Until the late 1970s, it was believed that the majority of gene transfer events in bacteria were mediated by plasmids. However, at that time, Don Clewell and co-workers isolated a strain of Enterococcus faecalis that could transfer tetracycline resistance in the absence of plasmid DNA (Tomich et al., 1979). The element responsible was an 18-kb segment of DNA that was integrated into the bacterial chromosome. As well as being capable of conjugative transfer to a new host, this element was also capable of intercellular transposition, so the term conjugative transposon was coined and this particular element was designated Tn916 (Franke and Clewell, 1981). Subsequently, it has become apparent that conjugative transposons are probably ubiquitous and are highly heterogeneous. This heterogeneity in form and function has led to some confusion and controversy about what a conjugative transposon actually is and how these elements should be named. This issue has more recently been addressed in a number of review articles (Burrus et al., 2002; Mullany et al., 2002; Osborn and Boltner, 2002). Therefore, for the purposes of this chapter, we define conjugative transposons in the loosest possible terms, as discrete DNA elements, usually integrated into the bacterial genome, which can transfer from a donor to a recipient cell by conjugation.

Because conjugative transposons have such a broad host range, they are very important in bacterial evolution (i.e., in disseminating genes between distantly related organisms, induction of deletion, rearrangements) and as a substrate for recombination events (Beaber et al., 2002; Mahairas and Minion, 1989; O'Keefe et al., 1999; Swartley et al., 1993).