INDIGENOUS MINIPLASMIDS IN STRAINS OF XANTHOMONAS-CAMPESTRIS PV VESICATORIA

Four sizes of miniplasmids, estimated on the basis of migration distances in agarose gels, were observed in strains of Xanthomonas campestris pv. vesicatoria. Estimated sizes of these miniplasmids were 2.5, 2.28, 1.8, and 1.7 kilobase pairs (kbp). Only one size of miniplasmid was observed within a bacterial strain. There was DNA homology between the two larger miniplasmids, 2.5 and 2.28 kbp, and between the two smaller miniplasmids, 1.8 and 1.7 kbp, but not between the two size groups. The two larger miniplasmids were linearized by BamHI; the two smaller miniplasmids were linearized by XhoI. Although all bacterial strains with miniplasmids also contained at least one larger plasmid, no homology of miniplasmid DNA to any larger plasmids or chromosomal DNA was detected. In bacterial strains isolated from diseased tissue, miniplasmids were detected only in race 2 strains and strains of the tomato group. Miniplasmids were observed in streptomycin-resistant and/or copper-resistant strains. However, not all strains resistant to copper contained a miniplasmid (e.g., race 1 strains) nor were all strains that contained a miniplasmid resistant to streptomycin. In the strains resistant to streptomycin, the size of the miniplasmid was inversely associated with the concentration of streptomycin tolerated by the strain. The miniplasmids did not hybridize with a copper-resistance determinant, avrBs1 (specifies pathogen race 2), or avrBs3 (specifies pathogen race 1). Miniplasmids that mobilized into copper- and streptomycin-sensitive strains of X. c. vesicatoria did not convert such strains to copper or streptomycin resistance.