New mechanistic insights from structural studies of the oxygen-sensing domain of Bradyrhizobium japonicum FixL

The FixL heme domain serves as the dioxygen switch in the FixL/FixJ two-com ponent system of Rhizobia. Recent structural studies of the Bradyrhizobium japonicum FixL heme domain (BjFixLH) have suggested an allosteric mechanism that is distinct from the classical hemoglobin model. To gain further insi ght into the FixL sensing mechanism, structures of BjFixLH bound to dioxyge n, imidazole, and nitric oxide have been determined. These structures, part icularly the structure of BjFixLH bound to its physiological ligand, dioxyg en, have helped to address a number of important issues relevant to the BjF ixLH sensing mechanism. On the basis of the oxy-BjFixLH structure, a conser ved arginine is found to stabilize the dioxygen ligand in a mode reminiscen t of the distal histidine in classical myoglobins and hemoglobins. The stru cture of BjFixLH bound to imidazole elucidates the structural requirements for accommodating sterically bulky ligands, Finally, the structure of BjFix LH bound to nitric oxide provides evidence for a structural intermediate in the heme-driven conformational change.