Kinetics of Electron Transfer between QA and QB in Wild Type and Herbicide-Resistant Mutants of Chlamydomonas reinhardtii

We have investigated the electron transfer kinetics for reduction of plastoquinone by photo­ system II in six mutant strains of Chlamydomonas reinhardtii by following the decay of the high fluorescence state after flash activation, and compared the separate reactions of the two-electron gate with those of a wild type strain. By analysis of the electron transfer kinetics, and separate measurement of the equilibrium constant for stabilization of the bound semiquinone after one flash, we have been able to deconvolute the contributions of rate constants and equilibrium constants for plastoquinone binding and electron transfer to the overall process. Two mutations, S 264 A and A 251 V, led to a marked slowing of kinetics for reduction of plastoquinone to the bound semiquinone. In S 264 A , the second electron transfer was also slower, but was normal in A 251 V. In mutant G 256 D , the electron transfer kinetics were normal after the first flash, but slowed after the second. In mutants L 257 F , V 219 I, and F 255 Y , the electron transfer kinetics after both flashes were similar to those in wild type. We discuss the results in terms of a model which provides a description of the mechanism of the two-electron gate in terms of measured kinetic and equilibrium constants, and we give values for these parameters in all strains tested.