Mairead Daly and Seamus Fanning
Antimicrobial resistance determinants may be transferred among bacteria via mobile genetic elements including plasmids, transposons, and the more recently explored integrons (1). Integrons are naturally occurring genetic elements found as part of the Tn21 transposon family or located on various broad host-range plasmids (2). The fundamental integron structure consists of a 5'-conserved segment (5'-CS) of 1.4-kbp and a 2-kbp 3'-CS (3). Between these conserved regions are DNA sequences of variable length and molecular complexity. These intervening sequences are known as gene cassettes, and several have now been characterized (Fig. 1). Acquisition and dissemination of these genes located within the integron structure, results in an increase in antimicrobial resistance (4).
Three classes of integron structure have been described. Class 1 integrons are of principal importance in clinical isolates. The 5'-CS of class 1 integrons includes an intI 1 gene of 1358 bp, which encodes a specific recombinase, a member of the DNA integrase family (5). This gene contains the attl recombination site, required for specifically integrating gene cassettes (Fig. 1). Classes 2 and 3 also contain integrase genes (intI 2 and intI 3), with the former showing 40% sequence identity to those of class 1, and the latter showing 61% (1). All three classes of integrons contain similar gene cassettes from the same families, which suggests the existence of a common pool of gene cassette with cross-specificity between the classes (1).
When the 3'-CS region is examined in detail, it contains several open reading frames (ORFs). These include qacEA1, which confers resistance to quaternary ammonium compounds, often associated with antiseptics, along with a sull gene expressing resistance to sulphonamide antimicrobial agents. The sull gene encodes the enzyme hydopteroate synthase. Transcription of the sull gene begins at a promoter located in
From: Methods in Molecular Biology, vol. 268: Public Health Microbiology: Methods and Protocols Edited by: J. F. T. Spencer and A. L. Ragout de Spencer © Humana Press Inc., Totowa, NJ
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