Tn602: a naturally occurring relative of Tn903 with direct repeats

We report the characterization of Tn602, a transposon encoding resistance to kanamycin and related aminoglycosides present on the R-plasmid pGD10. Tn602 is highly homologous to the previously characterized Tn903, present on the R-plasmid R6, in that it consists of a gene for aminoglycoside-phosphotransferase-3'-I (homologous to that of Tn903) flanked by copies of an IS-element homologous to IS903. Tn602 differs from Tn903 in the following respects: the flanking IS-elements (IS602) are in direct rather than inverted orientation as in Tn903; the fusion points between the IS-elements and the central region are different from those in Tn903; and several sequence changes, detected by the loss and acquisition of restriction sites, show the two repeats of IS602 to be nonidentical and different from IS903, IS102, and IS903.B. These structural details suggest that Tn602 and Tn903 evolved separately from related modules.