PsaC is the stromal subunit of photosystem I (PSI) which binds the two terminal electron acceptors FA and FB. This subunit resembles 2[4Fe-4S] bacterial ferredoxins but contains two additional sequences: an internal loop and a C-terminal extension. To gain new insights into the function of the internal loop, we used an in vivo degenerate oligonucleotide-directed mutagenesis approach for analysing this region in the green alga Chlamydomonas reinhardtii. Analysis of several psaC mutants affected in PSI function or assembly revealed that K35 is a main interaction site between PsaC and ferredoxin (Fd) and that it plays a key role in the electrostatic interaction between Fd and PSI. This is based upon the observation that the mutations K35T, K35D and K35E drastically affect electron transfer from PSI to Fd, as measured by flash-absorption spectroscopy, whereas the K35R change has no effect on Fd reduction. Chemical cross-linking experiments show that Fd interacts not only with PsaD and PsaE, but also with the PsaC subunit of PSI. Replacement of K35 by T, D, E or R abolishes Fd cross-linking to PsaC, and cross-linking to PsaD and PsaE is reduced in the K35T, K35D and K35E mutants. In contrast, replacement of any other lysine of PsaC does not alter the cross-linking pattern, thus indicating that K35 is an interaction site between PsaC and its redox partner Fd.