CHARACTERIZATION OF THE HRP REGULON OF PSEUDOMONAS SYRINGAE
S HUTCHESON, JD WEAVER, N THAREJA, I ROZENBAUM and K PAK
University of Maryland, College Park, MD 20742, USA
Background and objectives
Results and conclusions
In an attempt to identify new regulatory factors controlling expression of the hrp regulon, we initiated a search for constitutive up mutants. A constitutive up phenotype is likely be caused by: (i) enhanced hrpL expression caused by a promoter mutation; (ii) altered expression or activity of presently unidentified positive transcriptional factors, such as HrpX and HrpY recently identified in Erwinia amylovora; (iii) nonsense mutations in a negative-acting determinant; or (iv) mutations affecting signal transduction cascade-mediating apparent contact-dependent regulation, as described below. Several apparent up mutants were detected in a chemically mutagenized population of cells. Plant responses to these mutants were more rapid than the wild type, as expected for the constitutive up phenotype. The properties of the mutated genes and their role in transducing environmental signals known or proposed to affect hrp expression will be discussed.
Finally, the size and complexity of the HrpL-dependent regulon has not been determined in any P. syringae strain. A unifying feature of the hrp-encoded PTA and the proteins secreted by this apparatus is their regulation by HrpL. For example, all avr genes characterized thus far from P. syringae strains and the dsp genes of E. amylovora are regulated by HrpL. This opens the possibility of using this property to identify novel pathogenicity and host-range determinants in P. syringae strains. For these experiments, we developed a simple plate assay to identify HrpL-regulated genes. The properties of this genetic screen and its application to identifying P. syringae pathogenicity factors will be presented.