AN UNORTHODOX SENSOR PROTEIN (TORS) MEDIATES THE INDUCTION OF THE TORSTRUCTURAL GENES IN RESPONSE TO TRIMETHYLAMINE N-OXIDE IN ESCHERICHIA-COLI

We isolated and characterized three spontaneous mutations leading to t rimethylamine N-oxide (TMAO)-independent expression of the for operon encoding the TMAO-reductase anaerobic respiratory system in Escherichi a coli. The mutations lie in a new for regulatory gene, the torS gene, which probably encodes a sensor protein of a two-component regulatory system, One mutation, which leads to full TMAO-constitutive expressio n, is a 3-amino-acid deletion within the potential N-terminal periplas mic region, suggesting that this region contains the TMAO-detector sit e, for the other two mutations, a further induction of the tor operon is observed when TMAO is added, Both are single substitutions and affe ct the linker region located between the detector and the conserved tr ansmitter domains. Thus, as proposed for other sensors, the TorS linke r region might play an essential role in propagating conformational ch anges between the detector and the cytoplasmic signalling regions. The TorS histidine kinase is an unorthodox sensor that contains a receive r and a C-terminal alternative transmitter domain in addition to the d omains found in most sensors, Previously, we showed that TMAO inductio n of the tor operon requires the Torn response regulator and the TorT periplasmic protein, Additional genetic data confirm that torS encodes the sensor partner of TorR and TorT, First, insertion within torS abo lishes for operon expression whatever the growth conditions, Second, o verexpressed Torn bypasses the requirement for torS, whereas the torT gene product is dispensable for for operon expression in a torS consti tutive mutant, This supports a signal-transduction cascade from TorT t o TorR via TorS.