Figure 5.

A detailed model of the Rad5-mediate template switch pathway, assuming gap filling dependent on a subset of HR functions after the replication fork has passed. First, Siz1-dependent, SUMOylated PCNA stimulates Srs2 helicase to disrupt Rad51 filaments that have initiated D-loop formation. The role of Rad51 and other HR proteins still needs to be defined; they may rejoin and support downstream events in a remodeled complex. Depending on Rad18, PCNA is polyubiquitinated by Rad5-Mms2-Ubc13 (see Fig. 2), which, in an unknown way, promotes gap filling by DNA synthesis. (SUMOylation and polyubiquitination do not have to occur sequentially but may persist simultaneously on different subunits of the PCNA trimer.) The resulting hemicatenane structure is resolved by Sgs1-Top3-Rmi1 helicase, itself dependent on SUMOylation by Ubc9-Mms21. (Based on data from Branzei et al. 2008.)