Regulation of sulfate assimilation by amino acids in Lemna minor L.

Abstract

The effect of 2 mM arginine, asparagine and glutamine in the culture medium of Lemna minor L. on adenosine 5′-phosphosulfate sulfotransferase (APSSTase), a key enzyme of assimilatory sulfate reduction, was examined. These amino acids caused a 50–110% increase in extractable enzyme activity on a fresh weight basis. With asparagine and glutamine applied together at 2 mM this increase started 2 h after additionn and was completed after 6 h. Omission of these amino acids caused a decrease in APSSTase activity and by 24 h the original level was attained. ATP sulfurylase activity (EC 2.7.7.4) was not affected significantly by 24 h of cultivation of Lemna in the presence of amino acids and was consistently lower after 72 h than in controls which had nitrate as the sole nitrogen source. The extractable proteins were 10–30% higher in plants cultivated with amino acids for 72 h. 15N-density labelling experiments showed that the increase in APSSTase acitivity was predominantly or, perhaps, exclusively due to an increased synthesis, de novo. This regulation of APSSTase can be explained by assuming a mechanism aimed at producing sufficient amounts of sulfur amino acids during a period of increased protein synthesis.