SakA MAP kinase is involved in stress signal transduction, sexual development and spore viability in Aspergillus nidulans

The MAP kinase MpkA controls cell wall integrity, oxidative stress response, gliotoxin production and iron adaptation in Aspergillus fumigatus

The saprophytic fungus Aspergillus fumigatus is the most important air-borne fungal pathogen. The cell wall of A. fumigatus has been studied intensively as a potential target for development of effective antifungal agents. A major role in maintaining cell wall integrity is played by the mitogen-activated protein kinase (MAPK) MpkA. To gain a comprehensive insight into this central signal transduction pathway, we performed a transcriptome analysis of the ΔmpkA mutant under standard and cell wall stress conditions. Besides genes involved in cell wall remodelling, protection against ROS and secondary metabolism such as gliotoxin, pyomelanin and pseurotin A, also genes involved in siderophore biosynthesis were regulated by MpkA. Consistently, northern and western blot analyses indicated that iron starvation triggers phosphorylation and thus activation of MpkA. Furthermore, localization studies indicated that MpkA accumulates in the nucleus under iron depletion. Hence, we report the first connection between a MAPK pathway and siderophore biosynthesis. The measurement of amino acid pools and of the pools of polyamines indicated that arginine was continuously converted into ornithine to fuel the siderophore pool in the ΔmpkA mutant strain. Based on our data, we propose that MpkA fine-tunes the balance between stress response and energy consuming cellular processes.

The role of sho1 in polarized growth of Aspergillus fumigatus

Aspergillus fumigatus is an opportunistic pathogen that may cause severe invasive disease in immunocompromised patients. The filamentous fungi undergo polarized growth, searching for nutrients in the environment and causing invasive growth in tissue. Sho1 is a sensor of the high osmolarity glycerol pathway, and the sho1 mutant showed a decrease in growth rate. We found that sho1 is involved in the polarized growth of A. fumigatus. The sho1 mutation resulted in extended isotropic growth of germinating conidia followed by multiple germ tubes and wide hyphae with short intercalary cells by calcofluor white staining. The mechanism by which sho1 gene affected polarized growth is investigated. A reduced number of apical vesicles with greater dispersion were observed by transmission electron microscopy in the Spitzenkörper body of the sho1 mutant. Actin patches were distributed randomly at low density at early stages of mutant strain fungal development and reaggregated to the hyphal tip of later stages when long filamentous fungi formed. Actin patches located at the tip of polarized wild-type cells. RNA levels of polarized growth-related genes Rho GTPases were detected by real-time PCR. The sho1 gene did not affect the RNA expression when strains were cultured at 37°C for 6 h. At 17 h, the RNA expression of rho1, rho3 and CDC42 in the sho1 mutant were 0.18-, 0.18- and 0.33-fold of that in the wild type. The sho1 gene affected the polarized growth through affecting the expression of Rho GTPases, the distribution of actin cytoskeleton, vesicle quantity and distribution.

The small molecular mass antifungal protein of Penicillium chrysogenum--a mechanism of action oriented review

The β-lactam producing filamentous fungus Penicillium chrysogenum secretes a 6.25 kDa small molecular mass antifungal protein, PAF, which has a highly stable, compact 3D structure and is effective against a wide spectrum of plant and zoo pathogenic fungi. Its precise physiological functions and mode of action need to be elucidated before considering possible biomedical, agricultural or food technological applications. According to some more recent experimental data, PAF plays an important role in the fine-tuning of conidiogenesis in Penicillium chrysogenum. PAF triggers apoptotic cell death in sensitive fungi, and cell death signaling may be transmitted through two-component systems, heterotrimeric G protein coupled signal transduction and regulatory networks as well as via alteration of the Ca(2+) -homeostasis of the cells. Possible biotechnological applications of PAF are also outlined in the review.

Coordination of secondary metabolism and development in fungi: the velvet family of regulatory proteins

Filamentous fungi produce a number of small bioactive molecules as part of their secondary metabolism ranging from benign antibiotics such as penicillin to threatening mycotoxins such as aflatoxin. Secondary metabolism can be linked to fungal developmental programs in response to various abiotic or biotic external triggers. The velvet family of regulatory proteins plays a key role in coordinating secondary metabolism and differentiation processes such as asexual or sexual sporulation and sclerotia or fruiting body formation. The velvet family shares a protein domain that is present in most parts of the fungal kingdom from chytrids to basidiomycetes. Most of the current knowledge derives from the model Aspergillus nidulans where VeA, the founding member of the protein family, was discovered almost half a century ago. Different members of the velvet protein family interact with each other and the nonvelvet protein LaeA, primarily in the nucleus. LaeA is a methyltransferase-domain protein that functions as a regulator of secondary metabolism and development. A comprehensive picture of the molecular interplay between the velvet domain protein family, LaeA and other nuclear regulatory proteins in response to various signal transduction pathway starts to emerge from a jigsaw puzzle of several recent studies.

A novel family of dehydrin-like proteins is involved in stress response in the human fungal pathogen Aspergillus fumigatus

During a search for genes controlling conidial dormancy in Aspergillus fumigatus, two dehydrin-like genes, DprA and DprB, were identified. The deduced proteins had repeated stretches of 23 amino acids that contained a conserved dehydrin-like protein (DPR) motif. Disrupted DprAΔ mutants were hypersensitive to oxidative stress and to phagocytic killing, whereas DprBΔ mutants were impaired in osmotic and pH stress responses. However, no effect was observed on their pathogenicity in our experimental models of invasive aspergillosis. Molecular dissection of the signaling pathways acting upstream showed that expression of DprA was dependent on the stress-activated kinase SakA and the cyclic AMP-protein kinase A (cAMP-PKA) pathways, which activate the bZIP transcription factor AtfA, while expression of DprB was dependent on the SakA mitogen-activated protein kinase (MAPK) pathway, and the zinc finger transcription factor PacC. Fluorescent protein fusions showed that both proteins were associated with peroxisomes and the cytosol. Accordingly, DprA and DprB were important for peroxisome function. Our findings reveal a novel family of stress-protective proteins in A. fumigatus and, potentially, in filamentous ascomycetes.

