Beyond asexual development: modifications in the gene expression profile caused by the absence of the Aspergillus nidulans transcription factor FlbB

The role of the Flb protein family in the life cycle of Aspergillus niger

Genes flbA-E are involved in sporulation and vegetative growth in Aspergillus nidulans. Inactivation of either of these genes results in a fluffy phenotype with delayed or even abolished sporulation. Previously, a non-sporulating phenotype was obtained by inactivating flbA in Aspergillus niger, which was accompanied by lysis, thinner cell walls, and an increased secretome complexity. Here, we further studied the role of the flb genes of A. niger. Strains ΔflbA, ΔflbB and ΔflbE showed increased biomass formation, while inactivation of flbA-D reduced, or even abolished, formation of conidia. Strain ΔflbA was more sensitive to H2O2, DTT, and the cell wall integrity stress compounds SDS and Congo Red (CR). Also, ΔflbC was more sensitive to SDS, while ΔflbB, ΔflbD, and ΔflbE were more sensitive to CR. On the other hand, inactivation of flbE increased resistance to H2O2. Enzyme secretion was impacted when the Δflb strains were grown on xylose. Strain ΔflbE showed reduced xylanase, cellulase and amylase secretion. On the other hand, amylase secretion at the periphery of the ΔflbA colony was reduced but not in its center, while secretion of this enzyme was increased in the center of the ΔflbB colony but not at its periphery. Inactivation of flbC and flbD also impacted zonal cellulase and amylase activity. Together, the Flb protein family of A. niger function in biomass formation, sporulation, stress response, and protein secretion.

Identification and functional characterization of the putative members of the CTDK-1 kinase complex as regulators of growth and development in Aspergillus nidulans and Aspergillus fumigatus

IMPORTANCE

Aspergillus fumigatus has been included by the World Health Organization in the priority list of fungal pathogens because (i) it causes 90% of invasive aspergillosis cases, with a high mortality rate, and (ii) infections are becoming increasingly resistant to azole antifungals. A. nidulans is an opportunistic pathogen and a saprotroph which has served during the last 80 years as a reference system for filamentous fungi. Here, we characterized the role in morphogenesis and development of the putative transcriptional cyclin/kinase complex CTDK-1 in both aspergilli. The null mutants of the corresponding genes showed delayed germination, aberrant conidiophore development, and inhibition of cleistothecia production. While in higher eukaryotes this complex is formed only by a cyclin and a kinase, the fungal complex would incorporate a fungal-specific third component, FlpB, which would enable the interaction between the kinase (Stk47) and the cyclin (FlpA) and may be used as a target for antifungals.

bHLH transcription factor EcdR controls conidia production, pigmentation and virulence in Aspergillus fumigatus

Invasive Aspergillus fumigatus infection is a disease with high morbidity and mortality rates. Abnormalities in sporulation and pigmentation can significantly alter the pathogenicity of A. fumigatus, thus the mechanisms of conidiation and pigment biosynthesis have gained increasing attention. In Aspergillus oryzae, a novel predicted bHLH protein-encoding gene, ecdR, plays a role in asexual development, and its ortholog has also been characterized in A. nidulans. Herein, we determined its role in A. fumigatus by testing whether ecdR deletion affects asexual development, melanin synthesis, and regulation of virulence in this fungus. Our study shows that EcdR controls conidia and melanin production in A. fumigatus. In addition, we found that virulence in the ΔecdR strain was significantly reduced in the infection model of immunodeficiency mice.

The Role of Chromatin and Transcriptional Control in the Formation of Sexual Fruiting Bodies in Fungi

Fungal fruiting bodies are complex, three-dimensional structures that arise from a less complex vegetative mycelium. Their formation requires the coordinated action of many genes and their gene products, and fruiting body formation is accompanied by major changes in the transcriptome. In recent years, numerous transcription factor genes as well as chromatin modifier genes that play a role in fruiting body morphogenesis were identified, and through research on several model organisms, the underlying regulatory networks that integrate chromatin structure, gene expression, and cell differentiation are becoming clearer. This review gives a summary of the current state of research on the role of transcriptional control and chromatin structure in fruiting body development. In the first part, insights from transcriptomics analyses are described, with a focus on comparative transcriptomics. In the second part, examples of more detailed functional characterizations of the role of chromatin modifiers and/or transcription factors in several model organisms (Neurospora crassa, Aspergillus nidulans, Sordaria macrospora, Coprinopsis cinerea, and Schizophyllum commune) that have led to a better understanding of regulatory networks at the level of chromatin structure and transcription are discussed.

