Stable tetracycline resistance in Chlamydia suis is mediated by a family of genomic islands [the tet(C) islands] that are integrated into the chlamydial chromosome. The tet(C) islands contain several plasmid-specific genes, the tet(C) resistance gene and, in most cases, a novel insertion element (IScs605) encoding two predicted transposases. The hypothesis that IScs605 mediated the integration of the tet(C) resistance islands into the C. suis genome was tested using a plasmid-based transposition system in Escherichia coli. Both high- and medium-copy-number plasmids were used as carriers of IScs605 in these experiments. IScs605 integrated into a target plasmid (pOX38) when delivered by either donor plasmid, and integration of the entire donor plasmid was common. IScs605-mediated integration occurred at many positions within pOX38, with 36 of 38 events adjacent to a 5'-TTCAA-3' sequence. Deletions in each of the candidate transposase genes within IScs605 demonstrated that only one of the two ORFs was necessary for the observed transposition activity and target specificity. Analysis of progeny from the mating assays also indicated that IScs605 can excise following integration into a target DNA, and, in each tested case, the sequence 5'-AATTCAA-3' remained at the site of excision. Collectively, these results are consistent with the nucleotide sequence data collected for the tet(C) islands, and strongly suggest that a transposase within IScs605 is responsible for integration of these genomic islands into the C. suis chromosome.