Characterization of the termini and transposition products of Tn4399, a conjugal mobilizing transposon of Bacteroides fragilis.

We have isolated a 9.6-kilobase conjugal transposon, Tn4399 from Bacteroides fragilis, that is capable of mobilizing nonconjugal plasmids in cis. Here we characterize the ends of the transposon, its target-site requirements, and the products of transposition into the B. fragilis chromosome and two sets of B. fragilis-Escherichia coli shuttle vectors. With the exception of an additional cytosine residue in the left end, there are perfect 13-base-pair (bp) inverted repeats at the ends of Tn4399. Insertion of Tn4399 resulted in a 3-bp target-site repeat in 8 out of 12 independent transpositions and showed a high insertion-site specificity. A remarkable feature of Tn4399 insertions is the presence of an additional 5 bp located between the right inverted repeat and the target-site repeat. Four sequence variations of the 5 bp were found, with absolute conservation at positions 1, 2, and 5. Only two of the variations were present in junction fragments of all three copies of Tn4399 contained in the chromosome of the original donor strain, B. fragilis TM4.2321. Tn4399 appears to represent a new type of conjugal transposon. In contrast to Tn916 and Tn1545, described in streptococci, Tn4399 creates a target-site repeat and contains an additional 5 bp at the right end only, between the transposon and the target sequence. In addition, Tn4399 can mobilize plasmids in cis.