Study on the photosynthetic mechanism by spectroscopic measurements of cyanobacterial mutants

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We have examined the structure and reactions of photosystem II (PSII) complexes by analyzing the mutants of the thermophilic cyanobacterium Thermosynechococcus elongatus by means of light-induced Fourier transform infrared (FTIR) difference spectroscopy. The following results were obtained.1.Using the highly active PSII core complexes in which the CP43 protein is histidine tagged, the reaction mechanism of the water oxidation was studied. We have for the first time detected the reactions of substrate water in the individual S-state transitions (S_1→S_2,S_2→S_3,S_3→S_0,S_0→S_1) in the reaction cycle as the changes in FTIR bands of the water OH stretches (3500-3650 cm^<-1>). The changes in the amino acid side chains and polypeptide backbones were also identified using ^<13>C and ^<15>N isotope substitution. Furthermore, pH dependence of the individual S-state transitions was studied by measurements of flash-induced FTIR spectra of S-state cycle at various pHs. Remarkable inhibition in the acidic region was observed in the three S-state transitions other than the S_1→S_2, indicating the presence of proton release at these transitions.2.We have prepared the T.elongatus mutant in which D2-Tyr160 (YD) was replaced with Phe (D2-Y160F) and studied the role of Y_D, the peripheral electron donor to the primary electron donor chlorophyll P680. The FTIR difference spectrum upon light-induced P680 formation of the D2-Y160F mutant showed a clear downshift of carbomethoxy C=O stretching band at 1751 cm^<-1> observed in WT. This observation indicates that Y_D interacts with P680 by a hydrogen-bonding network and it is suggested that the oxidation status of Y_D controls the redox potential of P680 through this interaction.