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Abstract

The kinetics of electron transfer from the primary quinone, Q_A, to the secondary quinone, Q_B, have been measured by two methods. With chromatophores, the redox poising was adjusted so that the bound cytochrome had one heme reduced. A saturating flash was fired, populating the state (P-960⁺, Q_A⁻) and, in less than a microsecond, P-960⁺ was re-reduced by the cytochrome. The reoxidation of Q_A⁻ was followed by the ability of a second flash, given at a variable delay †t after the first, to photooxidize the primary donor P-960 (double flash method). With isolated reaction centers, the bound cytochrome was nearly entirely oxidized and Q_B was reconstituted with ubiquinone-9. The state (P-960⁺, Q_A⁻) was induced by a laser flash, and electron transfer from Q_A⁻ to Q_B was measured by the decay of an absorption shift at 830 nm. Control experiments done in presence of Q_B inhibitors showed that the kinetics were indeed attributable to Q_B reduction. In chromatophores and in reaction centers from Rhodopseudomonas viridis wild type, both methods gave very similar results, with a half-time of about 25 μs. No significant effect of pH was observed, between pH 5.5 and 9.0. The same measurements were also performed with materials from mutant cells selected for their resistance to the herbicide terbutryn. Chromatophores of the T4 mutant (Tyr L222→Phe), gave results analogous to the wild type. With the T1 mutant (Ser L223→Ala and Arg L217→ His), however, kinetics were greatly accelerated. Absorption shift measurements gave half-times of 8 μs at pH 8.0 and 4 μs at pH 6.0. The results of double flash experiments were globally consistent with absorption shifts. The faster kinetics in the T1 mutant are discussed in relation to the higher binding affinity for ubiquinone-9 at its QB site and to the rate of charge recombination between P-960⁺ and reduced quinones.