FNR regulon

The FNR (defective in fumarate and nitrate reduction) protein of E. coli is an oxygen – responsive transcriptional regulator required for the switch from aerobic to anaerobic metabolism[1].

"Type III mutants, originally frdB, were designated fnr because they were defective in fumarate and nitrate reduction and impaired in their ability to produce gas."

The switch from aerobic to nitrate and fumarate respiration or fermentation corresponds to a progressive decrease in ATP yields.

The oxygen-sensing domain of FNR contains a surface-exposed Fe-S cluster, which can react with cellular reductants, such as glutathione or thiol proteins.

Under anoxic conditions, the [4Fe-4S] FNR, bound to 4 cysteine residues binds to DNA target sites, and controls the expression of corresponding genes.

Under low oxygen the FNR dimer of E. coli recognizes this sequence and acts as a transcriptional activator.

Ferricyanide is able, in vivo and in vitro, to promote inactivation of FNR function or [4Fe–4S].FNR destruction by means of oxidizing the cluster.

[10] The fnr gene product, a pleiotropic transcriptional activator, is required for expression of the operons that encode nitrate and fumarate reductase complexes.