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Revised Manuscript Received June 26, 1995® abstract: The reaction mechanism of electron transfer from the interchangeable metalloproteins plastocyanin (Pc) and cytochrome C (,(Cyt) to photooxidized P700 in photosystem I (PSI) has been studied by laser-flash absorption spectroscopy using a number of evolutionarilydifferentiated organismssuch as cyanobacteria (Anabaena sp. PCC 7119 and Synechocystis sp. PCC 6803), green algae (Monoraphidium braunii), and higher plants (spinach). PSI reduction by Pc or Cyt shows different kinetics depending on the organism from which the photosystem and metalloproteins are isolated. According to the experimental data herein reported, three different kinetic models are proposed by assuming either an oriented collisional reaction mechanism (type I), a minimal two-step mechanism involving complex formation followed by intracomplex electron transfer (type II), or …