A diagram of the kinetic model for the FNR system. X1, fnr mRNA; X2, apoFNR and 2Fe-FNR; X3, 4Fe-FNR; X4, ClpXP protease; X5, iron–sulfur cluster assembly proteins (Isc); X6, molecular oxygen. The nucleotide and amino acid pools are assumed to be well regulated, and their nearly constant values are implicitly accounted for in the appropriate rate constants for transcription and translation. The fate of material lost from the system by degradation and/or dilution is not shown. (See the text for further details.)