Evolution of enzymes involved in carbon metabolism (phosphoenolpyruvate and ribulose-bisphosphate carboxylases, phosphoenolpyruvate carboxykinase) during the light-induced greening of Euglena gracilis strains Z and ZR.

Phosphoenolpyruvate carboxykinase activity decreases when Euglena gracilis Z and ZR undergo light-induced chloroplast development in batch "resting" medium lacking utilizable organic carbon and CO2. This enzyme is present in heterotrophically grown cells (Briand et al. 1981) and assures gluconeogenesis. It was consistently more active in strain ZR. Decreased carboxykinase activities were accompanied by parallel increases in the activities of ribulose bisphosphate carboxylase and phosphoenolpyruvate carboxylase. The rates of O2 evolution in light were much lower than those of CO2 fixed simultaneously. The incorporation of (14)CO2 into early C-4 dicarboxylic acids was higher in green cells than in etiolated cells, and it was even higher in green cells assayed in light in the presence of (DCMU). A hypothesis has been proposed, according to which there is a possible cooperation of phosphoenolpyruvate carboxylase in photosynthetic CO2 fixation, especially under conditions of limiting CO2.High temperatures (34° C) depress carboxylation enzyme activities to a greater extent than that of the carboxykinase without a great effect on cellular chlorophyll content. In the presence of 25 μm DCMU, however, chlorophyll accumulation is reduced without any detectable changes in enzyme activities in the Z strain. The ZR strain displayed its characteristic resistance to DCMU.