have defects in sexual differentiation and are partially sterile. We investigated how Csx1 is regulating this process in S. pombe. Csx1 associates with ste11⁺ mRNA and cells lacking Csx1 are sterile with a reduced amount of ste11⁺ mRNA. Overexpression of ste11⁺ mRNA completely rescues the mating deficiencies of csx1Δ cells.

Here, we present a novel mechanism of ste11⁺ mRNA positive regulation through the activity of Csx1, an RNA binding protein that also have key functions in the response to oxidative stress in fission yeast. This finding opens interesting question about the possible coordination of sexual differentiation and oxidative stress response in eukaryotes and the role of RNA binding proteins in the adaptation to environmental signals.

The fission yeast stress-responsive MAPK pathway promotes meiosis via the phosphorylation of Pol II CTD in response to environmental and feedback cues

The RRM-type RNA-binding protein Mei2 is a master regulator of meiosis in fission yeast, in which it stabilizes meiosis-specific mRNAs by blocking their destruction. Artificial activation of Mei2 can provoke the entire meiotic process, and it is suspected that Mei2 may do more than the stabilization of meiosis-specific mRNAs. In our current study using a new screening system, we show that Mei2 genetically interacts with subunits of CTDK-I, which phosphorylates serine-2 residues on the C-terminal domain of RNA polymerase II (Pol II CTD). Phosphorylation of CTD Ser-2 is essential to enable the robust transcription of ste11, which encodes an HMG-type transcription factor that regulates the expression of mei2 and other genes necessary for sexual development. CTD Ser-2 phosphorylation increases under nitrogen starvation, and the stress-responsive MAP kinase pathway, mediated by Wis1 MAPKK and Sty1 MAPK, is critical for this stress response. Sty1 phosphorylates Lsk1, the catalytic subunit of CTDK-I. Furthermore, a feedback loop stemming from activated Mei2 to Win1 and Wis4 MAPKKKs operates in this pathway and eventually enhances CTD Ser-2 phosphorylation and ste11 transcription. Hence, in addition to starting meiosis, Mei2 functions to reinforce the commitment to it, once cells have entered this process. This study also demonstrates clearly that the stress-responsive MAP kinase pathway can modulates gene expression through phosphorylation of Pol II CTD.

Hundreds of genes are newly expressed during meiosis, a process to form gametes, and the control of meiosis-specific gene expression is not simple. The master regulator of meiosis in fission yeast, Mei2, blocks an RNA destruction system that selectively degrades meiosis-specific mRNAs, highlighting the importance of post-transcriptional control in meiotic gene expression. Here we present another example of unforeseen regulation for meiosis. Ste11 is a key transcription factor responsible for the early meiotic gene expression in fission yeast. The ste11 gene is transcribed robustly only when serine-2 residues on the C-terminal domain (CTD Ser-2) of RNA polymerase II are phosphorylated. We show that the stress-responsive MAP kinase cascade transmits the environmental signal to stimulate CTD Ser-2 phosphorylation. Sty1 MAP kinase appears to phosphorylate and activate the catalytic subunit of CTDK-I, which in turn phosphorylates CTD Ser-2. We demonstrate further that Mei2, expression of which depends on Ste11, can activate the MAP kinase cascade, forming a feedback loop. Thus, we clarify here three important issues in cellular development: the physiological role of CTD Ser-2 phosphorylation, the molecular function of the stress-responsive MAP kinase pathway, and the presence of positive feedback that reinforces the commitment to meiosis.

