Excision and Circularization of Integrative Conjugative Element Tn 5253 of Streptococcus pneumoniae .

The integrative conjugative element (ICE) Tn 5253 of Streptococcus pneumoniae , conferring resistance to tetracycline and chloramphenicol, was found integrated at a 83-bp specific target site ( att B) located in the rbgA gene of the pneumococcal chromosome. PCR analysis of Tn 5253 -carrying strains showed evidence of precise excision of Tn 5253 from the pneumococcal chromosome with production of (i) circular forms of the ICE in which the ends were joined by a 84-bp sequence ( att Tn), and (ii) reconstituted chromosomal att B. When integrated into the chromosome, Tn 5253 was flanked by att L, identical to att B, and att R, identical to att Tn. Circular forms of Tn 5253 were present at a concentration of 3.8 × 10-4 copies per chromosome, whereas reconstituted att B sites were at 3.0 × 10-4 copies per chromosome. Deletion of int-xis of Tn 5253 abolished production of circular forms (<7.1 × 10-6 copies per chromosome) and was associated to the lack of Tn 5253 conjugal transfer suggesting, as expected, that Tn 5253 circular form acts as a conjugation intermediate.