Figure 1.

 Two models for Flp site-specific recombination. The two models for Flp recombination proposed recently (Qian and Cox 1995; Lee and Jayaram 1996; Lee et al. 1996) are shown schematically to depict their modes of operation during the generation of a Holliday junction from two linear DNA duplexes, and the subsequent resolution of this junction. The phosphodiesters at the cleavage/exchange positions are numbered 1–4. The Flp monomers bound adjacent to each of these positions are indicated by the appropriate subscript (F₁–F₄). The horizontal parallel arrows represent Flp-binding elements. (A) In the asymmetric trimer model, catalytic contributions from three Flp monomers F₁, F₂, and F₃ are sufficient to mediate strand cleavage and exchange at the left end of the strand exchange region (spacer). The directionality of Tyr-343 donation (or the cleavage mode; indicated by the curved arrow) conforms to trans-horizontal (F₂–F₁) plus trans-vertical (F₁–F₃), as defined by Chen et al. (1992). Note that F₄–F₁ (trans-diagonal) and F₁–F₃ (trans-vertical) cleavages would also yield exchange at the left. Resolution of the Holliday intermediate at the right may be mediated by the F₁–F₂–F₄ set or the F₂–F₃–F₄ set. (B) In the dimer of asymmetric dimers model, it is the whole tetramer of Flp (F₁–F₂–F₃–F₄) that is the catalytic unit in recombination. For strand exchange at the left, F₁ and F₃ provide the RHR triad domain, and F₂ and F₄ provide Tyr-343. Here, the cleavage is depicted to occur in the trans-horizontal fashion. The preponderance of experimental evidence supports this cleavage mode by Flp (Lee et al. 1994, 1996). For resolution at the right, the F₁–F₃ pair and the F₂–F₄ pair reciprocally reverse their catalytic contributions. Note that trans-diagonal mode of cleavage would also fit the model.