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. Author manuscript; available in PMC: 2012 Jan 1.
Published in final edited form as: Mol Microbiol. 2010 Nov 8;79(1):149–165. doi: 10.1111/j.1365-2958.2010.07437.x

Figure 1.

Figure 1

A representation of the FNR (fumarate nitrate reduction) system in E. coli. FNR regulates the transition between aerobic/anaerobic growth. The monomer forms combine to produce the dimeric 4Fe-FNR, an active transcription factor that regulates the adaptation of the cell to O2 limiting conditions. Aerobically, O2 inactivates FNR, but the cell continues to produce and reactivate it. This results in a 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 O2 results in a rapid buildup of 4Fe-FNR. The 4Fe-FNR form dimerizes to produce an active transcription factor.