Detection of a Novel, and Likely Ancestral, Tn-Like Element from a Human Saliva Metagenomic Library.

Tn is a conjugative transposon (CTn) and the first reported and most well characterised of the Tn/Tn family of CTns. Tn-like elements have a characteristic modular structure and different members of this family have been identified based on similarities and variations in these modules. In addition to carrying genes encoding proteins required for their conjugation, Tn-like elements also carry accessory, antimicrobial resistance genes; most commonly the tetracycline resistance gene, (M). Our study aimed to identify and characterise tetracycline resistance genes from the human saliva metagenome using a functional metagenomic approach. We identified a tetracycline-resistant clone, TT31, the sequencing of which revealed it to encode both (M) and (L). Comparison of the TT31 sequence with the accessory, regulation, and recombination modules of other Tn-like elements indicated that a partial Tn-like element encoding a truncated was cloned in TT31. Analysis indicated that a previous insertion within the truncated created the full length found in most Tn-like transposons; demonstrating that is, in fact, the result of a gene fusion event. Thus, we hypothesise that the Tn-like element cloned in TT31 likely represents an ancestral Tn.