Fig. 1.

Two-component signal transduction and the OmpR/PhoB subfamily of response regulators.

A. Schematic outline of a typical two-component system. The pathway features transfer of a phosphoryl group (red) between the conserved histidine kinase and receiver domains (HisK and REC, grey). Sensing of input stimuli by the HK modulates the kinase or phosphatase activity and regulates the phosphorylation level of the RR. The phosphorylated RR elicits the output response through the effector domain.

B. Dimer structure of the PhoB receiver domain (PDB ID: 1ZES). The receiver domain has a conserved (βα)₅ fold (teal blue) and OmpR/PhoB subfamily RRs appear to share a conserved dimeric structure once phosphorylated. The non-covalent phosphoryl analogue beryllofluoride () co-ordinates to the conserved aspartate residue (red), allosterically perturbing the α4-β5-α5 surface (green) and promoting dimerization.

C. Sequence alignment of the α4-β5-α5 region of E. coli RRs. Full-length RR sequences are aligned by clustalx (Thompson et al., 1997) and only the α4-β5-α5 region is shown. Sequence numbering is for ArcA. RRs include all E. coli OmpR/PhoB subfamily members and some representatives from other subfamilies. Among the highly conserved residues within the OmpR/PhoB subfamily are the highlighted residues that are involved in intermolecular interactions: hydrophobic contacts (blue); charged residues for salt bridge formation (orange). The pairing of charged residues is labelled by four pairs of letters a, b, c and d. A red highlight represents a pair of residues that are not conserved but could still complement each other with reversed charges. All these highlighted residues are not well conserved in RRs from other families.