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

The oxygen sensor regulator FNR (fumarate nitrate reductase regulator) of Escherichia coli is known to be inactivated by O2 as the result of conversion of a [4Fe‐4S] cluster of the protein into a [2Fe‐2S] cluster. Further incubation with O2 causes loss of the [2Fe‐2S] cluster and production of apoFNR. 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 glutathione, the O2 tension for the regulatory switch for FNR‐dependent gene regulation was decreased by a factor of 4–5 compared with the wild‐type, suggesting a role for glutathione in FNR function. In isolated apoFNR, glutathione could be used as the reducing agent for HS− formation required for [4Fe‐4S] assembly by cysteine desulfurase (NifS), and for the reduction of cysteine ligands of the FeS cluster in FNR. Air‐inactivated FNR (apoFNR without FeS) could be reconstituted to [4Fe‐4S]·FNR by the same reaction as used for apoFNR isolated under anaerobic conditions. The in vivo effects of glutathione on FNR function and the role of glutathione in the formation of active [4Fe‐4S]·FNR in vitro suggest an important role for glutathione in the de novo assembly of FNR and in the reductive activation of air‐oxidized FNR under anaerobic conditions.