title: Regulation of Sulfate Assimilation by Nitrogen Nutrition in the Duckweed Lemna minor L.
abstract: The effect of nitrate and ammonium on the extractable activity of two enzymes of assimilatory sulfate reduction, ATP sulfurylase (EC 2.7.7.4) and adenosine 5'-phosphosulfate sulfotransferase (APSSTase), was examined in Lemna minor L. cultivated under steady state conditions. Nitrate reductase (EC 1.6.6.1) was measured for comparison. Low nitrate concentrations (0.2 and 0.04 millimolar) caused a decrease in the specific activity of all three enzymes measured. Twenty-four hours after transfer to medium without a nitrogen source, the specific activity of APSSTase and nitrate reductase was at less than 30% of the original level, whereas ATP sulfurylase was still at about 80%. NH(4) (+) added to the nutrient solution caused a 50 to 100% increase in the specific activity of APSSTase within 24 hours, followed by a slow decrease. After 72 hours with NH(4) (+), the specific activity was still 25% higher than originally. During the same period, the extractable protein increased by 30% on a fresh weight basis, and total protein by 55 to 60%. Nitrate reductase activity decreased to less than 5%. After omission of NH(4) (+) from the nutrient solution extractable APSSTase activity rapidly decreased to the level of cultures with NO(3) (-) as a nitrogen source. Using [(35)S]SO(4) (2-) as a sulfur source, an increased incorporation of label into the protein fraction could be detected when NH(4) (+) was added to the nutrient solution. This indicated that more sulfate was assimilated and used for protein synthesis. The higher extractable activity of APSSTase with NH(4) (+) may be a regulatory mechanism involved in the formation of sufficient sulfur amino acids during a period of increased protein synthesis.
authors: Pflanzenphysiologisches Institut der Universität Bern, Bern, Switzerland.
authors: C
authors: C
authors: Brunold
authors: M
authors: M
authors: Suter