NORMAL GENOMIC

• Shevchik VE, Hugouvieux-Cotte-Pattat N
• PaeX, a second pectin acetylesterase of Erwinia chrysanthemi 3937.
• J Bacteriol. 2003; 185: 3091-100
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• Erwinia chrysanthemi causes soft-rot diseases of various plants byenzymatic degradation of the pectin in plant cell walls. Pectin is acomplex polysaccharide. The main chain is constituted of galacturonateresidues, and some of them are modified by methyl and/or acetylesterification. Esterases are necessary to remove these modifications and,thus, to facilitate the further degradation of the polysaccharidic chain.In addition to PaeY, the first pectin acetylesterase identified in the E.chrysanthemi strain 3937, we showed that this bacterium produces a secondpectin acetylesterase encoded by the gene paeX. The paeX open readingframe encodes a 322-residue precursor protein of 34,940 Da, including a21-amino-acid signal peptide. Analysis of paeX transcription, by usinggene fusions, revealed that it is induced by pectic catabolic products andaffected by catabolite repression. The expression of paeX is regulated bythe repressor KdgR, which controls all the steps of pectin catabolism; bythe repressor PecS, which controls most of the pectinase genes; and bycatabolite regulatory protein, the global activator of sugar catabolism.The paeX gene is situated in a cluster of genes involved in the catabolismand transport of pectic oligomers. In induced conditions, the twocontiguous genes kdgM, encoding an oligogalacturonate-specific porin, andpaeX are both transcribed as an operon from a promoter proximal to kdgM,but transcription of paeX can also be uncoupled from that of kdgM innoninduced conditions. PaeX is homologous to the C-terminal domain of theButyrivibrio fibriosolvens xylanase XynB and to a few bacterial esterases.PaeX contains the typical box (GxSxG) corresponding to the active site ofthe large family of serine hydrolases. Purified PaeX releases acetate fromvarious synthetic substrates and from sugar beet pectin. The PaeX activityincreased after previous depolymerization and demethylation of pectin,indicating that its preferred substrates are nonmethylatedoligogalacturonides. PaeX is mostly found in the periplasmic space of E.chrysanthemi. These data suggest that PaeX is mainly involved in thedeacetylation of esterified oligogalacturonides that enter the periplasmby the KdgM porin.

• Nasser W, Shevchik VE, Hugouvieux-Cotte-Pattat N
• Analysis of three clustered polygalacturonase genes in Erwiniachrysanthemi 3937 revealed an anti-repressor function for the PecSregulator.
• Mol Microbiol. 1999; 34: 641-50
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• Erwinia chrysanthemi 3937 secretes an arsenal of pectinolytic enzymesincluding several pectate lyases encoded by the pel genes. Wecharacterized a novel cluster of pectinolytic genes consisting of thethree adjacent genes pehV, pehW and pehX, whose products havepolygalacturonase activity. The high similarity between the three genessuggests that they result from duplication of an ancestral gene. Thetranscription of pehV, pehW and pehX is dependent on several environmentalconditions. They are induced by pectin catabolic products and thisinduction results from inactivation of the KdgR repressor which controlsalmost all the steps of pectin catabolism. The presence of calcium ionsstrongly reduced the transcription of the three peh genes. Theirexpression was also affected by growth phase, osmolarity, oxygenlimitation and nitrogen starvation. In addition, the pehX transcription isaffected by catabolite repression and controlled by the activator proteinCRP. PecS, which was initially isolated as a repressor of virulencefactors, acts as an activator of the peh transcription. We showed that thethree regulators KdgR, PecS and CRP act by direct interaction with thepromoter regions of the peh genes. Analysis of simultaneous binding ofKdgR, PecS, CRP and RNA polymerase indicated that the activator effect ofPecS results from a competition between PecS and KdgR for the occupationof overlapping binding sites. Thus, to activate peh transcription, PecSbehaves as an anti-repressor against KdgR.

• van Gijsegem F
• Relationship between the pel genes of the pelADE cluster in Erwiniachrysanthemi strain B374.
• Mol Microbiol. 1989; 3: 1415-24
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• In this paper, we have used filter hybridization and nucleotide sequencingto analyse the relationship between the three genes of the pelADE clusterin the Erwinia chrysanthemi (Ech) strain B374. This cluster encodes forthree of the five pectate lyase proteins that are involved in themaceration and soft-rotting of plant tissue, an important trait in Echpathogenicity. Southern hybridization revealed homology between each ofthe three pel genes. A 3560 bp DNA fragment containing the pelE and pelDgenes was sequenced. These two genes show extensive homology in the codingregions but only low homology in the 5' and 3' non-coding regions. Howeverboth genes exhibit sequences homologous to the Escherichia coliCAP-binding site consensus sequence upstream of the start codon and aninverted repeat sequence which may act as a rho-independenttranscriptional terminator after the translational stop. The pel genes ofEch B374 were also compared with the already sequenced pel genes of EC16,another Ech strain.