Excision and Circularization of Integrative Conjugative Element Tn of .

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