In C 4 plants the activity of phosphoenolpyruvate carboxylase (PEPC; EC 4.1.1.31) is regulated by phosphorylation/dephosphorylation which is mediated by light/dark signals. The study using protein kinase inhibitors showed that the inhibition pattern of maize PEPC-protein kinase (PEPC-PK) is similar to that of myosin light chain kinase, a Ca 2 + -calmodulin-dependent PK. The kinase activity was also inhibited by EGTA and the inhibition was relieved by Ca 2 + . These results suggest that PEPC-PK is Ca 2 + -dependent in contrast with previous observations by other research groups.