The hrpRS locus of Pseudomonas syringae pv. phaseolicola constitutes a complex regulatory unit

Summary The right part of the hrp cluster of Pseudomonas syringae pv. phaseolicola contains two regulatory genes, the previously described hrpS gene and an adjacent locus, hrpR. In this study we determined the sequence of hrpR and analysed the functional organization of the two genes. HrpR and HrpS show high sequence similarities to each other and to other response regulators of the two‐component regulatory system. This has recently also been described for the hrpRS system of the closely related pathogen Pseudomonas syringae pv. syringae. The results of our genetic analyses strongly indicate that hrpS expression is regulated by the hrpR gene product. DNA‐protein binding studies and site‐directed mutagenesis of the hrpR sequence provided further evidence that HrpR activates hrpS transcription by binding to an activator site. This HrpR binding site has mapped in a fragment which is located 378–609 nucleotides upstream of the hrpS transcription start site. The hrpS transcription start site maps 179 nucleotides upstream of the initiation codon ATG, as determined by primer extension analysis, and is preceded by a typical ‐12/‐24 promoter motif.