1887

Abstract

pv. syringae strain FF5 is a phytopathogen associated with a rapid dieback on ornamental pear trees. and the human pathogen produce the exopolysaccharide alginate, a copolymer of mannuronic and guluronic acid. In , the response regulator AlgR (AlgR1) is required for transcription of and , which encode key enzymes in the alginate biosynthetic pathway. In FF5, however, is not required for the activation of . Interestingly, mutants of remain nonmucoid, indicating an undefined role for this response regulator in alginate biosynthesis. In the current study, the promoter region was cloned from pv. syringae strain FF5, and sequence analysis of the promoter indicated the presence of potential binding sites for AlgR and , the alternative sigma factor encoded by . The promoter from FF5 () was cloned upstream of a promoterless glucuronidase gene (), and the transcriptional fusion was used to monitor expression in strains FF5.32 ( mutant of FF5) and PG4180.K2 ( mutant of pv. glycinea PG4180). Expression of the fusion was fourfold lower in both the and mutants as compared to respective wild-type strains, indicating that both AlgR and are required for full activation of transcription in pv. syringae. AlgR from was successfully overproduced in as a C-terminal translational fusion to the maltose-binding protein (MBP). Gel shift experiments indicated that MBP–AlgR binds strongly to the promoter region. Biological assays demonstrated that the mutant was significantly impaired in both pathogenicity and epiphytic fitness as compared to the wild-type strain. These results, along with the gene expression studies, indicate that AlgR has a positive role in the activation of in and contributes to both virulence and epiphytic fitness. Furthermore, the symptoms observed with wild-type FF5 suggest that this strain can move systemically in leaf tissue, and that a functional copy of is required for systemic movement.

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2004-08-01
2024-03-28
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