Tol-Pal proteins are critical cell envelope components of Erwinia chrysanthemi affecting cell morphology and virulence

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Abstract
Thetol-palgenes are necessary for maintaining the outer-membrane integrity of Gram-negative bacteria. These genes were first described inEscherichia coli, and more recently in several other species. They are involved in the pathogenesis ofE. coli,Haemophilus ducreyi,Vibrio choleraeandSalmonella enterica. The role of thetol-palgenes in bacterial pathogenesis was investigated in the phytopathogenic enterobacteriumErwinia chrysanthemi, assuming that this organism might be a good model for such a study. The wholeEr. chrysanthemi tol-palregion was characterized. Tol-Pal proteins, except TolA, showed high identity scores with theirE. colihomologues.Er. chrysanthemimutants were constructed by introducing auidA–kan cassette in theybgC,tolQ,tolA,tolB,palandybgFgenes. All the mutants were hypersensitive to bile salts. Mutations intolQ,tolA,tolBandpalwere deleterious for the bacteria, which required high concentrations of sugars or osmoprotectants for their viability. Consistent with this observation, they were greatly impaired in their cell morphology and division, which was evidenced by observations of cell filaments, spherical forms, membrane blebbing and mislocalized bacterial septa. Moreover,tol-palmutants showed a reduced virulence in a potato tuber model and on chicory leaves. This could be explained by a combination of impaired phenotypes in thetol-palmutants, such as reduced growth and motility and a decreased production of pectate lyases, the major virulence factor ofEr. chrysanthemi.