Role of glutathione in the formation of the active form of the oxygen sensor FNR ([4Fe-4S]center dot FNR) and in the control of FNR function

The oxygen sensor regulator FNR (fumarate nitrate reductase regulator) of E scherichia coli is known to be inactivated by O-2 as the result of conversi on of a [4Fe-4S] cluster of the protein into a [2Fe-2S] cluster. Further in cubation with O-2 causes loss of the [2Fe-2S] cluster and production of apo FNR. The reactions involved in cluster assembly and reductive activation of apoFNR isolated under anaerobic or aerobic conditions were studied in vivo and in vitro. In a gshA mutant of E. coli that was completely devoid of gl utathione, the O-2 tension for the regulatory switch for FNR-dependent gene regulation was decreased by a factor of 4-5 compared with the wild-type, s uggesting a role for glutathione in FNR function. In isolated apoFNR, gluta thione could be used as the reducing agent for HS- formation required for [ 4Fe-4S] assembly by cysteine desulfurase (NifS), and for the reduction of c ysteine ligands of the FeS cluster in FNR. Air-inactivated FNR (apoFNR with out FeS) could be reconstituted to [4Fe-4S].FNR by the same reaction as use d for apoFNR isolated under anaerobic conditions. The in vivo effects of gl utathione on FNR function and the role of glutathione in the formation of a ctive [4Fe-4S].FNR in vitro suggest an important role for glutathione in th e de novo assembly of FNR and in the reductive activation of air-oxidized F NR under anaerobic conditions.