Specific interactions of Erwinia chrysanthemi KdgR repressor with different operators of genes involved in pectinolysis.

The Erwinia chrysanthemi kdgR gene encodes a repressor that negatively regulates the expression of genes involved in pectinolysis and in pectinase secretion. The cloned kdgR gene was overexpressed in Escherichia coli by using a phage T7 system. Overproduced repressor was purified to homogeneity by two chromatographic steps. Gel retardation and DNase I protection experiments demonstrated the specific binding of the KdgR protein to the operators of pectinase genes (pelA, pelB, pelC, pelE), to the operator of genes involved in pectin catabolism (kdgT, ogl, kduI-kdgF) and to that of the outT gene involved in pectinase secretion. These interactions involved one (pelA, pelB, kduI-kdgF, outT) or several operator sites (pelC, pelE, ogl, kdgT) that generally overlap the promoter. Despite the presence of potential KdgR binding sites (KdgR-box) in the regulatory regions of four genes involved in pectin catabolism (kdgC, kduD, pem, kdgA) and in a pectinase secretion gene outC, no DNA-repressor complex could be observed by in vitro experiments. By using a missing contact experiment on the coding strand of ogl and pelE regulatory regions, a new KdgR-binding consensus was proposed. This new consensus, constituted by two half motifs (AATGAAAACT)N(NTCGATTTCTA), is well conserved in the operators which interact in vitro with the KdgR repressor. In contrast, this repressor-recognized motif is degenerated in the other operators that cannot interact in vitro with the repressor. These results suggest the existence of different regulation mechanisms mediated by the KdgR protein for the two classes of operators.