Light-dark regulation of sulfate assimilation in Lemna minor L. in the presence of o-acetyl-l-serine

The effect of light removal and addition of O-acetyl-l-serine (OAS) on sulfate assimilation in Lemna minor L. was analyzed by measuring the extractable activity of adenosine 5'-phosphosulfate sulfotransferase (APSSTase) and the in vivo incorporation of 35SO4 2-. After removal of light APSSTase activity decreased to 10% within 24 h in the absence and to 50% in the presence of OAS. Within 24 h total 35SO4 2- uptake decreased to 60% without and increased to 130% with OAS compared to light controls. The incorporation of 35S into cysteine increased 2 times without and 15 times with OAS, labelling of glutathione decreased to 65% and increased to 140%, the one of the protein fraction decreased to 30% and to 20% of the light control in the absence and presence of OAS. Our results indicate that OAS has a regulatory function on the assimilation of sulfate and that protein synthesis is inhibited in the dark