FIG 1.

Structural arrangement of prokaryotic transposable elements. The TE is indicated by a pale-yellow horizontal bar at the top of each section. ORFs are shown as horizontal arrows with arrowheads indicating the direction of expression: purple, transposition-associated genes; red, antibiotic resistance genes; and green, other passenger genes. The inverted terminal repeats found at the ends of the majority of TE are shown as gray arrows, and the direct target repeats generally produced by insertion are indicated by small black arrows. (A) Insertion sequence (IS), a short DNA segment encoding only the mobilization protein (transposase, TnpA), flanked by two imperfect inverted repeats (IRs) and generally containing a short flanking directly repeated duplication (DR) on the target of insertion. (B) The tIS (transporter IS) is structurally similar to an IS but contains passenger genes. These are presently restricted to the IS1595 and IS66 families. (C) Compound transposons are formed by two IS in either a direct or an inverted orientation, flanking a variety of passenger genes, including those for antibiotic resistance. (D) Transposons are more heterogeneous structures and include different sets of transposition-related genes that are specific to each transposon family and multiple antibiotic resistances, virulence, and other passenger genes. This is an example of a Tn3 family transposon with transposon, tnpA, and resolvase genes, tnpR.