RimO (SrrB) is required for carbon starvation signaling and production of secondary metabolites in Aspergillus nidulans

Depending on the prevailing environmental, developmental and nutritional conditions, fungi activate biosynthetic gene clusters (BGCs) to produce condition-specific secondary metabolites (SMs). For activation, global chromatin-based de-repression must be integrated with pathway-specific induction signals. Here we describe a new global regulator needed to activate starvation-induced SMs. In our transcriptome dataset, we found locus AN7572 strongly transcribed solely under conditions of starvation-induced SM production. The predicted AN7572 protein is most similar to the stress and nutritional regulator Rim15 of Saccharomyces cerevisiae, and to STK-12 of Neurospora crassa. Based on this similarity and on stress and nutritional response phenotypes of A. nidulans knock-out and overexpression strains, AN7572 is designated rimO. In relation to SM production, we found that RimO is required for the activation of starvation-induced BGCs, including the sterigmatocystin (ST) gene cluster. Here, RimO regulates the pathway-specific transcription factor AflR both at the transcriptional and post-translational level. At the transcriptional level, RimO mediates aflR induction following carbon starvation and at the post-translational level, RimO is required for nuclear accumulation of the AflR protein. Genome-wide transcriptional profiling showed that cells lacking rimO fail to adapt to carbon starvation that, in the wild type, leads to down-regulation of genes involved in basic metabolism, membrane biogenesis and growth. Consistently, strains overexpressing rimO are more resistant to oxidative and osmotic stress, largely insensitive to glucose repression and strongly overproduce several SMs. Our data indicate that RimO is a positive regulator within the SM and stress response network, but this requires nutrient depletion that triggers both, rimO gene transcription and activation of the RimO protein.

Systematic Characterization of bZIP Transcription Factors Required for Development and Aflatoxin Generation by High-Throughput Gene Knockout in Aspergillus flavus

The basic leucine zipper (bZIP) is an important transcription factor required for fungal development, nutrient utilization, biosynthesis of secondary metabolites, and defense against various stresses. Aspergillus flavus is a major producer of aflatoxin and an opportunistic fungus on a wide range of hosts. However, little is known about the role of most bZIP genes in A. flavus. In this study, we developed a high-throughput gene knockout method based on an Agrobacterium-mediated transformation system. Gene knockout construction by yeast recombinational cloning and screening of the null mutants by double fluorescence provides an efficient way to construct gene-deleted mutants for this multinucleate fungus. We deleted 15 bZIP genes in A. flavus. Twelve of these genes were identified and characterized in this strain for the first time. The phenotypic analysis of these mutants showed that the 15 bZIP genes play a diverse role in mycelial growth (eight genes), conidiation (13 genes), aflatoxin biosynthesis (10 genes), oxidative stress response (11 genes), cell wall stress (five genes), osmotic stress (three genes), acid and alkali stress (four genes), and virulence to kernels (nine genes). Impressively, all 15 genes were involved in the development of sclerotia, and the respective deletion mutants of five of them did not produce sclerotia. Moreover, MetR was involved in this biological process. In addition, HapX and MetR play important roles in the adaptation to excessive iron and sulfur metabolism, respectively. These studies provide comprehensive insights into the role of bZIP transcription factors in this aflatoxigenic fungus of global significance.

Biosynthesis of Antibacterial Iron-Chelating Tropolones in Aspergillus nidulans as Response to Glycopeptide-Producing Streptomycetes

The soil microbiome comprises numerous filamentous fungi and bacteria that mutually react and challenge each other by the production of bioactive secondary metabolites. Herein, we show in liquid co-cultures that the presence of filamentous Streptomycetes producing antifungal glycopeptide antibiotics induces the production of the antibacterial and iron-chelating tropolones anhydrosepedonin (1) and antibiotic C (2) in the mold Aspergillus nidulans. Additionally, the biosynthesis of the related polyketide tripyrnidone (5) was induced, whose novel tricyclic scaffold we elucidated by NMR and HRESIMS data. The corresponding biosynthetic polyketide synthase-encoding gene cluster responsible for the production of these compounds was identified. The tropolones as well as tripyrnidone (5) are produced by genes that belong to the broad reservoir of the fungal genome for the synthesis of different secondary metabolites, which are usually silenced under standard laboratory conditions. These molecules might be part of the bacterium-fungus competition in the complex soil environment, with the bacterial glycopeptide antibiotic as specific environmental trigger for fungal induction of this cluster.

