Protein phosphorylation and circadian rhythms

Abstract Phosphoenolpyruvate carboxylase (PEPc) (EC 4.1.1.31) catalyses the fixation of CO (as HCO-) into oxaloacetate. It plays an anaplerotic role in nonphotosynthetic tissues and in the leaves of C plants. In the leaves of C plants it is located in the mesophyll tissue and catalyses the first committed step in the C pathway of CO assimilation (see, for example, Andreo et al. 1987). In this article, however, we concentrate on the regulation of PEPc in plants that perform Crassulacean acid metabolism (CAM). In CAM, PEPc is responsible for the nocturnal fixation of atmospheric CO; the oxaloacetate produced is reduced to malate which is stored as malic acid in the vacuole. During the following light period, the malate is released from the vacuole and decarboxylated, and the resulting CO is fixed in the Calvin cycle (Osmond and Holtum 1981). Consideration of the metabolic pathways involved in CAM suggests that mechanisms must exist to permit flux through PEPc at night and reduce or eliminate it during the day. In this article we describe the role of phosphorylation in the regulation of PEPc in leaves of the CAM plant Bryophyllum (Kalanchoii) fedtschenkoi.