INDUCTION OF GLUCOSE-6-PHOSPHATE TRANSPORT ACTIVITY IN CHLOROPLASTS OF MESEMBRYANTHEMUM-CRYSTALLINUM BY THE C-3-CAM TRANSITION

To study possible changes in the transport metabolites between chlorop lasts and cytoplasm during CAM induction of Mesembryanthemum crystalli num, we compared substrate specificity of P-i translocator(s) in isola ted chloroplasts from the C-3 and CAM-induced plants. The [C-14]glucos e 6-phosphate (G6P) transport activity was significant only in the chl oroplasts of CAM-mode plants and not detectable in those of C-3-mode, while a similar high rate of [P-32]P-i uptake was observed with both t ypes of chloroplasts. Kinetic analysis of G6P uptake in the CAM chloro plasts showed a high V-max [10.6 mu mol (mg Chl)(-1) h(-1)] and a comp aratively low K-m value (0.41 mM); the latter was similar to K-i value s of P-i, 3-phosphoglycerate and phosphoenolpyruvate, 0.30, 0.34 and 0 .47 mM; respectively. On the other hand, [P-32]P-i uptake in the CAM c hloroplasts was inhibited competitively by G6P with a K-i value (8.4 m M) 20-fold higher than the K-m value for G6P uptake, while that in C-3 chloroplasts was not inhibited at all. These results suggest that a n ew G6P/P-i counterexchange mechanism is induced in the chloroplast env elope of CAM-induced M. crystallinum in addition to the ordinary type of P-i translocator, that cannot transport G6P, already present in the C-3-type chloroplasts.