PsaE is one of the photosystem I subunits involved in ferredoxin binding. The central role of arginine 39 of this 8-kDa peripheral polypeptide has been established by a series of mutations. The neutral substitution R39Q leads to a 250-fold increase of the dissociation constant K(d) of the photosystem I-ferredoxin complex, as large as the increase induced by PsaE deletion. At pH 8.0, this K(d) value strongly depends on the charge of the residue substituting Arg-39: 0.22 microM for wild type, 1.5 microM for R39K, 56 microM for R39Q, and more than 100 microM for R39D. The consequences of arginine 39 substitution for the titratable histidine were analyzed as a function of pH. The K(d) value of R39H is increased 140 times at pH 8.0 but only 5 times at pH 5.8, which is assigned to the protonation of histidine at low pH. In the mutant R39Q, the association rate of ferredoxin was decreased 3-fold compared with wild type, whereas an 80-fold increase is calculated for the dissociation rate. We propose that a major contribution of PsaE is to provide a prominent positive charge at position 39 for controlling the electrostatic interaction and lifetime of the complex with ferredoxin.