Characterization of the 13-kilobase ermF region of the Bacteroides conjugative transposon CTnDOT

The conjugative transposon CTnDOT is virtually identical over most of its l ength to another conjugative transposon, CTnERL, except that CTnDOT carries an ermF gene that is not found on MERL. In this report, we show that the r egion containing ermF appears to consist of a 13-kb chimera composed of at least one class I composite transposon and a mobilizable transposon (MTn). Although the ermF region contains genes also carried on Bacteroides transpo sons Tn4351 and Tn4551, it does not contain the IS4351 element which is fou nd on these transposons. In CTnDOT, insertion of the ermF region occurred n ear a stem-loop structure at the end of orf2, an open reading frame located immediately downstream of the integrase (int) gene of CTnDOT, and in a reg ion known to be important for excision of MERL and CTnDOT. The chimera that comprises the ennF region can apparently no longer excise and circularize, but it contains a functional mobilization region related to that described for the Bacteroides MTn Tn4399. Analysis of 19 independent Bacteroides iso lates showed that the ermF region is located in the same position in all of the strains analyzed and that the compositions of the ermF region are almo st identical in these strains. Therefore, it appears that CTnDOT-like eleme nts present in community and clinical isolates of Bacteroides were derived from a common ancestor and proliferated in the diverse Bacteroides populati on.