Abstract
Summary The role of reversible phosphorylation in the diurnal regulation of phosphoenolpyruvate carboxylase (PEPC, EC 4.1.31) in the facultative CAM plant Mesembryanthemum crystallinum was investigated. Labeling of detached leaves with 32Pi showed that PEPC was mostly phosphorylated at night and unphosphorylated during the day in CAM plants. Anion-exchange chromatography on Mono Q resulted in separation of phosphorylated and unphosphorylated forms of PEPC. Under all experimental conditions the phosphorylated enzyme was relatively insensitive to inhibition by L-malate, while the unphosphorylated enzyme was very sensitive. The fraction of phosphorylated, insensitive enzyme in night extracts varied considerably between experiments. It is suggested from these results that carbon flow through PEPC at night is regulated by the phosphorylation state of the enzyme pool.
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