The PsbP protein, an extrinsic subunit of photosystem II (PSII) in green plants, is known to induce a conformational change around the catalytic Mn4CaO5 cluster securing the binding of Ca2+ and Cl- in PSII. PsbP has multiple interactions with the membrane subunits of PSII, but how these affect the structure and function of PSII requires clarification. Here, we focus on the interactions between the N-terminal residues of PsbP and the a subunit of Cytochrome (Cyt) b(559) (PsbE). A key observation was that a peptide fragment formed of the first N-terminal 15 residues of PsbP, 'pN15', was able to convert Cyt b(559) into its HP form. Interestingly, addition of pN15 to NaCl-washed PSII membranes decreased PSII's oxygen-evolving activity, even in the presence of saturating Ca2+ and Cl- ions. In fact, pN15 reversibly inhibited the S-1 to S-2 transition of the OEC in PSII. These data suggest that pN15 can modulate the redox property of Cyt b(559) involved in the side-electron pathway in PSII. This potential change of Cyt b559, in the absence of the C-terminal domain of PsbP, however, would interfere with any electron donation from the Mn4CaO5 cluster, leading to the possibility that multiple interactions of PsbP, binding to PSII, have distinct roles in regulating electron transfer within PSII.