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 chr ysanthemi are pectinases that cleave pectin, a major constituent of th e plant cell wall, The cyclic AMP receptor protein (CRP) was identifie d as the main activator of the pectinolysis genes, Gel shift and DNase I footprinting experiments showed that the purified E, chrysanthemi C RP protein binds specifically to the promoter regions of seven pectino lysis genes (pelB, pelC, pelD, pelf, ogl, kdul and kdgT) whose express ion is positively regulated in vivo by CRP. In contrast, no interactio n was observed between CRP and the promoter-operator region of pelA, w hose expression is negatively regulated in vivo by CRP. Primer extensi on 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 posi tion 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 requi re another transcriptional activator in synergy with CRP. Investigatio n of the simultaneous binding of CRP and KdgR, the main repressor of p ectinolysis 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 the se two regulators is independent in the case of pelS, pelC, pelD, pelf and kdul. Taken together, our data suggest that the antagonistic effe cts 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 occupat ion of a common DNA region on the target genes.