ArnR binds a [4Fe-4S] cluster and indirectly senses anaerobicity in Corynebacteria

Corynebacteria are commercially and medically important Gram-positive bacteria that can switch from aerobic to anaerobic respiration in response to low O2 and the availability of nitrate as an alternative electron acceptor. The narKGHJI operon encoding the respiratory nitrate reductase is under the control of a novel regulator, ArnR, which plays a major role in the aerobic/anaerobic respiratory switch. ArnR was previously shown to be an iron-sulfur cluster protein that modulates its DNA binding according to availability of O2. However, previous data suggest that it does not do this directly in response to O2, but instead by sensing nitric oxide (NO), which builds up only under low O2 through the activity of nitrate reductase. Here, we report spectroscopic and mass spectrometric studies of C. glutamicum ArnR and its reactions with O2 and NO. We demonstrate that ArnR is a dimer that binds a [4Fe-4S] cluster in each subunit, and this form of the protein binds tightly to DNA. The [4Fe-4S] cluster of AnrR degrades only very slowly in the presence of O2, consistent with the ability of ArnR to repress nar transcription under aerobic conditions. Reaction with NO results in the formation of mono- and di-nitrosylated forms of the [4Fe-4S] ArnR dimer, which exhibit altered DNA-binding characteristics such that the di-nitrosyl form no longer binds to promoter DNA; i.e. cluster degradation is not required in order to modulate DNA binding. These data are consistent with previous literature and lead us to propose a model for AnrR regulatory function.