Expression from the Escherichia coli hcp-hcr operon promoter is optimally induced during anaerobic conditions in the presence of nitrite. This expression depends on transcription activation by FNR (fumarate and nitrate reduction regulator), which binds to a target centred at position −72.5 upstream of the transcript start site. Mutational analysis was exploited to identify the corresponding −10 and −35 hexamer elements. A DNA site for NarL and NarP, located at position −104.5, plays only a minor role, whereas NsrR binding to a DNA target centred at position +6 plays a major role in induction of the hcp-hcr operon promoter. Electrophoretic mobility-shift assays show that NsrR binds to this target. The consequences of this for the kinetics of induction of the hcp-hcr operon are discussed.

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