One-component, b-type heme-containing aerobic sensor and transcriptional regulator that responds to CO by activating the expression of the oxidation operon cox. Binds 1 heme group per subunit. The N-terminal sensor (heme-containing) domain is covalently linked to the C-terminal, DNA-binding response domain. Binding of an external ligand to the heme located in the N-terminal sensory domain displaces the Met-104 distal heme ligand, triggering a conformational change that activates the C-terminal DNA-binding domain. This protein undergoes a ligand switch upon reduction; the heme distal ligand is coordinated by a Cys residue in the inactive Fe(3+) (ferric) form, by Met-104 in the inactive Fe(2+) (ferrous) form, and by CO in the CO-bound active form.