Differential Expression of Aspergillus nidulans Ammonium Permease Genes Is Regulated by GATA Transcription Factor AreA

Abstract

The movement of ammonium across biological membranes is mediated in both prokaryotes and eukaryotes by ammonium transport proteins (AMT/MEP) that constitute a family of related sequences. We have previously identified two ammonium permeases in Aspergillus nidulans, encoded by the meaA and mepA genes. Here we show that meaA is expressed in the presence of ammonium, consistent with the function of MeaA as the main ammonium transporter required for optimal growth on ammonium as a nitrogen source. In contrast, mepA, which encodes a high-affinity ammonium permease, is expressed only under nitrogen-limiting or starvation conditions. We have identified two additional AMT/MEP-like genes in A. nidulans, namely, mepB, which encodes a second high-affinity ammonium transporter expressed only in response to complete nitrogen starvation, and mepC, which is expressed at low levels under all nitrogen conditions. The MepC gene product is more divergent than the other A. nidulans AMT/MEP proteins and is not thought to significantly contribute to ammonium uptake under normal conditions. Remarkably, the expression of each AMT/MEP gene under all nitrogen conditions is regulated by the global nitrogen regulatory GATA factor AreA. Therefore, AreA is also active under nitrogen-sufficient conditions, along with its established role as a transcriptional activator in response to nitrogen limitation.Ammonium transport proteins (AMT/MEP) have been identified in bacteria, fungi, plants, and animals and constitute a conserved family of polytopic membrane proteins (54). AMT/MEP proteins are predicted to contain 11 transmembrane (TM) helices with an N out -C in topology (26, 48). Bioinformatic topology predictions indicate that bacterial AMT/MEP proteins generally contain an additional N-terminal TM domain but that this region acts as a signal peptide which is removed from the mature protein (9). Aside from the small cytoplasmic loop between TM domains 3 and 4 that displays sequence similarity to a major facilitator superfamily motif (34), the AMT/MEP sequences represent a unique group of transport membrane proteins. Although over 300 members of the AMT/MEP/Rh family have currently been assigned based on amino acid sequence similarity (Pfam accession number PF00909), an ammonium transport function has not been confirmed for the vast majority of these sequences. Functional permeases have been described for bacteria, fungi, plants (reviewed in reference 54), and humans, where the rhesus blood group polypeptides, which display significant sequence identity to AMT/MEP proteins, have been shown to function as ammonium transporters in other systems (27,53,55,59). The structure of Escherichia coli AmtB has been determined up to 1.35-Å resolution, and structural analysis revealed that the protein functions as a trimer that recruits ammonium which is then channeled as ammonia (18,58). Genetic, molecular, and/or physiological evidence suggests that AMT/MEP proteins can function as homoand/or heterocomplexes (6,25,29,34).The presence of multiple ammonium ...

Characterization of Three Ammonium Transporters of the Glomeromycotan Fungus Geosiphon pyriformis

“…2A). Such a lack of MA transport had been reported in fungi only for one AMT that is, interestingly, from a member of the Glomeromycota (36), and for the structurally different AMT-like protein MepC of Aspergillus nidulans (72). This result was verified by an MA toxicity experiment; MA taken up by yeast cells is toxic and leads to growth reduction.…”

“…Outside the fungal kingdom, two plant AMTs, from Arabidopsis thaliana (78,79) and Lotus japonicus (80), and the mammalian Rh proteins, which are low-affinity AMTs (28), were reported to not transport MA, demonstrating that MA transport is not a universal feature of AMTs. Until recently, MepC of A. nidulans, which is only distantly related (31% to 35% identity) to the other AMTs of A. nidulans (72), was the only fungal AMT lacking MA transport capability. The second one, GiAMT2, was recently described from an AMF (36).…”

Amt2 Permease Is Required To Induce Ammonium-Responsive Invasive Growth and Mating in Cryptococcus neoformans

“…The extent of induction, however, varies considerably, with MEP2 being approximately 20-and 50-fold more highly expressed than MEP1 and MEP3, respectively, when proline is used as a nitrogen source (26). In Aspergillus nidulans, nitrogen-responsive transcription is mediated by the AreA factor and the meaA and mepA genes encode low-and high-affinity permeases, respectively (30,31). The meaA gene is constitutively expressed and does not respond to changes in external ammonium concentrations.…”

Nitrogen Metabolite Repression of Metabolism and Virulence in the Human Fungal Pathogen Cryptococcus neoformans

“…We subsequently gained insights into this paradox by proving that along with its established role as an activator of secondary catabolic gene expression in the absence of ammonium, Gat1/ Are1 is also functional in the presence of ammonium. Like A. nidulans AreA, C. neoformans Gat1/Are1 also regulates the expression of the major ammonium assimilation enzyme-encoding gene GDH1 as well as the permease-encoding genes AMT1 and AMT2 (Christensen et al 1998;Monahan et al 2002Monahan et al , 2006Rutherford et al 2008). Since the alternative pathway of ammonium assimilation (via GLN1 and GLT1) is not significantly impaired in the gat1/are1D mutant, we speculate that the basal levels of transcription of AMT1 and AMT2 may be insufficient for ammonium uptake.…”