Abstract
Coccolithophorids (Haptophyta) acquired their chloroplasts via the secondary endosymbiosis and thus possess four-layer envelopes. In a coccolithophorid, Emiliania huxleyi, we already reported that C4 compounds is photosynthetically produced via anaplerotic β-carboxylation reactions in addition to the C3 cycle. Pyruvate carboxylase (PYC) was suggested to play an important role in the production of C4 compound since PYC transcript remarkably increased under illumination. Based on the prediction of N-terminal targeting signal, PYC is expected to function in the chloroplast. In the present study, we could successfully detect PYC activity in the crude extract of E. huxleyi. The PYC activity was completely inhibited by the addition of avidin, a specific inhibitor of biotin carboxylases including PYC. Therefore, the activity we detected is actually derived from PYC, not from other β-carboxylation enzymes such as phosphoenolpyruvate carboxylase. Using immunofluorescence labeling, we showed that E. huxleyi PYC locates in the chloroplast. These data suggest the contribution of chloroplastic PYC to the active β-carboxylation during photosynthesis in E. huxleyi.
