In photosynthetic organisms, the transfer of electrons from cytochromes b6f to photosystem I (PSI) — which are both membrane-embedded complexes — is performed by the soluble metalloproteins cytochrome c6 (Cyt) and plastocyanin [1]. Plastocyanin and Cyt react with PSI following a similar kinetic model and exhibit similar rate constants when they are both isolated from the same organism, but vary from one another. In cyanobacteria, the donor proteins can be acidic, neutral or basic, and the role of electrostatic forces in their interaction with PSI varies accordingly. Thus, the slightly acidic donors of Synechocystis (pI = 5.6) exhibit repulsive electrostatic interactions with PSI, the reaction of the neutral (pI = 7.0) Arthrospira Cyt is independent of ionic strength, and finally the interaction of the positively charged Nostoc donors with PSI is driven by attractive electrostatic forces. In order to understand the factors responsible for the specificity of the interaction between the donor protein and PSI, the kinetics of the cross-reactions between Cyt and PSI from the three cyanobacteria have been investigated by laser flash photolysis. The ionic strength dependence of th
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