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Dissociation of oxygen from the heme domain of the bacterial oxygen sensor protein FixL constitutes the first step in hypoxia-induced signaling. In the present study, the photodissociation of the heme-[O.sub.2] bond was used to synchronize this event, and time-resolved resonance Raman (T[R.sup.3]) spectroscopy with subpicosecond rime resolution was implemented to characterize the heme configuration of the primary photoproduct. T[R.sup.3] measurements on heme-oxycomplexes are highly challenging and have not yet been reported. Whereas in all other known six-coordinated heme protein complexes with diatomic ligands, including the oxymyoglobin reported here, heme iron out-of-plane motion (doming) occurs faster than 1 ps after iron-ligand bond breaking; surprisingly, no sizeable doming is observed in the oxycomplex of the Bradyrhizobium japonicum FixL sensor domain (FixLH). This assessment is deduced from the absence of the iron-histidine band around 217 [cm.sup.-1] as early as 0.5 ps. We suggest that efficient ultrafast oxygen rebinding to the heme occurs on the femtosecond time scale, thus hindering heme doming. Comparing WT oxy-FixLH, mutant proteins FixLH-R220H and FixLH-R220Q, the respective carbonmonoxy-complexes, and oxymyoglobin, we show that a hydrogen bond of the terminal oxygen atom with the residue in position 220 is responsible for the observed behavior; in WT FixL this residue is arginine, crucially implicated in signal transmission. We propose that the rigid [O.sub.2] configuration imposed by this residue, in combination with the hydrophobic and constrained properties of the distai cavity, keep dissociated oxygen in place. These results uncover the origin of the 'oxygen cage' properties of this oxygen sensor protein. heme protein | molecular dynamics | ultrafast spectroscopy | vibrational spectroscopy