Role of AcndtA in cleistothecium formation, osmotic stress response, pigmentation and carbon metabolism of Aspergillus cristatus

As the dominant fungus during the fermentation of Fuzhuan brick tea, Aspergillus cristatus is easily induced to undergo a sexual cycle under low-salt stress. However, the underlying regulatory mechanism of sexual reproduction is unclear. Here, we report a P53-like transcription factor AcndtA, which encodes an NDT80 DNA binding protein and regulates fungal reproduction, pigmentation and the stress response. Both insertion and deletion mutants of AcndtA exhibited a complete blockade of cleistothecium formation, and overexpressing AcndtA strains (OE: AcndtA) exhibited significantly reduced cleistothecium production, indicating that AcndtA plays a vital role in sexual development. Osmotic stress tests showed that overexpression of AcndtA had a negative impact on growth and conidia production. Additionally, AcndtA insertion, deletion and overexpression mutants exhibited reduced pigment formation. All the above developmental defects were reversed by the re-introduction of the AcndtA gene in ΔAcndtA. Moreover, the growth of AcndtA mutants in carbon-limited medium was better than that of the WT and OE: AcndtA strains, indicating that AcndtA is involved in carbon metabolism. Transcriptional profiling data showed that AcndtA regulated the expression of several genes related to development, osmotic stress and carbon metabolism.

Transcription Factors in the Fungus Aspergillus nidulans: Markers of Genetic Innovation, Network Rewiring and Conflict between Genomics and Transcriptomics

Gene regulatory networks (GRNs) are shaped by the democratic/hierarchical relationships among transcription factors (TFs) and associated proteins, together with the cis-regulatory sequences (CRSs) bound by these TFs at target promoters. GRNs control all cellular processes, including metabolism, stress response, growth and development. Due to the ability to modify morphogenetic and developmental patterns, there is the consensus view that the reorganization of GRNs is a driving force of species evolution and differentiation. GRNs are rewired through events including the duplication of TF-coding genes, their divergent sequence evolution and the gain/loss/modification of CRSs. Fungi (mainly Saccharomycotina) have served as a reference kingdom for the study of GRN evolution. Here, I studied the genes predicted to encode TFs in the fungus Aspergillus nidulans (Pezizomycotina). The analysis of the expansion of different families of TFs suggests that the duplication of TFs impacts the species level, and that the expansion in Zn2Cys6 TFs is mainly due to dispersed duplication events. Comparison of genomic annotation and transcriptomic data suggest that a significant percentage of genes should be re-annotated, while many others remain silent. Finally, a new regulator of growth and development is identified and characterized. Overall, this study establishes a novel theoretical framework in synthetic biology, as the overexpression of silent TF forms would provide additional tools to assess how GRNs are rewired.

A Homeodomain-Containing Transcriptional Factor PoHtf1 Regulated the Development and Cellulase Expression in Penicillium oxalicum

Homeodomain-containing transcription factors (Htfs) play important roles in animals, fungi, and plants during some developmental processes. Here, a homeodomain-containing transcription factor PoHtf1 was functionally characterized in the cellulase-producing fungi Penicillium oxalicum 114-2. PoHtf1 was shown to participate in colony growth and conidiation through regulating the expression of its downstream transcription factor BrlA, the key regulator of conidiation in P. oxalicum 114-2. Additionally, PoHtf1 inhibited the expression of the major cellulase genes by coordinated regulation of cellulolytic regulators CreA, AmyR, ClrB, and XlnR. Furthermore, transcriptome analysis showed that PoHtf1 participated in the secondary metabolism including the pathway synthesizing conidial yellow pigment. These data show that PoHtf1 mediates the complex transcriptional-regulatory network cascade between developmental processes and cellulolytic gene expression in P. oxalicum 114-2. Our results should assist the development of strategies for the metabolic engineering of mutants for applications in the enzymatic hydrolysis for biochemical production.

