Globin-coupled sensors (GCSs) are multiple-domain transducers, consisting of a regulatory globin-like heme-binding domain and (a) linked transducer domain(s). GCSs have been described in both Archaea and bacteria. They are generally assumed to bind O2 (and perhaps other gaseous ligands) and to transmit a conformational change signal through the transducer domain in response to fluctuating O2 levels. In this study, the heme-binding domain, AvGReg178, and the full protein, AvGReg of the Azotobacter vinelandii GCS were cloned, expressed and purified. After purification, the heme iron of AvGReg178 was found to bind O2, as revealed by UV-visible and resonance Raman (RR) spectroscopy. This form was stable over many hours. In contrast, electron paramagnetic resonance and UV-visible spectroscopy revealed the predominant presence of a bis-histidine coordinate heme in ferric AvGReg. Differences in the heme pocket structure were also observed for the deoxygenated ferrous state of these proteins. The UV-visible and RR spectra showed that the deoxygenated ferrous derivatives of AvGReg178 and AvGReg are characterized by a penta-coordinate and hexa-coordinate heme iron, respectively. O2 binding isotherms indicate that AvGReg178 and AvGReg show a high affinity for O2 with P50 values at 20°C of 0.04 and 0.15 Torr, respectively. Kinetics of CO binding indicate that AvGReg178 carbonylation conforms to a monophasic process, comparable to that of myoglobin, whereas AvGReg carbonylation conforms to a three-phasic reaction, as observed for several proteins with bis-histidine heme iron coordination. Besides sensing ligands, in vitro data suggest that AvGReg(178) may have a role in O2-mediated NO-detoxification, yielding metAvGReg(178) and nitrate.