Nucleotide sequence and functional analysis of the tet (M)-carrying conjugative transposon Tn5251 of Streptococcus...

The Tn916-like genetic element Tn5251 is part of the composite conjugative transposon (CTn) Tn5253 of Streptococcus pneumoniae, a 64.5-kb chromosomal element originally called Omega(cat-tet) BM6001. DNA sequence analysis showed that Tn5251 is 18 033-bp long and contains 22 ORFs, 20 of which have the same direction of transcription. Annotation was possible for 11 out of 22 ORFs, including the tet(M) tetracycline resistance gene and int and xis involved in the integration/excision process. Autonomous copies of Tn5251 were generated during matings of Tn5253-containing donors with S. pneumoniae and Enterococcus faecalis. Tn5251 was shown to integrate at different sites in the bacterial chromosome. It behaves as a fully functional CTn capable of independent conjugal transfer to a variety of bacterial species including S. pneumoniae, Streptococcus gordonii, Streptococcus pyogenes, Streptococcus agalactiae, E. faecalis and Bacillus subtilis. The excision of Tn5251 produces a circular intermediate and a deletion in Tn5253 at a level of 1.2 copies per 10(5) chromosomes.