Transition Of Fungi Research Articles

A Temperature-Responsive Network Links Cell Shape and Virulence Traits in a Primary Fungal Pathogen

Author SummaryMicrobial pathogens of humans display the ability to thrive at host temperature. So-called “thermally dimorphic” fungal pathogens, which include Histoplasma capsulatum, are a class of soil fungi that upon being inhaled into the human lung, undergo dramatic changes in cell shape and virulence gene expression in response to host temperature. The ability of these pathogens to cause disease is exquisitely coupled to temperature response. Here we elucidate the regulatory network that governs the ability of H. capsulatum to switch from a filamentous form in the soil environment to a pathogenic yeast form at body temperature. The circuit is driven by three transcription regulators (Ryp1, Ryp2, and Ryp3) that control yeast-phase growth. We show that these factors, which include two highly conserved proteins of the Velvet family of unknown function, bind to specific regulatory DNA elements and directly regulate expression of virulence genes. We identify and characterize Ryp4, a fourth regulator of this pathway, and define DNA motifs that recruit these transcription factors to their temperature-responsive target genes. Our results provide a molecular understanding of how changes in cell shape are linked to expression of virulence genes in thermally dimorphic fungi.

Regulation of genes involved in cell wall synthesis and structure duringUstilago maydisdimorphism

The cell wall is the structure that provides the shape to fungal cells and protects them from the difference in osmotic pressure existing between the cytosol and the external medium. Accordingly, changes in structure and composition of the fungal wall must occur during cell differentiation, including the dimorphic transition of fungi. We analyzed, by use of microarrays, the transcriptional regulation of the 639 genes identified to be involved in cell wall synthesis and structure plus the secretome of the Basidiomycota species Ustilago maydis during its dimorphic transition induced by a change in pH. Of these, 189 were differentially expressed during the process, and using as control two monomorphic mutants, one yeast like and the other mycelium constitutive, 66 genes specific of dimorphism were identified. Most of these genes were up-regulated in the mycelial phase. These included CHS genes, genes involved in β-1,6-glucan synthesis, N-glycosylation, and proteins containing a residue of glycosylphosphatidylinositol, and a number of genes from the secretome. The possible significance of these data on cell wall plasticity is discussed.

Role of Hsl7 in Morphology and Pathogenicity and Its Interaction with Other Signaling Components in the Plant Pathogen Ustilago maydis

The phytopathogenic fungus Ustilago maydis undergoes a dimorphic transition in response to mating pheromone, host, and environmental cues. On a solid medium deficient in ammonium (SLAD [0.17% yeast nitrogen base without ammonium sulfate or amino acids, 2% dextrose, 50 μM ammonium sulfate]), U. maydis produces a filamentous colony morphology, while in liquid SLAD, the cells do not form filaments. The p21-activated protein kinases (PAKs) play a substantial role in regulating the dimorphic transition in fungi. The PAK-like Ste20 homologue Smu1 is required for a normal response to pheromone, via upregulation of pheromone expression, and virulence, and its disruption affects both processes. Our experiments suggest that Smu1 also regulates cell length and the filamentous response on solid SLAD medium. Yeast two-hybrid analysis suggested an Hsl7 homologue as a potential interacting partner of Smu1, and a unique open reading frame for such an arginine methyltransferase was detected in the U. maydis genome sequence. Hsl7 regulates cell length and the filamentous response to solid SLAD in a fashion opposite to that of Smu1, but neither overexpression nor disruption of hsl7 attenuates virulence. Simultaneous disruption of hsl7 and overexpression of smu1 lead to a hyperfilamentous response on solid SLAD. Moreover, only this double mutant strain forms filaments in liquid SLAD. The double mutant strain was also significantly reduced in virulence. A similar filamentous response in both solid and liquid SLAD was observed in strains lacking another PAK-like protein kinase involved in cytokinesis and polar growth, Cla4. Our data suggest that Hsl7 may regulate cell cycle progression, while both Smu1 and Cla4 appear to be involved in the filamentous response in U. maydis.

Horizontal transfer of a nitrate assimilation gene cluster and ecological transitions in fungi: a phylogenetic study.

