FIG. 4.
General structures of transposable elements and integrons. (A) Organization of a typical IS. Terminal inverted repeats are represented by gray bars labeled IRL (left inverted repeat) and IRR (right inverted repeat). A single open reading frame encoding the transposase tnp is between the inverted repeats. The transposase promoter (P) is partially contained within the IRL. (B) Organization of a composite transposon. One or more genes (light gray boxes labeled orf) are flanked by two usually identical insertion sequences (IS). Transposition is mediated by the IS-contained transposase recognizing the IRL of the first IS and the IRR of the second IS. (C) Organization of a Tn3-like complex transposon. Transposase and recombinase genes are represented by white boxes labeled tnpA and tnpR, respectively. Non-transposition-related coding sequences are represented by light gray boxes labeled orf. These coding sequences are frequently found in the context of integrons. (D) Organization of a typical class 1 integron. The 5′ conserved region consists of the integrase gene intI1 followed by the attI recombination site, represented by a small black box. A forward-directed promoter (Pc) within intI1 drives expression of gene cassettes. Gene cassettes (labeled orf1, orf2, and orf3) are separated by their attC sites (orange boxes). The 3′ conserved segment in most class 1 integrons consists of a truncated quaternary ammonium compound resistance gene (qacEΔ) fused with a sul1 sulfonamide resistance gene.