The Organization of the Fe-S Acceptors of Photosystem 1

Abstract

In higher plants, photochemical electron transfer from plastocyanin to ferredoxin is catalyzed by photosystem I (PS I), a membrane bound protein complex containing the reaction centre P700 and at least five phot oreducible electron acceptors (A o, A 1, X(A 2), A and B (1). Comparative Mossbauer studies have indicated that both centre X, A and B are 4Fe-4S centres (2). Quantitative analyses indicate that these 4Fe-4S centres most likely account for all the iron and acid labile sulfide (ALS) associated with PS I (3,4). The functional characterization of the individual PS I polypeptides is far from complete. SDS-Polyacrylamide gel electrophoresis ( SDS-PAGE) of purified PS I particles reveals polypeptide bands at 110, 18, 15, 10 and 8 kDa. The reaction centre chlorophyll P700 is bound to P700-chlorophyll a-protein 1, migrating as a green band at 110 kDa. Denaturation of P700-chlorophyll aprotein 1 produces either a single apoprotein band or a closely spaced doublet in the 70 kDa region (4,5). Two partially homologous chloroplast genes have been identified as coding for each of these apoprotein bands (6).