1. The PsaC subunit of photosystem I provides an essential lysine residue for fast electron transfer to ferredoxin.
- Author
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Fischer, Nicolas, Hippler, Michael, Sétif, Pierre, Jacquot, Jean-Pierre, and Rochaix, Jean-David
- Subjects
CHARGE exchange ,PHOTOSYNTHETIC pigments ,MUTAGENESIS ,CHLAMYDOMONAS reinhardtii ,PLANT molecular biology ,MOLECULAR biology - Abstract
PsaC is the stromal subunit of photosystem I (PSI) which binds the two terminal electron acceptors F
A 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 K35 T, K35 D and K35 E drastically affect electron transfer from PSI to Fd, as measured by flash-absorption spectroscopy, whereas the K35 R 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 K35 T, K35 D and K35 E 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]- Published
- 1998
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