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Mol. Cell. Biol., Aug 1997, 4282-4293, Vol 17, No. 8
AM Marini, S Soussi-Boudekou, S Vissers and B Andre
Ammonium is a nitrogen source supporting growth of yeast cells at an
optimal rate. We recently reported the first characterization of an NH4+
transport protein (Mep1p) in Saccharomyces cerevisiae. Here we describe the
characterization of two additional NH4+ transporters, Mep2p and Mep3p, both
of which are highly similar to Mep1p. The Mep2 protein displays the highest
affinity for NH4+ (Km, 1 to 2 microM), followed closely by Mep1p (Km, 5 to
10 microM) and finally by Mep3p, whose affinity is much lower (Km,
approximately 1.4 to 2.1 mM). A strain lacking all three MEP genes cannot
grow on media containing less than 5 mM NH4+ as the sole nitrogen source,
while the presence of individual NH4+ transporters enables growth on these
media. Yet, the three Mep proteins are not essential for growth on NH4+ at
high concentrations (>20 mM). Feeding experiments further indicate that
the Mep transporters are also required to retain NH4+ inside cells during
growth on at least some nitrogen sources other than NH4+. The MEP genes are
subject to nitrogen control. In the presence of a good nitrogen source, all
three MEP genes are repressed. On a poor nitrogen source, MEP2 expression
is much higher than MEP1 and MEP3 expression. High- level MEP2
transcription requires at least one of the two GATA family factors Gln3p
and Nil1p, which are involved in transcriptional activation of many other
nitrogen-regulated genes. In contrast, expression of either MEP1 or MEP3
requires only Gln3p and is unexpectedly down-regulated in a Nil1p-dependent
manner. Analysis of databases suggests that families of NH4+ transporters
exist in other organisms as well.
Copyright © 1997, American Society for Microbiology
A family of ammonium transporters in Saccharomyces cerevisiae
Laboratoire de Physiologie Cellulaire et de Genetique des Levures, Universite Libre de Bruxelles, Brussels, Belgium.
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