Characterization of a Negative Regulator of Exopolysaccharide Production by the Plant-Pathogenic Bacterium Pseudomonas solanacearum. Chris Cheng Kao. Department of Plant Pathology, University of Wisconsin, Madison 53706, U.S.A. Francoise Gosti, Yong Huang, and Luis Sequeira. Department of Plant Pathology, University of Wisconsin, Madison 53706, U.S.A. MPMI 7:121-130. Accepted 24 September 1993. Copyright 1994 The American Phytopathological Society.

Wild-type strains of the bacterial wilt pathogen Pseudomonas solanacearum exhibit reduced exopolysaccharide production and virulence when transformed with plas-mids carrying the epsR locus. To understand the function of epsR, we used mutagenesis and DNA sequencing to identify the gene responsible for the shutoff of exopolysaccharide production. The epsR gene encodes a 236-amino-acid polypeptide that, based on polypeptide sequence ho-mology, has significant similarity to other proteins of the luxR family of environmentally responsive, two-component regulatory systems. When a mutated copy of the epsR gene was marker-exchanged into the wild-type P. solanacearum chromosome, however, we observed no effect on growth in culture or on exopolysaccharide production. This suggests that the EpsR phenotype becomes apparent only via overproduction of the EpsR protein. By means of an antiserum directed against the EpsR protein, we detected the overproduction of EpsR in cell lysates of a strain of P. solanacearum harboring a multicopy plasmid with an active epsR gene but not in one harboring the same plasmid with a mutated epsR gene.