Identification of cytochromes involved in electron transport to trimethylamine N-oxide/dimethylsulphoxide reductase in Rhodobacter capsulatus

The role of cytochromes in the electron-transport pathway to trimethylamine N-oxide (TMAO)/dimethylsulphoxide (DMSO) reductase in the photosynthetic bacterium Rhodobacter capsulatus was investigated. Reduced-minus-oxidized difference spectra in intact cells with TMAO or DMSO as oxidant revealed cytochrome absorbance changes with a maximum at 559 nm and a shoulder between 548 nm and 556 nm. The former change indicates a role for a 6-type cytochrome and the latter for a c-type cytochrome, both of which are distinct from the cytochrome bc₁ complex. Cytochrome c-556 was identified in a bacterial periplasmic fraction as a redox component which couldbe oxidised by TMAO or DMSO. Cytochrome c-556 was the only cytochrome species which co-fractionated with TMAO/DMSO reductase following gel filtration of a post-chromatophore supernatant produced after French presstreatment of intact cells. The mid-point redox potential (pH 7.6) of cytochrome c-556 was + 105 mV (n = 1). It is suggested that TMAO/DMSO reductase and cytochrome c-556 form a structural and functional association in the periplasm of Rhodobacter capsulatus.