PMID:9234685

Citation	Marini, AM, Soussi-Boudekou, S, Vissers, S and Andre, B (1997) A family of ammonium transporters in Saccharomyces cerevisiae. Mol. Cell. Biol. 17:4282-93
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+ (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.
Links	PubMed PMC232281
Keywords	Amino Acid Sequence; Base Sequence; Biological Transport; Carrier Proteins/genetics; Carrier Proteins/metabolism; Cation Transport Proteins; Cloning, Molecular; DNA-Binding Proteins/genetics; DNA-Binding Proteins/physiology; Fungal Proteins/genetics; Fungal Proteins/physiology; GATA Transcription Factors; Gene Expression Regulation, Fungal/physiology; Genes, Fungal/genetics; Kinetics; Molecular Sequence Data; Mutation; Quaternary Ammonium Compounds/metabolism; RNA, Fungal/analysis; RNA, Messenger/analysis; Repressor Proteins; Saccharomyces cerevisiae/genetics; Saccharomyces cerevisiae/metabolism; Saccharomyces cerevisiae Proteins; Sequence Analysis, DNA; Sequence Homology, Amino Acid; Transcription Factors/genetics; Transcription Factors/physiology
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