A Pseudomonas aeruginosa strain (UCBPP-PA14) is infectious both in an Arabidopsis thaliana leaf infiltration model and in a mouse full-thickness skin burn model. UCBPP-PA14 exhibits ecotype specificity for Arabidopsis, causing a range of symptoms from none to severe in four different ecotypes. In the mouse model, UCBPP-PA14 is as lethal as other well-studied P. aeruginosa strains. Mutations in the UCBPP-PA14 toxA, plcS, and gacA genes resulted in a significant reduction in pathogenicity in both hosts, indicating that these genes encode virulence factors required for the full expression of pathogenicity in both plants and animals.

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We cloned the rpoN (ntrA and glnF) gene encoding 54 from the phytopathogen Pseudomonas syringae pv. maculicola strain ES4326. The P. syringae ES4326 rpoN gene complemented Pseudomonas aeruginosa, Escherichia coli, and Klebsiella aerogenes rpoN mutants for a variety of rpoN mutant phenotypes, including the inability to utilize nitrate as sole nitrogen source. DNA sequence analysis of the P. syringae ES4326 rpoN gene revealed that the deduced amino acid sequence was most similar (86% identity; 95% similarity) to the 54 protein encoded by the Pseudomonas putida rpoN gene. A marker exchange protocol was used to construct an ES4326 rpoN insertional mutation, rpoN::Km r . In contrast to wild-type ES4326, ES4326 rpoN::Km r was nonmotile and could not utilize nitrate, urea, C 4 -dicarboxylic acids, several amino acids, or concentrations of ammonia below 2 mM as nitrogen sources. rpoN was essential for production of the phytotoxin coronatine and for expression of the structural genes encoding coronamic acid. In addition, ES4326 rpoN::Km r did not multiply or elicit disease symptoms when infiltrated into Arabidopsis thaliana leaves, did not elicit the accumulation of several Arabidopsis defense-related mRNAs, and did not elicit a hypersensitive response (HR) when infiltrated into tobacco (Nicotiana tabacum) leaves. Furthermore, whereas P. syringae ES4326 carrying the avirulence gene avrRpt2 elicited an HR when infiltrated into Arabidopsis ecotype Columbia leaves, ES4326 rpoN::Km r carrying avrRpt2 elicited no response. Constitutive expression of ES4326 hrpL in ES4326 rpoN::Km r partially restored defense-related mRNA accumulation, showing a direct role for the hrp cluster in host defense gene induction in a compatible host-pathogen interaction. However, constitutive expression of hrpL in ES4326 rpoN::Km r did not restore coronatine production, showing that coronatine biosynthesis requires factors other than hrpL.The rpoN gene encodes the alternate sigma factor 54 , which works in conjunction with the NtrC class of transcriptional activators to control the expression of many genes in response to nutritional and environmental conditions (2, 54). For example, genes involved in nitrogen, hydrogen, and catabolite utilization are frequently regulated by 54 (6, 35, 48, 95). In the case of pathogenic bacteria, rpoN mediates expression of virulence-related factors such as pilin in Pseudomonas aeruginosa and flagellin in Vibrio anguillarum (24,61,86).For some phytopathogenic bacteria, rpoN has been implicated indirectly as a regulator of pathogenicity-related genes known as the hrp gene cluster (17, 18). Pseudomonas syringae pv. syringae strain 61, for example, contains a 25-kb hrp cluster consisting of several complementation groups comprising at least 27 genes (8, 26). Several hrp genes encode proteins that have a high degree of homology to components of the type III secretory pathway of Yersinia species which are responsible for translocating Yersinia outer membrane proteins into mammalian cells (13,15,55,83). By analogy, it is ...