Transcriptome analysis of nitrate assimilation in Aspergillus nidulans reveals connections to nitric oxide metabolism

Summary Nitrate is a dominant form of inorganic nitrogen (N) in soils and can be efficiently assimilated by bacteria, fungi and plants. We studied here the transcriptome of the short‐term nitrate response using assimilating and non‐assimilating strains of the model ascomycete Aspergillus nidulans . Among the 72 genes positively responding to nitrate, only 18 genes carry binding sites for the pathway‐specific activator NirA. Forty‐five genes were repressed by nitrate metabolism. Because nirA ‐ strains are N‐starved at nitrate induction conditions, we also compared the nitrate transcriptome with N‐deprived conditions and found a partial overlap of differentially regulated genes between these conditions. Nitric oxide (NO)‐metabolizing flavohaemoglobins were found to be co‐regulated with nitrate assimilatory genes. Subsequent molecular characterization revealed that the strongly inducible FhbA is required for full activity of nitrate and nitrite reductase enzymes. The co‐regulation of NO‐detoxifying and nitrate/nitrite assimilating systems may represent a conserved mechanism, which serves to neutralize nitrosative stress imposed by an external NO source in saprophytic and pathogenic fungi. Our analysis using membrane‐permeable NO donors suggests that signalling for NirA activation only indirectly depends on the nitrate transporters NrtA (CrnA) and NrtB (CrnB).