Antagonistic effect of CRP and KdgR in the transcription control of the Erwinia chrysanthemi pectinolysis genes

The main virulence factors of the phytopathogenic bacteria Erwinia chrysanthemi are pectinases that cleave pectin, a major constituent of the plant cell wall, The cyclic AMP receptor protein (CRP) was identified as the main activator of the pectinolysis genes, Gel shift and DNase I footprinting experiments showed that the purified E, chrysanthemi CRP protein binds specifically to the promoter regions of seven pectinolysis genes (pelB, pelC, pelD, pelf, ogl, kdul and kdgT) whose expression is positively regulated in vivo by CRP. In contrast, no interaction was observed between CRP and the promoter-operator region of pelA, whose expression is negatively regulated in vivo by CRP. Primer extension experiments demonstrated that each of the pelB, pelC, pelf and kdul genes is expressed from a unique sigma(70) promoter, whereas ogl and kdgT possess three and two functional promoters respectively. The position of the CRP binding site relative to the transcription start site suggests that CRP acts as a primary activator at the pelS (via the CRP binding site 1), pelC, pelf, pelD, kdgTP(1) and oglP(2) promoters. In contrast, transcription at the kdul, oglP(1) promoters seems to require another transcriptional activator in synergy with CRP. Investigation of the simultaneous binding of CRP and KdgR, the main repressor of pectinolysis genes, to the regulatory regions of pelS, pelC, pelD, pelf, ogl, kdul and kdgT genes showed that binding of KdgR is preferential and exclusive in the case of ogl and kdgT, whereas the binding of these two regulators is independent in the case of pelS, pelC, pelD, pelf and kdul. Taken together, our data suggest that the antagonistic effects of CRP and KdgR on the expression of the pectinolysis genes occur by different mechanisms, including direct competition between the two regulators or between the repressor and RNA polymerase for the occupation of a common DNA region on the target genes.