Metabolite control overrides circadian regulation of phosphoenolpyruvate carboxylase kinase and CO2 fixation in Crassulacean acid metabolism

Phosphoenolpyruvate carboxylase (PEPc) catalyzes the primary fixation of CO 2 in Crassulacean acid metabolism plants. Flux through the enzyme is regula ted by reversible phosphorylation. PEPc kinase is controlled by changes in the level of its translatable mRNA in response to a circadian rhythm. The p hysiological significance of changes in the levels of PEPc-kinase-translata ble mRNA and the involvement of metabolites in control of the kinase was in vestigated by subjecting Kalanchoe daigremontiana leaves to anaerobic condi tions at night to modulate the magnitude of malate accumulation, or to a ri se in temperature at night to increase the efflux of malate from vacuole to cytosol. Changes in CO2 fixation and PEPc kinase activity reflected those in kinase mRNA. The highest rates of CO2 fixation and levels of kinase mRNA were observed in leaves subjected to anaerobic treatment for the first hal f of the night and then transferred to ambient air. In leaves subjected to anaerobic treatment overnight and transferred to ambient air at the start o f the day, PEPc-kinase-translatable mRNA and activity, the phosphorylation state of PEPc, and fixation of atmospheric CO2 were significantly higher th an those for control leaves for the first 3 h of the light period. A nightt ime temperature increase from 19 degrees C to 27 degrees C led to a rapid r eduction in kinase mRNA and activity; however, this was not observed in lea ves in which malate accumulation had been prevented by anaerobic treatment. These data are consistent with the hypothesis that a high concentration of malate reduces both kinase mRNA and the accumulation of the kinase itself.