Oxygen Yield and Thermoluminescence Characteristics of a Cyanobacterium Lacking the Manganese-Stabilizing Protein of Photosystem II

Previous experiments have shown that a Synechocystis sp. PCC 6803 mutant (∆psbO) lacking the extrinsic manganese-stabilizing protein (MSP) exhibits impaired, but significant levels of H₂O-splitting activity [Burnap, R., & Sherman, L.A. (1991) Biochemistry 30, 440-446]. [¹⁴C] DCMU-binding experiments now show that the number and affinity of DCMU-binding sites (normalized to chlorophyll) are equivalent in ∆psbO and the wild type, suggesting equal concentrations of assembled reaction centers. A similar conclusion is reached on the basis of measurements of PSII electron transport (DPC-supported DCPIP reduction) by mutant and wild-type thylakoids. The pattern of flash O₂ yield by ∆psbO cells measured with a bare platinum electrode exhibits a period four oscillation (with a maximum on the third flash), indicating that the H₂O-splitting enzyme in ∆psbO retains the basic mechanistic features found in normal cells. However, the amplitude of these signals is smaller and more highly damped than those obtained from wild-type cells, suggesting the absence of MSP results in a higher miss probability and/or a reduction in the number of centers competent in oxygen evolution. Analysis of the rise kinetics of the ampermeric signal on the bare platinum electrode indicates that the S₃-[S₄]-S₀ transition is retarded by at least a factor of 5 in the mutant. Thermoluminescence emission peak temperatures indicate that the S₂Q_A. S₂Q_B, and S₃Q_B”charge pairs are significantly more stable with respect to recombination in the mutant. The intensities of the thermoluminescence emissions are also significantly reduced in the mutant. Taken together, the data suggest that functional consequences of the genetic removal of MSP are complex. Although the number of photochemically active PSII reaction centers is not much changed by the absence of MSP, the proportion of centers which are coupled to functional O₂-evolving enzymes appears to be reduced. For those centers which are effectively coupled to O₂ evolution, we find evidence of alterations in the kinetic properties of the enzyme due to the absence of MSP. These are (1) an increased miss factor, (2) a retardation of the S₃-[S₄]-So transition, and (3) an increase in the stabilization of the S₂ and S₃ states.