FNR is a global regulator that controls transcription of genes whose functions facilitate adaptation to growth under O2 limiting conditions. It has long been appreciated that the activity of FNR must be regulated by O2 availability, since FNR dependent gene expression is observed in vivo only under anaerobic conditions, while similar levels of this protein are present in both aerobic and anaerobic grown cells. Recent progress in this field has shown that anaerobically purified FNR contains a [4Fe-4S]2+ cluster and that this [4Fe-4S]2+ cluster is sufficiently unstable toward O2 to make it suitable as an O2 sensor. The presence of the [4Fe-4S] cluster increases dimerization of FNR which is correlated with an increase in site-specific DNA binding of FNR, a property expected of transcription factors of the FNR/CRP family. According to Mössbauer spectroscopy on purified FNR and cells containing overexpressed FNR, the [4Fe-4S]2+ cluster of FNR is converted by O2 to a [2Fe-2S]2+ in high yield. The [2Fe-2S]2+ cluster can be reconverted to the [4Fe-4S]2+ cluster on reduction with dithionite in vitro raising the possibility that the [2Fe-2S]2+ cluster is a biologically inactive intermediate which may be more readily available for reconstitution into the [4Fe-4S]2+ form than the Fe-free apoform. The ability to observe, by Mössbauer spectroscopy, the Fe-S clusters of FNR in cells containing high levels of FNR should be of value in further unraveling how FNR functions in vivo. Attempts to reduce the [4Fe-4S]2+ cluster of FNR with dithionite indicated that the redox potential of the +1/+2 couple is < or = -650 mV and that the [4Fe-4S]+ cluster form is, therefore, not likely to occur in vivo.