CTn12256, a chimeric Bacteroides conjugative transposon that consists of two independently active mobile elements

► Bacteroides conjugative transposon CTn12256 consists of two complete and independent conjugative transposons. ► The outside element is nearly identical to a cryptic 120 kbp CTn, CTnBf3 found in the chromosomal sequence of Bacteroides fragilis YCH46. ► The internal CTn is nearly identical to the well characterized 65 kbp CTn, CTnDOT. ► CTn3Bf determines the transfer, integration and host range of the element. ► Both CTns can excise and transfer independently under specific conditions but usually transfer together.