Roadmap of electrons from donor side to the reaction center dimer of photosynthetic purple bacteria with mutated cytochromes

Abstract

In photosynthetic bacteria, the absorbed light drives the canonical cyclic electron transfer between the reaction center and the cytochrome bc1 complexes via the pools of mobile electron carriers. If kinetic or structural barriers hinder the participation of the bc1 complex in the cyclic flow of electrons, then the pools of mobile redox agents must supply the electrons for the multiple turnovers of the reaction center. These conditions were achieved by continuous high light excitation of intact cells of bacterial strains Rba. sphaeroides and Rvx. gelatinosus with depleted donor side cytochromes c2 (cycA) and tetraheme cytochrome subunit (pufC), respectively. The graduate oxidation by ferricyanide reduced further the availability of electron donors of pufC. The electron transfer through the reaction center was tracked by absorption change of the dimer and by induction and relaxation of the bacteriochlorophyll fluorescence. The rate constants of the electron transfer (~ 3·103 s‒1) from the mobile donors of Rvx. gelatinosus bound either to the RC (pufC) or to the tetraheme subunit (wild type) were similar. The electrons transferred through the reaction center dimer were supplied entirely by the donor pool, their number amounted about 5 in wild type Rvx. gelatinosus and decreased to 1 by exhaustion of the pool in pufC oxidized by ferricyanide. The complex shape of the measured function of the yield of fluorescence versus oxidized dimer revealed the contribution of two competing processes: the migration of the excitation energy among the photosynthetic units and the availability of electron donors to the oxidized dimer. The experimental results were simulated and rationalized by a simple kinetic model of the two-electron cycling of the acceptor side combined with aperiodic one-electron redox function of the donor side.