Twenty strains of Pseudomonas syringae pv. tomato were examined for the presence of plasmid DNA. P. syringae pv. tomato plasmids were grouped into five size classes: class A ranged from 95 to 103 kilobases (kb); class B ranged from 71 to 83 kb; class C ranged from 59 to 67 kb; class D ranged from 37 to 39 kb; and class E was 29 kb. All strains contained at least two plasmids in classes A and B. The conjugative ability of P. syringae pv. tomato plasmids in three strains was demonstrated by mobilization of the nonconjugative plasmid RSF1010 into Pseudomonas syringae pv. syringae recipients. Plasmids from the three conjugative strains were labeled with TnS. Four conjugative plasmids were identified by their repeated transfer to P. syringae pv. syringae recipients. P. syringae pv. tomato strains varied in sensitivity to copper sulfate (CUSO4): MICs were 0.4 to 0.6 mM for sensitive strains, 1.2 mM for moderately resistant strains, and 1.6 to 2.0 mM for very resistant strains. One very resistant strain, PIT23, functioned as a donor of copper resistance. Recipient P. syringae pv. syringae strains PS51 and PS61 were inhibited by 0.1 mM CuS04, whereas the CuS04 MICs for transconjugant strains PS51(pPT23A) and PS61(pPT23C) were 1.8 and 2.6 mM, respectively. P. syringae pv. tomato strains PT12.2 and PT17.2 were inhibited by 0.6 mM copper sulfate, but their copper sulfate MICs were 2.6 and 1.8 mM, respectively, when they acquired pPI23C. Therefore, copper resistance in PT23 was controlled by two conjugative plasmids, designated pPT23A (101 kb) and pPT23C (67 kb).Many pathovars of Pseudomonas syringae contain indigenous plasmids (3,6,7,(26)(27)(28). Most of these plasmids are phenotypically cryptic; however, Comai and Kosuge (3) have demonstrated the involvement of an indigenous plasmid from Pseudomonas syringae pv. savastanoi in the production of indoleacetic acid, a compound essential for gall development in the olive plant.Although conjugative plasmids are important mediators of genetic exchange, they have been identified in only two of the 41 pathovars of P. syringae. pBPW1 is a 30-megadalton conjugative plasmid isolated from Pseudomonas syringae pv. tabaci BR2 (B. J. Staskawicz, M. Sato, and N. J. Panopoulos, Phytopathology 71:257, 1984), and pCG131 is a conjugative plasmid isolated from Pseudomonas syringae pv. syringae (14). Both of these plasmids are phenotypically cryptic. Since most P. syringae plasmids lack a selectable marker, they must be labeled with a drug resistance transposon or used to mobilize a nonconjugative drug resistance plasmid to demonstrate conjugative transfer.Plasmid-determined resistance to toxic metal ions has been demonstrated for many bacterial species (13,30) and is a useful selectable marker for these DNA molecules. Copper sprays are toxic to bacteria and are frequently applied for control of diseases caused by phytopathogenic species. Although copper resistance in bacteria is relatively rare, resistant species have been isolated from several diverse sources (2,8,9,21,32,33 Phytopathology,...