Photosystem I and Regulatory Proteins for its Biogenesis

Photosystem I (PSI) is a pigment-protein complex located in the thylakoid membrane of cyanobacteria and chloroplasts of algae and higher plants, which functions as a plastocyanin (or cytochrome c6)-ferredoxin oxidoreductase. The reducing potential of ferredoxin is utilized for a variety of biochemical processes such as the reduction of NADP, or the assimilation of nitrate or sulfate. In higher plants the complex consits of 14 different polypeptide subunits, nine of them are encoded in the nucleus (PSI-D/E/F/G/H/K/L/N/O) and the residual five (PSI-A/B/C/I/J) are plastid-encoded. Most of the information about the function of PSI derives from mutants impared in one or more subunit genes or from biochemical studies. In 2003, the crystal structure of PSI from a higher plant was determined. In contrast to the trimeric cyanobacterial PSI, the plant PSI was purified as a monomer. Compared to the structural information, much less is known about the regulation and assembly of PSI and only a few regulatory factors have been identified so far. In this review we describe the structure of PSI and the role of the individual subunits and present an overview on those factors which are required for PSI accumulation in proand eukaryotic photosynthetic organisms. _____________________________________________________________________________________________________________