Aspergillus nidulans transcription factor AtfA interacts with the MAPK SakA to regulate general stress responses, development and spore functions

Fungi utilize a phosphorelay system coupled to a MAP kinase module for sensing and processing environmental signals. In Aspergillus nidulans, response regulator SskA transmits osmotic and oxidative stress signals to the stress MAPK (SAPK) SakA. Using a genetic approach together with GFP tagging and molecular bifluorescence we show that SakA and ATF/CREB transcription factor AtfA define a general stress-signalling pathway that plays differential roles in oxidative stress responses during growth and development. AtfA is permanently localized in the nucleus, while SakA accumulates in the nucleus in response to oxidative or osmotic stress signals or during normal spore development, where it physically interacts with AtfA. AtfA is required for expression of several genes, the conidial accumulation of SakA and the viability of conidia. Furthermore, SakA is active (phosphorylated) in asexual spores, remaining phosphorylated in dormant conidia and becoming dephosphorylated during germination. SakA phosphorylation in spores depends on certain (SskA) but not other (SrrA and NikA) components of the phosphorelay system. Constitutive phosphorylation of SakA induced by the fungicide fludioxonil prevents both, germ tube formation and nuclear division. Similarly, Neurospora crassa SakA orthologue OS-2 is phosphorylated in intact conidia and gets dephosphorylated during germination. We propose that SakA-AtfA interaction regulates gene expression during stress and conidiophore development and that SAPK phosphorylation is a conserved mechanism to regulate transitions between non-growing (spore) and growing (mycelia) states.

Reactive oxygen species in phytopathogenic fungi: signaling, development, and disease

Reactive oxygen species (ROS) play a major role in pathogen-plant interactions: recognition of a pathogen by the plant rapidly triggers the oxidative burst, which is necessary for further defense reactions. The specific role of ROS in pathogen defense is still unclear. Studies on the pathogen so far have focused on the importance of the oxidative stress response (OSR) systems to overcome the oxidative burst or of its avoidance by effectors. This review focuses on the role of ROS for fungal virulence and development. In the recent years, it has become obvious that (a) fungal OSR systems might not have the predicted crucial role in pathogenicity, (b) fungal pathogens, especially necrotrophs, can actively contribute to the ROS level in planta and even take advantage of the host's response, (c) fungi possess superoxide-generating NADPH oxidases similar to mammalian Nox complexes that are important for pathogenicity; however, recent data indicate that they are not directly involved in pathogen-host communication but in fungal differentiation processes that are necessary for virulence.

The osmosensing signal transduction pathway from Botrytis cinerea regulates cell wall integrity and MAP kinase pathways control melanin biosynthesis with influence of light

Mitogen activated protein kinase (MAPK) signal transduction pathways are ubiquitous among eukaryotic organisms with evolutionary conserved modules. Although generally classified as osmotic and cell wall integrity pathways, functional divergences have been observed for HOG1- and SLT2-related MAPK pathways. Here we show that the osmotic signal transduction cascade is involved in cell wall integrity in the phytopathogenic ascomycete Botrytis cinerea. The deletion mutants of the upstream histidine kinase Bos1 and of the MAPK Sak1 showed modified tolerance to cell wall degrading enzymes and cell wall interfering agents, as well as increased staining of β1-3 glucan and chitin compared to the wild-type. The Sak1 MAPK was phosphorylated upon cell wall challenging. Sak1 interfered with the phosphorylation status of the SLT2 type MAPK Bmp3 hinting to cross talk between both MAPK pathways. All signal transduction components interfered with the expression of melanin biosynthesis genes in dark and bright, suggesting a coordinated control of melanin biosynthesis.

Involvement of the Aspergillus nidulans protein kinase C with farnesol tolerance is related to the unfolded protein response

Previously, we demonstrated that the Aspergillus nidulans calC2 mutation in protein kinase C pkcA was able to confer tolerance to farnesol (FOH), an isoprenoid that has been shown to inhibit proliferation and induce apoptosis. Here, we investigate in more detail the role played by A. nidulans pkcA in FOH tolerance. We demonstrate that pkcA overexpression during FOH exposure causes increased cell death. FOH is also able to activate several markers of endoplasmic reticulum (ER) stress and the unfolded protein response (UPR). Our results suggest an intense cross-talk between PkcA and the UPR during FOH-induced cell death. Furthermore, the overexpression of pkcA increases both mRNA accumulation and metacaspases activity, and there is a genetic interaction between PkcA and the caspase-like protein CasA. Mutant analyses imply that MAP kinases are involved in the signal transduction in response to the effects caused by FOH.

Overlapping and distinct functions of two Trichoderma virens MAP kinases in cell-wall integrity, antagonistic properties and repression of conidiation

We have studied the functions of the Trichoderma virens TmkB, a homologue of the yeast cell-wall integrity MAP kinase Slt2, using gene knockout. The functions of TmkB were compared to those of the pathogenicity MAP kinase homologue (TmkA). Like the tmkA loss-of-function mutants, tmkB mutants exhibited reduced radial growth and constitutive conidiation in dark as well as in liquid shake cultures. The tmkB mutants, in contrast to tmkA mutants, had cell-wall integrity defects, as shown by autolysis of the mycelia and increased sensitivity to cell-wall degrading enzymes. Interestingly, the tmkB mutants were not autolytic on the synthetic Vogels minimal medium. The tmkB mutants had attenuated ability to overgrow the plant pathogen Sclerotium rolfsii, while retaining the ability to overgrow Rhizoctonia solani and Pythium spp., a phenotype also exhibited by the tmkA mutants. This first functional analysis of a cell-wall integrity MAPK in Trichoderma spp., a group of economically important fungi, shows the importance of this signaling pathway in biocontrol. Common phenotypes of the TmkA and TmkB pathways suggest that the two MAPKs may share some substrates, perhaps subunits of key transcription factors, thus dependent on two phosphorylation events for their activity.

Role of the osmotic stress regulatory pathway in morphogenesis and secondary metabolism in filamentous fungi

Environmental stimuli trigger an adaptative cellular response to optimize the probability of survival and proliferation. In eukaryotic organisms from mammals to fungi osmotic stress, mainly through the action of the high osmolarity glycerol (HOG) pathway, leads to a response necessary for adapting and surviving hyperosmotic environments. In this review we show that the osmoadaptative response is conserved but not identical in different fungi. The osmoadaptative response system is also intimately linked to morphogenesis in filamentous fungi, including mycotoxin producers. Previous studies indicate that the response to osmotic stress is also coupled to the biosynthesis of natural products, including mycotoxins.

Analysis of mitogen-activated protein kinase phosphorylation in response to stimulation of histidine kinase signaling pathways in Neurospora

In eukaryotes, two-component regulatory systems have been demonstrated to regulate phosphorylation of mitogen-activated protein kinases (MAPKs). Here, we describe a method implementing preparation of a protein extract under denaturing conditions, followed by Western analysis using MAPK antibodies that can be used to observe the effects of components of two-component signaling pathways or other proteins on the phosphorylation status of MAPKs. The protein extraction method presented may also be used to concentrate cellular proteins for additional applications, such as metabolic labeling or analysis of other posttranslational modifications.

