In Pantoea stewartii subsp. stewartii, two regulatory proteins are key to the process of cell-cell communication
known as quorum sensing: the LuxI and LuxR homologues EsaI and EsaR. Most LuxR homologues function
as activators of transcription in the presence of their cognate acylated homoserine lactone (AHL) signal.
However, EsaR was initially found to function as a repressor in the absence of AHL. Previous studies
demonstrated that, in the absence of AHL, EsaR retains the ability to function as a weak activator of the lux
operon in recombinant Escherichia coli. Here it is shown that both the N-terminal and the C-terminal domains
of EsaR are necessary for positive regulation. A site-directed mutagenesis study, guided by homology modeling
to LuxR and TraR, has revealed three critical residues in EsaR that are involved in activation of RNA
polymerase. In addition, a native EsaR-activated promoter has been identified, which controls expression of a
putative regulatory sRNA in P. stewartii.