FNR-mediated oxygen-responsive regulation of the nrdDG operon of Escherichia coli.

Transcription of the nrdDG operon, which encodes the class III nucleotide reductase, which is only active under anaerobic conditions, was strongly induced after a shift to anaerobiosis. The induction was completely dependent on the transcriptional activator FNR and was independent of the ArcA-ArcB two-component response regulator system. The nrdD transcript start site was mapped to a position immediately downstream of two FNR binding sites. Transcription of the other two nucleotide reductase operons, nrdAB and nrdEF, did not respond to oxygen conditions in a wild-type background, but nrdAB expression was increased in the fnr mutant under anaerobic conditions.