AtfA bZIP-type transcription factor regulates oxidative and osmotic stress responses in Aspergillus nidulans

The aim of the study was to demonstrate that the bZIP-type transcription factor AtfA regulates different types of stress responses in Aspergillus nidulans similarly to Atf1, the orthologous 'all-purpose' transcription factor of Schizosaccharomyces pombe. Heterologous expression of atfA in a S. pombe Deltaatf1 mutant restored the osmotic stress tolerance of fission yeast in surface cultures to the same level as recorded in complementation studies with the atf1 gene, and a partial complementation of the osmotic and oxidative-stress-sensitive phenotypes was also achieved in submerged cultures. AtfA is therefore a true functional ortholog of fission yeast's Atf1. As demonstrated by RT-PCR experiments, elements of both oxidative (e.g. catalase B) and osmotic (e.g. glycerol-3-phosphate dehydrogenase B) stress defense systems were transcriptionally regulated by AtfA in a stress-type-specific manner. Deletion of atfA resulted in oxidative-stress-sensitive phenotypes while the high-osmolarity stress sensitivity of the fungus was not affected significantly. In A. nidulans, the glutathione/glutathione disulfide redox status of the cells as well as apoptotic cell death and autolysis seemed to be controlled by regulatory elements other than AtfA. In conclusion, the orchestrations of stress responses in the aspergilli and in fission yeast share several common features, but further studies are needed to answer the important question of whether a fission yeast-like core environmental stress response also operates in the euascomycete genus Aspergillus.

Molecular Genetics of Emericella nidulans Sexual Development

Many aspergilli that belongs to ascomycetes have sexuality. In a homothallic or self-fertile fungus, a number of fruiting bodies or cleistothecia are formed in a thallus grown from a single haploid conidia or ascospores. Genome-sequencing project revealed that two mating genes (MAT) encoding the regulatory proteins that are necessary for controlling partner recognition in heterothallic fungi were conserved in most aspergilli. The MAT gene products in some self-fertile species were not required for recognition of mating partner at pheromone-signaling stage but required at later stages of sexual development. Various environmental factors such as nutritional status, culture conditions and several stresses, influence the decision or progression of sexual reproduction. A large number of genes are expected to be involved in sexual development of Emericella nidulans (anamorph: Aspergillus nidulans), a genetic and biological model organism in aspergilli. The sexual development process can be grouped into several development stages, including the decision of sexual reproductive cycle, mating process, growth of fruiting body, karyogamy followed by meiosis, and sporulation process. Complicated regulatory networks, such as signal transduction pathways and gene expression controls, may work in each stage and stage-to-stage linkages. In this review, the components joining in the regulatory pathways of sexual development, although they constitute only a small part of the whole regulatory networks, are briefly mentioned. Some of them control sexual development positively and some do negatively. Regarding the difficulties for studying sexual differentiation compare to asexual one, recent progresses in molecular genetics of E. nidulans enlarge the boundaries of understanding sexual development in the non-fertile species as well as in fertile fungi.

Signaling by the pathogenicity-related MAP kinase of Cochliobolus heterostrophus correlates with its local accumulation rather than phosphorylation

Phosphorylated mitogen-activated protein kinases (MAPK) transmit signals by activation of their targets. The extent of signal transduction could depend on MAPK phosphorylation level, concentration, and subcellular localization. The pathogenicity MAPK Chk1 of the fungal corn pathogen Cochliobolus heterostrophus is required for central developmental functions, including appressoria formation, conidiation, melanization, virulence, and female fertility. We followed CHK1 transcript level, protein localization, quantity, phosphorylation, and expression of downstream genes during conidial germination on a surface inductive for appressoria formation and in suspension. The Chk1-GFP protein representing a translational fusion of Chk1 and GFP (green fluorescent protein) was very abundant in ungerminated conidia, accumulated in maturating appressoria and appressorial nuclei, but was uniformly distributed in suspension-grown hyphae. Expression of Chk1-dependent genes was upregulated in appressoria-forming hyphae but not in suspension. Despite Chk1 activation, there was no change in its phosphorylation and total protein quantity. Of all conditions tested, a temperature shift caused a decrease whereas hyperosmotic stress caused an increase in Chk1 phosphorylation. Activation of Chk1 during appressoria formation is apparently manifested by its local accumulation but not by significant changes in phosphorylation.

Transcriptional profiling for Aspergillusnidulans HogA MAPK signaling pathway in response to fludioxonil and osmotic stress

In filamentous fungi, the His-Asp phosphorelay signaling system and HOG pathway are involved in the action of the fungicides, fludioxonil, and iprodione, as well as osmotic and oxidative stress responses. Aspergillusnidulans response regulators (RRs), SskA and SrrA, and histidine kinase (HK), NikA, are involved in the growth inhibitory effects of these fungicides. To gain further insights into the molecular basis for these signaling systems, we performed DNA microarray analyses of fludioxonil and osmotic stress responses in A.nidulans. A global expression analysis revealed that a large number of genes were modulated by fludioxonil treatment in an SskA-dependent manner, whereas SrrA hardly contributed to this modulation. The fludioxonil up-regulated or down-regulated genes (FUGs or FDGs, respectively) are also dependent on the HogA MAPK cascade. We found that the SskA-HogA pathway regulates expression of atfA gene encoding a transcription factor involved in conidia stress tolerance. From the results of microarray analyses, AtfA-dependent FUGs largely overlapped with HogA-dependent FUGs, suggesting that AtfA functions downstream of the HogA MAPK. A series of microarray analyses showed that the inferred SskA-HogA-AtfA pathway is implicated in the transcriptional response to osmotic stress as well as fludioxonil. The srrAatfA null double mutant turns off the SrrA and SskA-HogA-AtfA pathways and showed sensitivity to osmotic stress but no resistance to fludioxonil. Our data revealed that the growth inhibitory effect of fludioxonil depends on factors other than AtfA in spite of the fact that AtfA functions downstream of the HogA MAPK cascade. The complexity of the stress response in the His-Asp phosphorelay system followed by the HogA MAPK cascade is discussed.

The nsdC gene encoding a putative C2H2-type transcription factor is a key activator of sexual development in Aspergillus nidulans

The formation of the Aspergillus nidulans fruiting body is affected by a number of genetic and environmental factors. Here, the nsdC (never in sexual development) gene-encoding a putative transcription factor carrying a novel type of zinc-finger DNA-binding domain consisting of two C(2)H(2)'s and a C(2)HC motif that are highly conserved in most fungi but not in plants or animals-was investigated. Two distinct transcripts of 2.6 and 3.0 kb were generated from nsdC. The 2.6-kb mRNA accumulated differentially in various stages of growth and development, while the level of the 3.0-kb mRNA remained relatively constant throughout the life cycle. While the deletion of nsdC resulted in the complete loss of fruiting body formation under all conditions favoring sexual development, overexpression of nsdC not only enhanced formation of fruiting bodies (cleistothecia) but also overcame inhibitory effects of certain stresses on cleistothecial development, implying that NsdC is a key positive regulator of sexual development. Deletion of nsdC also retarded vegetative growth and hyperactive asexual sporulation, suggesting that NsdC is necessary not only for sexual development but also for regulating asexual sporulation negatively. Overexpression of veA or nsdD does not rescue the failure of fruiting body formation caused by nsdC deletion. Furthermore, nsdC expression is not affected by either VeA or NsdD, and vice versa, indicating that NsdC regulates sexual development independently of VeA or NsdD.