Multistep regulation of protein kinase A in its localization, phosphorylation and binding with a regulatory subunit in fission yeast

The cAMP-PKA is the major glucose-sensing pathway that controls sexual differentiation in Schizosaccharomyces pombe. Sequencing from the pka1 locus of recessive sam mutants, in which cells are highly inclined to sexual differentiation, led to the identification of mutations in the pka1 locus in sam5 (pka1-G441E) and sam7 (pka1-G441R). Rst2 and Ste11 proteins were induced and localized to the nucleus of sam5 and sam7 mutants even under rich glucose conditions, indicating that the function of Pka1 was completely abolished by mutations. Pka1-G441E and Pka1-G441R mutant proteins reside in the cytoplasm, even under glucose-rich conditions, while wild-type Pka1 resides in the nucleus, indicating that the functionality of Pka1 is important for its nuclear localization. This is supported by the observation that the Pka1-T356A mutant, which partially lacks Pka1 function, was localized to both the cytoplasm and the nucleus, but an active phosphomimetic Pka1-T356D mutant prtotein was localized to the nucleus under glucose-rich conditions. In addition to the basal phosphorylation of Pka1 at T356, hyperphosphorylation of Pka1 was observed under glucose-starved conditions, and such hyperphosphorylation was not observed in pka1-G441E, pka1-G441R, pka1-T356A or pka1-T356D mutants. As these mutant proteins failed to interact with a regulatory subunit Cgs1, hyperphosphorylation of Pka1 mutant proteins was considered to be dependent on Cgs1 interaction. Consistent with a role for Cgs1 in Pka1 phosphorylation, we detected the formation of a Cgs1-Pka1 complex prior to Pka1 hyperphosphorylation. Together, these results indicate that nuclear localization of Pka1 depends on its activity and hyperphosphorylation of Pka1 depends on Cgs1 interaction.

Important characteristics of sequence-specific recombination hotspots in Schizosaccharomyces pombe

In many organisms, meiotic recombination occurs preferentially at a limited number of sites in the genome known as hotspots. In the fission yeast Schizosaccharomyces pombe, simple sequence motifs determine the location of at least some, and possibly most or all, hotspots. Recently, we showed that a large number of different sequences can create hotspots. Among those sequences we identified some recurring motifs that fell into at least five distinct families, including the well-characterized CRE family of hotspots. Here we report the essential sequence for activity of two of the novel hotspots, the oligo-C and CCAAT hotspots, and identify associated trans-acting factors required for hotspot activity. The oligo-C hotspot requires a unique 8-bp sequence, CCCCGCAC, though hotspot activity is also significantly affected by adjacent nucleotides. The CCAAT hotspot requires a more complex and degenerate sequence, including the originally identified seven nucleotide CCAATCA sequence at its core. We identified transcription factors, the CCAAT-binding factor (CBF) and Rst2, which are required specifically for activity of the CCAAT hotspots and oligo-C hotspots, respectively. Each of these factors binds to its respective motifs in vitro. However, unlike CRE, the sequence required for hotspot activity is larger than the sequence required for binding, suggesting the involvement of additional factors.

TOR and PKA pathways synergize at the level of the Ste11 transcription factor to prevent mating and meiosis in fission yeast

Background

In the fission yeast Schizosaccharomyces pombe, the TOR (target of rapamycin) and PKA (protein kinase A) signaling transduction pathways regulate the expression of genes required for cell growth and sexual differentiation in response to the nutritional environment. Inhibition of Tor2 signaling results in the induction of genes involved in sexual differentiation, and the cells undergo mating and meiosis, even under good nutritional conditions. The same phenotype is observed in mutants in which the PKA pathway is inactive. By contrast, Tor2 overexpression or mutations that hyperactivate PKA signaling impair sexual differentiation, even under poor nutritional conditions. Accordingly, a very important question is to understand the molecular mechanism by which these two pathways coordinately regulate gene expression in response to nutrients.

Methodology/Principal Findings

Here we demonstrate that TOR and PKA pathways operate coordinately to negatively regulate sexual differentiation by inhibiting the nuclear accumulation of the Ste11 transcription factor. However, the Tor2 pathway is unable to block the nuclear localization of Ste11 under good nutritional conditions when the PKA pathway is inactive. Using microarray analyses, we found that both pathways inhibit sexual differentiation by blocking ste11-dependent gene expression.

Conclusions/Significance

We conclude that both the PKA and the TOR pathways inhibit Ste11 nuclear accumulation to repress Ste11-dependent gene expression. However, the PKA pathway plays a quantitatively more important role than the TOR pathway in this process.

