The DNA nucleoid-associated protein Fis co-ordinates the expression of the main virulence genes in the phytopathogenic bacterium Erwinia chrysanthemi

Erwinia chrysanthemi strain 3937 is a necrotrophic bacterial plant pathogen. Pectinolytic enzymes and, in particular, pectate lyases (Pels) play a key role in soft rot symptoms but the efficient colonization of plants by E. chrysanthemi requires additional factors. These factors include the harpin HrpN, the cellulase Cel5, proteases (Prts), flagellar proteins and the Sap system, involved in the detoxification of plant antimicrobial peptides. HrpN and flagellum are mostly involved in the early steps of infection whereas the degradative enzymes (Pels, Cel5, Prts) are mainly required in the advanced stages. Production of these virulence factors is tightly regulated by environmental conditions. This report shows that the nucleoid-associated protein Fis plays a pivotal role in the expression of the main virulence genes. Its production is regulated in a growth phase-dependent manner and is under negative autoregulation. An E. chrysanthemi fis mutant displays a reduced motility and expression of hrpN, prtC and the sap operon. In contrast, the expression of the cel5 gene is increased in this mutant. Furthermore, the induction of the Pel activity is delayed and increased during the stationary growth phase in the fis mutant. Most of these controls occur through a direct effect because purified Fis binds to the promoter regions of fis, hrpN, sapA, cel5 and fliC. Moreover, potassium permanganate footprinting and in vitro transcription assays have revealed that Fis prevents transcription initiation at the fis promoter and also transcript elongation from the cel5 promoter. Finally, the fis mutant has a decreased virulence. These results suggest a co-ordinated regulation by Fis of virulence factors involved in certain key steps of infection, early (asymptomatic) and advanced (symptomatic) phases.