The bisZ gene of Escherichia coli was previously described as encoding a minor biotin sulfoxide (BSO) reductase in addition to the main cytoplasmic BSO reductase, BisC. In this study, bisZ has been renamed torZ based on the findings that (i) the torZ gene product, TorZ, is able to reduce trimethylamine N-oxide (TMAO) more efficiently than BSO; (ii) although TorZ is more homologous to BisC than to the TMAO reductase TorA (63 and 42% identity, respectively), it is located mainly in the periplasm as is TorA; (iii) torZ belongs to the torYZ operon, and the first gene, torY (formerly yecK), encodes a pentahemic c-type cytochrome homologous to the TorC cytochrome of the TorCAD respiratory system. Furthermore, the torYZ operon encodes a third TMAO respiratory system, with catalytic properties that are clearly different from those of the TorCAD and the DmsABC systems. The torYZ and the torCAD operons may have diverged from a common ancestor, but, surprisingly, no torD homologue is found in the sequences around torYZ. Moreover, the torYZ operon is expressed at very low levels under the conditions tested, and, in contrast to torCAD, it is not induced by TMAO or dimethyl sulfoxide.Escherichia coli can survive in various growth conditions owing to its ability to adapt in response to environmental changes. For example, in anaerobiosis and according to the exogenous electron acceptor present in the medium, this organism synthesizes the energetically more appropriate respiratory system (18). Sometimes, more than one respiratory system is produced for a given substrate. For instance, reduction of nitrate can be carried out by at least three respiratory systems (10). At high concentrations of nitrate, only the membranous NarG system is synthesized (46), whereas at very low concentrations, the periplasmic Nap system is produced (36). The operon encoding a third system (NarZ) is expressed during the early stationary phase under control of s , irrespective of the presence of nitrate (8). Accordingly, whatever the nitrate concentration and the growth phase, at least one of the nitrate reductases is synthesized in the cell (8,36).Trimethylamine N-oxide (TMAO) is reduced to the volatile compound trimethylamine (TMA) by at least two respiratory systems, the TorCAD and the DmsABC systems (5, 29). The torCAD operon, which encodes the periplasmic Tor system, is induced in the presence of TMAO by the TorS-TorR twocomponent regulatory system (25), whereas the membranous dimethyl sulfoxide (DMSO) reductase system, encoded by the dmsABC operon, is synthesized constitutively in anaerobiosis (5). The reason for the presence in the same host of several systems dedicated to a common substrate is still unclear, but one possibility is that they allow the cell to better adapt to changing environmental conditions during the different growth phases.The terminal reductase of the inducible Tor pathway, TorA, encoded by the torA gene, is a periplasmic molybdoenzyme of 90 kDa (29) that is thought to receive electrons from the membrane pool of menaquinone ...