Construction and characterization of a hrpG mutant rendering constitutive expression of hrp genes in Xanthomonas campestris pv. Campestris *

The expression of hrp genes in Xanthomonas is regulated by hrpG, an OmpR-type transcriptional activator, and can be induced by artificial mimic medium XVM2, but repressed in rich medium NYG. In this study, a site-specific mutation of hrpG in Xanthomonas campestris pv. campestris (Xcc) was generated by PCR-directed point mutagenesis and was characterized by its phenotype and transcriptional expression analyses. The results indicated that the E45K mutated hrpG rendered all the hrp genes expressing constitutively in the suppression medium NYG. Plant test showed that mutated hrpG enhanced the timing and intensity of hypersensitive reaction (HR) on the on the nonhost plant pepper, but did not affect the growth of Xcc in both culture medium and plants. The E45K mutation did not cause significant changes of production of exopolysaccharide cellulase and amylase, but in inhibited the secretion of extracellular protease. This mutant 8004 is more suitable than the wild type for identifying hrp-dependant eff...