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The PsaC subunit of photosystem I provides an essential lysine residue for fast electron transfer to ferredoxin
Author(s) -
Fischer Nicolas,
Hippler Michael,
Sétif Pierre,
Jacquot JeanPierre,
Rochaix JeanDavid
Publication year - 1998
Publication title -
the embo journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 7.484
H-Index - 392
eISSN - 1460-2075
pISSN - 0261-4189
DOI - 10.1093/emboj/17.4.849
Subject(s) - ferredoxin , biology , photosystem i , protein subunit , electron transfer , residue (chemistry) , photosystem ii , cytochrome b6f complex , lysine , biochemistry , photosynthesis , enzyme , photochemistry , gene , amino acid , chemistry
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 K 35 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 K 35 T, K 35 D and K 35 E drastically affect electron transfer from PSI to Fd, as measured by flash‐absorption spectroscopy, whereas the K 35 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 K 35 by T, D, E or R abolishes Fd cross‐linking to PsaC, and cross‐linking to PsaD and PsaE is reduced in the K 35 T, K 35 D and K 35 E mutants. In contrast, replacement of any other lysine of PsaC does not alter the cross‐linking pattern, thus indicating that K 35 is an interaction site between PsaC and its redox partner Fd.