Involvement of c-type cytochrome CymA in the electron transfer of anaerobic nitrobenzene reduction by Shewanella oneidensis MR-1

► Mtr respiration pathway is not involved in the NB bioreduction. ► CymA plays an important role in intracellular NB bioreduction by Shewanella. ► Some unknown protein are suggested to be involved in electron transfer. ► Novel electron transfer pathways of NB bioreducation are proposed. Effective bioreduction of nitrobenzene (NB) by electrochemically active bacteria, such as Shewanella and Geobacter, has been demonstrated. However, the mechanism behind such a bioreduction is unclear yet. In this work, the mechanism for anaerobic NB reduction by Shewanella oneidensis MR-1 was investigated at a gene level. The omcA-mtrCAB gene cluster, an important extracellular electron transfer chain in S. oneidensis MR-1 for the reduction of a variety of compounds, was found to be uninvolved in the NB bioreduction. Knockout of cymA, a tetraheme c-type cytochrome in the periplasmic space, led to a 67% loss in NB bioreduction efficiency in comparison with the wild strain, and caused accumulation of an intermediate, phenylhydroxylamine, at the initial stage of NB bioreduction. These results demonstrate that, unlike the previously reported pure extracellular reduction pathways for many compounds, intracellular reactions are involved in NB bioreduction by S. oneidensis MR-1, and CymA plays an important role in the electron transfer of the intracellular NB bioreduction. In addition, the participation of some unknown protein other than CymA in NB reduction is discovered, and the possible pathways of electron transfer in the NB reduction by S. oneidensis MR-1 are proposed.