Fig. 4.

Hypothetic possibilities for the evolution of a site-specific and a transposition system from each other. (A) A possible sequence of events potentially leading to the spontaneous development of a phage-encoded site-specific recombination system based on a transposable element. Most of these steps have already been described in different systems [IS insertion into a phage (36), partial deletion of an IS (37), and insertion adjacent to an IR (15)]. This series of events is only one of the numerous conceivable scenarios. Note that the third state is already able to fulfill the site-specific integration and excision by using an IR end as att site (13). (B) The reverse process resulting in a transposon is also possible. Note that the phage integrated into a secondary site is structurally analog to a transposon. Thick line, phage DNA; curved line, DNA of the host cell; black and white arrowheads, IRs; open box, coding sequence of the recombinase.