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Plastocyanin (Pc) is a copper (Cu)-containing blue protein, that functions as a mobile electron carrier between cytochrome (cyt) f and Photosystem 1 (PS1) in oxygenic organisms. The atomic structure is known and can be described as a β-barrel with hydrophobic residues in the interior of the protein. To increase the understanding about structure-function relationships, site-directed mutagenesis of Pc has proven to be very useful. Mainly two spectroscopic techniques, optical and EPR spectroscopy, have been used to investigate how the copper-site is affected by different mutations. The redox properties of the mutants have been investigated and factors that affect the reduction potential are discussed. Absorption and EPR spectra and reduction potentials for the surface mutants are similar to those of the corresponding wild-type. However, mutants around the Cu ion affected the mentioned properties. Comparisons are made with other cupredoxins. Five site-directed mutants of spinach Pc, Pc(Leu12His), Pc(Leu15His), Pc(Thr79His), Pc(Lys81His) and Pc(Tyr83His), have been modified by covalent attachment of a photoactive ruthenium (Ru)-complex at the surface-exposed histidine residues. The rates of the internal electron-transfer reactions exhibit an exponential dependence on the metal-to-metal separation with a decay factor of 1.1 A-1. A reorganization energy for the Cu-to-Ru electron-transfer reaction of 1.2 eV was determined. Interprotein electron-transfer reactions involving genetically modified Pc are described. Ionic-strength and pH dependencies indicated that electrostatic interactions are involved in the complex formation between Pc and PS 1, which was confirmed by mutations in the acidic patches of Pc. A very specific interaction was further verified by replacements of hydrophobic residues. Position 10, 12, 36, 87 and 90 were found to be very important for the formation of an active complex. A comparison between available structures of Pc and cyt c6, both effective donors to PS 1, is made. The physiological electron donor to Pc, cyt f, is briefly described.