Integrons Gather Genes into an Expression Site

Integrons are regions of DNA that gather together other DNA regions having the potential to encode proteins. Integrons then convert those regions into functional genes by placing them next to an active promoter, a region of DNA where RNA polymerase binds and begins making mRNA from an adjacent gene. Integrons do not move themselves, but instead they bring relatively small gene cassettes into an insertion site located next to a gene encoding a recombinase (integrase). To be moved into an integron, a region of DNA needs to have only a sequence that is related to the nucleotide sequence at the insertion site. Many of the “procured” genes encode proteins responsible for antibiotic resistance. Consequently, integrons, by accumulating sets of resistance genes, confer multidrug resistance (see Box 6-4). Integrons are involved in resistance to aminoglycosides, chloramphenicol, trimethoprim, rifampicin, erythromycin, fosfomycin, lincomycin, antiseptics of the quaternary ammonium family, and all known ß-lactams.¹⁴⁹