Conservation of cis-acting elements within the tor regulatory region among different Enterobacteriaceae

The Escherichia coli (Ec) torCAD operon encoding the trimethyl amine N-oxide (TMAO) reductase system is induced by both TMAO and anaerobiosis. The tor regulatory regions from bacteria related to Ec have been amplified by the polymerase chain reaction (PCR) using degenerate oligodeoxyribonucleotide primers based on conserved sequences of the tor products. The amplified regions from Salmonella enteritidis and Sa. typhimurium (St) were the same size as that from Ec and showed 82% identity with it. Interestingly, four boxes of a 10-nucleotide motif (5'-CTGTTCATAT) were found in direct repeat at the same location in the tor regulatory region of the three species. Although the amplified fragment from Shigella sonneï (Ss) was highly homologous to the Ec corresponding segment, the first tor box was missing. In Ec, the St and Ss tor promoters were still regulated by both TMAO and anaerobiosis, but their transcriptional activities were significantly lower than that of the Ec tor promoter. Deletion of the two first boxes of the Ec tor regulatory region inactivated the tor promoter while deletion of the region just upstream from the tor boxes led to a significant decrease in tor expression. Our results strongly suggest that the tor boxes, as well as specific sequences outside the tor boxes, play an important role in the expression of the tor operon.