Dissecting individual steps of nitrogen transcription factor cooperation in the Aspergillus nidulans nitrate cluster

In the ascomycete fungus Aspergillus nidulans, the transcriptional activation of nitrate assimilating genes (niiA, niaD) depends on the cooperativity between a general nitrogen status-sensing regulator (the GATA factor AreA) and a pathway-specific activator (the Zn-cluster regulator NirA). Because nitrate assimilation leads to intracellular ammonium formation, it is difficult to determine the individual contributions of NirA and AreA in this complex activation/inactivation process. In an attempt to find a suitable marker for the nitrogen status sensed by AreA, we determined the intracellular free amino acid levels on different nitrogen growth conditions. We show that the amount of glutamine (Gln) inversely correlates with all known AreA activities. We find that AreA mediates chromatin remodelling by increasing histone H3 acetylation, a process triggered by transcriptional activation and, independently of transcription, by nitrogen starvation. NirA also participates in the chromatin opening process during nitrate induction but its function is not related to histone acetylation. This chromatin remodelling function of NirA is dispensable only in nitrogen-starved cells, conditions that lead to elevated AreA chromatin occupancy and histone H3 hyperacetylation. Continuous nitrate assimilation leads to self-nitrogen metabolite repression but nitrate-activated NirA is partially compensating for lowered AreA activities under these conditions.