Defining the transcriptional responses of Aspergillus nidulans to cation/alkaline pH stress and the role of the transcription factor SltA

Fungi have developed the ability to overcome extreme growth conditions and thrive in hostile environments. The model fungus Aspergillus nidulans tolerates, for example, ambient alkalinity up to pH 10 or molar concentrations of multiple cations. The ability to grow under alkaline pH or saline stress depends on the effective function of at least three regulatory pathways mediated by the zinc-finger transcription factor PacC, which mediates the ambient pH regulatory pathway, the calcineurin-dependent CrzA and the cation homeostasis responsive factor SltA. Using RNA sequencing, we determined the effect of external pH alkalinization or sodium stress on gene expression. The data show that each condition triggers transcriptional responses with a low degree of overlap. By sequencing the transcriptomes of the null mutant, the role of SltA in the above-mentioned homeostasis mechanisms was also studied. The results show that the transcriptional role of SltA is wider than initially expected and implies, for example, the positive control of the PacC-dependent ambient pH regulatory pathway. Overall, our data strongly suggest that the stress response pathways in fungi include some common but mostly exclusive constituents, and that there is a hierarchical relationship among the main regulators of stress response, with SltA controlling pacC expression, at least in A. nidulans.

Identification and Characterization of Aspergillus nidulans Mutants Impaired in Asexual Development under Phosphate Stress

The transcription factor BrlA plays a central role in the production of asexual spores (conidia) in the fungus Aspergillus nidulans. BrlA levels are controlled by signal transducers known collectively as UDAs. Furthermore, it governs the expression of CDP regulators, which control most of the morphological transitions leading to the production of conidia. In response to the emergence of fungal cells in the air, the main stimulus triggering conidiation, UDA mutants such as the flbB deletant fail to induce brlA expression. Nevertheless, ΔflbB colonies conidiate profusely when they are cultured on a medium containing high H₂PO₄⁻ concentrations, suggesting that the need for FlbB activity is bypassed. We used this phenotypic trait and an UV-mutagenesis procedure to isolate ΔflbB mutants unable to conidiate under these stress conditions. Transformation of mutant FLIP166 with a wild-type genomic library led to the identification of the putative transcription factor SocA as a multicopy suppressor of the FLIP (Fluffy, aconidial, In Phosphate) phenotype. Deregulation of socA altered both growth and developmental patterns. Sequencing of the FLIP166 genome enabled the identification and characterization of PmtC^P282L as the recessive mutant form responsible for the FLIP phenotype. Overall, results validate this strategy for identifying genes/mutations related to the control of conidiation.

Rewiring of transcriptional networks as a major event leading to the diversity of asexual multicellularity in fungi

Complex multicellularity (CM) is characterized by the generation of three-dimensional structures that follow a genetically controlled program. CM emerged at least five times in evolution, one of them in fungi. There are two types of CM programs in fungi, leading, respectively, to the formation of sexual or asexual spores. Asexual spores foment the spread of mycoses, as they are the main vehicle for dispersion. In spite of this key dependence, there is great morphological diversity of asexual multicellular structures in fungi. To advance the understanding of the mechanisms that control initiation and progression of asexual CM and how they can lead to such a remarkable morphological diversification, we studied 503 fungal proteomes, representing all phyla and subphyla, and most known classes. Conservation analyses of 33 regulators of asexual development suggest stepwise emergence of transcription factors. While velvet proteins constitute one of the most ancient systems, the central regulator BrlA emerged late in evolution (with the class Eurotiomycetes). Some factors, such as MoConX4, seem to be species-specific. These observations suggest that the emergence and evolution of transcriptional regulators rewire transcriptional networks. This process could reach the species level, resulting in a vast diversity of morphologies.

UrdA Controls Secondary Metabolite Production and the Balance between Asexual and Sexual Development in Aspergillus nidulans

