Fitness drift of an atrazine-degrading population under atrazine selection pressure

Pseudomonas sp. ADP harbouring the atrazine catabolic plasmid ADP1 was subcultured in liquid medium containing atrazine as sole source of nitrogen. After approximately 320 generations, a new population evolved which replaced the initial population. This newly evolved population grew faster and degraded atrazine more rapidly than the initial population. Plasmid profiles and Southern blot analyses revealed that the evolved strain, unlike the ancestral strain, presented a tandem duplication of the atzB gene encoding the second enzyme of the atrazine catabolic pathway responsible for the transformation of hydroxyatrazine to N-isopropylammelide. This duplication resulted from a homologous recombination that occurred between two direct repeats of 6.2 kb flanking the atzB gene and constituted by the insertion sequences IS1071, ISPps1 and a pdhL homologous sequence. This study highlights the IS-mediated plasticity of atrazine-degrading potential and demonstrates that insertion sequences not only help to disperse the atrazine-degrading gene but also improve the fitness of the atrazine-degrading population.