Iron-sulfur cluster disassembly in the FNR protein of Escherichia coli by O 2 : 4Fe-4S to 2Fe-2S conversion with loss of biological activity

The transcription factor FNR (fumarate nitrate reduction) requires the presence of an iron-sulfur (Fe-S) cluster for its function as a global transcription regulator in Escherichia coli when oxygen becomes scarce. To define the oxidation state and type of Fe-S cluster present in the active form of FNR, we have studied anaerobically purified FNR with Mössbauer spectroscopy. Our data showed that this form of FNR contained a 4Fe-4S2+ cluster (delta = 0.45 mm/s; DeltaEQ = 1.22 mm/s) and that the 4Fe-4S2+ cluster was rapidly destroyed on exposure of FNR to air. Under these conditions, the yellow-green active form of FNR turned deep red; analysis of sulfide indicated that 70% of the labile sulfide was still present, suggesting that the Fe-S cluster had been converted into a different form. Little 3Fe-4S cluster was, however, detected by EPR. According to Mössbauer spectroscopy, the 4Fe-4S2+ cluster was converted in about 60% yield to a 2Fe-2S2+ cluster (delta = 0.28 mm/s; DeltaEQ = 0.58 mm/s) following 17 min of exposure to air. The 2Fe-2S2+ cluster form of FNR was much more stable to oxygen, but was unable to sustain biological activity (e.g., DNA binding). However, DNA binding and the absorption spectrum characteristic of the 4Fe-4S2+ cluster could be largely restored from the 2Fe-2S2+ form when Cys, Fe, DTT, and the NifS protein were added. It has yet to be determined whether the form of FNR containing the 2Fe-2S2+ cluster has any biological significance, e.g., as an in vivo intermediate that is more rapidly converted to the active form than the apoprotein.