Subunit Structure of Chloroplast Photosystem I Reaction Center

Abstract

Purified Photosystem I reaction center preparation catalyzed a plastocyanin-dependent cytochrome f photooxidation. Euglena cytochrome 552 replaced plastocyanin in all photochemical reactions catalyzed by the purified reaction center and was also photooxidized. A reaction center preparation lacking Subunit III was obtained by means of Triton X-100 treatment and DEAE-cellulose chromatography or sucrose gradient centrifugation. This preparation was incapable of cytochrome f photooxidation or of NADP photoreduction in the presence of plastocyanin. However, the reaction center lacking Subunit III photoreduced NADP when plastocyanin was replaced by N-methylphenazonium-3-sulfonate, serving as direct electron donor for P700. It was concluded that Subunit III mediates reduction of P700+ by plastocyanin. Mild treatment with sodium dodecyl sulfate yielded reaction center preparations which were partially depleted in Subunits II, III, IV, V, and VI. The NADP photoreduction activity and EPR-detectable bound ferredoxin were correlated to the presence of Subunits IV, V, and VI in the depleted preparations. Subunits IV, V, and VI might mediate photoreduction of soluble ferredoxin by the primary electron acceptor, in the reducing site of Photosystem I. The P700 reaction center (Subunit I) was further purified. Preparations with chlorophyll a to P700 ratios below 40 were obtained with the use of higher concentrations of Triton X-100, which caused antenna chlorophyll a to be partially solubilized. Removal of the sdlubilized chlorophyll by DEAE-cellulose chromatography resulted in preparations which contained 10 to 20 chlorophyll a molecules/P700. It is suggested that a unit of about 20 chlorophyll a molecules is specifically associated with the reaction center protein (Mr = 70,000) in the vicinity of P700. A model has been presented which depicts probable organization of the Photosystem I reaction center in the thylakoid membrane.