Abstract #75

Relief of ammonium ion toxicity by amino acid excretion. David Hess^1,2, Wen-Yun Lu^1,3, Joshua Rabinowitz^1,3, David Botstein^1,2. 1) Lewis-Sigler Institute for Integrative Genomics, Princeton University, Princeton NJ, 08544; 2) Department of Molecular Biology, Princeton University, Princeton NJ, 08544; 3) Department of Chemistry, Princeton University, Princeton NJ, 08544.
   We found two circumstances under which growth is impaired by nitrogen excess. One of these is limiting potassium, which allows excess ammonium ions to enter the cell; another is the overproduction of the high flux ammonium channels. The key feature of these steady-state (chemostat) cultures was the presence of excreted amino acids in the filtrate as detected by mass spectrometry. The amount of amino acids excreted increased in relation to the severity of growth impairment by ammonium suggesting this excretion used for detoxification. We also observed that as cells enter growth limitation by nitrogen toxicity their glucose utilization increases. This is accompanied by an increase in ethanol production suggesting that cells under the effects of nitrogen toxicity have higher energy needs. Microarray analysis of cells under the effect of nitrogen toxicity reveals a surprisingly specific response. As expected, genes under control of nitrogen catabolite repression are strongly repressed. Unexpectedly, the amino acid transporters regulated by the SPS pathway are strongly induced. These transporters provide a potential mechanism for the observed amino acid excretion. We hypothesize that the excretion of amino acids is necessary to eliminate excess nitrogen when yeast are grown in low potassium with ammonium as a nitrogen source.

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