Conservation of Plasmid DNA Sequences and Pathovar Identification of Strains ofXanthomonas campestris

Abstract

One hundred and seventeen different strains of Xanthomonas campestris, representing 26 different pathovars, were examined for plasmid content and restriction fragment length polymorphism of the plasmid DNAs. All strains tested of 10 pathovars contained plasmids. All strains tested of 13 pathovars contained no detectable plasmids, and strains of three pathovars were variable in plasmid content. Restriction endonuclease digests of plasmid DNAs from strains within a given plasmid-containing pathovar gave surprisingly similar, but not always identical, digestion profiles on agarose gels. When strains were purified by repeated single-colony isolations, the plasmid DNAs were found to be stable. In most cases, strains of X. campestris that contained plasmids could be differentiated at the pathovar level on the basis of their characteristic plasmid profiles. In no instance was the same plasmid profile seen in more than one pathovar. Plasmids that appeared to be similar by restriction fragment length profiles were confirmed to be simila rin DNA sequence by Southern hybridization analyses. All 60 strains tested of X. c. pv. glycines, X. c. pv. malvacearum, X. c. pv. phaseoli, and X. c. pv. vignicola could be accurately identified by Southern hybridizations of that profile. The apparent stability of the plasmids provides a natural genetic marker that can be strain specific and perhaps useful in epidemiological investigations.