Enzyme Regulation in Crassulacean Acid Metabolism Photosynthesis

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Abstract
Cell-free preparations of the Crassulacean acid metabolism (CAM) plant, Kalanchoë daigremontiana, were analyzed for thioredoxins and ferredoxin-thioredoxin reductase. Three distinct forms of thioredoxin were identified in Kalanchoë leaves, two of which specifically activated fructose 1,6-bisphosphatase (designated f1 and f2) and a third which activated NADP-malate dehydrogenase (thioredoxin m). The apparent molecular weight of both forms of thioredoxin f was 11,000 and that of thioredoxin m was 10,000. In parallel studies, ferredoxin and ferredoxin-thioredoxin reductase were purified from Kalanchoë leaf preparations. Kalanchoë ferredoxin-thioredoxin reductase was similar to that of C3 and C4 plants in molecular weight (31,000) and immunological cross-reactivity. Kalanchoë ferredoxin-thioredoxin reductase exhibited an affinity for ferredoxin as demonstrated by its binding to an immobilized ferredoxin affinity column. The purified components of the Kalanchoë ferredoxin-thioredoxin system could be recombined to function in the photoregulation of chloroplast enzymes. The data suggest that the ferredoxin/thioredoxin system plays a role in enzyme regulation of all higher plants irrespective of whether they show C3, C4, or CAM photosynthesis.