THERMOLUMINESCENCE AND FLASH-OXYGEN CHARACTERIZATION OF THE IC2 DELETION MUTANT OF SYNECHOCYSTIS SP PCC-6803 LACKING THE PHOTOSYSTEM-II 33 KDA PROTEIN

The psbO gene product of Photosystem II (PS II), the so-called 33 kDa extrinsic protein, is believed to be closely associated with the catalytic Mn cluster responsible for light-induced water oxidation. However, this protein is not absolutely required for water-splitting and its precise role remains to be clarified. We have used flash-induced thermoluminescence and oxygen evolution measurements to characterize the process of water oxidation in the IC2 mutant of Synechocystis sp. PCC 6803 from which the psbO gene had been deleted by Mayes et al. (Mayes, S.R., Cook, K.M., Self, S.J., Zhang, Z. and Barber, J. (1991) Biochim. Biophys. Acta 1060, 1-12). The thermoluminescence results show that the extent of charge stabilization in the S2Q(A)-and S2Q(B)- states is reduced in the IC2 mutant to about 25-30% of that observed in the wild-type, suggesting that functional oxygen evolution occurs in a proportion of the psbO-less mutant cells. The stability of the S2Q(A)-, but not that of the S2Q(B)-, charge pair is markedly increased in the mutant. This points to a structural change of the PS II reaction center complex in the absence of the psbO gene product which affects the redox properties of the Q(A) and Q(B) acceptors to a different extent. The flash-induced oscillation of the B thermoluminescence band, arising from the S2Q(B)- and S3Q(B)- charge recombinations, is largely dampened in the mutant. This indicates that the ability of the water-oxidizing complex to reach its higher oxidation states, S3 and S4, is limited when the psbO gene product is absent. In agreement with the thermoluminescence results, flash-induced oxygen evolution shows a decreased yield and largely dampened oscillation pattern in the mutant. These results indicate that although the psbO gene product is not an absolute requirement for water oxidation its absence disturbs the redox cycling of the water-oxidizing complex and retards the formation of its higher S states. The rapid loss of thermoluminescence intensity during strong illumination of the mutated organism confirms its high susceptibility to photoinhibition. This effect is most likely the consequence of the limited rate of electron donation from the psbO-less water-oxidizing complex to the PS II reaction centre where the accumulation of highly oxidizing species may damage their pigment and protein surroundings.