Identification of a high affinity NH4+ transporter from plants.

Abstract

Despite the important role of the ammonium ion in metabolism, i.e. as a form of nitrogen that is taken up from the soil by microorganisms and plants, little is known at the molecular level about its transport across biomembranes. Biphasic uptake kinetics have been observed in roots of several plant species. To study such transport processes, a mutant yeast strain that is deficient in two NH4+ uptake systems was used to identify a plant NH4+ transporter. Expression of an Arabidopsis cDNA in the yeast mutant complemented the uptake deficiency. The cDNA AMT1 contains an open reading frame of 501 amino acids and encodes a highly hydrophobic protein with 9‐12 putative membrane spanning regions. Direct uptake measurements show that mutant yeast cells expressing the protein are able to take up [14C]methylamine. Methylamine uptake can be efficiently competed by NH4+ but not by K+. The methylamine uptake is optimal at pH 7 with a Km of 65 microM and a Ki for NH4+ of approximately 10 microM, is energy‐dependent and can be inhibited by protonophores. The plant protein is highly related to an NH4+ transporter from yeast (Marini et al., accompanying manuscript). Sequence homologies to genes of bacterial and animal origin indicate that this type of transporter is conserved over a broad range of organisms. Taken together, the data provide strong evidence that a gene for the plant high affinity NH4+ uptake has been identified.

Trans‐root potential, xylem pressure, and root cortical membrane potential of ‘low‐salt’ maize plants as influenced by nitrate and ammonium

“…An ammonium-induced depolarization has also been reported for membrane potentials of root hair cells of barley and tomato seedlings (Ayling 1993) and individual epidermal and cortex cells of intact rice roots (Wang et al 1994). The ammonium effect can be ascribed to ammonium uptake via ion channels and/or specific carriers (Ninnemann, Jauniaux & Frommer 1994). Effects of ammonium on the electrical potentials in the root are presumably complicated by the fact that the uptake of other mineral nutrients such as potassium and magnesium is also affected (Gerendas & Sattelmacher 1994;Gerendas et al 1997).…”

An alternative PII protein in the regulation of glutamine synthetase in Escherichia coli

“…MEP1 and MEP2 of S. cerevisae (Marini et al, 1994, and GenBank Accession no. X83608), AMT1 of Arabidopsis (Ninnemann et al, 1994), and NrgA of B. subtilus (Wray et al, 1994). Dashes indicate gaps introduced to optimize the alignment.…”

“…Comparison of the amino-acid sequence of this ORF with proteins in the databases (GENPEPT, SWISS-PROT, and PIR) revealed that it is homologous to the ammonia transporters MEP1 (29% identical) and MEP2 (30% identical) of Saccharomyces cerevisae (Marini et al, 1994; GenBank Accession no. X83608, respectively), to AMT1 of Arabidopsis (26% identical) (Ninnemann et al, 1994), and to the hydrophobic protein NrgA of Bacillus subtilis (42% identical) (Wray et al, 1994). Interestingly, the latter gene product forms an operon together with a P II -like protein NrgB.…”

Quantitative detection of changes in the leaf‐mesophyll tonoplast proteome in dependency of a cadmium exposure of barley (Hordeum vulgare L.) plants

“…Although, the abundance of most proteins was not altered by Cd 2+ treatment, peptides 5 corresponding to AtPDR8, an ABC transporter shown to be implicated in pathogen defence [26,27] and heavy metal resistance [28] and the ammonium transporter AtAMT1.1 [29] were present in 3-to 5-fold higher amounts in Cd 2+ treated cell cultures. Despite the fact that the vacuole is the major Cd 2+ store no study has so far investigated whether the abundance of vacuolar membrane proteins changes in response to Cd 2+ stress.…”