The MAPKK kinase ChSte11 regulates sexual/asexual development, melanization, pathogenicity, and adaptation to oxidative stress in Cochliobolus heterostrophus

All fungi use multiple mitogen-activated protein kinase (MAPK) cascades to respond to external signals to regulate specialized responses. In this study, we cloned and characterized a putative MAPKKK gene ChSte11, orthologous to yeast STE11, of Cochliobolus heterostrophus. DeltaChste11 strains showed defects in conidiation, sexual development, melanization and the formation of appressoria. These mutants were significantly less virulent on corn plants than the wild type. Similar phenotypes were observed in mutants of Chk1-MAPK, a putative downstream protein kinase of ChSte11. These results suggested that ChSte11 regulates various morphological changes and pathogenicity via Chk1 MAPK. Both DeltaChste11 and Deltachk1 strains showed severe sensitivity to oxidative stress, hydrogen peroxide, and heavy metals, cupric or ferric cations. DeltaBmhog1 strains, mutants of the HOG1-type MAPK, did not show sensitivity to these forms of stress. Our results strongly suggested that the Ste11-type MAPKKK regulates not only various morphological changes and pathogenicity, but also adaptations to stress via Chk1-type MAPK in filamentous fungi.

Mitogen-activated protein kinase hog1 in the entomopathogenic fungus Beauveria bassiana regulates environmental stress responses and virulence to insects

Beauveria bassiana is an economically important insect-pathogenic fungus which is widely used as a biocontrol agent to control a variety of insect pests. However, its insecticide efficacy in the field is often influenced by adverse environmental factors. Thus, understanding the genetic regulatory processes involved in the response to environmental stress would facilitate engineering and production of a more efficient biocontrol agent. Here, a mitogen-activated protein kinase (MAPK)-encoding gene, Bbhog1, was isolated from B. bassiana and shown to encode a functional homolog of yeast HIGH-OSMOLARITY GLYCEROL 1 (HOG1). A Bbhog1 null mutation was generated in B. bassiana by targeted gene replacement, and the resulting mutants were more sensitive to hyperosmotic stress, high temperature, and oxidative stress than the wild-type controls. These results demonstrate the conserved function of HOG1 MAPKs in the regulation of abiotic stress responses. Interestingly, DeltaBbhog1 mutants exhibited greatly reduced pathogenicity, most likely due to a decrease in spore viability, a reduced ability to attach to insect cuticle, and a reduction in appressorium formation. The transcript levels of two hydrophobin-encoding genes, hyd1 and hyd2, were dramatically decreased in a DeltaBbhog1 mutant, suggesting that Bbhog1 may regulate the expression of the gene associated with hydrophobicity or adherence.

The nuclear Dbf2-related kinase COT1 and the mitogen-activated protein kinases MAK1 and MAK2 genetically interact to regulate filamentous growth, hyphal fusion and sexual development in Neurospora crassa

Ndr kinases, such as Neurospora crassa COT1, are important for cell differentiation and polar morphogenesis, yet their input signals as well as their integration into a cellular signaling context are still elusive. Here, we identify the cot-1 suppressor gul-4 as mak-2 and show that mutants of the gul-4/mak-2 mitogen-activated protein (MAP) kinase pathway suppress cot-1 phenotypes along with a concomitant reduction in protein kinase A (PKA) activity. Furthermore, mak-2 pathway defects are partially overcome in a cot-1 background and are associated with increased MAK1 MAPK signaling. A comparative characterization of N. crassa MAPKs revealed that they act as three distinct modules during vegetative growth and asexual development. In addition, common functions of MAK1 and MAK2 signaling during maintenance of cell-wall integrity distinguished the two ERK-type pathways from the p38-type OS2 osmosensing pathway. In contrast to separate functions during vegetative growth, the concerted activity of the three MAPK pathways is essential for cell fusion and for the subsequent formation of multicellular structures that are required for sexual development. Taken together, our data indicate a functional link between COT1 and MAPK signaling in regulating filamentous growth, hyphal fusion, and sexual development.

Distinct and combined roles of the MAP kinases of Cochliobolus heterostrophus in virulence and stress responses

Pathogenicity mitogen-activated protein kinases (MAPKs), related to yeast FUS3/KSS1, are essential for virulence in fungi, including Cochliobolus heterostrophus, a necrotrophic pathogen causing Southern corn leaf blight. We compared the phenotypes of mutants in three MAPK genes: HOG1, MPS1, and CHK1. The chk1 and mps1 mutants show autolytic appearance, light pigmentation, and dramatic reduction in virulence and conidiation. Similarity of mps1 and chk1 mutants is reflected by coregulation by these two MAPKs of several genes. Unlike chk1, mps1 mutants are female-fertile and form normal-looking appressoria. HOG1 mediates resistance to hyperosmotic and, to a lesser extent, oxidative stress, and is required for stress upregulation of glycerol-3-phosphate phosphatase, transaldolase, and a monosaccharide transporter. Hog1, but not Mps1 or Chk1, was rapidly phosphorylated in response to increased osmolarity. The hog1 mutants have smaller appressoria and cause decreased disease symptoms on maize leaves. Surprisingly, loss of MPS1 in a wild-type or hog1 background improved resistance to some stresses. All three MAPKs contribute to the regulation of central developmental functions under normal and stress conditions, and full virulence cannot be achieved without appropriate input from all three pathways.

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.

Comparative genomics of the environmental stress response in ascomycete fungi

Unicellular fungi thrive in diverse niches around the world, and many of these niches present unique and stressful challenges that must be contended with by their inhabitants. Numerous studies have investigated the genomic expression responses to environmental stress in 'model' ascomycete fungi, including Saccharomyces cerevisiae, Candida albicans and Schizosaccharomyces pombe. This review presents a comparative-genomics perspective on the environmental stress response, a common response to diverse stresses. Implications for the role of this response, based on its presence or absence in fungi from disparate ecological niches, are discussed.

