The PsaC subunit of photosystem I provides an essential lysine residue for fast electron transfer to ferredoxin.

PsaC is the stromal subunit of photosystem I (PSI) which binds the two terminal electron acceptors F_A and F_B. 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. [ABSTRACT FROM AUTHOR]