Oxygen sensing by the global regulator, FNR, the role of the iron-sulfur cluster

FNR is a global regulator that controls transcription of genes whose functi ons facilitate adaptation to growth under O-2 limiting conditions. It has l ong been appreciated that the activity of FNR must be regulated by O-2 avai lability, since FNR dependent gene expression is observed in vivo only unde r 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 t hat this [4Fe-4S](2+) cluster is sufficiently unstable toward O-2 to make i t suitable as an O-2 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 properly expected of transcription factors of the FNR /CRP family. According to Mossbauer spectroscopy on purified FNR and cells containing overexpressed FNR, the [4Fe-4S](2+) cluster of FNR is converted by O-2 to a [2Fe-2S](2+) in high yield. The [2Fe-2S](2+) cluster can be rec onverted to the [4Fe-4S](2+) cluster on reduction with dithionite in vitro raising the possibility that the [2Fe-2S](2+) cluster is a biologically ina ctive intermediate which may be more readily available for reconstitution i nto the [4Fe-4S](2+) form than the Fe-free apoform. The ability to observe, by Mossbauer spectroscopy, the Fe-S clusters of FNR in cells containing hi gh 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 dithionit e indicated that the redox potential of the +1/+2 couple is less than or eq ual to -650 mV and that the [4Fe-4S](+) cluster form is, therefore, not lik ely to occur in vivo. (C) 1999 Federation of European Microbiological Socie ties. Published by Elsevier Science B.V. All rights reserved.