The binding and electron transfer between plastocyanin (pc) or cytochrome c(6) (cyt
c(6)) and photosystem I (PSI) can be described by hydrophobic 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 of the electron donors and possessed
an altered pH dependence of electron transfer with pc and cyt c(6). 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 c(6) and PSI with respect to electron
transfer is deleterious to the cells.