Influence of association state and DNA binding on the O2-reactivity of [4Fe-4S] fumarate and nitrate reduction (FNR) regulator

The fumarate and nitrate reduction (FNR) regulator is the master switch for the transition between anaerobic and aerobic respiration in Escherichia coli. Reaction of dimeric [4Fe-4S] FNR with O-2 results in conversion of the cluster into a [2Fe-2S] form, via a [3Fe-4S] intermediate, leading to the loss of DNA binding through dissociation of the dimer into monomers. In the present paper, we report studies of two previously identified variants of FNR, D154A and I151A, in which the form of the cluster is decoupled from the association state. In vivo studies of permanently dimeric D154A FNR show that DNA binding does not affect the rate of cluster incorporation into the apoprotein or the rate of O-2-mediated cluster loss. In vitro studies show that O-2-mediated cluster conversion for D154A and the permanent monomer I151A FNR is the same as in wild-type FNR, but with altered kinetics. Decoupling leads to an increase in the rate of the [3Fe-4S](1+) into [2Fe-2S](2+) conversion step, consistent with the suggestion that this step drives association state changes in the wild-type protein. We have also shown that DNA-bound FNR reacts more rapidly with O-2 than FNR free in solution, implying that transcriptionally active FNR is the preferred target for reaction with O-2.