The active form of the oxygen sensor fumarate nitrate reductase regulator (FNR) of Escherichia colicontains a [4Fe-4S] cluster which is converted to a [2Fe-2S] cluster after reaction with air, resulting in inactivation of FNR. Reaction of reconstituted [4Fe-4S]ÆFNR with air resulted within 5 min in conversion to apoFNR. The rate was comparable to the rate known for [4Fe-4S]ÆFNR⁄[2Fe-2S]ÆFNR cluster conversion, suggesting that apoFNR is a product of [2Fe-2S]ÆFNR decomposition and a final form of air-inacti-vated FNR in vitro. .
