In an HmpX-strain, the synthesis of pectate lyases, which are pathogenicity determinants in E. chrysanthemi, was reduced in conditions of low oxygen tension. Using gus fusion in hmpX, it was shown that hmpX transcription was induced in coculture with tobacco cells. A putative function for HmpX is discussed.

The phytopathogenicity of Erwinia chrysanthemi is chiefly supported by the production of pectate lyase isoenzymes, encoded by the pel genes. One of these enzymes, PelA, encoded by the pelA 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 pelA gene expression, a pelA::uidA gene fusion was constructed. Expression of this fusion was analysed under various growth conditions and in various genetic backgrounds. Whatever the culture conditions, the pelA gene was weakly expressed. Moreover, pelA expression seems not to be regulated by the pecS gene product, but essentially by the kdgR gene product. In many plant-associated bacteria, genes involved in pathogenicity are induced by certain plant compounds. In this work, we demonstrate that E. chrysanthemi pel 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 pel 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.

The pefA gene from Erwinia chrysmthemi strain 3937, which encodes the acidic pectate lyase, PLa, has been sequenced and characterized. The structural gene consists of a 1179 bp open reading frame encoding a polypeptide of 41 555 Da, which includes an N-terminal signal peptide. The deduced amino acid sequence shows a protein very similar to some PLs already sequenced. Cloning of the pefA gene behind the focZ promoter of the vector pTZ19R allowed overexpression of PLa into a derivative of strain 3937 deleted of the other p f genes. The mature protein was obtained in milligram amounts from the supernatant of this strain and at homogeneous purity after two purification steps. Its biochemical properties were similar to those of other PLs. Polyclonal antibodies raised against the purified PLa cross-reacted with the basic pectate lyase PM, but not with PLe. The role of PLa in pathogenicity is discussed.