Background: A variety of bacterial adaptative cellular responses to environmental stimuli are mediated by two-component signal transduction pathways. In these phosphorelay cascades, histidine kinases transphosphorylate a conserved aspartate in the receiver domain, a conserved module in the response regulator superfamily. The main effect of this phosphorylation is to alter the conformation of the response regulator in order to modulate its biological function. The response regulator FixJ displays a typical modular arrangement, with a phosphorylatable N-terminal receiver domain and a C-terminal DNA-binding domain. In the symbiotic bacterium Sinorhizobium meliloti, phosphorylation of this response regulator activates transcription of nitrogen-fixation genes.Results: The crystal structures of the phosphorylated and of the unphosphorylated N-terminal receiver domain of FixJ (FixJN) were solved at 2.3 Angstrom and 2.4 Angstrom resolution, respectively. They reveal the environment of the phosphoaspartate in the active site and the specific conformational changes leading to activation of the response regulator, Phosphorylation of the conserved aspartate induces major structural changes in the beta 4-alpha 4 loop, and in the signaling surface alpha 4-beta 5 that mediates dimerization of the phosphorylated full-length response regulator. A site-directed mutant at this protein-protein interface decreases the affinity of the phosphorylated response regulator for the fixK promoter tenfold.Conclusions: The cascade of phosphorylation-induced conformational changes in FixJN illustrates the role of conserved residues in stabilizing the phosphoryl group in the active site, triggering the structural transition and achieving the post-phosphorylation signaling events. We propose that these phosphorylation-induced conformational changes underly the activation of response regulators in general.

Background: Two-component signal transduction pathways are sophisticated phosphorelay cascades widespread in prokaryotes and also found in fungi, molds and plants. FixL/FixJ is a prototypical system responsible for the regulation of nitrogen fixation in the symbiotic bacterium Sinorhizobium meliloti, In microaerobic conditions the membrane-bound kinase FixL uses ATP to transphosphorylate a histidine residue, and the response regulator FixJ transfers the phosphoryl group from the phosphohistidine to one of its own aspartate residues in a Mg2+-dependent mechanism.Results: Seven X-ray structures of the unphosphorylated N-terminal receiver domain of FixJ (FixJN) have been solved from two crystal forms soaked in different conditions. Three conformations of the protein were found, In the first case, the protein fold impairs metal binding in the active site and the structure reveals a receiver domain that is self-inhibited for catalysis, In the second conformation, the canonical geometry of the active site is attained, and subsequent metal binding to the protein induces minimal conformational changes. The third conformation illustrates a non-catalytic form of the protein where unwinding of the N terminus of helix ctl has occurred. Interconversion of the canonical and self-inhibited conformations requires a large conformational change of the beta 3-alpha 3 loop region,Conclusions: These unphosphorylated structures of FixJN stress the importance of flexible peptide segments that delineate the active site. Their movements may act as molecular switches that define the functional status of the protein, Such observations are in line with structural and biochemical results obtained on other response regulator proteins and may illustrate general features that account for the specificity of protein-protein interactions.