A high-affinity ammonium transporter from the mycorrhizal ascomycete Tuber borchii.
An ammonium transporter cDNA, named TbAMT1, was isolated from the ectomycorrhizal ascomycetous truffle Tuber borchii. The polypeptide encoded by TbAMT1 (52 kDa) functionally complements ammonium uptake-defective yeast mutants and shares sequence similarity with previously characterized ammonium transporters from Saccharomyces (Mep) and Arabidopsis (AtAMT1). Structural characteristics common to the Mep/Amt family and peculiar features of the Tuber transporter have been evidenced by a detailed topological model of the TbAMT1 protein, which predicts 11 transmembrane helices with an N terminus(OUT)/C terminus(IN) orientation. As revealed by uptake/competition experiments conducted in yeast, TbAMT1 is a high-affinity transporter with an apparent K(m) for ammonium of 2 microM. The TbAMT1 mRNA was very slowly, yet specifically upregulated in nitrogen-deprived T. borchii mycelia. Instead, a much faster return to basal expression levels was observed upon resupplementation of either ammonium or nitrate, which thus appear to be utilized as equally effective nitrogen sources by Tuber mycelia.
Montanini
Barbara
B
Dipartimento di Biochimica e Biologia Molecolare, Università di Parma, Parco Area delle Scienze 23/A, I-43100 Parma, Italy.
Moretto
Nadia
N
Soragni
Elisabetta
E
Percudani
Riccardo
R
Ottonello
Simone
S
D000595:Amino Acid Sequence; D001203:Ascomycota / Q000235:genetics / Q000378:metabolism*; D001483:Base Sequence; D002352:Carrier Proteins / Q000235:genetics / Q000302:isolation & purification / Q000378:metabolism*; D027682:Cation Transport Proteins; D018076:DNA, Complementary / Q000235:genetics / Q000302:isolation & purification; D004271:DNA, Fungal / Q000235:genetics / Q000302:isolation & purification; D005656:Fungal Proteins / Q000235:genetics / Q000302:isolation & purification / Q000378:metabolism*; D005800:Genes, Fungal; D005816:Genetic Complementation Test; D007700:Kinetics; D008958:Models, Molecular; D008969:Molecular Sequence Data; D010802:Phylogeny; D010940:Plant Proteins; D017433:Protein Structure, Secondary; D000644:Quaternary Ammonium Compounds / Q000378:metabolism*; D012331:RNA, Fungal / Q000235:genetics / Q000378:metabolism; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism
D002352:Carrier Proteins; D027682:Cation Transport Proteins; D018076:DNA, Complementary; D004271:DNA, Fungal; D005656:Fungal Proteins; D010940:Plant Proteins; D000644:Quaternary Ammonium Compounds; D012331:RNA, Fungal; D012333:RNA, Messenger; C088952:ammonium transporters, plant