The narL gene product activates the nitrate reductase operon and represses the fumarate reductase and trimethylamine N-oxide reductase operons in Escherichia coli.

Escherichia coli, which can utilize O2, nitrate, fumarate, or trimethylamine N-oxide (Me3NO) as terminal electron acceptor, preferentially utilizes the one with the highest redox potential. Thus O2 prevents induction of nitrate, fumarate, and Me3NO reductases, and nitrate curtails the induction of fumarate and Me3NO reductases. Under anaerobic conditions the narL gene product, in the presence of nitrate, is known to activate transcription of the narC operon, which encodes nitrate reductase. This study shows that the same product plays a role in the repression by nitrate of the operons (frd and tor) that encode fumarate and Me3NO reductases. In contrast, the anaerobic repression of ethanol dehydrogenase by nitrate does not require the narL product. Expression of narL does not require the fnr gene product, a pleiotropic activator that is required for full expression of narC, frd, and tor.