Functional analysis of a fungal endophyte stress-activated MAP kinase

The ability of fungi to sense and respond rapidly to environmental stress is crucial for their survival in the wild. One of the most important pathways involved in this response is the stress-activated MAP (mitogen-activated protein) kinase pathway. We report here on the isolation of the stress-activated MAP kinase, sakA, from the fungal endophyte Epichloë festucae. Complementation of the stress sensitivity and cell cycle defects of an Schizosaccharomyces pombe sty1Delta mutant with sakA confirmed it encodes a functional MAP kinase. Analysis of an E. festucae DeltasakA mutant revealed sakA is essential for growth under conditions of temperature and osmotic stress in culture, and for sensitivity to the fungicide fludioxonil. However, the DeltasakA mutant shows no increased sensitivity to hydrogen peroxide. Given sakA can rescue the sty1Delta mutant from sensitivity to oxidative stress, SakA has the potential to sense and transduce oxidative stress signals. The DeltasakA mutant is also defective in conidia formation, suggesting a role for SakA in asexual development of E. festucae. The detection of elevated hydrogen peroxide production in the DeltasakA mutant suggests there may be a link between MAP kinase and ROS (reactive oxygen species) signalling pathways in E. festucae.

Ssk2 mitogen-activated protein kinase kinase kinase governs divergent patterns of the stress-activated Hog1 signaling pathway in Cryptococcus neoformans

The stress-activated p38/Hog1 mitogen-activated protein kinase (MAPK) pathway is structurally conserved in many diverse organisms, including fungi and mammals, and modulates myriad cellular functions. The Hog1 pathway is uniquely specialized to control differentiation and virulence factors in a majority of clinical Cryptococcus neoformans serotype A and D strains. Here, we identified and characterized the Ssk2 MAPKKK that functions upstream of the MAPKK Pbs2 and the MAPK Hog1 in C. neoformans. The SSK2 gene was identified as a potential component responsible for the difference in Hog1 phosphorylation between the serotype D f1 sibling strains B-3501 and B-3502 through comparative analysis of meiotic maps showing their meiotic segregation patterns of Hog1-dependent sensitivity to the antifungal drug fludioxonil. Ssk2 is the only component of the Hog1 MAPK cascade that is polymorphic between the two strains, and the B-3501 and B-3502 SSK2 alleles were distinguished by two coding sequence changes. Supporting this finding, SSK2 allele exchange completely interchanged the Hog1-controlled signaling patterns, related phenotypes, and virulence levels of strains B-3501 and JEC21. In the serotype A strain H99, disruption of the SSK2 gene enhanced capsule and melanin biosynthesis and mating efficiency, similar to pbs2 and hog1 mutations. Furthermore, ssk2Delta, pbs2Delta, and hog1Delta mutants were hypersensitive to a variety of stresses and resistant to fludioxonil. In agreement with these results, Hog1 phosphorylation was abolished in the ssk2Delta mutant, similar to what occurred in the pbs2Delta mutant. Taken together, these findings indicate that Ssk2 is a critical interface connecting the two-component system and the Pbs2-Hog1 MAPK pathway in C. neoformans.

Response regulators SrrA and SskA are central components of a phosphorelay system involved in stress signal transduction and asexual sporulation in Aspergillus nidulans

Among eukaryotes, only slime molds, fungi, and plants contain signal transduction phosphorelay systems. In filamentous fungi, multiple sensor kinases appear to use a single histidine-containing phosphotransfer (HPt) protein to relay signals to two response regulators (RR). In Aspergillus nidulans, the RR SskA mediates activation of the mitogen-activated protein kinase SakA in response to osmotic and oxidative stress, whereas the functions of the RR SrrA were unknown. We used a genetic approach to characterize the srrA gene as a new member of the skn7/prr1 family and to analyze the roles of SrrA in the phosphorelay system composed of the RR SskA, the HPt protein YpdA, and the sensor kinase NikA. While mutants lacking the HPt protein YpdA are unviable, mutants lacking SskA (DeltasskA), SrrA (DeltasrrA), or both RR (DeltasrrA DeltasskA) are viable and differentially affected in osmotic and oxidative stress responses. Both RR are involved in osmostress resistance, but DeltasskA mutants are more sensitive to this stress, and only SrrA is required for H(2)O(2) resistance and H(2)O(2)-mediated induction of catalase CatB. In contrast, both RR are individually required for fungicide sensitivity and calcofluor resistance and for normal sporulation and conidiospore viability. The DeltasrrA and DeltasskA sporulation defects appear to be related to decreased mRNA levels of the key sporulation gene brlA. In contrast, conidiospore viability defects do not correlate with the activity of the spore-specific catalase CatA. Our results support a model in which NikA acts upstream of SrrA and SskA to transmit fungicide signals and to regulate asexual sporulation and conidiospore viability. In contrast, NikA appears dispensable for osmotic and oxidative stress signaling. These results highlight important differences in stress signal transmission among fungi and define a phosphorelay system involved in oxidative and osmotic stress, cell wall maintenance, fungicide sensitivity, asexual reproduction, and spore viability.

The thioredoxin system of the filamentous fungus Aspergillus nidulans: impact on development and oxidative stress response

Redox regulation has been shown to be of increasing importance for many cellular processes. Here, redox homeostasis was addressed in Aspergillus nidulans, an important model organism for fundamental biological questions such as development, gene regulation or the regulation of the production of secondary metabolites. We describe the characterization of a thioredoxin system from the filamentous fungus A. nidulans. The A. nidulans thioredoxin A (AnTrxA) is an 11.6-kDa protein with a characteristic thioredoxin active site motif (WCGPC) encoded by the trxA gene. The corresponding thioredoxin reductase (AnTrxR), encoded by the trxR gene, represents a homodimeric flavoprotein with a native molecular mass of 72.2 kDa. When combined in vitro, the in Escherichia coli overproduced recombinant proteins AnTrxA and AnTrxR were able to reduce insulin and oxidized glutathione in an NADPH-dependent manner indicating that this in vitro redox system is functional. Moreover, we have created a thioredoxin A deletion strain that shows decreased growth, an increased catalase activity, and the inability to form reproductive structures like conidiophores or cleistothecia when cultivated under standard conditions. However, addition of GSH at low concentrations led to the development of sexual cleistothecia, whereas high GSH levels resulted in the formation of asexual conidiophores. Furthermore, by applying the principle of thioredoxin-affinity chromatography we identified several novel putative targets of thioredoxin A, including a hypothetical protein with peroxidase activity and an aldehyde dehydrogenase.

