To study the function of the PsaF subunit of photosystem I (PSI), the interactions between plastocyanin or cytochrome c₆ and PSI isolated from wild-type and a PsaF-deficient mutant of Chlamydomonas reinhardtii have been examined using cross-linking techniques and flash absorption spectroscopy. We show that efficient electron transfer from both plastocyanin and cytochrome c₆ to PSI depends on PsaF. A remarkable feature of the PSI complex of C. reinhardtii is that both plastocyanin and cytochrome c₆ reduce P700⁺ with first-order kinetics and a half-time of 3 μs, which is unique among photosynthetic organisms examined.