GENETIC TRANSFER OF AMYLOVORAN AND STEWARTAN SYNTHESIS BETWEEN ERWINIA-AMYLOVORA AND ERWINIA-STEWARTII

DNA fragments with ems genes of Erwinia amylovora and cps genes of Erw inia stewartii were transferred to exopolysaccharide (EPS)-deficient m utants of the other species. The resulting EPSs were characterized by sensitivity to EPS-dependent bacteriophages, staining with amylovoran- specific fluorescein-isothiocyanate-labelled lectin and chemical techn iques, such as determination of the sugar composition and methylation analysis in order to distinguish between amylovoran and stewartan. Deg radation by the stewartan-dependent phage phi-K9 was used to detect st ewartan production, and staining with a lectin from Abrus precatorius detected amylovoran capsules. the patterns of sugar linkages were dete rmined by methylation analysis. Stewartan contained a significantly hi gher glucose to galactose ratio than amylovoran and produced a charact eristic signal from 6-linked glucose residues. By these criteria, most E. stewartii cps mutants displayed exclusively amylovoran synthesis w hen complemented with the ams cluster, and E. amylovora ams mutants co mplemented with E. stewartii cps genes produced stewartan. The complem entation to an EPS-positive phenotype may require most genes of the am s or the cps operon. An exception was an E. stewartii cpsK mutant that made predominantly stewartan when complemented with the ams cosmid. I R spectra showed that both amylovoran and stewartan were acylated when synthesized in E. amylovora, but not in E. stewartii. The amylovoran- producing E. stewartii merodiploids regained virulence to corn seedlin gs when mucoidy was restored by the ems cluster, but the stewartan-pro ducing E. amylovora ams(-)lcps(+) strains were weakly virulent on pear slices and avirulent on apple seedlings.