Escherichia coli SdiA is a quorum-sensing (QS) receptor that responds to autoinducers produced by other bacterial species to control cell division and virulence. Crystal structures reveal that E. coli SdiA, which is composed of an N-terminal ligand-binding domain and a C-terminal DNA-binding domain (DBD), forms a symmetrical dimer. Although each domain shows structural similarity to other QS receptors, SdiA differs from them in the relative orientation of the two domains, suggesting that its ligand-binding and DNA-binding functions are independent. Consistently, in DNA gel-shift assays the binding affinity of SdiA for the ftsQP2 promoter appeared to be insensitive to the presence of autoinducers. These results suggest that autoinducers increase the functionality of SdiA by enhancing the protein stability rather than by directly affecting the DNA-binding affinity. Structural analyses of the ligand-binding pocket showed that SdiA cannot accommodate ligands with long acyl chains, which was corroborated by isothermal titration calorimetry and thermal stability analyses. The formation of an intersubunit disulfide bond that might be relevant to modulation of the DNA-binding activity was predicted from the proximal position of two Cys residues in the DBDs of dimeric SdiA. It was confirmed that the binding affinity of SdiA for the uvrY promoter was reduced under oxidizing conditions, which suggested the possibility of regulation of SdiA by multiple independent signals such as quorum-sensing inducers and the oxidation state of the cell.

SdiA enhances cell division by regulating the ftsQAZ operon in Escherichia coli as a transcription activator. In addition, SdiA is suggested to play a role in detecting quorum signals that emanate from other species. It is therefore a homologue of LuxR, a cognate quorum-sensing receptor that recognizes a quorum signal and activates the quorum responses. To elucidate the role of SdiA and its functional and structural relationship to LuxR, structural studies were performed on E. coli SdiA. Recombinant SdiA was overexpressed, purified and crystallized at 287 K using the hanging-drop vapour-diffusion method. X-ray diffraction data from a native crystal were collected with 99.7% completeness to 2.7 Å resolution with an R merge of 6.0%. The crystals belong to the hexagonal space group P6 1 22 or P6 5 22, with unit-cell parameters a = b = 130.47, c = 125.23 Å .

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