High affinity nitrate assimilation genes in fungi occur in a cluster (fHANT-AC) that can be coordinately regulated. The clustered genes include nrt2, which codes for a high affinity nitrate transporter; euknr, which codes for nitrate reductase; and NAD(P)H-nir, which codes for nitrite reductase. Homologs of genes in the fHANT-AC occur in other eukaryotes and prokaryotes, but they have only been found clustered in the oomycete Phytophthora (heterokonts). We performed independent and concatenated phylogenetic analyses of homologs of all three genes in the fHANT-AC. Phylogenetic analyses limited to fungal sequences suggest that the fHANT-AC has been transferred horizontally from a basidiomycete (mushrooms and smuts) to an ancestor of the ascomycetous mold Trichoderma reesei. Phylogenetic analyses of sequences from diverse eukaryotes and eubacteria, and cluster structure, are consistent with a hypothesis that the fHANT-AC was assembled in a lineage leading to the oomycetes and was subsequently transferred to the Dikarya (Ascomycota+Basidiomycota), which is a derived fungal clade that includes the vast majority of terrestrial fungi. We propose that the acquisition of high affinity nitrate assimilation contributed to the success of Dikarya on land by allowing exploitation of nitrate in aerobic soils, and the subsequent transfer of a complete assimilation cluster improved the fitness of T. reesei in a new niche. Horizontal transmission of this cluster of functionally integrated genes supports the “selfish operon” hypothesis for maintenance of gene clusters.

Paracoccidioides brasiliensis and paracoccidioidomycosis: molecular approaches to morphogenesis, diagnosis, epidemiology, taxonomy and genetics.

Paracoccidioides brasiliensis is an amenable model to study the molecular and biochemical events that lead to morphological transition in fungi, because temperature seems to be the only factor regulating this process. It is the causative agent of paracoccidioidomycosis, a systemic mycosis that affects humans and that is geographically confined to Latin America, where it constitutes one of the most prevalent deep mycoses. With the help of molecular tools, events leading to the morphological transition have been traced to genes that control cell wall glucan and chitin syntheses, and other metabolic processes such as production of heat shock proteins and ornithine decarboxylase activity. Molecular diagnosis and epidemiology of paracoccidioidomycosis are also the focus of intensive research, with several primers being proposed as specific probes for clinical and field uses. Although P. brasiliensis is refractory to cytogenetic analysis, electrophoretic methods have allowed an approximation of its genomic organization and ploidy. Finally, the recognition of P. brasiliensis as an anamorph in the phylum Ascomycota, order Onygenales, family Onygenaceae, has been accomplished by means of molecular tools. This phylogenetic placement has revised the taxonomic position of this fungus, which was traditionally included within now-abandoned higher anamorph taxa, the phylum Deuteromycota and the class Hyphomycetes.

It infects me, it infects me not: phenotypic switching in the fungal pathogen Cryptococcus neoformans.

It infects me, it infects me not: phenotypic switching in the fungal pathogen Cryptococcus neoformans.

A nutrient-regulated, dual localization phospholipase A(2) in the symbiotic fungus Tuber borchii.

Important morphogenetic transitions in fungi are triggered by starvation-induced changes in the expression of structural surface proteins. Here, we report that nutrient deprivation causes a strong and reversible up-regulation of TbSP1, a surface-associated, Ca(2+)-dependent phospholipase from the mycorrhizal fungus Tuber borchii. TbSP1 is the first phospholipase A(2) to be described in fungi and identifies a novel class of phospholipid-hydrolyzing enzymes. The TbSP1 phospholipase, which is synthesized initially as a pre-protein, is processed efficiently and secreted during the mycelial phase. The mature protein, however, also localizes to the inner cell wall layer, close to the plasma membrane, in both free-living and symbiosis-engaged hyphae. It thus appears that a dual localization phospholipase A(2) is involved in the adaptation of a symbiotic fungus to conditions of persistent nutritional limitation. Moreover, the fact that TbSP1-related sequences are present in Streptomyces and Neurospora, and not in wholly sequenced non-filamentous microorganisms, points to a general role for TbSP1 phospholipases A(2) in the organization of multicellular filamentous structures in bacteria and fungi.