Novel reporter gene expression systems for monitoring activation of the Aspergillus nidulans HOG pathway

The Aspergillus nidulans high-osmolarity glycerol response (AnHOG) pathway is involved in osmoadaptation. We found that fludioxonil, a fungicide, causes improper activation of HogA mitogen-activated protein kinase (MAPK) in A. nidulans. Here we present novel reporter systems for monitoring activation of the AnHOG pathway. The promoter region of gfdB (glycerol-3-phosphate dehydrogenase), whose expression depends on the presence of HogA, was fused to a beta-glucuronidase uidA gene (GUS) to construct the reporter, which was introduced into A. nidulans wild type and hogADelta. Increased GUS activity was detected in the wild type only when it was treated with high osmolarity or fludioxonil, while reporter activity was scarcely stimulated in the hogADelta mutant. These results indicate that the reporter activity is controlled via HogA activation. Furthermore, we present possible applications of the reporter systems in screening new antifungal compounds.

Phosphopantetheinyl transferase CfwA/NpgA is required for Aspergillus nidulans secondary metabolism and asexual development

Polyketide synthases (PKSs) and/or nonribosomal peptide synthetases (NRPSs) are central components of secondary metabolism in bacteria, plants, and fungi. In filamentous fungi, diverse PKSs and NRPSs participate in the biosynthesis of secondary metabolites such as pigments, antibiotics, siderophores, and mycotoxins. However, many secondary metabolites as well as the enzymes involved in their production are yet to be discovered. Both PKSs and NRPSs require activation by enzyme members of the 4'-phosphopantetheinyl transferase (PPTase) family. Here, we report the isolation and characterization of Aspergillus nidulans strains carrying conditional (cfwA2) and null (DeltacfwA) mutant alleles of the cfwA gene, encoding an essential PPTase. We identify the polyketides shamixanthone, emericellin, and dehydroaustinol as well as the sterols ergosterol, peroxiergosterol, and cerevisterol in extracts from A. nidulans large-scale cultures. The PPTase CfwA/NpgA was required for the production of these polyketide compounds but dispensable for ergosterol and cerevisterol and for fatty acid biosynthesis. The asexual sporulation defects of cfwA, DeltafluG, and DeltatmpA mutants were not rescued by the cfwA-dependent compounds identified here. However, a cfwA2 mutation enhanced the sporulation defects of both DeltatmpA and DeltafluG single mutants, suggesting that unidentified CfwA-dependent PKSs and/or NRPSs are involved in the production of hitherto-unknown compounds required for sporulation. Our results expand the number of known and predicted secondary metabolites requiring CfwA/NpgA for their biosynthesis and, together with the phylogenetic analysis of fungal PPTases, suggest that a single PPTase is responsible for the activation of all PKSs and NRPSs in A. nidulans.

The response regulator RRG-1 functions upstream of a mitogen-activated protein kinase pathway impacting asexual development, female fertility, osmotic stress, and fungicide resistance in Neurospora crassa

Two-component systems, consisting of proteins with histidine kinase and/or response regulator domains, regulate environmental responses in bacteria, Archaea, fungi, slime molds, and plants. Here, we characterize RRG-1, a response regulator protein from the filamentous fungus Neurospora crassa. The cell lysis phenotype of Delta rrg-1 mutants is reminiscent of osmotic-sensitive (os) mutants, including nik-1/os-1 (a histidine kinase) and strains defective in components of a mitogen-activated protein kinase (MAPK) pathway: os-4 (MAPK kinase kinase), os-5 (MAPK kinase), and os-2 (MAPK). Similar to os mutants, Delta rrg-1 strains are sensitive to hyperosmotic conditions, and they are resistant to the fungicides fludioxonil and iprodione. Like os-5, os-4, and os-2 mutants, but in contrast to nik-1/os-1 strains, Delta rrg-1 mutants do not produce female reproductive structures (protoperithecia) when nitrogen starved. OS-2-phosphate levels are elevated in wild-type cells exposed to NaCl or fludioxonil, but they are nearly undetectable in Delta rrg-1 strains. OS-2-phosphate levels are also low in Delta rrg-1, os-2, and os-4 mutants under nitrogen starvation. Analysis of the rrg-1(D921N) allele, mutated in the predicted phosphorylation site, provides support for phosphorylation-dependent and -independent functions for RRG-1. The data indicate that RRG-1 controls vegetative cell integrity, hyperosmotic sensitivity, fungicide resistance, and protoperithecial development through regulation of the OS-4/OS-5/OS-2 MAPK pathway.

Le.MAPK and its interacting partner, Le.DRMIP, in fruiting body development in Lentinula edodes

Development in shiitake mushroom, Lentinula edodes, is a unique process and studies of the molecular basis of this process may lead to improvement in mushroom cultivation. Previous studies have identified a number of signal transduction genes related to mushroom development, but those genes have not been well characterized. The present work characterized a developmentally regulated MAP kinase, Le.MAPK, and its interaction with a novel gene, Le.DRMIP in the signal transduction pathway. The expression profiles of these two genes reveal their importance in fruiting body initiation and development; the Le.DRMIP transcript is localized predominantly in the developing young fruiting body and gills, which further signifies its role in cell differentiation during mushroom development.

Proteome map of Aspergillus nidulans during osmoadaptation

The model filamentous fungus Aspergillus nidulans, when grown in a moderate level of osmolyte (+0.6M KCl), was previously found to have a significantly reduced cell wall elasticity (Biotech Prog, 21:292, 2005). In this study, comparative proteomic analysis via two-dimensional gel electrophoresis (2de) and matrix-assisted laser desorption ionization/time-of-flight (MALDI-TOF) mass spectrometry was used to assess molecular level events associated with this phenomenon. Thirty of 90 differentially expressed proteins were identified. Sequence homology and conserved domains were used to assign probable function to twenty-one proteins currently annotated as "hypothetical." In osmoadapted cells, there was an increased expression of glyceraldehyde-3-phosphate dehydrogenase and aldehyde dehydrogenase, as well as a decreased expression of enolase, suggesting an increased glycerol biosynthesis and decreased use of the TCA cycle. There also was an increased expression of heat shock proteins and Shp1-like protein degradation protein, implicating increased protein turnover. Five novel osmoadaptation proteins of unknown functions were also identified.

Sensing the environment: lessons from fungi

All living organisms use numerous signal-transduction systems to sense and respond to their environments and thereby survive and proliferate in a range of biological niches. Molecular dissection of these signalling networks has increased our understanding of these communication processes and provides a platform for therapeutic intervention when these pathways malfunction in disease states, including infection. Owing to the expanding availability of sequenced genomes, a wealth of genetic and molecular tools and the conservation of signalling networks, members of the fungal kingdom serve as excellent model systems for more complex, multicellular organisms. Here, we review recent progress in our understanding of how fungal-signalling circuits operate at the molecular level to sense and respond to a plethora of environmental cues.

