FNR is a direct oxygen sensor having a biphasic response curve

Area of Science:

• Biochemistry
• Molecular Biology
• Genetics

Background:

• FNR (factor for inversion stimulation) is a crucial transcription regulator.
• It controls gene expression in response to anoxia (low oxygen conditions).
• FNR binds DNA site-specifically upon acquiring two [4Fe-4S]2+ clusters per dimer under anaerobiosis.

Purpose of the Study:

• To investigate the mechanism by which FNR senses and responds to oxygen.
• To characterize the structural and functional changes in FNR upon oxidation.
• To determine if FNR functions as an oxygen sensor.

Main Methods:

• Mossbauer spectroscopy to identify iron-sulfur cluster types.
• Electron Paramagnetic Resonance (EPR) spectroscopy to detect free ferric ions.
• Controlled oxidation experiments using varying amounts of oxygen and potassium ferricyanide (K3Fe(CN)6).

Main Results:

• Oxidation of the [4Fe-4S]2+ cluster in FNR by O2 yields a transcriptionally inactive, non-DNA-binding form containing a [2Fe-2S] cluster.
• Complete conversion requires 2.5-3.0 O2 molecules per [4Fe-4S]2+ cluster.
• Stable co-existence of [4Fe-4S]2+ and [2Fe-2S]2+ forms observed with sub-stoichiometric oxygen, without releasing detectable ferric ions.
• Potassium ferricyanide oxidation released ferric ions, unlike oxygen oxidation.

Conclusions:

• FNR's iron-sulfur clusters undergo distinct transformations upon oxygen exposure.
• The [4Fe-4S]2+ to [2Fe-2S] cluster conversion directly impacts FNR's DNA-binding and transcriptional activity.
• FNR functions as a sensitive oxygen sensor, translating oxygen levels into changes in gene regulation.