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Spectroscopic characterization of the newly discovered heme-PAS domain sensor protein BxRcoM-2 reveals that this protein undergoes redox-dependent ligand switching and CO- and NO-induced ligand displacement. The aerobic bacterium Burkholderia xenovorans expresses two homologous heme-containing proteins that promote CO-dependent transcription in vivo. These regulators of CO metabolism, BxRcoM-1 and BxRcoM-2, are gas-responsive heme-PAS domain proteins like mammalian neuronal PAS domain protein 2 (NPAS2) and the direct oxygen sensor from Escherichia coli ( EcDos). BxRcoM-2 was studied using electronic absorption, MCD, resonance Raman, and EPR spectroscopies. In the Fe(III) oxidation state, the heme is low-spin and six-coordinate with a cysteine(thiolate) as one of the two ligands. The sixth ligand is a histidine (His (74)), which is present in all states of the protein that were studied. Reduction to the Fe(II) oxidation state results in replacement of the cysteine(thiolate) with a neutral thioether ligand, Met (104). CO and NO bind to the Fe(II) BxRcoM-2 heme opposite the histidine ligand. Thus, BxRcoM-2 employs coordination state changes similar to those known for CO-sensing CooA, with redox-dependent loss of a cysteine(thiolate) ligand and displacement of a relatively weakly bound axial ligand by the effector gas molecule. Like EcDos, the weakly bound axial ligand that is displaced is methionine.

The Transcription Regulator RcoM-2 from Burkholderia xenovorans Is a Cysteine-Ligated Hemoprotein That Undergoes a Redox-Mediated Ligand Switch