Variety and dynamics of electron donors involved in primary reactions of bacterial photosynthesis

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1.New electron carrier proteins working as electron donors to the reaction center complex have been searched in purple photosynthetic bacteria.(1)New membrane-bound cytochrome c with a molecular mass of 50 kDa was found in a marine species, Rhodovulum sulfidophilum. Growth tests using mutants lacking this cytochrome showed that this cytochrome can completely support photosynthetic cyclic electron transport, as well as a popular electron carrier, cytochrome c_2. This new cytochrome was named cytochrome c_2m, taking into account its high sequence similarity with the cytochrome c_2 at the core region.(2)A c-type cytochrome with a molecular mass of 25 kDa was newly found as a potential electron donor to the reaction center in aβ-purple bacterium, Rubrivivax gelatinosus. Based on sequence analyses, this cytochrome was identified as a homolog of cytochrome c4, which physiological functions have not been well clarified. In R gelatinosus, growth tests for the mutants lacking this cytochrome showed that this cytochrome can contribute to the photosynthetic electron transfer. This is the first experimental evidence to show that cytochrome c_4 can work as a physiological electron donor to the reaction center.2.Redox midpoint potentials of two low-potential hemes in the tetrahemic cytochrome subunit bound to the reaction center have been raised about 60 mV by site-directed mutagenesis. These alterations resulted in changes in the rates of electron transfer from the high-potential hemes to the special pair of bacteriochlorophylls when peripheral redox potentials were lowered below the new midpoint potentials of the low-potential hemes. It was suspected that midpoint potentials of hemes are dynamically changed, depending on the redox states of the neighboring hemes.