Variable fluorescence of photosystem I particles and its application to the study of the structure and function of photosystem I

Chlorophyll a fluorescence in Photosystem I (PSI) particles isolated according to the method of Bengis and Nelson [J. Biol. Chem.252, 4564–4569 (1977)]was found to be dependent on the redox state of both P700 and X (an acceptor on the reducing side of PSI). Addition of dithionite plus neutral red to PSI caused an increase in fluorescence intensity and a shift of the main fluorescence peak from 689 to 674 nm. Addition of electron acceptors such as ferredoxin and methyl viologen decreased the fluorescence yield when added to PSI incubated under anaerobic conditions in the presence of excess dichlorophenol indophenol (DCIPH₂). The K_m for ferredoxin agreed with that determined from direct measurements of ferredoxin reduction, showing that X is a quencher of fluorescence. P700 was also found to be a quencher of fluorescence, since electron donors such as DCIPH₂, TMPD, and plastocyanin decreased fluorescence with K_m's nearly identical to those observed for P700⁺ reduction. Chemical modification of PSI (with ethylene diamine + a water-soluble carbodiimide) to make it positively charged increased the fluorescence yield and shifted the 689-nm peak to 674 nm. The K_m's for DCIPH₂ and ferredoxin were decreased. In contrast, modification of PSI with succinic anhydride, which increased the net negative charge, increased the K_m for ferredoxin. Salts affected the interaction of methyl viologen with PSI. Both anion and cation selectivity were observed. Limited proteolysis increased the K_m for both methyl viologen and ferredoxin, indicating that their binding site on PSI was altered. These results suggest that the binding site for ferredoxin is on either the 70- or the 20-kDa subunit of PSI.