(A) Class-1 integrons consist of a 5'-conserved region (black section) and an integrase (intI) that can capture gene cassettes (GC, colored rectangles) and insert them at the attI gene site. They are the most wide-spread class of these genetic elements and contain arrays ranging from one to five gene cassettes. The gene cassettes are separated by attC sites (trapezoids with black borders). Exposure to low doses of antibiotic (yellow tablets) activates the integrase (pacman symbols). (B) The integrase can rearrange the gene cassettes by either cutting them at attC sites (b1), or by inserting previously excised or free gene cassettes at the attI site (b2). The integrase acts by breaking and rejoining the bottom DNA strand (red). After DNA replication (gray arrows), daughter integrons are parental (derived from the blue top strand) or carry a changed gene cassette array (derived from the bottom strand). (C) Increased antibiotic concentrations exert selective pressure on the diversity of the generated integron. Those carrying the appropriate gene cassette (yellow rectangle) in the first position are favored (+) due to higher expression of resistance, while other gene cassette arrays are counter-selected (–).