Xanthomonas campestris pathovar campestris (Xcc) is a Gram-negative bacterium and the causal agent of black rot disease in cruciferous plants. The exopolysaccharides (EPS) are one of the important virulence factors, whose biosynthesis is regulated by the rpf (regulation of pathogenicity factors) genes cluster. Mutants of rpfB、rpfF、rpfG and rpfC were constructed by insertional mutagenesis. It has previously been shown that rpfF、rpfC and rpfG mutants are avirulent and exhibit a decreased level of EPS. Nevertheless, a rpfB mutant displays little phenotypical change. Xcc is motile by using a single polar flagellum. In this work, experiments were carried out to elucidate the role of rpf genes on the bacterial motility as well as on the expression of flagellar genes in the transcriptional and translational level.
The results showed that, 1) electron microscopy demonstrated that rpfC and rpfF mutants have no flagellum and rpfB, rpfG mutants have single flagellum. 2) Spot test result demonstrated that a rpfF mutant become resistant to the infection of bacteriophage L7. 3) In the transcriptional level, rpfC、rpfG and rpfF genes positively control the expression of flagellar genes flgG、flhF、fliC、fliQ、fliE and fliL promoter activity. rpfB positively regulate the promoter activity of flgB、fliQ、fliC and flhF genes and rpfC、rpfF positively regulate the promoter activity of flgB. 4) In the translational level, rpfC positively regulates the expression of FliC and FliA; rpfF positively regulates the expression of FliA、FliC and FlhF; rpfG positively regulates the expression of FliC and FliA.