The genus Aspergillus includes important plant pathogens, opportunistic human pathogens and mycotoxigenic fungi. In these organisms, secondary metabolism and morphogenesis are subject to a complex genetic regulation. Here we functionally characterized urdA, a gene encoding a putative helix-loop-helix (HLH)-type regulator in the model fungus Aspergillus nidulans. urdA governs asexual and sexual development in strains with a wild-type veA background; absence of urdA resulted in severe morphological alterations, with a significant reduction of conidial production and an increase in cleistothecial formation, even in the presence of light, a repressor of sex. The positive effect of urdA on conidiation is mediated by the central developmental pathway (CDP). However, brlA overexpression was not sufficient to restore wild-type conidiation in the ΔurdA strain. Heterologous complementation of ΔurdA with the putative Aspergillus flavus urdA homolog also failed to rescue conidiation wild-type levels, indicating that both genes perform different functions, probably reflected by key sequence divergence. UrdA also represses sterigmatocystin (ST) toxin production in the presence of light by affecting the expression of aflR, the activator of the ST gene cluster. Furthermore, UrdA regulates the production of several unknown secondary metabolites, revealing a broader regulatory scope. Interestingly, UrdA affects the abundance and distribution of the VeA protein in hyphae, and our genetics studies indicated that veA appears epistatic to urdA regarding ST production. However, the distinct fluffy phenotype of the ΔurdAΔveA double mutant suggests that both regulators conduct independent developmental roles. Overall, these results suggest that UrdA plays a pivotal role in the coordination of development and secondary metabolism in A. nidulans.

The role of the veA gene in adjusting developmental balance and environmental stress response in Aspergillus cristatus

veA belongs to the velvet regulatory system that regulates the development and secondary metabolism of many fungi. To identify the function of veA in Aspergillus cristatus, veA deletion mutants were constructed by homologous recombination via Agrobacterium tumefaciens-mediated transformation. Deletion of veA led to increased conidial production and reduced sexual sporulation. The regulatory role of veA in A. cristatus was not light-dependent, and this differed from its role in other Aspergilli. Furthermore, veA deletion mutants were more sensitive to environmental stressors, including salt, osmotic pressure, temperature and pH. In contrast, deletion of veA resulted in increased resistance to oxidative stress. veA also affected aerial vegetative growth. Transcriptomic analysis of the veA-null mutant and wild type indicated that most asexual and sexual development genes were upregulated and downregulated, respectively. These findings confirmed that veA has a positive effect on sexual development but represses conidial formation. Overall, these results suggested that the veA gene plays a critical role in maintaining a developmental balance between asexual and sexual sporulation and is involved in vegetative growth and environmental stress response in A. cristatus.

Neurons show the path: tip-to-nucleus communication in filamentous fungal development and pathogenesis

Multiple fungal species penetrate substrates and accomplish host invasion through the fast, permanent and unidirectional extension of filamentous cells known as hyphae. Polar growth of hyphae results, however, in a significant increase in the distance between the polarity site, which also receives the earliest information about ambient conditions, and nuclei, where adaptive responses are executed. Recent studies demonstrate that these long distances are overcome by signal transduction pathways which convey sensory information from the polarity site to nuclei, controlling development and pathogenesis. The present review compares the striking connections of the mechanisms for long-distance communication in hyphae with those from neurons, and discusses the importance of their study in order to understand invasion and dissemination processes of filamentous fungi, and design strategies for developmental control in the future.

Boron Tolerance in Aspergillus nidulans Is Sustained by the SltA Pathway Through the SLC-Family Transporters SbtA and SbtB

Microbial cells interact with the environment by adapting to external changes. Signal transduction pathways participate in both sensing and responding in the form of modification of gene expression patterns, enabling cell survival. The filamentous fungal-specific SltA pathway regulates tolerance to alkalinity, elevated cation concentrations and, as shown in this work, also stress conditions induced by borates. Growth of sltA⁻ mutants is inhibited by increasing millimolar concentrations of boric acid or borax (sodium tetraborate). In an attempt to identify genes required for boron-stress response, we determined the boric acid or borax-dependent expression of sbtA and sbtB, orthologs of Saccharomyces cerevisiae bor1, and a reduction in their transcript levels in a ΔsltA mutant. Deletion of sbtA, but mainly that of sbtB, decreased the tolerance to boric acid or borax. In contrast, null mutants of genes coding for additional transporters of the Solute Carrier (SLC) family, sB, sbtD or sbtE, showed an unaltered growth pattern under the same stress conditions. Taken together, our results suggest that the SltA pathway induces, through SbtA and SbtB, the export of toxic concentrations of borates, which have largely recognized antimicrobial properties.

