PMC

Figure 1. Schematic representation of class 1 (C1) integrons. In0 is the most basic integron that does not contain any gene cassettes. Different cassette arrays can be integrated in the variable region of the integron. Abbreviations: intI1, class 1 integrase gene; P_c and P2, gene cassette promoters; attI1, integron-associated recombination site; qacEΔ1, truncated version of a quaternary ammonium resistance gene; sulI, sulphonamide resistance gene; orf5, open reading frame; attC, recombination site of the gene cassette; GC, gene cassette; 5′-CS,5′ conserved segment of the integron; 3′-CS,3′ conserved segment of the integron.

Figure 2. Representation of the possible mechanisms contributing to mobility of class 1 (C1) integrons (in red). (A) Transposition of C1 integrons due to linkage with MITEs and a transposase acting in trans; (B) Transposition of C1 integrons due to linkage with a composite transposon with IS elements; (C) Transposition of C1 integrons due to linkage with a transposon; (D) Exchange of gene cassettes by double homologous recombination between the conserved end regions of C1 integrons; (E) Integration of C1 integrons/gene cassettes by homologous recombination between two adjacent DNA regions in the chromosome; (F) Transposition-like movement of C1 integrons containing two Inverted Repeats and a tni module provided in trans. Abbreviations: IRi, inverted repeat at the 5′-CS outer end of the C1 integron; IRt, inverted repeat at the 3′-CS outer end of the C1 integron; IR, inverted repeat; tnp module, transposition module including the tnpA, tnpR and tnpM genes; TnpA, transposase enzyme; TSD, target site duplication; RecA, enzyme responsible for homologous recombination; tni module, transposition module including the tniA, tniB, tniQ and tniR genes; Tni, TniA, TniB and TniQ enzymes.