Fission yeast TORC1 regulates phosphorylation of ribosomal S6 proteins in response to nutrients and its activity is inhibited by rapamycin

Cellular activities are regulated by environmental stimuli through protein phosphorylation. Target of rapamycin (TOR), a serine/threonine kinase, plays pivotal roles in cell proliferation and cell growth in response to nutrient status. In Schizosaccharomyces pombe, TORC1, which contains Tor2, plays crucial roles in nutrient response. Here we find a nitrogen-regulated phosphoprotein, p27, in S. pombe using the phospho-Akt substrate antibody. Response of p27 phosphorylation to nitrogen availability is mediated by TORC1 and the TSC-Rhb1 signaling, but not by TORC2 or other nutrient stress-related pathways. Database and biochemical analyses indicate that p27 is identical to ribosomal protein S6 (Rps6). Ser235 and Ser236 in Rps6 are necessary for Rps6 phosphorylation by TORC1. These Rps6 phosphorylations are dispensable for cell viability. Rps6 phosphorylation by TORC1 also responds to availability of glucose and is inhibited by osmotic and oxidative stresses. Rapamycin inhibits the ability of TORC1 to phosphorylate Rps6, owing to interaction of the rapamycin-FKBP12 complex with the FRB domain in Tor2. Rapamycin also leads to a decrease in cell size in a TORC1-dependent manner. Our findings demonstrate that the nutrient-responsive and rapamycin-sensitive TORC1-S6 signaling exists in S. pombe, and that this pathway plays a role in cell size control.

The selective elimination of messenger RNA underlies the mitosis-meiosis switch in fission yeast

The cellular programs for meiosis and mitosis must be strictly distinguished but the mechanisms controlling the entry to meiosis remain largely elusive in higher organisms. In contrast, recent analyses in yeast have shed new light on the mechanisms underlying the mitosis-meiosis switch. In this review, the current understanding of these mechanisms in the fission yeast Schizosaccharomyces pombe is discussed. Meiosis-inducing signals in this microbe emanating from environmental conditions including the nutrient status converge on the activity of an RRM-type RNA-binding protein, Mei2. This protein plays pivotal roles in both the induction and progression of meiosis and has now been found to govern the meiotic program in a quite unexpected manner. Fission yeast contains an RNA degradation system that selectively eliminates meiosis-specific mRNAs during the mitotic cell cycle. Mmi1, a novel RNA-binding protein of the YTH-family, is essential for this process. Mei2 tethers Mmi1 and thereby stabilizes the transcripts necessary for the progression of meiosis.

The S. pombe SAGA complex controls the switch from proliferation to sexual differentiation through the opposing roles of its subunits Gcn5 and Spt8

The SAGA complex is a conserved multifunctional coactivator known to play broad roles in eukaryotic transcription. To gain new insights into its functions, we performed biochemical and genetic analyses of SAGA in the fission yeast, Schizosaccharomyces pombe. Purification of the S. pombe SAGA complex showed that its subunit composition is identical to that of Saccharomyces cerevisiae. Analysis of S. pombe SAGA mutants revealed that SAGA has two opposing roles regulating sexual differentiation. First, in nutrient-rich conditions, the SAGA histone acetyltransferase Gcn5 represses ste11(+), which encodes the master regulator of the mating pathway. In contrast, the SAGA subunit Spt8 is required for the induction of ste11(+) upon nutrient starvation. Chromatin immunoprecipitation experiments suggest that these regulatory effects are direct, as SAGA is physically associated with the ste11(+) promoter independent of nutrient levels. Genetic tests suggest that nutrient levels do cause a switch in SAGA function, as spt8Delta suppresses gcn5Delta with respect to ste11(+) derepression in rich medium, whereas the opposite relationship, gcn5Delta suppression of spt8Delta, occurs during starvation. Thus, SAGA plays distinct roles in the control of the switch from proliferation to differentiation in S. pombe through the dynamic and opposing activities of Gcn5 and Spt8.