BcSAK1, a stress-activated mitogen-activated protein kinase, is involved in vegetative differentiation and pathogenicity in Botrytis cinerea

The gene bcsak1, encoding a mitogen-activated protein kinase (MAPK) of Botrytis cinerea, was cloned and characterized. The protein has high homology to the yeast Hog1 and to corresponding MAPKs from filamentous fungi, but it shows unique functional features. The protein is phosphorylated under osmotic stress, specific fungicides, and oxidative stress mediated by H(2)O(2) and menadione. Northern blot analyses indicate that only a subset of typical oxidative stress response genes is regulated by BcSAK1. In contrast to most other fungal systems, Deltabcsak1 mutants are significantly impaired in vegetative and pathogenic development: they are blocked in conidia formation, show increased sclerotial development, and are unable to penetrate unwounded plant tissue. These data indicate that in B. cinerea the stress-activated MAPK cascade is involved in essential differentiation programs.

Novel mitogen-activated protein kinase MpkC of Aspergillus fumigatus is required for utilization of polyalcohol sugars

The genome of Aspergillus fumigatus has four genes that encode mitogen-activated protein kinases (MAPKs), sakA/hogA, mpkA, mpkB, and mpkC. The functions of the MpkB and MpkC MAPKs are unknown for A. fumigatus and the closely related and genetically amenable species Aspergillus nidulans. mpkC deletion mutants of A. fumigatus were made and their phenotypes characterized. The mpkC deletion mutants were viable and had normal conidial germination and hyphal growth on minimal or complete media. This is in contrast to deletion mutants with deletions in the closely related MAPK gene sakA/hogA that we previously reported had a nitrogen source-dependent germination phenotype. Similarly, the growth of the mpkC deletion mutants was wild type on high-osmolarity medium. Consistent with these two MAP kinase genes regulating different cellular responses, we determined that the mpkC deletion mutants were unable to grow on minimal medium with sorbitol or mannitol as the sole carbon source. This result implicates MpkC signaling in carbon source utilization. Changes in mRNA levels for sakA and mpkC were measured in response to hypertonic stress, oxidative stress, and a shift from glucose to sorbitol to determine if there was overlap in the SakA and MpkC signaling pathways. These studies demonstrated that SakA- and MpkC-dependent patterns of change in mRNA levels are distinct and have minimal overlap in response to these environmental stresses.

Identification of OS-2 MAP kinase-dependent genes induced in response to osmotic stress, antifungal agent fludioxonil, and heat shock in Neurospora crassa

Two-component signal transduction comprising of OS-1 (histidine kinase), OS-4 (MAPKK kinase), OS-5 (MAPK kinase), and OS-2 (MAP kinase) plays an important role in osmotic regulation in Neurospora crassa. To identify the genes regulated downstream of OS-2 MAP kinase, quantitative real-time RT-PCR analysis was conducted in selected genes based on Hog1 MAP kinase regulated genes in yeast. In response to osmotic stress and fludioxonil, expression of six genes that for glycerol synthesis (gcy-1, gcy-3, and dak-1), gluconeogenesis (fbp-1 and pck-1), and catalase (ctt-1) was activated in the wild-type strain, but not in the os-2 mutant. A heat shock treatment also induced their expression in the same way. Consisting with the gene expression, the enzyme activity of glycerol dehydrogenase, but not glycerol-3-phosphate dehydrogenase, was increased in response to osmotic stress and fludioxonil in the wild-type strain. OS-2 was phosphorylated by the OS-1 cascade in response to relatively low osmotic stress and fludioxonil. However, OS-2 phosphorylation by heat shock and a higher osmotic stress was found in the os-1 mutant normally but not in the os-4 and os-5 mutants. These results suggested that non-OS-1 signaling activates OS-2 in an OS-4-dependent manner in such conditions.

A class III histidine kinase acts as a novel virulence factor in Botrytis cinerea

Filamentous ascomycetes contain large numbers of histidine kinases (HK) that belong to eleven classes. Members of class III from different species were previously shown to be involved in osmoregulation and resistance to dicarboximide and phenylpyrrole fungicides. We have inactivated the gene encoding the single group III HK, BOS1, in the economically important plant pathogen Botrytis cinerea. BOS1 inactivation had pleiotropic effects on the fungus. Besides the expected osmosensitivity and resistance to fungicides, null mutants presented additional characteristics indicating that BOS1 is necessary for normal macroconidiation and full virulence. On standard culture media, null mutants very rarely formed conidiophores and those few conidiophores failed to produce conidia. This defect could be partially restored with 1 M sorbitol, suggesting that another BOS1-independent signal cascade may be involved in macroconidiation. The mutants were not found to be hypersensitive to various oxidative stresses but were more resistant to menadione. Finally, pathogenicity tests showed that bos1-null mutants were significantly reduced in the ability to infect host plants. Appressorium morphogenesis was not altered; however, in planta growth was severely reduced. To our knowledge, this is the first class III HK characterized as a pathogenicity factor in a plant-pathogenic ascomycete.

The role of the sakA (Hog1) and tcsB (sln1) genes in the oxidant adaptation of Aspergillus fumigatus

The Hog1 MAP kinase pathway regulates stress adaptation in several fungi. To assess its role in stress adaptation in Aspergillus fumigatus, we constructed mutants in genes encoding the sensor histidine kinase (HK) tcsB as well as sakA, which are homologues of the Saccharomyces cerevisiae sln1 and Hog1, respectively. Compared to the wild type strain (Wt), growth of sakA (sakAtriangle up) mutant was reduced, and growth inhibition was increased when H(2)O(2), menadione, or SDS was added to the media. On the other hand, the tcsB mutant (tcsBtriangle up) was similar to the Wt strain in regard to growth and morphology, although a partial sensitivity to SDS was observed. Western blot analysis of Wt and the tcsBtriangle up strains indicated that when stressed with H(2)O(2), phosphorylation of Hog1p still occurs in the mutant. Since in Candida albicans, Hog1 regulates transcription of at least one histidine kinase, we performed RT-PCR of 6 histidine kinase genes as well as the ssk1 and skn7 response regulator genes of A. fumigatus. No significant differences in transcription were observed with the sakAtriangle up when compared to the Wt, indicating that the sakA does not regulate transcription of these genes. Our studies indicate that the A. fumigatus sakA is required for optimal growth of the organism with or without oxidant stress, while tcsB gene is dispensable.

