Transposon Transfer of Drug Resistance

Abstract

Bacterial DNA is a dynamic molecule, subject to extensive structural rearrangements. The main mechanism for restructuring of bacterial genomes is the illegitimate recombination between heterologous DNA sequences, a process made possible by transposable DNA elements. Bacterial transposons can be divided into three classes. Class I transposons comprise the insertion sequence (IS) elements as well as the composite transposons, class II transposons include the Tn 3-elements, and class III transposons form a group of elements possessing properties not found in classes I and II. Integration of transposons into foreign DNA can occur as “conservative transposition”, in which the element moves from a donor to a recipient site, or in a mode called “replicative transposition”, in which the transposon is copied as part of its movement. Besides insertion, transposable DNA can promote deletion, inversion, excision, or the fusion of two complete replicons. The mechanisms of illegitimate recombination described have contributed significantly to the evolution of drug resistance in clinically important bacteria. Molecular data suggest that many drug resistance genes have evolved in antibiotic-producing microorganisms. Mobilization of these determinants was initially made possible by transposable DNA elements. The rapid spread of determinants through bacterial populations then occurred via plasmids and bacteriophages. Mutations in resistance genes of transposons resulted in the evolution of determinants coding for resistance to new antimicrobials.