A representation of the FNR system in E. coli. FNR regulates the shift between aerobic and anaerobic growth. Dimeric 4Fe-FNR adapts the cell to oxygen-limiting conditions. Aerobically, oxygen inactivates FNR, but the cell continues to produce and reactivate it. This results in constant cycling of FNR between its three states—apoFNR, 4Fe-FNR, and 2Fe-FNR. Aerobic cycling is tuned so that the inactive apoFNR predominates. Under anaerobic conditions, the absence of oxygen results in rapid buildup of 4Fe-FNR. The 4Fe-FNR form dimerizes to produce an active transcription factor.