The cytochrome c₈ involved in the nitrite reduction pathway acts also as electron donor to the photosynthetic reaction center in Rubrivivax gelatinosus.

The purple photosynthetic bacterium Rubrivivax gelatinosus has, at least, four periplasmic electron carriers, i.e., HiPIP, two cytochromes c₈with low- and high-midpoint potentials, and cytochrome c₄ 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 c₈of 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 cd₁-type nitrite reductase. Phylogenetic analysis and the well-conserved nir operon gene arrangement suggest that the origin of the three cytochromes c₈ in this bacterium is NirM. The two other cytochromes c₈, of high and low potentials, proposed to be generated by gene duplication from NirM, have evolved to function in distinct pathways.