THE OXYGEN-RESPONSIVE TRANSCRIPTIONAL REGULATOR FNR OF ESCHERICHIA-COLI - THE SEARCH FOR SIGNALS AND REACTIONS

The FNR (fumarate and nitrate reductase regulation) protein of Escheri chia coli is an oxygen-responsive transcriptional regulator required f or the switch from aerobic to anaerobic metabolism. In the absence of oxygen, FNR changes from the inactive to the active state. The sensory and the regulatory functions reside in separate domains of FNR. The s ensory domain contains a Fe-S cluster, which is of the [4Fe-4S](2+) ty pe under anaerobic conditions. It is suggested that oxygen is supplied to the cytoplasmic FNR by diffusion and inactivates FNR by direct int eraction. Reactivation under anoxic conditions requires cellular reduc tants. In vitro, the Fe-S cluster is converted to a [3Fe-4S](+) or a [ 2Fe-2S](2+) cluster by oxygen, resulting in FNR inactivation. After pr olonged incubation with oxygen, the Fe-S cluster is destroyed. Reassem bly of the [4Fe-4S](2+) cluster might require cellular proteins, such as the NifS-like protein of E. coli. In this review, the rationale for regulation of alternative metabolic pathways by FNR and other oxygen- dependent regulators is discussed. Only the terminal reductases of res piration, and not the dehydrogenases, are regulated in such a way as t o achieve maximal H+/e(-) ratios and ATP yields.