THE HRPRS LOCUS OF PSEUDOMONAS-SYRINGAE PV PHASEOLICOLA CONSTITUTES ACOMPLEX REGULATORY UNIT

The right part of the hrp cluster of Pseudomonas syringae pv. phaseoli cola 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. Hr pR 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 relat ed pathogen Pseudomonas syringae pv, syringae. The results of our gene tic analyses strongly indicate that hrpS expression is regulated by th e hrpR gene product. DNA-protein binding studies and site-directed mut agenesis of the hrpR sequence provided further evidence that HrpR acti vates hrpS transcription by binding to an activator site. This HrpR bi nding site was mapped in a fragment which is located 378-609 nucleotid es upstream of the hrpS transcription start site. The hrpS transcripti on start site maps 179 nucleotides upstream of the initiation codon AT G, as determined by primer extension analysis, and is preceded by a ty pical -12/-24 promoter motif.