Photosynthetic oxygen evolution by plants, algae, and cyanobacteria is performed at the Mn4Ca cluster in photosystem II (PSI!) by light-driven water oxidation. It has been proposed that CP43-Arg357, which is located in the vicinity of the Mn4Ca cluster, plays a key role in the O-2 evolution mechanism; however, direct evidence for its involvement in the reaction has not yet been obtained. In this study, we have for the first time detected the structural coupling of CP43-Arg357 with the Mn4Ca cluster by means of isotope-edited Fourier transform infrared (FTIR) spectroscopy. Light-induced FTIR difference spectra upon the S-1 -> S-2 transition (S-2/S-1 difference spectra) of the Mn4Ca cluster were measured using isolated PSII core complexes from Synechocystis sp. PCC 6803 cells, where the Arg side chains were labeled with either [eta(1,2)-N-15(2)]Arg or [zeta-C-13]Arg. Bands due to Arg side chain vibrations, which were extracted by taking a double difference between the S-2/S-1 spectra of isotope-labeled and unlabeled samples, were found at 1700-1600 and 1700-1550 cm(-1) for [eta(1,2)-N-15(2)]Arg- and [zeta-C-13]Arg-labeled PSII, respectively. These frequency regions are in good agreement with those of the CN/NH2 vibrations of a guanidinium group in difference spectra between isotope-labeled and unlabeled Arg in aqueous solutions. The detected Arg bands in the S-2/S-1 difference spectra were attributed to CP43-Arg357, which is the only Arg residue located near the Mn4Ca cluster. The presence of relatively high frequency bands arising from unlabeled Arg suggested that the guanidinium N(eta)H2 is engaged in strong hydrogen bonding. These results indicate that CP43-Arg357 interacts with the Mn4Ca cluster probably through direct hydrogen bonding to a first coordination shell ligand of a redox-active Mn ion. This structural coupling of CP43-Arg357 may play a crucial role in the water oxidation reactions.