Physiological and molecular characterization of isolated diatom plastids
Lang, M., Wittpoth, C., Weyrauch, K., Kroth, P.G.
Institut für Biochemie der Pflanzen,
Heinrich-Heine-Universitüt Düsseldorf,
Universitätsstr.1,
D-40225 Düsseldorf,
Germany
Diatoms and other chromophytic chlorophyll a/c-containing algae have plastids with four surrounding envelope membranes. This typical structure is thought to reflect an evolution of these organisms by secondary endosymbiosis, i.e. by uptake of a eukaryotic photosynthetic cell by a eukaryotic host. We have investigated on the functional photosynthetic properties of isolated plastids from the diatoms Odontella sinensis and Coscinodiscus granii. Photosynthetic activity and CO2 fixation of the isolated plastids have been observed by measuring oxygen evolution. Up to 50 µmol O2/mg Chl x h have been obtained, which is about 40 % of the activity of the cells. Oxygen evolution was inhibited by addition of phosphate and restored after addition of 3-phosphoglycerate which indicates the presence of a phosphate translocator in diatom plastids. We have cloned and sequenced the gene for the phosporibulokinase (PRK) of Odontella, which is a key enzyme of the Calvin cycle and which is regulated via the ferredoxin/thioredoxin system in green algae and higher plants. Although conserved cysteines are present within the PRK sequence, so far we were not able to demonstrate regulation of the diatom enzyme in a stromal extract by bacterial thioredoxin or dithiothreitol. We also characterized the enzymatic activity of the plastidic malate dehydrogenase from diatoms. In contrast to the respective enzyme from green algae and higher plants it exclusively utilizes NAD+/NADH as substrate and, similarly to the PRK, was not affected by reducing agents like thioredoxin or dithiothreitol. This work was supported by a grant of the Deutsche Forschungsgemeinschaft to P.G.K. (SFB189, TP B3).
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