Masuda Shinji, Tsukatani Yusuke, Kimura Yasuaki, Nagashima Kenji V P, Shimada Keizo, Matsuura Katsumi
Department of Biology, Tokyo Metropolitan University, Minamiohsawa, Hachioji, Tokyo 192-0397, Japan.
Biochemistry. 2002 Sep 17;41(37):11211-7. doi: 10.1021/bi0258492.
The purple photosynthetic bacterium Rhodovulum sulfidophilum has an unusual reaction center- (RC-) bound cytochrome subunit with only three hemes, although the subunits of other purple bacteria have four hemes. To understand the electron-transfer pathway through this subunit, three mutants of R. sulfidophilum were constructed and characterized: one lacking the RC-bound cytochrome subunit, another one lacking cytochrome c(2), and another one lacking both of these. The mutant lacking the RC-bound cytochrome subunit was grown photosynthetically with about half the growth rate of the wild type, indicating that the presence of the cytochrome subunit, while not indispensable, is still advantageous for the photosynthetic electron transfer to support its growth. The mutant lacking both the cytochrome subunit and cytochrome c(2) showed a slower rate of growth by photosynthesis (about a fourth of that of the wild type), indicating that cytochrome c(2) is the dominant electron donor to the RC mutationally devoid of the cytochrome subunit. On the other hand, the mutant lacking only the cytochrome c(2) gene grew photosynthetically as fast as the wild type, indicating that cytochrome c(2) is not the predominant donor to the RC-bound triheme cytochrome subunit. We further show that newly isolated soluble cytochrome c-549 with a redox midpoint potential of +238 mV reduced the photooxidized cytochrome subunit in vitro, suggesting that c-549 mediates the cytochrome c(2)-independent electron transfer from the bc(1) complex to the RC-bound cytochrome subunit. These results indicate that the soluble components donating electrons to the RC-bound triheme cytochrome subunit are somewhat different from those of other purple bacteria.
紫色光合细菌嗜硫小红卵菌有一个不同寻常的与反应中心（RC）结合的细胞色素亚基，该亚基仅含三个血红素，而其他紫色细菌的亚基含有四个血红素。为了解通过该亚基的电子传递途径，构建并表征了嗜硫小红卵菌的三个突变体：一个缺少与RC结合的细胞色素亚基，另一个缺少细胞色素c2，还有一个同时缺少这两者。缺少与RC结合的细胞色素亚基的突变体通过光合作用生长，其生长速率约为野生型的一半，这表明细胞色素亚基的存在虽非必不可少，但对光合电子传递以支持其生长仍具有优势。同时缺少细胞色素亚基和细胞色素c2的突变体光合作用生长速率较慢（约为野生型的四分之一），这表明细胞色素c2是向因突变而缺少细胞色素亚基的RC提供电子的主要供体。另一方面，仅缺少细胞色素c2基因的突变体光合作用生长速度与野生型一样快，这表明细胞色素c2不是与RC结合的三血红素细胞色素亚基的主要供体。我们进一步表明，新分离的氧化还原中点电位为+238 mV的可溶性细胞色素c-549在体外还原了光氧化的细胞色素亚基，这表明c-549介导了不依赖细胞色素c2的从bc1复合体到与RC结合的细胞色素亚基的电子传递。这些结果表明，向与RC结合的三血红素细胞色素亚基提供电子的可溶性成分与其他紫色细菌的有所不同。
