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+ (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.
Mesh Terms:
Amino Acid Sequence, Base Sequence, Biological Transport, Carrier Proteins, Cation Transport Proteins, Cloning, Molecular, DNA-Binding Proteins, Fungal Proteins, GATA Transcription Factors, Gene Expression Regulation, Fungal, Genes, Fungal, Kinetics, Molecular Sequence Data, Mutation, Quaternary Ammonium Compounds, RNA, Fungal, RNA, Messenger, Repressor Proteins, Saccharomyces cerevisiae, Saccharomyces cerevisiae Proteins, Sequence Analysis, DNA, Sequence Homology, Amino Acid, Transcription Factors
Amino Acid Sequence, Base Sequence, Biological Transport, Carrier Proteins, Cation Transport Proteins, Cloning, Molecular, DNA-Binding Proteins, Fungal Proteins, GATA Transcription Factors, Gene Expression Regulation, Fungal, Genes, Fungal, Kinetics, Molecular Sequence Data, Mutation, Quaternary Ammonium Compounds, RNA, Fungal, RNA, Messenger, Repressor Proteins, Saccharomyces cerevisiae, Saccharomyces cerevisiae Proteins, Sequence Analysis, DNA, Sequence Homology, Amino Acid, Transcription Factors
Mol. Cell. Biol.
Date: Aug. 01, 1997
PubMed ID: 9234685
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