Hog1 mitogen-activated protein kinase plays conserved and distinct roles in the osmotolerant yeast Torulaspora delbrueckii

Torulaspora delbrueckii has emerged during evolution as one of the most osmotolerant yeasts. However, the molecular mechanisms underlying this unusual stress resistance are poorly understood. In this study, we have characterized the functional role of the high-osmolarity glycerol (HOG) mitogen-activated protein kinase pathway in mediating the osmotic stress response, among others, in T. delbrueckii. We show that the T. delbrueckii Hog1p homologue TdHog1p is phosphorylated after cell transfer to NaCl- or sorbitol-containing medium. However, TdHog1p plays a minor role in tolerance to conditions of moderate osmotic stress, a trait related mainly with the osmotic balance. In consonance with this, the absence of TdHog1p produced only a weak defect in the timing of the osmostress-induced glycerol and GPD1 mRNA overaccumulation. Tdhog1Delta mutants also failed to display aberrant morphology changes in response to osmotic stress. Furthermore, our data indicate that the T. delbrueckii HOG pathway has evolved to respond to specific environmental conditions and to play a pivotal role in the stress cross-protection mechanism.

ThHog1 controls the hyperosmotic stress response in Trichoderma harzianum

Trichoderma harzianumis a widespread mycoparasitic fungus, able to successfully colonize a wide range of substrates under different environmental conditions. Transcript profiling revealed a subset of genes induced inT. harzianumunder hyperosmotic shock. Thehog1gene, a homologue of the MAPKHOG1gene that controls the hyperosmotic stress response inSaccharomyces cerevisiae, was characterized.T. harzianum hog1complemented thehog1Δ mutation inS. cerevisiae, but showed different features to yeast alleles: improved osmoresistance by expression of thehog1allele and a lack of lethality when thehog1F315Sallele was overexpressed. ThHog1 protein was phosphorylated inT. harzianumunder different stress conditions such as hyperosmotic or oxidative stress, among others. By using a ThHog1-GFP fusion, the protein was shown to be localized in nuclei under these stress conditions. Two mutant strains ofT. harzianumwere constructed: one carrying thehog1F315Sallele, and a knockdownhog1-silenced strain. The silenced strain was highly sensitive to osmotic stress, and showed intermediate levels of resistance against oxidative stress, indicating that the main role of ThHog1 protein is in the hyperosmotic stress response. Stress cross-resistance experiments showed evidences of a secondary role of ThHog1 in oxidative stress. The strain carrying thehog1F315Sallele was highly resistant to the calcineurin inhibitor cyclosporin A, which suggests the existence of links between the two pathways. The two mutant strains showed a strongly reduced antagonistic activity against the plant pathogensPhoma betaeandColletotrichum acutatum, which points to a role of ThHog1 protein in fungus–fungus interactions.

Fungal heat-shock proteins in human disease

Heat-shock proteins (hsps) have been identified as molecular chaperones conserved between microbes and man and grouped by their molecular mass and high degree of amino acid homology. This article reviews the major hsps of Saccharomyces cerevisiae, their interactions with trehalose, the effect of fermentation and the role of the heat-shock factor. Information derived from this model, as well as from Neurospora crassa and Achlya ambisexualis, helps in understanding the importance of hsps in the pathogenic fungi, Candida albicans, Cryptococcus neoformans, Aspergillus spp., Histoplasma capsulatum, Paracoccidioides brasiliensis, Trichophyton rubrum, Phycomyces blakesleeanus, Fusarium oxysporum, Coccidioides immitis and Pneumocystis jiroveci. This has been matched with proteomic and genomic information examining hsp expression in response to noxious stimuli. Fungal hsp90 has been identified as a target for immunotherapy by a genetically recombinant antibody. The concept of combining this antibody fragment with an antifungal drug for treating life-threatening fungal infection and the potential interactions with human and microbial hsp90 and nitric oxide is discussed.

Group III histidine kinase is a positive regulator of Hog1-type mitogen-activated protein kinase in filamentous fungi

We previously reported that the group III histidine kinase Dic1p in the maize pathogen Cochliobolus heterostrophus is involved in resistance to dicarboximide and phenylpyrrole fungicides and in osmotic adaptation. In addition, exposure to the phenylpyrrole fungicide fludioxonil led to improper activation of Hog1-type mitogen-activated protein kinases (MAPKs) in some phytopathogenic fungi, including C. heterostrophus. Here we report, for the first time, the relationship between the group III histidine kinase and Hog1-related MAPK: group III histidine kinase is a positive regulator of Hog1-related MAPK in filamentous fungi. The phosphorylation pattern of C. heterostrophus BmHog1p (Hog1-type MAPK) was analyzed in wild-type and dic1-deficient strains by Western blotting. In the wild-type strain, phosphorylated BmHog1p was detected after exposure to both iprodione and fludioxonil at a concentration of 1 microg/ml. In the dic1-deficient strains, phosphorylated BmHog1p was not detected after exposure to 10 microg/ml of the fungicides. In response to osmotic stress (0.4 M KCl), a trace of phosphorylated BmHog1p was found in the dic1-deficient strains, whereas the band representing active BmHog1p was clearly detected in the wild-type strain. Similar results were obtained for Neurospora crassa Os-2p MAPK phosphorylation in the mutant of the group III histidine kinase gene os-1. These results indicate that group III histidine kinase positively regulates the activation of Hog1-type MAPKs in filamentous fungi. Notably, the Hog1-type MAPKs were activated at high fungicide (100 microg/ml) and osmotic stress (0.8 M KCl) levels in the histidine kinase mutants of both fungi, suggesting that another signaling pathway activates Hog1-type MAPKs in these conditions.

TmpA, a member of a novel family of putative membrane flavoproteins, regulates asexual development in Aspergillus nidulans

Asexual reproduction (conidiation) in Aspergillus nidulans is induced by environmental signals like exposure to air or nutrient starvation, and depends on brlA gene activation. The study of 'fluffy' mutants showing delayed asexual development and reduced brlA expression has defined the fluG pathway, involved in regulation of this differentiation process. Genetic characterization of a 'fluffy' mutant identified tmpA as a new gene involved in regulation of conidiation. TmpA defines a new family of putative transmembrane proteins of unknown function, widespread in filamentous fungi and plants, with homologues showing similarity to non-ribosomal peptide synthetases. The deletion of tmpA resulted in decreased brlA expression and conidiation in air-exposed colonies. This defect was suppressed when DeltatmpA mutants were grown next to wild-type or DeltafluG mutant colonies, even without direct contact between hyphae. In liquid culture, tmpA was essential for conidiation induced by nitrogen but not by carbon starvation, whereas the overexpression of different tmpA tagged alleles resulted in conidiation. The overexpression of fluG-induced conidiation independently of tmpA and DeltatmpADeltafluG double mutants showed an additive 'fluffy' phenotype, indicating that tmpA and fluG regulate asexual sporulation through different pathways. TmpA and its homologues appear to have diverged from the ferric reductase family, retaining overall transmembrane architecture, NAD(P), flavin adenine dinucleotide (FAD) and possibly haem-binding domains. Based on our results, we propose that TmpA is a membrane oxidoreductase involved in the synthesis of a developmental signal.