Heat-stabilities of Electron Transport Related to Photosystem I in a Thermophilic Blue-green Alga,Synechococcussp.

摘要

Heat-stabilities of photosystem I reactions in a thermophilic blue-green alga,Synechococcussp. were studied. All the reactions examined were highly resistant to heat as compared with those in ordinary higher plants and algae.Cytc-553 photooxidation in vivo was abolished by treatment at 75°C for 5 min. By contrast, P700 photooxidation was extremely resistant to heat and could not be completely inactivated by treatment of the cells or isolated thylakoids at about 100°C for 5 min. Photooxidation of added Cytc-553 by isolated thylakoid membranes was more heat-stable than was this activity in cells. This suggests that heat-treatment caused a perturbation in the structural integrity of the membranes which is required for efficient electron transfer from Cytc-553 to P700 in situ.At higher temperatures, the inactivation of Cytc-553 photooxidation in the membranes parallels the decrease in the rate of P700 photooxidation. Spectrophotometric studies with short flashes indicated that inactivation of the electron transport from Cytc-553 to methyl viologen is due to the denaturation of a secondary electron acceptor of photosystem I, A2, and possibly another acceptor, P43