A Family of Ammonium Transporters in Saccharomyces cerevisiae†

Abstract

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 microM), followed closely by Mep1p (K_m, 5 to 10 microM) and finally by Mep3p, whose affinity is much lower (K_m, ~ 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.

Notes

† Dedicated to the memory of Marcelle Grenson.