ABSTRACT
Two closely related phytopathogenic bacterial strains, Pseudomonas syringae pv. glycinea PG4180 and P. syringae pv. tomato DC3000, produce the chlorosis-inducing phytotoxin coronatine (COR) in a remarkably divergent manner. PG4180 produces COR at the virulence-promoting temperature of 18 degrees C, but not at 28 degrees C. In contrast, temperature has no effect on COR synthesis in DC3000. A modified two-component system consisting of the histidine protein kinase (HPK), CorS, the response regulator (RR), CorR, and a third component, CorP, governs COR biosynthesis in both strains. A plasmid-based component and domain swapping approach was used to introduce different combinations of RRs, HPKs and hybrid HPKs into corS mutants of both strains. Subsequently, expression levels of the COR biosynthetic cma operon were determined using RNA dot-blot analysis, suggesting that CorRSP of PG4180 mediates a thermoresponsive phenotype dependent on the genomic background of each strain. The reciprocal experiment demonstrated a loss of temperature dependence in the corS mutant of PG4180. The presence of corR from PG4180 led to more pronounced cma expression in DC3000 and was associated with thermoresponsiveness, while corS of PG4180 did not mediate a temperature-dependent phenotype in the DC3000 mutant containing native corR and corP. These findings were substantiated by RT-PCR experiments. The C-terminal domain of CorS of PG4180 mediated thermosensing, while the N terminus did not respond to temperature changes, suggesting cytosolic perception of the temperature signal.
