Oxygen regulated gene expression in Escherichia coli: control of anaerobic respiration by the FNR protein.

Molecular oxygen is an important regulatory signal in facultative anaerobic bacteria and controls the expression of a great variety of genes positively or negatively. The expression of anaerobic respiration and of related functions of E. coli is controlled by the positive gene regulator FNR, which activates transcription in the absence of O2. The regulated genes carry a FNR consensus sequence upstream of the promoter. Under the same conditions FNR represses some of the genes of aerobic respiration. The binding to the DNA occurs by an alpha-helix-turn-alpha-helix DNA-binding domain. FNR contains 5 cysteine residues, four of which are arranged in a cluster close to the N-terminal end. For the function of FNR as a O2-dependent regulator three of the cysteine residues in the cluster and the residue outside the cluster are essential. FNR binds iron as a cofactor which most likely is involved in the O2-sensing by the protein. The experiments indicate that the cysteine residues are responsible for the binding of the iron. From the protein in vivo two functional states can be differentiated, an aerobic or metal-depleted form and an anaerobic form. Only the anaerobic form acts as a gene activator or repressor. Sensing of O2 or of positive redox potentials by the iron ion is thought to cause the conversion of the two functional states. The FNR protein in addition contains a potential nucleotide binding domain. The significance and function of this site is not clear.