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The functionality of PsaF with respect to the photosystem I complex is diminished due to a displacement of the N-terminal domain of the subunit. This is evident in an experiment in which the psaJ chloroplast gene of the green alga Chlamydomonas reinhardtii was inactivated either in a wild-type or in a PsaF-deficient nuclear background. The N-terminal domain of PsaF is exposed in the lumenal space in the absence of PsaJ when the domain is cross-linked to Pc and Cyt c(sub 6). It is suggested that PsaJ maintains PsaF in a proper orientation and results in fast electron transfer from soluble donor proteins to P700(super +).

Cross-linking techniques and flash absorption spectroscopy were used to determine the interactions between plastocyanin or cytochrome c6 with photosystem I (PSI) from wild-type and a PsaF-deficient mutant of Chlamydomonas reinhardtii. Results reveal that efficient electron transfer from cytochrome c6 and plastocyanin to photosystem I of C. reinhardtii requires PsaF. It is also found that the PSI complex of C. reinhardtii is remarkably different from other photosynthetic organisms since its plastocyanin and cytochrome c6 reduce P700+ with first order kinetics and a half-time of 3 mus.

The process of electron transfer from the soluble protein plastocyanin to the multiprotein complex of photosystem I from spinach was examined. The individual dissociation constants of the complex between the proteins in all oxidized and reduced states was determined by monitoring the effect of increasing the amount of reduced plastocyanin on the kinetic components and the competition by oxidized plastocyanin. Findings indicated that a substantial portion of the energy for electron transfer is diverted to photosystem I.

The composition of light harvesting complex I (LHCI) of a tomato is examined. The findings indicate that several populations of the photosystem I that differ in their Lhca composition may exist.

The subunit compositions of the PSI-LHCI (photosystem I -light-harvesting chlorophyll protein complex I)supercomplex and the LHCI by immunoblot analysis and two-dimensional gel electrophoresis combined with mass spectrometry is examined. The possible localization and function of Lhca polypeptides in the LHCI is discussed.

The binding and electron transfer between plastocyanin (pc) or cytochrome c6 (cyt c6) and photosystem I (PSI) can be described by hydrophic as well as electrostatic interactions. The two α helices, l and l' in PsaB and PsaA, respectively, are involved in forming the hydrophobic interaction site at the oxidizing site of PSI. To obtain mechanistic insights into the function of the two negatively charged residues D612 and E613, present in α helix l of PsaB, we exchanged both residues by site-directed mutagenesis with His and transformed a PsaB deficient mutant of Chlamydomonas reinhardtii. Flash-induced absorption spectroscopy revealed that PSI harboring the changes D612H and E613H had a high affinity toward binding ol the electron donors and possessed an altered pH dependence of electron transfer with pc and cyt c6. Despite optimized binding and electron transfer between the altered PSI and its electron donors, the mutant strain PsaB-D612H/E613H exhibited a strong light sensitive growth phenotype, indicating that decelerated turnover between pc/cyt c6 and PSI with respect to electron transfer is deleterious to the cells. [ABSTRACT FROM AUTHOR]