Affiliation:
1. Department of Genetics, University of Melbourne, Parkville, Victoria, Australia 3010
Abstract
ABSTRACT
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 (
K
m
0
, 44.3 μM); in comparison, the
K
m
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.
Publisher
American Society for Microbiology
Subject
Molecular Biology,General Medicine,Microbiology
Cited by
35 articles.
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