Erwinia carotovora has two KdgR-like proteins belonging to the IcIR family of transcriptional regulators: identification and characterization of the RexZ activator and the KdgR repressor of pathogenesis

A novel Erwinia carotovora subsp. carotovora mutant designated RexZ, (regulator of exoenzymes) showed reduced production of the degradative exoenzymes. The rexZ gene product shows similarity to the KdgR regulatory protein from Erwinia chrysanthemi, described as the major repressor of the pectin catabolism pathway genes in the latter species. In vitro DNA—protein interaction experiments demonstrated that the synthesis of the RexZ protein is controlled by the cAMP—CRP (cAMP—receptor protein) complex. Western blot analysis also revealed the presence of a second KdgR homologue (distinct from RexZ) which, like RexZ, was present in all species of the genus Erwinia tested. The corresponding KdgR proteins from both E. carotovora subsp. carotovora and E. carotovora subsp. atroseptica share a high level of sequence identity with the KdgR homologues from E. chrysanthemi and Escherichia coli. Although the E. carotovora subsp. carotovora rexZ regulatory region displayed specific interactions with both the purified E. chrysanthemi KdgR repressor and the partially purified E. carotovora subsp. carotovora KdgR, in vivo quantification revealed that the cellular level of RexZ protein was unaffected by the presence of pectic compounds. This study shows that the complex regulatory network governing virulence in the erwinias involves two totally distinct, but highly conserved, members of the IcIR class of DNA binding proteins: RexZ and KdgR.