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

A Bacteroides conjugative transposon (CTn), CTn12256, which has an estimated size of more than 150 kbp appeared to contain most or all of a previously discovered 65 kbp CTn, CTnDOT. To determine whether the integrated CTnDOT was still intact and to identify the element into which CTnDOT had integrated, large segments of CTn12256 were cloned and sequenced. Results of this analysis revealed that an intact CTnDOT type CTn had integrated into another CTn that was most closely related to a putative CTn found in the genome sequence of Bacteroides fragilis YCH46 (CTn3Bf). The CTnDOT portion of CTn12256 (CTnDOT2) proved to be capable of excising from CTn12256 and to transfer itself independently of CTn12256. The CTn3Bf region rarely transferred independently of CTnDOT2, and the transferred CTn3Bf contained a large deletion flanking the site of CTnDOT2 insertion and was no longer transmissible. Nonetheless, genetic analysis showed that the CTn3Bf portion controls transfer of CTn12256.