The torYZ (yecK bisZ) operon encodes a third respiratory trimethylamine N-oxide reductase in Escherichia coli

The bisZ gene of Escherichia coli was previously described as encoding a mi nor biotin sulfoxide (BSO) reductase in addition to the main cytoplasmic BS O reductase, BisC. In this study, bisZ has been renamed torZ based on the f indings that (i) the torZ gene product, TorZ, is able to reduce trimethylam ine N-oxide (TMAO) more efficiently than BSO; (ii) although TorZ is more ho mologous to BisC than to the TMAO reductase TorA (63 and 42% identity, resp ectively), it is located mainly in the periplasm as is TorA; (iii) torZ bel ongs to the torn operon, and the first gene, torY (formerly yecK), encodes a pentahemic c-type cytochrome homologous to the TorC cytochrome of the Tor CAD respiratory system. Furthermore, the torn operon encodes a third TMAO r espiratory system, with catalytic properties that are clearly different fro m those of the TorCAD and the DmsABC systems. The torn and the torCAD opero ns may have diverged from a common ancestor, but, surprisingly, no torD hom ologue is found in the sequences around torn. Moreover, the torn 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.