The Oligogalacturonate-specific Porin KdgM of Erwinia chrysanthemi Belongs to a New Porin Family
Résumé
The phytopathogenic Gram-negative bacteria Erwinia chrysanthemi secretes pectinases, which are able to degrade the pectic polymers of plant cell walls, and uses the degradation products as a carbon source for growth. We characterized a major outer membrane protein, KdgM, whose synthesis is strongly induced in the presence of pectic derivatives. The corresponding gene was characterized. Analysis of transcriptional fusions showed that the kdgM expression is controlled by the general repres-sor of pectinolytic genes, KdgR, by the repressor of hexu-ronate catabolism genes, ExuR, by the pectinase gene repressor, PecS, and by catabolite repression via the cyclic AMP receptor protein (CRP) transcriptional activa-tor. A kdgM mutant is unable to grow on oligogalactu-ronides longer than trimers, and its virulence is affected. Electrophysiological experiments with planar lipid bilay-ers showed that KdgM behaves like a voltage-dependent porin that is slightly selective for anions and that exhibits fast block in the presence of trigalacturonate. In contrast to most porins, KdgM seems to be monomeric. KdgM has no homology with currently known porins, but proteins similar to KdgM are present in several bacteria. Therefore , these proteins might constitute a new family of porin channels.
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