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Evidence for Acquisition of Copper Resistance Genes from Different Sources in Citrus-Associated Xanthomonads

First and third authors: Department of Plant Pathology, University of Florida, Gainesville 32611; second author: United States Department of Agriculture–Agricultural Research Service, U.S. Horticultural Research Laboratory, 2001 S. Rock Rd., Fort Pierce, FL 34945; and fourth author: Soil and Water Science Department, Citrus Research and Education Center, University of Florida, Lake Alfred 33850.

We determined that multiple and independent introductions of copper resistance genes have taken place for strains of Xanthomonas citri subsp. citri from Argentina and strains of X. alfalfae subsp. citrumelonis from Florida. This study compared the partial nucleotide sequences of principal copper resistance genes copL, copA, and copB from X. citri subsp. citri and X. alfalfae subsp. citrumelonis to strains of other Xanthomonas spp. resistant to copper that were isolated from 12 different countries or territories. The survey confirmed that the copLAB gene cluster is present in many species of Xanthomonas from different parts of the world. Alignment of partial nucleotide sequences of copper resistance genes among the copper-resistant (Cu^R) strains of Xanthomonas detected homology of ≥92, ≥96, and ≥91% for copL, copA, and copB, respectively. Grouping of strains based on branching patterns of phylogenetic trees was similar for copL and copA but differed for copB. When the three genes were concatenated and analyzed using various phylogenetic methods, it appeared that the plasmid had been horizontally transferred and various populations were mutating based on selection pressure unique to geographic regions. Although high homology of the genes among the strains indicated that the copper resistance in xanthomonads has a common origin, the slight differences in nucleotide sequences within groups of strains indicated that Cu^R genes have been independently exchanged among species of Xanthomonas throughout the world.