Circadian rhythm of a TCA cycle enzyme is apparently regulated at the translational level in the dinoflagellate Lingulodinium polyedrum

Previously, the authors have reported that intracellular amounts of several metabolic-related enzymes from the photosynthetic dinoflagellate Lingulodinium polyedrum (formerly Gonyaulax polyedra) showed a daily rhythm under a 12:12 h LID cycle. This led the authors to hypothesize that a circadian clock controls metabolism, including the tricarboxylic acid (TCA) cycle. In this study, the authors investigated daily changes in the levels of mRNA, protein, and enzyme activity of several metabolic enzymes during 12:12 h LID, 8:16 h LID, and constant light conditions. The NADP-dependent isocitrate dehydrogenase (NADP-ICDH) in the TCA cycle exhibited circadian changes of protein abundance and enzyme activity under all conditions, whereas its mRNA level remained constant throughout the cycle. These results indicate that the rhythm of NADP-ICDH is regulated by a circadian control of protein synthesis or modification rather than by message levels and suggest that the TCA cycle may be controlled by the circadian clock system.