Development of Methods for Structural Characterization of Pantoea stewartii Quorum-Sensing Regulator EsaR

The LuxR family of proteins serves as quorum-sensing transcriptional regulators in proteobacteria. At high population densities, a small acyl-homoserine lactone (AHL) molecule, produced by a LuxI homologue, accumulates in the environment. The LuxR proteins bind to their respective AHL when the ligand accumulates to sufficient levels. Once bound to AHL, the holoproteins usually become functional as transcriptional activators. However, there is a subset of LuxR homologues, the EsaR subfamily, which is active without the AHL ligand and becomes inactivated once bound to it. EsaR is the best understood member of this subfamily. It controls virulence in the corn pathogen Pantoea stewartii ssp. stewartii.

Solubility issues have previously limited structural studies of LuxR homologues as the proteins could not be purified without the AHL ligand. A soluble recombinant EsaR protein, HMGE, is biologically active and can be purified in the absence and presence of AHL, unlike most other LuxR homologues. Using HMGE, amino acid substitutions and Förster resonance energy transfer (FRET), experimental methods were designed for determining the dimerization interface of EsaR and for testing the hypothesis that EsaR undergoes a conformational shift when presented with the AHL ligand.

To identify residues of the dimerization interface, heterodimerization assays were designed, involving either coexpression or coincubation of wild-type EsaR and variant HMGE proteins. In this assay, the inability of the proteins to copurify by nickel affinity chromatography would indicate that the modified residue(s) are important for dimerization of EsaR. To determine the conformational change that EsaR undergoes when bound to the AHL ligand, a FRET assay was developed to estimate the distances between amino acid residues in the absence and presence of AHL. Future work will have to include a few modifications to the methods and/or control experiments. This study provides the basis upon which the present methods can be further developed and later used for structural studies of EsaR.