Kinetic analysis of the oxidative conversion of the [4Fe-4S]2+ cluster of FNR to a [2Fe-2S]2+ Cluster.

The ability of FNR to sense and respond to cellular O(2) levels depends on its [4Fe-4S](2+) cluster. In the presence of O(2), the [4Fe-4S](2+) cluster is converted to a [2Fe-2S](2+) cluster, which inactivates FNR as a transcriptional regulator. In this study, we demonstrate that approximately 2 Fe(2+) ions are released from the reaction of O(2) with the [4Fe-4S](2+) cluster.

Superoxide destroys the [2Fe-2S]2+ cluster of FNR from Escherichia coli.

The oxygen sensing ability of the transcription factor FNR depends on the presence of a [4Fe-4S]2+ cluster. In the presence of O2, conversion of the [4Fe-4S]2+ cluster to a [2Fe-2S]2+ cluster inactivates FNR, but the fate of the [2Fe-2S]2+ cluster in cells grown under aerobic conditions is unknown. The present study shows that the predominant form of FNR in aerobic cells is apo-FNR (cluster-less FNR) indicating that the [2Fe-2S]2+ cluster, like the [4Fe-4S]2+ cluster, is not stable under these conditions.

The role of Fe-S proteins in sensing and regulation in bacteria.

Fe-S clusters are key to the sensing and transcription functions of three transcription factors, FNR, IscR and SoxR. All three proteins were discovered in Escherichia coli but experimental data and bioinformatic predictions suggest that homologs of these proteins exist in other bacterial species, highlighting the widespread nature of Fe-S-dependent regulatory networks. In addition, the nearly ubiquitous citric acid cycle enzyme, aconitase, plays a role in translational regulation in E.