Activates the expression of the CowN protein in response to carbon monoxide (CO). Is required to sustain N(2)-dependent growth in the presence of low levels of carbon monoxide (CO). Binds 1 heme group per subunit. The N-terminal sensor (heme-containing) domain is covalently linked to the C-terminal, DNA-binding response domain. Strains lacking this gene are not able to grow with N(2) as a nitrogen source in the presence of CO.