Regulation of Aerobic-to-Anaerobic Transitions by the FNR Cycle in Escherichia coli

The FNR (fumarate nitrate reduction) protein plays a central role in the global oxygen response of a variety of bacteria. In Escherichia coli, FNR is the master transcriptional regulator of the transition between aerobic and anaerobic growth. Regulation of FNR is achieved by cycling the molecule between three states in a process dependent on oxygen. In an effort to better understand the nature of this post-transcriptional cyclic regulatory mechanism, we formulated a kinetic model of the FNR protein and its regulation in E. coli. The values for the parameters of the model were fit to experimental data for the wild-type organism, and the model was validated by successfully predicting the behavior of fnr mutant strains characterized in the literature. We characterized the steady-state behavior of the FNR system by determining its sensitivity to changes in parameter values and its response to changes in the concentration of iron–sulfur cluster assembly proteins and the protease ClpXP. We also determined the steady-state induction characteristic that provides a direct estimate for the levels of the active form of FNR as a function of oxygen concentration. This result, in combination with reporter assays for expression of FNR target operons, gives an estimate for the equilibrium dissociation constant for the binding of active FNR to its recognition sequences in the DNA. Finally, we predicted the dynamics of the aerobic-to-anaerobic transition and determined distinct contributions to the dynamic profile of regulatory mechanisms operating at the transcriptional and post-translational levels.