A Large Fraction of PsaF Is Nonfunctional in Photosystem I Complexes Lacking the PsaJ Subunit

PsaJ is a small hydrophobic subunit of the photosystem I complex (PSI) whose function is not yet fully understood. Here we describe mutants of the green alga Chlamydomonas reinhardtii, in which the psaJ chloroplast gene has been inactivated either in a wild-type or in a PsaF-deficient nuclear background. Cells lacking one or both subunits grow photoautotrophically and contain normal levels of PSI. Flash-absorption spectroscopy performed with isolated PSI particles isolated from the PsaJ-deficient strain indicates that only 30% of the PSI complexes oxidize plastocyanin (Pc) or cytochrome c 6 (Cyt c 6) with kinetics identical to wild type, whereas the remaining 70% follow slow kinetics similar to those observed with PsaF-deficient PSI complexes. This feature is not due to partial loss of PsaF, as the PsaJ-less PSI complex contains normal levels of the PsaF subunit. The N-terminal domain of PsaF can be cross-linked to Pc and Cyt c 6 indicating that in the absence of PsaJ, this domain is exposed in the lumenal space. Therefore, the decreased amount of functional PsaF revealed by the electron-transfer measurements is best explained by a displacement of the N-terminal domain of PsaF which is known to provide the docking site for Pc and Cyt c 6. We propose that one function of PsaJ is to maintain PsaF in a proper orientation which allows fast electron transfer from soluble donor proteins to P700+.