Light-Dependent Regulatory Mechanisms Acting on Photosynthetic Phosphoenolpyruvate Carboxylase in C4 Plants

Abstract

Phosphoenolpyruvate carboxylase (EC 4.1.1.31; PEPC) catalyzes the β-carboxylation of PEP by HCO3− in the presence of a divalent cation. PEPC isozymes are widely distributed in plant tissues in which they are involved in a variety of physiological contexts. In C4 plants, a specific isoform (C4 PEPC) plays a key role in the primary CO2 fixation in the photosynthesis pathway. C4 PEPC is subject to a light-dependent transcriptional control resulting in the accumulation of high amounts of the corresponding protein in the mesophyll cell cytosol during greening of the etiolated C3 leaf. In vitro studies have shown that the enzyme activity is regulated by photosynthesis-related metabolites, e.g., feedback inhibition by L-malate and allosteric activation by sugar-P. During the last decade, a posttranslational process, i.e., reversible phosphorylation, acting on C4 PEPC has been established and shown to modulate both its functional and regulatory properties. The main purpose of the work reported here was to explore the light-dependent mechanisms which control the biosynthesis and covalent modification of C4 PEPC.