C 4-leaf phosphoenolpyruvate carboxylase (PEPC; EC 4.1, 1.31) undergoes reversible, light-induced increases in its activity—seryl phosphorylation-status in vivo. We now report that the PEPC-protein kinase activity in desalted crude extracts of light-adapted maize leaves is several-fold greater than that from the corresponding dark tissue when in vitro phosphorylation assays are performed with either endogeneous or purified dark-form maize PEPC as substrate, both in the absence or presence of okadaic acid, a potent inhibitor of the PEPC type 2A protein phosphatase(s). These and related results indicate that the PEPC protein-serine kinase(s) per se is reversibly light activated in vivo by either covalent modification, protein turnover, less likely, a tight-binding effector.
