Ammonium transport: unifying concepts and unique aspects

Ammonium uptake by cells has been studied for more than a century, but only recently a family of ammonium transporters (Mep/Amt) with 10-12 transmembrane domains has been defined. These proteins are probably ubiquitous, since homologues have been found in the major kingdoms of living organisms. Plants as well as yeast and some archaebacteria have multiple Mep/Amt paralogues, which can be distinguished by their affinity for ammonium and the ammonium analogue methylammonium. Most ammonium transporters are induced in nitrogen-starving conditions, both in prokaryotes and plants. In Saccharomyces cerevisiae, Escherichia coli and Azospirillum brasilense Mep/Amt proteins where shown to be necessary for growth when the external concentration of the diffusive ammonium form (NH 3 ) becomes limiting. Ammonium transporters also play an important role in pseudohyphal differentiation in yeast and efficient symbiotic interaction between Rhizobium etli and its host plant. In most bacteria, NH 4 + transport appears to be a uniport mechanism driven by the membrane potential, but, depending on the organism, a different mode of ammonium uptake may be operating. Current knowledge offers the basis to investigate further the physiological role of ammonium transporters in the natural habitat of organisms and their importance in plant--bacteria interactions.