Genetic Analysis of the Quorum Sensing Regulator EsaR

Pantoea stewartii subsp. stewartii is the causative agent of Stewart's wilt disease in maize plants. The bacteria are injected into the plant by corn flea beetles during feeding. They colonize the xylem and overproduce a capsular exopolysaccharide (EPS) at high cell densities. The production of EPS is regulated by an EsaI/EsaR quorum sensing mechanism, homologous to the LuxI/R system. Although activation of the EPS encoding genes by EsaR occurs after it complexes to the AHL (3-oxo-C6-HSL), unlike the LuxI/R system, this activation occurs by a different mechanism. At low cell densities, dimerized EsaR acts as a repressor. At a high cell population, derepression of the EPS genes occurs via an unknown mechanism once the AHL complexes to EsaR. Hence, a random mutagenesis genetic approach to isolate EsaR* variants that are immune to the effects of AHL has been utilized. Error-prone PCR and site-directed mutagenesis were used to generate desired mutants, which were subsequently screened for their ability to repress transcription in the presence of AHL. Several individual amino acids playing a critical role in the AHL-insensitive phenotype have been identified and mapped onto a homology model of EsaR. A separate study attempted to localize the dimerization region and analyze the stability of the N-terminal domain of EsaR. Truncations of EsaR at amino acids 169 and 178, without and with the extended linker region respectively, were generated using PCR. Dimerization assays similar to those by Choi and Greenberg in 1991 were performed but proved to be unsuccessful. However, the N-terminal domain is stable as determined by western blotting, which may facilitate its future structural analysis. Together, these efforts have contributed to the molecular understanding of AHL-dependent derepression of EsaR.