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

1.在紫光合细菌中发现了作为反应中心复合体电子供体的新的电子载体蛋白：(1)在海洋种红景天中发现了新的膜结合的细胞色素c，其相对分子质量为50 kDa。利用缺乏这种细胞色素的突变体进行的生长试验表明，这种细胞色素能够完全支持光合作用循环电子传递，以及一种受欢迎的电子载体细胞色素c_2。这种新的细胞色素被命名为细胞色素c_2m，因为它与核心区的细胞色素c_2具有很高的序列相似性。(2)在一株β紫色细菌中，新发现了一种c型细胞色素，其相对分子质量为25 kDa，是反应中心的潜在电子供体。根据序列分析，该细胞色素被鉴定为细胞色素C4的同系物，其生理功能尚未很好地阐明。在胶状根霉中，对缺乏这种细胞色素的突变体的生长测试表明，这种细胞色素可以促进光合作用的电子传递。这是首次有实验证据证明细胞色素C_4可以作为反应中心的生理性电子供体。2.通过定点突变，反应中心结合的四氢细胞色素亚基中的两个低电位血红素的氧化还原中点电位提高了约60 mV。当外周氧化还原电位降低到低电位血红素的新中点电位以下时，这些变化导致从高电位血红素到特殊的细菌-叶绿素对的电子转移速率发生变化。有人推测，血红素的中点电位是动态变化的，这取决于邻近的血红素的氧化还原状态。

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.