title: 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+ (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.
authors.affiliation: Laboratoire de Physiologie Cellulaire et de Génétique des Levures, Université Libre de Bruxelles, Brussels, Belgium.
authors.forename: A M
authors.initials: AM
authors.lastname: Marini
authors.forename: S
authors.initials: S
authors.lastname: Soussi-Boudekou
authors.forename: S
authors.initials: S
authors.lastname: Vissers
authors.forename: B
authors.initials: B
authors.lastname: Andre
mesh_terms: D000595:Amino Acid Sequence; D001483:Base Sequence; D001692:Biological Transport; D002352:Carrier Proteins / Q000235:genetics / Q000378:metabolism*; D027682:Cation Transport Proteins; D003001:Cloning, Molecular; D004268:DNA-Binding Proteins / Q000235:genetics / Q000502:physiology; D005656:Fungal Proteins / Q000235:genetics / Q000502:physiology; D050980:GATA Transcription Factors; D015966:Gene Expression Regulation, Fungal / Q000502:physiology; D005800:Genes, Fungal / Q000235:genetics; D007700:Kinetics; D008969:Molecular Sequence Data; D009154:Mutation; D000644:Quaternary Ammonium Compounds / Q000378:metabolism*; D012331:RNA, Fungal / Q000032:analysis; D012333:RNA, Messenger / Q000032:analysis; D012097:Repressor Proteins; D012441:Saccharomyces cerevisiae / Q000235:genetics / Q000378:metabolism*; D029701:Saccharomyces cerevisiae Proteins; D017422:Sequence Analysis, DNA; D017386:Sequence Homology, Amino Acid; D014157:Transcription Factors / Q000235:genetics / Q000502:physiology
chemical_list: D002352:Carrier Proteins; D027682:Cation Transport Proteins; D004268:DNA-Binding Proteins; D005656:Fungal Proteins; C096100:GAT1 protein, S cerevisiae; D050980:GATA Transcription Factors; C071664:GLN3 protein, S cerevisiae; C088951:MEP1 protein, S cerevisiae; C107270:MEP2 protein, S cerevisiae; C107271:MEP3 protein, S cerevisiae; D000644:Quaternary Ammonium Compounds; D012331:RNA, Fungal; D012333:RNA, Messenger; D012097:Repressor Proteins; D029701:Saccharomyces cerevisiae Proteins; D014157:Transcription Factors