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Quorum sensing (QS) is a mechanism for bacterial cell–cell communication using QS signals. N-acyl-homoserine lactones (AHLs), QS signals in Pseudomonas syringae pv. tabaci (Pta) 6605, are synthesized by an AHL synthase (PsyI) and recognized by the cognate transcription factor PsyR. To reveal the role of PsyR in virulence, we generated a ∆psyR mutant and complemented strains of Pta 6605 and found that the ∆psyR mutant is remarkably reduced in AHL production and ability to cause disease and propagate in host tobacco leaves. The phenotypes of complemented strains were restored to that of the wild type (WT). Because the ∆psyR mutant lost nearly all AHL production, we investigated the function of PsyR in the transcription of psyI and production of AHL. Electrophoretic mobility shift assays suggested that the recombinant PsyR protein binds the promoter region of psyI but not psyR without AHL. The addition of AHL did not significantly affect this binding. The binding core sequence of this region was identified as a 20-bp lux box-like sequence. To reveal the function of PsyR and AHL on psyI transcription, we constructed a psyI promoter::lacZYA chimeric reporter gene, and inserted it into the WT and ∆psyI mutant of Pta 6605. β-galactosidase activity increased in a bacterial density-dependent manner in the WT and also in a ∆psyI mutant after the addition of exogenous AHL. These results indicate that the solo PsyR binds the lux box in the psyI promoter and activates transcription in the concomitant presence of AHL. [ABSTRACT FROM AUTHOR]