Oxygen yield and thermoluminescence characteristics of a cyanobacterium lacking the manganese-stabilizing protein of photosystem II (original) (raw)

Previous experiments have shown that a Synechocystis sp. PCC 6803 mutant (ApsbU) lacking the extrinsic manganese-stabilizing protein (MSP) exhibits impaired, but significant levels of HzO-splitting activity [Burnap, R., & Sherman, L. A. (1991) Biochemistry30,440-446]. [ 14C]DCMU-bindingexperiments now show that the number and affinity of DCMU-binding sites (normalized to chlorophyll) are equivalent in ApsbU 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 0 2 yield by ApsbU 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 ApsbU 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 S~-[ S~] S O transition is retarded by at least a factor of 5 in the mutant. Thermoluminescence emission peak temperatures indicate that the SZQA-, S~QB-, and S3QB-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 02-evolving enzymes appears to be reduced. For those centers which are effectively coupled to 0 2 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. The H20-splitting reaction of oxygenic photosynthetic is catalyzed by the membrane-bound photosystem I1 (PSII) complex. The oxidative decomposition of two molecules of H20 involves the utilization of four oxidizing equivalents accumulated in the H20-splitting enzyme as a result of four successive charge-separation and-transfer events within the PSII reaction center. Accordingly, the H2O-splitting enzyme passes through a series of oxidation states, termed S states, which correspond, at least in part, to the stepwise oxidation ~~ This work was supported by DOE Grant DE-FG02-89-ER14028 (L.A.S.) and a grant on Photosynthetic Sciences from the Science and Technology Agency (Y.I.). R.L.B. thanks the Solar Energy Group at RIKEN in Tokyo for funding to perform the thermoluminescence measurements.