Haloarcula marismortui cytochrome b-561 is encoded by the narC gene in the dissimilatory nitrate reductase operon.

The composition of membrane-bound electron-transferring proteins from denitrifying cells of Haloarcula marismortui was compared with that from the aerobic cells. Accompanying nitrate reductase catalytic NarGH subcomplex, cytochrome b-561, cytochrome b-552, and halocyanin-like blue copper protein were induced under denitrifying conditions. Cytochrome b-561 was purified to homogeneity and was shown to be composed of a polypeptide with a molecular mass of 40 kDa. The cytochrome was autooxidizable and its redox potential was -27 mV. The N-terminal sequence of the cytochrome was identical to the deduced amino acid sequence of the narC gene product encoded in the third ORF of the nitrate reductase operon with a unique arrangement of ORFs. The sequence of the cytochrome was homologous with that of the cytochrome b subunit of respiratory cytochrome bc. A possibility that the cytochrome bc and the NarGH constructed a supercomplex was discussed.