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.