Conjugative transposons, found in bacteria, are hybrid elements that can both transpose and can move from cell to cell, like a transmissible plasmid. The first of these to be discovered, Tn916, confers tetracycline resistance and was found in the bacterium Enterococcus faecalis. Tn916 carries several genes needed for conjugative transfer and is therefore much larger than most transposons.
Tn916 jumps by the cut-and-paste mechanism. However, it differs in two respects from typical conservative transposons. First, the target sequence is not duplicated when Tn916 inserts itself. Second, it can excise itself precisely, leaving the host cell DNA intact.When moving from one bacterial cell to another,Tn916 is thought to excise itself temporarily from the DNA of the original cell. It then transfers itself into the recipient and, once inside, it transposes into the DNA of the new host cell (Fig. 15.24). Tn916 and related elements can enter many different groups of bacteria, both gram-positive and gram-negative. Because the host range of conjugative transposons is so broad they are partly to blame in the spread of antibiotic resistance genes among diverse groups of bacteria.
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