부산대학교 도서관

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 Mossbauer spectroscopy. Our data showed that this form of FNR contained a [[4Fe-4S].sup.2+] cluster ([Delta] = 0.45 mm/s; [Delta][E.sub.Q] = 1.22 mm/s) and that the [[4Fe-4S].sup.2+] 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 Mossbauer spectroscopy, the [[4Fe-4S].sup.2+] cluster was converted in about 60% yield to a [[2Fe-2S].sup.2+] cluster ([Delta] = 0.28 mm/s; [Delta][E.sub.Q] = 0.58 mm/s) following 17 min of exposure to air. The [[2Fe-2S].sup.2+] 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-4S].sup.2+] cluster could be largely restored from the [[2Fe-2S].sup.2+] 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-2S].sup.2+] cluster has any biological significance, e.g., as an in vivo intermediate that is more rapidly converted to the active form than the apoprotein.