Apical control of conidiation in Aspergillus nidulans

The infection cycle of filamentous fungi consists of two main stages: invasion (growth) and dispersion (development). After the deposition of a spore on a host, germination, polar extension and branching of vegetative cells called hyphae allow a fast and efficient invasion. Under suboptimal conditions, genetic reprogramming of hyphae results in the generation of asexual spores, allowing dissemination to new hosts and the beginning of a new infection cycle. In the model filamentous fungus Aspergillus nidulans, asexual development or conidiation is induced by the upstream developmental activation (UDA) pathway. UDA proteins transduce signals from the tip, the polarity site of hyphae, to nuclei, where developmental programs are transcriptionally activated. The present review summarizes the current knowledge on this tip-to-nucleus communication mechanism, emphasizing its dependence on hyphal polarity. Future approaches to the topic will also be suggested, as stimulating elements contributing to the understanding of how apical signals are coupled with the transcriptional control of development and pathogenesis in filamentous fungi.

Tip-to-nucleus migration dynamics of the asexual development regulator FlbB in vegetative cells

In Aspergillus nidulans, asexual differentiation requires the presence of the transcription factor FlbB at the cell tip and apical nuclei. Understanding the relationship between these two pools is crucial for elucidating the biochemical processes mediating conidia production. Tip-to-nucleus communication was demonstrated by photo-convertible FlbB::Dendra2 visualization. Tip localization of FlbB depends on Cys382 in the C-terminus and the bZIP DNA-binding domain in the N-terminus. FlbE, a critical FlbB interactor, binds the bZIP domain. Furthermore, the absence of FlbE results in loss of tip localization but not nuclear accumulation. flbE deletion also abrogates transcriptional activity indicating that FlbB gains transcriptional competence from interactions with FlbE at the tip. Finally, a bipartite nuclear localization signal is required for nuclear localization of FlbB. Those motifs of FlbB may play various roles in the sequence of events necessary for the distribution and activation of this transcriptionally active developmental factor. The tip accumulation, FlbE-dependent activation, transport and nuclear import sketch out a process of relaying an environmentally triggered signal from the tip to the nuclei. As the first known instance of transcription factor-mediated tip-to-nucleus communication in filamentous fungi, this provides a general framework for analyses focused on elucidating the set of molecular mechanisms coupling apical signals to transcriptional events.

A second component of the SltA-dependent cation tolerance pathway in Aspergillus nidulans

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SltB is a novel component of the cation stress responsive pathway.

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Loss of SltB function results in sensitivity to elevated extracellular concentrations of cations and to alkalinity.

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SltB is involved in signaling to transcription factor SltA.

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SltA regulates expression of sltB.

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The Slt pathway is unique to fungi from the pezizomycotina subphylum.

The transcriptional response to alkali metal cation stress is mediated by the zinc finger transcription factor SltA in Aspergillus nidulans and probably in other fungi of the pezizomycotina subphylum. A second component of this pathway has been identified and characterized. SltB is a 1272 amino acid protein with at least two putative functional domains, a pseudo-kinase and a serine-endoprotease, involved in signaling to the transcription factor SltA. Absence of SltB activity results in nearly identical phenotypes to those observed for a null sltA mutant. Hypersensitivity to a variety of monovalent and divalent cations, and to medium alkalinization are among the phenotypes exhibited by a null sltB mutant. Calcium homeostasis is an exception and this cation improves growth of sltΔ mutants. Moreover, loss of kinase HalA in conjunction with loss-of-function sltA or sltB mutations leads to pronounced calcium auxotrophy. sltA sltB double null mutants display a cation stress sensitive phenotype indistinguishable from that of single slt mutants showing the close functional relationship between these two proteins. This functional relationship is reinforced by the fact that numerous mutations in both slt loci can be isolated as suppressors of poor colonial growth resulting from certain null vps (vacuolar protein sorting) mutations. In addition to allowing identification of sltB, our sltB missense mutations enabled prediction of functional regions in the SltB protein. Although the relationship between the Slt and Vps pathways remains enigmatic, absence of SltB, like that of SltA, leads to vacuolar hypertrophy. Importantly, the phenotypes of selected sltA and sltB mutations demonstrate that suppression of null vps mutations is not dependent on the inability to tolerate cation stress. Thus a specific role for both SltA and SltB in the VPS pathway seems likely. Finally, it is noteworthy that SltA and SltB have a similar, limited phylogenetic distribution, being restricted to the pezizomycotina subphylum. The relevance of the Slt regulatory pathway to cell structure, intracellular trafficking and cation homeostasis and its restricted phylogenetic distribution makes this pathway of general interest for future investigation and as a source of targets for antifungal drugs.