Figure 23.

Template switch model of recombineering. (A) As before, a Red-processed dsDNA end (Beta bound to a ssDNA overhang generated by Exo) invades a replication fork and promotes annealing to the lagging strand template. (B) Beta captures the leading strand and promotes a template switch, such that the leading strand polymerase now uses the incoming strand as a template. A nick is introduced in the leading strand template by an unspecified nuclease. (C) The redirected polymerase completely resynthesizes the incoming strand, reestablishing a dsDNA end. The 3′ end of the invading strand is filled in and ligated to complete the recombination event. (D) The products of the reaction are an intact chromosome (after filling in and ligation) and the broken end containing the incoming substrate. This dsDNA end could be acted upon by the λ Red system and invade the leading strand template of another replisome (for small homology substrates), or by the RecA-dependent pathway for recombination (for long homology substrates) to complete the gene replacement.