Analysis of three clustered polygalacturonase genes in Erwinia chrysanthemi 3937 revealed an anti-repressor function for the PecS regulator

Erwinia chrysanthemi 3937 secretes an arsenal of pectinolytic enzymes inclu ding several pectate lyases encoded by the pel genes. We characterized a no vel cluster of pectinolytic genes consisting of the three adjacent genes pe hV, pehW and pehX, whose products have polygalacturonase activity. The high similarity between the three genes suggests that they result from duplicat ion of an ancestral gene. The transcription of pehV, pehW and pehX is depen dent on several environmental conditions. They are induced by pectin catabo lic products and this induction results from inactivation of the KdgR repre ssor which controls almost all the steps of pectin catabolism. The presence of calcium ions strongly reduced the transcription of the three peh genes. Their expression was also affected by growth phase, osmolarity, oxygen lim itation and nitrogen starvation. In addition, the pehX transcription is aff ected by catabolite repression and controlled by the activator protein CRP. PecS, which was initially isolated as a repressor of virulence factors, ac ts as an activator of the peh transcription. We showed that the three regul ators KdgR, PecS and CRP act by direct interaction with the promoter region s of the peh genes. Analysis of simultaneous binding of KdgR, PecS, CRP and RNA polymerase indicated that the activator effect of PecS results from a competition between PecS and KdgR for the occupation of overlapping binding sites. Thus, to activate peh transcription, PecS behaves as an anti-repres sor against KdgR.