Role of pathogenicity island‐associated integrases in the genome plasticity of uropathogenic Escherichia coli strain 536

The genome of uropathogenic Escherichia coli isolate 536 contains five well‐characterized pathogenicity islands (PAIs) encoding key virulence factors of this strain. Except PAI IV536, the four other PAIs of strain 536 are flanked by direct repeats (DRs), carry intact integrase genes and are able to excise site‐specifically from the chromosome. Genome screening of strain 536 identified a sixth putative asnW‐associated PAI. Despite the presence of DRs and an intact integrase gene, excision of this island was not detected. To investigate the role of PAI‐encoded integrases for the recombination process the int genes of each unstable island of strain 536 were inactivated. For PAI I536 and PAI II536, their respective P4‐like integrase was required for their excision. PAI III536 carries two integrase genes, intA, encoding an SfX‐like integrase, and intB, coding for an integrase with weak similarity to P4‐like integrases. Only intB was required for site‐specific excision of this island. For PAI V536, excision could not be abolished after deleting its P4‐like integrase gene but additional deletion of the PAI II536‐specific integrase gene was required. Therefore, although all mediated by P4‐like integrases, the activity of the PAI excision machinery is most often restricted to its cognate island. This work also demonstrates for the first time the existence of a cross‐talk between integrases of different PAIs and shows that this cross‐talk is unidirectional.