Plastocyanin (PC), a mobile electron transfer protein existing in the thylakoid lumen of photosynthetic organisms, is reduced by cytochrome f (cyt. f) and is oxidized by the reaction center chlorophyll (P700 + ) in the photosystem I (PSI) complex. Recent studies including computational simulation have indicated that the solvent-accessible site containing acidic residues (negative patch) of PC is an interacting site with cyt. f. Site-directed mutation studies have shown that the decrease in electrostatic properties of the negative patch decreases the rate of electron transfer from cyt. f to PC exponentially, and supported that cyt. f interacts with the negative patch of PC, although electron transfer from PC to P700 is suggested to follow through the Cu-coordinating solvent-accessible histidine site (hydrophobic patch). The molecular recognition site of PC was studied by following the spectral changes and electron transfer reactions using lysine peptides (Lsyptd = di-, tri-, tetra-, and pentalysine) and negative patch mutants of PC (M1 = Asp42Asn, M2 = Glu43Lys, M3 = Asp42Asn, Glu43Lys, M4 = Glu43Lys, Asp44Lys) by absorption spectra and stopped-flow measurements. We propose that lysine peptides serve as models of the PC interacting site of cyt. f.Lysine peptides caused changes in the absorption spectra of PC while glycine peptides and NaCl did not affect them. We attribute this effect to the binding of Lsyptd to the negative patch of PC. Lysine peptides also served as competitive inhibitors of the electron transfer from reduced cytochrome c (cyt. c) to oxidized PC, while they served as promoters for the electron transfer from [Fe(CN) 6 ] 4 - to oxidized PC. The electron transfer inhibiting effects are explained as competitive inhibition due to neutralization of the negative patch by formation of PC Lysptd complexes, while the electron transfer promoting effects are explained as due to formation of PC Lysptd complexes or Lysptd [Fe(CN) 6 ] 4 - complexes which then form an electron transferring complex, PC Lysptd [Fe(CN) 6 ] 4 - , without repulsion of the negative charges. The electron transfer rate from reduced cyt. c to oxidized PC and the inhibiting effect of Lysptd decreased upon decreasing the net charge of the negative patch, which demonstrates the negative patch is the dominant cyt. f recognition site for the electron transfer between cyt. f and PC. The inhibiting and promoting effects by Lysptd became significant when their net charges became more than four, suggesting the importance of having more than four net charges at the molecular recognition site. Association constants between Lysptd and PC were also obtained by fitting the obtained kinetic constants with different Lysptd concentrations.