FNR is a direct oxygen sensor having a biphasic response curve

FNR is a transcription regulator that controls the expression of target genes in response to anoxia. Anaerobiosis is accompanied by the acquisition of two [4Fe‐4S] 2+ clusters per FNR dimer and the ability to bind DNA site‐specifically. Oxidation of the [4Fe‐4S] 2+ form of FNR by O 2 produced a non‐DNA‐binding, transcriptionally inactive form which also contains an iron‐sulfur cluster, recently identified by Mossbauer spectroscopy as a [2Fe‐2S] cluster (Khoroshilova et al., 1997, PNAS. 94, 6078). Complete conversion needed at least 2.5–3.0 molecules of O 2 per [4Fe‐4S] 2+ cluster. Using sub‐stoicheiometric amounts of air‐saturated buffer, stable equilibria were established in which the [4Fe‐4S] 2+ and [2Fe‐2S] 2+ forms co‐exist and no EPR detectable free ferric ions were released. In contrast, a 20‐fold molar excess K 3 Fe(CN) 6 was required to oxidise the [4Fe‐4S] 2+ cluster and in this case, ferric ions were released. FNR is therefore a sensitive O 2 sensor.