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.