Molecular genetics and ecology of transposon-encoded streptomycin resistance in plant pathogenic bacteria

Plasmid-encoded streptomycin resistance in the plant pathogenic bacteria Erwinia amylovora, Pseudomonas syringae pvs. papulans and syringae, and Xanthomonas campestris pv. vesicatoria is conferred by the strA-strB aminoglycoside phosphotransferase genes. These genes are located on a transposable element designated Tn5393. The expression of the strA-strB genes is directed from a promoter within Tn5393 in P. syringae pv. syringae; the insertion sequence elements IS1133 and IS6100 are involved in the expression of strA-strB in E. amylovora and X. campestris, respectively. Epidemiological studies have shown that single plasmid species in E. amylovora, P. syringae pv. papulans, and X. campestris pv. vesicatoria are the main carriers of Tn5393 within local populations; however, Tn5393 is distributed among a large group of plasmids in P. syringae pv. syringae. The location of Tn5393 on stable indigenous plasmids in P. syringae pv. syringae suggests that this transposon may persist in this species. Streptomycin-resistant nontarget bacteria isolated from plants and soil from several agroecosystems contained Tn5393 sequences. These bacteria were postulated to be the source of streptomycin resistance in plant pathogenic bacteria. The strA-strB genes, but not Tn5393, are also widespread among commensal and pathogenic bacteria from humans and animals indicating that these bacteria, and plant pathogenic bacteria, share a common gene pool.