A Marriage of Old and New: Chemostats and Microarrays Identify a New Model System for Ammonium Toxicity

A mmonium (NH 4 +), the substance created by the fi xation of atmospheric nitrogen (N 2), occupies a central role in cellular nitrogen metabolism. However, even though it is an important metabolite, free ammonium is present only transiently in biological systems, where it is rapidly converted into amino acids—initially glutamate and, particularly, glutamine. From there, transamination reactions convert it into other amino acids and nitrogen-containing cellular components. The scarcity of free ammonium is not accidental. Ammonium and ammonia (NH 3 ; the term ammonium is used here generically) are highly reactive compounds with substantial cytotoxicity. Ammonium detoxifi cation is the primary function of the urea cycle in mammals, which converts the amine group from arginine into urea in the liver before it is excreted. Above-normal levels of ammonium (hyperammonemia), principally as a result of liver damage, can cause a variety of pathologies, the most serious of which is swelling of the brain, termed hepatic encephalopathy. In plants, high ammonium concentrations (in the low millimolar range) in the soil can retard growth, impair root development, and cause a yellowing of the leaves (chlorosis). Ammonium toxicity in plants is an interesting and troubling problem (for a more thorough discussion, see [1]). Ammonium levels in soils are generally rising, presaging increasingly severe effects on plant life. The rising concentrations come from the heavy agricultural use of ammonium-based fertilizers; at low concentrations, ammonium is an ideal nutrient for plants. This clearly illustrates the paradox of ammonium: in multicellular organisms, it is a nutrient, a metabolite, and a toxin. As for many other processes, a genetically tractable model system would help shed light on these contradictory roles. In microbes, however, ammonium is considered a preferred nitrogen source, one capable of supporting optimal growth rates even at high levels. In fact, Escherichia coli and Bacillus subtilis grow well in even molar concentrations of ammonium [2], making a bacterial model for ammonium toxicity unlikely. Similarly, the model eukaryote Saccharomyces cerevisiae avidly uses ammonium as a nutrient. Synthetic growth media commonly contain 76 mM ammonium sulfate (5 g/l), several-fold higher than the concentrations that inhibit the growth of most plants, and it has never been suggested that this " preferred " nitrogen source is harmful in any way. Thus, one might not expect the great similarities in cellular physiology between yeast and higher eukaryotes to extend to ammonium toxicity. Institute demonstrate for the fi rst time that …