Ammonium and the analogue
methylammonium are taken into the cell by active transport systems which constitute a family of transmembrane
proteins that have been identified in fungi, bacteria, plants, and animals. Two genes from Aspergillus nidulans,
mepA and meaA, which encode
ammonium transporters with different affinities have been characterized. The
MepA transporter exhibits the highest affinity for
methylammonium (Km, 44.3 microM); in comparison, the Km for MeaA is 3.04 mM. By use of targeted gene replacement strategies, meaA and
mepA deletion mutants were created. Deletion of both meaA and
mepA resulted in the inability of the strain to grow on
ammonium concentrations of less than 10 mM. The single meaA deletion mutant exhibited reduced growth at the same concentrations, whereas the
mepA deletion mutant displayed wild-type growth. Interestingly, multiple copies of
mepA were found to
complement the
methylammonium resistance phenotype conferred by the deletion of meaA. The expression profiles for
mepA and meaA differed; the
mepA transcript was detected only in
nitrogen-starved cultures, whereas meaA was expressed under both
ammonium-sufficient and
nitrogen starvation conditions. Together, these results indicate that MeaA constitutes the major
ammonium transport activity and is required for the optimal growth of A. nidulans on
ammonium as the sole
nitrogen source and that
MepA probably functions in scavenging low concentrations of
ammonium under
nitrogen starvation conditions.