Availability of O-2 is one of the most important regulatory signals in
facultatively anaerobic bacteria. Various two- or one-component senso
r/regulator systems control the expression of aerobic and anaerobic me
tabolism in response to O-2. Most of the sensor proteins contain heme
or Fe as cofactors that interact with O-2 either by binding or by a re
dox reaction. The ArcA/ArcB regulator of aerobic metabolism in Escheri
chia coli may use a different sensory mechanism. In two-component regu
lators, the sensor is located in the cytoplasmic membrane, whereas one
-component regulators are located in the cytoplasm . Under most condit
ions, O-2 can readily reach the cytoplasm and could provide the signal
in the cytoplasm. The transcriptional regulator FNR of E. coli contro
ls the expression of many genes required for anaerobic metabolism in r
esponse to O-2. Functional homologs of FNR are present in facultativel
y anaerobic Proteobacteria and presumably also in gram-positive bacter
ia. The target genes of FNR are mostly under multiple regulation by FN
R and other regulators that respond to O-2, nitrate, or glucose. FNR r
epresents a 'one-component' sensor/regulator and contains Fe for signa
l perception. In response to O-2 availability, FNR is converted revers
ibly from the aerobic (inactive) state to the anaerobic (active) state
. Experiments suggest that the Fe cofactor is bound by four essential
cysteine residues. The O-2-triggered transformation between active and
inactive FNR presumably is due to a redox reaction at the Fe cofactor
, but other modes of interaction cannot be excluded. O-2 seems to affe
ct the site-specific DNA binding of FNR at target genes or the formati
on of an active transcriptional complex with RNA polymerase.