An unsuspected autoregulatory pathway involving apocytochrome TorC and sensor TorS in Escherichia coli

Trimethylamine N-oxide (TMAO) respiration is carried out mainly by the Tor system in Escherichia coli. This system is encoded by the torCAD operon and comprises a periplasmic TMAO reductase (TorA) and a c-type cytochrome (TorC), which shuttles electrons to TorA. Expression of the tor operon is positively controlled by the TorS/TorR phosphorelay system in response to TMAO availability and negatively regulated by apocytochrome TorC. Interaction studies showed that, when immature, TorC can no longer bind TorA efficiently but can bind the periplasmic detector region of sensor TorS. ApoTorC negative autoregulation and TMAO induction are thus mediated by the same sensor protein. As apocytochromes related to TorC could not down-regulate the tor operon, we concluded that this negative control is highly specific. Moreover, the N-terminal half of apoTorC played no role in this control but the immature C-terminal domain of TorC strongly down-regulated the tor operon and interacted with the TorS detector region. This sophisticated autoregulatory pathway thus involves the C-terminal domain of apoTorC and allows optimal TorC biogenesis by preventing from saturation the c-type cytochrome maturation machinery.