Conjugative transposons are genetic elements that transpose from one place in the genome of a donor bacterium to a new place in the genome of a recipient bacterium. The transfer from donor to recipient requires intercellular contact. They have an extraordinarily broad host range, being able to transfer between bacteria of different species and genera. The best studied of these transposons, Tn916, and its close relative, Tn1545, transpose by a mechanism that involves excision of the transposon in the donor bacterium to form a circular intermediate. This circular form of the transposon then transfers to the recipient, where integration occurs. Unlike most transposable elements, transposition of Tn916 does not result in duplication of the target sequence. Rather, one of two 6 base-pair sequences flanking the transposon in the donor (called coupling sequences) is transferred with Tn916 to the recipient. Upon excision, either the original target sequence is restored, or 6 base pairs of the target are replaced by the coupling sequence introduced with the transposon. Two transposon-encoded proteins called Int and Xis are involved in excision and integration. Int is member of the integrase family of recombinases. Current evidence suggests that transposon excision is the rate-limiting step in conjugal transposition. Two factors have been shown to influence the frequency of conjugative transposition. These factors are the nature of the coupling sequences flanking the transposon and the presence of a transposon-encoded accessory protein. The most remarkable property of conjugative transposons like Tn916 is their ability to operate in wide variety of different bacterial hosts, yet how the frequency of excision, and thus transposition, is determined remains poorly understood. The recent establishment of an in vitro assay for transposon excision means that the hypothesis that coupling sequences and accessory proteins together determine the frequency of transposon excision can be rigorously tested. It is not clear whether accessory proteins play a significant role in determining the frequency of conjugative transposition. This research will remove major uncertainty from the study of these elements that are important mediators of gene transfer between many important species of bacteria and lead to a much greater understanding of how conjugative transposition occurs.
