Natural products in the predatory defence of the filamentous fungal pathogen Aspergillus fumigatus.

The kingdom of fungi comprises a large and highly diverse group of organisms that thrive in diverse natural environments. One factor to successfully confront challenges in their natural habitats is the capability to synthesize defensive secondary metabolites. The genetic potential for the production of secondary metabolites in fungi is high and numerous potential secondary metabolite gene clusters have been identified in sequenced fungal genomes. Their production may well be regulated by specific ecological conditions, such as the presence of microbial competitors, symbionts or predators. Here we exemplarily summarize our current knowledge on identified secondary metabolites of the pathogenic fungus Aspergillus fumigatus and their defensive function against (microbial) predators.

Peroxiredoxin Asp f3 Is Essential for Aspergillus fumigatus To Overcome Iron Limitation during Infection.

Aspergillus fumigatus is an important fungal pathogen that causes allergic reactions but also life-threatening infections. One of the most abundant A. fumigatus proteins is Asp f3. This peroxiredoxin is a major fungal allergen and known for its role as a virulence factor, vaccine candidate, and scavenger of reactive oxygen species. Based on the hypothesis that Asp f3 protects A. fumigatus against killing by immune cells, we investigated the susceptibility of a conditional aspf3 mutant by employing a novel assay. Surprisingly, Asp f3-depleted hyphae were killed as efficiently as the wild type by human granulocytes. However, we identified an unexpected growth defect of mutants that lack Asp f3 under low-iron conditions, which explains the avirulence of the Δaspf3 deletion mutant in a murine infection model. A. fumigatus encodes two Asp f3 homologues which we named Af3l (Asp f3-like) 1 and Af3l2. Inactivation of Af3l1, but not of Af3l2, exacerbated the growth defect of the conditional aspf3 mutant under iron limitation, which ultimately led to death of the double mutant. Inactivation of the iron acquisition repressor SreA partially compensated for loss of Asp f3 and Af3l1. However, Asp f3 was not required for maintaining iron homeostasis or siderophore biosynthesis. Instead, we show that it compensates for a loss of iron-dependent antioxidant enzymes. Iron supplementation restored the virulence of the Δaspf3 deletion mutant in a murine infection model. Our results unveil the crucial importance of Asp f3 to overcome nutritional immunity and reveal a new biological role of peroxiredoxins in adaptation to iron limitation. IMPORTANCE Asp f3 is one of the most abundant proteins in the pathogenic mold Aspergillus fumigatus. It has an enigmatic multifaceted role as a fungal allergen, virulence factor, reactive oxygen species (ROS) scavenger, and vaccine candidate. Our study provides new insights into the cellular role of this conserved peroxiredoxin. We show that the avirulence of a Δaspf3 mutant in a murine infection model is linked to a low-iron growth defect of this mutant, which we describe for the first time. Our analyses indicated that Asp f3 is not required for maintaining iron homeostasis. Instead, we found that Asp f3 compensates for a loss of iron-dependent antioxidant enzymes. Furthermore, we identified an Asp f3-like protein which is partially functionally redundant with Asp f3. We highlight an unexpected key role of Asp f3 and its partially redundant homologue Af3l1 in overcoming the host's nutritional immunity. In addition, we uncovered a new biological role of peroxiredoxins.

Yeast two-hybrid screening reveals a dual function for the histone acetyltransferase GcnE by controlling glutamine synthesis and development in Aspergillus fumigatus.

The acetyltransferase GcnE is part of the SAGA complex which regulates fungal gene expression through acetylation of chromatin. Target genes of the histone acetyltransferase GcnE include those involved in secondary metabolism and asexual development. Here, we show that the absence of GcnE not only abrogated conidiation, but also strongly impeded vegetative growth of hyphae in the human pathogenic fungus Aspergillus fumigatus. A yeast two-hybrid screen using a Saccharomyces cerevisiae strain whose tRNA molecules were specifically adapted to express A. fumigatus proteins identified two unprecedented proteins that directly interact with GcnE. Glutamine synthetase GlnA as well as a hypothetical protein located on chromosome 8 (GbpA) were identified as binding partners of GcnE and their interaction was confirmed in vivo via bimolecular fluorescence complementation. Phenotypic characterization of gbpA and glnA deletion mutants revealed a role for GbpA during conidiogenesis and confirmed the central role of GlnA in glutamine biosynthesis. The increase of glutamine synthetase activity in the absence of GcnE indicated that GcnE silences GlnA through binding. This finding suggests an expansion of the regulatory role of GcnE in A. fumigatus.

Nitrate Assimilation in Fusarium fujikuroi Is Controlled by Multiple Levels of Regulation.

Secondary metabolite production of the phytopathogenic ascomycete fungus Fusarium fujikuroi is greatly influenced by the availability of nitrogen. While favored nitrogen sources such as glutamine and ammonium are used preferentially, the uptake and utilization of nitrate is subject to a regulatory mechanism called nitrogen metabolite repression (NMR). In Aspergillus nidulans, the transcriptional control of the nitrate assimilatory system is carried out by the synergistic action of the nitrate-specific transcription factor NirA and the major nitrogen-responsive regulator AreA. In this study, we identified the main components of the nitrate assimilation system in F. fujikuroi and studied the role of each of them regarding the regulation of the remaining components. We analyzed mutants with deletions of the nitrate-specific activator NirA, the nitrate reductase (NR), the nitrite reductase (NiR) and the nitrate transporter NrtA. We show that NirA controls the transcription of the nitrate assimilatory genes NIAD, NIIA, and NRTA in the presence of nitrate, and that the global nitrogen regulator AreA is obligatory for expression of most, but not all NirA target genes (NIAD). By transforming a NirA-GFP fusion construct into the ΔNIAD, ΔNRTA, and ΔAREA mutant backgrounds we revealed that NirA was dispersed in the cytosol when grown in the presence of glutamine, but rapidly sorted to the nucleus when nitrate was added. Interestingly, the rapid and nitrate-induced nuclear translocation of NirA was observed also in the ΔAREA and ΔNRTA mutants, but not in ΔNIAD, suggesting that the fungus is able to directly sense nitrate in an AreA- and NrtA-independent, but NR-dependent manner.