γ-Glutamyl transpeptidase (GgtA) of Aspergillus nidulans is not necessary for bulk degradation of glutathione

Stress Responses Elicited by Glucose Withdrawal in Aspergillus fumigatus

Glucose is a widely used carbon source in laboratory practice to culture Aspergillus fumigatus, however, glucose availability is often low in its “natural habitats”, including the human body. We used a physiological−transcriptomical approach to reveal differences between A. fumigatus Af293 cultures incubated on glucose, glucose and peptone, peptone (carbon limitation), or without any carbon source (carbon starvation). Autolytic cell wall degradation was upregulated by both carbon starvation and limitation. The importance of autolytic cell wall degradation in the adaptation to carbon stress was also highlighted by approximately 12.4% of the A. fumigatus genomes harboring duplication of genes involved in N-acetyl glucosamine utilization. Glucose withdrawal increased redox imbalance, altered both the transcription of antioxidative enzyme genes and oxidative stress tolerance, and downregulated iron acquisition, but upregulated heme protein genes. Transcriptional activity of the Gliotoxin cluster was low in all experiments, while the Fumagillin cluster showed substantial activity both on glucose and under carbon starvation, and the Hexadehydro-astechrome cluster only on glucose. We concluded that glucose withdrawal substantially modified the physiology of A. fumigatus, including processes that contribute to virulence. This may explain the challenge of predicting the in vivo behavior of A. fumigatus based on data from glucose rich cultures.

Strategies Shaping the Transcription of Carbohydrate-Active Enzyme Genes in Aspergillus nidulans

Understanding the coordinated regulation of the hundreds of carbohydrate-active enzyme (CAZyme) genes occurring in the genomes of fungi has great practical importance. We recorded genome-wide transcriptional changes of Aspergillus nidulans cultivated on glucose, lactose, or arabinogalactan, as well as under carbon-starved conditions. We determined both carbon-stress-specific changes (weak or no carbon source vs. glucose) and carbon-source-specific changes (one type of culture vs. all other cultures). Many CAZyme genes showed carbon-stress-specific and/or carbon-source-specific upregulation on arabinogalactan (138 and 62 genes, respectively). Besides galactosidase and arabinan-degrading enzyme genes, enrichment of cellulolytic, pectinolytic, mannan, and xylan-degrading enzyme genes was observed. Fewer upregulated genes, 81 and 107 carbon stress specific, and 6 and 16 carbon source specific, were found on lactose and in carbon-starved cultures, respectively. They were enriched only in galactosidase and xylosidase genes on lactose and rhamnogalacturonanase genes in both cultures. Some CAZyme genes (29 genes) showed carbon-source-specific upregulation on glucose, and they were enriched in β-1,4-glucanase genes. The behavioral ecological background of these characteristics was evaluated to comprehensively organize our knowledge on CAZyme production, which can lead to developing new strategies to produce enzymes for plant cell wall saccharification.

The DUG Pathway Governs Degradation of Intracellular Glutathione in Aspergillus nidulans

Glutathione (GSH) is an abundant tripeptide that plays a crucial role in shielding cellular macromolecules from various reactive oxygen and nitrogen species in fungi. Understanding GSH metabolism is of vital importance for deciphering redox regulation in these microorganisms. In the present study, to better understand the GSH metabolism in filamentous fungi, we investigated functions of the dugB and dugC genes in the model fungus Aspergillus nidulans These genes are orthologues of dug2 and dug3, which are involved in cytosolic GSH degradation in Saccharomyces cerevisiae The deletion of dugB, dugC, or both resulted in a moderate increase in the GSH content in mycelia grown on glucose, reduced conidium production, and disturbed sexual development. In agreement with these observations, transcriptome data showed that genes encoding mitogen-activated protein (MAP) kinase pathway elements (e.g., steC, sskB, hogA, and mkkA) or regulatory proteins of conidiogenesis and sexual differentiation (e.g., flbA, flbC, flbE, nosA, rosA, nsdC, and nsdD) were downregulated in the ΔdugB ΔdugC mutant. Deletion of dugB and/or dugC slowed the depletion of GSH pools during carbon starvation. It also reduced accumulation of reactive oxygen species and decreased autolytic cell wall degradation and enzyme secretion but increased sterigmatocystin formation. Transcriptome data demonstrated that enzyme secretions-in contrast to mycotoxin production-were controlled at the posttranscriptional level. We suggest that GSH connects starvation and redox regulation to each other: cells utilize GSH as a stored carbon source during starvation. The reduction of GSH content alters the redox state, activating regulatory pathways responsible for carbon starvation stress responses.IMPORTANCE Glutathione (GSH) is a widely distributed tripeptide in both eukaryotes and prokaryotes. Owing to its very low redox potential, antioxidative character, and high intracellular concentration, GSH profoundly shapes the redox status of cells. Our observations suggest that GSH metabolism and/or the redox status of cells plays a determinative role in several important aspects of fungal life, including oxidative stress defense, protein secretion, and secondary metabolite production (including mycotoxin formation), as well as sexual and asexual differentiations. We demonstrated that even a slightly elevated GSH level can substantially disturb the homeostasis of fungi. This information could be important for development of new GSH-producing strains or for any biotechnologically relevant processes where the GSH content, antioxidant capacity, or oxidative stress tolerance of a fungal strain is manipulated.

Autolytic hydrolases affect sexual and asexual development of Aspergillus nidulans

Radial growth, asexual sporulation, and cleistothecia formation as well as extracellular chitinase and proteinase formation of Aspergillus nidulans were monitored in surface cultures in order to study the physiological role of extracellular hydrolase production in carbon-stressed cultures. We set up carbon-stressed and carbon-overfed experimental conditions by varying the starting glucose concentration within the range of 2.5 and 40 g/L. Glucose starvation induced radial growth and hydrolase production and enhanced the maturation of cleistothecia; meanwhile, glucose-rich conditions enhanced mycelial biomass, conidia, and cleistothecia production. Double deletion of chiB and engA (encoding an extracellular endochitinase and a β-1,3-endoglucanase, respectively) decreased conidia production under carbon-stressed conditions, suggesting that these autolytic hydrolases can support conidia formation by releasing nutrients from the cell wall polysaccharides of dead hyphae. Double deletion of prtA and pepJ (both genes encode extracellular proteases) reduced the number of cleistothecia even under carbon-rich conditions except in the presence of casamino acids, which supports the view that sexual development and amino acid metabolism are tightly connected to each other in this fungus.

Phi Class of Glutathione S-transferase Gene Superfamily Widely Exists in Nonplant Taxonomic Groups

Glutathione S-transferases (GSTs) constitute a superfamily of enzymes involved in detoxification of noxious compounds and protection against oxidative damage. GST class Phi (GSTF), one of the important classes of plant GSTs, has long been considered as plant specific but was recently found in basidiomycete fungi. However, the range of nonplant taxonomic groups containing GSTFs remains unknown. In this study, the distribution and phylogenetic relationships of nonplant GSTFs were investigated. We identified GSTFs in ascomycete fungi, myxobacteria, and protists Naegleria gruberi and Aureococcus anophagefferens. GSTF occurrence in these bacteria and protists correlated with their genome sizes and habitats. While this link was missing across ascomycetes, the distribution and abundance of GSTFs among ascomycete genomes could be associated with their lifestyles to some extent. Sequence comparison, gene structure, and phylogenetic analyses indicated divergence among nonplant GSTFs, suggesting polyphyletic origins during evolution. Furthermore, in silico prediction of functional partners suggested functional diversification among nonplant GSTFs.