To obtain a functional map of Tn5252, a 47.5-kb streptococcal conjugative transposon, a series of defined deletion and insertion mutations were introduced within the transposon. Interruptions at several regions were found to affect the conjugal transposition functions of the element in filter-mating experiments. The nucleotide sequence of the left terminus of Tn5252 showed two open reading frames, ORF1 and ORF2, adjoining the att site. The organization of this region and the structure of the predicted integrase encoded by ORF1 were found to be similar to those of other site-specific recombination systems.