Erwinia chrysanthemi 3937 synthesizes an exopolysaccharide (EPS) composed of rhamnose, galactose, and galacturonic acid. Fourteen transcriptional fusions in genes required for EPS synthesis, named eps, were obtained by Tn5-B21 mutagenesis. Eleven of them are clustered on the chromosome and are repressed by PecT, a regulator of pectate lyase synthesis. In addition, expression of these fusions is repressed by the catabolite regulatory protein, CRP, and induced in low osmolarity medium. The three other mutations are located in genes that are not regulated by pecT. A 13-kb DNA fragment containing pecT-regulated eps genes has been cloned. All the genes identified on this fragment are transcribed in the same orientation and could form a large operon. The promoter region of this operon has been sequenced. It contains a JUMP-start sequence, a sequence required for the expression of polysaccharide-associated operons. E. chrysanthemi 3937 produces a systemic soft rot on its host Saintpaulia ionantha. An eps mutant was less efficient than the wild-type strain in initiating a maceration symptom, suggesting that production of EPS is required for the full expression of the E. chrysanthemi virulence.

Erwinia chrysanthemi synthesizes and secretes pectate lyases that attack components of the plant cell wall and, therefore, play a major role in the pathogenesis of soft rot disease. We isolated a new mutant (designated pec-1), by Tn5 mutagenesis, that displays weak pectate lyase production and decreased motility and mucoidicity. Maceration and pathogenicity tests done on different plant organs showed that the pec-1 strain displays a reduced virulence compared to that of the parental strain. The Tn5 insertion was localized between the pelL and the out loci and defines a new regulatory region. Sequencing of the pec-1::Tn5 insertion revealed that pec-1 is tightly linked to the pecT regulatory gene that also controls pectate lyase synthesis. Moreover, the pecT mutation is dominant over the pec-1 mutation, suggesting that these two loci are involved in the same regulatory network. We demonstrated, by Northern blot analysis, that the pec-1::Tn5 insertion provokes derepression of pecT transcription and defines a cis-acting element. Introduction of the pecT gene in trans of a pecT::uidA fusion induced a decrease of pecT::uidA transcription, indicating a negative autoregulation. Band shift experiments confirmed that the PecT repressor specifically interacts with the pecT regulatory region. We also demonstrated that the PecT protein interacts with the regulatory region of the pelD gene encoding a pectate lyase. Therefore, the abolition of the pecT autoregulation in the pec-1 mutant provokes an overproduction of the PecT repressor that is responsible for the decrease of pectate lyase synthesis. Mutagenesis of the pecT regulatory region revealed the presence of two sites in which insertions reproduced the pec-1 phenotype. This result suggests that pecT autoregulation requires the presence of two functional operator sites. From this study, we propose that the PecT repressor binds to these two sites, generating a loop that blocks pecT transcription.

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Advanced material with antibacterial properties would be a promising way to improve the disinfection process in food plants. Our objective was to combine the bactericidal effect of TiO 2 with the mechanical strength of TiN coatings. A TiO 2 rutile film was obtained after annealing of a supplied 316 stainless steel with a TiN coating. This TiO 2 upperlayer displays a photocatalytic activity under UV light exposure. The substrates with the TiN coating and the TiO 2 upperlayer are more hydrophobic than the 316 control. The adhesion of either Listeria or Pseudomonas, on 316-TiN is characterized by the presence of clusters of cells, while the oxidation of the TiN surface leads to a more hydrophilic layer where cells are individualized. After UV illumination of the adherent cells and subsequent growth, the residual bacterial population present on 316-TiO 2 is lower than that present on the 316-TiN. The bactericidal effect is more important on Listeria than on Pseudomonas.