Efficient respiration on TMAO requires TorD and TorE auxiliary proteins in Shewanella oneidensis

Respiration on trimethylamine oxide (TMAO) allows bacterial survival under anoxia. In Shewanella oneidensis, Tor is the system involved in TMAO respiration and it is encoded by the torECAD operon. The torA and torC genes encode TorA terminal reductase and the TorC c-type cytochrome, respectively. Sequence analysis suggests that TorD is the putative specific chaperone of TorA, whereas TorE is of unknown function. The purpose of this study was to understand whether TorD and TorE are two accessory proteins that affect the efficiency of the Tor system by chaperoning TorA terminal reductase. Moreover, by deleting each gene, we established that the absence of TorD drastically affects the stability of TorA, while the absence of TorE does not affect TorA stability or activity. Since TMAO reduction was affected in the ΔtorE mutant, TorE could be an additional component of the TorC-TorA electron transfer chain during bacterial respiration. Finally, a fitness experiment indicated that the presence of TorE, as expected, confers a selective advantage in competitive environments.