Ammonium is a nitrogen source supporting growth of yeast cells at an o
ptimal rate, We recently reported the first characterization of an NH4
+ transport protein (Mep1p) in Saccharomyces cerevisiae. Here we descr
ibe the characterization of two additional NH4+ transporters, Mep2p an
d Mep3p, both of which are highly similar to Mep1p, The Mep2 protein d
isplays the highest affinity for NH4+ (K-m, 1 to 2 mu M), followed clo
sely by Mep1p (K-m, 5 to 10 mu M) and finally by Mep3p, whose affinity
is much lower (K-m, similar to 1.4 to 2.1 mM), A strain lacking all t
hree MEP genes cannot grow on media containing less than 5 mM NH4+ as
the sole nitrogen source, while the presence of individual NH4+ transp
orters enables growth on these media. Yet, the three Mep proteins are
not essential for growth on NH4+ at high concentrations (>20 mM), Feed
ing experiments further indicate that the Mep transporters are also re
quired to retain NH4+ inside cells during growth on at least some nitr
ogen sources other than NH4+. The MEP genes are subject to nitrogen co
ntrol, In the presence of a good nitrogen source, all three MEP genes
are repressed, On a poor nitrogen source, MEP2 expression is much high
er than MEP1 and MEP3 expression, High-level MEP2 transcription requir
es at least one of the two GATA family factors Gln3p and Nil1p, which
are involved in transcriptional activation of many other nitrogen-regu
lated genes, In contrast, expression of either MEP1 or MEP3 requires o
nly Gln3p and is unexpectedly down-regulated in a Nil1p-dependent mann
er, Analysis of databases suggests that families of NH4+ transporters
exist in other organisms as well.