FIGURE 1.

Structure and motility functions of PAIs. (A) Insertion and excision processes of PAIs. PAIs are chromosomal fragments of pathogenic bacteria that encode biological functions involved in virulence. Their insertion in the chromosome is due to the presence of att sites at a chromosomal acceptor site (attB) and in the episomal PAI (attP). They are recognized by integrases, which catalyze a recombination of att sites. It results in the insertion of the episomal element at the attB site and the formation of direct repeated sequences (DRS) also named attL (left DRS) and attR (right DRS) in the ends of the inserted PAI. The excision of the PAI results from recombination between the direct repeats attL and attR. Catalyzed by integrases and recombination directionality factors (RDFs) also called excisionases, it generates an episomal element that contains one of the att sites (attP), while the other att site remains in the chromosome (attB). (B) Horizontal transfer of PAIs via conjugative plasmids, ICEs, and phages harboring att sites. Episomal PAIs can be inserted at att sites in conjugative plasmids, ICEs and phages as described above and then transferred into a bacterial recipient via conjugation for ICE-type and plasmid-type navettes or via transduction for phage-type navettes.