226 Biochimica et Biophysica Acta, 504 (1978) 226--230 © Elsevier/North-Holland Biomedical Press BBA Report BBA 41308 EVIDENCE THAT THE INTERMEDIATE ELECTRON ACCEPTOR, A2, IN PHOTOSYSTEM I IS A BOUND IRON-SULFUR PROTEIN J.H. GOLBECK, B.R. VELTHUYS and B. KOK Martin Marietta Laboratories, 1450 South Rolling Road, Baltimore, Md. 21227 (U.S.A.) (Received May 29th, 1978) Summary 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 en- riched 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~. 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 ~ A2 were completely lost. These results provide evidence that the intermediate electron acceptor, A2, 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 A2 is different from that of P-430. Identification of the components comprising the primary electron acceptor complex of Photosystem I has proceeded largely through the successful application of electron spin resonance and optical techniques. Electron spin resonance techniques have uncovered the existence of three components functioning on the reducing side of Ph0tosystem I: two membrane-bound iron-sulfur centers (ESR Centers A and B) [1,2] and one, as yet unidentified ESR component denoted as "X" [3,4]. Until very recently, optical techniques revealed only one component, P-430 [5, 6]. We have previously described a method [7] to denature the bound iron- s~ilfurproteins in a purified Photosystem I particle without destroying the primary electron donor, P-700 (as indicated by the ascorbate-minus-ferri- cyanide difference spectrum). In the course of denaturation, we observed a Abbreviations: DPIP, 2,6-dlchlorophenolindophenol.