TN5252: a model for complex streptococcal conjugative transposons.

Abstract

Recent widespread dissemination of multiple antibiotic resistance among clinical streptococci has been due mainly to a novel class of chromosome-borne mobile elements capable of intracellular as well as intercellular transmission. The streptococcal element, Tn5253, originally discovered as a heterologous insertion in the chromosome of Streptococcus pneumoniae BM6001 (2) carrying chloramphenicol and tetracycline resistance determinants, is capable of conjugal transfer to a number of streptococcal species. Later studies investigating the nature of its relationship to other similar elements showed that Tn5253 was a composite of two independent conjugative transposons, the tetracycline resistance containing Tn5251 (18 kb) inserted within Tn5252 (47 kb) carrying the cat determinant, with no significant homology between the two elements detectable in blot hybridization experiments (1). Of the four classes of conjugative transposons found in bacteria, Tn5252 is the prototype of a distinct class of streptococcal conjugative transposons. The region encoding recombinative functions in Tn5252 has been studied in some detail (6). However, little is known about the actual mechanism of cell-to-cell contact and DNA transfer through the membrane or about the genes involved. To enlarge our understanding of the biology of Tn5252, a preliminary genetic map was obtained following insertion mutagenesis using a heterologous reporter DNA segment. DNA sequence determination of a region of about 7 kb at the left end of the element carrying two of these insertions affecting transfer functions enabled the identification of several open reading frames (ORFs) displaying homologies to DNA processing genes such as lambdoid integrases, excisionases, DNA relaxases, and transcriptional regulators (6, GenBank accession number L29324). Interruption of the ORFs exhibiting homologies to integrases and DNA relaxases led to a tra − phenotype (6). On the other hand, two other insertions affecting conjugal transferrability of the element were at the right end of the element. DNA sequence data from about 11 kb comprising these loci was obtained. Here we summarize our recent molecular analysis of this region predicted to carry genes involved in transport of DNA during conjugal transfer of Tn5252.