Tn5251 belongs to the Tn916-Tn1545 family of conjugative transposons (CT) and was found integrated into CT Tn5252, to form the composite element Tn5253 of Streptococcus pneumoniae. We show that Tn5251 is identical in structure and size to Tn916. DNA sequence analysis of a 4,419-bp segment containing the tet(M) gene showed that only 73 nucleotides out of 4,419 were different in the two CT. Essentially all differences (66/73) were clustered in a 688-bp segment of tet(M), which was 90% identical to Tn916 and 100% identical to the tet(M) genes of Tn1545 from S. pneumoniae and pOZ101 from Neisseria gonorrhoeae. DNA sequence analysis of the Tn5251/Tn5252 junction fragments allowed us (i) to determine Tn5251 termini, (ii) to define the 6-bp coupling sequences flanking the CT, and (iii) to infer the structure of the integration site (attB) of Tn5251 into Tn5252. Conjugal transfer of Tn5251 independent from Tn5253 could not be detected, even if we could show excision and formation of Tn5251 circular intermediates at a level of 5.4 copies per 10(6) chromosomes.