Evidence that the intermediate electron acceptor, A2, in Photosystem I is a bound iron-sulfur protein

Abstract Absorption changes accompanying the formation of light-induced P -700 + were investigated in a highly enriched Photosystem I preparation where an intermediate electron acceptor preceding P -430 could be detected. In an enriched Photosystem I particle, light-induced reversible absorption changes observed at 700 nm in the presence of dithionite resembled those previously seen at 703 nm and 820 nm [9], thus indicating the presence of a backreaction between P -700 + and A − 2 . After this same Photosystem I particle was treated to denature the bound iron-sulfur centers, the photochemical changes that could be attributed to P -700 A 2 were completely lost. These results provide evidence that the intermediate electron acceptor, A 2 , is a bound iron-sulfur protein. Additional studies in the 400–500 nm region with Photosystem I particles prepared by sonication indicate that the spectrum of A 2 is different from that of P -430.