The phytopathogenicity of is chiefly supported by the production of pectate lyase isoenzymes, encoded by the genes. One of these enzymes, PelA, encoded by the gene, seems to represent only a small part of the total pectate lyase activity, but is required for full bacterial pathogenicity. To study the regulation of gene expression, a :: gene fusion was constructed. Expression of this fusion was analysed under various growth conditions and in various genetic backgrounds. Whatever the culture conditions, the gene was weakly expressed. Moreover, expression seems not to be regulated by the gene product, but essentially by the gene product. In many plant-associated bacteria, genes involved in pathogenicity are induced by certain plant compounds. In this work, we demonstrate that genes are induced in the presence of plant extracts, but only in synergy with known pectate lyase inducers (KDG: 2-keto-3-deoxygluconate; DKII: 2,5-diketo-3-deoxygluconate). However, different genes did not exhibit the same sensitivity to plant signal molecules. Partial purification of inducing plant compounds suggested that plant extracts contain at least one molecule involved in pectate lyase induction. This compound is thermoresistant, and has a low molecular mass and a very hydrophilic behaviour.


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