Abstract
The purple photosynthetic bacterium Rubrivivax gelatinosus has, at least, four periplasmic electron carriers, i.e., HiPIP, two cytochromes c 8 with low- and high-midpoint potentials, and cytochrome c 4 as electron donors to the photochemical reaction center. The quadruple mutant lacking all four electron carrier proteins showed extremely slow photosynthetic growth. During the long-term cultivation of this mutant under photosynthetic conditions, a suppressor strain recovering the wild-type growth level appeared. In the cells of the suppressor strain, we found significant accumulation of a soluble c-type cytochrome that has not been detected in wild-type cells. This cytochrome c has a redox midpoint potential of about + 280 mV and could function as an electron donor to the photochemical reaction center in vitro. The amino acid sequence of this cytochrome c was 65% identical to that of the high-potential cytochrome c 8 of this bacterium. The gene for this cytochrome c was identified as nirM on the basis of its location in the newly identified nir operon, which includes a gene coding cytochrome cd 1-type nitrite reductase. Phylogenetic analysis and the well-conserved nir operon gene arrangement suggest that the origin of the three cytochromes c 8 in this bacterium is NirM. The two other cytochromes c 8, of high and low potentials, proposed to be generated by gene duplication from NirM, have evolved to function in distinct pathways.
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