Abstract

Photosystem I (PSI) catalyzes the photooxidation of plastocyanin in the thylakoid lumen and the photoreduction of ferredoxin in chloroplast stroma or cyanobacterial cytoplasm (1). Optical and EPR spectroscopy and resolution-reconstitution biochemistry have been used to identify redox centers in PSI and to characterize kinetics of electron transfer. Light-induced oxidation of the P700 reaction center of PSI leads to charge separation. This is followed by a rapid spatial displacement of electrons through a series of low potential redox centers. The PSI complexes of cyanobacteria, algae and plants are similar in their structural and functional characteristics. Cyanobacterial PSI contains 11 different proteins, at least 89 chlorophyll a molecules, 10-12 s-carotenes, two phylloquinones (PhQ), and three [4Fe-4S] clusters. A structural model for the cyanobacterial PSI at 4A. resolution is available from X-ray crystallography (2). In recent years, role of proteins in influencing redox centers and electron transfer in PSI has been investigated using molecular genetics in cyanobacteria and algae (1). Here we describe a mutational strategy to understand function of the PhQs (vitamin Ki) in PSI.

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