Regulation of Sulfate Assimilation by Nitrogen Nutrition in the Duckweed Lemna minor L.

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%. NH4+ 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 NH4+, 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 NH4+ from the nutrient solution extractable APSSTase activity rapidly decreased to the level of cultures with NO3− as a nitrogen source. Using [35S]SO42− as a sulfur source, an increased incorporation of label into the protein fraction could be detected when NH4+ 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 NH4+ may be a regulatory mechanism involved in the formation of sufficient sulfur amino acids during a period of increased protein synthesis.