FUNCTION AND ORGANIZATION OF PHOTOSYSTEM-I POLYPEPTIDES

Photosystem I functions as a plastocyanin:ferredoxin oxidoreductase in the thylakoid membranes of chloroplasts and cyanobacteria. The PS I c omplex contains the photosynthetic pigments, the reaction center P700, and five electron transfer centers (A(0), A(1), F-X, F-A, and F-B) th at are bound to the PsaA, PsaB, and PsaC proteins. In addition, PS I c omplex contains at least eight other polypeptides that are accessory i n their functions. Recent use of cyanobacterial molecular genetics has revealed functions of the accessory subunits of PS I. Site-directed m utagenesis is now being used to explore structure-function relations i n PS I. The overall architecture of PSI complex has been revealed by X -ray crystallography, electron microscopy, and biochemical methods. Th e information obtained by different techniques can be used to propose a model for the organization of PS I. Spectroscopic and molecular gene tic techniques have deciphered interaction of PS I proteins with the s oluble electron transfer partners. This review focuses on the recent s tructural, biochemical and molecular genetic studies that decipher top ology and functions of PS I proteins, and their interactions with solu ble electron carriers.