Identification of a circular intermediate in the transfer and transposition of Tn4555 a mobilizable transposon from Bacteroides spp.

Transmissible cefoxitin (FX) resistance in Bacteroides vulgatus CLA341 was associated with the 12.5-kb obilizable transposon Tn4555 which encoded the 13-lactamase gene cfxA. Transfer occurred by a onjugation-like mechanism was stimulated by growth of donor cells with tetracycline (TC) and required the resence of a Bacteroides chromosomal Tcr element. Transconijugants resistant to either FX TC or both drugs ere obtained but only Fx Tcr isolates could act as donors of Fxr in subsequent matings. Transfer of Fxr could e restored in FxF Tc' strains by the introduction of a conjugal Tcr element from Bacteroidesfragilis V479-1. covalently closed circular DNA form of Tn4555 was observed in donor cells by Southern hybridization and he levels of this circular transposon increased significantly in cells grown with TC. Both the cfxA gene and the n4555 mobilization region hybridized to the circular DNA suggesting that this was a structurally intact ransposon unit. Circular transposon DNA purified by CsCI-ethidium bromide density gradient centrifugation as used to transform Tcs B. fragilis 638 and FXr transformants were obtained. Both the circular form and he integrated Tn4555 were observed in transformants but the circular form was present at less than one copy er chromosomal equivalent. Examination of genomic DNA from Fxr transformants and transconjugants evealed that Tn4555 could insert at a wide variety of chromosomal sites. Multiple transposon insertions were resent in many of the transconjugants indicating that there was no specific barrier to the introduction of a second transposon copy. Originally published Journal of Bacteriology Vol. 175 No. 9 May 199