A family of ammonium transporters in Saccharomyces cerevisiae

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+ (K-m, 1 to 2 mu M), followed closely 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 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.