The MeaB bZIP transcription factor is needed for proper nitrosative stress response induced by nitrite in Aspergillus fumigatus.

Background: MeaB is a bZIP-type transcription factor in fungi. This protein is known to regulate nitrogen metabolism, contribute to nitrite susceptibility and determine virulence in aspergilli. We hypothesized that MeaB is required for proper nitric oxide (NO) metabolism of fungi. Here, we tested this hypothesis on the human pathogenic fungus Aspergillus fumigatus using a transcriptomics approach.

Results: Deletion of the meaB gene increased nitrite, diethylamine NONOate and menadione-sodium bisulfite stress sensitivity, but not that of terc-butyl hydroperoxide or H₂O₂. The conidia of the ΔmeaB mutant showed no altered susceptibility to human macrophages. When the transcriptome of the ΔmeaB strain was compared with the wild-type strain (Af293), genes involved in siderophore biosynthesis or glucanases were enriched in the up-regulated gene set, whereas genes encoding heme-binding proteins or chitinases were enriched in the down-regulated gene set. The 90 mM NaNO₂-induced stress elicited a response in the ΔmeaB gene deletion mutant that was very similar to that of the wild-type strain in the presence of 135 mM NaNO₂. These stress responses included the downregulation of mitotic cell cycle and ribosomal protein genes, and the upregulation of nitrosative stress response (fhpA, fhpB, gnoA), nitrate utilization (niaD, niiA), several iron acquisition and ergosterol biosynthesis genes, as well as the alternative oxidase gene aoxA. These stress treatments also altered the transcriptional activity of secondary metabolite cluster and carbohydrate-active enzyme (CAZyme) genes. Nitrite treatment upregulated arginine metabolism genes only in the wild-type strain. The observed transcriptional changes were associated with reduced growth, increased redox imbalance, increased sterol content in both strains and increased nitrite sensitivity of the ΔmeaB mutant on arginine as sole C and N source.

Conclusions: Transcriptomic data implies that MeaB affected fine-tuned regulation of arginine metabolism genes, and the alteration in arginine dependent processes (including siderophore production and possibly NO homeostasis) was responsible for the altered phenotype of the ΔmeaB mutant. Our results also suggest that, although inhibition of A. fumigatus defense against nitrosative stress may not be an effective antifungal therapy for all A. fumigatus strains, a combined approach based on disruption of both iron and NO homeostasis is promising.