(a) Tn10, an example of a composite transposon. The IS elements are inserted in opposite orientation and form inverted repeats (IRs). Each IS element carries a transposase, but only one is usually functional. (b) Tn3, an example of a simple transposon. Short inverted repeats contain no transposase. Instead, simple transposons encode their own transposase. The resolvase is a protein that promotes recombination and resolves the cointegrates (see Figure 15-9).