IDENTIFICATION OF A HIGH-AFFINITY NH4+ TRANSPORTER FROM PLANTS

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 an d plants, little is known at the molecular level about its transport a cross biomembranes. Biphasic uptake kinetics have been observed in roo ts of several plant species. To study such transport processes, a muta nt yeast strain that is deficient in two NH4+ uptake systems was used to identify a plant NH4+ transporter. Expression of an Arabidopsis cDN A in the yeast mutant complemented the uptake deficiency. The cDNA AMT 1 contains an open reading frame of 501 amino acids and encodes a high ly hydrophobic protein with 9-12 putative membrane spanning regions. D irect uptake measurements show that mutant yeast cells expressing the protein are able to take up [C-14]methylamine. Methylamine uptake can be efficiently competed by NH4+ but not by K+. The methylamine uptake is optimal at pH 7 with a K-m of 65 mu M and a K-i for NH4+ of similar to 10 mu M, is energy-dependent and can be inhibited by protonophores . The plant protein is highly related to an NH4+ transporter from yeas t (Marini et al., accompanying manuscript). Sequence homologies to gen es of bacterial and animal origin indicate that this type of transport er is conserved over a broad range of organisms. Taken together, the d ata provide strong evidence that a gene for the plant high affinity NH 4+ uptake has been identified.