HIGHER-PLANT PHOSPHOENOLPYRUVATE CARBOXYLASE KINASE IS REGULATED AT THE LEVEL OF TRANSLATABLE MESSENGER-RNA IN RESPONSE TO LIGHT OR A CIRCADIAN-RHYTHM

Phosphoenolpyruvate carboxylase is regulated by reversible phosphoryla tion in response to light in C-3 and C-4 plants and to a circadian osc illator in CAM plants. Increases in phosphoenolpyruvate carboxylase ki nase activity require protein synthesis. This requirement has been ana lysed by quantifying translatable mRNA for this protein kinase using i n vitro translation of isolated RNA followed by direct assay of kinase activity. In leaves of the CAM plant Bryophyllum (Kalanchoe) fedtsche nkoi, in normal diurnal conditions, kinase mRNA was 20-fold more abund ant at night than in the day. In constant environmental conditions (co ntinuous darkness, CO2-free air, 15 degrees C) kinase mRNA exhibited c ircadian oscillations. The circadian disappearance of kinase mRNA and kinase activity was delayed by lowering the temperature to 4 degrees C and accelerated by raising the temperature to 30 degrees C. The appea rance of kinase mRNA and activity was blocked by cordycepin and puromy cin. In maize and barley, kinase mRNA increased in response to light. For all three plants, the phosphoenolpyruvate carboxylase kinase activ ity generated during in vitro translation was Ca2+-independent. These results demonstrate that phosphoenolpyruvate carboxylase kinase activi ty is regulated at the level of translatable mRNA in C-3, C-4 and CAM plants.