Fig. 7.

From early leptotene to SC formation. (A, Upper) From left to right: pictures corresponding to the different stages described in the paper. (A, Lower) Cartoon of the experimentally observed proteins (indicated above) involved in the corresponding transitions. (B) Proposed model for Zip2-Zip4-mediated coalignment-to-synapsis transition. From left to right: Zip2-Zip4-Mer3-Msh4–mediated recruitment of SC central component Sme4/Zip1 (red arrows) and SC central components (black line). The resulting ensemble, bound to axes components, is released from the axes through bridge formation before relocalization halfway between axes. A signal for SC nucleation then occurs which disassembles the bridges and brings axes closer together, allowing SC initiation. During this process, the Zip2-Zip4-Mer3-Msh4 ensemble now gets associated only with SC central components. SC is thus nucleated by centrally located Zip2-Zip4-Mer3-Msh4, and SC initiation occurs at sites of recombination specifically because of their association with Zip2-Zip4. Recombination complexes are thereby automatically localized on SC central elements. Instead of recombination complexes being “moved” to the SC, the SC comes to the recombination complex.