THE TORR GENE OF ESCHERICHIA-COLI ENCODES A RESPONSE REGULATOR PROTEIN INVOLVED IN THE EXPRESSION OF THE TRIMETHYLAMINE N-OXIDE REDUCTASE GENES

Expression of the Escherichia coli torCAD operon encoding the trimethy lamine N-oxide (TMAO) reductase system is induced by both TMAO and ana erobiosis. A torR insertion mutant unable to express the torA gene had previously been isolated. The torR gene was cloned and sequenced. It encodes a 25,000-Da protein which shares homology with response regula tors of two-component systems and belongs to the OmpR-PhoB subclass. O verproduction of TorR mimicks the presence of the inducer TMAO while t he anaerobic control is unchanged, suggesting that TorR mediates only the TMAO induction. The overproduced TorR protein was purified to more than 90%. The torR gene is located just upstream of the torCAD operon , with an opposite transcription direction. The torR-torCAD intergenic region is unusual in that it contains four direct repeats of a 10-nuc leotide motif. Part or all of these motifs could be involved in the bi nding of TorR. The gene encoding the sensor partner does not seem to b e adjacent to torR, since the divergent open reading frame found immed iately downstream of torR exhibits none of the features of a protein h istidine kinase.