GENETICS OF GALACTOSE METABOLISM OF ERWINIA-AMYLOVORA AND ITS INFLUENCE ON POLYSACCHARIDE SYNTHESIS AND VIRULENCE OF THE FIRE BLIGHT PATHOGEN

Galactose metabolism mutants of Erwinia amylovora were created by tran sposon insertions and characterized for their growth properties and in teraction with plant tissue. The nucleotide sequence of the gate gene was determined. The gene, which encodes UDP-galactose 4-epimerase, sho ws homology to the galE genes of Escherichia coli, Neisseria gonorrhoe ae, Rhizobium meliloti, and other gram-negative bacteria. Cloned DNA w ith the galE and with the galT and galK genes did not share borders, a s judged by the lack of common fragments in hybridization with chromos omal DNA. These genes are thus located separately on the bacterial chr omosome. In contrast to the gal operon of E. coli, the galE gene of E. amylovora is constitutively expressed, independently of the presence of galactose in the medium. The function of the galE gene but not of t he galT or galK gene is required for bacterial virulence on pear fruit s and seedlings. In the absence of galactose, the galE mutant was defi cient in amylovoran synthesis. Subsequently, the galE mutant cells eli cited host defense reactions, and they were not stained by fluorescein isothiocyanate-labelled lectin, which efficiently binds to amylovoran capsules of E. amylovora. The mutation affected the side chains of ba cterial lipopolysaccharide, but an intact O antigen was not required f or virulence. This was shown with another mutant, which could be compl emented for virulence but not for side chain synthesis of lipopolysacc haride.